Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
Understanding and optimizing charge collection and therefore the solar cell fill factor is arguably one of the most crucial aspects for further development of organic solar cells. High fill factors and good charge-carrier collection are commonly associated with a good electronic quality of the semiconducting thin film composing the active layer, i.e. long charge-carrier lifetimes and high charge-carrier mobilities of the organic material blend. The problem with translating the fill factor into a quantitative assay of electronic quality is the complex dependence of the fill factor on a multitude of parameters such as active-layer thickness, internal electric fields and charge-carrier mobilities and lifetimes. Further improvements therefore require a solid understanding of the physical mechanisms affecting the fill factor, as well as rapid screening methods to allow photovoltaic laboratories worldwide to compare their results and identify the most promising materials.
We first present a refined expression for the charge-carrier collection coefficient that describes the competition between the extraction and recombination of charge carriers in organic solar cells. Compared to the previous study of Bartesaghi et al. [1] a clearer mapping between charge-carrier collection coefficient and solar cell fill factor is achieved. A complete physical picture of the fill factor is developed by including a detailed discussion of the model’s assumptions and deviations that come to place in the presence of imperfect contacts or space charges resulting from doping or unbalanced charge-carrier transport.
Based on this analysis, we then show a quantitative measure of electronic quality from nothing more than thickness, fill factor, open-circuit voltage and short-circuit current. These are parameters that are nearly always measured and reported in literature for a large range of materials used in organic photovoltaics. Thereby a valuable and easy-to-access figure of merit available to the whole organic photovoltaics community without further need for equipment is found. Eventually, this allows the monitoring of the development of organic semiconductor blend films with good electronic properties which promote organic solar cell fill factors and device efficiencies to higher levels.
[1] D. Bartesaghi et al., Nat. Comm., 2015, 6, 7083
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