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 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 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.