Geminate Charge Recombination in Alternating Polyfluorene Copolymer/Fullerene Blends

De, Swati; Pascher, Torbjörn; Maiti, Manisankar; Jespersen, Kim G.; Kesti, Tero; Zhang, Fengling; Inganäs, Olle; Yartsev, and Arkady P.; Sundström, Villy

doi:10.1021/ja068909q

Cited by 147 publications

(274 citation statements)

References 45 publications

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“…Time scales of hundreds of picoseconds to nanoseconds have been reported for this process in various polymers. [9][10][11]13,17,41 We are not able to distinguish charges that are bound in the CT state from free ones using the broad polaron absorption band (similar spectral signatures have also been noted in TA studies of some other polymers). 9,10 On the basis of a model by Laquai et al, 9−11 both are expected even at the earliest time delays, since they are instantaneously generated and do not interconvert.…”

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confidence: 55%

“…A similar flat band extending next to the GSB with a peak at lower wavelengths has been observed in blends of other donor− acceptor copolymers with PCBM, and was also assigned to the charged polymer. 9,17 The anion of PCBM is known to absorb in the 1030 nm region, and is thus outside our measuring window. 13 It has often been reported that CS occurs quantitatively in less that 100 fs in polymer:fullerene bulk heterojunctions.…”

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confidence: 99%

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Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

Paraecattil

Beaupré

Leclerc

et al. 2012

J. Phys. Chem. Lett.

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PBDTTPD is a conjugated polymer with high power conversion efficiency if used in organic solar cells together with fullerene derivatives. We have investigated for the first time the excited state dynamics of pristine PBDTTPD thin film as well as the ultrafast evolution of charge carriers in PBDTTPD:PCBM bulk heterojunction blend using femtosecond transient absorption spectroscopy. In the latter, charges appear within the time resolution of the experiment (<100 fs), but clean spectral signatures allowed to directly follow slower ∼1 ps charge separation. Only the slower quenching component competes with exciton−exciton and exciton−charge annihilation, leading to a reduced yield of charge carriers at high laser fluence. Our excellent measuring sensitivity made it possible to reduce pump power to a point where annihilation is quasi suppressed. In this case >80% of charges survive after 1 ns; the rest recombines (most probably geminately) on the 200 ps time scale. The photophysics of PBDTTPD has only been little explored. 7,8 We report here, to our knowledge, the first transient absorption (TA) study of the ultrafast processes occurring in thin films of pristine PBDTTPD and PBDTTPD:PCBM blend. TA spectroscopy has been used on numerous occasions to understand the charge carrier dynamics in polymer:fullerene blends. 9−17 It is, however, becoming clear that the important energy delivered by pulsed laser excitation in such experiments is not necessarily representative of solar irradiation: The high density of excited species and charges generated at high pump fluence leads to bimolecular loss mechanisms that are not observed under solar cell operating conditions. 10−12,17−20 Excellent measuring sensitivity of our apparatus has allowed us to reduce the excitation power to a point where those loss mechanisms become negligible on the subnanosecond time scale. Moreover, we have taken advantage of the annihilation processes that occur with increasing fluence to gain valuable insight about the evolution of excited and charged species in pristine and blended PBDTTPD. Figure 1A depicts the steady-state absorption and fluorescence spectra of the investigated thin films. Pristine PBDTTPD has a broad and structured absorption band ranging from ∼360−700 nm. The presence of defined vibronic features, uncommon for conjugated donor−acceptor copolymers, has been attributed to a planar backbone conformation of the polymer in the ground state (density functional thoery calculations), 21 and to insignificant torsional relaxation in the excited state (two-dimensional electronic spectroscopy). 8 The quantum simulations also suggest delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and that several electronic bands might be present under the broad envelope of the absorption spectrum. 21 The emission spectrum of pristine PBDTTPD film is weakly Stokes shifted with a maximum at 662 nm and a vibronic shoulder around 720 nm. For the 1:2 PBDTTPD:PCBM blend cast from o-dichlorobenze...

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confidence: 55%

mentioning

confidence: 99%

Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

Paraecattil

Beaupré

Leclerc

et al. 2012

J. Phys. Chem. Lett.

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“…51 However, they were not able to determine whether the product of the charge transfer was bound CT states or free charge carriers. De et al 52 and Hwang et al 53 were among the first to be able to determine ultrafast free charge carrier generation in polymer:fullerene blends using TA measurements in the subnanosecond time regime. Both of them observed the generation of free carriers within ~ 200 ps.…”

Section: Ultrafast Generation Of Free Charge Carriersmentioning

confidence: 99%

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Gao

Inganäs

2014

Phys. Chem. Chem. Phys.

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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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“…11 In most organic donor/acceptor systems the geminate processes should be present to some degree and will have to be considered, relative to other types of recombination.…”

Section: Introduction a Backgroundmentioning

confidence: 99%

Mobility and fill factor correlation in geminate recombination limited solar cells

Andersson¹,

Müller²,

Badada³

et al. 2011

Journal of Applied Physics

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Empirical data for the fill factor as a function of charge carrier mobility for two different polymer:fullerene systems is presented and analyzed. The results indicate that charge extraction depth limitations and space charge effects are inconsistent with the observed behavior, and the decrease in the fill factor is, instead, attributed to the field-dependent charge separation and geminate recombination. A solar cell photocurrent limited by the Onsager-Braun charge transfer exciton dissociation is shown to be able to accommodate the experimental observations. Charge dissociation limited solar cells always benefit from increased mobilities, and the negative contribution from the reduced charge separation is shown to be much more important for the fill factor in these material systems than any adverse effects from charge carrier extraction depth limitations or space charge effects due to unbalanced mobilities. The logarithmic dependence of the fill factor on the mobility for such a process is also shown to imply that simply increasing the mobilities is an impractical way to reach very high fill factors under these conditions since unrealistically high mobilities are required. A more controlled morphology is, instead, argued to be necessary for high performance.

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Geminate Charge Recombination in Alternating Polyfluorene Copolymer/Fullerene Blends

Cited by 147 publications

References 45 publications

Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Mobility and fill factor correlation in geminate recombination limited solar cells

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