Snaith group

Abstract:

Perovskite inks play critical roles in determining film quality and device performance, and ink stability is desired to ensure high device reproducibility. Here, we reveal the instability issue of current cesium-formamidinium lead triiodide (CsxFA1-xPbI3) inks whose aggregation and precipitation tendencies are induced by excessively strong solvent-lead-halide coordination. By modulating coordination strength between precursor salts and solvents, we identify solvent coordination-dispersion equilibrium as the governing factor for ink stability and develop a stable ink that exhibits a remarkable increase in the shelf life. It effectively tunes ink drying and film crystallization, resulting in blade-coated perovskite films with excellent uniformity and low defect density. This enhancement led to increased aperture efficiency of ambient-fabricated p-i-n perovskite modules to 23.5%. The resultant devices also exhibit high durability, and 99% of the initial PCE was retained after 1700 hours of maximum power point tracking following the ISOS-L-2 standard protocol.

Abstract:

Long-term stability of the perovskite top cell remains a hurdle to commercializing perovskite–silicon tandem (PST) solar cells. While accelerated tests provide valuable insights into degradation kinetics, they fail in predicting real-world degradation behavior. Keeping stressors constant, accelerated tests neglect dynamic conditions in actual operational environments, like diurnal and seasonal temperature and irradiance variability. We address this challenge by integrating a degradation function into our energy yield (EY) modelling software which integrates degradation in collection efficiency (and thus photocurrent) over time due to light and heat exposure, bridging the gap between accelerated testing and in real-world stability assessment. By linking the EY model to measurable material parameters like activation energy governing degradation pathways, this approach enables physically grounded degradation modelling. Based on degradation observed under accelerated tests, the model predicts PST operational lifetimes in diverse climates, highlighting the substantial discrepancy between lifetimes measured under accelerated testing and real-world locations. Applied to a PST solar cell, we show that an operational lifetime (T90,Agg) of about 1400 h under ISOS-L2 (1 Sun, 85 °C), translates to several months in-field (about 26 months in arid Phoenix and 42 months in temperate Seattle), demonstrating strong climate dependence. We also provide a practical mapping from ISOS-L2 to real-world lifetimes and estimate the minimum stability threshold needed for deployment as around 4000 h T90,Agg under ISOS-L2, translating to more than 5 years of operation across the investigated locations. After device-specific parameterization from appropriate aging tests, this device-agnostic framework allows stability-aware EY modelling to predict real-world degradation.

Abstract:

Impurities in formamidinium iodide affect perovskite thin films differently depending on fabrication route. By comparing solution processing with thermal vapour deposition, this study reveals distinct mechanisms in which impurities influence nucleation, growth, and final film quality and stability. While metal halide perovskites have yielded remarkable power conversion efficiencies in photovoltaic applications, uncertainty concerning their long-term stability remains a significant barrier to widespread deployment. Previous studies have demonstrated that trace impurities present in perovskite precursor materials can influence the crystallisation dynamics of perovskite thin-films and hence, affect crystal structure, film morphology and optoelectronic properties. However, the nature of the impurities in formamidinium iodide (FAI) and their effect(s) on film quality and device performance remain underexplored. In this work, we carry out a detailed analysis of the impurities present in commonly used commercial FAI sources, and probe their impact on the composition, structure, and optoelectronic quality of the resulting perovskite thin-films and devices. We find that while some FAI impurities can improve the optoelectronic properties of solution-processed perovskite thin-films, in vapour-processed films, their presence alters the sublimation behaviour of FAI, favouring irreversible degradation pathways which lead to the formation of sym -triazine. While sym -triazine does not directly incorporate into the perovskite films, the impurity-driven variation in sublimation behaviour results in films which can deviate from the target stoichiometry, even under otherwise optimised conditions; and thus, do not fully convert into the desired photoactive phase, eventually causing poor material stability. Our results highlight the importance of understanding and controlling impurity concentrations in perovskite precursor materials as a route to enhancing both performance and process reproducibility in perovskite solar cells.

Abstract:

Perovskite solar cells (PSCs) are emerging as a particularly promising technology to enhance the world’s renewable energy generation capacity. As PSCs are transitioning from research to industrial-scale production, there is an important opportunity to establish sustainable manufacturing pathways. Here, we present a closed-loop framework for the development of environmentally sustainable PSCs and highlight strategies to achieve this vision. First, we analyse the sourcing of raw materials and compare two established PSC fabrication techniques, vapour-phase deposition and solution processing, evaluating their respective advantages and limitations in terms of economic feasibility and environmental impact. Second, we examine solution processing methods, focusing on solvent system design for the preparation of high-quality perovskite films and on the use of non-hazardous or less-hazardous solvents. Third, we examine potential lead-release concerns during PSC operation and discuss approaches to minimize associated environmental risks. Fourth, we summarize effective recycling methods for main PSC components to support a circular production model. Finally, we identify key challenges and outline future research directions to achieve fully sustainable, closed-loop PSC technologies.

Abstract:

The complex and varied relationship found in intermolecular interactions within the photo-active layers plays a decisive role in determining the photovoltaic energy conversion and overall device performance of organic solar cells (OSCs). Among different approaches, the ternary blend strategy serves as an effective technique to control the morphology within the active layer in OSCs. In this work, PM6:L8-BO is used as the main host system (binary) while the fullerene molecules PC61BM and PCBC6 are introduced to form ternary OSCs. The results highlight the important role of fullerenes in enhancing the performance of binary non-fullerene acceptor-based cells by suppressing trap-assisted recombination and optimizing the active layer morphology. The improved film phase microstructure, enabled by fullerene derivatives with higher lowest unoccupied molecular orbital (LUMO) energy levels in comparison to the host acceptor (L8-BO), facilitates more efficient charge collection and reduced non-radiative recombination. This results in an increase in the fill factor (FF) and open circuit voltage (Voc) in the ternary OSCs. Consequently, power conversion efficiencies (PCEs) of binary OSCs were increased from 17.28% to 18.10% and 18.38% for the PC61BM- and PCBC6-based ternary OSCs, respectively. Furthermore, the addition of the fullerene molecules in the active layer provided the devices with enhanced long-term photo and thermal stability. The ternary OSCs demonstrated degradation pathways distinct from those of binary cells (ISOS-L1-I and ISOS-D2-I protocols), as identified through in situ ultraviolet-visible (UV-Vis) absorption and Raman spectroscopy. Molecular dynamics (MD) simulations, for the first time, reveal the significant role of fullerene molecules as morphology regulators in non-fullerene acceptor (NFA)-based systems. Their presence ensures improved dispersion of blend components and promotes more uniform and isotropic thermal and mechanical behaviour. Finally, mini-modules with active areas of 3.8 cm2 were fabricated, achieving PCEs of 12.90%, 13.32%, and 13.70% for the binary and ternary cells using PC61BM-and PCBC6-based ternary cells, respectively. Our results demonstrate that regulation of the morphology of the photo-active layer in OSCs through fullerene incorporation reduces the non-radiative energy loss pathways, enabling high-efficiency, stable and scalable OSCs.