Solar energy is a promising and popular research area as one of the types of renewable energy sources. The existing highly efficient multi-junction solar cells have already reached their theoretical efficiency limit. Therefore, today's global scientific community is concentrating on developing and implementing more efficient and cost-effective approaches to their production.
One of the directions of photovoltaics is the creation of lattice-mismatched A3B5 solar cells on a silicon substrate. However, this approach has a significant disadvantage: a high dislocation density in the components due to the mismatch of the lattice constants leads to poor quality of the layers of the A3B5 compounds and their considerable degradation, which limits their use for the photovoltaic conversion of solar energy.
Alexander Gudovskikh, professor at the Photonics Institute of the St. Petersburg Electrotechnical University “LETI”, proposed a solution to this problem. His team published the research results in the journal Physica Status Solidi (a) – Applications and Materials Science.
The approach proposed by ETU researcher “LETI” uses silicon substrates that are formed by combining atomic layer deposition technology in the initial stage of growth using organometallic vapor phase epitaxy (MOVPE). The main difference from previous approaches is the growth of the nucleation layer, which is carried out by plasma-assisted atomic layer deposition (PE-ALD) at relatively low temperatures with subsequent epitaxial growth of the top junction based on A3B5 quantum structures using MOVPE.
“Existing photocell formation processes use high temperature annealing (900-1000 ° C) of the silicon substrate in the early stages of growth for oxide removal and surface reconstruction, which further degrades the life of the substrate,” said Alexander Gudovskikh. Professor of the Photonics Department of the ETU “LETI” says.
The new technology enables the temperature of the epitaxial growth of GaP on Si substrates to be reduced to 600-750 ° C and the formation of GaP / Si structures with a GaP nucleation layer by PE-ALD at a temperature of 380 ° C.
Investigations of the structural and electronic properties and the temperature stability of the new material have shown that GaP / Si structures, which were obtained by PE-ALD without additional hydrogen plasma, have better photoelectric properties than those produced with high H2 plasma power and the formation of defects in the silicon induce subsurface layer.
“Annealing at temperatures of 550–600 ° C leads to a decrease in the defect concentration caused by the hydrogen plasma. Therefore, the GaP / Si interfaces produced by both types of PE-ALD processes have a similar quality after annealing. The thermal treatment of the GaP / Si structures at temperatures of 725–750 ° C leads to the diffusion of phosphorus from GaP into Si and the formation of an isotype n-GaP / np-Si heterojunction with improved photoelectric properties, ”comments Alexander Gudovskikh .
The new epitaxial GaP growth technology on Si substrates will increase the efficiency of double junction solar cells to record values: over 30% for non-concentrated AM0 radiation and over 35% for concentrated AM1.5D radiation. The high efficiency and optimal costs of the new technology offer broad application prospects: from solar energy elements installed on the earth's surface to the power supply of space vehicles.
The work was carried out jointly with colleagues from the Academic University or Alferov University, the Ioffe Institute and Paris-Sud University (France) and funded by a grant from the Russian Science Foundation.
In 2018, the Photonics Department started a master's program in renewable solar energy, in which students can master modern solar technology and learn the most important principles of photovoltaics. It gives insights into the underlying physical principles and materials science aspects of photovoltaics, solar module development technology, equipment, design and maintenance of solar power plants.