Electron spin resonance in PPV-photodiodes: detection via photoinduced current

Abstract

Optically (ODMR) and electrically (EDMR) detected magnetic resonance experiments were performed on poly-(paraphenylene-vinylene) (PPV) films and photodiodes based on PPV. The photoinduced short circuit current ISC was found to be dependent on the carriers spin polarization in the space charge region of the A1/PPV/ITO-photodiode. A large decrease in ISC of up to ten percent was observed in the temperature range between 1.5 and 293 K, when ESR conditions were fulfilled. The effect is at least two orders of magnitude stronger than the enhancement of photoluminescence (PL) induced by ESR, where the recombination of thermalized non-geminate excess charge carriers takes place. The ESR signal shows a strong dependence on the applied voltage, the behaviour of which is similar to a diode current-voltage characteristic in a range between −10 and +10 V. In this voltage range the dark current is a singly injected current, so that only the photoinduced current and not the dark current is subjected by ESR. The ESR-induced change of ISC is discussed in terms of the recombination of non-thermalized, non-geminate polaronic pairs. The inequilibrium steady-state magnetization in the spin system is believed to be on account of the difference in the generation and emission rates of particular spin states in a polaronic pair, from which the large effect of ESR on polaronic recombination by means of the intersystem crossing resulted.