Some fundamental questions in the organic solar cell (OSC) community are related to the role of bulk and interfacial morphology on key processes such as charge generation, recombination, and extraction...

Fused‐ring core nonfullerene acceptors (NFAs), designated “Y‐series,” have enabled high‐performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid‐state NMR, crystallography, and molecular modeling approach to elucidate the atomic‐scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self‐assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused‐ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular‐level understanding of BHJs enabled by this approach will guide the engineering of next‐generation NFAs for stable and efficient OSCs.

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High electrical conductivity of metal oxide thin films needs uniform surface coverage, which has been the issue for the thin films based on electrospun nanofibers (NFs) that have advantage over the sputtered/spin-coated films with respect to large surface area and mechanical flexibility. Herein, we investigated a reduction in the sheet resistance of electrospun indium tin oxide (ITO) NF films with improved surface coverage. We found that the surface coverage depends significantly on the electrospinnable polymer concentration in the precursor solutions, especially after post-hot-plate annealing following the infrared radiation furnace treatment. The postannealing process increases crystallinity and oxygen vacancies. However, with a higher PVP content, it makes the surface of ITO NFs more prominently rough as a result of the formation of larger sphere-shaped ITO particles on the NF surface, which gives rise to poor surface coverage. A less poly(vinylpyrrolidone) (PVP) content in ITO NF films by electrospinning for short deposition times was found to improve surface coverage even after postannealing. The sheet resistance notably decreases, down to as low as 350 Ω/sq, with a high transmittance of over 90%. Our study provides an understanding on how to achieve high electrical conductivity of ITO NF films with high surface coverage, which can be utilized for the optoelectronic and sensing applications.

Phone: þ82 2 940 8675, Fax: þ82 2 942 9160Understanding the effects of interlayers in organic photovoltaic devices is necessary to investigate its potential to maximize efficiencies. Here, we investigate how a solutionprocessed copper (I) iodide (CuI) hole transport layer (HTL) prepared onto indium tin oxide (ITO) substrates affects the photovoltaic performances of the devices based on Poly(3hexylthiophene-2,5-diyl) (P3HT):Phenyl-C61-butyric acid methyl ester (PCBM), with its different thicknesses. We found that the film morphology and roughness depends sensitively on the concentration of CuI solutions used for thin films. The surface of CuI films spun with high concentration solutions consists of large grains with high roughness, resulting in a direct short in the devices. For the devices with a thin CuI layer, the power conversion efficiency (PCE) of the devices under illumination was observed to decrease with time. The X-ray photoelectron spectroscopy (XPS) suggests that the indium ions might be diffused into the active layer across the thin CuI layer. Our results highlight that the CuI layer thickness is a key parameter in affecting not only efficiency but also device yield and short-term photostability.