Charge carrier generation in nonfullerene organic solar cells with different stoichiometric ratios of donor and acceptor materials.

Titas Klepeckas, Vidmantas Jašinskas, Andrius Gelžinis and Vidmantas Gulbinas.

Center for Physical Sciences and Technology, Saulėtekio av.3, Vilnius, 10257, Lithuania

Introduction of nonfullerene electron acceptors in organic solar cells (OSCs) has significantly boosted their operation efficiency. Currently the record reported efficiency already surpassed 20% making the OSCs an attractive photoelectric technology again.  Nevertheless, understanding of the important details of their performance still lags slightly behind. In particular, important questions remain regarding charge carrier generation, which is considered as a two-step process involving formation of charge transfer (CT) states and their separation into pairs of free charge carriers.  These processes in some cases are more efficient than one could expect considering energy levels of electron donor and acceptor molecules. Among large number of suggested explanations, the idea that aggregation of acceptor molecules in the central part of the acceptor grains creates cascaded landscape for the charge separation [1, 2] seems particularly rational, nevertheless requires additional confirmation and understanding of its details.

The carrier generation process, in particular cascaded carrier separation, is very sensitive to the stoichiometric ratio of donor and acceptor materials in the active layer. This balance is also a crucial aspect in maximizing organic solar cell efficiency.  Therefore, this research aims to investigating the charge carrier generation, their drift dynamics, and factors limiting charge extraction in active layers of PM6:Y6 OSCs with different stoichiometries. By employing time-resolved fluorescence and transient absorption spectroscopy techniques, together with  photocurrent measurements, this study aims to provide a deeper understanding of the fundamental processes that influence the performance of the  next-generation nonfullerene organic solar cells.

We demonstrate that charge carrier generation processes indeed strongly and even qualitatively depend on the stoichiometric ratio. In blends dominated by donors, excited donors rapidly transfer their energy to acceptors which subsequently generate CT states, but their separation into charge pairs is slow and inefficient. In contrary, when acceptor molecules dominate, excited donors rapidly form CT states and free charge carriers, which, however, experience geminate recombination.  Optimal stoichiometric ratio enables fast and efficient carrier generation and tenuous geminate recombination.