Dopant-Free PolymericHole-Transport Materials forPerovskite Solar Cells: Simple Is Best!

A series of (BDD-X)_n conjugated polymers, comprised of 5,7-bis­(2-ethylhexyl)­benzo­[1,2-<i>c</i>:4,5-<i>c</i>′]­dithiophene-4,8-dione (BDD) and X = B (<b>P1</b>), X = TBT (<b>P2</b>), and X = TBTBT (<b>P3</b>), where T = thiophene and B = benzo­[<i>c</i>]­[1,2,5]­thiadiazole, have been synthesized and applied as dopant-free hole-transport layer materials in perovskite solar cells (PSCs). We explored the effect of the molecular structure of the block X on the optical and electronic properties of the polymers, the nanoscale morphology of their films, and the impact of all these parameters on the performance of the polymers in PSCs. As a result, using the polymer <b>P1</b> with the simplest molecular architecture provided a power conversion efficiency (PCE) of 20.1% in solar cells, thus outperforming devices assembled with the more sophisticated polymers <b>P2</b>–<b>P3</b> or the reference poly­(triarylamine)-based hole-transport materials. The enhanced device performance is attributed to a better HOMO alignment of <b>P1</b> with respect to the perovskite valence band, a low concentration of defects and suppressed carrier recombination at the <b>P1</b>/perovskite interface and, most importantly, a highly uniform film structure, as revealed by atomic force microscopy and infrared scattering near-field optical microscopy (IR s-SNOM) techniques. The supramolecular interactions of the building blocks of polymers <b>P1</b>–<b>P3</b> with the perovskite films, resulting in the passivation of surface defects, were further studied by density functional theory calculations.