Simple synthesis and molecular engineering of low-cost and star-shaped carbazole-based hole transporting materials for highly efficient perovskite solar cells

Perovskite solar cells (PrSCs) have emerged as a very promising technology in the field of photovoltaics by demonstrating power conversion efficiencies (PCEs) soaring from 3.9% to above 22% within the past eight years. To date, perovskite solar cells mainly depend on spiro- OMeTAD to perform a key role as a hole transporting material (HTM). However, the complicated multi-step synthetic procedures and high-cost purification process for spiro- OMeTAD limited its potential for commercial application. Herein, three new carbazole-based HTMs with a starburst structure, coded as SGT-405 (3,6), SGT-410 (3,6) and SGT-411 (3,6) via tuning the substitution position from the (2,7) to the (3,6) position of the carbazole moiety, have been successfully synthesized via three-step synthesis from commercially available reagents and investigated for highly efficient perovskite solar cells. By adopting this strategy, among them, molecularly engineered carbazole derivative SGT-405 (3,6) exhibits significantly increased T g (192.7 °C), improved film forming ability, reduced hole reorganization energy and enhanced hole mobility compared to its parent molecule SGT-405 (2,7) and spiro- OMeTAD . Owing to the promising properties of SGT-405 (3,6), meso-porous type PrSCs employing SGT-405 (3,6) showed a remarkable PCE of 18.87%, which is better than that of the photovoltaic device employing spiro- OMeTAD (17.71%). To the best of our knowledge, the achieved PCE (18.87%) is the highest value reported for devices with the structure of FTO/compact TiO 2 /meso-porous TiO 2 /CH 3 NH 3 PbI 3−x Cl x /HTM/Au employing small-molecular HTMs. Meanwhile, owing to the simple synthesis of SGT-405 (3,6), compared with SGT-405 (2,7) previously developed by our group, synthesis cost was much lowered, resulting in low cost compared to the spiro- OMeTAD and SGT-405 (3,6), by approximately three times. Furthermore, the long-term device stability of PrSCs was enhanced for SGT-405 (3,6) to some extent compared to those of other HTMs studied here due to the good uniform capping layer of SGT-405 (3,6) on top of the perovskite layer and the prevention of moisture penetration into the perovskite layer. Therefore, SGT-405 (3,6) is a promising low-cost and efficient non-spiro type HTM with potential to replace expensive spiro- OMeTAD for PrSCs.