Nanometer-Thick ITIC Bulk Heterojunction Films as Non-Fullerene Acceptors in Organic Solar Cells

TY Huang and AP Le Brun and B Sochor and CM Wu and Y Bulut and P Müller- Buschbaum and SV Roth and YL Yang, ACS APPLIED NANO MATERIALS, 7, 17588-17595 (2024).

DOI: 10.1021/acsanm.4c02865

The nanomorphology of bulk heterojunctions (BHJs) plays a critical role in determining the performance of non-fullerene organic solar cells (OSCs). Thermal annealing is commonly used to reorganize the donor and acceptor phases within the BHJs. In this study, we investigate the vertical morphology of BHJ blend films incorporating the poly(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3 & tprime;-di(2-octyldodecyl)-2,2 ';5 ',2 '';5 '',2 & tprime;-quaterthiophen-5,5 & tprime;-diyl) (PffBT4T-2OD) polymer as the donor and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))- 5,5,11,11-tetrakis(4-hexylphenyl)-dithieno2,3-d:2 ',3 '-d '-s-indaceno1,2-b:5,6-b 'dithiophene (ITIC) as the acceptor. Neutron reflectivity patterns and scattering length density profiles reveal that the surface of the BHJ films became diffuse when the annealing temperature was above 150 degrees C. We further find that mitigated agglomeration of PffBT4T-2OD side chains exhibits minimal impact on morphology post-annealing. Instead, ITIC molecules trigger aggregations, accompanied by interface diffusion and increased film roughness. X-ray scattering confirms a 5-fold increase in aggregated ITIC nanodomains after annealing. Our findings highlight that unstable ITIC phases dominate the BHJ morphology of thin films, leading to the thermal instability of OSCs. This study enhances our understanding of the BHJ morphology and offers insights into improving the performance of energy conversion devices.

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