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

• Published in: Advanced electronic materials Vol. 7; no. 5
• Main Authors: Wilken, Sebastian, Scheunemann, Dorothea, Dahlström, Staffan, Nyman, Mathias, Parisi, Jürgen, Österbacka, Ronald
• Format: Journal Article
• Language: English
• Published: 01.05.2021
• Subjects: aggregation; charge recombination; morphology; organic photovoltaics; phase separation
• ISSN: 2199-160X; 2199-160X
• DOI: 10.1002/aelm.202001056

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 are 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 appear to be less important. The P3HT:PCBM blend is revisited to study the effect of morphology on charge recombination in organic solar cells. It is shown that the presence of high‐quality aggregates is key to suppress recombination to such an extent that Shockley‐type devices with scalable thickness become possible. Instead, the size of phase separation is found to be of less importance.