Abstract
The thermodynamic limit for the efficiency of solar cells is predominantly defined by the energy bandgap of the used semiconductor. In case of organic solar cells both energetics and kinetics of three different species play role: excitons, charge transfer states and charge separated states. In this work, we clarify the effect of the relative energetics and kinetics of these species on the recombination and generation dynamics. Making use of detailed balance, we develop an analytical framework describing how the intricate interplay between the different species influence the photocurrent generation, the recombination, and the open-circuit voltage in organic solar cells. Furthermore, we clarify the essential requirements for equilibrium between excitons, CT states and charge carriers to occur. Finally, we find that the photovoltaic parameters are not only determined by the relative energy level between the different states but also by the kinetic rate constants. These findings provide vital insights into the operation of state-of-art non-fullerene organic solar cells with low offsets.
Highlights
Organic solar cells based on donor−acceptor (D-A) bulk heterojunctions (BHJs) have seen a drastic increase in the device performance, with power conversion efficiencies (PCEs) currently exceeding 17%,1,2 with 20% in sight and even 25% predicted.[3]
To demonstrate the physical meaning and how the energetics and kinetics among excitons, charge transfer (CT) states, and free charge carriers collectively determine the device performance of organic solar cells, we conduct analytical simulations based on the developed theoretical framework
These findings clearly demonstrate that the charge generation yield (CGY) critically depends on both the relative energetics and kinetics between the different species
Summary
Organic solar cells based on donor−acceptor (D-A) bulk heterojunctions (BHJs) have seen a drastic increase in the device performance, with power conversion efficiencies (PCEs) currently exceeding 17%,1,2 with 20% in sight and even 25% predicted.[3]. While the encounter between free electrons and holes follows a diffusion-limited Langevin-like process, albeit geometrically constricted,[14] every charge encounter is expected to result in a CT state, which may subsequently dissociate back into free charge carriers or recombine.[15−17] This interrelation has been suggested to induce a mutual equilibrium between CT states and free charge carriers in the so-called reduced Langevin systems, where CT state dissociation is efficient.[18] Such an equilibrium further implies that VOC in organic solar cells is entirely defined by the energetics and the (radiative and nonradiative, NR) recombination kinetics of CT states This has been observed in fullerene acceptor-based BHJs, characterized by a large energy offset between excitons and CT states.[18−21]. We demonstrate how the relative energetics and kinetics critically determine the overall PCE in these solar cells, suggesting that a PCE above 20% may be obtained at energetic offsets of 0.1 eV
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