Due to the finite nature, health and environmental hazards currently associated with the use of fossil energy resources, there is a global drive to hasten the development and deployment of renewable energy technologies. One such area encompasses perovskite solar cells (PSCs) that have shown photoconversion efficiencies (PCE) comparable to silicon-based photovoltaics, but their commercialisation has been set back by short-term stability and toxicity issues, among others. A tremendous potential to overcome these drawbacks is presented by the emerging applications of graphene derivative-based materials in PSCs as substitutes or components, composites with other functional materials, and enhancers of charge transport, blocking action, exciton dissociation, substrate coverage, sensitisation and stabilisation. This review aims to illustrate how these highly capable carbon-based materials can advance PSCs by critically outlining and discussing their current applications and strategically identifying prospective research avenues. The reviewed works show that graphene derivatives have great potential in boosting the performance and stability of PSCs through morphological modifications and compositional engineering. This can drive the sustainability and commercial viability aspects of PSCs. This review discusses how incorporating graphene derivatives as components of perovskite, charge transport and blocking layers of perovskite solar cells affects substrate coverage, defect passivation, morphology and charge extraction/transport. For example, 18% of the total global deaths in 2018 were from pollution-related ailments emanating from the use of fossil-based energy resources.2 According to 2018 statistics from the World Health Organization (WHO), the use of fossil-based energy resources has mostly affected children .3 Economic and health costs  due to air pollution from burning fossil fuel energy resources ascended to $2.9 trillion in 2018.4 Since fossil-based energy resources are non-renewable and causing alarming damaging outcomes, other alternatives need to be developed to dependable levels.As the global village advances in machinery and technology that sustains and enables humankind to live comfortably, environmental and health issues are also rising from the extensive dependency on highly polluting fossil-based energy resources. Fossil resources currently provide 81% of the global total energy supply.5 For instance, in Europe, where 70% of the global total installed solar cells are located, annual PV installation rose from 58 MW to 222 GW between 2000 and 2015.6 In this ambit, perovskite solar cells (PSCs) in particular, are not only a rising energy technology but a cheaper option as well. In 2016 the hypothetical levelized cost of electricity (LCOE) for a perovskite solar cell (PSC) with PCE of 20% and a projected PSC life span > 15 years was calculated to be 3.5 US cents per kW h, whilst the LCOE for traditional energy resources, such as oil and gas, was in the range US 7.04–11.90 cents per kW h.7,8 PSCs have emerged with great potential to compliment market-leading Si PV due to their high PCE and potentially low manufacturing costs.8,9 Additionally, in 2017, Song et al.10 stated that the production cost of a standard PSC was US $31.70 m−2  components). This means the manufacturing cost of PSCs, after excluding BOM component expenditure, is lower than that of other thin-film solar cells (CIGS: US $29 m−2 and CdTe: US $27 m−2).8,10 This analysis also infers that the major contributor to PSC costs is the BOM components.One hugely important renewable energy technology is photovoltaics, with a global energy output growth of 60% between 2000 and 2016.10,11 Examples of expensive BOM components in PSCs include noble metals, such as Au, conductive substrates, and hole transporting layers (HTLs), such as spiro-OMeTAD.12,13 The use of lower-cost, alternative sustainable materials or strategies (such as remanufacturing) will be crucial in minimising LCOE and improving the chance of commercial success.Additives and substitution of BOM components with lower-cost alternatives is a promising approach in balancing the possible trade-off between cost and PCE. In a PSC, 76% of the total expenditure is from materials; however, the cost of the material in the perovskite unit cell structure and active material contributes only 7.6% to this.6 One strategy to help alleviate this problem is the greater use of eco-friendly materials that can reduce both bio-incompatible waste accumulation and recycling costs. Coupled with this waste problem is the growing use of primary materials, with renewable energy technologies requiring significant amounts of critical raw materials. To ensure rapid deployment of renewable technologies is not disrupted, materials with inherent natural abundance are needed. Graphene derivatives are one such class of materials as they can be obtained from the abundant and naturally occurring graphite and biomass. To date, sustainable additives such as graphene derivatives have principally shown potential in aiding exciton dissociation, conductivity and long-term stability in PSCs , HC(NH2)2+ (FA+), Cs+ or mixed cations, such as FAMA, which occupy the cubo-octahedral site shared with twelve halide anions X−. B is the smaller cation  which is stabilised by the octahedral setup and shared with six halide anions, X−.14–20 An organic–inorganic hybrid perovskite is usually used in solar cells, and the methylammonium lead iodide (MAPbI3) is the most common , long charge-carrier lifetime and long charge-diffusion path (>1 μm).24–29 A further advantage is the compatibility with low-cost solution processing techniques. In addition to quality (morphology and uniformity), the film thickness of the perovskite layer is an important factor for maximising the efficiency of devices. The high absorption coefficient of perovskites results in thin-films of ca. 400 nm being suitable for efficient photon capture. This is advantageous because it results in less use of materials; even though perovskites are typically composed of earth-abundant materials, this still benefits cost and helps reduce materials wastage.Perovskites have the general formula ABXt common . Perovsk21 Interfacial electron transfer from the perovskite conduction band to the HTL and/or ETL surface states, together with interfacial electron transfer from the conduction band of the ETL to the HTL and/or perovskite, are possible recombination mechanisms in PSCs , while holes are extracted at the hole transport layer (HTL) prior to transportation to the anode and cathode, respectively .21 Inter in PSCs .30 The c31 to a certified value of 25.5% by 2019,32,33 which is comparable to the 26% PCE of monocrystalline Si-based solar cells.33–43 PSCs are well-positioned to be successfully commercialised since their PCE has surpassed other thin film-based solar cells (CIGS: 23% and CdTe: 22%).33,44,45 This significant improvement of PCE within the last decade is mainly attributed to morphological tailoring and compositional engineering of the PSC device layers.46 Relative to the first two photovoltaic generations, PSCs have the potential to harness light at lower costs and are also associated with facile fabrication procedures.47The PCE of PSCs has dramatically increased from about 10% in 2012 19,40 PSCs have a great potential in finding a niche in the world energy markets because they are highly efficient, printable, can be noble metal- and HTL-free.48 For example, PSCs have high potential in wearable devices due to the associated facile fabrication, high PCE and low costs.39,49,50 To date, the technology has already advanced to mini- and standard-sized modules.19 For instance, Yeo et al.51 reported PCEs in the 8.1–10% range for scalable flexible and rigid modules with an area of 10 cm2. However, PSCs are still not found substantially in the global markets because the technology is in the laboratory/industrial scale transition phase.21,48,52 Roll-to-roll technology is more practical for optimum substrate coverage at large-scale due to low fabrication costs, sample nature, device structure and manufacturing steps involved. The transformation from laboratory to large-scale roll-to-roll deposition still needs more insights on several aspects, such as solvent selection, annealing temperature, and film thickness control and deposition rate, to avoid a substantial decrease in PCE.53,54Over 12 000 articles on PSCs had been reported by 2019, and since then, companies have been focusing on the commercialisation of the technology.43 such as DMF,43 γ-butyrolactone (GBL),55 chlorobenzene and toluene,12,19 and Ag and Pb as counter electrodes.21,42 Notwithstanding the fact that the toxicity of Pb(ii) salts is a problem, studies are yet to provide a lucrative prospective substitute material to Pb-based PSCs.56 Possible substitutes being considered include elements in the same periodic group, Sn and Ge; however, their toxicity, stability and band gap still need tailoring to achieve comparable absorption and PCEs.15,57 There is also the possibility of solvent-free approaches58 although control over uniformity and morphology can prove difficult.Additionally, the commercialisation and large-scale production of PSCs is also hindered by toxicity issues due to the use of solvents,14,28,59,60 The reviewed research has shown interesting stability profiles of PSCs in terms of shelf-life, continuous illumination, short circuit (increased accumulation of charge), bending cycles and defined relative humidity (RH) , interface state(s), photosensitivity, and thermal, oxygen and moisture effects that cause perovskite degradation.9,61–66 Moisture (RH of >55%) catalyses perovskite degradation to a polar state via H+ extraction from the perovskite and facilitates the dissolution of water-soluble components as MAI, causing a colour change.15,67–69 Oxygen detrimental effects occur when the molecules diffuse into the perovskite, and become trapped in the halide vacancies where interaction with an excited state forms reactive superoxide species.15,62,64,69 Ultraviolet and visible irradiation can also cause device degradation through accelerated ion migration.28,57,64,70–72 The blue to the ultraviolet region of the solar spectrum also triggers the dissolution of the perovskite organic component.73 Perovskite degradation to PbI2 can be thermally induced altering the physicochemical properties, such as a reduced optical absorption due to an enlarged band gap.15,63 Perovskite phase separations (cause photo-inactive states and current blocking) and phase transformations, such as FA0.9Cs0.1PbI3 to FA>0.9Cs<0.1PbI3, deteriorate PCE.63,64 Additionally, studies have shown that I2 vapour does not only cause localised degradation but also induces decomposition of neighbouring perovskite regions during operational conditions.74,75 Interestingly, under illumination, I2 is more easily released from PbI2 than from perovskite,74 meaning that once some perovskite decomposes to PbI2 more problems are initiated.Another key aspect hindering commercialisation is the stability of the perovskite layer with, to date, the best-reported device lifetimes of approximately one year.ity (RH) . In-dept76 With the current state of the art, real-world devices will need to be fabricated in such a way as to limit humidity and oxygen exposure. Thus, more work is needed to develop new charge selective carriers, and engineer new perovskite materials, interfacial modifications, and novel configurations to achieve long-term stability.15In research laboratories, device fabrication is carried out in a glove box to ensure a controlled atmosphere, but this may not be easily achieved in large-scale industrial production. In addition, real-world PSCs will be applied in RH environments that exceed the laboratory average of 30–50%.2.12) layer on top of the compact ETL, whereas planar devices just have a compact ETL layer.77–79 Perovskites are suitable for planar structural solar cells due to their ability to transport holes and electrons simultaneously, and have long exciton diffusion lengths (100–1000 nm) and favourable excited-state lifetimes (100 ns).56,80 PSC devices can be manufactured in both n–i–p (transparent conducting electrode (TCE)/ETL/perovskite/HTL/cathode 24,36,41TL/anode ) configulectrode .28i.e., defects; imperfect crystals and undesirable interfacial states) and recombination.83,84 The carrier lifetime, charge extraction and charge transfer issues that cause charge recombination and thus lower photocurrent in PSCs, particularly for planar architecture, have been linked to the ETL used.44 Other common issues related to the introduction of charge transport layers in PSCs include the cost of materials, deterioration of electronic characteristics due to poor