Understanding Multi-Length Scale Fractal Networks in Organic Photovoltaic Active Layers and Their Impacts on Charge Trapping

A comprehensive understanding of the active layer morphology is crucial for establishing the processing-structure-performance relationship for high-performance organic photovoltaics (OPVs). However, similar chemical structures of donor (D) and acceptor (A) materials constituting the OPV active layer make conventional morphology characterization techniques ineffective. Additionally, it remains challenging to correlate device performances with complex phase-separated structures at different length scales. Herein, we understand the active layer morphology from a new angle, focusing on the fractal nature of donor and acceptor networks. The unique feature of a fractal structure is its self-similarity at multi-length scales, allowing us to understand the variation in mesoscale phase-separated structures from subtle changes in local intermolecular packing patterns. A detailed morphology characterization is performed using a combination of grazing-incidence X-ray and neutron scattering techniques. Firstly, we selectively deuterate Y6, a prototypical acceptor molecule, to enhance its contrast under neutron beams. This allows us to probe a hitherto hidden short-range aggregation of Y6 molecules and identify its vital role in establishing percolation pathways within the D:A intermixed domains[1]. Having established the detrimental impact of isolated acceptor domains on electron trapping, we then explore the role of domain shapes on inter-domain connectivity, unraveling a robust positive correlation between the fractal dimension (D_f) of acceptor domains and the density of deep electron traps. We propose that forming highly-crystalline acceptor domains while maintaining a relatively low D_f is essential for simultaneously suppressing bimolecular and deep trap-assisted recombination. Taking advantage of the multi-length scale self-similar nature of the fractal object, we further develop effective additive and molecular structure selection rules, allowing us to fine-tune inter-molecular aggregation patterns, which in turn determine the mesoscale fractal dimension of the acceptor domain through bottom-up effects[2]. Our findings underscore the importance of both crystalline and amorphous morphology at multi-length scales to enhance the performance of OPV devices, surpassing 20% efficiency and advancing towards industrialization.
[1] G. Cai, Y. Li, Y. Fu, H. Yang, L. Mei, Z. Nie, T. Li, H. Liu, Y. Ke, X.-L. Wang, J.-L. Brédas, M.-C. Tang, X. Chen, X. Zhan & X. Lu. Deuteration-enhanced neutron contrasts to probe amorphous domain sizes in organic photovoltaic bulk heterojunction films. Nature Communications 15, 2784 (2024).
[2] Y. Fu, L. Xu, Y. Li, E. J. Yang, Y. Guo, G. Cai, P. F. Chan, Y. Ke, C.-J. Su, U. S. Jeng, P. C. Y. Chow, J.-S. Kim, M.-C. Tang & X. Lu. Enhancing inter-domain connectivity by reducing fractal dimension: the key to passivating deep traps in organic photovoltaics. Energy & Environmental Science (2024, in press).