The rapid refinement of perovskite solar cells made them the rising star of the photovoltaic world and generated enormous interest in the academic community.
Since their methods of operation are still relatively new, there are great opportunities to further study the basic physics and chemistry around perovskites.
Additionally, as has been shown in recent years, improvements in the engineering of perovskite formulations and manufacturing routines have led to significant increases in energy conversion efficiency.
Advances in these solar cell materials provide high efficiency at low cost. However, more R&D is needed to prove that the emerging perovskite technology is stable, durable and reliable, compared to traditional silicon wafer-based solar panels.
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What is perovskite?
The mineral perovskite, named after Russian mineralogist Count Lev Perovski, was first used in solar cells only in 2009. Since then, the ever-increasing energy performance of these so-called perovskite solar cells has been demonstrated in scientific laboratories.
Perovskite is a material that has the same crystal structure as the mineral calcium titanium oxide, the first discovered perovskite crystal.
Perovskite compounds typically have the chemical formula ABX3, where “A” and “B” represent cations and X is an anion that binds to both. It is possible to combine a large number of different elements to form perovskite structures.
Thanks to this compositional flexibility, scientists can design perovskite crystals with a wide variety of physical, optical and electrical characteristics. Perovskite crystals are now found in ultrasound devices, memory chips and now solar cells.
Energy applications of perovskites
All photovoltaic solar cells rely on semiconductors, materials located between electrical insulators like glass and metallic conductors like copper, to convert light energy into electricity. Sunlight excites electrons in the semiconductor material, which flow to the conductive electrodes and produce an electric current.
Silicon has been the main semiconductor material used in solar cells since the middle of the last century because its semiconductor properties match the spectrum of sunlight well and it is relatively abundant and stable.
However, the large silicon crystals used in conventional photovoltaic panels require an expensive, multi-step, and energy-intensive manufacturing process.
In the search for an alternative, scientists have exploited the ability of perovskites to create semiconductors with properties similar to those of silicon. Perovskite solar cells can be made using simple techniques, such as printing, for a fraction of the cost and energy. Thanks to the compositional flexibility of perovskites, they can also be tuned to perfectly match the solar spectrum.
In 2012, researchers first discovered how to fabricate a stable thin-film perovskite solar cell with photon-to-electron conversion efficiency greater than 10%, using lead halide perovskites as the light-absorbing layer.
Since then, the efficiency of perovskite solar cells converting sunlight into electrical energy has skyrocketed, with the lab’s record standing at more than 25%.
Researchers are also combining perovskite solar cells with conventional silicon solar cells: record efficiencies for these “perovskite-on-silicon” tandem cells are currently 29% (beating the 27% record for conventional silicon cells) and increasing rapidly.
With this rapid increase in cell efficiency, perovskite solar cells and perovskite tandem solar cells may soon become cheap and highly efficient alternatives to conventional silicon solar cells.
Types of Perovskite Solar Cells
All solar cells, regardless of their composition, have certain things in common.
All must have at least one negative layer and one positive layer of photovoltaic material; and they must have front and rear conductive electrodes to carry solar-charged electrons from the negative layer along a wire to generate electricity before returning them to the positive layer. Once mounted on a solar module, the cells are sealed in an encapsulation layer to protect them from the weather.
There are basically two different types of perovskite solar cells: thin-film cells with perovskite as the sole photovoltaic material, and tandem cells, which have multiple layers of perovskite or a thin layer of perovskite on traditional crystalline silicon.
To complicate things a bit, there are also thin film tandem cells with a layer of perovskite on copper indium gallium selenide (CIGS), which is an advanced thin film solar technology.
Advantages and disadvantages of perovskite.
Advantage.
The use of perovskite as a semiconductor in photovoltaic modules has significant advantages:
- The raw materials needed to produce this semiconducting perovskite are really cheap.
- Also, you only need a very thin layer of perovskite in a solar cell, resulting in even lower material costs.
- Perovskite can be applied with a relatively simple deposition process (layering on a given substrate), so that no expensive machinery required.
