A simple spiro-type hole transporting material for efficient perovskite solar cells

We developed a cost-effective spiro-type 4,4 ',4 '',4 '''\-(2H,2 ' H,4H,4 ' H-3,3 '-spiro-bi[thieno[3,4-b][1,4] dioxepine]-6,6 ',8,8 '-tetrayl) tetrakis(N,N-bis(4-methoxyphenyl)aniline) hole transporting material (PST1) for perovskite solar cells (PSCs) that works efficiently even without a cobalt dopant. The PST1 is obtained by employing facile synthetic routes and tends to crystallize in the solid state. An X-ray diffraction study of PST1 revealed a unique quasi-spiro molecular configuration and found multiple CH/pi and pi-pi intermolecular contacts. For the first time, the crystal structure of 2,2 ',7,7 '-tetrakis(N,N '-di-p-methoxyphenylamine)-9,9 '-spirobifluorene (spiro-OMeTAD) is also studied for comparison. The device based on PST1 exhibited a PCE of 13.44%, and a comparable 12.74% PCE was achieved for its undoped form, which paves the way for developing new low cost hole transporting materials and final industrialization of perovskite solar cells.