| As a new generation of photovoltaic application technology,organic solar cells have gained rapid development in the past decades.Due to advantages of low cost,lightweight,solution processing,and flexibility,organic solar cells are expected to have a wide range of applications in printable and wearable devices.Among them,all small molecule organic solar cells exhibit good commercialization potential owing to the well-defined chemical structure of the small molecules and excellent batch-to-batch reproducibility,and the highest efficiency has exceeded 17% in the latest studies.However,the aggregation and crystallization properties of small molecular materials are difficult to adjust,which brings challenges to the morphology control of the active layer.According to the above considerations,the following works have been carried out.π-bridge units are important parts of the conjugated backbone of molecules,which has great influences on improving the solubility,aggregation,and crystallization behavior of molecules.Two small porphyrin-based donors,Por-BR and Por-TR,were synthesized by introducing alkoxy benzene and alkyl thiophene as π-bridges.Due to the aggregation effect of alkoxy,a large pure phase region is formed in the active layer based on Por-Br,so the efficiency is not good.The Por-TR-based device has a smooth active layer surface,and finally achieves11.26% efficiency with such a high open-circuit voltage of 0.904 V.This chapter shows that the rational selection of π-bridge units in molecular design can effectively improve the morphology of the blend film,and ultimately improve the performance of the device.Non-fullerene acceptors with asymmetric end groups have gained significant development in recent years.Such materials have larger dipole moments,which helps to enhance intermolecular interactions and provides additional opportunities to improve molecular packing.Meanwhile,the asymmetric conjugated backbones will contribute to the formation of asymmetric electron push-pull within the molecule,which may improve the photovoltaic performance of the material.Inspired by the success of asymmetric nonfullerene acceptors design,a small donor Por-DPP-TR based on porphyrin units and its symmetric counterpart PTR were synthesized.The asymmetrically conjugated backbone brings a significantly enhanced intramolecular dipole to the Por-DPP-TR,increasing the interaction between molecules.Finally,Por-DPP-TR exhibits strong crystallizability.Among the binary devices,the device based on the Por-DPP-TR:6TIC active layer achieves an efficiency of 13.28% thanks to the ordered morphology and enhanced carrier transmission capability.In addition,the isogenous porphyrin donor Zn P-TSEH was employed as the third components to prepare ternary device.The morphology of the ternary blend film is further improved,and clear phase separation of the bicontinuous network is formed.The device based on Por-DPP-TR:Zn P-TSEH:6TIC achieved an efficiency of 16.31%.In the current study of all-small-molecule organic solar cells,16.31% is one of the highest efficiencies.Recently,the symmetry breaking strategy of donor units has been regarded as an effective method to regulate the molecular interaction,crystallization property,and stacking mode.To further explore structural design and asymmetric structures,in this section,two porphyrin cores with asymmetric side chains were synthesized by a one-flask method using dipyrrole methane and different arylaldehydes,and two A-D-A type small molecule donors named DDPor-DPP and DAPor-DPP were prepared using these as donor units.Having fabricated binary devices with 6TIC,both donors exhibited good photovoltaic performance,thanks to the optimiazed photophysical and electrochemical properties caused by the fine-tuned chemical structures.Among them,the DAPor-DPP-based device achieved a higher efficiency of 16.62% due to the more efficient carrier transport properties,lower charge recombination,and suitable phase separation. |
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