Solar cells are energy conversion devices that transform infinite solar energy into electrical energy. Currently, they are the most promising technology among various next-generation new and renewable energy resources. In line with this, we introduce perovskite solar cells (PSCs), which are expected to be the potential alternatives to expensive silicon solar cells.
1. History of PSCs
Hybrid organic-inorganic perovskite materials used in PSCs as absorbers were first synthesized in 1978 by Professor Weber and his team at the University of Stuttgart in Germany. Later, studies of fundamental physical properties made slow progress. However, for the first time in 2009, Professor Miyasaka and his team at the Toin University of Yokohama in Japan proved that a hybrid organic-inorganic perovskite material could be utilized as a light absorber material for solar cells. Its initial efficiency was only about 3%–4%, but in 2012, the research team led by Professor Nam-gyu Park at Sungkyunkwan University, South Korea, successfully increased the efficiency to 9.7% or higher.
Professor Park’s team synthesized nanostructured hybrid organic-inorganic perovskites, thereby significantly improving solar cell efficiency. Since then, the efficiency has been steadily upgraded, reaching 25.5% as of 2020 (Korea University and Ulsan National Institute of Science and Technology) and almost keeping up with the best silicon solar cell efficiency (26.1%) established by ISFH in about 20 years. The time has come to replace expensive silicon solar cells.
Figure 1—Best Research-Cell Efficiency Chart by the US National Renewable Energy Laboratory (NREL)
2. Advantages and disadvantages
In addition to high efficiency, PSCs are much more cost-effective than silicon solar cells because of their solution processability at low temperatures (less than 200°C), which ensures lower manufacturing costs. According to journals of Royal Society of Chemistry, United Kingdom, the production of a silicon module (1 m2) costs about USD 63, while for a PSC, a module of the same size costs only about USD 39. In other words, the process costing of PSCs is far more affordable than that of silicon solar cells (Source: Sofia et al., Sustainable Energy Fuels 4, p. 852 ).
Moreover, the material itself is lighter, more flexible, and translucent. In recent years, Dr. Jang-won Seo and his research team of the Korea Research Institute of Chemical Technology (KRICT), South Korea, have succeeded in developing a flexible PSC with an efficiency of 20.7%.
Figure 2—Flexible PSC developed by Dr. Jang-won Seo and his research team of the KRICT, South Korea
A Polish company, Saule Technologies, has also recently unveiled semitransparent PSCs. Both technologies above are expected to be widely used in flexible/semitransparent solar cell applications in the future.
Figure 3—Semi-transparent PSCs developed by Saule Technologies, Poland
Source: Nature, 570, p. 429 (2019)
On the other hand, PSCs have poor moisture stability. In general, hybrid organic-inorganic perovskites break down into metal halides and molecular halide ions in the presence of moisture. When decomposed this way, they lose all useful properties as absorber materials for solar cells.
In addition, lead (Pb), which is highly toxic to the human body, is used as the main component in most high-efficiency PSCs. It is contradictory to use lead, toxic material, as a new and renewable energy resource. Therefore, experts foresee that these two issues will pose significant challenges for commercializing PSCs in the future.
3. Development trends
1) Securing moisture stability through materials development and process optimization
As mentioned above, PSCs are highly susceptible to moisture. However, the recent development of new materials in conjunction with the evolution of process technologies has mitigated this issue to some extent. For instance, the UK-based Oxford Photovoltaics Limited (Oxford PV) has developed highly moisture-resistant tandem solar cells with a capping layer to pair silicon substrates with perovskite cells. Furthermore, its efficiency reached up to 28%, exceeding silicon solar cells’ highest efficiency of 26.1%. This has reinforced the possibility of commercial-scale perovskite/silicon tandem solar cells. In addition, the aggressive development of materials using highly moisture-resistant elements is in progress.
Figure 4—Perovskite-silicon tandem solar cells developed by Oxford PV in the United Kingdom
Source: Nature Materials 17, 372 (2018) (https://www.nature.com/articles/s41563-018-0071-z)
2) Development of lead-free perovskite materials
All high-efficiency PSCs contain lead, meaning this element, which can adversely impact the environment in the long term, must be substituted to realize commercialization. The most dominant attempt was designing a solar cell by replacing lead with tin (Sn) or bismuth (Bi). However, compared to lead-based solar cells, these have failed to show significant efficiency improvement. Another attempt was seeking an entirely new type of lead-free perovskite material.
In 2016, a team led by Professor Karunadasa at Stanford University, United States, succeeded in synthesizing lead-free single-crystal perovskite in the form of A2B’B’’X6 (A = alkaline cation, B’ = monovalent cation, B’’ = divalent cation, and X = halide anion). Because of the long carrier recombination life time, this material showed excellent light absorption. In addition, it consists of four elements (A, B’, B’’, and X), and their infinite combinations may lead to the creation of diverse substances. Therefore, there is room for discovering high-performance solar cell materials as they grow more popular as next-generation PSC absorber materials.
Figure 5—Typical lead-free perovskite Cs2AgBiBr6 and long carrier recombination life time characteristics
Source: Slavney et al., Journal of the American Chemical Society 138, p. 2138 (2016)
3) Large-area perovskite process technology development
Just because a material has good properties for a specific application does not necessarily mean it is commercially available. For technology commercialization, not only should the material possess good physical properties, but the process must also be economical, easy to use, and hazard-free. Hybrid organic-inorganic perovskites also exhibit good properties as absorber materials for solar cells but are difficult to mass-produce because of lower stability compared to silicon. Therefore, to achieve future commercialization, we must maximize the process’s economic feasibility and availability and substitute harmful elements such as lead. In particular, an urgent demand is seen for process technology for manufacturing large-area devices.
In recent years, Professor Dauskardt and his team at Stanford University have developed a new method for producing PSCs at speeds up to 12 m/min, much faster than the silicon solar cell process (Source: Rolston et al., Joule 4, 1 ). The research team used a robot with two nozzles—one for spraying a liquid solution of perovskite materials and the other for releasing plasma to remove the liquid from the adsorbed perovskite solution. Along with increased efficiency of about 18%, this method allows the perovskite thin films to be manufactured quickly and cost-effectively at approximately USD 0.25 per sq ft. Therefore, this newly developed method is expected to accelerate the commercialization of PSCs significantly.
Figure 6—A PSC produced using spray method developed by Professor Dauskardt and his team at Stanford University, United States
Source: Joule 4, 2675 (2020) (https://www.cell.com/joule/fulltext/S2542-4351(20)30509-2)
Dr. Jung-Hoon Lee
Senior Researcher Scientist, Computational Science Research Center, Korea Institute of Science and Technology