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Photovoltaic systems are an efficient solution to the rising energy needs of emerging economies and greenhouse gas-related issues across the planet.
Crystalline silicon (c-Si)-based photovoltaic (PV) devices have fulfilled the criterion of low-cost manufacturing of clean energy conversion due to abundantly available raw materials and no apparent environmental health or safety issues. However, perovskite solar cells (PSC) are currently considered the rising stars of the photovoltaic industry due to their potential in increasing efficiency and lowering the costs of solar energy.
What are Perovskite Solar Cells?
A PSC is a solar cell made of perovskite material. The most common perovskite materials used for manufacturing solar PSCs are hybrid organic-inorganic lead (Pb) or tin halide-based perovskite material.
How are PSCs a Cut Above c-Si solar cells?
The power conversion efficiency (PCE) of PSCs has increased manifold within a very short period, with exhaustive research about PSCs starting from 2012.
Based on lab calculations, the PCEs of the photovoltaic devices using PSCs have increased from 3.8% in 2009 to 25.2% in 2020, exceeding the maximum efficiency achieved in traditional mono- or poly-crystalline silicon cells.
PSCs are currently the fastest-advancing solar cell technology in the photovoltaic industry. The idea is to improve relevant photovoltaic parameters that affect the PCE, such as the short circuit current (Jsc), open circuit potential (Voc), and the fill factor (FF).
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Until now, there has been no significant deficiency observed for PSCs concerning any of the above parameters, which explains the high efficiency achieved by PSCs.
Bandgap value is one of the vital parameters to evaluate the overall performance of a solar cell, and it should be close to the Shockley-Quiser (SQ) limit. The perovskite bandgap is much closer to the SQ limit.
The perovskite structure with the Pb-halide bonding renders the unique properties such as large absorption with a thin layer, direct bandgap with less recombination loss, and an extra high lifetime of carriers.
Perovskite materials possess intrinsic properties such as broad absorption spectrum, fast charge separation, long transport distance of electrons and holes, and long carrier separation lifetime that make them particularly exciting materials for solid-state solar cells.
Why are Perovskite Materials a Better Alternative to Silicon?
The properties that render perovskite materials as better candidates for solar cell technology are as follows:
- Charge generation: The excellent absorption properties and low exciton binding energy helps PSCs to absorb light across almost all visible wavelengths. This results in a high population of charge carriers (both electrons and holes) within the same absorber material, which has significant implications on the transport, recombination, and extraction of charges. This leads to high PCEs in PSCs.
- Charge Transport: Unlike other photovoltaic systems, PSCs have to transport both electrons and holes for extraction efficiently. The excellent charge-transport properties of the material, with long charge carrier diffusion lengths, L, of more than 5 μm, and an associated lifetime of ∼1 μs in single-crystal and poly-crystalline thin films ensures the outstanding PCEs of these cells. PSCs’ diffusion length values are several orders of magnitude larger than those of most organic solution-processed material-based solar cells. The diffusion lengths for both electrons and holes seem to be well balanced in perovskite solar cells, which is very important for thin-film solar cells for the efficient extraction of both kinds of carriers.
- Charge Recombination/ Energy utilization: Perovskite materials can efficiently separate and transport electrons and holes due to the fast exciton dissociation, very high charge diffusion lengths of perovskite, and a probable self-doping at the interface. Studies have found that the unique properties of perovskites, such as fast exciton dissociation and considerable diffusion lengths, together with the shallow trap states caused by impurities, largely reduce the charge carrier loss within the perovskite bulk. The energy utilization is quite high and almost equivalent to other leading monolithic crystalline technologies such as c-Si-based solar cells.
What is Holding PSCs Back to be Fully Commercialized?
The overall cost of PSC manufacturing is higher, and cheaper perovskite solar cells have a shorter lifespan. The presence of moisture significantly deteriorates PSCs. The substantial encapsulation required to protect the perovskite absorber can add to the cell cost and weight. Other factors reportedly affecting the instability of PSCs are thermal stress, heating under applied voltage, photo influence (ultraviolet and visible light), and mechanical fragility.
Very high PCE has been achieved using small cells, which is excellent for lab testing. However, for commercial applications, such small sizes would not suffice. Therefore, the scaling factor is a disadvantage yet to be solved.
