Charge losses in bulk-heterojunction organic solar cells on nanosecond timescale and the role of charge transfer states (Conference Presentation)

Abstract

Electron-hole recombination determines photocurrent generation yields in polymer:fullerene blends, but the nature of this process and its timescales are not completely understood. In this study, we use a combination of spectroscopy techniques to probe how film structure and interface energetics control the charge generation and recombination dynamics in several polymer:fullerene blends. By varying film composition and comparing between different LUMO-LUMO offset systems, we identify geminate electron-hole recombination on the nanosecond timescale only in the blends consisting of finely-intermixed polymer:fullerene phases and low LUMO-LUMO offsets, whereas the formation of pure fullerene phases leads to the suppression of the geminate recombination process. Charge transfer state photoluminescence and electroluminescence data show similar dependencies confirming that geminate electron-hole recombination is controlled by interfacial enthalpic energy offsets and the density ratio between pure-fullerene and intermixed polymer-fullerene phases. Our results also indicate that electron-hole association probed with electroluminescence and photoluminescence derives from different interfaces playing distinct roles in the photocurrent generation process.