Publication date: 11th March 2026
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 [1], time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to investigate the vertical distribution of sequentially deposited PM6/Y5 films and bulk heterojunction (BHJ) 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. The depth-profiles revealed that blend-casting samples present a donor-rich surface layer, approximately 2.4 nm wide and that TA does not affects the vertical composition. On the other hand, the depth profiles obtained for the SD-films show that SD inverts the vertical composition within the active layer, promoting an acceptor-rich surface layer. It was found that TA of the PM6 bottom layer helps to suppress Y5 diffusion, and TA of the bilayer after the acceptor deposition further promotes vertical phase separation resulting in a vertical composition gradient. 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 sequentially deposited and blend films. Depth-dependent molecular orientation was assessed by comparing NEXAFS spectra acquired in total electron yield (TEY) and fluorescence yield (FY). PM6:Y5 blends measured in TEY mode presented a carbon K-edge spectra dominated by the spectral features of the donor PM6, while for SD-films carbon K-edge spectra are dominated by Y5 spectral features, clearly reflecting the donor-rich and acceptor-rich surfaces, in agreement with ToF-SIMS results. Furthermore, nitrogen K-edge NEXAFS spectra were employed to selectively probe the acceptor orientation. In SD-processed samples, NEXAFS nitrogen K-edge spectra recorded in TEY and FY modes have shown 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.
The authors thank Dr Leif K E Ericsson, Dr Cleber F N Marchiori at Karlstad University, Sweden, for their assistance with the NEXAFS experiments.
E M acknowledges the Swedish Research Council for financial support of the project (Grant Nr. 2021−04798). This work was partially supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by The Knut and Alice Wallenberg Foundation (Grant Nr. WISE-AP01-PD08). For financial support of the research infrastructure the authors thank the Knut and Alice Wallenberg Foundation (Grant Nr. 2016.0059).
P D acknowledges the European Union for the financial support in the framework of the Smart Growth Operational Programme, Measure 4.2; Grant Nos. POIR.04.02.00-00-D001/20, “ATOMIN 2.0—ATOMic scale science for the INnovative economy.
Research at the National Synchrotron Radiation Centre SOLARIS is supported by the Ministry of Science and Higher Education, Poland, under contract no. 1/SOL/2021/. The study was carried out using research infrastructure purchased with the funds of the European Union in the framework of the Smart Growth Operational Programme, Measure 4.2; Grant No. POIR.04.02.00-00-D001/20, ‘ATOMIN 2.0—ATOMic scale science for the INnovative economy’.
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