What are perovskite?
Perovskites are a class of materials that share a similar structure, which display a myriad of exciting properties like superconductivity, magnetoresistance and more. These easily synthesized materials are considered the future of solar cells, as their distinctive structure makes them perfect for enabling low-cost, efficient photovoltaics. They are also predicted to play a role in next-gen electric vehicle batteries, sensors, lasers and much more.
How does the PV market look today?
In general, Photovoltaic (PV) technologies can be viewed as divided into two main categories: wafer-based PV (also called 1st generation PVs) and thin-film cell PVs. Traditional crystalline silicon (c-Si) cells (both single crystalline silicon and multi-crystalline silicon) and gallium arsenide (GaAs) cells belong to the wafer-based PVs, with c-Si cells dominating the current PV market (about 90% market share) and GaAs exhibiting the highest efficiency.
Thin-film cells normally absorb light more efficiently than silicon, allowing the use of extremely thin films. Cadmium telluride (CdTe) technology has been successfully commercialized, with more than 20% cell efficiency and 17.5% module efficiency record and such cells currently hold about 5% of the total market. Other commercial thin-film technologies include hydrogenated amorphous silicon (a-Si:H) and copper indium gallium (di)selenide (CIGS) cells, taking approximately 2% market share each today. Copper zinc tin sulphide technology has been under R&D for years and will probably require some time until actual commercialization.
What is a perovskite solar cell?
An emerging thin-film PV class is being formed, also called 3rd generation PVs, which refers to PVs using technologies that have the potential to overcome current efficiency and performance limits or are based on novel materials. This 3rd generation of PVs includes DSSC, organic photovoltaic (OPV), quantum dot (QD) PV and perovskite PV.
A perovskite solar cell is a type of solar cell which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting active layer. Perovskite materials such as methylammonium lead halides are cheap to produce and relatively simple to manufacture. Perovskites possess intrinsic properties like broad absorption spectrum, fast charge separation, long transport distance of electrons and holes, long carrier separation lifetime, and more, that make them very promising materials for solid-state solar cells.
Perovskite solar cells are, without a doubt, the rising star in the field of photovoltaics. They are causing excitement within the solar power industry with their ability to absorb light across almost all visible wavelengths, exceptional power conversion efficiencies already exceeding 20% in the lab, and relative ease of fabrication. Perovskite solar cells still face several challenge, but much work is put into facing them and some companies, are already talking about commercializing them in the near future.
What are the advantages of Perovskite solar cells?
Put simply, perovskite solar cells aim to increase the efficiency and lower the cost of solar energy. Perovskite PVs indeed hold promise for high efficiencies, as well as low potential material & reduced processing costs. A big advantage perovskite PVs have over conventional solar technology is that they can react to various different wavelengths of light, which lets them convert more of the sunlight that reaches them into electricity.
Moreover, they offer flexibility, semi-transparency, tailored form factors, light-weight and more. Naturally, electronics designers and researchers are certain that such characteristics will open up many more applications for solar cells.
What is holding perovskite PVs back?
Despite its great potential, perovskite solar cell technology is still in the early stages of commercialization compared with other mature solar technologies as there are a number of concerns remaining.
One problem is their overall cost (for several reasons, mainly since currently the most common electrode material in perovskite solar cells is gold), and another is that cheaper perovskite solar cells have a short lifespan. Perovskite PVs also deteriorate rapidly in the presence of moisture and the decay products attack metal electrodes. Heavy encapsulation to protect perovskite can add to the cell cost and weight. Scaling up is another issue - reported high efficiency ratings have been achieved using small cells, which is great for lab testing, but too small to be used in an actual solar panel.
A major issue is toxicity - a substance called PbI is one of the breakdown products of perovskite. This is known to be toxic and there are concerns that it may be carcinogenic (although this is still an unproven point). Also, many perovskite cells use lead, a massive pollutant. Researchers are constantly seeking substitutions, and have already made working cells using tin instead. (with efficiency at only 6%, but improvements will surely follow).
