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The efficiency with which photogenerated free carriers are extracted in excitonic solar cells is governed by the interactions between a number of material specific and device level properties and parameters: recombination dynamics, faster and slower carrier mobilities, contact potentials, etc. This complexity is inherent in disordered excitonic semiconductors, and understanding the underlying structure-property relationships is an ongoing endeavour in the field of organic solar cells. Questions such as – do higher mobility semiconductors necessarily deliver improved charge collection; how does supressed bimolecular recombination lead to fill factor maintenance especially at higher input irradiances [1]; or, how do we achieve high fill factors and efficiencies in thick junctions more suited to viable manufacturing [2]?
In my talk I will examine some of the latest thinking in this area [3], and present data from a suite of emerging experimental techniques such as intensity dependent photocurrent, resistant dependent photovoltage and related extraction methodologies. I will also discuss recent findings on the interrelationship between the efficiency with which free charges are extracted and charge separated state formation – the two possibly connected through entropic considerations [4].
[1] “The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells”, B. Philippa, M. Stolterfoht, P. L. Burn, G. Juska, P. Meredith, R. D. White & A. Pivrikas, Scientific Reports, 4, 5695 (2014).
[2] "Efficient, large area, and thick junction polymer solar cells with balanced mobilities and low defect densities." A. Armin, M. Hambsch, P. Wolfer, H. Jin, J. Li, Z. Shi, P. L. Burn, and P. Meredith, Advanced Energy Materials, 5, 1401221 (2015).
[3] “Impact of charge transport on current-voltage characteristics and power conversion efficiency of organic solar cells”, U. Wurfel, D. Neher, A. Spies & S. Albrecht, Nature Communications, 6:6951 (2015).
[4] “Slower carriers limit charge generation in organic semiconductor light harvesting systems”, M. Stolterfoht, A. Armin, S. Shoaee, I. Kassal, P.L. Burn & P. Meredith, Nature Communications, 7, 11944 (2016).
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