alignment of energy levels and charge extraction , slow electron mobility, high hydrophilicity of components causing poor chemical stability, and short-term stability and poor perovskite quality .21,85,86Studies on planar PSCs have highlighted the need for critical crystallisation and growth control to minimise morphological deviations , 0D from impurities and 2D .Rs) from 27.8 to 15.2 Ω.93 This was ascribed to enhanced crystallinity of the perovskite in the composite. Additionally, surface potentials of a PSC were lowered with an increase of both Jsc and Voc by lowering the HOMO level through the addition of polyaniline (PANI) to a GO/poly:polystyrene sulfonate (PEDOT:PSS) composite.94 Graphene derivatives have great potential in improving some of the most significant drawbacks of optimising PSCs. Despite this great potential, the use of graphene-based materials in PSCs is still at primitive stages;42 and, hence, needs constant review to reveal their potential holistically.Graphene derivatives have shown potential in improving substrate coverage, defect passivation, and charge extraction/transport .96 Furthermore, the development of several graphene derivatives has generated numerous and sometimes confusing terms, such as transferred, suspended, epitaxial grown, chemical vapour deposition (CVD)-grown, isolated-freestanding, and nanoplatelets, among others,97,98 which represent GO-based materials with different physicochemical properties. The major graphene derivative examples applicable in PSCs include GO ,33 the relative increase in performance of graphene derivative-based devices from their first use, has been attributed to several factors that are inclusive of chemical inertness, increased charge transfer pathways, and surface area. For instance, a relatively large surface area of 202 m2 g−1 in a graphene derivative-based PSC (graphene derivative applied on top of mesoporous ZrO2 and TiO2) created a high number of interconnected channels that allowed perovskite precursor penetration and decreased the photoluminescence (PL) lifetime (from 1.03 to 0.698 ns).100A comprehensive summary of the use of graphene derivatives in PSCs is presented in 21 Hence, GO is more appropriate for large-scale PSC development than graphene because solution processable GO synthesised via several chemical exfoliation methods96 can disperse well in several solvents compatible with PSC fabrication, and allows for potential further functionalisation.The use of graphene grown by CVD in PSCs potentially introduces high costs for large-scale manufacturing because of complicated transfer steps of graphene onto the target substrate. In addition, it is associated with both poor contact and minimum film thickness control.25 fast charge decay is affected by fast transport of free carriers from the perovskites to respective contacts.101 The electron and hole mobility need to be balanced in PSCs, and this can be attained through the synthesis of composites, such as polymer-perovskite,102,103 metal oxide-perovskite104,105 and graphene derivative-perovskite composites.46,106 For instance, GO-perovskite composites were reported to increase charge separation (PL coefficient due to non-radiative surface-state relaxation of perovskite was one-tenth of that of the composites), charge mobility (from 29 to 35 cm2 V−1 s−1) and recombination resistance (from 8865 to 99 978 Ω cm2).80 Also, the addition of RGO to mesoporous TiO2 [(m-TiO2)–RGO] was reported to double the electron diffusion coefficient from that of a m-TiO2 ETL.26 In another study, the electron mobility of m-TiO2 was raised from 2.5 × 10−7 to 4.2 × 10−7 cm2 V−1 s−1 in a RGO–m-TiO2 ETL.107 Doping of PEDOT:PSS with GO was reported to improve the hole mobility from 5.55 × 10−5 to 1.57 × 10−4 cm2 V−1 s−1.108 These studies demonstrate the potential of graphene derivatives in promoting charge mobility via the provision of additional pathways for effective excited-state charge carriers and charge transport away from the PSC active layer .36,107 Graphene derivatives can participate in defect passivation at both exterior and interior film surfaces through bonding of their functionalities  scale" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>O) to uncoordinated ions on the perovskite surface.109,110 Hence, graphene derivatives, such as GO, have lucrative research prospects for defect passivation in PSCs.Since the mechanism of PSCs is inclined to the charge transport layer used for quenching charge carriers,ve layer . This is44 In recent times, other positive prospects have shown the inclusion of NiO–RGO in a glass/FTO/Sr–TiO2/Al2O3–graphene/NiO/perovskite/NiO–RGO/spiro-OMeTAD/Au configuration, achieving 95% of the theoretical photocurrent density of 27.2 mA cm−2 , and facilitate fast charge mobilities . This highlights the suitability and potential benefits of using RGO composites in PSC technology.The conventional and inverted planar architectures of graphene derivative-based PSCs have achieved PCE values of 18.2 and 13.8%, respectively. mA cm−2 .111 ThisVoc, an increased conductivity leads to a high charge collection efficiency and Jsc.21,112 On the one hand, the valence band position of the HTL should align with the HOMO of the perovskite layer , and increase hole extraction (shortened PL lifetime from 0.36 to 0.29 ns) between polyvinyl acetate and carbon in a FTO/compact TiO2 (c-TiO2)/m-TiO2/perovskite/polyvinyl acetate/GO/carbon cell.13 On the other hand, the LUMO of the perovskite should align with the LUMO of the ETL , by reacting GO with molecules such as NH3 85 and doping21,115 with elements such as B99 and F.114 For example, silver trifluoromethane sulfonate doped GO achieved a lowered Rs (from 16.44 to 12.11 Ω cm2) and electron/hole recombination due to WF modulation.116 Mann et al.117 also reported RGO WF modulation to 5.34 eV via sulfur doping that improved functionality of PEDOT:PSS to achieve a 19% increase in the PCE and improved stability (stable PCE after 10 days for sulfur–RGO/PEDOT:PSS versus continuous decline for 30 days in control PEDOT:PSS devices). The graphene derivative dopant must increase performance without compromising other parameters, such as transmittance and charge extraction. For illustration, Kim et al.118 altered sheet sizes of GO by sulfur-doping, and this influenced the extraction of charge carriers in a PSC , highlighting the importance of optimising sheet sizes. Composite synthesis was also reported to modulate the WF; for example, MoO3 and Ag raised the WF of GO from 4.23 to 4.71 eV119 and 4.95 eV,103 respectively. Another reported functional composite example is GO–Li, with a lowered GO WF of 4.3 eV (from 4.9 eV).120 This was rationalised by the ability of Li to donate a valence electron to GO and the associated induction of dipoles by Li+; therefore, the Fermi level shifts towards vacuum and the WF is minimised.Other important aspects of a PSC device are linked to conductivity and band alignment of constituent materials. It is difficult to balance the band alignment effects of GO with conductivity properties towards effective charge separation; hence, more research is needed. While the high work function (WF) effects of graphene derivatives promote better band alignment, ohmic contact and building of a potential difference towards higher te layer .56,99 Fo the ETL . Theoret95,121 Whilst it is a good fabrication strategy to utilise the high dispersibility of GO in promoting substrate coverage with perovskite, the subsequent reduction process may introduce complications in PSCs. On the other hand, despite aggregation effects and poor dispersion-ability from prior GO reduction, fabrication may require the use of both surfactants (such as sodium dodecylbenzene sulfonate)22 and chemical reductants (such as hydrazine hydrate),22,121 which may be difficult to handle at large-scale due to their toxic and explosive properties. A moderate annealing temperature in air , UV radiation and chemical treatment are possible routes to reduce GO towards improved device conductivity.24,56,122,123 Annealing graphene derivatives at high temperatures (above 500 °C) for PSCs is not suitable for most flexible substrates and perovskites (stability and contact issues). Typical contact problems can be improved by using volatile pore-forming agents that evaporate upon thermal treatment, leaving voids occupied by the perovskite.21GO is transformed to a semiconducting character by tuning the associated WF through reduction.et al.46 studied the effect of the oxygen content of GO  used as an additive to the HTL (spiro-OMeTAD) of PSCs and comprehended that 10% oxygen content was the optimum with a negligible drop in PCE after 18 days of storage under a RH of 20%. Cho et al.124 investigated the most effective location of RGO in a PSC and obtained the best results when it was used as an ETL component . A similar study reported a 20% PCE improvement through a simultaneous inclusion of RGO in the ETL and active layers of a PSC.107 From the WF discussion, it is possible that the oxygen concentration influences RGO performance in PSCs. This view is supported by density functional theory (DFT) calculations that showed that the epoxy oxygen concentration tunes the charge extraction ability as follows: if <33% then both holes and electrons can be extracted, for the 30–66% range electron transport is permissible, but if >60% no extraction is conceivable.125 However, since it is rare to find only one particular type of oxygen species in a graphene derivative after synthesis, it is not easy to attain such tailoring due to the presence of other moieties.The effective role of graphene derivatives in PSCs still needs further in-depth understanding. For example, Kim 115,126,127 HTL,56,81,82,115,126,128 buffer and active layer additives,56,81 and transparent conducting electrodes (TCE)  .22,81,123.17,129 and energy consumption, natural brittleness, and the scarcity of indium (in the case of ITO glass), which is estimated to be 0.05 ppm and 0.072 ppm in the continental and oceanic crust, respectively.130The role of a TCE, commonly designed to be the layer facing the light source, is to collect electrons from a semiconductor and transfer them to the external circuit. The most common TCEs are fabricated from indium- and fluorine-doped tin oxide (ITO and FTO). The problems associated with these commonly used TCEs include high fabrication costs 131,132 The thermal stability of RGO promotes heat dissipation within the PSC,95 hence, reducing the risk of thermally-induced device degradation. While the use of pristine graphene as both front and back electrodes can work,78,133,134 its intrinsic low wettability, due to its hydrophobic nature, causes the formation of poor perovskite films, limiting its usage. Other graphene derivatives, such as GO and RGO, are suitable PSC TCE alternatives due to their good mechanical strength (Young modulus of 1 TPa), ability to facilitate heat dissipation, and lower perovskite degradation and ion migration rates at elevated temperatures. P-Type graphene derivative-based TCEs, such as GO, are relatively stable in air and therefore easily produced.135 GO reduced by chemical or laser means is suitable for TCEs fabricated by spin-coating and result in PSCs with low charge impedances.Theoretical simulations conducted with COMSOL and SCAPS-1D software have predicted graphene-based materials to be effective as either top or bottom TCEs in PSCs as they offer better heat dissipation and stability than their metallic counterparts.95Despite the usefulness of graphene-based derivatives as TCEs, their practical application has been greater in other PSC layers, as reviewed in the subsequent sections. This is probably due to limitations in light transmittance to the perovskite layer, because graphene derivatives absorb visible light, and difficulties in deposition onto substrates. The drawbacks of replacing common TCEs in PSCs with solution-processed GO include lower conductivities and PCEs.21,81 Examples of graphene derivative-based TCEs in a flexible PSC are single-walled carbon nanotubes (SWCNTs)–RGO,81 SnO2–GO,136 Au–Cl3-doped graphene137 (with the use of 3-aminopropyl-triethoxy silane as adhesion promoter), and Ag nanowires–GO  and improved uniformity.29 The optimum performance of the composites was subject to the wt% of GO (optimum = 0.5 wt%). This means that even if the inclusion of graphene derivatives has benefits, maximum gains to overall PSC performance require optimisation of the amounts of components to avoid the formation of inhomogeneous films that culminate in a disrupted crystal quality. In summary, solution-processed graphene derivatives have the potential to improve crystallisation, morphology and charge transport in PSCs.The role of an active layer in a solar cell is to harness