The SunShot Initiative supports research and development projects aimed at increasing the efficiency and lifetime as well as evaluating new materials for hybrid organic-inorganic perovskite solar cells.
This field has been dominated by absorber materials based on methylammonium lead halide perovskites. Perovskite solar cells have shown remarkable progress in recent years with rapid increases in conversion efficiency, from initial reports of 2-3% in 2006 to 20% in 2015. Perovskite solar cells may offer the potential for an earth-abundant and low-energy-production solution to truly large-scale manufacturing of photovoltaic (PV) modules. While perovskite solar cells have achieved very high efficiencies in a very short amount of time, a number of challenges remain before perovskite solar cells can become a competitive commercial technology.
Although organic-inorganic perovskite materials have been studied for more than a century, initial studies on methylammonium lead halides for semiconductor applications, including thin-film transistors and light-emitting diodes, started in the last two decades. The first application of hybrid organic-inorganic perovskite absorbers in solar cells occurred in 2006 with first publication in 2009 by Miyasaka and coworkers. However, these cells were of rather poor efficiency (<4%) due in part to the liquid electrolyte used, which limited both device stability and the open circuit voltage due to compromised interfacial chemistry and energetics. The application of a solid-state hole transport material (HTM), spiro-MeOTAD (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’- spirobifluorene), by two separate groups improved the efficiency to 10% by 2012. Subsequent improvements in performance and stability have come through continued investigation of mixed halide perovskites, improved contact materials, new device architectures, and improved deposition processes; with 20% efficiency having been reported in late 2014.
Hybrid organic-inorganic perovskite solar cells have demonstrated competitive efficiencies with potential for higher performance, however the current stability of perovskite solar cells is limited compared to leading PV technologies. To increase stability, researchers are studying the relevant degradation mechanisms in both the perovskite materials and the contact layers. Improved cell durability is paramount for development of commercial perovskite solar products.
Additional barriers to commercialization are the potential environmental impacts related to the lead-based perovskite absorber currently being used and developed. As such, significant efforts toward the evaluation, reduction, mitigation, and potential elimination of toxicity and environmental concerns are being investigated through the study of current and future materials. Current materials discovery efforts are evaluating lead-free perovskite structures in order to reduce or eliminate potential environmental concerns.
Furthermore, a potential high-value application for perovskite solar cells exists in high performance tandem device architectures. Perovskite solar cells have demonstrated very low energy losses, allowing for high open-circuit voltage; as such, they may serve as excellent wide gap absorbers in tandem devices with low gap absorbers, such as crystalline or multi-crystalline silicon cells. A silicon based tandem device architecture using a low-cost, high-performance, wide band gap perovskite cell might offer a cost-effective path toward high efficiency modules. Researchers are presently evaluating wider band gap perovskite absorbers and contact materials for the tunnel junction between the sub-cells, as well as mechanical tandem architectures.
A final challenge lies in scale-up and optimization of the deposition processes for reproducible perovskite solar cell performance. Relatively high throughput manufacturing could be enabled if the processes were demonstrated to be scalable and reproducible, leading to high performance, high volume production of perovskite PV modules to meet and potentially exceed the SunShot levelized cost of electricity (LCOE) target of $0.06 per kilowatt-hour.
Learn more about the DOE SunShot PV R&D awardees and the projects involving perovskites:
- University at Buffalo, The State University of New York (Photovoltaics Research and Development: Small Innovative Projects in Solar)
- University of Colorado Boulder (Photovoltaics Research and Development: Small Innovative Projects in Solar)
- Duke University (Next Gen III)
- National Renewable Energy Laboratory (Next Gen III)
- Stanford University (Next Gen III)
- University of Nebraska-Lincoln (Next Gen III)
- University of Washington (Next Gen III)
- Low cost and ease of manufacturing: Perovskites hold the promise of being a very low-cost technology with great ease of manufacture and decreased capital expenditures (CapEx).
- Abundant and low-cost materials: Perovskites rely on stable and abundant resource materials, such as methylammonia, lead, and iodine; thereby enabling very high volume production.
- Tunable band gap: the band gap of perovskite solar cells can be modified through modifying the composition of the perovskite material, which is potentially enabling for higher efficiency tandem PV applications.
Hybrid organic-inorganic halide perovskite solar cells are currently only produced in research laboratory-scale quantities. The scale-up of perovskite cells is an important area of research and is required to enable high-efficiency, low-cost production of perovskite solar cells. The coating over large areas, either through solution (printing or coating) or vacuum deposition methods, poses many challenges for maintaining uniformity for the thin film devices. Furthermore, the relationship between the record cell efficiencies being reported (< 1 cm2) and large area modules is not yet clear. This will be highly dependent on film quality, which has been a challenge even on the reproducible fabrication of small area devices.