Boosted Electron Transport and Enlarged Built-In Potential by Eliminating the Interface Barrier in Organic Solar Cells

A smart interface modification strategy was employed to simultaneously improve short-circuit current density (J(SC)) and open-circuit voltage (V-oc) by incorporating a poly[(9,9-bis(3 '-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-diocty1)-fluorene] (PFN) interlayer between a TiO2 film and an active layer, arising from the fact that PFN effectively eliminated the interface barrier between TiO2 and the fullerene acceptor. The work function (WF) of TiO2 was apparently reduced, which facilitated effective electron transfer from the active layer to the TiO2 electron transport layer (ETL) and suppressed charge carrier recombination between contact interfaces. Electron injection devices with and without a PFN interlayer were fabricated to prove the eliminated electron barrier, meanwhile photoluminescence (PL) and time-resolved transient photoluminescence (TRTPL) were measured to probe much easier electron transfer from [6,6] phenyl C71-butyric acid methyl ester (PC71BM) acceptor to TiO2 ETL, contributing to enhanced J(SC). The shift in vacuum level altered the WF of PC71BM, which enlarged the internal electrical field at the donor/acceptor interface and built-in potential (V-bi) across the device. Dark current characteristics and Mott-Schottky measurements indicated the enhancement of Vbi, benefiting to increased V-oc. Consequently, the champion power conversion efficiency for a device with a PFN interlayer of 0.50 mg/mL reached to 7.14%, which is much higher than the PCE of 5.76% for the control device.