Synthesis and characterization of some new ruthenium (II) complexes as photosensitizers in dye-sensitized solar cells

New ruthenium (II) complexes, [Ru(DHZ)2(bpy)], [Ru(SCN)2(bpy)(DMSO)2], [Ru(SCN)2(dmbpy)(DMSO)2] and [RuCl2(salen)], where bpy = 2,2'bipyridine, DHZ = 1,5-diphenylthiocarbazone, dmbpy = 4,4'-dimethyl-2,2' bipyridine and salen = 2,2'ethylenebis(nitrilomethylidene)diphenol were synthesized and characterized by elemental analysis, FTIR, UV-Vis spectroscopy and thermal analysis. From data of these investigations the structural formula and the mode of bonding were obtained. These complexes were successfully applied to sensitization of nano-crystalline TiO2 based solar cells (DSSCs). The photovoltaic efficiencies of the studied DSSCs increase in the following order [Ru(DHZ)2(bpy)]< [Ru(SCN)2(bpy)(DMSO)2]< [Ru(SCN)2(dmbpy)(DMSO)2]< [RuCl2(salen)]. This increase is in agreement with the light harvesting of these complexes as indicated from their absorption spectra. Ferrioxalate complex enhanced the performance of some investigated cells. Therefore, a mechanism of this improvement has been postulated. Polyaniline as well as iodine doped polyaniline modified FTO electrode has been tested as promising counter electrodes. The efficiencies of the cells using iodine doped polyaniline is higher than that of polyaniline, which is assignable to the high conductivity of iodine.


INTRODUCTION
Dye-sensitized solar cells (DSSCs) have attracted considerable attention in recent years due to the possibility of low-cost conversion of photovoltaic energy [1,2]. DSSCs possess four components: (i) dye sensitizer to harvest solar energy and generate excitons, (ii) nanostructure metal oxide material to transport electrons efficiently, (iii) redox couple, usually comprised of iodide/ triiodide (I -/I -3) to reduce the oxidized dye back to its neutral state and (iv) counter electrode to accept the positive charge from the redox electrolyte. The following reactions summarize the working principles of the cell in a lucid manner, where S refers to the sensitizer [3][4][5]:

c o m
Due to the non-toxicity, availability and low-cost characteristics, TiO2 has been the mostly preferred as the semiconductor for the photoelectrode. Although a lot of dyes have been tested and investigated, ruthenium complexes have proved to be the most effective consistently [6]. Therefore, we reported herein, the synthesis and characterization of some new ruthenium (II) complexes and study of their photovoltaic properties in DSSCs. Recently, nanostructured conducting polymers have been utilized as a potential catalyst for counter electrodes in DSSCs [7,8]. Among conductive polymers, polyaniline (PANI) is especially promising because its extremely low cost, good conductivity and good electro-catalytic activity [9][10][11][12]. Therefore, polyaniline prepared electrochemically (EC-PANI), polyaniline prepared chemically (C-PANI), iodine-doped polyaniline (I2-PANI) as well as graphite have been tested in our DSSCs.
Independently, ferrioxalate complex has been used in order to improve the performance of the cells.

Materials and methods
All chemicals and solvents used were of analytical grade and used without any purifications.  [14,15] were prepared according to the previously published work.

Preparation of polyaniline modified FTO electrode
C-PANI was prepared according to previously published work [16][17][18][19][20]. Accurately, 0.5gm of aniline-HCl was dissolved in 15ml of distilled water(solution A) and 1.5gm of ammonium persulphate was dissolved in 15ml of distil-water (solution B), the two solutions A and B were stirred for 1h and then solution B was added to solution A drop wise and stirred for 1h, then the solution is kept to complete polymerization. FTO electrode was immersed in the polymer solution for one day. The polymer attached to the nonconductive side of FTO was removed. Finally, the electrode was dried at 60ºC. The prepared polymer was characterized by X-ray diffraction (XRD) and Fourier transient infrared (FT-IR).
FTO acted as a working electrode in three electrode system (a platinum as a counter electrode and Ag/AgCl as a reference electrode) on an electrochemical station (Autolab PGSTAT302F, Metrohm UK, and Ireland). The anodic deposition was controlled in a solution containing 0.3ml of aniline and 0.5ml of HCl (37%). The EC-PANI was prepared by controlling the number of cycles (72) with a potential range from 1 V to -0.3 V and a scan rate of 0.05 V/S, the resulting electrode was washed with distilled water and dried at 60 °C.

