Absorption phenomena in organic thin films for solar cell applications investigated by photothermal deflection spectroscopy

Goris, Ludwig; Haenen, Ken; Nesládek, Miloš; Wagner, Patrick; Vanderzande, Dirk; Schepper, L. De; D’Haen, Jan; Lutsen, Laurence; Manca, Jean

doi:10.1007/s10853-005-0576-0

Cited by 155 publications

(146 citation statements)

References 24 publications

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“…Previous studies of PPV and polyfluorene-based donor/acceptor blends 29,35,36,86,87 have reported significant red-shifted exciplex-like emission and CT absorption, suggesting that the BRP states observed in these systems are radiatively coupled to the ground state. A sub-band gap absorption has also been reported for P3HT/PCBM blend films by Fourier-transform photocurrent spectroscopy.…”

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

confidence: 91%

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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confidence: 91%

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

et al. 2008

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“…As a result, highly sensitive approaches such as photothermal deflection spectroscopy (PDS) have to be employed. 35 PDS measures the absorption by measuring the change in refractive index due to heating of the surrounding medium by light absorption in the sample. 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.…”

Section: Absorptionmentioning

confidence: 99%

“…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%

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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“…The red shift in the C6 absorption peak after blending with PCBM molecules (at different concentrations) may be attributed to the formation of a charge transfer complex (CTC) in the C6:PCBM blend films as a result of a significant interaction between conjugated C6 dye and PCBM molecules in the ground state. In general, the red shift in the absorption spectrum, as an indication of the formation of a CTC in the blends, has been frequently stated in the literature [18][19][20]. Moreover, Figure 3 shows the contribution of PCBM molecules in the absorption of the C6:PCBM blend films.…”

Section: Optical Properties Of Blend Filmsmentioning

confidence: 54%

Investigation of PCBM Concentration on the Performance of Small Organic Solar Cell

Ismail

Soga

Jimbo

2012

ISRN Renewable Energy

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We have fabricated bulk heterojunction organic solar cells using coumarin 6 (C6) as a small organic dye, for light harvesting and electron donating, with fullerene derivative [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM), acting as an electron acceptor, by spin-coating technique of the blend solutions. We have studied effect of PCBM concentration on photocurrent and performance parameters of the solar cells. We found that the optical absorption of the dye increased with increasing its concentration in the active layer blends. The higher concentrations of PCBM in active layer enhanced the photocurrent of the solar cells, as a result of improving charge carrier separation and electron transport in solar cell active layer. The improved charge carrier separation between C6, as a donor, and PCBM, as an acceptor, was indicated through the formation of bulk heterojunction by blending C6 with PCBM. The formation of C6:PCBM bulk heterojunction blend was confirmed through the symbatic behavior of the corresponding solar cell and, also, through the homogeneity and smoothing in the atomic force microscopy images of the C6:PCBM blend films. For the same reasons, the performance parameters of the C6:PCBM solar cell improved by modification of the PCBM concentration in the solar cell active layer.

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Absorption phenomena in organic thin films for solar cell applications investigated by photothermal deflection spectroscopy

Cited by 155 publications

References 24 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

Investigation of PCBM Concentration on the Performance of Small Organic Solar Cell

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