sunlight and, in turn, create charge carriers. Several graphene derivative-perovskite composites for use in the PSC active layer have been reported for a variety of reasons . For ins3.3−.110 Buffering layers are utilised in blocking holes and electrons from collecting at the anode and cathode, respectively. This ensures a balanced charge extraction towards the appropriate electrodes.81 Common buffering layers in PSCs are TiOx and PEDOT:PSS. Although the PSS component of PEDOT:PSS lowers the overall conductivity,139 its hygroscopic nature is manipulated for dispersion in water as a counter ion.101 The acidic and hygroscopic nature of PEDOT:PSS,101,140 as well as moisture sensitivity of TiOx, are some of the common setbacks of current blocking materials. Moisture in PSCs will not only deteriorate the perovskite at atmospheric conditions but also reduce electrode conductivity.141 Additionally, PEDOT:PSS has an intrinsic inefficient electron blocking ability.36Buffering in PSCs is important in several ways; for example, in minimisation of the reaction between extracted charges and X140,142 For example, the wider band gap of GO induces an effective blocking layer in PEDOT:PSS–GO composites –GO was reported to promote both oxidation resistance and moisture adsorption, thereby inducing better device stability (51 Another example is a poly(methyl methacrylate) (PMMA)–GO composite buffer that was reported to block electrons and increase carrier transport selectivity through shunt resistance (Rsh) enhancement and a decrease in Rs, respectively.145 Also, the strong chemical interactions between NiO, RGO and the perovskite in a NiO–RGO blocking layer were envisaged to boost stability against reactions with air and water . However, the HTL can improve the PCE by reducing recombination rates and enhancing Voc. The Voc in PSCs is mostly influenced by recombination at interfaces. This effect is large in HTL-free devices due to the longer-lived duration of charges at interfaces. Another driver for HTL use is that the surface morphology of the HTL positively influences electrical properties such as Rs and Rsh. This is important because a large Rsh infers small charge carrier recombination in the active layer and high selectivity of charge collection.101,152 An efficient hole selective collection will occur when current leakage in the reverse bias is decreased.152The role of HTLs is to extract holes generated in the perovskite layer.83 A commonly used HTL for the p–i–n configuration is PEDOT:PSS due to its tuneable conductivity and high optical transmittance within the visible region.82,115 Also, since the WF of GO (4.9 eV) and that of PEDOT:PSS (5.1 eV) align well, GO is able to decrease the associated Rs.56,140 Copper phthalocyanine27 and spiro-OMeTAD142,145,148 are also well reported functional HTLs. Owing to a compatible energy level with perovskites, spiro-OMeTAD is common in Au-based PSCs though it requires a complicated multi-step synthesis, is expensive and associated with low intrinsic charge carrier mobility, and thus requires dopants to perform well.12,15,21,54To be a suitable HTL in PSCs, the material requires an appropriate energy level, low deep-trap state density, sufficient charge extraction rate and transfer capabilities.80,153 but has a lower conductivity than RGO, rendering it an inferior transporter.154 The GO HTL can tailor optical properties,155 crystallisation and morphology of perovskites.140 A uniform, compact and void-free GO layer is important for device performance, but some studies have encountered difficulties in achieving this with GO.56,115 Unlike N–RGO, RGO has been reported in some instances to cause deterioration of perovskite morphology.95 Cracks and voids in the perovskite layer are recombination centres with poor uniformity leading to increased shunt paths, current leakages, and low voltage.21,115,156 This phenomenon is subject to the deposition method and is a signal of the importance of uniform layers throughout a PSC. Furthermore, inhomogeneity of GO layers on TCEs produces non-uniform electrical characteristics due to either direct contact between the perovskite and TCE or insulating behaviour that results in poor hole transport.GO functions as a hole acceptor and transporter,et al.56 overcame this shortfall by a repeated spin-coating of GO and PEDOT:PSS to reduce surface roughness. Sequential solution deposition has achieved decent, repeatable, and easily controlled deposition, proving better than both vacuum deposition and vapour-assisted solution-processing.142,157 Nouri et al.115 achieved a uniform thin film by first cleaning FTO with ethanol to increase its hydrophilicity, and then spin-coating twice with GO dispersed in isopropanol at 200 rpm (PCE increased from 4.4 to 10.2%). Feng et al.85 reported PEDOT:PSS/GO–NH3 as a HTL with improved optical transparency, texture and complete substrate coverage morphology. NH3 treatment of GO used as a HTL was also reported to increase the PCE of GO-based inverted PSCs, by 12%85 and 18%,158 and the PSC device was more stable than that from pristine GO due to advanced Voc and FF, and preferred crystal orientations, respectively  and Rsh (from 152 to 10 669 Ω cm−2) but a decrease in conductivity (from 4820 to 1521 S cm−1) upon B-doping of GO. Habib et al.160 reported an optimised ratio of PANI : GO of 1 : 0.5 with a sheet resistance of 0.067 Ω cm−2 and PCE boost of 125% due to reduction of current leakage from smooth surfaces (root mean square of 12.65 nm) in the bulky materials.Lee ectively . The enh38,161 This means that PEDOT:PSS limits lifetime and cell performance due to its hygroscopic nature.101 Encapsulation improves device stability and can maintain the integrity of PSC layers over time.74 The use of graphene derivatives as encapsulant materials has the potential to contribute significantly to enhancing perovskite device longevity. An illustration of this is the reported lowering of the hygroscopic character of PEDOT:PSS through the synthesis of an annealed composite with GO.162 However, depending on their chemical nature, graphene derivatives may pose drawbacks in their long-term utilisation as HTLs since they may introduce considerable moisture in the active layer. This is detrimental to PSC devices because moisture dissolves the methylamine and/or hydrates the perovskite and causes it to decompose to PbI2 in a few hours or days when exposed to air and moisture.38,85,101 Possible ways to keep moisture out of the perovskite layer and thereby minimise perovskite degradation include employing a hydrophobic HTL such as hydrophobic RGO. Another graphene derivative-based HTL with minimal hydrophilicity is thiolated GO as in the perthiolated trisulfur-annulated hexa-peri-hexabenzocoronene–GO composite form.83 Additionally, a defect-free monolayer of some graphene derivatives is impermeable to all gases, preventing O2 diffusion into the perovskite.163 For instance, F–RGO HTL was able to minimise moisture and O2 attack on the perovskite layer resulting in 72% PCE retention after 30 days .146 Similar reports detail an isopropanol ultrasonically-treated graphene-based encapsulant enhancing the stability of a PSC by 90% within 60 h by reducing oxygen oxidation and moisture attack.164 In addition, the high thermal conductivity of graphene helps with thermal stress on PSCs, leading to additional stability benefits.95,109 A graphene derivative–carbon paste has been reported as a suitable encapsulant of PSC modules of areas 0.09, 1 and 4 cm2 that achieved champion PCEs of 15.81%, 14.06% and 13.86%, respectively.165 Additionally, graphene-based encapsulants are compatible with roll-to-roll fabrication through a dry transfer method that has attained PCE retention of 82.4% after aging in ambient air for 3700 h.163 This demonstrates that graphene derivatives, as encapsulants, are beneficial and conducive to the future scaling-up of PSCs. Other hydrophobic materials, such as a cross-linked compacted poly(methyl)methacrylate (PMMA) layer on top of the HTL,145 have been used to modify the perovskite surface and seal the active layer from atmospheric effects and therefore increase stability. The compatibility of graphene derivatives with common PSC encapsulants, such as PMMA, has been illustrated. For example, a PSC encapsulated with a PMMA–RGO composite displayed excellent performance under operational conditions because the OH− moieties on RGO interact with the CO on PMMA.95,109 However, strict and effective encapsulation may introduce not only additional cost (60% more)38,166 but also hinder optical transparency.167 Hence, a careful cost-effective sealing innovation needs to be sought for moisture insensitive designs involving graphene derivative HTLs. In a study to compare stability and PCE upon inclusion of GO in PSC devices, the glass/FTO/NiO/GO/perovskite/GO/TiOx/Al cell structure was more stable but with a 25% lower PCE than the glass/FTO/PEDOT:PSS/MAPbIx3−Clx/PCBM/Al configuration.168 Hence, further understanding of the degradation mechanism through focused studies on individual PSC layers rather than the overall PSC device may provide solutions to both stability and PCE enhancement.169The lifetime of a PSC is subject to pH, humidity level, permeability, density and hydrophobicity of the HTL.101 The perovskite quality boost occurs via heterogeneous nucleation (nucleation at foreign nuclei or surface), and this process is influenced by the surface or interface contact energy (surface chemistry and morphology effects).170 This infers that improved wettability  is associated with a small contact energy barrier and, together with a uniform large surface area, promotes continuous crystal growth. For illustration, GO was modified with 4-fluoro phenyl-hydrazine hydrochloride (4FPH) to form a 4FPH–GO composite with increased passivation of trap states close to the perovskite surface (disorder parameter that represents broadness of trap state densities decreased from 151 to 81 meV) and improved charge diffusion length (from 400 to 700 nm) for effective hole collection, and hydrophilicity for easy coating , leading to better spreading of the HTL on the perovskite.For HTL purposes, graphene can be chemically modified as an interlayer material to simplify the electrode solution coating process by improving wettability during the coating process. The enhanced wettability from derivatized graphene reduces the nucleation barrier to form a better quality perovskite layer without pinholes. coating .152 Furt80 Hence, GO absorption of 2.3% of incident light in the 200–800 nm range may affect the overall PCE.26 Hence, a high concentration of graphene derivative additives introduced into the HTL and poor dispersion can cause reduced film quality (light transmittance) and ultimately lower PCEs.21,80 This aspect is closely related to the film thickness effect as reported with functionalised GO and composites.36,135,152,153 Despite the possible increase of Rs due to the high oxygen concentration in GO,101 the quenching effect has also been reported to increase with composition.25 For illustration, a 6.7% PCE was reported for a single GO layer (80% quenching effect improvement) in a PSC device, whilst a bilayer recorded 11.5% (91% quenching effect boost) due to the formation of an effective Schottky barrier, minimising recombination effects.171 In brief, there is a trade-off between optical transmittance and electrical resistance. On the one side, the electrical resistance is lowered by thick graphene derivative sheets, whilst on the other side, thin films have higher optical transmittance.22,120,140,143 Resistivity of graphene was shown to decrease from 1050 to 140 Ω sq−1 when the thickness was increased from 1 to 2 layers.127 SCAPS-1D simulations have also shown the effect of thickness on Voc up to 1 V and an optimum thickness of 30 nm, and a decline in FF with thickness increase.132 Since several works have reported Voc above 1 V, this effect is likely to be dependent on the material properties , FF and Voc.36 Additionally, graphene derivatives have generally been able to improve the perovskite crystallisation process producing larger crystals, TCE surface coverage and in-plane orientation towards effective hole extraction.25,101To sum up, the review of graphene derivatives in the context of HTLs shows their potential in increasing hole transport and their electron blocking effect, thereby improving the electrical properties of the device . PCBM facilitates a fast charge transfer from the perovskite (approx. 