Self-Organization Schemes towards Thermodynamic Stable Bulk Heterojunction Morphologies: A Perspective on Future Fabrication Strategies of Polymer Photovoltaic Architectures
Research efforts to improve our understanding of electronic polymers are developing fast because of their promising advantages over silicon in photovoltaic solar cells. A major challenge in the development of polymer photovoltaic devices is the viable fabrication strategies of stable bulk heterojunction architecture that will retain functionality during the expected lifetime of the device. Block copolymer self-assembly strategies have attracted particular attention as a scalable means toward thermodynamically stable microstructures that combine the ideal geometrical characteristics of a bulk heterojunction with the fortuitous combination of properties of the constituent blocks. Two primary routes that have been proposed in the literature involve the coassembly of block copolymers in which one domain is a hole conductor with the electron-conducting filler (such as fullerene derivatives) or the self-assembly of block copolymers in which the respective blocks function as hole and electron conductor. Either way has proven difficult because of the combination of synthetic challenges as well as the missing understanding of the complex governing parameters that control structure formation in semiconducting block copolymer blends. This paper summarizes important findings relating to structure formation of block copolymer and block copolymer/nanoparticle blend assembly that should provide a foundation for the future design of block copolymer-based photovoltaic systems. 1. Introduction Solar cells from silicon (Si) represent the essential part of the solar photovoltaic (PV) cells on the market. But the Si technology together with other metal composites like CdTe and GaAs has major drawbacks such as high process cost, hazard to the environment, and pollution in addition to a limited availability [1, 2]. Polymer-based PV cells can be viewed as an alternative or at least a complement to those standard technologies [3–5]. The active layer of the polymer composite is easy to process and can be deposited on a solid surface from a solution. The process cost is drastically reduced compared to standard inorganic cells, and large surface areas can be obtained. The cells are flexible with a shape designed almost at will according to the needs but nevertheless have a strong mechanical resistance [6–8]. The main challenge that remains to be faced is the possibility of enhancing the efficiency [8, 9] which is continuously improving by using the appropriate polymer system with the right architecture and morphology [9, 10]. The active polymer layer collects the incident light
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