| Work Detail |
After years of theoretical studies on gallium phosphide and titanium solar cells, a group of Spanish researchers has now attempted to build the first intermediate band device based on this combination of materials and has discovered that it can achieve greater external quantum efficiency at wavelengths greater than 550 nm.
A group of scientists led by the Complutense University of Madrid (Spain) has manufactured for the first time an intermediate band (IB) solar cell based on gallium phosphide (Gap) and titanium (Ti).
IB solar cells are believed to have the potential to overcome the Shockley-Queisser limit, i.e. the maximum theoretical efficiency that a solar cell with a single pn junction can achieve. It is calculated by examining the amount of electrical energy extracted per incident photon.
The devices are typically designed to provide a large photogenerated current while maintaining a high output voltage. They incorporate an energy band that is partially filled with electrons within the band gap of a semiconductor. In this cell configuration, photons with insufficient energy to push electrons from the valence band into the conduction band use this intermediate band to generate an electron-hole pair.
“Our group has been researching these cells for more than 15 years,” Javier Olea Ariza, lead author of the research, told pv magazine . “We published the first article in the series in 2009, and in the latest one we have moved on to manufacturing the first real devices. The devices do not yet work well and their current efficiency is very poor. Although further work is needed, these cells have the theoretical potential to achieve efficiencies of around 60%.”
In the article “ Optoelectronic properties of GaP:Ti photovoltaic devices ,” recently published in Materials Today Sustainability , Olea Ariza and colleagues explain 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 50 nm thick GaP:Ti absorber, a p-type GaP layer and metal contacts of gold (Au) and germanium (Ge). The GaP substrates were provided by the Polish research institute Lukasiewicz-Itme. “The GaP:Ti layer was modeled as a very thin GaP surface layer with a constant Ti concentration,” the scientists explained.
They then performed a series of transmittance and reflectance measurements, as well as spectroscopic ellipsometry, and found that at wavelengths longer than 550 nm there is a broad band that could be related to increased light absorption as a result of the incorporation of Ti.
“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,” said Olea Ariza, noting that there is still a long way to go before this technology can reach commercial maturity. “There is no point in thinking about it until we have a laboratory prototype where we have solved the problems and it has high efficiency.”
Looking ahead, the scientists said they want to achieve improved surface passivation through gas-forming annealing processes, as well as reducing the depth profile tails of Ti using a deposition technique rather than Ti ion implantation.
“In future work, we will look into obtaining thicker GaP:Ti layers for integration into high-efficiency photovoltaic devices,” they noted. “However, we also suggest using deposition techniques (such as sputtering) instead of ion implantation to achieve this thickness, in order to avoid implantation tails.”
The research group included scientists from the Institute of Optics of the Spanish National Research Council (IO-CSIC, Madrid) and the Autonomous University of Madrid. |
