Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy
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- 作者：Hideo Ohkita ; Steffan Cook ; Yeni Astuti ; Warren Duffy ; Steve Tierney ; Weimin Zhang ; Martin Heeney ; Iain McCulloch ; Jenny Nelson ; Donal D. C. Bradley ; James R. Durrant
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：March 12, 2008
- 年：2008
- 卷：130
- 期：10
- 页码：3030 - 3042
- 全文大小：277K
- 年卷期：v.130,no.10(March 12, 2008)
- ISSN：1520-5126
文摘
We report herein a comparison of the photophysics of a series of polythiophenes with ionizationpotentials 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]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to anonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), givemore crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed bytransient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristinepolymers, no triplet formation is observed, but rather a short-lived (~100 ns), broad photoinduced absorptionfeature 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, power-law decay phase assigned to bimolecular recombinationof these polarons, was observed to vary by over 2 orders of magnitude depending upon the polymeremployed. 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 radicalpairs and, for the more crystalline polymers, to the direct observation of the geminate recombination ofthese bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic modelfor charge generation in polythiophene/PCBM blend films analogous to that reported to explain theobservation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we findan excellent correlation between the free energy difference for charge separation (
GCSrel) and yield of thelong-lived charge generation, with efficient charge generation requiring a much larger GCSrel than thatrequired to achieve efficient PL quenching. We suggest that this observation is consistent with a modelwhere the excess thermal energy of the initially formed polaron pairs is necessary to overcome theirCoulombic binding energy. This observation has important implications for synthetic strategies to optimizeorganic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blendfilms, a large GCSrel (or LUMO level offset) is required to achieve efficient charge dissociation.
GCSrel) and yield of thelong-lived charge generation, with efficient charge generation requiring a much larger GCSrel than thatrequired to achieve efficient PL quenching. We suggest that this observation is consistent with a modelwhere the excess thermal energy of the initially formed polaron pairs is necessary to overcome theirCoulombic binding energy. This observation has important implications for synthetic strategies to optimizeorganic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blendfilms, a large GCSrel (or LUMO level offset) is required to achieve efficient charge dissociation.