A new model shows how transport resistance limits solar cell efficiency and how better fill factor predictions could help us develop more powerful photovoltaics
Organic solar cells are a rapidly advancing third-generation solar technology, offering a combination of high efficiency, low production cost, and mechanical flexibility. With reported power conversion efficiencies now exceeding 20%, organic solar cells are beginning to rival traditional silicon-based cells. They also enable new applications, such as integration into building materials and wearable electronics.
A key metric for evaluating solar cell performance is the fill factor, which measures how close a cell comes to delivering its theoretical maximum power. Fill factors are influenced by both the materials used and the design of the device. Historically, recombination losses have been considered the primary limitation to maximising fill factors. This is where electrons and holes recombine before contributing to the electrical current.
Recent research has highlighted another critical variable called transport resistance. In organic semiconductors, which typically have low electrical conductivity, charge carriers move slowly through the material. This slow transport increases the likelihood of recombination before the charges reach the electrodes, leading to significant fill factor losses even in highly efficient devices. If the electrons and holes were runners in a race, recombination losses are runners giving up before they finish. In organic materials, charge transport occurs via hopping: it is like the runners moving through mud, it is much harder – increasing the transport resistance – and they are more likely to give up before the end.
To address this, the authors developed an analytical model that incorporates the nature of this slow transport, energetic disorder, and systematically evaluates transport resistance using experimental data. By analysing current-voltage characteristics and light intensity-dependent open-circuit voltage across a range of temperatures, the model distinguishes between losses due to recombination and those due to charge transport.
This refined approach enables more accurate predictions of fill factors and offers practical strategies to minimise transport-related losses. Improving fill factors not only enhances the performance of organic solar cells but also provides insights applicable to other emerging photovoltaic technologies, helping to guide the future design of third-generation solar power.
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Efficient charge generation at low energy losses in organic solar cells: a key issues review by Ye Xu, Huifeng Yao, Lijiao Ma, Jingwen Wang and Jianhui Hou (2020)
