Solar Cells Using Ferroelectric Perovskites: The Next Generation Photovoltaics
Perovskite materials are those which are having chemical formula of ABX3(where X = I, Cl, Br, or O). The ferroelectric perovskite materials have drawn a major attention in the field of photovoltaics. These materials can bring revolution in the field of solar cell industry because of their ferroelectric polarization driven carrier separation and above band gap generated photo voltage. In principle, using perovskite materials the power conversion efficiency (PCE) can be enhanced beyond the maximum value (~34%) reported in traditional silicon-based bipolar heterojunction solar cells.
Apart from the conventional Si-based solar cell, the organic-inorganic hybrid solar cells tried to take over in this industry but none sustained. Tremendous efforts have been given to find reasonable solution until Michael M. Lee et al. reported on perovskite organo-metallic halide solar cells. The chemical formula of the organo-metallic halide is CH3NH3MX3where, M = Pb or Sn and X = Cl, Br, and I. The rapid improvement in perovskite organo-metallic halide solar cells has made them highly demanded and huge attention has been paid to them by the academic community. The PCE % has been reported till date is around 20 % . Figure 1 depicts how the PCE % of perovskite halide solar cells have improved in time. Unfortunately, the metal (Pb) which is used mostly in these devices is toxic. Also, the organic material is unstable in nature.
Modified solar panels designed with the ferroelectric perovskite oxide materials open the way to provide potentially sustainable power that can be price competitive and efficient. When pre-poled ferroelectric materials are illuminated by ultraviolet/visible/infrared radiation, generation of electron-hole pairs is occurred. These charge carriers are separated by the internal electric field due to the ferroelectric polarization and therefore photo voltage and photo current is produced. This is an added advantage over conventional semiconductor p-n junction based photovoltaic cell where an external bias voltage is needed for its operation.
Solar cell devices using ferroelectric perovskite oxides have three major benefits. First, a solar cell device that is thinner than today’s silicon-based solar panel can be produced. Second, they use cheaper materials than today’s high cost thin film solar panel. Third, the materials used are ferroelectric which can switch polarity (see fig. 2). In principle, the PCE % can exceed the theoretical energy-efficiency limits of today’s silicon solar cell materials. Hence, there is a great need to explore ferroelectric perovskite oxide materials for solar cell applications.
The photovoltaic effect in ferroelectric BaTiO3was observed back in 1956, and renewed interest has been started very recently. According to Glass et al. the generated photocurrent density (J) is proportional to the intensity of light (I). It is written as J= kαI, where kis Glass coefficient and αis optical absorption coefficient. Many reports suggest that photocurrent is also affected by incident light wavelength, poling voltage, thermal effect, space charge, electrode geometry, thickness of the active layers etc. The results are enormous and promising but the magnitude of the photo generated voltage and current in ferroelectrics are marginally low value. It is necessary to enhance the photovoltaic effect of these materials so as to envisage its potential to the application point of view. Ferroelectrics with higher remnant polarization could provide one such possibility.
In this context, recent report on enhanced polarization of heteroepitaxially constrained thin films of multiferroic BiFeO3gains more importance. The BiFeO3is reported to exhibit larger polarization value, higher than Pb0.5Zr0.5TiO3(PZT), both in polycrystalline and epitaxial form. The polarization value is highly dependent on its crystallinity and orientations. This gives extra degrees of freedom to change the polarization by fabricating it in polycrystalline and epitaxial form. We are planning to study the photovoltaic effect on the ferroelectric BiFeO3films grown in different crystalline forms.
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Dr. Chiranjib Nayek
Chief R&D Officer, AlienSolar
He did his Ph.D in physics from Indian Institute of Technology Madras and joined United Arab Emirates University as post doctoral fellow. He is working in AlienSolar as the Chief Research and Development Officer (CRADO). His soul interest is on renewable energy and their maximum usability as energy resource both in day-to-life and scientific benefits.
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