The question how the nanoscale blend morphology of an organic solar cell determines the free charge carrier recombination rate remains largely unanswered. In a recent paper published in Advanced Electronic Materials, we provide new insight in this topic by revisiting the P3HT:PCBM blend system that dominated organic solar cell research a decade ago. Even though much better performing materials are known today, P3HT:PCBM still sets the benchmark in terms of reduced recombination losses.

In our study, we try to understand why this is so and which intrinsic properties and/or processing conditions lead to these exceptionally low losses. We looked at various morphological features such as phase separation, aggregation and phase purity. Among other methods, we used highly complex techniques such as high-resolution three-dimensional electron tomography to characterize the morphology. The key result is that it is not the size of the donor and acceptor domains that matters, but their internal energetic structure. In particular, an equal occurrence of amorphous, molecularly intermixed, as well as pure phases of high crystalline quality is necessary. In this way, charge carrier generation and separation on the one hand and charge carrier recombination on the other hand are spatially separated.

Electron Tomography
In-depth characterization of differently processed P3HT:PCBM blends unsing electron tomography

Thus, we were able to postulate a model morphology that can serve as a guideline for the development of future absorber materials. This is especially important for modern non-fullerene systems, as they are particularly susceptible to losses through recombination due to comparatively low charge carrier mobilities.

S. Wilken, D. Scheunemann, S. Dahlström, M. Nyman, J. Parisi, R. Österbacka, How to Reduce Charge Recombination in Organic Solar Cells: There are Still Lessons to Learn from P3HT:PCBM, Adv. Electron. Mater. 7, 2001056 (2021)