Photovoltaics is considered as one of the key sustainable solutions to respond to the increasing global energy demand. At present, silicon solar cells are commercially the most viable photovoltaic technology, having however restrictions for example regarding maximum conversion efficiency as well as application scenarios.
An alternative photovoltaic technology has been studied at the Optoelectronics Research Centre of Tampere University, namely multijunction solar cells incorporating III‒V semiconductors. Among all photovoltaic technologies, this technology has yielded the world record efficiencies of over 47 %. Owing to their material properties, these solar cells are suitable also for more demanding conditions such as for space power stations used on satellites. Another typical application is terrestrial concentrator systems.
“The potential of reaching higher conversion efficiencies using multijunction solar cells lies in the device configuration having multiple junctions that are optimized to exploit different spectral ranges of the solar spectrum. The goal is to ensure that a larger fraction of the solar energy can be converted into electricity than what would be possible with conventional silicon solar cells having only one junction,” Marianna Vuorinen explains.
New material systems require process development
Vuorinen’s dissertation focuses on studying advanced multijunction solar cells incorporating so called dilute nitride materials. These materials enable to convert the sunlight energy at the spectral domains where established materials are not efficient or cannot be integrated in multijunction solar cells architectures.
“To reach best possible performance and ultimately a high conversion efficiency, fabrication processes must be optimized to minimize power losses. Developing new material systems always requires adaptation of the fabrication processes, and this is where my dissertation focuses. The more diverse the semiconductor compounds forming the device are, the more demanding their processing becomes,” Vuorinen states.
It’s all in the details
Processing of the cell components studied in the dissertation is performed under highly regulated conditions in cleanroom laboratory, in which the humidity and possible contamination particles are being controlled. Multijunction solar cells are fabricated using conventional processing methods used in semiconductor industry, yet they need to be adapted for each type of device architecture in order to achieve the target performance.
“Basically, solar cell processing includes fabrication of different device features utilizing patterning and thin films. Fabrications requires high level of precision, since the smallest features are of the order of few micrometers, that is, only a fraction of the thickness of a hair. In addition, solar cell performance is very sensitive to fabrication defects. Even a small dust particle may deteriorate the fabrication process significantly,” Vuorinen particularizes.
Building blocks for semiconductor device fabrication
Vuorinen’s dissertation focuses especially on optimization of one essential process method, wet etching of dilute nitride compound semiconductors and multijunction solar cells incorporating them. The dissertation includes also development of fabrication process for a novel, advanced electrical contact. These fabrication building blocks have also applications for other type of devices than multijunction solar cells.
“Optimization of the fabrication focused on finding a suitable combination of the processing methods that ensures compatibility with the range of materials forming the solar cells. Altogether the research methodology for processing of III‒V solar cells is multidisciplinary, as it combines knowledge from the fields of physics, chemistry, and material sciences,” Vuorinen clarifies.
After dissertation Vuorinen will use and advance her knowledge concerning semiconductor processing working in industry.
Public defence on Friday 29 November
The doctoral dissertation of MSc (Tech) Marianna Vuorinen in the field of physics titled Advanced Processing of III‒V Multijunction Solar Cells will be publicly examined at the Faculty of Engineering and Natural Sciences at Tampere University at 12 o’clock on Friday 29 November 2024 at Hervanta campus, in the auditorium K1702 of the Konetalo building (Korkeakoulunkatu 6, Tampere). The Opponent will be Professor Markku Sopanen from Aalto University. The Custos will be Professor Mircea Guina from the Faculty of Engineering and Natural Sciences of Tampere University.