title: Interface and material engineering for highly stable and efficient halide perovskite solar cells.
Creators: name: Tabi, Grace
Publisher: name: The Australian National University
Year: 2024

description: The field of halide perovskite solar cells (PSCs) has witnessed remarkable progress, holding great promise for clean and efficient energy generation. Nonetheless, addressing issues related to stability and performance is essential for their widespread commercial viability. This thesis focuses on developing interface and material engineering systems to enhance the performance and reliability of halide PSCs. Here are the key findings: 1. We explore the potential of LiI doping in mixed-cation mixed-halide PSCs, aiming to address critical questions raised in the existing literature regarding the influence of extrinsic ions, such as lithium and iodide ions, on PSC performance, ion migration, and hysteretic behaviour. Our findings reveal that lithium ions can be incorporated into the interstitial positions of the perovskite crystal structure at low LiI doping concentrations. However, at higher LiI doses, lithium ions segregate into a non-perovskite phase. Furthermore, we propose the first study of ion migration through the changes in the transient behaviour within the LiI doped devices using chronopotentiometry (CP) measurements. The observed scan-dependent hysteresis is intricately linked with higher LiI dosages and their corresponding alterations in transient ionic responses, as confirmed through CP measurements and numerical simulations. Our optimized LiI formulations achieved improved film quality, reduced recombination losses, and limited ion migration. Consequently, optimal LiI doped PSCs attained a power conversion efficiency (PCE) of 21.5%, outperforming the reference cells at 20.4% PCE, all while minimizing current-voltage hysteresis and enhancing ambient stability. 2. Long-term stability of PSCs remains a significant challenge in PSCs. Our research aims to bridge the gap between efficiency and stability by exploring the impact of incorporating poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TRFE) as an additive in the 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) hole transport layer (HTL). P(VDF-TRFE) forms robust coordination bonds with the HTL critical components, mitigating 4-tert-butylpyridine (TBP) evaporation and improving film quality. This integration leads to increased conductivity and a reduction in non-radiative recombination at the HTL/perovskite interface, resulting in efficient charge transport. The inclusion of P(VDF-TRFE) significantly enhances PCE, reaching 24.1% from a reference PCE of 21.4%. Unencapsulated devices with P(VDF-TRFE) demonstrate enhanced stability, retaining over 90% PCE after 45 days in ambient conditions in the dark and 94% PCE after 1080 hours of continuous light-soaking in a nitrogen environment. 3. Low conductivity in some passivation layers of PSCs can offset efficiency gains. To address this issue, we explore localized contact strategies inspired by their success in silicon photovoltaics. We chose a 1.65 eV wide bandgap and a constant bulk recombination rate for tandem application suitability. Leveraging advanced 3D numerical device models, we optimized contact geometries. This strategy can achieve PCEs of 24% (28% PCE with bandgap = 1.55 eV) for 5nm hole sizes at a high defect density of 2.5 x 10^12 cm^-2 compared to 14% PCE of the planar device by reducing losses at the recombination-active interface while maintaining a balance between open-circuit voltage and fill factor. In addition, we demonstrate that a large hole size of 1000 nm radius and pitch of ~9000 nm, compatible with industrial fabrication processes, is feasible with the localized contact strategy to achieve a high efficiency of over 23%. Importantly, our study provides valuable design guidelines for both small- and large-area fabrication processes. In summary, this research pioneers engineering systems and material enhancements in halide PSCs, providing a foundation for highly stable and efficient renewable energy solutions to meet global energy demands.