Toward 'greener,' inexpensive solar cells

In hopes of one day replacing silicon-based photovoltaic cells, which are relatively expensive and require a lot of energy to make, scientists have turned to hybrid organic-inorganic lead halide perovskites that can be developed at a lower cost using less energy than silicon. The efficiency of these materials has increased rapidly over the past several years. But further improving the efficiency requires stacking two sub-cells with the top one exhibiting a wide bandgap and the bottom a low bandgap to form a tandem cell. The bandgap refers to the lowest energy of light that a semiconductor can absorb. The performance of low-bandgap perovskite cells has been lagging for many years. Dewei Zhao, Ren-Gen Xiong, Yanfa Yan and colleagues sought a way to change this.

For their solar cells, the researchers developed a new precursor solution combining formamidinium tin iodide and methylammonium lead iodide. The resulting tin-lead perovskite cells had low bandgaps and up to 15 percent power conversion efficiency. Other teams recently reported low-bandgap cells with about 13.6 percent efficiency. Additionally, the cells contained 60 percent less lead than the lead-based, single-junction (non-tandem) perovskite solar cell holding the current record efficiency of 22.1 percent. The reduction in lead content and improved efficiency for a low-bandgap cell represent a significant step toward practical, more environmentally friendly perovskite tandem solar cells, the researchers say.