A new alcohol‐soluble polymer PFN‐ID is successfully synthesized by combining N,N‐di(2‐ethylhexyl)‐6,6′‐dibromoisoindigo and an amino‐containing fluorene subunits, and applied to polymer solar cells (PSCs) with PTB7‐Th:PC71BM as an active layer. The n‐type backbone of the PFN‐ID improves electron transfer performance and thus optimizes device performance. The PSCs with PFN‐ID as cathode interfacial layers (CILs) have significantly improved compared to the device without the interface layer, especially the optimum power conversion efficiency (PCE) of PSCs reaches up to 9.24%, which is 1.62 times higher than that of devices without CILs. The I–V curves show that the introduction of the n‐type backbone leads to a significant increase in the conductivity of PFN‐ID compared to PFN. The UV photoelectron spectroscopy and Mott–Schottky curves further confirm that PFN‐ID can decrease the work function of Al electrode, and increase its built‐in potential, giving higher open‐circuit voltage. The resulting conventional PSCs using PFN‐ID as cathode interlayer achieve high photovoltaic performance, and the research results can provide a new strategy for the advancement of PSCs.
Comprehensive Summary
All‐small‐molecule organic solar cells (ASM OSCs) are promising for commercial application due to the well‐defined chemical structures, convenient purifying process and low batch‐to‐batch variation. However, the similarity of molecule structures between small molecule donors and acceptors makes a hard regulation of their blend morphology, which will limit the efficiency. One of the efficient approaches is structural tuning, among which the π‐bridge engineering is considered as a good method to improve the blend morphology. Herein, we synthesized two porphyrin‐based small‐molecule donors, Por‐BR and Por‐TR, by introducing alkoxybenzene and alkylthiophene as π bridges. The Por‐BR‐based active layer produces poor morphology and does not achieve satisfying device efficiency because of the excessive aggregation tendency. As for Por‐TR, an efficiency of 11.26% is achieved with such a high open circuit voltage of 0.904 V. This study shows that altering π‐bridge units can facilitate the improvement of film morphology, finally increase the device performance, and also provides a sample for molecule designing in terms of structure‐property correlation.
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