Sequential deposition enables suppression of spontaneous donor enrichment of the surface in spin-coated films

Abstract Sequential deposition (SD) through independent processing of donor and acceptor materials, has emerged as a promising strategy to enable better control over the active layer morphology of organic solar cells. In this work, time-of-flight secondary ion mass spectrometry was employed to investigate the vertical distribution of SD PM6/Y5 films and bulk heterojunction PM6:Y5 films coated from a blend solution in one-step process. The influence of thermal annealing (TA) on the vertical distribution of the components was also evaluated. Our results show that SD inverts the vertical distribution within the active layer, while TA helps to suppress Y5 diffusion into the PM6 layer. Additionally, near edge x-ray absorption fine structure spectroscopy (NEXAFS) was used to investigate the molecular orientation of the donor PM6 and the acceptor Y5, in SD and blend films. Depth-dependent molecular orientation was assessed by comparing NEXAFS spectra acquired in total electron yield and fluorescence yield. Nitrogen K-edge NEXAFS spectra were employed to selectively probe the acceptor orientation. In SD-processed samples, we found that Y5 retains its face-on orientation when deposited on top of PM6, despite the combined effects of film formation dynamics and interfacial intermixing inherent to the process.