Glucose, a widely distributed biomaterial in nature, is presented as a new cathode interfacial material for highly efficient inverted organic solar cells. The interactions between glucose and the indium tin oxide (ITO) substrate as well as the formation mechanisms of the glucose interlayer were investigated by molecular dynamics simulation and relevant experimental tests. The results revealed that the In–OH coordination between the oxygen atom of glucose and the indium of ITO is the key factor for the formation of interfacial dipoles, thereby reducing the work function of the ITO cathode for efficient charge transfer. With PM6:Y6 as the active layer, the power conversion efficiency (PCE) of the organic solar cells was significantly increased from 1.99 to 15.42% after ITO was modified by a glucose interlayer through the traditional spin-coating method. More importantly, glucose can be adsorbed on the ITO surface by a simple immersion process, and the devices based on the modified ITO by immersed glucose achieved a PCE of 14.48%, which is comparable to that of the traditional spin-coating method. Furthermore, we found that the OSCs with the ITO cathodes modified with glucose derivatives including sorbitol and sodium gluconate by different preparation methods also exhibited high performance. The overall performance of the devices with ITO modified by a simple and low-cost immersion method can be maintained at more than 93% of that prepared with the traditional spin-coating method. The results demonstrated that low-price glucose and its derivatives are good candidates as ITO interlayer materials for OSCs, and the effectiveness of the immersion process paves a way for simplifying the manufacture of low-cost and large-area organic solar cells.
Three small-molecule acceptors (Por-PDI, TEHPor-PDI, and BBOPor-PDI) with different side chains were synthesized by using a porphyrin core as the electron-donating unit and connecting electron-withdrawing perylene diimide dimers via acetylene bridges. The bulk heterojunction organic solar cells based on the three acceptors and a polymer donor provided power conversion efficiencies (PCEs) of 3.68-5.21 % when the active layers were fabricated with pyridine additives. Though the synthesis of Por-PDI is easier with fewer reaction steps and higher yields, the devices based on Por-PDI showed the best performance with a PCE of 5.21 %. The more ordered intermolecular packing due to the reduced steric hindrance at the porphyrin core of Por-PDI could contribute to the more balanced hole/electron mobilities, higher maximum charge generation rate, and less bimolecular recombination in Por-PDI devices, which are beneficial for the higher PCE.
Two conjugated A–D–A (A = electron acceptor, D = electron donor) porphyrins 2OH–2BT–ZnPOR and 2C60–2BT–ZnPOR are designed and synthesized for single-component organic solar cells (SCOSCs). In the two molecules, two benzothiadiazoles (BTs) are connected to a porphyrin core as the electron-withdrawing units through π-bridges to build a conjugated A–D–A main chain, which can narrow the energy levels, broaden the absorption, and promote the intramolecular charge transfer of the porphyrins. While there are no other electron-acceptor side groups in 2OH–2BT–ZnPOR, two fullerene units are symmetrically connected to 2OH–2BT–ZnPOR in 2C60–2BT–ZnPOR. Though the 2OH–2BT–ZnPOR-based SCOSCs exhibit a very poor device performances, the 2C60–2BT–ZnPOR-based SCOSC shows a power conversion efficiency (PCE) of 1.51% and a high open-circuit voltage (V OC) of 1.05 V, which is also much higher than those of previously reported porphyrin-based SCOSCs. The much higher performance of 2C60–2BT–ZnPOR SCOSCs can be ascribed to the delocalized π-electrons induced by the conjugated A–D–A main chain and the promoted charge dissociation induced by the fullerene units in 2C60–2BT–ZnPOR.
The development of the third component is very important for ternary organic solar cells (OSCs). Herein, we synthesized two porphyrin electron acceptors (Zn-Por-IC4F and Zn-Por-IC) with an A-2π-D-2π-A (A: electron acceptor unit, D: electron donor unit) structure as the third component for PTB7-Th:Y6 devices. Zn-Por-IC4F and Zn-Por-IC in the ternary OSCs can improve the crystallization of Y6 and the exciton dissociation, therefore enhancing their fill factors (FF). As a result, the power conversion efficiencies (PCEs) of the PTB7-Th:Y6:Zn-Por-IC4F and PTB7-Th:Y6:Zn-Por-IC ternary OSCs are improved to 11.6 and 10.67%, which are increased by 21 and 11.5%, respectively, compared with that of PTB7-Th:Y6 binary devices with a PCE of 9.57%. This is the first report that porphyrins are employed as electron acceptors in ternary OSCs, though porphyrins have been widely used as donor materials of OSCs, which may open up an avenue for the applications of porphyrins in OSCs and provide guidelines for designing photovoltaic materials.
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