Solving the NIR dilemma for organic photovoltaics

点击翻译按钮获取中文摘要

Being able to convert sunlight efficiently into electrical energy would solve all the world's energy needs. It would also greatly reduce the production of carbon dioxide. Photovoltaic devices for achieving this conversion are becoming increasingly efficient, but the technology is still a long way off being put into broad application for large area collection of sunlight. One of the major problems is that the materials that currently work best in terms of photoinduced charge-separation are only able to collect a part of the solar spectrum. Sunlight incorporates a large amount of red and near-infrared light, and the proportion of such light reaching the earth increases when the sun is low in the sky (for example, in winter, and in mornings and evenings in summer), due to the scatter of light of shorter wavelengths. However, the photon energy content of such long-wavelength light is too low for current organic photovoltaic (OPV) devices.The proposed work will produce and study new materials which absorb such low-energy photons of light, and which then come together to form a single photon of twice the energy. This can be achieved by generating low-energy triplet excited states, which can then undergo a process of triplet-triplet annhilation (TTA) to generate higher energy singlet states. In this way, the normally useless red and NIR light is converted into useful green or orange light that can be used with the OPVs. The process is known as upconversion. A key new feature of our work will be that the singlet states once formed will be trapped by dopants that have lower-energy than the host materials and emit with high efficiency. This will circumvent the current problems such as wastage of the upconverted photons at defect sites.By preparing and studying new materials for these three purposes (triplet generation, hosts for TTA, and singlet traps), we shall optimise the process and identify the best systems to be used. The up-converting systems will be amenable for use with a whole range of OPV systems, by effectively bolting onto the OPV to collect the unharvested low-energy light and radiate it back into the OPV. This has the potential to lead to large increases in the efficiency of such systems.