Testing two concepts to increase the efficiency of p-type dye-sensitized solar cells

廉价的太阳能是减少二氧化碳排放和防止全球变暖的关键因素之一。染料敏化太阳能电池(DSC)价格低廉，能量回收期短，在漫反射光下表现出良好的性能，这使得它们在商业应用中非常有吸引力。到目前为止，DSC的光电转换效率不超过13%。虽然带光阳极的DSC(n-DSCs)适合于商业应用，但最高效的带光电阴极的DSC(p-DSCs)的功率转换效率只有十分之一左右，对于商业应用来说太低了。建立高效的p-DSC不仅为商业应用提供了一种新型的DSC，而且也是构建具有光活性阳极和光活性阴极的高性能DSC(NP-DSC)的里程碑。这些器件可以比它们各自的模块拥有更高的功率转换效率。在这一应用中，提出了两个概念来提高p-DSC的光电转换效率。第一个概念是在p-DSC中结合使用离子液体和过渡金属氧化还原介体。离子液体的阴离子应该比阳离子更接近半导体中的注入空穴，这似乎是合理的，比如作为氧化还原介体的带正电的过渡金属络合物。因此，离子液体的使用可能会降低p-DSCs的高暗电流，从而限制p-DSCs的功率转换效率。第二个概念是氧化还原介体在染料处的临时对接。带有Lewis碱性基团的染料可以配位在过渡金属络合物上。染料与带正电的氧化还原介体的紧密接触会提高染料向氧化还原介体的优先电子转移速率，同时也会增加染料与半导体之间的非理想电子复合速率。如果从染料转移到氧化还原介体的电子位于过渡金属络合物的反键d轨道上，则染料与氧化还原介体之间的键应该被削弱。因此，如果染料提供了强烈的配位配体，则在染料上配位的还原氧化还原介体可以被溶液中氧化形式的氧化还原介体所取代。这一替换将恢复器件中的初始配置。如果将临时氧化还原介体对接在染料上或使用离子液体与过渡金属氧化还原介体结合作为提高p-DSC效率的总体概念，后续项目将专注于器件性能的优化，以构建高效的p-DSC和超过13%光电转换效率的新一代DSC，NP-DSCs。

Cheap solar energy sources are one of the key factors to reduce carbon dioxide emission and prevent global warming. Dye-sensitized solar cells (DSCs) are cheap, possess a short energy payback time, and show a good performance at diffuse light, which make them very attractive for commercial applications. So far, DSCs do not exceed 13% light-to-electricity conversion efficiency. While DSCs with a photoanode (n-DSCs) are suitable for commercial applications, the most efficient DSC with a photocathode (p-DSCs) shows only about one-tenth power conversion efficiency, significantly too low for commercial applications. Building a high efficient p-DSC would not only provide a new type of DSC for commercial applications, it is also a milestone in the construction of high-performance DSCs with a photoactive anode and a photoactive cathode (np-DSC). These devices can possess a significantly higher power conversion efficiency than their individual modules. Within this application, two concepts are addressed to increase the light-to-electricity conversion efficiency of p-DSCs.The first concept is the usage of ionic liquids in combination with transition metal redox mediators in p-DSCs. It seems reasonable that the anions of ionic liquids should be closer to injected holes in the semiconductor than cations, like positively charged transition metal complexes which serve as redox mediator. Thus, the usage of ionic liquids might decrease the high dark current of p-DSCs which limits the power conversion efficiency of p-DSCs. The second concept is a temporary docking of the redox mediator at the dye. A dye with a Lewis basic group as building block can coordinate at a transition metal complex. The close contact of the dye and the positively charged redox mediator should enhance the preferred electron transfer rate from the dye to the redox mediator while the undesired electron recombination rate between dye and semiconductor should be increased. If the electron transferred from the dye to the redox mediator resides in an antibonding d-orbital of the transition metal complex, the bond between the dye and the redox mediator should be weakened. As a result, the reduced redox mediator coordinated at the dye can be replaced by the oxidized form of the redox mediator in solution if the dye offers a strongly coordinating ligand. This replacement will restore the initial configuration in the device.If the concept of temporary redox mediator docking at the dye or the usage of ionic liquids in combination with transition metal redox mediators can be shown as general concept to increase the efficiency of a p-DSC, a follow-up project will focus on device performance optimizations to construct highly efficient p-DSCs and a new generation of DSCs, np-DSCs exceeding 13% light-to-electricity conversion efficiency.