Identification and Mitigation of Degradation Mechanisms Induced by Heat and Light in NiOx-based Perovskite Solar Cells

Perovskite solar cells (PSCs) offer an attractive solution to increase the efficiency of PV panels via perovskite/silicon tandems thanks to their potential cost-effectiveness and high efficiency. In inverted PSC architecture (p-i-n), commonly used hole contact materials include NiOx, self-assembled monolayers (SAMs), and semiconducting polymers. With the aim of bringing tandem solar cells to the commercial market, NiOx stands out as the preferred choice due to the ease of scalability and uniform coverage achievable by sputtering on both flat and textured substrates. Additionally, NiOx exhibits advantageous optoelectronic characteristics, thanks to its wide bandgap, charge carrier selectivity and minimal parasitic absorption losses. The commercialization of tandem solar cells demands rigorous testing, including damp heat and light soaking tests. However, PSCs are susceptible to various degradation mechanisms when exposed to elevated temperatures, light, or humidity. This thesis aims to investigate these degradation mechanisms and propose some mitigation strategies.NiOX-based PSCs stability under damp heat testing was first investigated. It was found that the presence of NiOx was responsible for the degradation at elevated temperatures in absence of oxygen. Thicker NiOx layers exacerbate the problem, leading to S-shaped J-V curves. Mitigation strategies, such as NiOx thickness reduction and the use of SAM interlayers, successfully limit degradation, with encapsulated devices retaining 94% of their initial efficiency after 1000 hours of damp-heat testing.Alternatively, we show that a modified sputtering process in an argon-oxygen atmosphere and the introduction of cesium into NiOx resulted in a reduction of Ni3+ species, a smoother surface and denser NiOx layer, ensuring less than a 5% relative efficiency change in PSCs after more than 5000 h of test, complying with IEC 61215 damp-heat test requirements.Moreover, the thesis investigated the light-soaking stability, the most challenging test for PSCs, especially at elevated temperatures. It identifies that perovskite composition plays a key role in device degradation. Excess PbI2 accelerates perovskite decomposition, ionic movements can cause halide segregation, structural damage (amorphization), and formation of non-photoactive phases. Besides, light and heat can accelerate the loss of volatile perovskite components, even in the presence of a SnO2 ALD barrier layer and an ITO electrode. Inhomogeneous electric field can lead to perovskite degradation, particularly at the device boundaries. Redox reactions at the NiOx/perovskite interface pose a significant problem and metal electrodes are vulnerable to chemically react with the I2 vapor. Finally, the study also revealed that the incorporation of an additive used for perovskite/C60 interface defects passivation, as well as the use of a hydrophilic SiOx nanoparticles layer on the SAM/perovskite interface (used as wetting agent), can result in significant degradation.In conclusion, achieving long-term operational stability in PSCs necessitates meticulous material control, effective interface engineering, and protective layers implementation. The thesis provides some valuable insights into these aspects.