Bulk and Surface Modifications for Achieving Highly Efficient Perovskite-Silicon Tandem Solar Cells

Abstract

The efficiency of perovskite-silicon tandem solar cells has rapidly increased in recent years, yet challenges in material stability, defect passivation, and scalability remain barriers to commercialization. This thesis explores bulk and surface passivation strategies alongside scalable deposition techniques to enhance the performance of 1.67 eV wide-bandgap perovskite solar cells suitable for tandem integration. The fabrication was carried out using a hybrid deposition approach, where the perovskite absorber was formed via co-evaporation of inorganic precursors followed by solution-based organic cation deposition. A 1:0.1 PbI₂:CsBr co evaporation ratio, found to provide better conformality on textured silicon substrates in another study, was adopted, ensuring alignment with tandem device requirements. In the initial phase, spin-coating was used to evaluate bulk and surface passivation strategies for 1.67 eV perovskite films. 2,3,4,5,6-Pentafluorobenzyl phosphonic acid (pFBPA) was tested as a bulk additive, but its effects on film crystallinity and device performance required further study. In terms of surface passivation, pFBPA, Piperazinium Iodide (PI), and Propane-1,3 Diammonium Iodide (PDAI₂) were investigated, with trends in Voc enhancement but reductions in other performance parameters, ultimately lowering device efficiency. Additionally, for PI, a rinsing step was introduced to assess its impact on passivation effectiveness. However, batch-to batch variability in spin-coated devices led to a transition to blade-coating, a more scalable deposition method offering improved film uniformity and process control. With blade-coating, the perovskite precursor concentration was optimized to achieve the highest possible efficiency for the baseline device. 0.41 M was identified as the most effective concentration. Additionally, to maintain the 1.67 eV bandgap, the organohalide concentration ratio was adjusted during blade-coating, ensuring bandgap consistency across samples. MACl, which was included in the precursor solution during spin-coating, was further investigated under blade-coating, where it was found to enhance crystallinity, increase grain size, and passivate defects, leading to longer charge carrier lifetimes. Solvent engineering for blade-coating was explored, comparing isopropanol (IPA) with and without N-methyl pyrrolidone (NMP) additions. 1% NMP initially improved device performance, but prolonged illumination studies revealed ion migration effects, leading to bandgap shifts and stability concerns. PDAI₂ was revisited for blade-coated surface passivation. Higher concentrations led to insulating effects, while lower concentrations showed incomplete coverage. A rinsing step was introduced, which mitigated unwanted film aggregation and improved device performance. These results highlight the importance of precise control over passivation strategies, deposition conditions, and solvent selection in optimizing perovskite films for tandem integration. The study provides key insights into defect mitigation, scalable processing, and film stability, laying the groundwork for further advancements in high-efficiency, industrially viable perovskite-silicon tandem solar cells.