First attempt to build solar cells based on gallium phosphide and titanium – pv magazine International

October 21, 2024
Emiliano Bellini

A group of scientists led by the Universidad Complutense de Madrid in Spain have produced, for the first time, an intermediate-band (IB) solar cell based on gallium phosphide (Gap) and titanium (Ti).

The IB solar cells are believed to have the potential to surpass that Shockley-Queisser limit – 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 generally designed to deliver a large photogenerated current and maintain a high output voltage. They contain an energy band that is partially filled with electrons within the band gap of a semiconductor. In this cell configuration, photons whose energy is insufficient to push electrons from the valence band to the conduction band use this intermediate band to create an electron-hole pair.

“Our group has been researching these cells for over 15 years,” said lead author of the research Javier Olea Ariza PV Magazine. “We published the first article in the series in 2009 and have moved on to making the first real devices in our latest article. The devices are not yet working well and their current efficiency is very poor. Although more work is needed, these cells have the theoretical potential to achieve efficiencies of around 60%.”

In the newspaper “Optoelectronic Properties of GaP:Ti Photovoltaic Devices,” recently published in Materials today sustainabilityOlea Ariza and his colleagues explained that GaP has a band gap of 2.26 eV, which they say is “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 GaP surface layer with a constant Ti concentration,” the scientists explained.

They then performed a series of transmission and reflectance measurements as well as spectroscopic ellipsometry and found that there is a broad band at wavelengths above 550 nm that could be related to increased light absorption as a result of 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,” said Olea Ariza, pointing out that this technology still has a long way to go us can reach commercial maturity. “There is no point in thinking about it until we have a laboratory prototype in which we have solved the problems and which has high efficiency.

Looking forward, the scientists said they want to achieve better surface passivation through forming gas annealing processes as well as a reduction in Ti depth profile tails by using a deposition technique instead of Ti ion implantation.

“In future work, we aim to obtain thicker GaP:Ti layers that can be integrated into high-efficiency photovoltaic devices,” they said. “However, we also suggest using deposition techniques (such as sputtering) instead of ion implantation to achieve this thickness to avoid the implantation tails.”

The research group included scientists from the Institute of Optics of the Spanish National Research Council (IO-CSIC, Madrid) and the Universidad Autónoma de Madrid.