Hybrid organic-inorganic perovskite materials, particular, the CH₃NH₃PbI₃ have attracted significant attention because of their strong absorption, high carrier mobility, long diffusion lengths, and low fabrication costs. Due to all these advantages, the certified power conversion efficiency (PCE) of perovskite solar cells (PSC) reached over 20% during last years [1]. It is also known that PSC performance demonstrate a specific dependence under varied light intensity from 10 to 1000 W/m² [2]. At the same time the problems of PSC photostability when operating under high humidity conditions are still unsolved.

A possible way to increase the performance and stability of PSC is the modification of the interface properties using surface passivation of mesoporous TiO₂-photoelectrode. The process leads to the decrease of oxygen vacancies at the TiO₂/photoelectrode interface and results in the reducing the interfacial recombination processes. The modification of the perovskite/TiO₂ interface can also impact to the optimization of interface optoelectronic structure and improve the charge transfer efficiency. For this purpose the low cost cadmium sulfide (CdS) interlayer with high electron conductivity could be introduced to the PSC structure. It should be mentioned that CdS is also an excellent hole-blocking material that is sufficient for the improvement of PSC performance.

In this paper we report the application of thin CdS layers between TiO₂-based photoelectrode and perovskite material prepared using chemical bath deposition (CBD) method. It is low cost and low temperature method, which allows to control CdS infliction by alteration of the process parameters such as reagent concentrations, temperature profile, and the deposition time. We have prepared and characterized a series of CdS/TiO₂/compact layer/FTO/glass samples, fabricated at different process parameters using home-made CBD deposition system. The structural, morphological, and optical properties of TiO₂ layers were studied before and after CdS deposition using X-ray diffraction, scanning electron microscopy, and UV-vis NIR analysis. The band alignment at CdS/TiO₂ interface was estimated using X-ray photoelectron spectroscopy. EIS measurements were also conducted. The J-V characteristics and energy conversion efficiencies of PSCs under investigation were measured at AM1.5 incident light illumination (1000 W/m²) using Abet Technologies Solar Simulator and Semiconductor Characterization System 4200-SCS (Keithley, USA). Neutral-density filters were used to perform the characterization of the PSCs at varying illumination intensities ranging from 10 to 1000 W/m². All PSC samples were fabricated under ambient conditions. The effect of the thickness of CdS interlayer on the PSC efficiency was investigated.

We have found that insertion of CdS interlayer in PSCs leads to the increase of the efficiency by around 15% and to the improvement of photostability and stability under ambient conditions. The effects were assigned to the favorable electronic structure of perovskite/CdS/TiO₂ interface and the passivation of the mesoporous TiO₂-photoelectrode surface. The PCE values of PSCs under varied light intensities (> 100 W/m²) were noticeably increased with insertion of sulfide-based interlayer (see picture below). The obtained results are of great interest because they open the opportunity to improve PSC performance under the outdoor conditions.