Efficiency record of 17.6% on flexible CIGS solar cell on plastic developed at EMPA

Flexible thin film solar cells on polymer film with a new record efficiency of 17.6% have been developed by the scientists at EMPA.

Flexible thin film solar cells on polymer film with a new record efficiency of 17.6% have been developed by the scientists at the Swiss Federal Laboratories for Material Science and Technology (EMPA). The conversion efficiency record has been independently certified by the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany.

Scientists under the leadership of Dr. Ayodhya N. Tiwari at the Laboratory of Thin Film and Photovoltaics, EMPA in Switzerland have been developing thin film solar cells based on Cu(In,Ga)Se2 semiconductor material. The research group at EMPA working in close collaboration with FLISOM Company, has developed a process that resulted in a remarkably high 17.6% efficiency solar cell which is an independently certified highest efficiency record for any type of flexible solar cell on polymer film reported up to now.

This development is challenging because most of the polymer films used as substrate, lack thermal stability for growth of high electronic and structural quality CIGS solar cell layers at high temperatures. High thermal expansion coefficient of polymer causes a large stress in the layers deposited at high substrate temperature, resulting in cracks and delamination of the solar cells from the substrate. Adrian Chirila and other colleagues, working under the supervision of Dr. Tiwari have been developing a vacuum evaporation process for growth of high quality CIGS absorber layers at sufficiently low temperature of about 450 °C. This is suitable for polyimide film as a flexible substrate for roll-to-roll manufacturing.

Moving from a previous record value of 14.1% to a new record of 17.6% was achieved by reducing the optical and electronic losses in the CIGS solar cell structure. The most important factor was the optimisation of the composition gradient of Ga across the CIGS layer thickness and an appropriate incorporation of Na for doping during the final stage of the growth process. Consequently, an optimum band gap grading and larger grain size in CIGS layer resulted in a substantial increase in the efficiency of flexible solar cells. The photovoltaic measurements performed under the standard test condition at ISE Freiburg confirmed 17.6% efficiency with Voc = 688 mV, Isc = 34.8 mA/cm2, FF = 73.6%.

The low temperature process for CIGS deposition offers a unique advantage that the same process and equipment can be used for polymer as well as metal foils. Flexible CIGS solar cells on metal foils with highest efficiency of ca 17.5% are generally grown at high temperatures above 550 °C, while lower efficiencies were obtained on polymer films because of lower deposition temperature. This successful development has closed the efficiency gap between the solar cells on polymer and metal foils. This solar cell processing can be adapted for roll-to-roll manufacturing of monolithically connected solar modules on polymer films. Lower thermal budget and roll-to-roll manufacturing of high efficiency flexible CIGS solar cells will pave the way for substantial reduction in production cost of next generation of solar modules produced on large industrial scale in future.

This November, Dr. Tiwari Ayodhya will be speaking at the 3rd Thin Film Solar Summit USA about enhancing thin film efficiency and the developments that will allow the industry to go beyond the 12% mark. For details about his participation visit www.thinfilmtoday.com/us

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