Wide band gap (WBG) perovskite solar cells (PSCs) have attracted substantial importance for their role as the top cell in the construction of perovskite based tandem solar cells (TSCs) to overcome the Shockley–Queisser single junction limit. At present, WBG perovskite solar cells suffer from severe open-circuit voltage (V_OC) loss due to surface and bulk defects (e.g. Pb clusters, halide vacancies) that cause non-radiative recombination, limiting the performance of devices. Surface treatments with various passivation materials deposited from solutions with different optimized concentrations have been applied to passivate these defects, but the correlation between the treatment methods and the concentration of passivation material has not been investigated yet. To address this, we applied three different surface treatment methods: no annealing (NA), high temperature annealing (HTA) for two minutes at 100^o C and room temperature annealing (RTA) for one hour at room temperature^[1]. 2-thiopheneethylammonium chloride (TEACl) was used as a surface passivant on a methylammonium-free WBG (E_g = 1.77eV) Cs_0.17FA_0.83Pb₁Br_1.2I_1.8 absorber, which after optimization was employed in the realization of p-i-n perovskite solar cells. High performing devices were made through the formation of (TEA)₂Pb(X)₄ 2-D layer at different concentrations varying the surface treatment method achieving record power conversion efficiency (PCE) of 19.9%.

X-ray diffraction (XRD) analysis performed on perovskite thin films with TEACl deposited from a 0.8 mg/ml solution and treated with RTA and HTA showed the formation of a 2-D layer at 2-theta angle of 5.6^o. The 2-D peak intensity for RTA decreased as we moved to concentration of 0.5 mg/ml while no 2-D peak was observed at a concentration of 0.2 mg/ml. HTA showed lower 2-D peak intensity as compared to RTA. To study the effect of these 2-D layers with varying intensities, complete cells with different TEACl concentration were fabricated, and it was established that cells with higher 2-D peak intensity showed increase in PCE for RTA. Conversely, for NA treatment the PCE increased with decrease in concentration of TEACl, attributed to reduced charge carrier extraction as judged by evolution of the fill factor and short circuit current of the devices. For HTA, a similar trend was observed with peak efficiency at 0.5 mg/ml, indicating a need for presumably thicker layers for annealing at high temperatures.

For further investigation, photoluminescence quantum yield (PLQY) analysis was performed which indicated up to ten-fold higher PL intensity with passivation in comparison to the reference. Passivated films with NA and RTA gave up to five times higher signal as compared to HTA at a concentration of 0.8 mg/ml with implied VOC of 1.326V for NA case. To analyze the carrier recombination pathways in complete cells, PLQY was performed on passivated layers with electron transport layer (ETL) on top and similar trends for PL intensity and V_OC were observed for RTA. It was confirmed that at higher concentrations of TEACl we have reduced non radiative recombination hence better performing devices with RTA. A similar trend was found for NA, highlighting the need for compromise between good passivation and charge carrier extraction. Transient-PL confirmed the reduced non-radiative recombination with RTA at 0.8 mg/ml concentration along with extended carrier lifetime compared to the films deposited from lower concentration solutions.

To summarize, high performing WBG perovskite solar cells were realized at low concentration with NA, at intermediate concentration with HTA and at high concentration with RTA, achieving 80 mV increase in V_OC in comparison to non-passivated samples and peaking at 19.9% efficiency.