Observation of a Charge Transfer State in Low‐Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies

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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“…As a result, the measured spectrum is usually composed of the PL from both the pristine materials and CT states. [45][46][47] …”

Section: Absorptionmentioning

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 transfer recombination is more visible in the photoluminescence from this state, which is in direct competition with the generation of the photocurrent. Studies of the steady-state field dependence of photoluminescence quenching and photocurrent generation in some low bandgap polymer/fullerene systems [17][18][19] yield results indicating that the collection of charge carriers and the suppression of photoluminescence do not always have the same field dependence, arguing that there are contributions from both geminate and bimolecular recombination in such systems or that there could be more than one route of photocurrent generation.…”

Section: B Charge Generationmentioning

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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“…After the initial electron transfer an intermolecular, so called, charge transfer (CT) state is composed of an electrostatically bound electron-hole pair with a very different dissociation probability in different molecular systems. The properties of CT states have been investigated in a range of experimental studies 12,14,[29][30][31][32][33][34][35][36][37] addressing different aspects of CT phenomenon. These studies have spawned a variety of models explaining the process of electron and hole detachment in organic heterojunction systems.…”

Section: Introductionmentioning

confidence: 99%

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

et al. 2016

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Charge separation and recombination are key processes determining the performance of organic optoelectronic devices. Here we combine photoluminescence and photovoltaic characterisation of organic solar cell devices with ultrafast multi-pulse photocurrent spectroscopy to investigate charge generation mechanisms in the organic photovoltaic devices based on a blend of an alternating polyquinoxaline copolymer with fullerene. The combined use of these techniques enables the determination of the contributions of geminate and bimolecular processes to the solar cell performance. We observe that charge separation is not a temperature-activated process in the studied materials. At the same time, the generation of free charges shows a clear external-field and morphology dependence. This indicates that the critical step of charge separation involves the non-equilibrium state that is formed at early times after photoexcitation, when the polaronic localisation is not yet complete. This work reveals new aspects of molecular level charge dynamics in the organic light-conversion systems.2 Introduction:

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Observation of a Charge Transfer State in Low‐Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies

Cited by 79 publications

References 22 publications

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Mobility and fill factor correlation in geminate recombination limited solar cells

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

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