Effects of Delocalized Charge Carriers in Organic Solar Cells: Predicting Nanoscale Device Performance from Morphology

Adam G. Gagorik, Jacob W. Mohin, Tomasz Kowalewski and Geoffrey R. Hutchison “Asymmetric Surface Potential Energy Distributions in Organic Electronic Materials via Kelvin Probe Force Microscopy” Advanced Functional Materials, 2015, 25, pp. 1996–2003. Online.

toc_scaledMonte Carlo simulations of charge transport in organic solar cells are performed for ideal and isotropic bulk heterojunction morphologies while altering the delocalization length of charge carriers. Previous device simulations have either treated carriers as point charges or with a highly delocalized mean-field treatment. This new model of charge delocalization leads to weakening of Coulomb interactions and more realistic predicted current and fill factors at moderate delocalization, relative to point charges. It is found that charge delocalization leads to significantly increased likelihood of escaping interface traps. In isotopic two-phase morphologies, increasing the domain sizes leads to slight decreases in predicted device efficiencies. It was previously shown that tortuous pathways in systems with small domain sizes can decrease device performance in thin film systems. However, the diminishing effects of Coulomb interactions with delocalization and efficient separations of excitons by small domains make morphological effects less pronounced. The importance of delocalization, which has largely been ignored in past simulations, as a parameter to consider and optimize when choosing materials for organic solar cells is emphasized.