How to produce flexible CIGS solar cells with record efficiency

Sep 21, 2011
This is a flexible CIGS solar cell developed at Empa. Credit: Empa

High-performance flexible and lightweight solar cells, say, on plastic foils, have excellent potential to lower the manufacturing costs through roll-to-roll processing and the so called "balance-of-system" cost, thus enabling affordable solar electricity in the near future. Thus far, however, flexible solar cells on polymer films have been lacking behind in performance compared to rigid cells, primarily because polymer films require much lower temperatures during deposition of the absorber layer, generally resulting in much lower efficiencies.

The research team at Empa's Laboratory for Thin Film and , led by Ayodhya N. Tiwari, has been involved in the development of CIGS solar cells on both glass and with a special focus on reducing the deposition temperature of the CIGS layer. The group has repeatedly increased efficiency of flexible CIGS solar cells over the past years – first at ETH Zurich and now since three years at Empa. With their current record value of 18.7% Tiwari and his team nearly closed the efficiency gap to cells based on multi-crystalline silicon (Si) wafers or CIGS cells on glass. The scientific details of their novel low-temperature technology and the multi-layered device have recently been published in Nature Materials.

"To achieve such high efficiency values, we had to reduce the recombination losses of photo-generated charge carriers", said Tiwari. CIGS layers grown by co-evaporation at temperature of around 450 °C have a strong composition grading because of inadequate inter-diffusion of intermediate phases and preferential diffusion of gallium (Ga) towards the electrical back contact.

To overcome this problem doctoral students Adrian Chirilă and Patrick Bloesch developed novel processes for optimizing the solar cell performance. To achieve an appropriate composition profile in the CIGS layer – for enabling more efficient charge carrier collection and reduced interface recombination – Chirilă and colleagues developed an innovative growth process by carefully controlling the Ga and indium (In) evaporation flux during different stages of the evaporation process.

Such high-efficiency CIGS solar cells up to now were developed only on glass substrates with processes where CIGS layers are grown at temperatures of 600 °C or above. In contrast, polymer foils cannot withstand such high temperatures. The low-temperature process now developed by Tiwari and Co. not only yielded an 18.7%-efficiency cell on polymer foils but also another record of 17.7% on steel foil without any diffusion oxide or nitride barrier layer commonly used in high-temperature processes. Both efficiencies were independently certified by the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany. "We have thus shown that this low-temperature process is also applicable on low-cost metal foils such as aluminum or Mild-steel, achieving comparably high-efficiency cells and indicating a severe cost reduction potential with this technology", said Tiwari.

Scientists at FLISOM, a start-up company, and Empa have been collaborating to further develop low-temperature processing, and FLISOM is scaling up the technology for roll-to-roll manufacturing of monolithically interconnected solar modules and commercializing the technology.

Explore further: Intel wireless charging in a bowl coming sooner than later

Provided by Swiss Federal Laboratories for Materials Science and Technology

not rated yet
add to favorites email to friend print save as pdf

Related Stories

Recommended for you

Team improves solar-cell efficiency

Sep 19, 2014

New light has been shed on solar power generation using devices made with polymers, thanks to a collaboration between scientists in the University of Chicago's chemistry department, the Institute for Molecular ...

Calif. teachers fund to boost clean energy bets

Sep 19, 2014

The California State Teachers' Retirement System says it plans to increase its investments in clean energy and technology to $3.7 billion, from $1.4 billion, over the next five years.

Idealistic Norwegian sun trappers

Sep 19, 2014

The typical Norwegian owner of a solar heating system is a resourceful man in his mid-fifties. He is technically skilled, interested in energy systems, and wants to save money and protect the environment.

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

omatranter
3 / 5 (2) Sep 21, 2011
Fools, it should be obvious to all that the only technology that can save us from fictional Global warming caused freezing is Repulsive Neutrons, you ask for proof?

http://www.youtub...B6o-bbKY

No one has ever seen God. John 1:18
Or Repulsive Neutrons, Ha
nicknick
5 / 5 (1) Sep 21, 2011
Indium and Gallium are rare elements and already quite expensive. Let us save them for the future generations and the semiconductor industry and manufacture silicon solar cells instead.