Solvation effect in precursor solution enables over 16% efficiency in thick 2D perovskite solar cells

High-quality thick light harvesting film is important for two-dimensional (2D) perovskite solar cells (PVSCs). The relationship between the structure of the second spacer cation (SSC+) in the precursor solution and the morphology of the perovskite film was studied. By applying structurally symmetric guanidinium (GA(+)) as a SSC+ and utilizing the solvation effect of GA(+) in N,N-dimethylformamide (DMF), the solvent of the precursor, we realized the fabrication of 2D BA(2)MA(4)Pb(5)I(16) (BA(+) = butylammonium, MA(+) = methylammonium) perovskite film with oriented crystal grains grown throughout the 530 nm thick film on a PEDOT:PSS substrate, which favored the fast transport of the charge carriers. The inverted planar-structured PVSCs exhibit a hero PCE of 14.94%. The device efficiency was further improved to 16.26%, one of the highest efficiencies reported for 2D PVSCs, through interface optimization via adding MACl into the precursor solution. The unsealed device exhibits moisture stability by retaining 90% of its initial PCE after 800 hours of exposure to air with a humidity of 55 +/- 5%. This work provides a new approach toward high-performance 2D PVSCs in terms of both PCE and stability.