Field-dependent exciton dissociation in organic heterojunction solar cells

Abstract

In organic heterojunction solar cells, the generation of free charge carriers takes place in a multistep process which involves charge transfer (CT) states, that is, bound electron-hole pairs at the interface between donor and acceptor molecules. Past efforts to model the CT-state dissociation during solar cell operation were not able to consistently reproduce the experimentally observed field and temperature dependence. This discrepancy between model and experiment was partly due to the field-dependent free charge carrier collection process, which plays an important role in the widely used bulk heterojunction cell configuration and superimposes a possible field-dependent charge carrier generation process. In order to distinguish between generation and collection of free charge carriers, we propose the planar heterojunction cell configuration as a model system to study the field-dependent charge carrier generation process in organic heterojunction solar cells. We apply this model system to check current CT-state dissociation models against experimental data. Although the models can quantitatively account for the photocurrent's dependence on the applied voltage and the device thickness, they fail to account for the virtually negligible temperature dependence of the field-dependent charge-generation process. This discrepancy is traced back to a common feature of the models: an Arrhenius-like temperature dependence, distinctive of all processes involving a thermally activated jump over an energy barrier. As a solution to the problem, we introduce an exciton dissociation model based on a field-dependent tunnel process and demonstrate its consistency with the experimental observations. Our results indicate that the current microscopic picture of the charge-generation process in organic heterojunction solar cells being limited by the CT-state dissociation process needs to be reconsidered.

“Hot or cold”: how do charge transfer states at the donor–acceptor interface of an organic solar cell dissociate?

“…10,53 The present model provides a straightforward explanation. Early on, the missing temperature dependence of photogeneration in OSCs as well as in single compound conjugated polymers has been taken as a signature of the failure of the point charge model for explaining eh-dissociation.…”

“…53 Arkhipov et al conjectured that delocalization of an initially quite mobile charge on a preferentially polymeric donor is helpful. a coulomb bound CT state, with the fact that its subsequent dissociation requires little if any thermal energy, as evidenced by temperature-dependent photocurrent measurements.…”

Decreased Recombination Through the Use of a Non‐Fullerene Acceptor in a 6.4% Efficient Organic Planar Heterojunction Solar Cell

“…[ 33,34 ] As our current data set cannot reveal the most applicable of the various models, we here select a rather basic model where V ( ) CT η is described by observed at reverse bias ( Figure 5 ). This is indicative of voltage dependent exciton dissociation (as opposed to recombination), which likely lies at the root of the non-zero J -V slope of the photocurrent at reverse bias.…”

Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent

“…A considerable amount of such work has been done in polymer-based films, including Monte Carlo simulations of hopping conduction of photoexcited carriers, [36][37][38] numerical modeling of the time-resolved photocurrent dynamics on various time scales, [39][40][41] and modeling of the photocurrent in solar cells. 36,[42][43][44][45][46] In this paper, we present numerical modeling of nanosecond (ns) time-scale time-resolved photocurrent dynamics in ADT films and ADT-based D/A composites. For our studies, we chose a fluorinated ADT derivative functionalized with triethylsilylethynyl (TES) side groups, ADT-TES-F, as the donor and either PCBM or a fluorinated Pn derivative (Pn-TIPS-F8) functionalized with triisopropylsilylethynyl (TIPS) side group as acceptors (Fig.…”