Mechanisms of spin-dependent dark conductivity in films of a soluble fullerene derivative under bipolar injection

Abstract

We report room-temperature pulsed electrically detected magnetic resonance measurements of the dark conductivity of films of the fullerene derivative [6,6]-phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester (PCBM) under bipolar (electron-hole) and unipolar (electron-rich) injection conditions. Directly after material deposition, no detectable spin-dependent processes are observed, yet after storage under ambient conditions for more than a day, two distinct spin-dependent mechanisms are found under bipolar injection, suggesting the involvement of degradation-induced electronic states. Spin-Rabi beat oscillation measurements show that at least one of these processes is due to weakly spin-coupled pairs with $s=1/2$. The absence of these signals when hole injection is impeded by a barrier suggests that they are due to spin-dependent recombination. The presence of recombination confirms that fullerenes are both electron and hole acceptors, with important consequences for the design, operation, and understanding of plastic solar cells. Electron-hole recombination can occur within homogeneous domains of either the donor or the acceptor of the bulk heterojunction structure, constituting an important dissipative channel in addition to the established interfacial bimolecular recombination loss.