Doctoral dissertation

Timo Aho: Diffractive reflector on the backside of solar cells enables more efficient solar cells

Timo Aho.
Solar cells that employ light-guiding structures, such as diffractive back reflectors, show an increased current generation, thus higher efficiency. Moreover, the implementation of such photon recycling abilities usually requires thinning of the solar cells, which brings advanced functionalities to the solar cells such as flexibility and lighter weight. These are instrumental for power generation in advanced satellites or other emerging systems, for example in powering up drones. To this end, in his doctoral dissertation Timo Aho demonstrated advanced high-efficiency solar cell concepts utilizing back reflector technologies.

Energy demand will increase in the future and thus carbon free energy production will be essential from the environmental point of view. To this end, more efficient and larger scale exploitation of solar radiation will continue to play a major role for renewable energy production.

“This need is fueled by the deployment of novel solar cell materials and concepts targeting increased efficiency of solar energy harvesting. At the same time, wider use of solar energy requires advanced functionalities, for example flexible solar cells that can be folded on curved surfaces,” Timo Aho explains.

The focus of Aho’s research was on the next generation solar cells and their fabrication methods. According to the results, solar cells employing back reflectors are producing increased current generation: The unabsorbed light passing through the solar cells is reflected back into the active area, leading to higher energy production. In this way, the thickness of the active layer can be reduced, thus saving the fabrication costs of the solar cell material. In addition, it enables the use of absorbing materials that cannot be very thick due to material property constrains.

“By using diffractive reflectors as a part of advanced next generation solar cells, it is possible to reach 50% efficiency, which is seen as the next milestone in the photovoltaic research community,” Timo Aho says.

The advanced solar cells that are studied in Timo Aho’s thesis are relatively expensive to fabricate when compared to silicon based solar cells, that are dominating the consumer markets.

“The main application area of these next generation solar cells is currently in space industry such as satellites and space stations. These type of flexible and light-weight solar cells which have high efficiency, are essential for example to the development of larger drones used in security and sensing applications,” Aho clarifies.

“In the consumer markets, these advanced solar cells are most likely to be installed first to large solar parks. After manufacturing techniques have developed further and the prices have decreased, it is possible that also consumers are investing in the next generation solar cells due to the superior efficiency,” Timo Aho explains.

The research was carried out in the framework of the H2020 SPACE project “TFQD” and ERC AdG Project “AMETIST”.

The doctoral dissertation of M.Sc. (Tech) Timo Aho in the field of photonics titled Advanced III‒V Solar Cells with Back Reflectors will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University on Friday 27 November 2020 at 12:00 in Tietotalo building auditorium TB109, Korkeakoulunkatu 1, Tampere. The Opponent will be Professor Antonio Martí from Technical University of Madrid via remote connection. The Custos will be Professor Mircea Guina from the Optoelectronics Research Centre of the Faculty of Engineering and Natural Sciences at Tampere University.

The dissertation is available online at

The public can follow the event via remote connection.

Photo: Antti Tukiainen