Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency.

Tandem devices combining perovskite and silicon solar cells are promising candidates to achieve power conversion efficiencies above 30% at reasonable costs. State-of-the-art monolithic two-terminal perovskite/silicon tandem devices have so far featured silicon bottom cells that are polished on their front side to be compatible with the perovskite fabrication process. This concession leads to higher potential production costs, higher reflection losses and non-ideal light trapping. To tackle this issue, we developed a top cell deposition process that achieves the conformal growth of multiple compounds with controlled optoelectronic properties directly on the micrometre-sized pyramids of textured monocrystalline silicon. Tandem devices featuring a silicon heterojunction cell and a nanocrystalline silicon recombination junction demonstrate a certified steady-state efficiency of 25.2%. Our optical design yields a current density of 19.5 mA cm-2 thanks to the silicon pyramidal texture and suggests a path for the realization of 30% monolithic perovskite/silicon tandem devices.

Interface passivation for 31.25%-efficient perovskite/silicon tandem solar cells.

Silicon solar cells are approaching their theoretical efficiency limit of 29%. This limitation can be exceeded with advanced device architectures, where two or more solar cells are stacked to improve the harvesting of solar energy. In this work, we devise a tandem device with a perovskite layer conformally coated on a silicon bottom cell featuring micrometric pyramids-the industry standard-to improve its photocurrent. Using an additive in the processing sequence, we regulate the perovskite crystallization process and alleviate recombination losses occurring at the perovskite top surface interfacing the electron-selective contact [buckminsterfullerene (C60)]. We demonstrate a device with an active area of 1.17 square centimeters, reaching a certified power conversion efficiency of 31.25%.

Understanding and Mitigating the Degradation of Perovskite Solar Cells Based on a Nickel Oxide Hole Transport Material during Damp Heat Testing.

The development of stable materials, processable on a large area, is a prerequisite for perovskite industrialization. Beyond the perovskite absorber itself, this should also guide the development of all other layers in the solar cell. In this regard, the use of NiOx as a hole transport material (HTM) offers several advantages, as it can be deposited with high throughput on large areas and on flat or textured surfaces via sputtering, a well-established industrial method. However, NiOx may trigger the degradation of perovskite solar cells (PSCs) when exposed to environmental stressors. Already after 100 h of damp heat stressing, a strong fill factor (FF) loss appears in conjunction with a characteristic S-shaped J-V curve. By performing a wide range of analysis on cells and materials, completed by device simulation, the cause of the degradation is pinpointed and mitigation strategies are proposed. When NiOx is heated in an air-tight environment, its free charge carrier density drops, resulting in a band misalignment at the NiOx/perovskite interface and in the formation of a barrier impeding hole extraction. Adding an organic layer between the NiOx and the perovskite enables higher performances but not long-term thermal stability, for which reducing the NiOx thickness is necessary.

Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells.

Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.

Light Management: A Key Concept in High-Efficiency Perovskite/Silicon Tandem Photovoltaics.

The remarkable recent progress in perovskite photovoltaics affords a novel opportunity to advance the power conversion efficiency of market-dominating crystalline silicon (c-Si) solar cells. A severe limiting factor in the development of perovskite/c-Si tandems to date has been their inferior light-harvesting ability compared to single-junction c-Si solar cells, but recent innovations have made impressive headway on this front. Here, we provide a quantitative perspective on future steps to advance perovskite/c-Si tandem photovoltaics from a light-management point of view, addressing key challenges and available strategies relevant to both the 2-terminal and 4-terminal perovskite/c-Si tandem architectures. In particular, we discuss the challenge of achieving low optical reflection in 2-terminal cells, optical shortcomings in state-of-the-art devices, the impact of transparent electrode performance, and a variety of factors which influence the optimal bandgap for perovskite top-cells. Focused attention in each of these areas will be required to make the most of the tandem opportunity.

Probing Photocurrent Nonuniformities in the Subcells of Monolithic Perovskite/Silicon Tandem Solar Cells.

Perovskite/silicon tandem solar cells with high power conversion efficiencies have the potential to become a commercially viable photovoltaic option in the near future. However, device design and optimization is challenging because conventional characterization methods do not give clear feedback on the localized chemical and physical factors that limit performance within individual subcells, especially when stability and degradation is a concern. In this study, we use light beam induced current (LBIC) to probe photocurrent collection nonuniformities in the individual subcells of perovskite/silicon tandems. The choices of lasers and light biasing conditions allow efficiency-limiting effects relating to processing defects, optical interference within the individual cells, and the evolution of water-induced device degradation to be spatially resolved. The results reveal several types of microscopic defects and demonstrate that eliminating these and managing the optical properties within the multilayer structures will be important for future optimization of perovskite/silicon tandem solar cells.