Perovskite precursor concentration for enhanced recombination suppression in perovskite solar cells

Abstract

Perovskite is a light harvesting material with general structure of ABX3, where A is a monovalent cation (methylammonium (MA+), formamidinium (FA+), caesium (Cs+)), B is a divalent metallic cation (lead (Pb2+)or (Sn2+)) and X is a halide (iodine (I−), bromine (Br−) or chlorine (Cl−). Perovskite precursor concentration was varied from 30 ​wt% to 60 ​wt% at step size of 5 ​wt% and deposited via one-step spin coating of lead iodide (PbI2) and methylammonium iodide (CH3NH3I) at 3:1 ​M ratio. The effects of precursor concentration on the quality of films and devices fabricated were determined through optical, micro-structural, structural, electrochemical and electrical analyses. The optical characteristics show that 40 ​wt% is the optimized sample. The micro-structural analysis reveals increase in grain size as the concentrations increases from 30 ​wt% to 40 ​wt% and decrease as precursor concentration further increased from 45 ​wt% to 60 ​wt%. XRD patterns indicate increase in peaks intensity as the concentration increases from 35 ​wt% to 40 ​wt%. However, decrease in peak intensities occurs as the concentration increases beyond 40 ​wt%. Electrochemical (cyclic voltammetry and electrochemical impedance spectroscopies (EIS)) analyses reveal optimal suppression of carrier recombination by 40 ​wt% device, which is attributed to presence of larger grain size. The champion cell (40 ​wt% device) shows 12.43% power conversion efficiency (PCE). Optimization of precursor concentration provides an effective way for suppressing surface charge recombination due to trap-assisted recombination in perovskite devices thereby enhancing the quality and performance of perovskite solar cells.