Quadrimolecular recombination kinetics of photogenerated charge carriers in the composites of regioregular polythiophene derivatives and soluble fullerene

Abstract

Light-induced electron spin resonance (LESR) measurements have been performed on the composites of regioregular polythiophene derivatives and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in order to study the recombination kinetics of photogenerated charge carriers. We adopt two regioregular polymers with different side chains; head-to-tail poly(3-hexylthiophene) (RR-P3HT) and head-to-head poly(3-dodecynylthiophene-2,5-diyl) [HH-P3(C≡CDec)Th]. In both systems, two LESR signals due to positive polarons on the polymer (g∼2.002) and fullerene radical anions (g∼2.000) have been observed. Quadrimolecular recombination (QR) kinetics, previously reported for RR-P3HT/C60 composites, where two positive polarons and two radical anions recombine simultaneously, has been confirmed in both systems by the observation of Iex0.25 dependence of the LESR intensity on the excitation light intensity (Iex) and the decay curve of the LESR intensity. This process implies the formation of doubly-charged states such as bipolarons or polaron pairs on the polymer to attract two radical anions. Temperature dependence of the QR rate constant, γ, in both systems has exhibited a crossover of the transport mechanism from low temperature tunneling to high temperature hopping process, as in the case of RR-P3HT/C60 composites. In the RR-P3HT/PCBM composites, γ has exhibited marked dependencies on the PCBM concentration or annealing, which may be related to the change of the crystallinity of the phase-separated polymer and fullerene domains as well as their interface structures, affecting the carrier mobilities or the trap states at the interface. Associated change of the molecular orientation of RR-P3HT crystalline domains with the lamellar structure has been further confirmed from the anisotropic LESR signals of the cast films on the substrates, exhibiting a qualitative agreement with the reported x-ray or optical analyses. In the HH-P3(C≡CDec)Th/PCBM composite, γ has been smaller than those in the RR-P3HT/PCBM composites, reflecting the difference of local structures due to the different molecular structure. Furthermore, the hyperfine-determined LESR linewidth of the positive polaron has exhibited a smaller value than those in the RR-P3HT composite, implying the larger extension of the polaron wave function on the polymer chain, which is consistent with the highly coplanar structure of this polymer.

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

“…These values are significantly smaller than the activation energy for charge recombination in P3HT/PC 70 BM composite E a = 0.16 eV obtained in the present work. Although a slightly different recombination order was assumed for P3HT/PC 60 BM (quadrimolecular recombination, p = 4), 35 it is quite close to p = 3.5 used in the present work. This small difference does not influence the derived activation energy substantially.…”

“…In the frame of this approach charge generation and recombination can be formally described as bulk processes by the following rate equation, similar to the equation proposed earlier: 34,35…”

“…This method allows to determine the apparent recombination order by observing the LEPR signal intensity dependence on excitation light intensity and by measuring LEPR signal decay after light is turned off. 34,35 We found that LEPR signal in P3HT/PC 70 BM can be observed in a wide temperature range up to room temperature, in sharp contrast to LEPR signal in P3HT/PC 60 BM, which disappears upon temperature increase approximately at 200 K. Thus, the goal of this work was to describe the features of recombination in P3HT/PC 70 BM and to trace the differences of charge recombination rate in P3HT/PC 70 BM and in P3HT/PC 60 BM.…”

“…Most of the recombination studies of OPV materials were done on composites containing fullerene C 60 or its derivatives, usually [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM) (Figure ), as electron acceptor and conductive polymers as electron donor . Free charge recombination in such composites was studied by various methods: transient absorption spectroscopy, − electron paramagnetic resonance (EPR), , photogenerated charge carrier extraction by linearly increasing voltage, double injection current transient technique and time-of-flight measurements. , It is known that recombination in this system can not be described by Langevin bimolecular recombination model . Measurements of the recombination rate in polymer/fullerene composites result in much lower value than predicted by Langevin theory .…”

“…Many different mechanisms were proposed to explain this recombination process. Among them are quadrimolecular recombination, , trap-limited bimolecular recombination, − ,, trimolecular recombination, two-dimensional Langevin recombination, and slow back electron transfer caused by finite reaction radius . While an exact recombination mechanism is still under discussion, a trap-limited bimolecular recombination model is most commonly assumed at present.…”