- Perovskite layers can be deposited at low temperatures, which keeps production costs low.
- Relatively little power is needed to make a perovskite cell, and so the solar cell quickly harvests the energy that was needed to make the cell.
With the current state of perovskite solar cell technology, the same module efficiency can be achieved in glass or foil that is currently achieved with other technologies.
Disadvantages.
- Perovskites break down over time when exposed to moisture, light, heat, and oxygen, which means that additional technologies to stabilize the cells must be developed for widespread use.
- The best perovskites for power generation contain lead, which is a pollutant; however, the industry is working on ways to reduce the potential toxicity of perovskite.
- Perovskite cells are not yet ready for commercial scale.
What problems do perovskites face?
The biggest current problem in the field of perovskites is long-term instability.
This has been shown to be due to degradation pathways involving external factors, such as water, light and oxygen, as well as intrinsic instability, such as degradation by heating, due to the properties of materials.
Various strategies have been proposed to improve stability, in particular by modifying the choice of components.
Many recently released high efficiency systems use inorganic components.
Stability has also been improved through the use of surface passivation and the combination of 2D-layered perovskites, which exhibit better intrinsic stability, but lower yield, than conventional 3D perovskites.
These efforts (along with factors such as better encapsulation) have significantly improved the stability of perovskites since their initial introduction, and shelf life is on track to meet industry standards, with recent work showing cells capable of withstanding a 1000 hour damp heat test.
Another issue that has yet to be fully resolved is the use of lead in perovskite compounds. Although it is used in much lower quantities than currently found in lead or cadmium batteries, the presence of lead in commercial products is problematic. Exposure to toxic lead compounds (via perovskite leaching into the environment) remains a concern, and some studies have suggested that large-scale application of perovskites would require complete containment of degradation products. In contrast, other life cycle assessments found the toxic impact of lead to be negligible compared to other cellular materials (such as the cathode).
There is also the possibility of using an alternative to lead in perovskite solar cells (such as tin-based perovskites), but the energy conversion efficiency of these devices is still significantly lower than that of tin-based devices. lead, and the record for perovskite-based pewter currently stands at 9%. Some studies have also concluded that tin may actually have higher environmental toxicity than lead and that less toxic alternatives are needed.
Another major performance issue is the current-voltage hysteresis commonly seen in devices. Factors influencing hysteresis remain a matter of debate, but are most commonly attributed to migration of mobile ions in combination with high levels of recombination. Methods for reducing hysteresis include varying cell architecture, surface passivation, and increasing lead iodide content, as well as general strategies for reducing recombination.
For the cost per watt to be truly low, perovskite solar cells must achieve the much-heralded trio of high efficiency, long life, and low manufacturing cost. This has not yet been achieved in other thin-film technologies, but perovskite-based devices currently show enormous potential to do so.
The future of perovskites.
Future perovskite research will likely focus on reducing recombination through strategies such as passivation and defect reduction, as well as increasing efficiency through the inclusion of 2D perovskites and materials better optimized interface. Charge removal layers are capable of switching from organic materials to inorganic materials, to improve both efficiency and stability. Improving stability and reducing the environmental impact of lead will likely remain areas of great interest.
Although the commercialization of stand-alone perovskite solar cells still faces hurdles in terms of fabrication and stability, their use in c-Si/perovskite tandem cells has progressed rapidly (with efficiencies above 25%) and it is It is likely that perovskites will enter the photovoltaic market for the first time under this structure.
Findings.
- Perovskites are materials with specific crystal structures that exhibit a photovoltaic (electricity of light) effect.
- These materials have the potential to revolutionize the solar industry by dramatically increasing the efficiency and reducing the manufacturing cost of photovoltaic panels.
- Scientists have been working hard to perfect these materials since 2009, and commercially available solar cells may soon be within reach.
- The benefits of perovskites for making solar cells are hard to overstate, but there are drawbacks, such as the presence of lead in these materials, that need to be overcome before they can become widespread.