Abstract
The in‐depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter‐related via the kinetics of charge transfer states—being singlet or triplet states. Although high‐charge‐photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low‐mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta‐analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field‐dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a “spin‐related factor” that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge‐generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin‐related factor.
Highlights
It is intuitive to understand the nongeminate recombination loss as a voltage-dependent process
We provide a model to explain the role of spin in the interplay between charge photogeneration and recombination in various donor:acceptor systems and introduce a spin-related factor that quantifies the relative strength of triplet and singlet charge transfer (CT) state loss channels
We present a large amount of published and unpublished data from our own group and others, of over 20 systems, where the fielddependent charge-generation efficiency, the recombination coefficient, and mobilities have been measured
Summary
It is intuitive to understand the nongeminate recombination loss as a voltage-dependent process. We will unify previous models for suppression of bimolecular recombination by introducing a “spin-related factor” that is dependent on the ratio of the back electron transfer rate of the triplet CT states to the decay rate of the singlet CTs. From the metaanalysis, it will be shown that geometrical confinement plays no role on the suppression of bimolecular recombination. We provide a model to explain the role of spin in the interplay between charge photogeneration and recombination in various donor:acceptor systems and introduce a spin-related factor that quantifies the relative strength of triplet and singlet CT state loss channels. When CTS dissociation is much slower than the two loss rates (a very poor generation efficiency), the total recombination rate is entirely encounter limited and given by In such a case, the total reduction factor relative to the Langevin rate is γ 1⁄4 γen, merely due to the geometrical confinement of the electrons and holes in their respective domains.
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