Critical Assessment of the Use of Excess Lead Iodide in Lead Halide Perovskite Solar Cells

Abstract

It is common practice in the lead halide perovskite solar cell field to add a small molar excess of lead iodide (PbI2) to the precursor solution in order to increase device performance. However, recent reports have shown that an excess of PbI2 can accelerate performance loss. In addition, PbI2 is photoactive (bandgap ~2.3 eV), which may lead to parasitic absorption losses in a solar cell. Here, we show that devices using small quantities of excess PbI2 exhibit better device performance as compared to stoichiometric devices, both initially and for the duration of a stability test under operating conditions, primarily by enhancing charge extraction. However, photolysis of PbI2 negates the beneficial effect on charge extraction by leaving voids in the perovskite film and introduces trap states that are detrimental for device performance. We propose that although excess PbI2 provides a good template for enhanced performance, the community must continue to seek other additives or synthesis routes that fulfill the same beneficial role as excess PbI2 without the photolysis that negates these beneficial effects under long-term device operation.