Design Principles for Organic Small Molecule Hole-Transport Materials for Perovskite Solar Cells: Film Morphology Matters

High-efficiency n-i-p perovskite solar cells generally incorporate organic hole-transport layer materials such as spiro-OMeTAD or PTAA, which have intrinsically low charge carrier mobility and therefore require doping to improve transport properties. However, using dopants is known to affect badly the operational stability of perovskite solar cells. Therefore, the development of suitable dopant-free hole-transport materials is the critical issue for realizing perovskite solar cells with high efficiency and long operational lifetimes. Herein, a series of small molecules with triazatruxene, benzodithiophene, triphenylamine, and dithienosilole electron donor core units were designed and explored as solution-processed dopant-free hole-transport materials for perovskite solar cells. The best performance has been obtained using the triazatruxene-based molecule TAT-2T-CNA with terminal alkyl cyanoacetate groups and a 2,2'-bithiophene pi-conjugated bridge, which has enabled device efficiency of 20.1% with negligible hysteresis, along with a substantially improved V-OC and FF values as compared to the reference devices assembled with PTA as a hole-transport material. The superior performance of TAT-2T-CNA is attributed to the optimal optoelectronic properties of this material and, most importantly, defectless film morphology. Using scanning near-field infrared microscopy (IR-SNOM) technique was shown to be particularly useful for the detection and visualization of defects in thin films of hole-transport materials, which strongly correlate with the device performance. The results obtained in this work are expected to provide new insights facilitating the rational design of efficient dopant-free hole-transport materials for efficient and stable perovskite solar cells.