Self-doping n-type Polymer as Cathode Interface Layer Enables Efficient Organic Solar Cells by Increasing Built-in Electric Field and Boosting Interface Contact
The self-doped polymer cathode interface materials for organic solar cells have been widely investigated to enhance the ohmic contact between the electrode and the photosensitive active layer. Here, a novel polymer named PBTA-FN with self-doping effect was successfully synthesized by incorporating benzotriazole (BTA) as an electron-deficient group and a fluorene containing an amino group. In favor of the n-type backbone and amine-based groups, an obvious n-type doping was obtained, resulting in the dramatically improved conductivity of the PBTA-FN. Subsequently, the PBTA-FN and PFN as cathode interface layers (CILs) have been successfully applied in the organic solar cells based on PBDB-T-2F:IT-4F. A notable power conversion efficiency of 12.18% and 11.03% can be achieved with PBTA-FN and PFN as CILs, respectively. The PBTA-FN shows a better planarity than PFN from the results by density functional theory. The self-doping behaviour of PBTA-FN was determined by electron paramagnetic resonance, which exhibits a higher mobility and carrier density. The water contact angle results on surface of active-layer/ PBTA-FN bilayer suggest the PBTA-FN surface polarity is improved, which is attributed to a larger interface dipole. Thus, the PBTA-FN can reduce the work function of Al electrode and enhance the built-in electric potential, which further confirmed by ultraviolet photoelectron spectroscopy and Mott Schottky curves, and the related device produce a higher Voc (0.88 V) than that of PFN (0.86 V). This work provides a deeper understanding of the PBTA-FN interlayer mechanism and has potential application in optoelectronic devices.