Perovskite solar cells (PSC) have shown remarkable progress in the last few years. The improvement has been so steep that people now consider it as the rising star of the photovoltaic sphere.
Perovskite is often used interchangeably with perovskite structure. The former is a mineral commonly used in the Ural Mountains. It was found by Lev Perovski and got its name from him. The perovskite structure is the compound having the same form as perovskite.
When it comes to harnessing solar energy, the most commonly found perovskite solar cells are based on Group IV and specifically lead. They offer several advantages over other types.
In this article, we discuss the basics of perovskite solar cells.
What are perovskite solar cells?
Perovskite bears the same crystal structure as the titanium oxide (the first discovered perovskite crystal). Most compounds in this category follow the ABX3 chemical formula. Here ‘A’ and ‘B’ are cations whereas ‘X’ is an anion binding to both. Perovskite structures are flexible, and depending on your end needs, you can opt for any composition available.
In the mid-2000s, scientists first discovered perovskite’s ability to act as a solar cell material because of a lab test. A perovskite solar cell includes the perovskite compound as the light-harvesting active layer. In 2006, it had an efficiency of 3% which has now ramped up to over 25% in 2020.
Perovskite solar cell structure
The structure of a perovskite solar cell depends on the placement of perovskite material or on how the top and bottom electrode function. Most of these are thin-film where most electron or hole transport takes place in the perovskite.
The resulting electrons, known as photo-generated electrons, get coated onto a mesoporous scaffold. These are then extracted into the circuit. Once the light absorption and charge-generation processes are complete, the carriers begin charging selective contacts.
Fabrication process
One of the primary reasons perovskite solar cells are gaining so much traction is because of their easy integration. Initially, these were based on solid-state dye-sensitized solar cells (DSSCs) along with a mesoporous TIO2 scaffold. With further inroads, manufacturers can now mould them into any thin-film architecture.
The new ones still follow the same TIO2 architecture or have shifted to AI2O3 scaffold in a meso-super structured tuning. The latter is difficult to manufacture, owing to higher temperature requirements and UV instability. It has given rise to a planar architecture, which is in line with what other thin-film solar cells employ.
Typically manufacturers use the spin-dye coating process for manufacturing PSCs, but there are several other methods too. The spin-dye process involves one or two steps. For the former, a precursor solution is coated (usually in a polar solvent), and then quenched using a non-polar solvent to complete the spin coating process.
In the two-step spin-dye process, manufacturers use two different components, a metal halide and another one organic. Both of them are spin-coated to form separate but subsequent films. Alternatively, VASP (Vacuum-assisted Solution Process) can be employed where metal halide films are coated and annealed in a chamber consisting of an organic compound vapour for consistent coating.
Efficiency
PSCs are tunable, and you can achieve a varied level of efficiency depending on the halide levels in the structure. Irrespective of what you opt for, a perovskite solar cell can reach maximum efficiency of 33% currently.
The first published records of usage of perovskite as a solar cell material dates back to 2009. It mentions that PSC’s efficiency is barely anything to talk about at a measly 4%. In the year 2012, the solid-state components replaced the liquid-phase ones, thereby improving the efficiency to 10%. By 2014, further improvements in stability and performance saw the same go up to 20%.
Significance of perovskite solar cells
Here are the advantages that perovskite solar cells have on offer –
- Perovskite can be easily created in lab conditions and demands no rare resources.
- The modern-day PSC is manufactured as a thin-film product, thereby ensuring limited usage of resources.
- It offers a wide bandgap providing easy tuning and higher conversion rates.
- Specific PSCs provide a smaller footprint compared to most other solar cell types.
- These do not require high-cost complex machinery to manufacture.
- Unlike most other thin-film solar cell types, PSC is highly defect-tolerant and results in high manufacturing yields, especially while working with 300W or higher boards.
- Manufactures can manufacture them as the traditional rectangular slabs or give the custom shapes as per requirements.
Reasons for degradation of perovskite solar cell
Here are the factors that can lead to a degradation in the perovskite solar panel –
Intrinsic factors
The PSC manufacturing process inadvertently leads to vacancies in the overall perovskite structure. If these exceed the maximum limit, it can lead to fluctuation in the performance levels of the attached device due to ion migration going on within the film.
Such unwanted occurrences can lead to the formation of a local electric field and cause deprotonation of the organic cations. Also, it can result in ions from conductive contacts migrating via the perovskite layer, which can create shunt pathways and lead to short circuits.
Extrinsic factors
Exposure to UV
Even though exposure to UV radiation is innocuous by itself, it can lead to a significant dent when combined with other factors. Especially in the MAPbl3 case, it can cause the compound to downgrade to Pbl2. TIO2, the electron transport layer, is also known to be reactive to UV and leads to faster degradation of the PSC.
Elevated temperature
Most PSC types are susceptible to a higher temperature and it causes faster degradation. The reduction is slower if water or oxygen is not affecting the cell yet.
Exposure to moisture
Organic cations used in PSCs are hygroscopic, and exposure to moisture can lead to the formation of weak hydrogen bonds with the cations. These affect the structural stability of the crystal and lead to the creation of a hydrated phase. It is reversible initially, but if the user ignores it for long, the perovskite starts decomposing and wearing off.
High oxygen content in the ambience
If the perovskite solar cell is in an oxygen-rich ambience for long, users can experience the creation of large Pbl2 structures within the body. These can often lead to structural failure and ultimately affect performance.
Leading global manufacturers of perovskite solar cells
In 2019, the global PSC market size stood at USD 494.7 million. Experts further expect it to reach USD 3714.2 million by the end of 2026, with an astonishing 33% CAGR in 2021-26. The race of producing perovskite SC is led by North America, with Europe and Japan taking the second and the third spot, respectively.
Conclusion
Perovskite solar cells come laced with a plethora of benefits. From being easy to manufacture to non-usage of costly manufacturing processes, they are worth the attention that they receive globally. Keeping in mind the business model, these can demand up to 50% less cost than other models when it comes to manufacturing.
With so much on their plate, the future of PSCs will likely revolve around optimizing the manufacturing process, curtailing defects, and working towards achieving higher efficiency levels. The usage of 2D perovskite and optimized manufacturing processes can lead to a lower impact on the environment.
Even after years of development, perovskite solar cells are still a niche market. It is time that the government initiates steps to convert it into a mass product.
