Abstract
AbstractSuppressing 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.
Highlights
Sebastian Wilken,* Dorothea Scheunemann, Staffan Dahlström, Mathias Nyman, Jürgen Parisi, and Ronald Österbacka efficiencies approaching 20%.[1,2,3] How
We find that the presence of aggregates of high crystalline quality and purity is crucial for organic photovoltaics (OPVs) to function as Shockley-type devices without recombination losses even at high thickness
While the performance of the CB and DCB systems is very similar in thin devices, significant differences become apparent with increasing thickness
Summary
Sebastian Wilken,* Dorothea Scheunemann, Staffan Dahlström, Mathias Nyman, Jürgen Parisi, and Ronald Österbacka efficiencies approaching 20%.[1,2,3] How-. By rials that are available, there are still only a few systems that maintain their full performance at junction thicknesses of 300 nm and more.[4,5,6] Compatibility with thick active layers, as well as a genchanging small details in the annealing procedure, two model morphologies eral tolerance to thickness variations, is are prepared that vary in phase separation, molecular order, and phase purity, as revealed by electron tomography and optical spectroscopy Both systems behave very with respect to charge generation and transport, but differ significantly in bimolecular recombination. Introduction tion strength.[12,13] Conceptually, free charge recombination is a bimolecular process and can be described by the rate equa-
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