While major challenges indeed exist, perovskite solar cells are still touted as the PV technology of the future, and much development work and research are put into making this a reality. Scientists and companies are working towards increasing efficiency and stability, prolonging lifetime and replacing toxic materials with safer ones. Researchers are also looking at the benefits of combining perovskites with other technologies, like silicon for example, to create what is referred to as “tandem cells”.
Commercial activity in the field of perovskite PV
In September 2015, Australia-based organic PV and perovskite solar cell (PSC) developer Dyesol declared a major breakthrough in perovskite stability for solar applications. Dyesol claims to have made a significant breakthrough on small perovskite solar cells, with “meaningful numbers” of 10% efficient strip cells exhibiting less than 10% relative degradation when exposed to continuous light soaking for over 1000 hours. Dyesol was also awarded a $0.5 million grant from the Australian Renewable Energy Agency (ARENA) to commercialize an innovative, very high efficiency perovskite solar cell.
Also in 2015, Saule Technologies signed an investment deal with Hideo Sawada, a Japanese investment company. Saule aims to combine perovskite solar cells with other currently available products, and this investment agreement came only a year after the company was launched.
The latest perovskite solar news:
Empa and Solaronix design new manufacturing processes for commercialization of perovskite solar cells
An Empa team led by Frank Nüesch, Head of Empa's Functional Polymers Department, has been working in recent years on new manufacturing processes for perovskite solar cells in order to produce them not only faster but also cheaper. To this end, the researchers collaborated with Solaronix, a company based in western Switzerland, as part of a project of the Swiss Federal Office of Energy (SFOE). Together they produced a functional perovskite cell on a laboratory scale with a surface area of 10x10cm.
For the production of this novel perovskite cell, the so-called slot-die process is used. Here, the material layer is applied to a substrate of glass and then structured by removing excess material with a laser. "With the new coating process, we can not only coat faster, but also determine the thickness of the layers more flexibly," says Nüesch. In the future, the slot-die process will make it possible to coat meter-long webs relatively easily and quickly. The coating speed is then also the central element in a possible industrialization of perovskite cell production.
Panasonic Corporation has achieved an energy conversion efficiency of 16.09% for a perovskite solar module (Aperture area 802 cm2: 30 cm long x 30 cm wide x 2 mm thick) by developing lightweight technology using a glass substrate and a large-area coating method based on inkjet printing.
This was carried out as part of the project of the New Energy and Industrial Technology Development Organization (NEDO), which is working on the "Development of Technologies to Reduce Power Generation Costs for High-Performance and High-Reliability Photovoltaic Power Generation" to promote the widespread adoption of solar power generation.
The US Department of Energy (DOE) will allocate up to USD$125.5 million in financing for research and development (R&D) projects in the solar field. The research will target reducing the cost of solar technology, which in turn will enhance the competitiveness of the domestic photovoltaic (PV) production and improve the grid reliability.
Among other projects, the DOE funds will see USD$15 million go to 8-12 projects that aim to prolong the lifespan of PV systems and cut hardware costs for plants using traditional silicon solar cells, as well as thin-film, tandem and perovskite cells.
Scientists from Japan’s Gifu University and the Tokyo Institute of Technology have identified a chalcogenide perovskite material with light absorption attributes strong enough to offer the potential for a theoretical maximum conversion efficiency of 38.7% in a silicon perovskite tandem solar cell.
The material – BaZrTiS3 – has a light absorption coefficient exceeding 105cm-1, the highest of all solar cell materials including chalcogenide perovskites such as SrZrS3, BaHfS3 and SrHfS3, the scientists claim.
A new paper claims that scientists have reached a consensus on the procedures for the testing of perovskite cells, which they say will lead to better reproducibility and comparability of data produced by different laboratories, and ultimately a better understanding of the degradation pathways affecting perovskite solar cells, and the most effective ways to mitigate them.
Perovskites still have a way to go to convince the PV industry of their potential for long term durability. The class of materials’ sensitivity to moisture and other conditions they would be sure to encounter in the field has continued to be a stumbling block.