Efficient all polymer active layers with long-range ordered 1D p-n nanoheterojunctions confirmed by TEM tomography

Abstract

Achieving precise control over the 3D morphology of the active layer in all-polymer solar cells (APSCs) is crucial for improving power conversion efficiency and stability. The ideal configuration involves a vertically aligned p-n nanoheterojunction with a 10-20 nm exciton diffusion length, but the inherent freedom of polymer chains poses challenges due to their stochastic nature. Extensive exploration of spatial analytical techniques, from molecular to macroscale levels, is also needed to reveal the polymer behavior governing the active layers. Meanwhile, halogenated solvents are used for high-efficiency APSC fabrication due to their favorable solubility with each polymer component. However, recent efforts aim to harness non-halogenated solvents, mitigating toxicity and environmental hazards. Here, we explore the generation of uniform thin film morphologies based on solution-processable crystallization-driven polymer assemblies. Pre-assembled n-type crystalline nanowires (NWs) via heating and cooling solutions of non-halogenated 1,2,4-trimethylbenzene formed very well-aligned, uniform thin film structures with well-dissolved noncrystalline p-type polymers and showed efficiency comparable to the performance of thin films prepared with chlorobenzene (CB) halogenated solvent. X-ray scattering and transmission electron microtomography confirm organized, long-range aligned one-dimensional NWs with closely interfaced p-n nanoheterojunctions through strong intra- and inter-polymeric pi-stacking interactions. Notably, the resulting thin film morphology resembles that of blended film casting with CB. Significantly, the long-term stability of the crystalline NWs-based thin film morphology under light exposure surpasses that of blend films due to robust intermolecular packing in the NWs. Using crystallization-driven polymer assemblies in non-halogenated solvents allows for precise control of the 3D morphology of the photoactive layer, thus improving the efficiency and stability of all-polymer solar cells.