Orientation‐Engineered Small‐Molecule Semiconductors as Dopant‐Free Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells

Crystallized p-type small-molecule semiconductors have great potential as efficient and stable hole transporting materials (HTMs) for perovskite solar cells (PSCs) due to their relatively high hole mobility, good stability and tunable HOMOs depending on the molecular orientation. Here, a thienoacene-based organic semiconductor, 2,9- diphenyldinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DPh-DNTT), was thermally evaporated and employed as the dopant-free HTM that can be scaled up for large-area fabrication. By controlling the deposition temperature, the molecular orientation was modulated into a dominated face-on orientation possessing π-π stacking direction towards the surface normal to maximize the out-of-plane carrier mobility. With an engineered face-on orientation, the DPh-DNTT film shows an improved out-of-plane mobility of 3.3 ×10-2 cm2V-1s-1, outperforming the HTMs reported so far. Such orientation-reinforced mobility contributes to a remarkable efficiency of 20.2% for CH3NH3PbI3 inverted PSCs with enhanced stability. The results reported here provide insights into engineering the orientation of molecules for the dopant-free organic HTMs for PSCs.