Closing the loop for perovskite solar modules
- Posted by R2ES Laboratory
- On 25 Giugno 2021
- 0 Comments
Perovskite solar cell modules are expected to enter the market in the near future, but their implications in terms of sustainability compared to other photovoltaic devices are still debated. Now a study lays the groundwork for their eco-design.
ince the first demonstration in 2009, perovskite solar cells (PSCs) have emerged as a disruptive class of photovoltaic (PV) technologies that have progressed so rapidly1 that their commercial readiness is expected in the very near term2. Compared to current established silicon and other thin-film PV technologies, PSCs are highly competitive due to a favourable combination of characteristics including tunable band gap, strong absorption and high-power conversion efficiency (PCE). Furthermore, the large availability and low cost of raw materials and simple solution-based processing techniques for their manufacturing, the flexibility of structural configuration and compatibility with several supporting materials3 complete the outline of this promising technology. Writing in Nature Sustainability, Tian and co-workers4 implement a full life cycle assessment (LCA) to track the environmental impacts of six state-of-the-art perovskite solar modules (PSMs) along the whole product value chain. As a result, this study presents a detailed analysis of the materials, components, processes and life cycle phases that are all major factors driving the environmental footprint of PSMs. Technologies exploiting renewable energy sources are not always intrinsically sustainable, especially if they are not designed based on energy and material efficiency criteria. The life cycle environmental footprint of PV technologies is primarily determined by the manufacturing and end-of-life (EoL) phases, as the environmental impacts generated during the operational phase can be reasonably considered negligible. Thus, these two phases matter most to the sustainability of PV devices and emerging PV technologies5. In order to achieve more sustainable devices, we need to conceive a system value chain in compliance with the circular economy principles: keeping materials and products on the market as long as possible and, after EoL, recycling, reusing or transforming them efficiently so that actual waste is minimized, and valuable resources are recovered to the maximum (Fig. 1). We can accomplish that by practicing eco-design: applying LCA for the assessment of the environmental and economic burden associated with all technological options to design the product value chain ‘from cradle to grave’. Using sustainability-relevant metrics like primary energy use, carbon footprint, energy pay-back time (EPBT), greenhouse gases (GHG) emission factors and levelized cost of energy, such an approach allows identifying the best set of materials and processes to obtain the most sustainable eco-profile depending on the final use. In this context, recycling, and recovery EoL solutions play a crucial role, and LCA implementation to anticipate economic and environmental hotspots can support more transparent and evidence-based decisions to promote the development of eco-friendly technologies. The added value of the work by Tian and colleagues is the focus on the environmental, energy and economic advantages associated with various materials recycling options that are integrated in the life cycles of two commercial-scale single-junction modules, and four laboratory-scale modules selected due to their promising module efficiency and stability. Compared with the recycling approaches for commercialized PV technologies, strategies for recycling PSCs materials are much more straightforward since they take advantage of the selective dissolution properties of the perovskite and transport layers6. Furthermore, substrates can be recovered and reused directly, avoiding grinding before reusing. Metal electrodes can also be directly retrieved after cleaning if they are chemically stable and insoluble in the chosen solvent. From a closed-loop recycling perspective, fabricating PSC solar modules with recycled materials that are identified as environmental hotspots in manufacturing PSCs might allow substantial improvement ranging from 50% up to 90% both in terms of primary 755news & viewsenergy consumption (embedded in raw materials and consumed as direct energy in the assembling processes) and in carbon footprint values compared to using virgin materials, depending on the configuration of solar modules.Building on these results and further performing a sensitivity analysis on the recycling levels of critical steps, such as the use of solvents7, substrates and metal electrodes3, Tian and colleagues pinpoint the need for developing alternative strategies. They also illustrate how filling the gap between available techniques for material recovery and the potentiality of the optimum recovery practice can help lowering the EPBT (between 34% and 75%) and GHG emission factors (between 41% and 73%). As the recycling strategies are modelled based on laboratory-scale processes, they will be subject to further optimization during industrial scale-up. However, the results provide conservative estimates and serve as upper bounds on the prevalent sustainability metrics.For commercial-scale module design, trade-off relationships (such as those among cost-effectiveness, large availability, versatility and recycling of raw materials) should always be carefully analysed in order to select proper materials when looking for improved PCE and effective recycling. One example is the evaluation of the solar cell substrate that can be made of fluorine- or indium-doped tin oxide (FTO and ITO) glass that is critical to the eco-profiles of perovskite solar modules. Indeed, even if FTO is a cheap and low energy-intensive material, it represents a sizable contribution in terms of the device mass being the support of PSCs. Due to complex procedures and low material recovery rate in the recycling process, FTO is a hotspot that we should carefully consider in terms of environmental implications and to find a ‘breakeven’ point and make a balanced choice.Although this study provides an informative picture and inspiring results, the environmental implications associated with the use of lead8 for the best-performing PSCs, and the encapsulation methods for improving stability and prolonging lifetimes9 or consolidating PSCs’ PV performances10, need to be modelled more precisely and robustly. But ultimately, the valuable work of Tian et al. tells us that to achieve the challenging goal of sustainable manufacturing of PV technologies, the search for improved stability and PCE must be supported by economic and environmental LCA studies to boost eco-efficiency.