Effects of doping with Na, K, Rb, and formamidinium cations on (CH3NH3)0.99Rb0.01Pb0.99Cu0.01I3−x(Cl, Br)x perovskite photovoltaic cells

Naoki Ueoka,Takeo Oku,Atsushi Suzuki

doi:10.1063/5.0029162

Abstract

The effects of doping with alkali metals and formamidinium cations on CH3NH3(Rb)Pb(Cu)I3(Cl,Br) perovskite photovoltaic cells were investigated. Lattice constants were slightly decreased and increased by K and Na doping, respectively. This indicated that Na atoms occupied interstitial sites in the perovskite crystal. The conversion efficiencies for most of the devices decreased after one year. However, the conversion efficiency for the CH3NH3(Na,Rb)Pb(Cu)I3(Cl,Br) photovoltaic cell was unchanged after one year. The Na-doped perovskite provided the smallest carrier trap density, which suggested that lattice defects were suppressed by Na doping. First principles calculations supported these experimental results.

Highlights

There have been many studies on applying perovskite halide compounds in photovoltaic devices,1 and photoconversion efficiencies of perovskite solar cells have exceeded 20%
J–V characteristics under illumination recorded in the reverse scan and external quantum efficiency (EQE) spectra of the fabricated perovskite photovoltaic cells are shown in Figs. 1(a) and 1(b), respectively
EQE values were increased by NaI addition in the wavelength range of 300 nm–500 nm, as observed in Fig. 1(b), which was attributed to optical absorption by alkali metal elements as reported previously

Summary

INTRODUCTION

There have been many studies on applying perovskite halide compounds in photovoltaic devices, and photoconversion efficiencies of perovskite solar cells have exceeded 20%. Introducing organic cations with larger ionic radii than CH3NH3+ (MA+), such as HC(NH2)2+ (FA+), CH3CH2NH3+ (EA+), and C(NH2)3+ (GA+), should stabilize the structure and improve the photovoltaic performance of perovskite solar cells. The MAPbI3 compound transforms from cubic to tetragonal during cooling to room temperature, and partial defects induce distortion of the perovskite structure. Another approach to controlling the t-factor is element substitution at the Pb site, which stabilizes the structure and lowers the toxicity. The highest photovoltaic performance was obtained for MA(Rb)Pb(Cu)I3(Cl,Br) perovskites by adding RbI and CuBr2.25 In the present study, changing the distortion of perovskite crystals containing alkali metals or formamidinium was evaluated and ion migration were investigated. Microstructures of the devices were investigated by x-ray diffraction (XRD) using an x-ray diffractometer (Bruker D2 PHASER), optical microscopy (Nikon Eclipse E600), and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (Jeol, JSM-6010PLUS-LA)

EXPERIMENTAL PROCEDURES

RESULTS AND DISCUSSION

CONCLUSIONS