Compositional and interfacial engineering for improved light stability of flexible wide-bandgap perovskite solar cells†

Metal halide perovskites, due to their facile bandgap tunability, are excellent materials for highly efficient multi-junction solar cell architecture. However, commonly used mixed halide wide-bandgap (WBG) perovskite compositions suffer from performance-damaging phase instability as a result of prolonged light exposure. Here, we demonstrate a dual strategy to suppress this effect in flexible WBG (E_g = 1.76 eV) perovskite solar cells (PSCs). First, we optimized the perovskite precursor solution by synergistic addition of lead thiocyanate (Pb(SCN)₂) and 4-fluoro-phenethylammonium iodide (4FPEAI). This modification led to a successful reduction in non-radiative recombination and suppression of ionic mobility. Next, we incorporated a carbazole-based self-assembling molecule, equipped with three anchoring sites, 4-((5H-diindolo[3,2-a:3′,2′-c]carbazole-5,10,15-triyl)tris(butane-4,1-diyl))tris(phosphonic acid) (TRIPOD-C4) as a hole-transporting layer. Such a molecular design promoted uniform surface packing with the p-type material, greatly improving hole extraction efficacy. The combined effect of these two developments led to a T85 of 1200 hours in the light-soak aging test of a flexible WBG perovskite solar cell (PSC) at 65 °C (illumination with LED light of 600 mW cm⁻²). Moreover, we report a large-area (1 cm²) flexible WBG PSC of 15% efficiency and a flexible all-perovskite tandem device reaching 22.5% efficiency.