How to Reduce Charge Recombination in Organic Solar Cells: There Are Still Lessons to Learn from P3HT:PCBM

TL;DR

This study investigates how nanoscale morphology influences charge recombination in P3HT:PCBM organic solar cells, revealing that high-quality crystalline aggregates significantly reduce recombination and enhance device performance.

Contribution

The paper demonstrates that specific morphological features, particularly high-quality P3HT aggregates, are crucial for minimizing recombination in organic solar cells, providing new insights into morphology-performance relationships.

Findings

High-quality P3HT aggregates support charge delocalization.

Optimized morphology leads to long hole diffusion lengths.

Recombination rates are significantly lower in systems with pure, crystalline aggregates.

Abstract

Suppressing charge recombination is key for organic solar cells to become commercial reality. However, there is still no conclusive picture of how recombination losses are influenced by the complex nanoscale morphology. Here, new insight is provided by revisiting the P3HT:PCBM blend, which is still one of the best performers regarding reduced recombination. By changing small details in the annealing procedure, two model morphologies were prepared that vary in phase separation, molecular order and phase purity, as revealed by electron tomography and optical spectroscopy. Both systems behave very similarly with respect to charge generation and transport, but differ significantly in bimolecular recombination. Only the system containing P3HT aggregates of high crystalline quality and purity is found to achieve exceptionally low recombination rates. The high-quality aggregates support charge delocalization, which assists the re-dissociation of interfacial charge-transfer states formed upon the encounter of free carriers. For devices with the optimized morphology, an exceptional long hole diffusion length is found, which allows them to work as Shockley-type solar cells even in thick junctions of 300 nm. In contrast, the encounter rate and the size of the phase-separated domains appears to be less important.