Observation of the subgap optical absorption in polymer-fullerene blend solar cells

Goris, Ludwig; Poruba, A.; Hodakova, L.; Vaněček, M.; Haenen, Ken; Nesládek, Miloš; Wagner, Patrick; Vanderzande, Dirk; Schepper, L. De; Manca, Jean

doi:10.1063/1.2171492

Cited by 165 publications

(159 citation statements)

References 11 publications

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“…A sub-band gap absorption has also been reported for P3HT/PCBM blend films by Fourier-transform photocurrent spectroscopy. 88 This may be assigned to ground state CT absorption indicative of the radiative coupling. However, no distinct red-shifted emission was observed up to 850 nm for the polythiophene/PCBM blends studied herein, indicating that the BRP states proposed here are not strongly radiatively coupled to the ground state.…”

Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 99%

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

et al. 2008

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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

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Section: -----<<< Scheme 1 >>>-----mentioning

confidence: 99%

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

et al. 2008

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“…By comparing PDS signal of the polymer:fullerene blend with those of the individual components, weak absorption from the CT state was observed (Figure 2a), indicating interaction between polymer and fullerene in the ground state. [35][36][37] In addition to PDS, we can also detect the CT state absorption by measuring the external quantum efficiency (EQE) of the solar cell in the CT state absorption regime, where neither polymer nor fullerene absorbs. The EQE in the CT state absorption regime results from the photocurrent generated by absorption of the CT state.…”

Section: Absorptionmentioning

confidence: 99%

“…38 Both techniques demonstrated photocurrent signal below the bandgap of pristine materials, indicating absorption from the CT state. [37][38][39][40][41] Photoluminescence Photon absorption by the polymer (fullerene) generates excitons, which transfer electrons to the fullerene and leave holes in the polymer (or the other way around). The electrons in the fullerene might radiatively recombine with the holes in the polymer at the donor/acceptor interfaces, generating additional photoluminescence (PL) which is absent in the PL spectra of the individual components.…”

Section: Absorptionmentioning

confidence: 99%

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Gao

Inganäs

2014

Phys. Chem. Chem. Phys.

204

252

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Charge generation in organic solar cells is a fundamental yet heavily debated issue. This article gives a balanced review of different mechanisms proposed to explain efficient charge generation in polymer-fullerene bulk-heterojunction solar cells. We discuss the effect of charge-transfer states, excess energy, external electric field, temperature, disorder of the materials, and delocalisation of the charge carriers on charge generation. Although a general consensus has not been reached yet, recent findings, based on both steady-state and transient measurements, have significantly advanced our understanding of this process.2

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“…23 The technique is sufficiently sensitive to measure much lower absorption than standard optical absorption techniques and reveals the CT absorption at energies below the bulk exciton absorption. The measurement technique is described elsewhere and is performed either in a modified FTIR spectrometer 24 or by a standard photoconductivity measurement system with lock-in amplifier. 23,25 All the measurements were made at room temperature, and the maximum monochromatic illumination intensity is <1 mW/cm 2 , so that no significant sample heating occurs.…”

Section: Background and Measurement Methodologymentioning

confidence: 99%

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

et al. 2014

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Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of 50 meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer exciton and the charge transfer ground state below about 0.25 eV adversely affects the cell performance, while a HOMO band offset below about 0.2−0.3 eV also degrades cell performance but by a different mechanism.

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Observation of the subgap optical absorption in polymer-fullerene blend solar cells

Cited by 165 publications

References 11 publications

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

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

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

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