Benzamide-based dopant-free small-molecule hole transport materials for inverted perovskite solar cells

Linear and star-shaped derivatives were synthesized through condensation reactions between spiro-types arylamines and terephthaloyl dichloride (X17), 2,3,5,6-tetrafluoroterephthaloyl dichloride (X18), benzene-1,3,5-tricarbonyl trichloride (X19). Inverted planar perovskite solar cells (PSCs) were fabricated by dopant-free X17, X18 and X19 as hole-transport materials (HTMs). They exhibit good charge mobility and suitable the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) energy levels. The imide-based molecules have been shown to be effective for defect passivation in inverted PSCs. The photoelectric conversion efficiencies (PCEs) are increased in an order of X19 (18.84 %) > X18(17.15 %) > X17(15.03 %), indicating the star-shaped derivatives with multi-imides groups and a large conjugated central core contribute to high performance. Their hole mobility values are 3.63 x 10(-4) cm(2) V--(1) S--(1) (X17), 3.74 x 10(-4) cm(2) V--(1) S--(1) (X18) and 4.12 x 10(-4) cm(2) V--(1) S--(1) (X19). Atomic force microscopy (AFM) results showed smooth substrate with root-mean-square (RMS) roughness being 1.39 nm, 1.63 nm and 1.21 nm for X17, X18, and X19, respectively. Meanwhile, the HTMs exhibit high glass-transition temperatures (T-g) of 138 degrees C for X17, 166 degrees C for X18, and 145 degrees C for X19. The unencapsulated devices stability based on X17, X18, and X19 retained the initial efficiency as follows: 73 % for X19, 48 % for X18, and 37 % for X17 after 30 days of aging in a nitrogen atmosphere. In comparison, device incorporating poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) showed an efficiency of 18.58 %, and retained less than 65 % of the initial efficiency under the same condition.