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In organic solar cells (OSCs), the energy of the charge-transfer (CT) complexes at the donor-acceptor interface, ECT, determines the maximum open-circuit voltage ( VOC). The coexistence of phases with different degrees of order in the donor or the acceptor, as in blends of semi-crystalline donors and fullerenes in bulk heterojunction layers, influences the distribution of CT states and the VOC enormously. Yet, the question of how structural heterogeneities alter CT states and the VOC is seldom addressed systematically. In this work, we combine experimental measurements of vacuum-deposited rubrene/C60 bilayer OSCs, with varying microstructure and texture, with density functional theory calculations to determine how relative molecular orientations and extents of structural order influence ECT and VOC. We find that varying the microstructure of rubrene gives rise to CT bands with varying energies. The CT band that originates from crystalline rubrene lies up to ≈0.4 eV lower in energy compared to the one that arises from amorphous rubrene. These low-lying CT states contribute strongly to VOC losses and result mainly from hole delocalization in aggregated rubrene. This work points to the importance of realizing interfacial structural control that prevents the formation of low ECT configurations and maximizes VOC. [ABSTRACT FROM AUTHOR]

The energy landscape in organic semiconducting materials greatly influences charge and exciton behavior, which are both critical to the operation of organic electronic devices. These energy landscapes can change dramatically depending on the phases of material present, including pure phases of one molecule or polymer and mixed phases exhibiting different degrees of order and composition. In this work, ultraviolet photoelectron spectroscopy measurements of ionization energies (IEs) and external quantum efficiency measurements of charge-transfer (CT) state energies ( ECT) are applied to molecular photovoltaic material systems to characterize energy landscapes. The results show that IEs and ECT values are highly dependent on structural order and phase composition. In the sexithiophene:C60 system both the IEs of sexithiophene and C60 shift by over 0.4 eV while ECT shifts by 0.5 eV depending on molecular composition. By contrast, in the rubrene:C60 system the IE of rubrene and C60 vary by ≤0.11 eV and ECT varies by ≤0.04 eV as the material composition varies. These results suggest that energy landscapes can exist whereby the binding energies of the CT states are overcome by energy offsets between charges in CT states in mixed regions and free charges in pure phases. [ABSTRACT FROM AUTHOR]

Structural order and intermolecular interactions strongly determine the interfacial energy landscape in organic photovoltaics. The finding that the ionization energy and the energy of charge‐transfer states can vary by approximately 0.5 eV depending on material order and composition may explain how excitons are able to separate efficiently in organic photovoltaics, where phases of pure materials and mixed phases are commonly present. This is reported by Kenneth R. Graham, Aram Amassian, and co‐workers in article number 1601211. [ABSTRACT FROM AUTHOR]