0.4 ns), and its hydrophobic nature enhances PSC stability.140 Due to the low cost of graphene derivatives and their ability to introduce additional percolation channels for electron transport, solution-processed graphene derivatives have the potential to advantageously replace the popular PCBM.56,81 Hence, the basis of adding graphene derivatives to the active layer as ternary components of polymers and fullerenes is to ultimately increase carrier mobility.The function of an ETL in a PSC is to extract and transport generated electrons away from the perovskite active layer. The three common classes of ETLs for PSCs are metal oxides, n-type conjugated polymers, and fullerenes.172 ETLs are important in lowering interfacial energy barriers between electrodes and the perovskite active layer. Hence, electron/hole recombination rates are lowered by the ETL through electron extraction and transfer enhancement.173 The tuning of the ETL surface energy is a feasible control protocol for wettability, recrystallization and nano-structural parameters of perovskites.173 With reference to the PSC structure, the n–i–p type is largely influenced by the electrical, optical and structural features of an ETL.35 Common ETLs for the n–i–p structure are oxides, such as TiO2,34 SnO2 and ZnO.35,44,174Although a report indicates that PSCs can also function without an ETL (with PCE of 10%),2 require a high sintering temperature (ca. 450 °C) for optimum crystallinity and charge mobility.21,37,175 A high sintering temperature hinders applicability in widespread areas, such as wearable and flexible electronics. ZnO is a good alternative because it requires a low sintering temperature and time; however, its non-optimised dimensions (diameter: ∼80 nm and length: ∼1 μm) negatively affect perovskite crystal growth in PSCs.77 A m-TiO2 layer may hinder electron transport due to roughened grain boundary scattering.26,176 Ternary n-type semiconducting metal oxides with similar characteristics as TiO2, such as high Hall electron mobility and small effective mass, are also effective ETLs that have the potential to replace TiO2 in PSCs.153 For instance, the TiO2 hysteresis shortfall has been resolved by the use of SnO2 despite its low thermal stability, and non-optimal optical and electrical properties.35,84The processing temperature of an ETL during PSC fabrication influences device performance. For instance, both c- and m-TiO25 such as increased semiconducting properties,121,173 device stability and charge extraction.27 For illustration, the inclusion of RGO within TiO2 layers of PSCs produced a PCE boost of 21%.27 A typical example is the overturning of the slow electron diffusion rate limitation in TiO2, due to high resistance and low charge collection efficiency, by the use of GO as an interfacial layer between the TCE and TiO2.26,34,37,124 Although all values were lower than the FTO control (Jsc of 17.49 mA cm−2 and FF of 63%), altering the % of GO in c-TiO2 has also been reported to proportionally enhance Jsc and FF due to increased electron transport through the creation of thermodynamically favourable energy paths and lowered recombination.22 Xie et al.177 reported a Voc boost of up to 1.12 V through RGO inclusion in a MoO3 ETL .178 In another example, Li neutralised GO was reported to increase electron extraction from the perovskite as an ETL.120 In short, the electron conductivity of an ETL plays an important role towards effective charge extraction from active layers in PSCs, and compositional effects must be investigated in each case when graphene derivatives are involved.Interestingly, graphene derivatives are compatible with metal oxides that function in ETLs to form meso-super-structured PSCs with preferred properties ,25 such MoO3 ETL . Additio3.6Voc and Jsc , perovskite/CIGS (25% share) and perovskite/Si (58% share).179 Perovskite/Si TSCs have reached a certified value of 29.5%.33 Illustratively, graphene has been shown as a suitable material for TCEs (PCE of 8.3%),182 electrodes (PCE of 18.1%),183 and as an additive to the HTL (PCE of 6.02%)182 of TSCs. Graphene derivatives can hypothetically slow down charge thermalisation processes.165 However, there are currently scarce applications of graphene derivatives in TSCs. Theoretically, since a tandem structure will require an efficient PSC, the demonstrated positive effects of graphene derivatives in various layers of PSCs  are stacked narrow band gap and wide band gap subcells with high potential to suppress the Shockley–Queisser limitation of single solar cells and increase  and Jsc .165,179, of PSCs  infers t4.114 The PSC fabrication methods involving graphene derivatives include electro-spraying,152 radio-frequency magnetron sputtering,128 screen-printing,185 lamination,119 doctor blade (blade coating)21 and spin-coating82 .187 The roll-to-roll method has been successfully employed for PSCs with Li–GO as a TiO2/perovskite interfacial material for a module with an area of 50.6 cm2 and PCE of 12.6% (relative to 13.5% for an area of 0.01 cm2).30 Screen-printing is another scaling-up technique in which the paste composition and conditions can tune properties.190 The technique is suitable for scaling-up thin film deposition, usually at a speed greater than 6 m min−1 but with long drying and sintering durations at high temperatures for better porosity.191 An investigation of the use of near-infrared heating was reported to substantially reduce the processing time at a speed of 2 m min−1.191 The ink of graphene derivatives can be cheaply produced in high yields using non-toxic solvents to support integration into large PSC modules with competitive PCE.30 Transfer of graphene derivatives through polymer carriers has led to performance deterioration; hence, it is difficult to fabricate effective TCEs.110,192 Despite knowledge gaps, graphene derivatives, particularly the oxide forms, are compatible with flexible substrates because of their high dispersion in solvents and the ease with which they can be coated; hence, future research directions can explore their use in improving the cost, stability, and performance of PSCs.Most graphene derivatives utilised in PSC fabrication, as summarised in oating82 . The spi76 An analysis of PSC stability in terms of the decline in PCE upon graphene derivative inclusion as a p–i–n HTL component, solely a p–i–n HTL, the hole blocking layer component, active layer constituent, and ETL n–i–p constituent/blocking layer, after roughly 30 days show PCE declines of 12, 10, 50, 30 and 35%, respectively  perspective,187 Reduced defect intensity in perovskites and interface passivation induced by graphene derivatives also lessens charge traps; therefore, promoting performance.60,95 Regulation of perovskite crystal growth by using graphene derivatives improves reproducibility of deposition of the active layer.165 Graphene derivatives in PSC charge carriers  promote performance by reducing electron/hole recombination.30,221,222 This is achieved through WF and band gap modulation that create appropriate energy level alignment between perovskite and charge carriers . Graphene derivatives also generate high Jsc and FF via interfacial chemical modifications that promote high electron extraction and transport from the ETL due to relatively higher conductivity. Additionally, graphene derivatives as dopants to the ETL suppress octahedral tilt in the perovskite monolayer to create nanoscale ferritic distortion with irreversible polarisation, thereby increasing electron extraction from the perovskite and minimising electron/hole recombination.30From the reviewed articles , it is c124 On the other hand, composites of GO, such as GO–Li, have also been reported to reduce moisture attack and enhance both stability and life-span through passivation of titania oxygen vacancies by acting as reactive centres and blocking channels to moisture attack.110,120 Hence, the current review suggests the need to monitor and balance the basic necessities, particularly cost and performance, of a functional PSC device.Future use of graphene derivatives in TCEs, though currently less common than pristine graphene, is motivated by the low-cost processing, controllable work function, and resistance to chemical degradation. The hydrophilicity of graphene derivatives has positive prospects for easy dispersions in polar solvents, making them applicable to low-temperature solution processing. RGO is a better option than pristine GO for PSC applications because it is associated with a smaller energy mismatch to the perovskite; hence, promoting electron extraction. The inclusion of graphene derivatives in PSCs needs critical optimisation since it may enhance other relevant parameters at the expense of PCE, as summarised below. For example, graphene oxide (GO and RGO) applications in PSCs still need a conductivity boost through doping or composite synthesis, development of practical, simplified large-scale synthesis, improved film transfer protocols, and optimisation insights towards thin and uniform materials. The film thicknesses of both perovskite and graphene derivatives are crucial factors that also influence the PCE. It is critical to note two contradicting phenomena discussed earlier; firstly, the need for high hydrophilicity during fabrication and sacrificial reaction centre/blocking purposes; secondly, the hydrophobicity requirement in the perovskite for long-term stability and functionality of PSCs. Also, on the one hand, GO-based composites have been reported in some studies to disrupt the homogeneity of spiro-OMeTAD, thereby causing a detrimental effect on PSCs by creating shunt pathways.21,78Graphene derivatives are promising alternatives that can positively impact the costs involved in large-scale PSC manufacturing. Relevant current research gaps that involve the use of graphene derivatives in PSCs towards scalable, low temperature processed PSCs include:(i) Improved mechanical infiltration of perovskite solution into graphene derivatives within short intervals and with better reproducibility.(ii) Attainment of long-term stability under light exposure. Charge transport material should not cause photocatalytic degradation upon light exposure.(iii) Increasing the temperature tolerance.(iv) Enlargement of the active cell area.63 Graphene derivatives have demonstrated, through an in-depth PSC degradation profile analysis under operational conditions (60% PCE retention (15.8%) relative to control PCE retention of 25% (13.6%) after 24 h light soaking),61 to improve long-term stability. This was attributed to a reduction in localised inhomogeneous light-induced decomposition of perovskite back to precursors, preventing I2-induced m-TiO2 degradation and inhibited diffusion of I2 and Au across the whole device.Photo-induced decomposition is still challenging in PSCs because the perovskite/light interaction is strong; hence, future guidelines include using functionalised additives that also improve chemical interactions between ions.109 hence, as a future research direction, focus on in-depth degradation studies of typical devices is essential. Graphene-based materials are good candidates for PSC encapsulants due to their commendable optical properties.135,163,183 For instance, 60 μm of a GO encapsulant of a AgNW TCE achieved 83.8% transmittance at 550 nm (preserving 98.4% of the average transmittance of the pristine AgNW TCE).122 However, increasing the RGO loading can be detrimental as shown by Nouri et al.,27 RGO loadings of 0, 0.05 and 0.8 wt% in the ETL achieved PCEs of 13.1%, 13.5% and 12,1%, respectively, due to reduced transparency. Hence, since charge carriers and encapsulants should permit sufficient light to reach the active layer, the future use of graphene derivatives in PSCs should critically investigate concentration dynamics. Also supporting this strategy, is the proven ability of small amounts of graphene derivatives in boosting PCE without altering optical properties.223 The realisation of suitable encapsulation materials and protocols prospectively supports the drive to achieve the approximated low 3.5 US cents per kW h target, which is competitive with current commercial solar cells and fossil energy resources upon scaling-up.165The use of graphene derivatives as PSC encapsulant materials is an underexplored area;From the summarised past research, the attainability of the next stride of PSCs in commercial markets is highly possible if long-term stability, defect concentration minimisation, scalability and processability goals are met. This can be achieved through studies that provide a fundamental and thorough understanding of the effect of graphene derivatives on interfacial parameters and perovskite crystallisation. The modulation of the WF of graphene derivatives has also shown essential benefits in the functioning of PSCs. The current literature has presented a vast number of positive attributes linked with the inclusion of graphene derivatives in PSCs, but further in-depth studies and industrial up-scaling are required.There are no conflicts to declare.


