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Abstract:

Mobile ions limit halide perovskite device performance, yet quantifying ionic properties remains challenging. Frequency-domain electrical techniques are restricted to operational devices, and the resulting signals are often dominated by interfacial recombination which obscures ionic contributions. Here, we introduce intensity-modulated photoluminescence spectroscopy (IMPLS) as a fully optical alternative, where the amplitude and phase of the photoluminescence intensity is measured as a function of excitation modulation frequency. IMPLS is demonstrated on a Cs_0.07(FA_0.83MA_0.17)_0.93Pb-(I_0.83Br_0.17)₃ film. Fitting the data with an optical equivalent circuit model reveals two characteristic lifetimes: τ_char = 2.1 ms and 77 s, likely corresponding to defect formation and ionic diffusion, respectively. The diffusion feature is consistent with intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements on corresponding full devices. Importantly, IMPLS enables contact-free characterization of slow processes for all perovskite sample types, including films and devices, significantly expanding the techniques available for understanding mobile ions in these materials.

Abstract:

All-perovskite tandem solar cells (2J-PSCs) reach the highest power-to-weight ratios, making them promising candidates for space applications. To determine their potential for future deep space missions, this study assesses the performance of 2J-PSCs under low-intensity and low-temperature (LILT) conditions, akin to those found near Saturn, the Asteroid belt, or in eclipse. Temperature-dependent current density-voltage (J–V) measurements under varying solar intensities (AM0, 0.1 AM0, 0.01 AM0) reveal that the 2J-PSCs, comprising 1.80 eV high-bandgap and 1.27 eV low-bandgap perovskites, exhibit significant efficiency losses at lower temperatures and low light levels. In contrast, 1.54 eV single-junction PSCs (1J-PSCs) exhibit resilient performance, maintaining or even increasing their power conversion efficiency at low temperatures. The main performance problem of the 2J-PSCs is then identified as a demixing of the 1.80 eV perovskite due to its high Br ratio at temperatures below 250 K. This demixing at low temperatures leads to a significant increase in ion-induced performance losses as well as current imbalances between the two subcells in the monolithic tandem. Together, this causes severe S-shapes in solar cell operation and impedes the operation of the monolithic interconnected tandem solar cells. Notably, these limitations vanish upon heating, leading to a recovery of performance.

Abstract:

The stability of perovskite‐based tandem solar cells (TSCs) is the last major scientific/technical challenge to be overcome before commercialization. Understanding the impact of mobile ions on the TSC performance is key to minimizing degradation. Here, a comprehensive study that combines an experimental analysis of ionic losses in Si/perovskite and all‐perovskite TSCs using scan‐rate‐dependent current–voltage (J–V) measurements with drift‐diffusion simulations is presented. The findings demonstrate that mobile ions have a significant influence on the tandem cell performance lowering the ion‐freeze power conversion efficiency from >31% for Si/perovskite and >30% for all‐perovskite tandems to ≈28% in steady‐state. Moreover, the ions cause a substantial hysteresis in Si/perovskite TSCs at high scan speeds (400 s−1), and significantly influence the performance degradation of both devices through internal field screening. Additionally, for all‐perovskite tandems, subcell‐dominated J–V characterization reveals more pronounced ionic losses in the wide‐bandgap subcell during aging, which is attributed to its tendency for halide segregation. This work provides valuable insights into ionic losses in perovskite‐based TSCs which helps to separate ion migration‐related degradation modes from other degradation mechanisms and guides targeted interventions for enhanced subcell efficiency and stability.

Abstract:

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfill ambitions for net-zero carbon dioxide equivalent (CO2eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TWp in 2021 to 8.5 TWp by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the PVs community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.

Abstract:

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin−lead (Sn−Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn−Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn−Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn−Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.