Effects of thiophene substituents on hole-transporting properties of dipolar chromophores for perovskite solar cells

We present a theoretical investigation of thiophene substituent effects on the electrochemical properties of dipolar chromophores (TCNE, TCNE22 and TCNE24) as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). Herein, the material properties in crystalline phases are explored by using the first-principle calculations combined with Marcus theory. The results show that the increased number of thiophene substituents for TCNE, TCNE22 and TCNE24 results in a redshift of the absorption spectrum (27-46 nm). Furthermore, both TCNE22 and TCNE24 have maximum absorption peaks at a wavelength of 400 nm. Most importantly, the molecular planarity is improved effectively, which generates strong intermolecular face-to-face pi-pi packing interaction. The higher hole mobility of TCNE24 (2.069 x 10(-1) cm(2) V-1 s(-1)) with four thiophene substituents is obtained due to the face-to-face pi-pi packing. The new designed TCNE24 not only has excellent spectral property, but also has strong hole mobility. Therefore, TCNE24 is a promising organic small-molecule HTMs. Our work provides theoretical guidance for designing higher-performance HTMs in PSCs.