The results showed that all the studied systems are thermodynamically stable with large binding energies of about 3.72 eV–6.86 eV. Interestingly, all the TM-intercalated graphene/MoS2 heterostructures are ferromagnetic and their total magnetic moments increase with TM concentration. Furthermore, TM concentration-dependent spin polarization is obtained for the graphene layer and MoS2 layer due to the charge transfer between TM atoms and the layers. A significant band gap is opened for graphene in these TM-intercalated graphene/MoS2 heterostructures (around 0.094 eV–0.37 eV). With the TM concentration increasing, the band gap of graphene is reduced due to the enhanced spin polarization of graphene. Our study suggests a research direction for the manipulation of the properties of 2D materials through control of the intercalation concentration of TM atoms.The electronic and magnetic properties of graphene/MoS The CDD plots of 1Cr@(G/MoS2) and 1Mn@(G/MoS2) are plotted in 2 monolayers gain electrons. Detailed charges transferred to graphene and MoS2 monolayers in all the studied 1TM@(G/MoS2) heterostructures are summarized in To explicitly elucidate the bonding characteristics of the 2) heterostructures. Interestingly, the band gap of graphene is opened in these systems, with a significant band gap of about 0.37 eV, 0.28 eV, 0.30 eV, and 0.094 eV for TM = V, Cr, Mn, and Fe, respectively, which is much larger than those of G/MoS2 heterostructures without intercalation [2)s. For example, the magnetic moments of 1TM@(G/MoS2)s are 3.93 µB, 4.70 µB, 3.69 µB, and 2.00 µB for TM = V, Cr, Mn, and Fe, respectively. In addition, the charge transfer from the TM to the two-sided monolayers causes spin polarization in the graphene (MoS2) monolayer, whose local magnetic moment is around 0.02(0.48), 0.004(0.20), 0.06(0.26), and 0.06(0.24) for TM = V, Cr, Mn, and Fe, respectively. Particularly, 1Fe@(G/MoS2) is a Dirac half-metal, having a Dirac cone in the majority-spin channel while exhibiting a 94 meV gap in the minority-spin channel  . For 2TM@(G/MoS2)s and 3TM@(G/MoS2)s, nine and six isomers with different TM atom arrangements were tested, respectively (see 2), the two Fe atoms prefer to sit a bit further away, i.e., they sit in two adjacent hollow positions in graphene along the armchair direction. For the latter, similar conformations were found for the most stable 3V@(G/MoS2) and 3Mn@(G/MoS2), where the three TM atoms are located on three adjacent hollow sites of the graphene forming a “<”-shaped pattern (see 2), the shape of the three Cr atoms was found to be almost linear, with two of the end Cr atoms located at the edge sites of the graphene, leaving the middle Cr atom located at the hollow site (see 2), two of the three Fe atoms sit far from each other, forming an “L” shape (see 2)s, no significant structure deformation is found for the graphene layer or MoS2 layer. Except for 2(3)Fe@(G/MoS2) and 3Cr@(G/MoS2), with TM atoms sitting at a different height along the z axis, all the TM atoms are found to stay in one plane. Moreover, the G/MoS2 interlayer distances (d) of these nTM@(G/MoS2)s are found to increase with the number of TM atoms (see d1TM@(G/MoS2) < d2TM@(G/MoS2) < d3TM@(G/MoS2).Furthermore, to explore the effect of different TM ratios on the electronic and magnetic properties of these TM-intercalated G/MoSvely see . For thevely see a–c, whertern see e,g. As fsite see f. For 3Fhape see h. Similatoms see , i.e., d2)s and 3TM@(G/MoS2)s are 6.03/5.70 eV, 4.08/3.79 eV, 4.02/3.72 eV, and 5.23/5.15 eV for TM = V, Cr, Mn, and Fe, respectively, which are smaller than those of the 1TM@(G/MoS2)s. We note that the Ebs per TM atom for these nTM@(G/MoS2)s is related to the ratio of intercalated TM atoms, i.e., the higher the TM ratio, the smaller the Ebs per TM atom. Our results show that single TM atom intercalation is the most energetically favorable in all systems. This stability related to the TM ratio is consistent with the transferred charge between the TM atoms and the two face layers, i.e., ΔQ1 + ΔQ2 decreases as n increases from 1 to 3 s and 3TM@(G/MoS2)s are shown in 2)s, the systems intercalated with two or three TM atoms are magnetic, with their magnetic moments mainly contributed to by 3d orbitals of TM atoms. Our results show that the magnetic moments of 2TM@(G/MoS2)s and 3TM@(G/MoS2)s are nearly double or triple those of 1TM@(G/MoS2)s. For the former, they are around 5.81 µB, 10.51 µB, 6.83 µB, and 5.12 µB for TM = V, Cr, Mn, and Fe, respectively, while for the latter, they are 9.64 µB, 14.73 µB, 10.63 µB, and 8.26 µB for TM = V, Cr, Mn, and Fe, respectively. Moreover, the spin polarization of graphene and MoS2 monolayers increases as the number of TM atoms increases. On one hand, the local moments of graphene and MoS2 monolayers in 2TM@(G/MoS2)s/3TM@(G/MoS2)s are increased to 0.03(0.03)/0.02(0.33), 0.02(1.00)/0.14(0.58), 0.006(1.16)/0.10(1.29), and 0.05(0.29)/0.06(0.19) for TM = V, Cr, Mn, and Fe, respectively. On the other hand, the larger spin polarization reduces the band gap in graphene, as shown in 2)s/3TM@(G/MoS2)s are 0.30/0.21 eV, 0.21/0.0 eV, 0.24/0.16 eV, and 0.26/0.27 eV for TM = V, Cr, Mn, and Fe, respectively. Therefore, we can conclude that the electronic and magnetic properties can be manipulated by varying the ratio of intercalated TM atoms, which is feasible in experiments through control of electrochemical voltage [The PDOS and spin density plots of these 2TM@ interactions [−6 eV per unit cell, and the ionic force on all relaxed atoms was less than 0.01 eV/Å. A vacuum layer of about 15 Å was used along the z direction to avoid interactions between neighboring cells. To consider the 3d electron’s strong correlation effect, a GGA + U method [2 heterostructure (G/MoS2) was constructed as periodic slabs with the lattice parameters a = b = 12.35 Å, in which a 5 × 5 slab of graphene and 4 × 4 slab of the MoS2 monolayer were employed. One, two, and three TM atoms were intercalated into the interlayer gap of G/MoS2 heterostructures, which is denoted as nTM@ (G/MoS2) . A Monkhorst–Pack grid of 5 × 5 × 1 was used for geometrical optimization of all the systems, and much denser k-point grids of 15 × 15 × 1 were used to explore electronic properties.All the spin-polarized density functional theory (DFT) calculations were performed with the Vienna ab initio simulation package (VASP) . The ionractions . The excractions . The eneU method  was adopU method ,48. The 2 heterojunctions were systematically studied. Our results revealed that all the nTM@(G/MoS2)s  are very stable and have large binding energies. On one hand, all the studied systems are ferromagnetic, and their magnetic moments increase with n. In addition, spin polarization was observed in the graphene layer and MoS2 layer, and the degree of polarization increased with TM concentration. On the other hand, a significant band gap is opened for graphene upon the introduction of TM atoms, and the band gap is found to reduce with n due to the enhanced spin polarization of graphene. These results show that intercalation at different concentrations is a powerful approach for manipulating the electronic and magnetic properties of 2D vdW heterostructures and is thus expected to be widely applicable to other 2D layer materials and beyond.In conclusion, the electronic and magnetic properties of TM-intercalated G/MoS


Traditional humidity sensors for respiration monitoring applications have faced technical challenges, including low sensitivity, long recovery times, high parasitic capacitance and uncalibrated temperature drift. To overcome these problems, we present a triple-layer humidity sensor that comprises a nanoforest-based sensing capacitor, a thermistor, a microheater and a reference capacitor. When compared with traditional polyimide-based humidity sensors, this novel device has a sensitivity that is improved significantly by 8 times within a relative humidity range of 40–90%. Additionally, the integration of the microheater into the sensor can help to reduce its recovery time to 5 s. The use of the reference capacitor helps to eliminate parasitic capacitance, and the thermistor helps the sensor obtain a higher accuracy. These unique design aspects cause the sensor to have an excellent humidity sensing performance in respiration monitoring applications. Furthermore, through the adoption of machine learning algorithms, the sensor can distinguish different respiration states with an accuracy of 94%. Therefore, this humidity sensor design is expected to be used widely in both consumer electronics and intelligent medical instrument applications. Abnormal respiration is usually a sign of an individual’s physical problems. Many illnesses and health conditions, including heart disease, pneumonia, bronchitis, sleep apnea and hyperpyrexia caused by infection, will cause changes in a person’s respiratory frequency and depth3. Humidity sensing represents a promising approach to establish a relationship between human respiration and electrical signals. Therefore, the demand for humidity sensors to be used in respiration monitoring is growing at a tremendous rate5. However, applications of conventional humidity sensors are generally limited by their large size, low sensitivity, nonnegligible temperature drift and slow responses7. Following the development of microelectromechanical systems (MEMSs) in recent decades, miniaturized humidity sensors designed using several different principles, including capacitive, resistive, resonant and optical sensors, have been developed13. The resistive devices are small, simple and low-cost, but this device type has the disadvantage of low test accuracy because of its poor antijamming capability14. Optical sensor devices have high test accuracy, but their micromachining processes do not match well with integrated circuit techniques16. Among the different design principles, the capacitive-type device is favored because this device offers both better accuracy over a wide relative humidity (RH) range and integration convenience17.Respiration, which is the exchange of gases between a body and its surrounding environment, is a critical process for supporting human life and activity18. The performance of capacitive humidity sensors is restricted in terms of the following aspects. First, the relative scarcity of high-quality humidity-sensitive materials and the parasitic capacitance from the substrate limit both the sensitivity and the accuracy of these sensors. To date, the sensitive materials used in these devices mainly include porous silicon, ceramics and organic materials25. Organic materials such as polyimide (PI) have been widely used. However, PI is a porous material that contains semienclosed nanostructures26, and the full output range of PI-based humidity sensors is usually only 1 to 2 pF, which means that high-precision and high-cost application-specific integrated circuits (ASICs) are required to match these sensors28. Porous silicon and ceramics, such as anodic aluminum oxide, exhibit better sensitivity because they offer better hygroscopicity, but the preparation processes for these materials are relatively complex and are not compatible with conventional complementary metal-oxide-semiconductor (CMOS) processing29. In addition, Gerwen’s research30 indicates that parasitic capacitance appears when the individual electric field lines pass through the substrate, which will then affect both the true value test for the humidity-sensitive capacitance and the accuracy of the sensors31. In