Morphological Effect on Performance of Organic Photovoltaics---In Terms of Entropy and Helmholtz Energy

Organic photovoltaics (OPVs) are promising alternatives to conventional silicon solar cells, but the current major challenge is their low performance. Morphology---phase separation and crystallinity of organic semiconductors---is a key factor to improve performance, and depends not only on materials but also on manufacturing processes, {\it e.g.}, thermal annealing. At present, however, optimization scheme of morphologies is not established. In our previous study, we examined temperature dependence of morphologies and effects of morphology on performance by device-scale simulations. Bulk heterojunction morphologies were generated by reptation, and current density--voltage characteristics and transient absorption spectroscopy were simulated by Dynamic Monte Carlo (DMC); we elucidated the existence of the optimal annealing temperature for efficiency.\footnote{E. Kawashima, {\it et al.}, {\it Phys. Chem. Chem. Phys.}, \textbf{2016}, 18, 26456--26465} In this presentation, we show Helmholtz energy $F$ and entropy $S$ of charge separation evaluated by graph theory and DMC simulations. We revealed that (i) $S$ drastically decreases $F$, (ii) $F$ attains a maximum at e--h distance of {\it ca.} 6 nm, and (iii) charge separation efficiency is determined by barrier height of $F$.