Dr. Akash Bhatnagar
AG Light for high-voltage photovoltaics
Zentrum für Innovationskompetenz, Martin Luther Universität Halle-Wittenberg, 06120 Halle (Saale), Germany
Dr. Akash Bhatnagar
Junior Research Group Leader
Light for High Voltage Photovoltaics,
The Centre for Innovation Competence SiLi-nano®, Halle, Germany
The ever-growing demand for renewable energy cannot be overstated. Photovoltaic effect, which involves conversion of light to electricity has been considered as one of the most promising pathways. The field of photovoltaics is largely dominated with the use of semiconductor-based devices. The photovoltaic device primarily consists of a pn-junction with a space charge region at the interface between p- and n-type material . The maximum efficiency obtainable from the pn-junction devices is defined by the Shockley-Quisser limit and has been calculated to be around 30% . The innate limitation of the photovoltaic energy conversion in semiconductors has triggered the pursuit to find new materials that have a fundamentally different mechanism for charge separation.
Figure 1. (a) Piezo force microscope 3.5 x 3.5 µm2 image acquired with Park XE-100. The brown and yellow colors show different ferroelectric domains. (b) Measurement geometry for BPV effect. (c) Angular dependent photovoltaic current. (d) Enhancement in Voc as the temperature is reduced in the case of 71° domains measured across 15 and 100 µm gaps [Taken from 4].
In this context, the photovoltaic effect in ferroelectric materials, i.e. bulk photovoltaic effect (BPV), has attracted a lot of attention. Ferroelectric materials can exist in different states of polarizations, or domains (Figure 1a), and can be switched from one state to the other by the application of electric field. The region in between the two domains is referred to as the domain wall. The photovoltaic effect in these materials is driven by the symmetry of the material rather a junction. As a result, the photocurrent generated is determined by a tensor which is largely analogous to the piezoelectric tensor of the material . The photocurrent when measured with a linearly polarized light (Figure 1b), exhibits a dependency on the angle which the electric field of linearly polarized light makes with the direction of current flow (Figure 1c). Interestingly, the resultant open circuit voltages (Voc) are much higher than the band gap of the material and are in the range of 10–20V! Even higher voltages can be achieved if the overall conductivity of the samples is suppressed by lowering the temperature (Figure 1d) .
Another interesting aspect is the switch-ability of the effect. Unlike semiconductor-based photoeffect, the direction of photocurrent can be effectively reversed and under certain conditions the magnitude can be massively reduced (Figure 2). Such tunability arises from the ordering of domains which can manipulated by the application of electric field. Conversely, the BPV effect can be also utilized to determine the ordering of domains and estimate the net polarization.
Figure 2. Photovoltaic current measured after each step of domain rearrangement upon application of switching field [Taken from 5].
However, there are some severe drawbacks that need to be addressed to widen the applicability of BPV effect. For instance, the band gap of ferroelectric material is typically above 2.8 eV, due to which only a small fraction of solar spectra can be utilized. Also, the current densities reported until now are way too low for any meaningful application. Nevertheless, with rapid development in thin film deposition and computational techniques, new material systems can be proposed and realized which would be able to overcome such limitations.
1) Sze, S.M. (1981) Physics of semiconductor devices, John Wiley & Sons, INC
2) Shockley, W., and Queisser, H.J. (1961) Detailed balance limit of efficiency of p-n junction solar cells. J. Appl. Phys., 32 (3), 510–519.
3) Belinicher, V.I., Malinovskii, V.K., and Sturman, B.I. (1977). Photogalvanic effect in a crystal with polar axis. Soviet Physics – JETP 46 (2): 362–366
4) Bhatnagar, A., Roy Chaudhuri, A., Heon Kim, Y. et al. (2013). Role of domain walls in the abnormal photovoltaic effect in BiFeO3. Nature Communications 4 (May): 2835.
5) Knoche, D.S., Yun, Y., Ramakrishnegowda, N. et al. Domain and Switching Control of the Bulk Photovoltaic Effect in Epitaxial BiFeO3 Thin Films. Sci Rep 9, 13979 (2019)