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Producing Flexible CIGS Solar Cells with Record Efficiency
By EmpaWednesday, September 21, 2011
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Flexible CIGS solar cells developed at Empa.
The technology yielding flexible solar cells with an 18.7% world record
efficiency developed by scientists at Empa, the Swiss Federal Laboratories
for Materials Science and Technology, has now been published in "Nature
Materials". Key to the breakthrough is the control of the energy band gap
grading in the copper indium gallium (di)selenide semiconductor, also known
as CIGS, the layer that absorbs light and converts it into electricity.
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.
Record-breaking team
The research team at Empa's Laboratory for Thin Film and Photovoltaics, led
by Ayodhya N. Tiwari, has been involved in the development of
high-efficiency CIGS solar cells on both glass and flexible substrates 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 deposition 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 Chirilã 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 – Chirilã and colleagues developed an innovative growth
process by carefully controlling the Ga and indium (In) evaporation flux
during different stages of the evaporation process.
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