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Contribution of the different components of the photovoltaic power plant to the life cycle greenhouse gas emissions per kWh of low voltage electricity produced, at inverter; with installation on a rooftop in Central Europe with an electricity yield of 919 kWh per kWp and year including average degradation of 10.5 % with a lifetime of 30 years; *optimistic lifetime of 30 years for PSC layer. • Key parameters for the environmental impacts are the module efficiency and lifetime of the modules as well as the degradation rate of the cell efficiency • For GHG emissions and Energy Payback Time the deciding factor is the electricity demand during manufacturing (deposition process) • The toxicity impacts of PSC solar cells are related to the use and emission of heavy metals (mainly Pb and Sn) • Resource depletion is dominated by the use of indium for transparent conductive oxides (TCO ) for SHJ and PSC solar cells, current mono-Si and poly-Si cells do not utilise indium containing TCOs and cause lower resource depletion • 3rd generation solar cells using perovskites have the potential for improved performance compared to current photovoltaic technologies if the cells can be stabilised Matthias Stucki and René Itten Zurich University of Applied Sciences, Institute of Natural Resource Sciences Grüental, 8820 Wädenswil, Switzerland matthias.stucki@zhaw.ch 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 Mono-Si REF, eff: 16.5% Mono-Si ITRPV, eff: 26% Poly-Si REF, eff: 16% Poly-Si ITRPV, eff: 20% *PSC PESS, eff: 15% *PSC OPT, eff: 20% *SHJ-PSC PESS, eff: 26% *SHJ-PSC OPT, eff: 30% Greenhouse gas emissions in kg CO 2 -eq per kWh electricity Inverter Mounting system Module production Cell production PSC Layer SHJ Layer Wafer Rest 0% 50% 100% 150% 200% 250% 300% 350% 400% 5 10 15 20 25 30 GHG emissions per kWh of electricity Lifetime perovskite (PSC) in years Mono-Si REF, eff: 16.5% Mono-Si ITRPV, eff: 26% Poly-Si REF, eff: 16% Poly-Si ITRPV, eff: 20% PSC PESS, eff: 15% PSC OPT, eff: 20% SHJ-PSC PESS, eff: 26% SHJ-PSC OPT, eff: 30% Introduction and Methods Figure 3. Life cycle greenhouse gas emissions per kWh of low voltage level electricity produced, at inverter, depending on the lifetime relative to mono-Si-REF silicon with a given lifetime of 30 years; installation on a rooftop in Central Europe with an electricity yield of 919 kWh per kWp and year including average degradation of 0.7 % per year; lifetime variable for PSC and SHJ-PSC tandem. Prospective Scenarios and Results Table 1. Summary of different prospective scenarios with abbreviation, technology, parameters for cell and module efficiency, wafer thickness, kerf loss and description including references for parameter values [1–10] The model approach applied uses process-based LCA data in combination with attributional allocation. The key parameters for wafer based crystalline silicon technologies are subject to prospective future scenarios based on expected trends. A similar modelling approach was applied in Louwen et al. [2], Frischknecht et al. [3] and Rufer & Braunschweig [4]. These key parameters were modelled based on future projections in the International Technology Roadmap for Photovoltaics (ITRPV) for mono- Si single-junction solar cells [5], Burschka et al [6] and Yang et al. [7] for non-bifacial perovskite single-junction cells and Werner [8], Albrecht et al. [9], Bush et al. [10] and Almansouri et al. [11] for monolithic two terminal SHJ-PSC tandem cells. The parameters for the different solar cell types have been summarised in Table 1. A relative decrease in efficiency of 8.5 % from cell to module was assumed for all solar cell types. This corresponds to the current cell to module efficiency ratio for mono-Si solar cells [12]. In this study, the environmental impacts of monolithic silicon heterojunction organometallic perovskite tandem cells (SHJ-PSC) and single junction organometallic perovskite solar cells (PSC) were compared with the impacts of crystalline silicon based solar cells using a prospective life cycle assessment with a time horizon of 2025. This approach provides a result range depending on key parameters like efficiency, wafer thickness, kerf loss, lifetime and degradation, which are appropriate for the comparison of these different solar cell types with different maturity levels. Abbre- viation Technology Efficiency in % Thickness in micrometer Description Cell Module Wafer Kerf Mono- Si REF Mono-crystalline silicon, single-junction 16.5 15.1 295 145 Reference scenario for the current market average according to IEA PVPS [12] Mono- Si ITRPV Mono-crystalline silicon, single-junction 26.0 23.8 140 60 Future scenario according to the ITRPV [5] Poly-Si REF Poly-crystalline silicon, single-junction 16.0 14.7 295 145 Reference scenario for the current market average according to IEA PVPS [12] Poly-Si ITRPV Poly-crystalline silicon, single-junction 20.0 18.3 150 60 Future scenario according to the ITRPV [5] PSC PESS Perovskite single-junction 15.0 13.8 n.a. n.a. Pessimistic scenario with low efficiency for pervovskite single-junction cell [6] PSC OPT Perovskite single-junction 20.0 18.3 n.a. n.a. Optimistic scenario with high efficiency for perovskite single-junction cell [7,11,15] SHJ- PSC PESS Monolithic two terminal tandem cell using perovskite and silicon heterojunction tandem 26.0 23.8 295 145 Pessimistic scenario with low efficiency for monolithic two terminal tandem cell using perovskite and silicon heterojunction tandem cell [8–10] SHJ- PSC OPT Monolithic two terminal tandem cell using perovskite and silicon heterojunction tandem 30.0 27.5 120 60 Optimistic scenario with low efficiency for monolithic two terminal tandem cell using perovskite and silicon heterojunction tandem cell [11,16] Figure 3. Schematic representation of the silicon heterojunction perovskite tandem cell (Bush et al. [10])