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Installing PV systems inside a building seems to defy common sense, but it is justifiable on practical grounds. Since PV cells absorb a certain range of wavelengths from solar radiation to generate electricity, they can also do the same with light produced by indoor lighting fixtures. While artificial sources of light have their own spectrums that allow for light of different colors, these spectrums generally fall within the same range of wavelengths used by PV cells. Recently, a team of researchers from three countries have developed a type of flexible perovskite PV cell that achieves a new high in conversion efficiency in an indoor environment compared with the devices of similar types.Its members are from Italy’s Tor Vergata University of Rome, Colombia’s Universidad Surcolombiana (South Colombian University), and Germany’s Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (FEP).
From the standpoint of the amount of light being casted on a surface, conventional lighting fixtures located in buildings such as homes, stores, and offices can provide 100-500lx. In contrast, direct sunlight in most outdoor conditions shines around 110,000lx. The huge discrepancy in the strength of illuminance between the sun and artificial light sources affects the performance of PV systems. Generally speaking, conventional PV modules (or solar panels) that comprise of crystalline silicon cells are unable to generate electricity in an environment that is illuminated by just indoor lighting fixtures.
Perovskite, which is a class of compounds, has lately emerged as an alternative to crystalline silicon in the development to PV cells. It is also the material that the international team has chosen when devising a solution for PV generation in an indoor setting.
Unlike crystalline silicon, perovskite as an active layer in a PV cell can generate electricity with light of a much wider range of strengths. In other words, perovskite cells can operate under different brightness conditions from natural sun light to low-level indoor lighting. Perovskite also has a few advantages over the crystalline silicon in the aspect of cell manufacturing. For instance, perovskite compounds are made of materials that are abundant in supply and are less costly to produce than crystalline silicon. Moreover, perovskite can be deposited on curved or flexible substrates (e.g., glass panel or plastic tape). This advantage allows for the further diversification of cell and module designs. Buildings can incorporate curved solar panels as walls or roofs, and strips of flexible solar cells can be attached to the body of a car for supporting various automotive systems. There is even the possibility of flexible solar cells embedded in clothing for charging electronics. On account of these attributes, perovskite has become a new favorite material in the development of PV cells during these recent years.
Regarding conversion efficiency, perovskite is close to parity with crystalline silicon. Thanks to continuous R&D, the average conversion efficiency rate of perovskite cells has leapt from 3% to 24% within a decade. As for the flexible perovskite cell designed by the international research team, it has reached the conversion efficiency rates of 20.6% and 22.6% under 200lx and 400lx respectively. It also has a transmittance of more than 80%.
Moreover, the international research team uses a flexible glass substrate that can withstand being bent more than 1,600 times at a curvature of 20.5mm.
Earlier, another team composed of scientists from Linköping University in Sweden, the Chinese Academy of Sciences, and the University of Science and Technology Beijing created an organic PV cell that is also designed for operating indoors. Specifically, 1cm2 of this organic PV cell has a conversion efficiency rate of 26.1% with an open-circuit voltage of 1.10V under an LED illumination of 1,000lx. A larger version of the cell (4cm2) has a lower but still impressive conversion efficiency rate of around 23% under the same condition. Although both the large and small versions of the organic PV cell that was jointly developed by Swedish and Chinese scientists can achieve a fairly high level of conversion efficiency, they were tested in a much brighter environment. In contrast, the latest flexible perovskite cell that was jointly developed by researchers from FEP, University of Rome Tor Vergata, and Universidad Surcolombiana can reach the 20% level in the setting of low ambient light. The latter team claimed that considering all the factors, its flexible perovskite cell ranks higher in conversion efficiency.
The same team also emphasized that even though its indoor PV solution maintains a relatively low level of power density, it is well suited for most indoor applications such as supporting small sensors and IoT devices. According to FEP, the flexible perovskite cell can achieve the power densities of 16.7μW/cm2 and 35μW/cm2under 200lx and 400lx of LED illumination respectively.