Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

Abstract

Abstract:We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt% 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C 61 (PCBM). Three polymers are observed to give amorphous films, attributed to a non-planar geometry of their backbone whilst the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (~ 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt% PCBM resulted in 70 -90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer + and PCBM − polarons, assayed by the appearance of a long-lived, powerlaw decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over two orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find a excellent correlation between the free energy difference for charge separation (ΔG CS rel ) and yield of the long-lived charge generation yield, with efficient charge generation requiring a much larger ΔG CS rel than that required to achieve efficient PL 3 quenching. We suggest this observation is consistent with a model where the excess thermal energy of the initially formed polarons pairs is necessary to overcome their coulomb binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large ΔG CS rel (or LUMO level offset) is required to achieve efficient charge dissociation.4

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…Based on this observation, they proposed that the excess energy after the exciton dissociation provide extra kinetic energy for the CT state dissociation. 67 This hot CT state theory was further supported by their subsequent work on other polymer:fullerene blends as well as polymer:perylene diimide blends ( Figure 6). [69][70][71][72][73][74] In addition to the above work, there are some other reports supporting the hot CT state theory.…”

“…67 The authors found that the yield of the dissociated polarons varied by two orders depending on polythiophenes used, in spite of efficient PL quenching for all the blends. They observed a strong dependence of the yield of dissociated polarons on the energy difference between the singlet exciton and the dissociated polarons.…”

“…67 In this hot CT state theory, the primary kinetic competition is between the thermalisation and the dissociation. As an internal conversion process, the thermalisation process is ultrafast, usually on the order of several hundred femtoseconds.…”

Parameters Influencing the Efficiency of Electron Injection in Dye-Sensitized Solar Cells

“…64 This requirement comes from the relatively low dielectric constant of such organic materials, resulting in relatively strong coulomb attraction of electrons and holes and causing geminate recombination losses to become a significant loss pathway in such devices. This situation can be contrasted with the electron injection dynamics we report here for DSSCs.…”

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

“…A possibly more favorable free energy of charge generation through improved band alignment for ordered P3HT chains remains to be investigated. 56 Recent results state a rise only of the HOMO energy level and no change of the LUMO level through chain ordering. 57 For a possible photovoltaic application, we compared RR-P3HT:Si-ncs BHJs with aggRR-P3HT/poly-Si PHJs.…”