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-dioctyl)-fluorene] (PFN) interlayer between a TiO 2 film and an active layer, arising from the fact that PFN effectively eliminated the interface barrier between TiO 2 and the fullerene acceptor. The work function (WF) of TiO 2 was apparently reduced, which facilitated effective electron transfer from the active layer to the TiO 2 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 (PC 71 BM) acceptor to TiO 2 ETL, contributing to enhanced J sc . The shift in vacuum level altered the WF of PC 71 BM, 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 V bi, 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.