Dye sensitized solar cells using liquid crystal charge transport layers
Photovoltaic (PV) devices have been extensively studied and developed for many decades now, with several inorganic device technologies being produced at industrial scales. A very concise way to sum up the vast variety of work performed in this area is to classify the historic development in three stages. The first generation PV cells included p-n junction solar cells, in particular crystalline silicon (Cr-Si). The second generation of PV devices were developed due to the widespread interest in lowering the material costs and complexity of bulk fabrication, through the development of thin film technologies such as amorphous-silicon, CIGS and cadmium Tellurium (CdTe) devices. Although, Cr-Si and thin film technologies have become considerably mature, and device efficiencies of a single junction III-VI GaAs as high as 27.6% have been reported, the complexity and costs of fabrication have left considerable room for innovation. There is still a need to achieve competitive performance with fossil fuels in cost per unit watt, and at the same time make the technology scalable. This brings us to what is widely considered as the third generation of solar cells, that started with the first demonstration of a organic solar cell by Tang etal. in 1986, followed by widespread efforts to fabricate PV devices using low cast carbon based organic materials.
Figure 1. A schematic of a typical working DSSC device (Hagfeldt, et al., 2010) .
Dye sensitised solar cells (DSSCs) are an organic-inorganic hybrid that were first realised by Michael Graetzel and Brian O’Reagan in 1991. In their seminal work, a mesoporous Titania (TiO2) layer was utilised, and was sensitised with a organo-metal Ruthenium based dye. The use of nano-crystalline mesoporous TiO2 can be seen as their greatest contribution, as it provided a large surface area for dye adhesion, while also allowing the iodide/tri-iodide (I-/I3-) based redox electrolyte to fill the TiO2 nano-pores allowing the electrolyte to regenerate the large amount of adsorbed oxidized dye. A schematic of a DSSC device is shown in Figure 1.
Figure 2. Charge transport process and relative energy levels of the different components.
The basic device operation of almost all DSSCs is similar. Photo-excitation of an electron from the HOMO of the dye to the LUMO is followed by charge transfer into the conduction band of the mesoporous titania. Electrons then flow out of the TiO2 from the anode and the circuit is completed through the external load, supplying power. The oxidized dye needs to be regenerated, and hence an electrolyte / hole transport layer is necessary. Traditionally, an I-/I3- liquid redox electrolyte has been used for this purpose, but more recently researchers have studied many different liquid and solid hole transporters. An energy level illustration is provided in Figure 2.
One of the main problems associated with liquid (particularly I-/I3- ) based electrolytes is that it is corrosive to metallic contacts and sealing materials, and also that the low viscosity of the liquid leads to leakage problems, making these cells unstable for long term operation, and limit the operational lifetime. In our research group, we are interested in applying Liquid Crystal (LC) based sol-gel materials as charge transport layers in DSSCs. We are investigating both polymer dispersed liquid crystals (PDLCs) and low molecular weight mesogens. It is anticipated that the self organising, self healing and thermal/photo controlled micro-fluidic properties of LC materials could potentially lead to more stable and possibly more efficient liquid crystal based DSSC devices.
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