Solar energy is an inexhaustible and environmentally sustainable natural resource. Therefore, photovoltaic technology has the potential to minimize the use of CO2-emitting fossil fuels in today’s global energy system. Conventional polycrystalline p-n-silicon photovoltaic cells are commercially available with energy conversion efficiencies in the 10–20% range. However, the production of ultrapure silicon is expensive and requires a considerable amount of energy. Thus, it takes approximately one year until polycrystalline silicon solar cells harvest the energy required to build them.
Dye-sensitised solar cells (DSCs), initially reported by O’Regan and Grätzel, have been subject of intensive research in the last two decades as they show great potential becoming an alternative to silicon solar cells in the future. Apart from the potentially lower fabrication costs and construction from environment-friendly components, DSCs can perform better under diffuse light conditions and at elevated temperatures. Moreover, DSCs may be better integrated into future commercial products, as they can be produced in manifold shapes, color and transparency. Device fabrication costs could further be reduced by automatic continuous screen printing, ideally on flexible substrates to enable broad application.
Researchers in the Spiccia group aim for developing new DSC materials, cell designs and deposition techniques on various substrates.
DSCs typically consist of a sensitised mesoporous semiconductor on a conductive substrate as a working electrode and a platinized counter electrode, bridged by an electrolyte containing the reduced and oxidized forms of the redox mediator (Figure 1). The latter mediates charge transport between the two electrodes and is also responsible for the regeneration of the photo-oxidized dye, following electron injection into the TiO2. The application of a cobalt(II)/(III) tris(2,2-bipyridine) based electrolyte has recently accelerated the transition to non-iodide based electrolytes, which are less corrosive and exhibit weaker absorption of visible light. In our group, transition metal, such as cobalt, iron and manganese based redox couple have been successfully developed and promising efficiencies were obtained in both n-type and p-type DSCs. By solidifying the electrolyte, the stability of the devices can be further improved. More importantly, the replacement of organic solvent in the electrolyte with water paves the way for environmentally friendly devices, which keeps us in the leading role in this area worldwide.
Beside DSCs, we are also working on improving hybrid organic-inorganic perovskite solar cells due to the enormous potential of methylammoinum lead halides ([CH3NH3PbX3]n). There is intensive research into this photovoltaic technology since 2012 and the topic has been selected as one of the top 10 breakthroughs of 2013 as announced by the journal Science. In this new type of all-solid-state hybrid solar calls, an organic-inorganic halide perovskite material with a similar crystal structure to CaTiO3 in the form of ABX3 is employed to harvest the solar energy. Here, A is an organic cation, B is a metal, and X is a halide. Such hybrid trihalide perovskite materials have a broad light absorption range from 300 to 800 nm and a high molar extinction coefficient. Therefore, only a very thin photoactive layer is needed (~ 300 nm) for an efficient perovskite solar cell. Compared to DSCs, which require a film thickness of 10-20 µm, the perovskite thin-film solar cell would save a lot of material and hence fabrication costs.