Thin Films and Photovoltaics  
General Information
Tandem Cells


CuIn1-xGaxSe2 (CIGS) is one of the most promising materials for highly efficient thin film solar cells. By changing the gallium to indium ration, the bandgap can be shifted between 1.0 and 1.7 eV to adjust the devices to various needs. This makes those cells also interesting for application in multijunction solar cells.

In our group, efficiencies up to 18.8% on glass substrates were achieved. Flexible thin film solar cells on polymer film with a new record efficiency of 20.4% have been independently certified by Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany.

The 20.4% efficiency solar cell is an independently certified highest efficiency record for any type of flexible solar cell on polymer film reported up to now. Furthermore, it represents the highest reported efficiency of a CIGS solar cell on any type of substrate. This solar cell processing can be adapted for roll-to-roll maufacturing of monolithically interconnected solar modules on polymer films.

The research in our group focuses on alternative materials and processes for buffer layers and back contacts, as well as the development of thin film tandem solar cells.

An In2S3 buffer layer can increase the efficiency by better utilization of the light in the visible range of the solar spectrum which leads to an increased current density. Here, different deposition methods like PVD, flash evaporation, and ultrasonic spray pyrolysis, are investigated.

For back contacts, transparent conductive oxides like ITO or ZnO:Al are investigated, which can be used for bifacial illumination using a mirror system (higher current with same are of absorber, thus potential to decrease cost) as well as for production of tandem cells using the combination of CGS with CIS. Furthermore, or group works on highly reflective back contacts like TiN, which should enable a decrease of the absorber thickness while maintaining the current density, and thus reduce cost per watt.

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