Symmetry-breaking co-assembly of conjugated molecules boosts perovskite photovoltaics

In perovskite photovoltaics, self-assembled molecules (SAMs) have demonstrated the ability to enhance interface quality, reduce charge recombination, and improve energy-level alignment. However, most symmetrical molecules for photovoltaic applications tend to self-aggregate, which hinders uniform film formation, reduces the active surface area, and limits interface contact and device efficiency. In this work, we propose a symmetry-breaking co-assembly (SBC) strategy to improve the performance of the widely employed SAM of 2-[3,6-Dimethoxy-9H-carbazol-9-yl)ethyl] phosphonic acid (MeO-2PACz) by coupling with another small conjugated molecule, dibenzo[b,d]thiophene-4-carboxylic acid (DTCA). The broken symmetry at the molecular level enables the synthesis of co-SAM layers with significantly improved uniformity and coverage. A quantitative protocol based on atomic force microscope-infrared spectroscopy (AFM-IR) has been developed to determine the surface coverage of SAM layers. When the surface coverage of co-SAM layers is maximized, the interfacial chemical reaction under electrical stress and the non-radiative recombination loss are effectively suppressed, resulting in power conversion efficiencies (PCEs) of 26.32% (certified as 25.67%) and 25.34% for areas of 0.08 cm2 and 1 cm2, respectively. The encapsulated device retains 93% of its initial PCE after operating at the maximum power point (MPP) for 1,150 hours, as evaluated following the ISOS-L-1 protocol. These results underscore the effectiveness of the SBC strategy in advancing perovskite photovoltaics, and the coverage-maximizing methodology may be generalized to other research domains involving SAMs. Perovskite solar cells need uniform self-assembled molecule layers, but symmetric molecules can clump and weaken interfaces. Luo et a. paired two molecules to break symmetry, improving coverage, efficiency and long-term stability.