A group of scientists led by the Universidad Complutense de Madrid in Spain has fabricated an intermediate band (IB) solar cell based on gallium phosphide (Gap) and titanium (Ti) for the first time.
The IB solar cells are believed to have the potential to exceed the Shockley-Queisser limit – the maximum theoretical efficiency that a solar cell with a single p-n junction can reach. It is calculated by examining the amount of electrical energy that is extracted per incident photon.
The devices are commonly designed to provide a large photogenerated current and maintain a high output voltage. They incorporate an energy band that is partly filled with electrons within the bandgap of a semiconductor. In this cell configuration, photons with insufficient energy to push electrons from the valence band to the conduction band utilize this intermediate band to generate an electron-hole pair.
“Research on these cells has been going on for over 15 years in our group,” the research's lead author, Javier Olea Ariza, told pv magazine. “We published the first article in the series in 2009 and, in our latest article, we have moved on to making the first real devices. The devices do not work well yet and their current efficiency is very poor. Although more work is needed, these cells have the theoretical potential could reach efficiencies of around 60%.”
In the paper “Optoelectronic properties of GaP:Ti photovoltaic devices,” which was recently published in Materials Today Sustainability, Olea Ariza and his colleagues explained that GaP has a bandgap of 2.26 eV, which they describe as “remarkably close” to the theoretical optimum.
They built the 1 cm2 cell with a GaP:Ti absorber with a thickness of 50 nm, a p-type GaP layer, and metal contacts made of gold (Au) and germanium (Ge). The GaP substrates were provided by the Polish research institute Łukasiewicz-Itme. “The GaP:Ti layer was modeled as a very thin surface GaP layer with a constant Ti concentration,” the scientists explained.
They then conducted a series of transmittance and reflectance measurements, as well as spectroscopic ellipsometry, and found that, at wavelengths above 550 nm, there is a broad band that could be related to enhanced light absorption as a consequence of the Ti incorporation.
“The results confirm that the GaP:Ti material has a very high absorption coefficient at energies below 550 nm, which is one of the goals of this work,” Olea Ariza said, noting that there is still a long way to go before this technology may reach commercial maturity. “It doesn't make sense to think about it until we have a laboratory prototype in which we have solved the problems and it has high efficiency.
Looking forward, the scientists said they want to achieve better surface passivation via forming-gas annealing processes, as well as the reduction of the Ti depth profile tails by using a deposition technique instead of Ti ion implantation.
“In future work, we will seek the obtention of thicker GaP:Ti layers to be integrated in highly efficient photovoltaic devices,” they said. “Nevertheless, we also suggest the use of deposition techniques (like sputtering) instead of ion implantation to reach this thickness, in order to avoid the implantation tails.”
The research group included scientists from the Spanish National Research Council's Institute of Optics (IO-CSIC, Madrid) and the Universidad Autónoma de Madrid.
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