What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends

Abstract

In solar energy harvesting devices based on molecular semiconductors, such as organic photovoltaics (OPVs) and artificial photosynthetic systems, Frenkel excitons must be dissociated via charge transfer at heterojunctions to yield free charges. What controls the rate and efficiency of charge transfer and charge separation is an important question, as it determines the overall power conversion efficiency (PCE) of these systems. In bulk heterojunctions between polymer donor and fullerene acceptors, which provide a model system to understand the fundamental dynamics of electron transfer in molecular systems, it has been established that the first step of photoinduced electron transfer can be fast, of order 100 fs. But here we report the first study which correlates differences in the electron transfer rate with electronic structure and morphology, achieved with sub-20 fs time resolution pump-probe spectroscopy. We vary both the fullerene substitution and donor/fullerene ratio which allow us to control both aggregate size and the energetic driving force for charge transfer. We observe a range of electron transfer times from polymer to fullerene, from 240 fs to as short as 37 fs. Using ultrafast electro-optical pump-push-photocurrent spectroscopy, we find the yield of free versus bound charges to be weakly dependent on the energetic driving force, but to be very strongly dependent on fullerene aggregate size and packing. Our results point toward the importance of state accessibility and charge delocalization and suggest that energetic offsets between donor and acceptor levels are not an important criterion for efficient charge generation. This provides design rules for next-generation materials to minimize losses related to driving energy and boost PCE.

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

“…As the field increases, the balance shifts toward charge collection (Figure 7b). [69,70] In fact, there is a growing body of work emphasizing the importance of aggregation and charge delocalization, [69][70][71][72][73][74][75][76][77][78][79] and the necessity for high purity in the separate donor/acceptor domains. The possibility for exciton traps was illustrated also by TD-DFT calculations.…”

Temperature Dependent Epitaxial Growth of C60 Overlayers on Single Crystal Pentacene

“…[10][11][12][13][14] Accordingly, accurate control of the crystal quality of molecular heterojunctions is desired for advancing further development of next-generation organic electronic devices. [10][11][12][13][14] Accordingly, accurate control of the crystal quality of molecular heterojunctions is desired for advancing further development of next-generation organic electronic devices.…”

Quantum modeling of ultrafast photoinduced charge separation

“…This clearly indicates that direct generation of CT excitons, although possible with very small cross sections, is not an important charge generation mechanism in these materials. Therefore, ultrafast charge separation in these devices is influenced by several factors including: frontier level alignment (which provides a driving force for charge separation), kinetic competition between generation and recombination pathways, and complex morphology (affecting exciton diffusion length and charge collection) [96,97].…”