Self-doped alcohol-soluble small molecules as cathode interface layers for organic solar cells

Abstract

Studying interfacial engineering is crucial to developing organic solar cells (OSCs). Herein, we report a novel, alcohol-soluble, self-doped small molecule, 3,3′-(indolo[3,2-b]indole-5,10-diyl)bis(N,N-dimethylpropan-1-amine) (IDID1), which features a backbone composed of two linked indole units and two nitrogen-containing carbon chains as side chains. The nitrogen-containing side chains impart a polarity, making it soluble in polar solvents such as methanol, thereby enhancing its environmental compatibility. Notably, IDID1 forms an interfacial dipole layer, which effectively reduces the work function (WF) of the Al electrode. Furthermore, an n-type self-doping effect is observed in IDID1, which is attributed to the transfer of the lone pair of electrons present on the nitrogen atoms to the central conjugated unit. The self-doping effect has been empirically demonstrated to significantly enhance the electron collection efficiency and transport capacity, thereby ultimately resulting in an increased electron mobility. In the present study, orthotropic devices were fabricated utilizing IDID1 as the cathode interfacial layers (CILs). The optimal performance of the devices was achieved at a IDID1 concentration of 0.6 mg·mL⁻¹, with a power conversion efficiency (PCE) of 8.88 %. This represents a 175 % improvement in performance compared to devices without CILs.