[Image above] Example of perovskite tin solar cells. Perovskite solar cell technology is advancing rapidly and looks promising to play a big role in the photovoltaic industry in the near future. Credit: University of Oxford Press Office, Flickr (CC BY 2.0)
In the past decade, investment in renewable energy sources and the resulting expansion in capacity has exploded—from 414 gigawatts of energy capacity in 2009 to just over 1,650 gigawatts by the end of this year.
Solar power in particular received an outsized share of the investments, drawing in $1.3 trillion, or over half of all investments during this decade. These investments did not just help construct facilities for solar energy storage, though. They also helped significantly advance the capabilities of solar technologies, especially in regard to perovskite tandem solar cells.
Perovskite materials are compounds with the same type of crystal structure as calcium titanium oxide, the mineral traditionally referred to as perovskite. When arranged in tandem with silicon, the traditional solar cell material, perovskite materials can greatly increase the power conversion efficiency of solar cells. For example, within the past six years, perovskite tandem solar cells improved from below 14% power conversion efficiency to as high as 29.1%, surpassing the record of single junction perovskite and silicon solar cells.
Despite impressive gains in power conversion efficiency, perovskite solar cell technologies are still in the early stages of commercialization. But knowing the history of recent developments can help manufacturers pursue commercialization, which is why a new review paper published in Chemical Reviews is such a boon for the industry.
The staggering 116-page review is written by two researchers in the Advanced Technology Institute at the University of Surrey (U.K.): first author Hui Li, visiting professor and Advanced Newton Fellow of The Royal Society; and corresponding author Wei Zhang, senior lecturer in energy technology.
“We are excited to offer this review that is showing great potential for moving our planet towards green energy,” Li says in a University of Surrey press release.
The review contains a wealth of information that can only be briefly touched on here. Below are just a few highlights contained within the many pages.
Developing tandem solar cells: The early years
Before diving into the specifics of perovskite solar cells, Li and Zhang provide a broad overview of tandem solar cell development to give context for the perovskite research.
The concept of arranging solar cells in tandem as a way to increase total conversion efficiency was proposed as early as 1955. (See this article by Texas Instruments researcher E. D. Jackson, published in Transactions of the Conference on the Use of Solar Energy.) However, the idea was rejected at the time because the use of tunnel junctions for connecting the cells was not yet realized.
In 1979, Bedair et al. fabricated the first monolithically interconnected tandem solar cell using an epitaxy method in combination with the epitaxial tunnel junction, an approach that is commonly used today. But the material they used—AlGaAs/GaAs—was sensitive to oxygen.
To overcome this weakness, Olson et al. from the National Renewable Energy Laboratory began work on GaInP/GaAs in 1984, and by 1996, GaInP/GaAs cells achieved power conversion efficiencies of over 30%.
Researchers assumed power conversion efficiencies could be further improved if the band gaps of GaInP and GaAs were decreased to extend the absorption region to wavelengths beyond 900 nm. This possibility led to the development of GaInP/GaInAs/Ge, which today are the most successful example of solar cells for space and concentrator applications—the record power conversion efficiency of three-junction GaInP/GaInAs/Ge cells is 41.6% at 364 suns.
Development of (Al)GaAs/Si cells began as well in the 1980s and 1990s. In recent years, interest in this cell type has increased as a way to replace the expensive germanium substrates in GaInP/GaInAs/Ge with low-cost silicon substrates, and some studies have achieved promising results.
Currently, the record power conversion efficiencies are 39.2% and 47.1%, held by six-junction solar cells composed of Al0.2Ga0.3In0.5P/Al0.2Ga0.8As/GaAs/Ga0.8In0.2As/Ga0.7In0.3As/Ga0.4In0.6As under 1 sun and 143 suns illumination, respectively.
Advantages of perovskite tandem solar cells
As mentioned earlier, researchers are eager to combine perovskite materials with silicon to improve the power conversion efficiency of solar cells. Li and Zhang elucidate the reasons for this ability in the paper.
Metal halide perovskites have tunable band gaps in the region of 1.17−3.10 eV by compositional engineering. Typical band gaps for highly efficient perovskite cells are about 1.60 eV, close to the ideal band gap regions of 1.67−1.75 eV for top cells in a tandem arrangement. Recently, improvements in stability and power conversion efficiency enhancement of low band gap perovskites made it possible to use such perovskites as bottom cells in all-perovskite tandem solar cells as well.
In addition to tunable band gaps, perovskite materials have high structure tolerance, which can reduce the strict requirement of lattice match between subcells. Perovskite materials also exhibit good electrical and defect tolerance, mainly because the dominant intrinsic defects only create shallow levels within the valence band or conduction band.
Overall, “Perovskite [tandem solar cells] have great potential to enhance [power conversion efficiencies] and reduce the costs of PVs, while still benefiting from well-established fabrication technologies and [power conversion efficiency] enhancement strategies of [perovskite solar cells],” Li and Zhang write.
Perovskite tandem solar cell designs
Li and Zhang spend the majority of the paper (69 pages) diving deep into the device architectures and designs for perovskite tandem solar cells, including semi-transparent perovskite solar cells, silicon-based versus all-perovskite tandem solar cells, and two- versus three- versus four-junction solar cells.
Among all the various types of perovskite tandem solar cells mentioned, they conclude that perovskite/silicon, perovskite/copper-indium-gallium-selenide, and all-perovskite tandem solar cells are among the most promising due to their high power conversion efficiencies, feasible fabrication methods, easy fabrication on flexible and light substrates, high throughput, and low cost.
At this stage, Li and Zhang believe perovskite/silicon tandem solar cells are the most promising candidate because the dominant position of silicon solar cells in the photovoltaic market “represent a direct entry pathway for commercialization.” However, the ultimate goal is the all-perovskite tandem solar cells owing to their high power conversion efficiency, low-cost fabrication methods, and compatibility with flexible devices once their long-term stability is resolved.
Prospects and future opportunities
In the last section of the paper, Li and Zhang project a very bright future for perovskite solar cells.
“We expect that the trend in [power conversion efficiencies] enhancement is highly likely to continue in the future, which eventually will overwhelm the increase in fabrication cost,” they write.
Improved power conversion efficiency is not the only step to commercialization, however. Li and Zhang conclude the section by highlighting several challenges and strategies they believe should be “seriously considered” for future developments, including
- Developing scalable fabrication methods like blading, vacuum-solution, and coevaporation to directly deposit perovskite solar cells on textured bottom cells;
- Achieving high stability of bottom perovskite solar cells;
- Improving light management to decrease the optical loss; and
- Conducting further assessment of lead impact on the environment and the human body before drawing conclusions about the lead toxic issue in perovskite tandem solar cells.
“We believe that most of these issues will be resolved in the future and great progress will be achieved in perovskite [tandem solar cells] both in academical and industrial fields by a combined effort of fundamental investigations, device engineering and interface modification, understanding and improving materials and device stability, and development of large-scale fabrication technologies and equipment,” they write.
The paper, published in Chemical Reviews, is “Perovskite tandem solar cells: From fundamentals to commercial deployment” (DOI: 10.1021/acs.chemrev.9b00780).