Conformational and Compositional Tuning of Phenanthrocarbazole-Based Dopant-Free Hole-Transport Polymers Boosting the Performance of Perovskite Solar Cells

Abstract

Conjugated polymers are regarded as promising candidates for dopant-free hole-transport materials (HTMs) in efficient and stable perovskite solar cells (PSCs). Thus far, the vast majority of polymeric HTMs feature structurally complicated benzo[1,2-<i>b</i>:4,5-<i>b</i>']dithiophene (BDT) analogs and electron-withdrawing heterocycles, forming a strong donor-acceptor (D-A) structure. Herein, a new class of phenanthrocarbazole (<b>PC</b>)-based polymeric HTMs (<b>PC1</b>, <b>PC2</b>, and <b>PC3</b>) has been synthesized by inserting a <b>PC</b> unit into a polymeric thiophene or selenophene chain with the aim of enhancing the π-π stacking of adjacent polymer chains and also to efficiently interact with the perovskite surface through the broad and planar conjugated backbone of the <b>PC</b>. Suitable energy levels, excellent thermostability, and humidity resistivity together with remarkable photoelectric properties are obtained via meticulously tuning the conformation and elemental composition of the polymers. As a result, PSCs containing <b>PC3</b> as dopant-free HTM show a stabilized power conversion efficiency (PCE) of 20.8% and significantly enhanced longevity, rendering one of the best types of PSCs based on dopant-free HTMs. Subsequent experimental and theoretical studies reveal that the planar conformation of the polymers contributes to an ordered and face-on stacking of the polymer chains. Furthermore, introduction of the "Lewis soft" selenium atom can passivate surface trap sites of perovskite films by Pb-Se interaction and facilitate the interfacial charge separation significantly. This work reveals the guiding principles for rational design of dopant-free polymeric HTMs and also inspires rational exploration of small molecular HTMs.