Novel hybrid materials for improved photovoltaic device efficiencies

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In principle, photovoltaic (PV) devices could meet all our energy requirements in a sustainable way: the earth's surface receives enough light energy per hour to power the entire human population for one year. However, the current capital expense of conventional photovoltaics is too great to be competitive, and the volume in which they can be produced is much too small to make a serious dent in our electricity generating needs. As a consequence solar photovoltaics (PVs) account for < 0.1 % of our energy generation. Whereas conventional silicon-based PVs have high manufacturing costs and involve energy-intensive fabrication, organic PVs can be processed using high-volume roll-to-roll printing technology, promising large-area, cheap photovoltaic films of flexible backing material that could be used in principle to coat windows, walls and roofs. Such devices are already being produced commercially for high-mobility, low-power applications such as rucksacks comprising flexible organic PV devices to charge mobile phones and similar products. The external power efficiencies of polymer-based PVs have increased dramatically from sub-1% in the mid-90s to <6% in a period of 10-15 years. One of the most promising systems has involved a blend of two molecular materials P3HT (a polythiophene long-chained molecule) and PCBM (a functionalized fullerene). The preparation of these involves limited segregation that leads to a bicontinuous structure on the lengthscale of around 10 nm, such that electrons and holes can be separated relatively efficiently at an interface between the two materials. The efficiency of the P3HT/PCBM system is limited by the relatively large band-gap of P3HT, which renders the optical absorption inefficient at the red end of the solar spectrum. Consequently, new polymer materials are being developed, but are usually patented by companies and expensive to synthesise, that have lower band-gaps extending the absorption performance to red/infra-red wavelengths. We are developing an alternative approach using hybrid structures to increase the local electric field and to enhance the optical absorption. These hybrid materials will then be used to fabricate prototype photovoltaic devices allowing an assessment of the power conversion (solar-to-electricity) efficiencies. In the first instance, we hope to develop photovoltaic devices with efficiencies of greater than 6 %, comparing favourably with the market leaders. This will allow further capital investment to be sought with the long-term aim of larger scale applications being realised once efficiencies reach 9-10 %.