Solving the NIR dilemma for organic photovoltaics

如果能够有效地将太阳光转化为电能，将解决世界上所有的能源需求。这也将大大减少二氧化碳的产生。用于实现这种转换的光伏器件正变得越来越有效，但该技术距离大面积收集太阳光的广泛应用还有很长的路要走。其中一个主要问题是，目前在光致电荷分离方面工作得最好的材料只能收集一部分太阳光谱。太阳光包含大量的红光和近红外光，当太阳在天空中较低时（例如，在冬季和夏季的早晨和晚上），由于较短波长的光的散射，到达地球的这种光的比例增加。然而，这种长波长光的光子能量含量对于当前的有机光伏（OPV）器件来说太低了。拟议的工作将生产和研究吸收这种低能量光子的新材料，然后这些光子聚集在一起形成能量为两倍的单个光子。这可以通过产生低能量的三重激发态来实现，其然后可以经历三重-三重湮没（TTA）过程以产生更高能量的单重态。通过这种方式，通常无用的红光和NIR光被转换成可以与OPV一起使用的有用的绿色或橙子光。这个过程被称为上转换。我们工作的一个关键新特征是，一旦形成单重态，就会被掺杂剂捕获，这些掺杂剂的能量比主体材料低，并且发射效率高。通过制备和研究用于这三个目的（三重态生成、TTA主体和单重态陷阱）的新材料，我们将优化工艺并确定最佳的系统。上转换系统将适用于整个OPV系统，通过有效地固定在OPV上以收集未收集的低能量光并将其辐射回OPV。这有可能导致此类系统的效率大幅提高。

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.