Fabrication of DSSCs
Fluorine doped tine oxide glass (FTO) was cut into pieces of 2.5 cm x 2.5 cm size and cleaned [32].The photoanode was prepared by taking 200µLof TiO2 paste and spin coated with speed 1500rpm for 50 seconds, then the substrate was heated at 80ºC and calcined for 60 min at 500°C. The photoanode is then immersed in a solution of 1x10 -4 M sensitizers for 24h. For studying the effect of the addition of K3[Fe(C2O4)3], the photoanode is immersed in a solution of 1x10 -4 M of both Ru complexes and iron complex. The DSSC was assembled by sandwiching the electrolyte (0.5M of KI and 0.05M of I2 in ethylene glycol) between photoanode and counter electrode.
The current-voltage characteristics of the DSSCs were measured using solar simulator with an AM1.5 spectral filter, and the intensity was adjusted to 1 sun(100 mW/cm 2 )using calibrated cell. The output current and voltage were measured using a multimeter in the presence of variable resistance. The active surface area of the developed cell was 2.56cm 2 .

Instruments and measurements
The

RESULTS AND DISCUSSIONS
The synthesis of the precursor complex [RuCl2(DMSO)2] was achieved as a yellow precipitate by the action of microwave on a solution of RuCl3 in DMSO at 135 °C as previously reported [13]. The reactions of this yellow complex with the corresponding ligand yield the studied complexes through the substitution of DMSO or Clas illustrated in scheme 1.

FTIR and electronic spectra
The FTIR spectroscopy has been shown to be a powerful tool to give structural information about the complexes. Figure 1 shows the typical I.R spectra of the studied complexes in range 4000-500 cm -1 . The characteristic vibrational frequencies of [Ru(DHZ)2(bpy)] have been identified by comparing the peaks of the complex with that of the free ligands (Fig. 1a). The strong band at 1438 cm -1 assigned to ν-(N=N) in the free dithizone is shifted to 1444 cm -1 in the complex; also, the two bands assigned to ν-(C=S) and ν-(C=N) were shifted from 891 cm -1 and 1589 cm -1 to 763 cm -1 and 1596 cm -1 , respectively indicating the coordination of dithizone with ruthenium [24]. The coordination of ruthenium with 2,2'-bipyridine is indicated by the shift of the two characteristic bands of free 2,2'bipyridine from 1578 cm -1 and 1557 cm -1 to 1520 cm -1 and 1500 cm -1 , respectively in the complex [25,24]. Finally, the disappearance of the two characteristic bands of ν-SO at 1091cm -1 and 1013 cm -1 for DMSO indicate the complete substitution of DMSO [26].  Fig. 1 (b,c), which show nearly the same peaks for the two complexes due to the similarity in their structures. The spectrum shows an intense and sharp peak at 2107 cm -1 , which is assignable for coordinated SCN in the complex [27,24,28]. The coordination of ruthenium with 2,2'-bipyridine is clarified by the shift of characteristic peaks for the free 2,2'-bipyridine from 1578cm -1 , 1557cm -1 to 1464cm -1 , and 1445cm -1 in [Ru(SCN)2(bpy)(DMSO)2] and to 1558 cm -1 ,1540 cm -1 in [Ru(SCN)2(dmbpy)(DMSO)2], which indicate the coordination of 2,2'bipyridine [25,24]. Finally, the presence of coordinated DMSO in the complex is indicated by the presence of two characteristic peaks for SO at 1088cm -1 and 1013cm -1 of DMSO [26].

