Effects of the PbBr2:PbI2 Molar Ratio on the Formation of Lead Halide Thin Films, and the Ratio's Application for High Performance and Wide Bandgap Solar Cells.

Md Abdul Kuddus Sheikh,Ho-Seok Nam,Rahim Abdur,Sung-Min Lee,Hyunseung Lee,Jaegab Lee,Son Singh,Jae-Hun Kim

doi:10.3390/ma15030837

Md Abdul Kuddus Sheikh, Ho-Seok Nam + Show 6 more

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https://doi.org/10.3390/ma15030837

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Journal: Materials	Publication Date: Jan 22, 2022
Citations: 4	License type: CC BY 4.0

Affiliation: Kookmin University, Incheon National University

Abstract

We investigate the effects of the molar ratio (x) of PbBr2 on the phases, microstructure, surface morphology, optical properties, and structural defects of mixed lead halides PbI2(1−x)Br2x for use in solar cell devices. Results indicate that as x increased to 0.3, the surface morphology continued to improve, accompanied by the growth of PbI2 grains. This resulted in lead halide films with a very smooth and continuous morphology, including large grains when the film was formed at x = 0.3. In addition, the microstructure changed from (001)-oriented pure PbI2 to a highly (001)-oriented β (PbI2-rich) phase. The plausible mechanism for the enhanced morphology of the lead halide films by the addition of PbBr2 is proposed based on the growth of a Br-saturated lead iodide solid solution. Furthermore, iodine vacancies, identified by X-ray photoelectron spectroscopy, decreased as the ratio of PbBr2 increased. Finally, an electrical analysis of the solar cells was performed by using a PN heterojunction model, revealing that structural defects, such as iodine vacancies and grain boundaries, are the main contributors to the degradation of the performance of pure PbI2-based solar cells (including high leakage, low stability, and high hysteresis), which was significantly improved by the addition of PbBr2. The solar cell fabricated at x = 0.3 in air showed excellent stability and performance. The device lost merely 20% of the initial efficiency of 4.11% after 1500 h without encapsulation. This may be due to the dense microstructure and the reduced structural defects of lead halides formed at x = 0.3.

Highlights

PbI2 is a highly photoconductive semiconductor with a direct bandgap, high absorption coefficient, and good flexibility [1,2,3,4,5,6]
We investigated the effects of the molar ratio (x) of PbBr2 on the crystal structure, the preferred orientation, absorption spectra, and the morphology of a lead halide layer formed on the meso-TiO2 substrate
In this paper, we investigate the compositional effects of the molar ratio (x) of PbBr2 on the crystal structure, preferred orientation, optical property, microstructure, surface morphology, and defects of the mixed lead halides PbI2(1−x)Br2x formed on meso-TiO2 substrates

Summary

Introduction

PbI2 is a highly photoconductive semiconductor with a direct bandgap, high absorption coefficient, and good flexibility [1,2,3,4,5,6]. PbI2 single crystals have commonly been fabricated by the Bridgeman-based technique, vapor growth, and the hot wall technique [1,9,11,13,14,15,16,17,18], while polycrystalline thin films are formed by vacuum evaporation or the spin/spray coating of PbI2 solutions [19,20,21,22,23,24] Among these techniques, the deposition of PbI2 polycrystalline layers from a solution is very simple, reliable, and economical. It is difficult to grow high quality PbI2 films, partly due to the decomposition of PbI2 during the crystal growth at high temperatures, and partly due to the weak Van Der Waals (VDW) bonding between the layers that leads to the formation of PbI2 slabs with interlayer voids and defective packing structures, resulting in very rough surface morphologies [2,5,23,25]

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