Lead-free absorber materials for solar cell applications

Abstract

With the rise of industry and technology development based on the utilization of fossil fuels as a main energy source as well as environmental and air pollution greenhouse gases have drastically increased with the alert and demand to alter the sources of energy. One way to do so is to reduce the consumption of fossil fuels and replace it by other sources such as renewable energies. Most interesting and promising is solar energy. After the discovery of the photovoltaic effect and applying it for making solar cells, an effective way to convert solar energy into electric energy has become achievable. Throughout history three different solar cell generations can be observed mostly based on the evolution of solar cells by the decrease of the solar cell thickness and the lowering of the active layer thickness from µm to nm. Beyond the decrease in thickness of the solar cells, the price of solar cells has dropped over the years. Still, the silicon-based technology is complex and cost-intensive. The third generation of solar cells are thin films with their most interesting and well-researched sub-group: perovskite solar cells. This new way of cheap and easy-to-fabricate solar cell has interested both, scientists and industry. A decade of lead-based perovskite solar cells in the scientific community has shown an increase in power conversion efficiency from 3.8 to 26 % and enormous interest of world scientists investigating the excellent photovoltaic properties of these lead-based perovskites (e.g. CH3NH3PbI3) but also the possibility of large commercialization. Organic–inorganic lead halide perovskite absorbers have excellent photovoltaic properties, such as suitable bandgap, high optical absorption, and long carrier lifetime. Unfortunately, underlying issues are the presence of toxic lead and the cell instability under ambient atmosphere (e.g. O2 and H2O). Bismuth-based lead-free double perovskites (e.g. Cs2AgBiBr6) have been considered as alternatives to the lead-based perovskites for solar cell applications. Trivalent cations, such as Bi3+ along with monovalent cations, Ag+, have been concurrently introduced to the B-sites of halide perovskites, leading to B cation double perovskites with the general chemical formula of A2B’B’’X6. These Pb-free double perovskites have been reported to have promising photovoltaic properties, including long carrier recombination lifetime, good stability against air and moisture, and low carrier effective masses. Thus, they are a potential alternative to the toxic lead halide perovskites. Nevertheless, device development is still in its infancy, and its performance is affected by severe hysteresis. In this work the realization of the synthesis and deposition of the double perovskite is presented via different deposition routes such as vacuum vapor and solution deposition. The double perovskite thin films have been optimized and characterized using different surface and material characterization methods. Afterwards, hysteresis-free planar and mesoporous double perovskite solar cells with no s-shape in the device characteristics and increased device open circuit voltage have been realized for the first time. This has been achieved by fine-tuning the material deposition parameters and layer optimization using several modification routes such as different temperature annealings, the thicknesses of mesoporous and perovskite layer, ozone and TiCl4 treatments leading to better infiltration of the double perovskite solution into mesoporous TiO2–ending with a significant improvement in solar cell performance. Except of device and interface engineering, to improve the material properties, compositional engineering has been conducted using mixing elements such as organic cation methylammonium, inorganic cation antimony and halide anion iodine. Finally, dimensional engineering has been achieved by adding large organic cations to the double perovskite crystal structure resulting in the new white emissive 2D and quasi 2D lead-free double perovskite materials.