addition, in practical applications, the ambient temperature affects the dielectric constant of humidity-sensitive materials, which causes the capacitance to change and thus causes inaccurate measurement results33. Furthermore, when humidity sensors of this type are used in high RH environments for long periods, the condensation of water vapor usually occurs on the sensor surface. This water condensation process means that desorption of the water molecules takes a relatively longer time and thus leads to longer recovery times for these sensors35. To address these challenging issues, a humidity-sensitive device configuration that can not only eliminate parasitic capacitance and calibrate the temperature drift but can also increase sensitivity and reduce the recovery time of the humidity sensors is strongly needed.A typical capacitive humidity sensor usually consists of interdigital electrodes (IDEs) that are covered with a layer of humidity-sensitive materialIn this work, a novel triple-layer humidity sensor comprising a sensing capacitor, a reference capacitor, a microheater and a thermistor is reported. In this sensor, the sensing capacitor uses in situ integrated nanoforests as the humidity-sensitive material. Because these nanoforests are hygroscopic and are composed of fully open nanostructures, this humidity sensor exhibits high sensitivity. In addition, a reference capacitor is integrated into the structure to prevent environmental jamming and parasitic capacitance generation, a thermistor is added to provide temperature compensation, and a microheater is used to further accelerate the desorption of water molecules. With all these specific design features, the proposed sensor demonstrates excellent sensitivity, fast recovery speeds, high accuracy and improved antijamming capability. Furthermore, the humidity sensor was implanted into an N95 mask for monitoring different respiratory states, and an accuracy for monitoring respiratory states ranging up to 94% was achieved through the adoption of machine learning algorithms. The proposed sensor is expected to have wide-ranging applications in consumer electronics and medical instruments.37,CPI represents the capacitance of the humidity-sensitive layer, which is given theoretically asn is the number of IDEs, lFinger and hFinger are the length and thickness of the IDEs, respectively, and WGap represents the distance between two adjacent electrodes. εPI represents the dielectric constant of the PI layer, which is a variable value that changes with the RH of the environment, as shown in Eq.  results in the inaccurate determination of the humidity-sensitive layer capacitance (CPI) and thus also influences the sensitivity of the sensor.Therefore, the existence of these parasitic capacitances . With this design, the differential capacitance (CN) is not related to the parasitic capacitance (because CP1 + CP2 − CP1’ − CP2’ ≈ 0). In addition, the basic differential capacitance of the sensor can be adjusted by varying the structural parameters of the reference capacitor. In this design, a microheater is used to reduce the device recovery time, while a thermistor (Rt) is integrated into the device beside the heater to calibrate the temperature drift and monitor the microheater’s working state. The microheater and the thermistor are both designed to be serpentine to provide uniform heating and high resistance. The ground electrode is set to prevent electromagnetic interference in practical applications, while the electrode surfaces in the device are designed to have large-scale roughness to assist with wire bonding.Here, 2 layer was deposited on a Si substrate; then, a 0.4-μm-thick Al layer was sputtered and patterned to form the microheater, the thermistor and their ground electrodes. A 2-μm-thick SiO2 film was subsequently deposited on the Al layer to act as an insulating layer, on which contact holes were then patterned. Subsequently, another Al layer with a thickness of 2 μm was sputtered and patterned to form the IDEs and the top electrodes. The top and ground electrodes were connected via the contact holes. Later, a 2-μm-thick Si3N4 film was deposited and patterned on one group of IDEs, thus forming the reference capacitor. Subsequently, a PI layer with a thickness of ~8 μm was spin coated onto the other group of IDEs and patterned. Finally, a reactive ion etching (RIE) process using oxygen (O2) plasma was used to bombard the PI pattern and form the nanoforest-based sensing capacitor. During the etching step, the applied radio-frequency (RF) power was 200 W, and the O2 flow rate was 50 sccm. The O2 plasma treatment period was 30 min. Using the process described above, the nanoforest-based triple-layer humidity sensor was realized.The fabrication process of this humidity sensor is illustrated in Fig. 3N4 layer are exactly the same (430 μm × 820 μm). SEM images of the nanoforest and the results of the hydrophilicity testing (the contact angle for a water droplet on the nanoforests is approximately 1°) are presented in the Supplementary Information  in Sample-1 has the highest sensitivity of 0.11 pF/%RH at 40–90% RH  of this device remains stable (at approximately 3.85 pF) under different RH conditions, thus meaning that the differential capacitance (CN = CH − CR) can be used in this sensor. The following capacitance response results were all processed using the difference method.The humidity-sensing properties of the nanoforest-based humidity sensors are illustrated in Fig.  RH Fig. , which iIn practical applications of humidity sensors, repeatability is a critical performance indicator. As shown in Fig. Temperature drift is another important performance characteristic for humidity sensors. Figure Humidity sensors are often used in complex gas environments, and the sensor specificity is thus particularly important. To verify the specificity of this type of sensor, its response was tested in different gas environments containing carbon dioxide (100 ppm), ethylene glycol (500 ppm) and ethanol (100 ppm), and the results were compared with the sensor’s response at 90% RH. As Fig. Because aluminum has a positive temperature coefficient, the resistance of the aluminum-based thermistor increases with increasing temperature. In the experiment, the chamber was set to different temperatures , and the resistance of the thermistor varied accordingly between 897.7 Ω and 1039.3 Ω, showing a linear tendency  and medical diagnosis 43. In the experiments, we placed our humidity sensors inside several masks with breathing holes. Then, data were recorded at different respiratory states when volunteers were wearing these masks . FigureFigure Machine learning plays an important role in the sensing field due to its powerful data analysis and mining capabilities. In this work, humidity testing data were analyzed using machine learning algorithms to improve the accuracy of respiratory state prediction for volunteers. The flow chart for the respiratory state recognition algorithm based on machine learning is shown in Fig. nth layers of the water molecules, which, as a result, causes a 5 pF increase in capacitance. Based on the synergy between chemisorption and physisorption, the relationship between relative humidity and capacitance obtained experimentally in Fig. Figure In contrast, during the desorption process, when the microheater is operating, the temperatures of the nanoforest and the surrounding environment are increased. The movement of the water molecules thus becomes more vigorous, and both the chemical and hydrogen bonds breakdown more rapidly, which results in the faster evaporation of water molecules. As a result, the sensor’s recovery time is only 5 s, which is much shorter than that of previously reported capacitive and resistive humidity sensors, as listed in Table. In summary, we have proposed a novel humidity sensor that contains a sensing capacitor, a reference capacitor, a thermistor and a microheater to improve the sensing performance. When compared with traditional PI-based humidity sensors, the sensitivity of the proposed device with in situ integration of the nanoforests is enhanced significantly. In addition, by adding a microheater, a thermistor and a reference capacitor to the sensor, the proposed humidity device gains the ability to self-improve its signal accuracy and self-reduce its response time, in addition to its self-detection ability. Theoretical analyses and experimental tests were performed to investigate the effects of the number of IDEs and the heating temperature on the humidity sensing performance, and the advantages of the difference method used in the device were demonstrated. Based on the excellent performance demonstrated by the sensor and by using machine learning algorithms, we used the humidity sensor for the respiratory monitoring of human breathing. It is expected that such a highly sensitive humidity sensor will be used widely in the future in a variety of commercial applications.2, N2) used in the device calibration process were stored in steel cylinders purchased from Guangyun Gas Co., Ltd., Wuxi, China. The ethylene glycol and ethanol used in the experiments were of analytical purity and were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The masks used for the insertion of the sensors were purchased from 3 M China.PI (ZKPI-5100) was purchased from POME Technology, Beijing, China. The gases  and a focused ion beam (FIB) system  were used to investigate the humidity sensor structures. The humidity sensor was secured in a window-type TO package that was then connected to a printed circuit board layer through wire bonding. To evaluate the humidity-sensing capability of the sensor, it was placed in the chamber of a temperature-humidity generator . Capacitance signals were collected using a high-precision multimeter . The working temperatures of the microheater at different voltages were tested using an infrared imaging measurement system . The working voltage for the microheater was applied using a DC power supply . To evaluate the specificity of the humidity sensor in different gas environments, the sensor was placed in a chamber in which the gas concentrations could be controlled .Supplementary information of MICRONANO-01916R1


