Nanographene (NG)-based Hole Transporter with - interface modifier for Thermally Stable Perovskite Solar Cells

We report here thermally stable HTM for perovskite solar cell (PSC) which is based on Nanographene(NG) with functional substitution groups (coded NG-HTMs) to enhance its hole mobility and thermal property. Hole mobility of NG-HTM is optimized by changing functional groups and enhanced from 5.68x10-3 cm2V-1s-1 to 9.51x10-3 cm2V-1s-1 which shows much higher than spiro-MeOTAD (4.69x10-4 cm2V-1s-1). However, interface problem was detected when NG-HTM was applied to NIP perovskite solar cell due to interface property which originated from new chemical structure of NG-HTMs. NG-selective pi-interface modifier (pi-IM) which induces strong pi-pi interaction between perovskite and NG core significantly enhances charge extraction efficiency from perovskite layer. Therefore, power conversion efficiency (PCE) is significantly improved from 9.8% (stabilized PCE, w/o pi-IM) to 23.06% (w/ pi-IM) which shows higher than the spiro-MeOTAD-based device (20.56% (w/o pi-IM) and 19.38% (w/ pi-IM). In DSC measurement, our optimized NG-HTM shows 137.6 oC of Tg with very weak endothermic signal (6.00 mu W/mg), while the spiro-MeOTAD shows clear endothermic signal (153.75 mu W/mg) at 75.3 oC. Therefore, NG-HTM based device maintains 83.6% of initial PCE after 350 h at 75oC + 1000 h at 85 oC which is significant improvement compared to completely degraded spiro-MeOTAD device (which maintains 24.5% of initial PCE). These findings highlight the importance of core structure and proper substitution groups to design for new HTM and molecular-level design of interfacial modifier for highly efficient and stable PSCs.