power conversion efficiency of PSCs has risen due to considerable amount of research and focus on architectures, compositions, and manufacturing techniques. For commercialization of perovskite cell technology, it should be able to survive at minimum 25 years without degradation in yield in an external environment. In a controlled environment, high-efficiency solar cells are synthesized but when they are exposed to the ambient atmosphere, efficiency drops rapidly. Light absorbers are very fragile/sensitive to water, because of this, solar cells are unstable [38]. The degradation also depends on oxygen, UV radiation, and temperatures.
The rate of degradation of perovskite is very rapid (few minutes) when there is presence of water. The products formed after the degradation of perovskite are CH3NH3I and PbI2. So to increase the lifetime of PSCs it should be kept away from moisture. An HTM with long alkyl chain was used, which would help to prevent water from passing from HTM to perovskite [48]. A carbon material layer of 10 μm thick was used as back contact to retain water. After these arrangements, the stability got increased and power conversion efficiency remained as it is for 1008 hours [49].
The formation of perovskite CH3NH3PbI3 film is carried (annealing) at 100°C for 10 minutes [38]. During its annealiation process when the temperature crosses 120°C, the material(CH3NH3PbI3) starts to degrade [50]. When temperature crosses 300°C, the perovskite splits into PbI2, CH3NH2, and HI. Sometimes, TiO2 can also bring instability because it can trap electrons in the crystals as it is nonstoichiometric, and this generally happens in an inert environment. So in seal-packed solar cells, there is a huge problem to avoid perovskite to get oxidized [38]. One solution is to dope TiO2 with aluminium, which can greatly reduce the oxidation [51].
To some extent, the stability of the PSC has increased but it is still far away from long-term use and commercialization. For improving the stability the stability of PSCs, it is necessary to understand the degradation mechanisms and find out the state at which degradation reactions originated or accelerated [52].
2.7 Conclusion
The efficiency of the PSC in the laboratory has reached 23.1%, which is comparable to the single crystalline solar cell. Because of high achievable efficiency, the researchers of different backgrounds of science have contributed in the investigation of PSCs. The higher efficiency of PSCs has been achieved due to high coefficient of absorption and long diffusion lengths. Perovskite solar cell has desired properties, enormous potential, low cost, and high efficiency. Despite achieving high efficiency, there is much difference between the stability of PSCs (1000 hours) and silicon solar cells (20– 25 years). The future challenge is to increase stability. Another drawback is it has toxic material, like Pb, which is very harmful for human beings, as well as the environment. There are some problems that are not solved yet, such as the materials, to increase the scalability, processing of materials, and so on [38].
In total, there are three major concerns, which should be overcome before its commercialization. First, the degradation mechanism should be explored as humidity, heat, and UV light are the main reasons for degradation and brings instability in perovskite. Second, Pb is very harmful to nature, hence it should be replaced with some safer option. Third, PSCs has very high efficiency in small areas of 0.1 cm2, so this area must be increased such that efficiency and stability remains the same so that we can perform high scale [53]. Researches need to focus and improve on the perovskite and electron-hole transport materials in the future, so that we can use PSCs in place of the conventional solar cell. If these problems are solved, surely perovskite material is projected to have a significant part of the solar cell industry.
Acknowledgment
The authors acknowledge the Chemical Engineering Department of Nirma University for providing the necessary help, infrastructure, and support for the preparation of this chapter/work.
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