The device characterization reveals a clear electrical improvement when the graphene monolayer is placed as part of the electrode. The current–voltage characteristic of the solar cell with graphene shows an improved FF and Voc provided by the front electrode modification. Improved conductance values up to 5.5 mS are achieved for the graphene-based electrode, in comparison with 3 mS for bare ITO. In addition, the device efficiency improves by around 1.6% when graphene is incorporated on top. These results so far open the possibility of noticeably improving the contact technology of non-conventional photovoltaic technologies and further enhancing their performance.In this work, a new design of transparent conductive electrode based on a graphene monolayer is evaluated. This hybrid electrode is incorporated into non-standard, high-efficiency crystalline silicon solar cells, where the conventional emitter is replaced by a MoO Graphene is an attractive candidate for a new generation of transparent conductive electrodes (TCEs) with huge impact in different research field domains, such as displays, touch screens, or solar cells, and it is recently setting foot in the commercial market ,2,3,4. GNowadays, the PV market continues to be dominated by crystalline-silicon technology, which requires new non-conventional solutions to reduce costs. In this sense, silicon-heterojunction (SHJ) technology is considered a reliable low-temperature and high-efficiency solution. In this scenario, materials used in other emerging technologies , or alkaline salts) are being intensively studied as alternative charge-carrier collectors to traditional doped amorphous silicon layers. The main advantages of using these materials include the following: (i) simple deposition techniques at low-temperature; (ii) no hazardous gas precursors are employed; and (iii) less parasitic absorption compared with traditional heterojunctions. In this respect, successful high-efficiency solar cells implementing such materials have already been reported ,20,21. Regarding the use of graphene in SHJ technology, hybrid concepts have already been studied, demonstrating the possibility to enhance the photovoltaic performance ,23. Partx) hole-selective layer replaced the p-doped amorphous silicon layer of conventional heterojunction solar cells. The graphene monolayers form part of the transparent electrode and are transferred at the end of the fabrication route, just before the last metalization step  coating, and finally transferred to the desired substrate. In the transfer process, environments such as O2 plasma, UV-O3 activation, and high-temperature annealing processes were avoided. The main reason is that they can negatively affect the device, as non-conventional heterostructures are often less stable [The graphene monolayers were obtained by chemical-vapor-deposition (CVD) by the Spanish company Graphenea S.L. . The CVDs stable . Hence, s stable . Raman sx layer was thermally evaporated on the front side. The MoOx film was coated by an ITO transparent electrode that was much thinner than normal (<20 nm). This stack still worked pretty well as an anti-reflection coating, since the refractive index of MoOx is similar to that of ITO at the wavelength of interest [x layer and also serves as a transparent electrode. Interestingly, it can be kept much thinner because the final sheet resistance of the device will be further reduced by the use of graphene. Consequently, much less indium is needed in this structure compared with standard front transparent electrodes [2 and 4 cm2 were defined by photolithography. Finally, the devices were completed by evaporating a 2 μm-thick Ag grid contacting 4.5% of the device area. A set of solar cells was fabricated, skipping the graphene transfer step to serve as a reference. The solar cells were fabricated on n-type (2 Ω·cm) 280 μm-thick flat c-Si wafers with a (100) orientation. The wafers were first dipped in diluted HF (1%) to etch the native silicon oxide. Then, intrinsic and n-doped amorphous silicon layers (i/n stack) were deposited on the rear side by plasma-enhanced CVD to obtain a good reference electron-selective contact [interest . The ITOectrodes . The reaectrodes . Next, s2, AM1.5 g solar spectrum, 25 °C) were measured using an ORIEL 94021A  solar simulator. The external-quantum efficiency curves (EQE) of the solar cells were measured using a commercial instrument, QEX10 . Quasi-steady-state open-circuit (QSSVoc) measurements were acquired with a system made in-house [The current density vs. voltage (J–V) electrical characteristics of complete devices were measured in a four-probe configuration using a 2601B Source Meter . The J-V curves under standard test conditions  of the graphene-coated cell (31 mA/cm2) compared with the reference device (32 mA/cm2). This difference could a priori be related to optical absorbance by the graphene layer. However, EQE measurements indicate that the main difference in photocurrent collection is observed at wavelengths between 800 nm and 1100 nm. The behavior in this near-infrared region of the EQE curve is generally associated with rear surface recombination and the quality of the back reflector. A possible explanation is that the rear contact suffered some degradation during the wet transfer of the graphene sheet. The rear side was protected by a thick photoresist to avoid any damage from the reactive material used in this process. Thus, degradation could be related to the thermal step (150 °C) that is also involved in the graphene transference. In order to confirm this, we submitted reference devices to a similar thermal step, and we observed a quite similar degradation  of the graphene-coated solar cell is significantly higher compared with the reference device. This might not be expected, as the series resistance should have no direct influence on the measured open-circuit voltage. There is no current flowing through the solar cell in open-circuit conditions. On the other hand, the Voc value may indeed be related to the quality of the selective contact. Generally, thicker ITO electrodes coat the MoOx layer to provide high lateral conductance and good contact with the metallic grid. However, in this work, the ITO thickness was reduced below 20 nm to investigate graphene as an alternative indium-free transparent electrode. In silicon heterojunction technology, it is known that the electrode work function can significantly impact the band alignment at the corresponding contact [x/silicon interface could be modified by the addition of a graphene layer, given that the ITO electrode is very thin. Actually, it has been reported that graphene improved the quality of MoOx hole-selective contacts for perovskite solar cells [oc of the graphene-coated solar cell. Excitation of surface-plasmon-polaritons (SPP) on graphene monolayers has been demonstrated [oc by 80 mV compared with the reference solar cell. This, together with the higher FF value, translates into an overall 1.6% increase in the power conversion efficiency (PCE) due to the graphene incorporation.Finally, the open-circuit voltage  by the incorporation of graphene. Consequently, the JV curve of the graphene-coated solar cell shows better Voc and FF values for a very remarkable absolute increase in PCE of 1.6%. Hence, this investigation has identified possible applications of graphene-based electrodes in non-conventional solar cells. Furthermore, this use could also be interesting for applications demanding flexible or transparent electronics betting, on a reduced environmental impact.The goal of this research was to evaluate the effect of incorporating a graphene layer on the front transparent electrode of n-type silicon solar cells with non-conventional MoO


Oral diseases present a global public health problem that imposes heavy financial burdens on individuals and health-care systems. Most oral health conditions can be treated in their early stage. Even if the early symptoms of oral diseases do not seem to cause significant discomfort, prompt treatment is essential for preventing their progression. Biomaterials with superior properties enable dental therapies with applications in restoration, therapeutic drug/protein delivery, and tissue regeneration. Graphene nanomaterials have many unique mechanical and physiochemical properties and can respond to the complex oral microenvironment, which includes oral microbiota colonization and high masticatory force. Research on graphene nanomaterials in dentistry, especially in caries, periodontitis therapy, and implant coatings, is progressing rapidly. Here, we review the development of graphene and its derivatives for dental disease therapy. Oral diseases constitute a public health problem worldwide with high prevalence (3.4 billion cases) and incidence (4.35 billion cases) . Oral diThe goal of dentistry is, through an achievable plan, to maintain or rehabilitate a person’s oral health, promoting general health. Filling, also called restoration, is the main treatment method for caries, which requires various materials, such as composite resins and adhesives. These materials are exposed to saliva, oral microbiota, high masticatory force, and abrasion, which can lead to failure of the treatment, the formation of secondary caries, fracture of the restoration, and microleakage caused by the shrinkage of the resin or the dissolution of adhesive ,8. MoreoApplications of nanotechnology have greatly contributed to the development of dentistry, and one of the more prominent aspects is the innovative development and application of nanomaterials in dental practice ,21. Grap2 -hybridized carbon atoms with the arrangement of a honeycomb lattice [Graphene, the thinnest and strongest material known, is a single atomic sheet made up of sp lattice ,26. It w lattice . Graphen lattice ,30,31,32 lattice ,34,35. N lattice ,28,36,37 lattice ,39,40,41Graphene oxide (GO) and reduced graphene oxide (rGO) are two main graphene derivatives. GO was first discovered in 1859 and presented many oxygen-containing functionalities, such as hydroxyl epoxy and carboxyl groups, which contribute to the covalent or noncovalent combination of GO with biomolecules and other nanomaterials ,43,44. GChemical vapor deposition, chemical-based methods, and micromechanical exfoliation of graphite are three main techniques used to synthesize graphene ,51. The The oxidization of natural graphene results in graphite oxide, followed by sonication exfoliation to produce GO, the reduction of which forms rGO . The cheRecently, graphene and its derivatives have given rise to great interest in the fields of biomedicine and dentistry. Similar to other inorganic nanomaterials, assessments of cytotoxicity are critical to their further clinical application. The biological toxicity of graphene and its derivatives has been systematically reviewed . The toxDecreasing the restoration failure caused by bulk or marginal fractures, as well as reducing the risk of secondary caries, to minimize the demand to replace restorations is a crucial goal of contemporary dentistry ,76. The Graphene-based materials have been introduced into dentistry to improve the performance of dental materials. Evidence has indicated that graphene can enhance the mechanical and physicochemical properties of biomaterials, being biocompatible and noncytotoxic in the form of few-layer graphene (FLG), generally 1 to 6 layers. Therefore, FLG should be an ideal material that can be incorporated into dental polymers, thereby increasing their strength and durability. Malik S et al.  fabricatIn addition to GICs, bioactive calcium silicate bone cement is widely used in dentistry for endodontic treatments such as the management of perforations, retrograde root filling, and pulp capping due to its strong sealing ability and mineralization inducing ability, but it still has some problems in clinical application, including a long setting time and unsatisfactory physico-mechanical properties. Dubey et al.  tried toUnlike GICs and bioactive calcium silicate bone cement, PMMA, known as “organic glass”, is widely used as a denture material due to its superior heat resistance, high strength, and low cost. However, it suffers from brittleness and poor impact resistance. A previous study demonstrated that incorporation of G–AgNp improved the flexural strength of PMMA resin . FurtherIn addition, GO could improve the performance of adhesive and silane primers and protect Ti substrates ,88,89. SStreptococcus mutans is a key cariogenic pathogen and can synthesize insoluble EPS using dietary sucrose, thereby facilitating bacterial adhesion–cohesion and accumulation on the tooth surface, subsequently promoting the formation of caries-causing acidogenic biofilms [S. mutans-targeting materials is of great importance