Exploring the Effect of Lewis-Base Additives on the Performance and Stability of Mesoscopic Carbon-Electrode Perovskite Solar Cells

Carbon-electrode perovskite solar cells (C-PSCs) are arising as a promising way to fabricate low-cost and stable devices, with the potential of upscaling. However, the power conversion efficiencies (PCEs) of C-PSCs lag behind those of typical metal electrode PSCs; therefore, it is necessary to increase the efficiency of such devices. A straightforward method to achieve this is to employ in C-PSCs the chemical and device-engineering methods that have been successfully implemented in high-performance metal electrode PSCs. In this report, we have exploited the beneficial effect of the Lewis acid-base adduct approach and have introduced the compounds of thiourea or urea (U) as additives in the Cs-containing mixed-cation, mixed-halide perovskite precursor solution. The as-prepared films containing the two Lewis bases have proven to be homogeneous and dense, free of cracks and pinholes, while at the same time presenting a significant increase in the absorbance compared to the pristine film. In order to establish the optimum conditions, a range of additive contents have been used and studied. The corresponding C-PSCs, of triple mesoscopic structure, that have been prepared with the thiourea- and urea-containing perovskites, without the presence of any hole transport layer (HTL-free), exhibit enhanced performance, with maximum improvement of the PCE by 14 and 46% for thiourea and urea for 5 and 10% of additive content, respectively. The highest PCE obtained (13%) exceeds that of the reference device by 9%. Electrochemical impedance spectroscopy (EIS) has been used to study the interfaces of the highest-performing devices. Additionally, the stability study that has been carried out has revealed the high stability of all devices, which retain over 78% of their initial PCE, while the thiourea-containing devices, through time-induced recrystallization phenomena, can deliver a higher PCE after 48 days of storage.