Spin-dependent charge carrier recombination in PCBM

We present room temperature pulsed electrically detected magnetic resonance (pEDMR) measurements on [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) (electron acceptor) thin film unipolar and bipolar devices. Our study aimed at identifying the dominating spin-dependent transport and recombination processes therein. Experimentally, the devices were operated under a constant positive bias, and the resultant transient current response was then monitored after the application of a short resonant microwave pulse excitation. The measurements did not reveal any observable signal for unipolar electron devices which suggests that spin-dependent transport mechanisms are not dominant in PCBM. However, under bipolar injection, at least two pronounced spin-dependent signals were detected whose magnitudes increased as the devices degraded upon exposure to air. Electrical detection of spin-Rabi beat oscillation revealed that one of these two signals is due to weakly coupled pairs of spins with s$=$1/2. We therefore attribute this signal to electron-hole recombination. This observation shows that while PCBM is a poor hole conductor, hole injection can be significant.