Abstract
Conjugated donor–acceptor molecules with intramolecular charge transfer absorption are employed for single-component organic solar cells. Among the five types of donor–acceptor molecules, the strong push–pull structure of DTDCPB resulted in solar cells with high JSC, an internal quantum efficiency exceeding 20%, and high VOC exceeding 1 V with little photon energy loss around 0.7 eV. The exciton binding energy (EBE), which is a key factor in enhancing the photocurrent in the single-component device, was determined by quantum chemical calculation. The relationship between the photoexcited state and the device performance suggests that the strong internal charge transfer is effective for reducing the EBE. Furthermore, molecular packing in the film is shown to influence photogeneration in the film bulk.
Highlights
The calculated exciton binding energy (EBE) in the solid state with subtraction of Ex from energy gap (Eg) showed a negative value for DTDCPB and BCNDTS because the Ex corresponds to the absorption peak, whereas the real EBE is estimated from absorption band edge
Several materials with intramolecular charge transfer absorption were utilized as single-component organic solar cells
The electronic state calculation for the solid state suggests that these intramolecular charge transfer (CT) molecules have lower EBE, which corresponds to the photovoltaic performance
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. If a photoexcited state, such as a CT exciton, can be produced directly from the absorbed photons, photogeneration from the film bulk would be expected with no photon energy loss from the LUMO offset at the D–A interface. There are several reports of single-component systems based on polymers having donor and acceptor units [10,11,12] In such a linked system, the dissociated charges are relaxed in a secondary process following photoexcitation, resulting in photon energy loss in the same manner as in the BHJ system. For the intramolecular CT molecules, the highest occupied molecular orbital (HOMO) and LUMO are overlapped, and the photoexcited state is delocalized between the donor and acceptor unit In this case, the charges forming a CT state are expected to be separated from the delocalized photoexcited state to the surrounding molecules of the same type with smaller energetic barrier. The presented results suggest that molecules with a strong CT-state are promising absorbers in organic solar cells and detectors
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