Reverse-Graded 2D Ruddlesden-Popper Perovskites for Efficient Air-Stable Solar Cells

2D Ruddlesden-Popper perovskites (RPPs) have emerged as a promising solar cell material. A group of novel RPPs with cyclohexane methylamine (CMA) as a spacer cation is presented. Unlike previously reported RPPs, the deposited films of (CMA)(2)(MA)(n-1)PbnI3n+1 (MA is CH3NH3+, n = 1, 2, 3, horizontal ellipsis ) exhibit multiple phases with reverse-graded quantum well (QW) distribution; small n (n = 2) RPPs are located at the surface and large n (n >= 10) RPPs at the bottom. This has three advantages: (a) The outer, more moisture resistant, small n RPPs create a stable barrier that protects the vulnerable large n RPP lattice from being attacked by water molecules. (b) It forms a type-II band alignment between different phases, which favors self-driven charge transport. (c) The natural structure of graded QWs expands the range of photon collection. Attributed to these properties, the best efficiency of 15.05%, with high open-circuit voltage (V-oc) of 1.10 V for a first-generation solar cell containing (CMA)(2)(MA)(8)Pb9I28, is achieved. A notable enhancement in short wavelength is observed in the Incident photon-to-current conversion efficiency spectra. This device shows significantly improved long-term stability, retaining approximate to 95% of the initial efficiency after 4600 h exposure in ambient conditions with 40-70% relative humidity.