Solar cells made from molecular semiconductors currently trap too little sunlight to be useful, and this project aims to break that bottleneck by building their internal structure in a new way. The problem is that these cells rely on a nanoscale junction between two different semiconductors to generate electricity from light. Excitons—the packets of energy created when light hits the material—must reach that junction before they decay, which means the junction needs an enormous surface area packed into a tiny volume. Existing methods for creating that nanostructure have hit a wall, and cell performance has stalled. The team’s approach is to separate the task of shaping the nanoscale architecture from the task of choosing the semiconductors. They will use sacrificial polymer templates—specifically, the controlled structures formed by di-block copolymers—to create the desired nanoscale morphology first, then replace the polymer with active semiconductors. This should allow a stable manufacturing process, better control over the cell’s internal geometry, and the freedom to use semiconductors that better match the solar spectrum. The process is designed for low-temperature manufacturing, which is critical for keeping costs down. The target efficiency is 10%.
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This project will produce manufacturable nanoscale architectures for heterojunction solar cells. Though routed strongly within 'science', the objectives are to achieve engineering solutions to allow the breakthrough needed in this field (target efficiency 10%). Excitonic solar cells based on molecular semiconductors require the presence of a heterojunction between electron and hole-accepting semiconductors in order to separate charges from photogenerated excitons. Large heterojunction interfacial areas are required if all photogenerated excitons are to reach the heterojunction before decaying, and this requires a complex nanoscale architecture. Current methods to achieve this nanostructure and limited and solar cell performance of such devices has stalled. We propose therefore to develop generic routes to separate the control of the nanoscale morphology from the selection of the donor and acceptor semiconductors. This will represent a critical advance in allowing a stable process window, and should allow improved photovoltaic performance through better morphology control and the ability to use semiconductors better matched to the solar spectrum. These routes will be compatible with low temperature processing (this is critical for low-cost manufacturing). The general principle we will use is to separate the processes needed to form the desired nanoscale architecture from the subsequent formation of the active semiconductor-semiconductor heterojunctions at which charge separation is achieved.Central to our approach is the use of 'sacrificial' polymer structures that provide excellent control of nanoscale morphology, and their later replacement with active semiconductors. We will use the controlled nanoscale structures produced using di-block copolymers
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