Current-voltage scans yield ambiguous efficiency values due to the erratic current vs. voltage (IV) curves of perovskite solar cells showing hysteretic behavior. A lot more research is needed to understand this aspect of PSCs fully.
In the case of some PSCs, PbI is one of the breakdown products. This is a toxic compound, and it may be carcinogenic. Pb, used in many variants of PSCs, is a massive pollutant. Substitutions are rigorously worked on, and tin-based PSCs have already been introduced. However, the PCE of these tin PSCs needs to be improved.
A team at Iowa State University’s Microelectronics Research Center substituted cations in a variant of PSC with inorganic materials such as cesium. They also replaced iodine (I) with bromine (Br) in the perovskite absorber material, making the cells moisture-sensitive. However, it also reduced the overall efficiency .
Another team from the National Renewable Energy Laboratory (NREL) have devised a method to sequester the Pb used for making Pb-based perovskite solar cells to reduce the possible harmful leakage. They achieved this by using lead-absorbing films on the back and front of the solar cell, which under situations of extreme solar cell damage in a laboratory environment, sequestered a Pb leakage of ~96%. The team also reported that the long-term operational stability of the solar cells is not affected by the lead-absorbing layers .
An international team of scientists, led by Professor Qi Yabing at the Okinawa Institute of Science and Technology in Japan, with collaborators in China, France, and the US, have discovered an inorganic perovskite material (CsPbI3) that can replace silicon in terms of efficiency in solar cells manufacturing.
The researchers found that CsPbI3 is usually studied in its alpha phase with a crystal structure resulting in a black color and is particularly good at absorbing sunlight. However, it is unstable, and the structure rapidly degrades into a form less efficient at absorbing sunlight.
The researchers studied the beta phase of this material instead, which is more stable but less efficient in converting sunlight to electrical energy. The cracks often found in thin-film solar cells were the source of this lower absorbing efficiency. Repairing these cracks led to an increase in conversion efficiency, from 15% to 18%. With a small magnitude, it is almost within the ranges of certified efficiency values .
There is also exhaustive research going on to look at the benefits of combining perovskites with other technologies, such as c-Si, which creates “tandem cells”. In such structures, the advantages of the two technologies are used to develop better-performing cells .
The Potential for Perovskite Solar Cells
The main driving factors for fast device performance improvements during research on PSCs have been innovations in material or processing technologies.
The main points to explore would be:
- The nature of the charge carrier transport showing signs of low mobility but high diffusion lengths
- The main source of losses in the bulk or interfaces
- The role of mobile ions, which might compromise long-term stability
These aspects will be crucial to thoroughly understand the way PSCs work and to replace silicon-based solar cells in full-scale commercialization.
References and Further Reading
- Yixin Zhao, Kai Zhu. (2016) Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications. Chem. Soc. Rev., 45, 655-689. Available at: https://doi.org/10.1039/C4CS00458B
- Stefaan De Wolf, Jakub Holovsky, Soo-Jin Moon, Philipp Löper, Bjoern Niesen, Martin Ledinsky, Franz-Josef Haug, Jun-Ho Yum, Christophe Ballif. (2014) Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 5, 1035−1039. Available at: https://doi.org/10.1021/jz500279b
- Ranjan, R. (2020) Scientists Introduce An Innovative Way to Stabilize Perovskite Solar Cells at High Temperatures. [Online] Mercom India. Available at: https://mercomindia.com/scientists-introduce-way-to-stabilize-perovskite-cells/ (Accessed on 3 June 2020).
- AZoM. (2020) Researchers Achieve Potential Breakthrough in Hybrid Perovskite Solar Cells. [Online] Available at: https://www.azom.com/news.aspx?newsID=52997 (Accessed on 3 June 2020).
- Yong Wang et al. (2019) Thermodynamically stabilized β-CsPbI3–based perovskite solar cells with efficiencies >18%. Science. Vol. 365, Issue 6453, pp. 591-595. Available at: https://doi.org/10.1126/science.aav8680
- Peleg, R. (2016) Perovskite solar cells - a true alternative to silicon? [Online] Available at: https://www.nanowerk.com/spotlight/spotid=45249.php (Accessed on 3 June 2020).
- Tze-Chien Sum, Nripan Mathews. Halide Perovskites: Photovoltaics, Light Emitting Devices, and Beyond.