Charge Recombination in P3HT/PC₇₀BM Composite Studied by Light-Induced EPR

Abstract

A composite of conductive polythiophene P3HT and soluble fullerene derivative PC 70 BM (the material widely used as the active layer in organic photovoltaics devices) was studied by light-induced EPR (LEPR). In contrast to P3HT/PC 60 BM composite, LEPR signal in P3HT/PC 70 BM can be detected in a wide temperature range up to room temperature. This signal was attributed to charge carriers P3HT + and PC 70 BM − . The dependence of the intensity of LEPR signal on light intensity and the decay of LEPR signal upon switching light off are interpreted in frame of trap-limited bimolecular recombination model with finite rate of back electron transfer in [P3HT + PC 70 BM − ] encounter complex at polymer−fullerene interface. The apparent recombination order was found to be close to p = 3.5 for the temperature range from 100 K to room temperature. For temperatures above 150 K Arrhenius behavior of effective recombination rate constant was obtained with the activation energy E a = 0.16 ± 0.01 eV, which is larger than analogous values for P3HT/PC 60 BM reported previously. This difference is attributed to the change of polymer/fullerene interface induced by the replacement of PC 60 BM by PC 70 BM.

Light-induced electron spin resonance as a probe for charge separation and recombination in hybrid donor:acceptor systems

“…In line with that, several L-ESR studies of polymer:fullerene systems state that (almost) no signals were detected in dark [29,43]. In other cases, significant dark signals were observed [31,32,44]. Usually, a dark signal is attributed to the presence of defect states capturing charge carriers.…”

“…The L-ESR method has been applied to prove charge separation in various types of organic donor:acceptor systems. For example, P3HT:PCBM blends [31,32,46] and poly[2-methoxy-5-(3 0 ,7 0 -dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV):PCBM blends [29,43,47] have been studied. Al-Ibrahim et al [44] investigated charge transfer in blends of PCBM and poly(3-alkylthiophenes) with different alkyl chain length.…”

“…Modelling of L-ESR decay curves was also addressed in a recent study of the P3HT:PC 70 BM system by Lukina et al [32]. The authors used a power law to describe the decay of the carrier density, n, after switching off the excitation source (which was a halogen lamp in this case):…”

“…where p is the recombination order, and k is a constant considered as effective recombination rate [32]. Integration of the equation yields:…”

“…Alternative methods are for example, photo-induced absorption (PIA) spectroscopy [11,[25][26][27][28] or light-induced electron spin resonance (L-ESR) [11,29,30]. Both, PIA and L-ESR are not only useful to study charge transfer, but enable also obtaining valuable information about the recombination of charge carriers [26,[30][31][32].…”