17.6% efficiency 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 the Swiss Federal Laboratories for Material Science and Technology. The 17.6% PV conversion efficiency of the flexible solar cells has been independently certified by the Fraunhofer Institute for Solar Energy Systems.

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Figure 1: Independently certified measurement of a 17.6% efficiency flexible CIGS solar cell under standard test condition performed at ISE Friburg, Germany.
(June 30, 2010) — 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 17.6% photovoltaic conversion efficiency of the flexible solar cells has been independently certified by the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany. 
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Figure 2: Quantum efficiency measurement of a 17.6% efficiency flexible CIGS solar cell under standard test condition performed at ISE Friburg, Germany.

 Scientists under the leadership of Dr. Ayodhya N. Tiwari at the Laboratory of Thin Film and Photovoltaics, EMPA, Dübendorf, Switzerland (http://www.empa.ch/tfpv) have been developing thin film solar cells based on Cu(In,Ga)Se2 (CIGS) semiconductor material, and the research group, while working at ETH Zurich in 2005, had developed 14.1% efficiency flexible CIGS solar cells on polymer film. Now, collaborating with FLISOM Company (www.flisom.ch), the group’s process results in a 17.6% efficiency solar cell.

Development of high efficiency solar cells on plastic film 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 (CTE) 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. Doctoral student Adrian Chirila and 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 450C that is suitable for polyimide film as a flexible substrate for roll-to-roll manufacturing. 

A large improvement in the efficiency of flexible cell, 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 Friburg confirmed 17.6% efficiency with Voc = 688 mV, Isc = 34.8 mA/cm2, FF = 73.6%. 

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Figure 3: Scanning electron microscopic image of the cross-section of a typical CIGS solar cell.
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. The successful development of 17.6% efficiency flexible CIGS solar cell on polymer with a low temperature process 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. 
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Figure 4: Image of flexible CIGS solar cells on lightweight polymer film.

Preliminary results of the flexible CIGS solar cell development were presented at the recent IEEE Photovoltaic Specialist Conference, June 21-25, Honolulu, HI, where Adrian Chirila also received a Student Award for the development of high-efficiency flexible CIGS solar cells and monolithically connected solar modules developed in collaboration with FLISOM Company.

For more information, contact Prof. Dr. Ayodhya N. Tiwari, Head, Laboratory for Thin Films and Photovoltaics (Abt. 130), EMPA (Swiss Federal Laboratories for Material Science and Technology), Überlandstrasse 129, CH-8600 Dübendorf, Switzerland; +41-44-8234130 (Direct); ayodhya.tiwari@empa.ch; www.empa.ch/tfpv

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