High-efficiency Polymer Solar Cells Based On Dual Cathode Interlayer Strategy

Organic solar cells?OSC?have been widely studied by researchers because of their low cost,light weight,easy fabrication,and feasibility of fabricating large-area flexible devices.In recent years,thanks to the synthesis of new active layer materials,especially the rapid development of non-fullerene receptors,the power conversion efficiency?PCE?of single-junction OSCs has exceeded 15%,and the PCE of tandem OSCs has exceeded 17%,which has exceeded the minimum standard of commercialization.The OSCs can be classified into conventional structure and inverted structure according to the relative positions of the electron transport layer?ETL?and the hole transport layer?HTL?in the OSCs.For the conventional OSCs,due to the strong chemical activity of its commonly used low work function electrode,the stability of the device is often weaker than that of the OSC with inverted structure.For the inverted OSC,since the preparation of the commonly used electron transport layer often requires high temperature annealing,it is disadvantageous to prepare flexible devices.In this paper,from the perspective of interface optimization,we mainly attempt to improve the PCE and solve the stability problem of the OSCs.1.High-Efficiency and Stable Organic Solar Cells Enabled by Dual Cathode Buffer LayersIn Chapter 2,we prepared devices with different cathode interface layers and electrodes using PTB7:PC₇₁BM as the active layer.When the commonly used alcohol-soluble interface modifying materials?PDIN,PFN,and ZrAcac?are used as the cathode interface layer?alco-CIL?and Al as the electrode,the PCE of the device is only about 8%.When alco-CIL/BCP is used as the double-layer cathode interface layer and Ag as the electrode,the device achieves a power conversion efficiency?PCE?of 10.11%,and the stability is improved to varying degrees.Due to the stronger reflection of the Ag electrode,the light absorption in the active layer of the alco-CIL/BCP/Ag-based device is stronger than that of the alco-CIL/Al-based device,so the short-circuit current of the device is improved.In addition,the energy level alignment based on the double-layer interface layer is more conducive to charge transport and extraction;alcohol treatment of the active layer by alco-CIL can passivate the interface defects of the active layer and reduce charge recombination;double protection of the double-layer cathode interlayer to the active layer also reduce the defects generated on the surface of the active layer during electrode evaporation.Finally,due to the complex formed between BCP and Ag and the double blocking effect of alco-CIL/BCP on Ag atoms,devices based on double-layer cathode interlayer have better long-term stability than devices based on single-layer cathode interlayer.2.Efficient organic solar cell based on ethanolamine modified tin dioxideTin oxide?SnO₂?is a promising electron transport layer?ETL?material owing to its high electron mobility,high transmittance and feasibility of low-temperature processing.However,the performance of OSCs using SnO₂ as ETL still lagged that of zinc oxide?ZnO?based OSCs due to the high density of the deep trap centers on the surface.Here,we find that the performance of low-temperature?<150??solution processed SnO₂ based OSCs can be greatly improved by simply applying ethanolamine?EA?solution to wash the surface of SnO₂ films.EA can effectively reduce the deep trap centers on the surface of SnO₂ induced by hydroxyl groups,and particularly it can reinforce the built-in potential by forming a negative interface dipole.Due to the merit of EA modification on SnO₂ film surface,the charge recombination loss in OSCs is significantly reduced and the electron transport/extraction is substantially improved.Finally,OSCs based on EA modified SnO₂ demonstrated a high power conversion efficiency?PCE?of 12.45%comparable to those based on ZnO.By contrast,OSCs based on pristine SnO₂ only gives an efficiency of 10.71%.Our work demonstrates a simple and promising way to fabricate low-temperature solution processed SnO₂ that is compatible with flexible substrates using roll-to-roll technique for highly efficient inverted OSCs.