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-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.

Trap-Free Space-Charge-Limited Hole Transport in a Fullerene Derivative

“…Bipolar injection, resulting in subsequent recombination has been observed previously in devices of the fullerene detivative [60]PCBM with electrically detected magnetic resonance [41]. It has been observed by Gadisa et al that bipolar injection into a similar [60]PCBM device leads to electroluminescence [42].…”

Time-independent, high electron mobility in thin PC 61 BM films: Relevance to organic photovoltaics

“…The hole mobility in PC 61 BM is much lower than the electron mobility [30][31][32], therefore holes may be considered as being stationary during the extraction time of electrons. This assumption is supported by the fact that the fast decay corresponds to 50% of the total field reduction measured by time-integrated photocurrent, which includes two equal contributions of electrons and holes.…”

Discrimination between spin-dependent charge transport and spin-dependent recombination in π-conjugated polymers by correlated current and electroluminescence-detected magnetic resonance

“…For room-temperature conditions, this observation appears to be universal for poly(phenylenevinylene) (PPV) based materials [13], fullerenes [16,17], small molecule devices based on phthalocyanine [18], or blend materials [19,20]. This universal observation of spin beating conflicts with earlier hypotheses that processes involving higher spin-manifolds such as triplet excitons (s=1) or trions (s=3/2) could be responsible for the observed room-temperature spineffects [21][22][23][24], since those spin manifolds would be expected to display Rabi-nutation frequencies that have not been observed in experiment.…”