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| Future energy generation: Perovskite solar cells | | |
Er.Loveneesh Talwar, Dr. R.Jaiganesh, Dr.A. Nazar Ali
Several new solar cell or photovoltaic technologies have been researched in the last few years, with respect to finding an effective alternative to silicon-based solar cells. Research and development in this area generally aims to provide higher efficiency and lower costs per watt of electricity generated. Some in the solar cell industry identify different “generations” of solar cell technology. The third generation is somewhat ambiguous in the technologies that it encompasses, though generally it tends to include, among others, non semiconductor technologies, quantum dot technologies, multi-junction cells, hot-carrier cells, dye-sensitized solar cells and up conversion technologies.
Perovskite solar cells are new 3rd-generation solar cells that appear to have a very good chance of contributing to large scale solar energy production based on their high power conversion efficiency and compatibility with scalable processes. Perovskite solar cells warrant discussion because never before in the history of solar cell research has such rapid progress in increasing the power conversion efficiency been witnessed as that which has occurred for these solar cells. Organic and inorganic lead halide perovskite is regarded as one of the ideal materials for photovoltaics. Because the development of solid-state Perovskites Solar Cells (PSC) in 2012, the Power Conversion Efficiency of PSCs has presented an unprecedentedly rapid enhancement from 9.7% to 25.2% in less than 10 years. The high performance of these devices was found to be due to their superior optoelectronic properties, such as high absorption coefficient, long charge carrier lifetimes and diffusion lengths, and high defect tolerance, which originate from the characteristic chemical bonding nature, crystal structure, and resulting electronic band structures of metal halide perovskite materials.Many unique features of Perovskites have also been studied to explain the optoelectronic properties of perovskite materials but still require further study to understand properties such as Rashba splitting, large polaron formation, and GB effects. In particular, understanding the nature of the defects in Perovskite materials appears to be further enhancing the performance of PSCs.
Recently, various organic cations have also been developed to broaden the compositional window of perovskite materials. However, most of these fundamental studies have still focused on MAPbI3 and, therefore, further studies on their extended systems will be required to design more ideal perovskite materials. The development of various process techniques has enabled the formation of high-quality perovskite films with compact morphologies and low defect densities. Control over the nucleation and growth has been found to be critical toward the formation of uniform and highly crystalline perovskite films, which has been enabled using antisolvent-assisted one-step, two-step, and adduct methods. However, the density of the defects in solution-processed films is still high, and thus, the development of effective methods to passivate the defects in the bulk and surface/GBs has been essential to achieve high efficiencies of >20%. Understanding the nature of the defects in state-of-the-art perovskite materials will facilitate the development of effective strategies to passivate detrimental defects. In full devices, the electron transport layer (ETL) and hole transport layer (HTL), and their interfaces with the perovskite layer, are equally important to achieve high efficiency. Although various charge transporting materials have been adopted in the device, a mesoporous TiO2 or planar SnO2 layer for the ETL and spiroMeOTAD for the HTL are still widely used in high-efficiency devices. Recently, interface engineering to improve the energy level alignment and electrical coupling has also been actively studied. Not only achieving high efficiency but also resolving the stability issue is of critical importance to enable the practical use of high-efficiency PSCs. All the issues covered in this review, including the structural design, materials chemistry, process engineering, and device physics, are closely related to the operational stability of the device. While working toward the enhancement of the efficiency of these devices, it is necessary to simultaneously consider the durability of the PCE. For example, the durability of defect passivation agents and interface engineering layers under prolonged operational conditions should be considered. Such an approach will ultimately contribute to the development of commercially viable perovskite photovoltaic devices. The unique features of perovskite materials enable the use of other approaches to further enhance the PCE toward and beyond the Shockley−Queisser limit. Owing to the tunable band gap and low-temperature fabrication processes of PSCs, their application as the top subcell in tandem devices is easily achievable. At an initial stage, however, tandem devices were not able to surpass the PCE of their corresponding single junction devices, but intense research efforts on the development of interconnecting layers and high band gap Perovskite materials have allowed a rapid enhancement in the PCE to reach 29.1% based on a perovskite/silicon tandem structure. Furthermore, the development of low-band gap Perovskite materials based on a Sn/Pb mixed composition has made it possible to fabricate perovskite/perovskite tandem solar cells. Adjusting the band gap of these materials, while minimizing the potential loss, will facilitate further enhancements in the PCE to >30%. However, accurate measurement techniques used to characterize tandem devices also need to be developed. To reduce the thermalization loss, the possibility of utilizing hot carriers has also been explored. While promising hot carrier lifetimes have been observed, the design of the device used to extract the hot carriers is still challenging. The photon recycling effect is another approach used to reduce the potential loss due to charge recombination; however, there still persists an active debate on the occurrence of the photon recycling effect in PSCs. Recently, PSCs have also been tested under concentrated sunlight, where an enhancement in the PCE was observed. However, an improvement in the photostability of perovskite materials seems to be a prerequisite for application in concentrator solar cell applications.
At present, the most challenging issue in perovskite solar cells is the long term stability, which must be cleared up before putting it into practical applications. As we know that the stability of perovskite solar cells upon the severe environment, e.g., thermal treatment, light illumination, humidity, etc., appears to be the bottleneck that impedes their further commercialized.
Among them, the humidity is demonstrated to be one of the possible causes of the degradation of perovskite. The commercialization of perovskite solar cells requires extensive research and development of new perovskite materials that are not only very effective in photoelectric conversion but also non-toxic and stable. Thus, in the next few years, more efforts are required for the development of inorganic lead-free perovskite solar cells. |
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