Excitonic Solar Cells Using 2D Perovskite of (BA)₂(FA)₂Pb₃I₁₀

Using substrates at room temperature or at 110 °C, two-dimensional (2D) perovskite films and solar cells using mixed butylammonium (BA) and formamidinium (FA) cations, i.e., (BA) 2 (FA) n −1 Pb n I 3 n +1 ( n = 3 nominally), with the same molar ratio of components, but two concentrations of 0.25 mol/L (M) and 1.0 mol/L (M) based on PbI 2, are fabricated with adding thiourea in the precursor solution, respectively. No matter what the substrate temperature is, the 1.0 M concentration of solution results in more preferential out of plane alignment with respect to the contacts and larger crystalline grain size than that from the 0.25 M solution. However, devices from the 0.25 M solution using the hot-cast technique present better performance averagely, with the best power conversion efficiency (PCE) of 7.33%. In incident-photon-to-current efficiency (IPCE) spectra of devices from the 0.25 M solution, we observe the dominant contribution from exciton absorption, which is lacking in devices from the 1 M solution. Our work demonstrates the highest PCE reported for mixed BA and FA based 2D perovskite solar cells with n < 5 and manifests that the effective exciton dissociation will be crucial for FA based 2D perovskite solar cells.