Perovskite Solar Cells

Solar energy is set to play a big role in future energy generation and achieving climate change goals [1]. At the moment, silicon-based technologies dominate the market [2]. However, recent events have illustrated the vulnerability of supply chains, and any disturbance to silicon supply lines could seriously endanger the feasibility of achieving Net Zero by 2050. The perovskite solar cell (PSC) field has progressed tremendously in the decade since the seminal papers by Miyasaka [3] and Park [4] first explored lead halide perovskites as light-absorbing materials in solar cells. Lab-scale efficiencies of >25% have been achieved, rivalling silicon. [5] In addition, halide perovskites are finding use in multi-junction solar cells [6] and other optoelectronic applications, such as LEDs [7] and photodetectors [8]. The rapid progress on halide perovskites was largely possible due to the simplicity with which these materials can be (solution) processed, allowing many research groups around the world to contribute to the development of PSCs. Stability, lead toxicity and cost (both environmentally and economically) have drawn significant attention as factors to improve towards commercialization, and good progress has been made in all these aspects. However, further improvements are still needed for PSCs to claim a spot among the commercially successful PV technologies [9].

The presence of lead in halide perovskites may be a barrier towards their commercialization, as lead is toxic and has been banned from products such as petrol and paint [10]. A popular replacement for lead is tin, which has shown promising device performance [11]. The review article “The Low-Dimensional Three-Dimensional Tin Halide Perovskite: Film Characterization and Device Performance” authored by C. Gai, J. Wang, Y. Wang and J. Li provided a constructive review on the use of small amounts of 2D perovskite to stabilise and enhance the efficiency of 3D tin perovskites [12]. They reported that a surface layer of 2D perovskite can protect tin perovskites from oxidation and simultaneously passivate surface traps, leading to a significantly higher open circuit voltage and device stability. Nevertheless, the authors concluded that tin perovskites still have a long way to go before they can rival Pb-based perovskites.

L.-C. Chen, C.H. Tien, Y.-C. Jhou and W.-C Lin published an article on mixed lead/tin perovskite entitled “Co-Solvent Controllable Engineering of MA_0.5FA_0.5Pb_0.8Sn_0.2I₃ Lead–Tin Mixed Perovskites for Inverted Perovskite Solar Cells with Improved Stability” [13]. The authors modified the archetypical lead halide perovskite MAPbI₃ (MA = methyl ammonium), replacing MA partly with FA (formamidinium) to increase stability and partially replace lead with more environmentally friendly tin. They found that the ratio of dimethylformamide to dimethyl sulfoxide in the precursor solution is key to achieving good film morphology and efficient devices.

The long term stability of PSCs is another key area of improvement towards commercialising PSCs. Transport layers can play an important role in providing environmental stability [14]. Z. Shadrokh, S. Sousani, S. Gholipour, Z. Dehghani, Y. Abdi and B. Roose presented a paper entitled “Stannite Quaternary Cu₂M(M = Ni, Co)SnS₄ as Low Cost Inorganic Hole Transport Materials in Perovskite Solar Cells” [15]. The authors presented a facile and mild synthesis method for earth abundant inorganic hole transport materials (HTMs). The authors found that Cu₂NiSnS₄ is a promising material for use in PSCs, showing encouraging efficiency and enhanced environmental stability compared to organic HTMs. They predicted that the HTM can be further optimised by performing compositional engineering.

When moving towards commercialisation, it is also important to assess what costs (environmental and economic) are associated with each component of the PSC, and decide what materials are suitable for large-scale production. [16] The article “Emerging Photovoltaic (PV) Materials for a Low Carbon Economy” authored by I. Celik, R. H. Ahangharnejhad, Z. Song, M. Heben and D. Apul attempted to do just this [17]. The authors found that the perovskite absorber is the most eco-efficient material in the whole device stack. ITO is identified as an environmental hotspot and organic HTMs are the most expensive materials. Commercial PSCs will likely have to find alternatives for these materials.

Much research has been carried out in recent years to tackle the remaining hurdles for bringing PSCs to the market. Now that PSCs are showing more and more convincingly that they can fulfill stability and cost requirements needed for use in the field [18], it is expected that industry will start to play a much bigger role in future advances in the field. Several companies are in advanced stages of development and it is likely that we will see the first PSC-based products on the market within the next five years. The focus of these companies is as diverse as the perovskite material itself, as some take advantage of the low processing temperature of perovskites to produce lightweight or flexible modules, while others use the tunability of the perovskite bandgap to construct tandems, which can harvest a larger portion of the solar spectrum more efficiently, or are exploiting high-throughput roll-to-roll processes for large-scale manufacturing, aiming to rival silicon. Industry seems less concerned about lead content in PSCs than the academic community, citing research that lead is only a small contributor to environmental impacts [18] and having faith in module packaging that not only keeps oxygen and moisture out, but also lead inside of the module. It is also important to note that the lead content in PSCs is below regulatory limits for standard modules. However, for lightweight applications this is not the case, and lead will have to be effectively captured at the end of life of modules in all cases [19]. Another challenge then is managing the public opinion by presenting convincing evidence that the benefits of PSCs far outweigh potential negative impacts. Advocates of tin-based perovskites have their work cut out for them, as they will need to match the efficiency and stability of their lead-based counterparts before industry seriously considers them. In addition, tin is far from benign itself [20].

The future for PSCs is looking very promising. Having made phenomenal progress in the last decade through academic research, they have now built up significant backing by industry. PSCs are on track to contribute to establishing a clean and sustainable energy future, and making sure that this bright future is also obtainable for those living in low- and lower-middle income countries [21].

This Special Issue is composed of four papers (one review article) spanning various topics in PSCs. The contributing authors shared some valuable insights on recent developments to bring PSCs from the lab to market. The guest editor briefly summarised the details of each work, and highlighted the following three key areas in PSC development: lead-free materials, stability and cost. Lastly, trends and future developments were discussed. The guest editor would like to thank the contributions of all authors and reviewers and hopes this Special Issue will be of use to researchers in the PSC field.