A dithieno[3,2-b:2′,3′-d]pyrrole-cored four-arm hole transporting material for over 19% efficiency dopant-free perovskite solar cells

Dopant-free hole transporting materials (HTMs) have been in the limelight in the quest for stable perovskite solar cells (PVSCs). Herein, we report a novel dopant-free HTM featuring a dithieno[3,2-b:2 ',3 '-d]pyrrole core flanked with two-fold triphenylamine arm substituted thiophene bridges. This star-shaped molecule shows a suitable ionization potential (5.13 eV), good thermal stability (high glass-transition temperature of 120.3 degrees C) and a high hole mobility (6.50 x 10(-5) cm(2) V-1 s(-1)). The molecule is more hydrophobic than reference spiro-OMeTAD in a film. By capping our dopant-free HTM on the perovskite layer, the fabricated n-i-p device can achieve a maximum power conversion efficiency of 19.42%, which is comparable to the device based on doped spiro-OMeTAD (19.59%). This star-shaped dithieno[3,2-b:2 ',3 '-d]pyrrole based molecule is thus a promising candidate HTM for upscaling the commercial manufacture of PVSCs.