Search

With continuously increasing power conversion efficiency, metal halide perovskite solar cells have emerged as promising candidates for high-efficiency silicon based tandem solar cells in two-terminal monolithic integration and four-terminal mechanical stack architectures. The stability of perovskite solar cells is currently one of the major challenges for perovskite/silicon tandem devices and is improving rapidly. However, different degradation rates of perovskite cells and silicon cells in a tandem solar module can affect the overall module degradation. The lifetime energy yield and economic viability of perovskite/silicon tandem modules strongly depend on the degradation rates of perovskite cells. In this paper we present a simulation study of the long term power and energy yield of perovskite/silicon tandem modules under different perovskite cell degradation scenarios. We also estimate the efficiency and cost requirements for the economic feasibility of two- and four-terminal tandem modules. We determine that to maintain 80% of the initial power in a tandem module after 25 years, the maximum permissible perovskite top cell degradation rates are 0.9% per year in a two-terminal configuration and 1.3% per year in a four-terminal configuration for a realistic perovskite cell degradation scenario. We project that a future perovskite/silicon tandem module can produce over 10% more lifetime energy than a single-junction silicon module in 2025 assuming a tandem cell efficiency reaches of 28% and a modest perovskite cell degradation rate of 2% per year. Finally, we estimate the levelized cost of energy for both two- and four-terminal tandem modules. In the case of a degradation rate of 2% per year of the perovskite cell and 50% additional cost for the tandem structure compared to single-junction modules, we find that power conversion efficiencies of 28.7% and 27.6% enable the economic viability of two- and four-terminal perovskite/silicon tandem modules. Our study demonst

The stability of perovskite cells has been the major challenge to the commercialization of the technology. Here, we study the thermal stability of mesoporous perovskite cells using poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) for the hole transport layer. We find evidence that gold diffusion into the perovskite active layer is prevented by the PTAA layer when the cell is aged at 85~{mathbf {o}}C in the dark, but not under light. This seriously affects the operational stability of perovskite cells. We therefore develop thermally stable semi-transparent perovskite cells, which retain more than 90% of the initial efficiency after 160 hours operating at 85 {mathbf {o}}\textbf{C} under light.

The realization of very high efficiency, stable perovskite solar cells fabricated on a large scale at low cost, has the potential to further lower the cost of photovoltaics. This necessitates an understanding of the properties required of the perovskite material, including the carrier mobility. Perovskite cells also feature mobile ionic species, and the impact of these ions on cell performance- A nd in particular, to what extent and under what circumstances they may limit device performance-is not well understood. Here, we employ an advanced numerical model that allows for the presence of mobile ionic species to probe the relationship between carrier mobility, the presence of ionic species as well as different possible recombination mechanisms within the cell. We show that a high electron and hole conductivity throughout the device is key to avoiding transport losses. For devices operating significantly below their radiative limit, achieving a sufficiently high conductivity requires high carrier mobilities of at least 10cm2/V-s. It is shown that the presence of a single mobile ionic species can lead to effective doping of the perovskite bulk, which is detrimental to cell performance by lowering the conductivity of one type of carrier. The results also indicate that increasing cell VOC closer to its radiative limit is also beneficial for reducing transport losses and pushing cell performance closer to its theoretical limit.

Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Here, we describe a two-terminal perovskite/Si tandem design that increases the Si cell’s output in the simplest possible manner: by placing a perovskite cell directly on top of the Si bottom cell. The advantageous omission of a conventional interlayer eliminates both optical losses and processing steps and is enabled by the low contact resistivity attainable between n-type TiO2 and Si, established here using atomic layer deposition. We fabricated proof-of-concept perovskite/Si tandems on both homojunction and passivating contact heterojunction Si cells to demonstrate the broad applicability of the interlayer-free concept. Stabilized efficiencies of 22.9 and 24.1% were obtained for the homojunction and passivating contact heterojunction tandems, respectively, which could be readily improved by reducing optical losses elsewhere in the device. This work highlights the potential of emerging perovskite photovoltaics to enable low-cost, high-efficiency tandem devices through straightforward integration with commercially relevant Si solar cells.

It is highly probable that perovskite solar cells (PSCs) are mixed electronic-ionic conductors, with ion migration being the driving force for PSC hysteresis. However, there is much that is not understood about the interaction of ion migration with other processes in the cell. The key question is: what factors of a PSC are influenced when ions are free to move? In this contribution, we employ a numerical drift-diffusion model of PSCs to show that the migration of both anions and cations in interaction with trap-mediated recombination in the bulk and/or at the surfaces of the perovskite absorber can manifest both current-voltage hysteresis and unusual nonmonotonic PSC photovoltage transients. We identify that a key mechanism of this interaction is the influence of the net ionic charge throughout the perovskite bulk - which varies as the ions approach new steady-state conditions - on the distribution of electrons and holes and subsequently the spatial distribution of trap-mediated recombination modeled after Shockley Read Hall (SRH) statistics. Relative to intrinsic recombination mechanisms, SRH recombination can be highly sensitive to local asymmetries of the electron-hole population. We show that this sensitivity is key to replicating nonmonotonic transients with multiple time constants, the forms of which may have suggested multiple processes. This work therefore supports the conceptualization of the hysteretic behavior of PSCs as dominated by the interplay between ion migration and trap-mediated recombination throughout the perovskite absorber.

Multi-junction tandem design has been proven to be an effective means to further improve the efficiency of solar cells. However, its share in the photovoltaics market at present is tiny, since the most efficient tandem device comprises III-V semiconductors, which entail the use of expensive fabrication processes. The advent of perovskite solar cells, which have revitalized the PV field with their unprecedented pace of development, promises to address this bottleneck. Perovskite materials could not only serve as the top subcell absorber for commercial solar cells including Si and copper indium gallium selenide, but could work efficiently as bottom subcells owing to highly tuneable bandgaps which extend down to the range of ~1.2 to 1.5 eV. The highest-efficiency perovskite tandem to date was achieved by pairing a perovskite top cell with a Si bottom cell in a four-terminal configuration, yielding 26.4%. This review gives an overview of recent progress on the main tandem structures, and describes the detailed design improvements that have resulted in new record efficiencies. Ultimately, commercialization of these tandem solar cells relies on the scalability of perovskite technology. We, therefore, highlight the development of large-scale tandems and approaches to produce perovskite modules. We also point out the critical aspects that will require further effort and provide guidelines for future developments. The potential obstacles that will hamper the commercialization of perovskite tandems, if not adequately addressed, namely device stability and toxicity, are then critically examined. Finally, the substantial opportunities that perovskite materials open up for other solar devices with a tandem configuration are mentioned, which are attracting increasing attention.

Interfacial carrier recombination is one of the dominant loss mechanisms in high efficiency perovskite solar cells, and has also been linked to hysteresis and slow transient responses in these cells. Here we demonstrate an ultrathin passivation layer consisting of a PMMA:PCBM mixture that can effectively passivate defects at or near to the perovskite/TiO2 interface, significantly suppressing interfacial recombination. The passivation layer increases the open circuit voltage of mixed-cation perovskite cells by as much as 80 mV, with champion cells achieving Voc ∼ 1.18 V. As a result, we obtain efficient and stable perovskite solar cells with a steady-state PCE of 20.4% and negligible hysteresis over a large range of scan rates. In addition, we show that the passivated cells exhibit very fast current and voltage response times of less than 3 s under cyclic illumination. This new passivation approach addresses one of the key limitations of current perovskite cells, and paves the way to further efficiency gains through interface engineering.