Wide-bandgap perovskites for multijunction solar cells: improvement of crystalline quality of Cs0.1FA0.9PbI1.4Br1.6 by using lead thiocyanate

Abstract

Top cells based on mixed iodide–bromide lead perovskites (Cs_0.1FA_0.9PbI_1.4Br_1.6) for multijunction solar cells (MJ SCs) were fabricated and characterized. Various lead thiocyanate (Pb(SCN)₂) concentrations (0%, 1%, 2%, 3%, and 4%) were added into perovskite precursor solution to fabricate high quality Cs_0.1FA_0.9PbI_1.4Br_1.6 thin films. The bandgap energy of Pb(SCN)₂-added Cs_0.1FA_0.9PbI_1.4Br_1.6 thin films was 1.87 eV determined from Tauc plots. The average grain size of the 2% Pb(SCN)₂-added perovskite thin film was 2.27 μm, more than 16 times larger than that of the perovskite thin film without Pb(SCN)₂ (0.134 μm). The intensity of the two main peaks (14.38° and 28.97°) was intensified with Pb(SCN)₂-added perovskite thin films. The photoluminescence (PL) revealed higher intensity of perovskite thin films with Pb(SCN)₂ due to a decrease in nonradiative recombination. The carrier lifetimes of Cs_0.1FA_0.9PbI_1.4Br_1.6 thin films sharply increased with the presence of Pb(SCN)₂, from 25.92 ns at 0% Pb(SCN)₂ to 224.27 ns at 2% Pb(SCN)₂. The thickness of a Cs_0.1FA_0.9PbI_1.4Br_1.6-based absorber layer cell is 400 nm which is thin enough to obtain both high transparency and large current density. As a result, the addition of 2% of Pb(SCN)₂ to the precursor of Cs_0.1FA_0.9PbI_1.4Br_1.6 perovskite solar cells resulted in a notable improvement in power conversion efficiency (PCE), increasing from 9.37% to 13.66%. This improvement was accompanied by a significant increase in both open-circuit voltage (V_oc), which reached 1.27 V, and current density (J_sc,JV), which reached 14.19 mA cm⁻².