Interlayer Engineering for Flexible Large-Area Planar Perovskite Solar Cells

Hybrid metal halide perovskite solar cells (PSCs) have consistently demonstrated high power conversion efficiency (PCE), although the best performing PSCs mostly employ high-temperature (500 °C) processed compact and mesoporous TiO₂. Instead, low-temperature processed PSCs are desirable for implementation on flexible substrates and tandem solar cells. Here, we present a new method to achieve high efficiency flexible planar PSCs based on a low-temperature processed nonaqueous sol–gel route that synthesized TiO₂ and a guanidinium iodide (GuaI) salt passivation treatment of the perovskite film. We fabricate both rigid and flexible triple-cation perovskite (Cs_0.05(MA_0.17FA_0.83)_0.95Pb­(I_0.85Br_0.15)₃, E_g ∼ 1.58 eV) PSCs, achieving PCEs of 19.8% and 17.0% on glass and polyethylene naphtholate (PEN) substrates, respectively. At the same time, rigid and flexible high-bandgap double cation (FA_0.85Cs_0.15Pb­(I_0.7Br_0.3)₃, E_g ∼ 1.72 eV) PSCs reached a PCE of 18.0% and of 15.8%. Moreover, large area (1 cm²) ∼1.58 eV and ∼1.72 eV PSCs achieved PCEs of 18.2% and 16.7% PCE on glass substrates and of 16.2% and 13.9% on PEN substrates, demonstrating the high uniformity of all the solar cell layers.