Improved perovskite solar cell efficiency by tuning the colloidal size and free ion concentration in precursor solution using formic acid additive

Improving the quality of the perovskite active layer is crucial to obtaining high performance perovskite solar cells (PSCs). In this work, by introducing formic acid into the formamidinium lead iodide (FAPbI3) precursor solution, we managed to achieve reduced colloidal size in the solution, leading to more uniform deposition of FAPbI3 film with lower trap state density and higher carrier mobility. The solar cells based on the FAPbI3 absorber layer modified with formic acid show significantly better photovoltaic performance than that on the reference FAPbI3 film without formic acid. The device performance shows a close correlation with the colloidal size. Within the range studied from 6.7 to 1.0 nm, the smaller the colloidal size is, the higher the solar cell efficiency. More specifically, the cell efficiency is improved from 17.82% for the control cell without formic acid to 19.81% when 0.764 M formic acid was used. Formic acid has also been added into a CH3NH3PbI3 (MAPbI3) precursor solution, which exhibits a similar effect on the resulting MAPbI3 films and solar cells, with efficiency improved from 16.07% to 17.00%.