Perovskite solar cell stability under long term operationAjdič, Žan (Avtor) Jošt, Marko (Mentor) perovskite solar cellslong term stabilityAl2O3parylenemoisturelightstatistical analysismaximum power point trackingPerovskite solar cells are a novel thin-film photovoltaics technology that has gained lots of attention over the last 2 decades. Following extensive device optimization, power conversion efficiency (PCE) now rivals the one of the dominating silicon technology. However, despite their exceptional performance potential, the commercialization of perovskite solar cells remains limited by insufficient long-term stability. The ionic nature of perovskites makes them susceptible to degradation under environmental stressors such as moisture, oxygen, light, and elevated temperatures. Therefore, improving the operational stability and lifetime of PSCs has become a primary research challenge, especially as efficiencies approach theoretical limits. This doctoral research focuses on the p‑i‑n configuration of perovskite solar cells using a formamidinium–cesium‑based (FACs) mixed‑halide perovskite absorber. The FACs devices are known for their robustness and were thus chosen as the main composition tested, focusing on an absorber with the chemical formula FA0.83Cs0.17Pb(I0.83Br0.17)3. The perovskite solar cells had an inverted (p-i-n) planar structure and a layer configuration of glass/ITO/MeO-2PACz/FACs/C60/SnO2/Cu. Overall more than 1500 devices was fabricated with reference devices demonstrating PV performance parameters with median values of open‑circuit voltage (VOC) at 1.09 V, short‑circuit current density (JSC) at 19.9 mA/cm², fill factor (FF) at 74.5%, and power conversion efficiency (PCE) at 16.4%. Champion devices have exceeded 18%. The core of the doctoral dissertation are three published papers. The first one analyzes the role of a thin Al₂O₃ layer deposited by atomic layer deposition. It is investigated both as an ultrathin interfacial passivation layer at three relevant interfaces and as a moisture-blocking capping layer. The influence of Al₂O₃ thickness is systematically studied at the perovskite/C60, C60/SnO2, and SnO2/Cu interfaces. Ultrathin Al₂O₃ layers (1–2 nm) at the perovskite/C60 and C60/SnO2 interfaces improve cell efficiency and water resistance, respectively, while deposition at the SnO2/Cu interface results in an S-shaped J-V curve. When applied as a 30 nm thick capping layer, Al₂O₃ significantly improves resistance to moisture ingress without compromising device efficiency after recovery from high-temperature exposure during the ALD process. The improvement against water ingress is evaluated by visually monitoring degradation when cells are immersed in water. Furthermore, with the capping layer, long-term stability improves at least fourfold compared to non-capped devices. To further enhance long-term stability, Parylene F‑VT4 polymer is investigated as an additional encapsulation barrier. We test three different thicknesses: 2, 5, and 15 µm. Combined Al2O3/parylene encapsulation substantially improves resistance to moisture and prolongs operational lifetimes, with an optimal parylene thickness of 5 µm, which performs best in all tests: water test, damp-heat test, and long-term stability. Compared to the reference capped devices, stability improves by 1350%, 1000%, and 300%, respectively. Excessively thick polymer coatings were found to delaminate and fail mechanically. The Parylene results were also published in a scientific journal. The third paper analyzes in detail long-term stability of perovskite solar cells. For this, a custom white LED (WLED) testing system was developed in the laboratory. This setup enables simultaneous maximum power point tracking (MPPT) of up to 144 solar cells. Stability is quantified using the t80 metric, defined as the time required for the output power to decline to 80% of its maximum value. A comprehensive statistical analysis was conducted on more than 500 perovskite solar cells to ensure robust conclusions beyond single-device demonstrations. We examine the effects of moisture, light intensity, electrical bias voltage, and perovskite composition, particularly the effect of bromide content. In the long-term MPP tracking tests, we first confirm that light is the primary degradation contributor by performing cyclic tests and tests under different light intensities. Photoluminescence measurements reveal photo-induced halide phase segregation in mixed-halide perovskites, leading to band gap redshift and accelerated degradation. Devices without bromide ions exhibit significantly improved stability, with t80 lifetimes increased by up to a factor of five. We also note that during the aging process, perovskite usually degrades faster in electrically inactive areas, where the perovskite is effectively held in open-circuit condition. We show that there is a linear correlation between t80 time and the bias voltage applied to the cell during stability tracking. By testing a large number of samples, we demonstrate with statistical significance that long-term stability measurements show significantly higher spread (both batch-to-batch and intra-batch) than J-V measurements. In the final content chapter, we investigate the effects of different materials used for selected layers, such as the hole transport layer and back contacts, as well as the effects of process or fabrication parameters, including ITO thickness, the type of antisolvent used during perovskite spin-coating deposition, and the amount of PbI2 excess in the perovskite composition. We confirm beneficial effects on long-term stability by switching the back electrode material from copper to silver and by using a different antisolvent (anisole or toluene) instead of typically used ethyl acetate. Too much PbI2 excess accelerates degradation, while introducing a new monolayer – 4PADCB – increases PCE by 1%; however, in some cases, it reduces long-term stability. The results are also not conclusive for the ITO thickness therefore some of the experiments will be repeated together with performing additional test further exploring the effect of cell structure and process parameters on stability. Overall, this doctoral research demonstrates that stable, high-efficiency perovskite solar cells can be achieved through interface engineering, careful material selection, and advanced encapsulation strategies. The systematic, statistically robust approach presented here offers valuable insights into degradation mechanisms and practical methods for extending the operational lifetime of perovskite solar cells, bringing this promising technology closer to commercial viability.20262026-10-02 13:35:04Doktorsko delo/naloga189302sl