ELECTRON-SPIN-RESONANCE IN PPV-PHOTODIODES - DETECTION VIA PHOTOINDUCED CURRENT

Optically (ODMR) and electrically (EDMR) detected magnetic resonance e xperiments were performed on poly-(paraphenylene-vinylene) (PPV) films and photodiodes based on PPV. The photoinduced short circuit current I-SC was found to be dependent on the carriers spin polarization in th e space charge region of the Al/PPV/ITO-photodiode. A large decrease i n I-SC of up to ten percent was observed in the temperature range betw een 1.5 and 293 K, when ESR conditions were fulfilled. The effect is a t least two orders of magnitude stronger than the enhancement of photo luminescence (PL) induced by ESR, where the recombination of thermaliz ed non-geminate excess charge carriers takes place. The ESR signal sho ws a strong dependence on the applied voltage, the behaviour of which is similar to a diode current-voltage characteristic in a range betwee n -10 and +10 V. In this voltage range the dark current is a singly in jected current, so that only the photoinduced current and not the dark current is subjected by ESR. The ESR-induced change of I-SC is discus sed in terms of the recombination of non-thermalized, non-geminate pol aronic pairs. The inequilibrium steady-state magnetization in the spin system is believed to be on account of the difference in the generati on and emission rates of particular spin states in a polaronic pair, f rom which the large effect of ESR on polaronic recombination by means of the intersystem crossing resulted.