Polymide Flexible Solar Cells reach 14.1%, Swiss research

Zurich, Switzerland: Flexible Solar Cells on Polymer Foil Reach 14.1% Efficiency

The Swiss Federal Institute of Technology (ETH) has set a new record for thin film solar cells on plastic (polyimide) foil.

The research team of Professor Ayodhya Nath Tiwari, Dr. Dominik Rudmann and David Bremaud developed polycrystalline thin films solar cells of Cu(In,Ga)Se2 (called CIGS) on very thin polyimide foils at the Thin Film Physics Group, Laboratory for Solid State Physics, ETH Zurich. A multilayer structure of about 4 micron thickness was grown with a combination of thin film deposition methods.

The achievement of a solar cell conversion efficiency of 14.1% under standard AM1.5 illumination condition was certified by the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany, which is a renowned European agency for certification of solar cell and module efficiencies. These researchers have surpassed earlier record of 12.8% for CIGS cells on polyimide established in 1999.


A thin film solar cell on plastic (polyimide) foil
A significant improvement in the efficiency occurred because of change in the CIGS deposition process in such a way that the CIGS phase formation is not inhibited at low temperature of deposition, while Na is added with a post deposition annealing method to passivate the grain boundaries in CIGS, and therefore, improving the electronic properties of CIGS layer and efficiency of the solar cells.

During the 31st IEEE Photovoltaic Specialist Conference-2005, Orlando, USA and the 20th European Photovoltaic Solar Energy Conference-2005, Barcelona, Spain they presented papers towards the development of solar cells with efficiency exceeding 15% which requires application of anti reflection coating for reduction of about 13% reflection losses from the top surface.

Such high efficiency of solar cells on plastic (polyimide) foils transforms to a very high, above 2.5 W/g, specific power (ratio of electrical power generated to the weight of the solar cell), which offers several advantages related to portability. Another feature of these solar cells on plastic foil is very small radius of curvature - less than 1 cm - for roll-ability.

Such highly efficient, lightweight and roll-able solar electricity generators will enable a large variety of terrestrial and space applications technically possible which are otherwise either not viable or not attractive with other rigid, fragile, heavy solar cells based on wafers and glass substrates. Besides the value added consumer electronic applications to provide power for recharging batteries in laptops, i-pods, mobile phones, solar jackets, etc., high specific power and roll-able solar electricity generators can be integrated in tents and canopies. These solar cell integrated tents would be a valuable tool to provide emergency services when established infrastructure fails because of any disaster and natural calamities.

CIGS solar cells are known for highly stable long term performance not only for terrestrial - but according to recent studies- also for space applications. CIGS solar cells exhibit superior radiation (electron and proton) tolerance compared to conventional Si or GaAs solar cells. This means that the performance of such solar cells in space, when the satellite passes through the van Allen radiation belt of high energy charged (electron and proton) particles, will not degrade significantly. Flexible and lightweight solar cells can facilitate lightweight and simple solar power generating arrays that are easy to deploy in space. Such lightweight and highly efficient solar power generating structures have potential to save millions of Euro only in the launching cost of the satellites.

Potentially lower manufacturing cost (€/W) of polycrystalline thin film solar cells makes them also interesting compared to expensive wafer based solar cells.

The next step is to transfer the research results to large volume industrial production. The roll-to-roll manufacturing of solar modules will enable reduction in the manufacturing cost and energy pay-back time.

Further details: Dr. Ayodhya Nath Tiwari, Thin Film Physics Group, Laboratory for Solid State Physics ETH (Swiss Federal Institute of Technology) Zurich Tel: +41-44-4451474 or email at tiwari@phys.ethz.ch