for controlling the pathologic condition. Although widely explored biocides, such as chlorhexidine, present a strong ability to inhibit S. mutans, they also suppress the growth of beneficial bacteria, disrupting oral microbiota homeostasis [Dental caries is a disease that relies on cariogenic biofilms, which are highly organized microbial communities embedded in a cohesive matrix of extracellular polymers, mainly extracellular polysaccharide (EPS) . Streptobiofilms . Therefoeostasis . CurrentS. mutans in vitro [S. mutans, and the antibacterial properties improved with increasing FG content. When 4 wt% FG was added, the antibacterial rate of S. mutans reached 85.27%. Abundant evidence has demonstrated that graphene and its derivatives are promising anti-caries nanomaterials due to their impressive ability to inhibit cariogenic bacteria and prevent tooth demineralization [S. mutans using the 3--2,5-diphenyl tetrazolium bromide (MTT) reduced test, colony forming unit counting, growth curve observation, and live/dead fluorescent staining [S. mutans. Consistent with this result, the addition of 0.25% graphene nanoplatelets doped with silver nanoparticles into adhesives showed optimized antibiofilm properties against S. mutans without affecting the standard adhesion characteristics regarding bond strength, leakage expression and durability at the resin–dentin interface [S. mutans, graphene oxide–copper nanocomposites (GO–Cu) and graphene/zinc oxide nanocomposites (GZNC) could also reduce the biomass of the S. mutans biofilm and suppress cariogenic properties of S. mutans, such as acid production and glucan formation [Abundant evidence has demonstrated that graphene and its derivatives are promising anti-caries nanomaterials due to their impressive ability to inhibit cariogenic bacteria and prevent tooth demineralization ,97,98,99in vitro ,93,94,96lization . A studystaining  found thormation .S. mutans [S. mutans cells using GOPEI complexes, and a reduction in virulence-associated gene (gtfBCD and gbpB) expression due to AsvicR was observed. Both GO and AsvicR alone could significantly suppress biofilm formation and EPS production. In contrast, GOPEI–AsvicR exhibited much more remarkable inhibitory effects with respect to virulence-associated gene expression, biofilm aggregation, and EPS accumulation, which may be related to silencing the expression of the vicR gene and the physical effect of GO. They suggested preserving nanographene oxide with AsvicR RNA as a more effective strategy for dental caries management [S. mutans [Interestingly, there are studies that combine GO with new technologies to achieve its antibacterial effect . When io. mutans . Wu S et. mutans  developenagement  , pulp regeneration, and apical induction are the main and effective treatments for teeth with pulpitis or apical periodontitis. RCT is a fundamental step to remove infected tissue and pathogens in the root canal system. To reach the goals of RCT, filling materials used to occupy complex root canal systems are supposed to have the desired sealing ability, biocompatibility, and antibacterial properties. Persistent infection in confined areas of the root canal system is a main reason for root canal treatment failure and posttreatment apical periodontitis. Owing to the less accessible areas of the root canal system, mechanical debridement and chemical irrigation during RCT are often not enough to eliminate bacteria from the root canal system. Therefore, it is necessary to develop new strategies, such as agitation and activation methods, to improve efficacy.Enterococcus faecalis , Ag–GO particles and GO–ICG–PDT could effectively reduce the biovolumes of faecalis , which htic acid . These ftic acid . Recentltic acid . Akbari tic acid  fabricatPulp regeneration, a promising technique to treat pathological or iatrogenic dental pulp lesions caused by caries or pulpectomy, is expected to replace RCT in the future . In a piPeriodontitis is a chronic infectious disease of tooth supporting tissues, including gingiva, cementum, alveolar bone, and the periodontal ligament. Peri-implant diseases are inflammatory conditions that influence the soft and hard around dental implants. They are considered a special subtype of periodontal diseases, which could result in failures of dental implants. The primary goal of periodontitis and peri-implantitis treatment is to control infection and to prevent tissue destruction by removing multispecies biofilms on the root/implant surface and reducing tissue invasion.Porphyromonas gingivalis is regarded as a keystone periodontal pathogen, so it is the target bacteria for many new dental drugs and materials development research. GO nanosheets could effectively suppress the viability of P. gingivalis in a dose-dependent manner, stopping growth at a concentration of 40 μg/mL [Fusobacterium. nucleatum via a similar mechanism [P. gingivalis, F. nucleatum, and S. mutans [Aggregatibacter actinomycetemcomitans, P. gingivalis, and Prevotella intermedia). This is associated with ROS generation and the downregulation of biofilm genes  [S. mutans, F. nucleatum, and P. gingivalis reformation on implants well, and the effect was significantly better than that of single use [40 μg/mL . The mec40 μg/mL . GO nanoechanism . Thus, w. mutans . The und. mutans . GQDs cond inpA) . The comngle use . Taken tThe regeneration of lost tooth/implant-supporting tissues is an ambitious purpose of periodontal regenerative therapy. In stem cell-based therapy of dentistry regeneration, cells and/or growth factors often need to be delivered to the injured site by scaffolds. Periodontal ligament stem cells (PDLSCs) are readily available mesenchymal stem cells (MSCs) with promising applications in regenerative therapy. GO-based materials, such as GO combined with fibroin, GO-coated Ti substrates and GO-applied scaffolds, are regarded as promising biomaterials for tissue engineering due to their biological compatibility and bioactivity to promote the proliferation of PDLSCs ,122,123.Several lines of evidence have suggested the involvement of rGO in the proliferation of stem cells ,125. ForDental implants are now a common way to repair permanent tooth loss caused by oral diseases, especially dental hard tissue diseases and periodontitis. Ti is the most commonly used material in implantology, mainly due to its good biological compatibility. However, it has been demonstrated to release alloy particles and ions into peri-implant tissues, which could result in bone loss and osseointegration failure of the implant. Osseointegration is the “gold standard” used to assess the success of implants, and the interactions between the material surface and cells can be regulated by the characteristics of an implant surface. Therefore, considerable efforts have been made to optimize the properties of Ti implant surfaces to improve osteointegration. GO is regarded as a promising candidate to functionalize the surfaces of implants for the regulation of the interactions between implant surfaces and cells, and several groups have explored whether and how GO coatings of dental implants could improve osseointegration .In general, GO coatings can significantly enhance bone marrow mesenchymal stem cell (BMSC) adhesion, spreading, proliferation, and osteogenic differentiation in vitro and promote bone-implant osseointegration in vivo . More neInterestingly, GO was also studied as an efficient carrier to deliver therapeutic proteins, such as BMP-2 and SP . La W etS. mutans supplemented with sucrose for 8 days [Candida albicans biofilm growth and removal [Ti implants modified with graphene nanocoating (GN) can maintain their quality and electrochemical and structural integrity under biologically relevant stresses such as microbial-rich environments and inflammatory macrophages. Specifically, Ti implant surfaces with GN exhibited higher polarization resistance and lower corrosion rates after exposure to r 8 days . GN cove removal . However removal . In addi removal . This ma removal . Further removal .MPCR-TNZ, the multipass caliber-rolled Ti alloy of Ti13Nb13Zr, has been reported to have strong fatigue characteristics and high mechanical strength. For further applications in dental practice, Jung HS et al. obtained DEX/rGO–MPCR-TNZ by modifying the MPCR-TNZ surface with rGO followed by the loading of osteogenic DEX via π-π stacking on the graphitic domain of rGO. They found that DEX/rGO–MPCR-TNZ significantly stimulated MC3T3-E1 cell growth and osteoblast differentiation, showing remarkable expression of osteogenic markers, including Runx2, OPN, OCN and Col-1. rGO–MPCR-TNZ also exhibited an increase in calcium nodule deposition and ALP activity. The prototype implant of rGO–MPCR-TNZ was successfully implanted onto an artificial bone block with mechanical performance and structural characteristics similar to those of a jawbone, and the rGO on the surface remained stable even after implantation, confirming the feasibility of rGO–MPCR-TNZ for applications in clinical dentistry. Shin YC et al.  preparedTaken together, to date, there have been studies using graphene, GO, and rGO as coatings for Ti implants. Evidence from both in vitro and in vivo studies has suggested that GO coatings are beneficial for peri-implant bone formation and stabilization. The rGO coating also showed the potential to promote osteoblast differentiation; however, the pro-osteogenic effect of the rGO coating needs further investigation in vivo. Although graphene coatings have good stability in bacteria-rich and inflammatory environments, the ability of graphene coatings to cope with mechanical wear needs to be improved, and their osseointegration is unclear. Here, we summarize the applications of graphene and its derivatives mentioned above in dentistry for reference by subsequent researchers .S. mutans and P. gingivalis, which is beneficial for caries and periodontal disease therapy, as well as the success of implantation. However, many materials have this effect, and the current opinion is that dental caries and periodontal disease are not infections caused by a single pathogenic bacterium but result from oral microbiota dysbiosis and the imbalance of oral microbiota and host interactions. If graphene and its derivatives can selectively inhibit harmful bacteria without influencing beneficial bacteria, they will become a modern and powerful weapon for dentistry. Moreover, although graphene and GO can promote the differentiation and proliferation of DPSCs and PDLSCs, which is beneficial to the regeneration of dental pulp and periodontal tissues, whether they can act on specific genes or proteins to precisely regulate the host immune inflammatory response and maintain the balance between oral microbes and the host remains to be studied. In addition, the development of dentistry is inextricably linked to the delivery of biological drugs, including peptides, monoclonal antibodies, and nucleic acids. Graphene and its derivatives have shown great promise in the development of drug delivery systems, especially the delivery of drugs for targeted cancer therapy. However, there are currently only several lines of evidence that GO can be applied to oral biopharmaceutical delivery.Taken together, graphene and its derivatives can improve the mechanical properties of dental materials and present good biocompatibility; it may be an ideal material for caries filling, especially for severe caries filling. Existing evidence indicates that graphene and GO can inhibit the growth of both