Abstract
Reduced Langevin recombination has been observed in organic solar cells (OSCs) for many years, but its origin is still unclear. A recent work by Burke et al. (Adv. Energy Mater.2015, 5, 1500123-1) was inspired by this reduced Langevin recombination, and they proposed an equilibrium model of charge-transfer (CT) states that correlates the open-circuit voltage of OSCs with experimentally available device parameters. In this work, we extend Burke et al.’s CT model further and for the first time directly correlate the reduced Langevin recombination with the energetic and dynamic behavior of the CT state. Recombination through CT states leads in a straightforward manner to a decrease in the Langevin reduction factor with increasing temperature, without explicit consideration of the temperature dependence of the mobility. To verify the correlation between the CT states and reduced Langevin recombination, we incorporated this CT model and the reduced Langevin model into drift-diffusion simulations of a bilayer OSC. The simulations not only successfully reproduced realistic current–voltage (J–V) characteristics of the bilayer OSC, but also demonstrate that the two models consistently lead to same value of the apparent Langevin reduction factor.
Highlights
High-performance organic solar cells (OSCs) have demonstrated efficient photogeneration of charge carriers with >90% quantum efficiencies,[1] but their power conversion efficiencies are still lower than those of their inorganic counterparts
The drift-diffusion model provides a way to assess the implications of local, CT-state-based interface recombination for the macroscopic performance of the entire device, in particular for the current−voltage (J−V) characteristics
Considering typical Voc values for the bilayer pentacene/C60 OSC (∼0.37 V),[43] we identified corresponding γ values of around 10−2 or τCT values between 1 and 10 ns from the set of curves displayed in Figure 3
Summary
High-performance organic solar cells (OSCs) have demonstrated efficient photogeneration of charge carriers with >90% quantum efficiencies,[1] but their power conversion efficiencies are still lower than those of their inorganic counterparts. Another research direction is dedicated to a thorough assessment of the relative impact of the second rate.[13] This direction is currently gaining momentum because of increasing experimental evidence that the recombination process involves a charge-transfer (CT) state formed at the interface.[14,15] In strong contrast to the encounter-limited model, the opencircuit voltage was found to be determined by the energy, ECT, of this interface state.[2,16,17] Burke et al recently suggested that reduced Langevin recombination implies the existence of an equilibrium between free carriers and populated CT states.[18]. As this approach allows for the simulation of the full current−voltage characteristics, it can be used to predict the impact of the CT states on device parameters, such as the fill factor and the temperature-dependent open-circuit voltage
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