Thermal behavior
The thermal behavior of the studied complexes was carried out within the temperature range from room temperature up to 1000 °C on a dynamic air atmosphere using thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The observed mass losses are based on the TG results, and the calculated mass losses are based on the molecular weight of the proposed formula. The TG curve of [Ru(DHZ)2(bpy)].1.5 H2O (Fig. 3 a) shows three steps of decomposition. The first step at 38-216 °C corresponds to the loss of 1.5 water molecules of hydration (calc. 3.4 %, found 3.31 %). The anhydrous complex decomposes in a major stage consisting of three steps corresponds to the stepwise decomposition of the complex ended with oxide formation. The percentage weight of the residue agrees well with the RuO2 formation (calc. 16.75 %, found 17.12 %). The [Ru(SCN)2(bpy)(DMSO)2] complex was found to decompose in three steps (Fig. 3 b), associated with the stepwise decomposition of the complex. The residue may belong mostly to corresponding to un-assignment ligand decomposition. The weight loss percentage of the residue could be assigned to a mixture of the ruthenium oxide and metallic ruthenium.
Based on the above arguments the chemical structures as shown in Fig. 4 are suggested for the prepared complexes.

Photovoltaic performance of the studied DSSCs
The performance of a solar cell is defined by several parameters: open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF). The output efficiency (%Ƞ) calculated using the following equations:    (table 1). This absorption is associated with the delocalized electronic system of the dithizone ligand [36]. The presence of this delocalization may be responsible for the low efficiency exhibited by the dithizone complex, the presence of such low laying energy level may enhance the deactivation of the excited state formed upon light absorption and consequently competes with the electron injection to the conduction band of TiO2 (reaction 2) as explained in Fig.6.

Fig. 4 Proposed structures of the studied complexes; (a) [Ru(DHZ)2(bpy)], (b) [Ru(SCN)2(bpy)(DMSO)2], (c) [Ru(SCN)2(dmbpy)(DMSO)2] and (d) [RuCl2(salen)] -
The low cell performances obtained (table 2) may result from the relatively poor interconnection between the nanocrystalline TiO2 (reaction 2) and sensitizers under investigation. This poor interconnection is associated with the absence of any anchoring group in the complex, like the carboxylic or phosphonic group, which is the most decisive factors in bringing the relative orientation of energy level of donor and acceptor during the attachment on the metal oxide, which leads to increase the electron injection efficiency.
Sensitizers also show slower electron injection or self-quenching if they undergo aggregation, which can be encountered either before or during processing of dye adsorption onto metal oxide.

Effect of addition of K3[Fe(C2O4)3]
Additives   The formed oxalate radical, which is confirmed by electron paramagnetic resonance [38] adds another route in the reduction of the oxidized sensitizer to its neutral state; this is of course, beside the role of I -/I3couple as shown in the scheme (2).

Fig. 7 J-V curve of DSSCs of [Ru(DHZ)2(bpy)] in absence and in presence of K3[Fe(C2O4)3] using C-PANI as a counter electrode.
It is well known that electron injection from the sensitizer into the TiO2 typically happens in femtosecond to picosecond time scale whereas charge recombination in the microsecond to millisecond time scale [39]. The reduction of oxidized sensitizer molecules occurs in the ms or µs range, which in by several orders of magnitude slower than injection rate. Also, the transient state formed by  4ns [40]. Upon comparison of the times mentioned above, it is clear that the ferrioxalate complex will play an enhancement effect of the performance of the DSSCs if there is a good interaction between it and the sensitizer. Therefore, this point will be subject to further investigations in the future. Specially, we have recently proved an enhancement effect of this ferrioxalate complex on the photo colorization and photodegradation of an azo dye through UV/H2O2 advanced oxidation process [41].

Fig. 8 UV-Vis spectroscopy of [Ru(DHZ)2(bpy)] in absence and in presence of K3[Fe
In   However, the separation of polymer from FTO substrates offers a high interfacial resistance is an unfavorable factor for charge transportation.
Independently, cathodic reduction peak for (C-PANI) is higher than (EC-PANI), which means that electrocatalytic activity of (C-PANI) is better than (EC-PANI) [17]. I2-PANI occupies the top position in the above series, which can be associated to the high conductivity of iodine doped in this polymer.

Conclusion
It is clear that the [RuCl2(salen)] -2 complex showed the highest efficiency compared to other complexes, which is due to the good light