A data-driven identification of morphological features influencing the fill factor and efficiency of organic photovoltaic devices

Abstract

The performance of organic solar cells is strongly dependent on the morphology of the bulk heterojunction active layer. There has been intense efforts to identify and quantify morphological traits that correlate with various stages of the photo physics. While it is generally accepted that donor domain size affects exciton dissociation efficiency and connectivity affects charge collection, identifying morphology trait(s) that correlate with fill factor and total efficiency have remained elusive. In this work, we utilize correlation analysis on a large set of two dimensional bulk heterojunction morphologies to identify traits that are correlated with fill factor and efficiency. A large dataset of bulk heterojunction morphologies using a phase-field model of phase separation was first created. A comprehensive suite of morphology descriptors were evaluated for each of these morphologies using a recently developed graph based approach. Following this, a morphology aware excitonic-drift-diffusion based device model was used to compute current-voltage curves, fill factors, efficiencies as well as spatial distributions of exciton generation, dissociation, and charge collection for each of the morphologies. We find that (for a given material system with a specified HOMO-LUMO gap, and assuming perfect contact with electrodes) device efficiency primarily depends on the short circuit current, and has almost no dependence on the fill factor. Interestingly, we find that the fill factor is largely insensitive to many of the investigated descriptors. It is only weakly dependent on the contact area mismatch – the difference between the fraction of anode in direct contact with donor and the fraction of cathode in direct contact with acceptor. The fill factor is maximized when this quantity is nearly balanced. Since morphologies with a higher fraction of the electrodes in contact with the desirable material show higher short circuit current, we conclude that designing morphologies for a high short circuit current will necessarily lead to reasonably high fill factors.