Layered Ruddlesden–Popper perovskites with various thicknesses for stable solid-state solar cells

The present research comes up with optimizing the layers thickness of a Ruddlesden–Popper perovskite with the general formula of (S$_{0.97}$S’$_{0.03}$)$_2$[Cs$_{0.05}$(FA$_{0.097}$MA$_{0.03}$)$_{0.95}$]$_{n-1}$Pb$_n$(I$_{0.97}$Br$_{0.03}$)$_{3n+1}$ for efficient, stable solar cell applications. Such a triple-cation quasi-two-dimensional (2D) structure simultaneously contains two spacers, namely 5-ammonium valeric acid iodide (S) and tetra-$n$-octylammonium bromide (S'). Systematic studies showed that morphology, crystal structure, optical properties, photovoltaic performance, and internal resistances of this compound depended upon the value of the $n$ integer. Field emission scanning electron microscopy set forth that the deposited films were composed of various morphologies depending on the $n$ value. An increase in the n value resulted in improving the light absorption, reducing the band gap energy, and blue-shifting the photoluminescence peak. So as to fabricate solar cells, CuInS$_2$ nanoparticles were employed as a novel hole-transporting material. The device based on the film having $n$ = 4 value showed the highest power conversion efficiency of 10.2%. Electrochemical impedance spectroscopy demonstrated that the improved performance of this cell was mainly thanks to its low series resistance (11.68 $\Omega$), high charge recombination resistance (922.35 $\Omega$), and long electron lifetime (8.05 $\mu$s) as compared to all the fabricated cells. Moreover, this cell displayed a maximum external quantum efficiency of 82