We designed and synthesized the DPPEZnP-TEH molecule, with a porphyrin ring linked to two diketopyrrolopyrrole units by ethynylene bridges. The resulting material exhibits a very low energy band gap of 1.37 eV and a broad light absorption to 907 nm. An open-circuit voltage of 0.78 V was obtained in bulk heterojunction (BHJ) organic solar cells, showing a low energy loss of only 0.59 eV, which is the first report that small molecule solar cells show energy losses <0.6 eV. The optimized solar cells show remarkable external quantum efficiency, short circuit current, and power conversion efficiency up to 65%, 16.76 mA/cm(2), and 8.08%, respectively, which are the best values for BHJ solar cells with very low energy losses. Additionally, the morphology of DPPEZnP-TEH neat and blend films with PC61BM was studied thoroughly by grazing incidence X-ray diffraction, resonant soft X-ray scattering, and transmission electron microscopy under different fabrication conditions.
A new category of deep-absorbing small molecules is developed. Optimized devices driven by mixed additives show a remarkable short-circuit current of ≈20 mA cm(-2) and a highest power conversion efficiency of 9.06%. A multi-length-scale morphology is formed, which is fully characterized by resonant soft X-ray scattering, high-angle annular dark film image transmission electron microscopy, etc.
Solvent vapor annealing (SVA) studies on the morphology and performance of a porphyrin-based deep-absorption organic solar cells consisting of a strongly segregated bulk heterojunction (BHJ) blend, are presented. It is seen that the solvent vapor annealing of a well-mixed BHJ blends induces molecular motion, leading to a phase separated morphology governed by a spinodal decomposition mechanism. The earlier stage of solvent vapor swelling (<10s) led to an obvious phase separation but not device performance. The device performance showed a dramatic increase in short circuit current and fill factor between 15-20s of SVA. Thus, phase purity is a critical parameter in determining the performance of this binary blend. SVA on a thermally annealed BHJ thin film showed two distinctive processes, a crystal dissolution and a recrystallization, accompanied by phase mixing and then phase separation. The final morphology of SVA films that were initially thermally annealed showed a reduced length scale of phase separation, in comparison to SVA on as-cast films. Thus preformed donor crystallites appear to lock-in the morphology, even in a small molecule blend setting. The best performing device was obtained by a slight SVA (10s) of films that were initially thermally annealed, reaching a power conversion efficiency of 8.48%. This suggests that the localized morphological optimization and domain size reduction are most important factors in dictating organic photovoltaic device efficiencies.
The marriage of non-fullerene acceptors (NFAs) and ternary solar cell architecture has brought about great advances in organic photovoltaics. The primary effort, however, has been focusing on low-bandgap NFAs that exploit complementary absorption and energy-level cascade. Here we report a wide-bandgap NFA IDT-T that functions as an energy-level mediator, a fluorescence resonance energytransfer donor, an electron acceptor, and a crystallization modulator, which contribute synergistically in a ternary blend to yield high organic photovoltaic device performance.
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