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 (<i>J</i><sub>sc</sub>) and open-circuit voltage (<i>V</i><sub>oc</sub>) by incorporating a poly­[(9,9-bis­(3′-(<i>N</i>,<i>N</i>-dimethylamion)­propyl)-2,7-fluorene)-<i>alt</i>-2,7-(9,9-dioctyl)-fluorene] (PFN) interlayer between a TiO<sub>2</sub> film and an active layer, arising from the fact that PFN effectively eliminated the interface barrier between TiO<sub>2</sub> and the fullerene acceptor. The work function (WF) of TiO<sub>2</sub> was apparently reduced, which facilitated effective electron transfer from the active layer to the TiO<sub>2</sub> 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<sub>71</sub>BM) acceptor to TiO<sub>2</sub> ETL, contributing to enhanced <i>J</i><sub>sc</sub>. The shift in vacuum level altered the WF of PC<sub>71</sub>BM, which enlarged the internal electrical field at the donor/acceptor interface and built-in potential (<i>V</i><sub>bi</sub>) across the device. Dark current characteristics and Mott–Schottky measurements indicated the enhancement of <i>V</i><sub>bi</sub>, benefiting to increased <i>V</i><sub>oc</sub>. 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.