Hole conducting- and/or hole transport materials are useful in a wide range of electronic devices and applications, such as in organic electroluminescent (EL) devices, organic light-emitting devices (OLEDs) and in solar cells.
Dye-Sensitized Solar Cells
A dye-sensitized solar cell (DSC, also sometimes referred to as DSSC or DYSC) is a particular type of solar cell in which a hole transport material (HTM) may prove useful. DSCs have gained a great interest as cost-effective alternatives to silicon-based photovoltaic devices. In the DSC, light is absorbed by a dye molecule anchored to a mesoporous wide band-gap semiconductor, normally TiO2. Upon light absorption the photoexcited dye injects an electron into the conduction band of the semiconductor and its resulting oxidized state is regenerated by a redox mediator in a surrounding electrolyte. So far the best cell performances in DSCs obtained with ruthenium based dyes and the iodide/triiodide redox couple have been about 11% conversion efficiency.
Copper Based HTMs
Fukuzumi and coworkers (Journal of the American Chemical Society, 2005, 127(26):9648-9654) have constructed the first dye-sensitized solar cells using copper complexes as redox couples to compare the photoelectrochemical responses with those using the conventional I3−/I− couple. The maximum η value attained was 2.2% for DSC using [Cu(dmp)2]2+/+ under the weak solar light irradiation of 20 mW/cm2 intensity.
In addition, Peng Wang and coworkers (Chemical Communications, 2011, 47(15):4376-4378) have employed a bis(2,9-dimethyl-1,10-phenanthroline) copper(I/II) redox shuttle demonstrating a 7.0% efficiency. DSCs free of a corrosive iodine electrolyte were demonstrated by Wang et al by virtue of a mesoporous titania thin-film, coated with a high-absorption-coefficient organic photosensitizer. However, the utilized copper redox shuttle was also found to display very low electron transfer rates on several noble metals, carbon black and conducting oxides, resulting in a poor fill factor.
Cul and CuSCN can be used as a solution-processable, inorganic hole conductor in solid state DSCs (ssDSCs) and methylammonium lead iodide perovskite solar cells. These solar cells can provide power conversion efficiencies as high as 6.0% for DSCs and show to be very stable, providing a better phorcurrent stability and fill factors in comparison with spiroOMeTAD based solar cells upon continuous 2 h illumination. However, despite these advantages, the efficiency obtained with Cul is still lower because of the exceptionally high voltages (Voc) obtained in spiro-OMeTAD solar cells. Future studies are aimed toward determining whether the high recombination seen in Cul-based solar cells can be reduced and higher Voc obtained. Despite these potential difficulties, Cul represents a promising low-cost hole conductor for perovskite solar cells.
US 2006/008580 discloses organic hybrid solar cells in which copper based organometallic complexes may be used as HTMs. The organic hybrid solar cells further comprises a substrate material, an electrode material, a dye material, and a semiconductive oxide layer, and the semiconductive oxide layer of the organic hybrid cell has been vapor deposited.
The high efficiency obtained with hybrid solar cells based on dye and/or perovskite sensitizers demonstrate their potential for implementation as commercial solar cells. However, the use of organic hole conductors may represent a potential hurdle to the future commercialization of this type of solar cell because of their relatively high cost. For example, the current commercial price of high purity spiro-OMeTAD is over ten times that of gold and platinum. While increased demand would undoubtedly lower this cost dramatically in any large scale commercial endeavor, it is likely to remain expensive due to the synthetic methods and high purity needed for photovoltaic applications. Thus, development of alternative hole conductors and/or HTMs is a promising avenue to further improve the performance of solar cells, as spiro-OMeTAD likely does not represent the ideal hole-conducting material for this system. Thus, in order to increase the efficiency of ssDSSC and to decrease their cost, a new kind of hole conducting material and/or HTM is required.