On the Design of Steep Optical Absorbers for Vacuum-Processed Organic Solar Cells: One Isopropyl Group Makes the Difference.

The design of vacuum-processed organic solar cells requires a balance between optimizing energetic and morphological properties for efficient photovoltaic performance. In this study, we investigate two structurally similar molecular donors, DCV-iPr and DCV-Me, which exhibit stark differences in photovoltaic performance despite their minimal chemical differences. Through a combined experimental and theoretical approach-including crystal structure prediction, vapor deposition simulations, GIWAXS measurements, and advanced electronic structure calculations-we establish how molecular packing influences optical absorption and photovoltaic efficiency. Our findings show that steric effects introduced by the isopropyl group in DCV-iPr lead to a brick-wall molecular arrangement, favoring J-like excitonic interactions and resulting in sharp optical absorption with a reduced Stokes shift. In contrast, DCV-Me forms a more H-like aggregation, leading to broadened absorption and higher voltage losses. While DCV-iPr demonstrates enhanced photovoltaic performance, we identify substantial remaining voltage losses associated with charge-transfer excitations at the donor-acceptor interface. This work provides key design guidelines linking molecular packing to optical absorption properties and highlights the need for alternative acceptor materials with steeper absorption onsets to further optimize vacuum-processed organic solar cells.