The term perovskite refers to a group of materials that share the crystal structure with CaTiO3 discovered in the Ural mountains in the 19th century. This family represents an enormous range of possible compositions of materials that possess unique physical and chemical properties due to their special crystal structure. Halide perovskites in particular have been identified as exceptional absorbers of light, and their synthesis has been shown to be simple and possible to carry out on low temperatures. They also possess a range of other intrinsic and attractive properties which make them an excellent candidate for photovoltaic applications. These include a steep absorption onset, low exciton binding energies with high charge diffusion lengths, and a direct band gap. Simply put, these materials combine all the features required to prepare a highly efficient solar cell with an ultra-thin photoactive layer. These properties, along with the ability to process the material from solution with relative ease and simplicity, have attracted tremendous scientific interest.
Historically, perovskite solar cells originate from dye-sensitized solar cells (DSSCs). In 2009, Miyasaka et al. used perovskite absorbers to replace the conventional dyes in such photovoltaic devices. In this seminal work, the researchers employed the conventional liquid electrolyte used in DSSCs, and it led to a rapid degradation of the perovskite, which is soluble in polar solvents, and a low power conversion efficiency (PCE) of 3.8%.
The first ever DIN A4 paper sized ( 210 × 297 mm) printed, flexible perovskite solar module
A major breakthrough came a few years later when the liquid electrolyte was replaced by a solid state organic semiconductor. In 2012, two reports demonstrated a large increase of the PV efficiency by using spiro-OMeTAD as the hole transport material (HTM). Kim et al. and Lee et al. reported solid state devices reaching a PCE of 9.7% and 10.9% respectively. The latter work also reported that devices based on an insulating Al2O3 scaffold were capable of delivering an even better performance compared to those using semiconducting TiO2, which is traditionally used in DSSCs. This was an important observation, implying that the perovskite material is capable of transporting both electrons and holes to their respective electrodes without significant losses.
These reports attracted the attention of many scientists working on DSSCs and organic photovoltaics (OPV), and solidified perovskite solar cells as a separate field of thin film photovoltaic technology. The joint effort of researchers led to huge boost in achieved PCEs in the following years, reaching the current certified record of 22.7%, which is higher than that of the established thin film technologies CIGS and CdTe. While this is a remarkable improvement indeed, there are quite a few challenges to be overcome to develop a technology that is mature enough to enter the market.
The National Renewable Energy Laboratory (NREL) efficiency chart is a plot of compiled values of highest confirmed conversion efficiencies for research cells, from 1976 to the present, for a range of photovoltaic technologies.
As most perovskite thin films are polycrystalline in nature, it is crucial to understand the role of each parameter that contributes to the formation and crystallization of these layers. It has been shown that the atmosphere, the humidity, the utilized solvent, the temperature, the composition and the age of the precursor, among many other parameters will determine the final quality of the perovskite.
Devices made in laboratories have a tiny active area -- less than 1 cm2 in most of the cases, and it mitigates the effect caused by the lack of control on processing conditions. However, when transferring the technology from the laboratory to an industrial scale, it is critical to develop appropriate methodology. The two main approaches for device fabrication can be categorized as vacuum and solution processing.
Vacuum processing usually yields a material with higher reproducibility, and it is also intrinsically additive -- meaning that any sort of materials that are compatible with vacuum deposition may be used to fabricate these solar cells. This feature makes them interesting in tandem architectures, in particular for the application in silicon / perovskite tandem cells. The vacuum based technology however means higher expected costs for both the equipment and processing, and it leads to a higher price per module area.
Solution processing, on the other hand, offers the possibility of low-cost and high-throughput fabrication. Notably, the highest reported efficiencies have been reached via solution processing. Ink-jet printing in particular enables the design of free-form modules with negligible waste, and an easier and less capital intensive way for upscaling when compared with other printing technologies.
There are engineering challenges in finding the appropriate solvent orthogonality for deposition, but eventually it offers lower costs associated with CAPEX and OPEX, and it can lead to a significant reduction in the price of electricity. The challenges that most of the companies face while trying to commercialize the technology are finding the appropriate approach for upscaling, and to overcome the sensitivity of the material to environmental effects, such as heat and humidity. The latter issues can be tackled through the adequate engineering of the materials and by using proper encapsulation methodologies. It is only a matter of time and investment until perovskites will be available on the market, which we expect to happen in the next 2-3 years.
Building integrated photovoltaics (BIPV) is forecast to be one of the largest markets for perovskites for several reasons. There is a growing demand on the development of zero-energy buildings, however, architects are often limited by their choices when it comes to photovoltaics. The vast surfaces available and the additional benefit in offsetting installation costs also supports the high potential in this application. Besides, perovskite materials offer good low-light performance, which means that shadows and cloudy weather will have less impact on their performance, making them viable to be implemented on facades and windows. Applications in portable electronics, internet of things are possible, and on a longer scale electric vehicles, space technology and even utility scale energy production are areas that might benefit from the development of perovskite photovoltaics.
Written by David Forgacs, Knowledge Manager, Saule Technologies.