Through judicious molecular engineering, novel dopant-free star-shaped D–π–A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15. The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, π-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.

This review discusses the role of chemical design on the self-assembly of donor–acceptor polymers on surfaces. Three major structural factors are highlighted including molecular weight, side chain engineering and backbone curvature of the polymers.