High-Efficiency Y6 Homojunction Organic Solar Cells Enabled by a Secondary Hole Transport Layer.

Y6 homojunction solar cells are prepared using the exciton/electron-blocking material poly[9,9-di-n-octylfluorene-alt-N-(4-sec-butylphenyl)diphenylamine] (TFB) as a secondary hole transport layer material in conjunction with PEDOT:PSS. Using this device architecture, a maximum power conversion efficiency (PCE) of 2.57% is achieved, which is the highest reported thus far for a solution-processed small molecule homojunction organic photovoltaic (OPV) device. The devices display an unexpectedly low thickness dependence, with the average PCE only decreasing by ≈17% when the Y6 active layer thickness is increased from 80 to 300 nm. Time-resolved photoluminescence measurements show that the TFB does not contribute to charge generation through photoinduced hole or electron transfer. However, transient absorption spectroscopy on thin films of neat Y6 and a 1:1 blend of Y6:TFB shows that the TFB enhances the formation of the long-lived Y6 intermolecular charge-transfer state in the blend film. It is found that careful selection of the electron transport layer (ETL) is required to avoid unintended charge generation at the interface with Y6 so as to ensure that the device is a true homojunction. The improved efficiency of this architecture is attributed to the electron-blocking and hole-extraction effects of the TFB layer.