Quantifying Bimolecular Recombination in Organic Solar Cells in Steady State

Wetzelaer, Gert Jan A.H.; Kaap, Niels J. van der; Koster, L. Jan Anton; Blom, Paul W. M.

doi:10.1002/aenm.201300251

Cited by 66 publications

(95 citation statements)

References 28 publications

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“…22,23 Most importantly, in some blends, the recombination rate has been found to be several orders of magnitude less than what is predicted by the Langevin model. 8,[24][25][26][27][28][29][30][31][32][33] As a result, many of these studies have determined the Langevin reduction factor (ζ), where…”

mentioning

confidence: 99%

“…Accurately quantifying the recombination rate coefficient and the electron and hole mobilities under the same conditions itself is not trivial, but the largest difficulty is in controlling and limiting the number of independent variables between samples. For example, in some BHJ films the the recombination rate was found to be proportional to the mobility of the slowest charge carrier, 31,37 and as a result the minimum mobility model was proposed, 37 where…”

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confidence: 99%

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Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

2016

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Understanding how the complex intermolecular-and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than ±10% as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.

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Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

2016

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“…Contrary to several previous works [71,72] , the Langevin prefactor  is given by its theoretical expression  = e(µ n +µ p )/ε, which constitutes an upper estimation of the Langevin recombination.…”

Section: Drift Diffusion Simulations and Calibrationmentioning

confidence: 81%

Impact of Blend Morphology on Interface State Recombination in Bulk Heterojunction Organic Solar Cells

Bouthinon

Clerc

Vaillant

et al. 2015

Adv Funct Materials

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“…Langevin recombination has been observed in many organic solids [48,49,50,51], although exceptions apply. Especially, many efficient organic semiconductors blends [52] exhibit "NonLangevin" recombination dynamics, where the recombination rate is substantially lower than predicted by Langevin [49,53,54].…”

Section: Ii5 Charge Transport and Recombinationmentioning

confidence: 99%

“…Since recombination in a BHJ occurs at the donor:acceptor interface, the faster carrier (f) has to wait at the interface for the slower carrier (s) before recombination is possible. As a result, Blom and Koster proposed that the arrival of the slower carrier should be the recombination rate-limiting step, giving r ∝ s , the mobility of the slower carriers [58,199,200]. Following Braun's thesis that dissociation and recombination are opposite processes, this reasoning would suggest that CT-state dissociation yield in BHJs should also depend on the slower-carrier mobility.…”

Section: Introductionmentioning

confidence: 99%

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

Stolterfoht¹

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Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability and mechanical flexibility. In contrast to traditional banded semiconductors, organic semiconductors are intrinsically disordered systems and exhibit therefore much lower charge carrier mobilities. They are also low dielectric constant systems -as such, their photo-excitations are excitonic at room temperature with the electron-hole pair remaining Coulombically-bound. Blending organic semiconductors with differing electron affinities, so-called electron acceptors and donors, creates molecular heterojunctions which deliver the required driving force for exciton separation. These so-called bulk heterojunctions (BHJs) are the dominant architecture for creating organic photovoltaic cells and photodetectors. However, not least because of an incomplete understanding of the underlying physical mechanism that control the conversion of photons to free charges, and the subsequent extraction of these free charges to the electrodes, these organic optoelectronic systems still lag behind their inorganic counterparts.Motivated by these factors, the work described in this thesis advances the fundamental understanding of the loss mechanisms associated with charge photogeneration and charge transport phenomena in BHJ organic solar cells and photodetectors, and presents new experimental methodologies to study these processes. The transport of photogenerated charge carriers in the percolated donor: acceptor pathways towards the extracting electrodes has been studied in-depth via existing and newly developed transient photovoltage and steady-state characterization techniques. A simple but conclusive understanding has been developed which allows for minimization of the detrimental recombination of free charge carriers for given device parameters such as film thickness, applied voltage and the mobility of the slower carrier type (either electrons or holes). The models' predictions have been experimentally validated for many different (> 25) BHJ solar cell systems. Inspired by the need to selectively optimize the processes which control the photocurrent output of the cell, such a charge photogeneration and extraction, a technique to quantify and disentangle both efficiencies has been developed as a next step. Corroborated by transient absorption spectroscopy, the dynamics of the photocarrier generation process was examined. The results suggest that the so-called charge-transfer state (CTS) separation limits the conversion of photons to free charges and the photocurrent output from short-circuit to the maximum power point, strongly depending on the slower carrier ii mobility. These findings were explained by the ability of the slower carriers to leave the donor: acceptor interface via 1) a high enough mobility, 2) a sufficiently large domain size, and 3) enough conduction pathways (entropy). This work also shows that the dynamics of charge extraction and CTS dissociation are simi...

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Quantifying Bimolecular Recombination in Organic Solar Cells in Steady State

Cited by 66 publications

References 28 publications

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

Impact of Blend Morphology on Interface State Recombination in Bulk Heterojunction Organic Solar Cells

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

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