Structure-Property-Performance Relationships for Organic Bulk Heterojunction Solar Cells

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Organic semiconductors combine the semiconductor properties traditionally associated with inorganic materials with the more desirable properties of plastics such as low cost, flexibility and ease of processing and patterning. Moreover, the organic syntheses of these materials allow for great flexibility in the tuning of their electronic and optical properties. By combining these properties, organic semiconductors such as conjugated polymers and small molecules have been demonstrated as the active layer in a wide range of optical and electronic devices including photovoltaic solar cells. The leading design of organic solar cells is based on the bulk heterojunction , in which organic blends comprising an electron donating component (usually a conjugated polymer) and an electron accepting component (such as a fullerene derivative or a conjugated polymer) that are dissolved in the same solvent and then spin-coated from the solution to form a thin film, sandwiched between two different electrodes. Recent developments in materials and device fabrication processes are leading to rapid improvements in performance of these devices. For example, solar conversion efficiencies up to 5-6 % were reported for solution-processed organic solar cells. Despite their significantly improved device performance, a number of scientific challenges remain to more fully understand, quantify, and predict the behaviour of the organic bulk heterojunction solar cells. In particular, the dependence of device performance on the thin film morphology of organic blends and the factors affecting the development of specific thin film structures from blend solutions are still poorly understood. To understand the interplay between the organic blend thin films and devices, it is therefore important to identify the structure-property relationship of the organic materials and its effects on device performance.Our work at Imperial College London seeks to provide key fundamental and technological insights into this issue. We aim to clarify the structure-property-performance relationships of organic bulk heterojunction solar cells. We will achieve this aim through a combined application of scanning probe microscopic techniques and optical and electrical techniques to identify the nature of organic nanostructures and interfaces in terms of their chemical, physical, optical, and electrical properties, and thus to clarify the role of these organic nanostructures and interfaces on device performance. It is our vision that only the fully complementary information obtained by measurements of these properties will be able to provide the necessary powerful tool to study organic nanostructures and interfaces and thus to further develop organic bulk heterojunction solar cells.