Pushing to 20% efficient solution grown CIGS thin film solar cells

Successful deposition of thin film solar cells, such as CdTe and CIGS, have historically been limited to devices grown using vacuum techniques, such as evaporation, sputtering and close space sublimation. These techniques allow close process control of the material properties, and have produced the highest performing devices in the last few years. However, with an aim to reducing process costs, solution processed absorbers have been slowly closing the gap between atmospheric and vacuum processed solar cells in the last few years. CZTSSe and perovskite solar cells are produced with the highest efficiency using atmospheric techniques, and recently, Solar Frontier has announced a Fraunhofer certified 17.7% efficient solution processed CIGS device, and an “in-house” record of 18.7%. It is clear then that the potential for cost reduction by atmospheric processes is close to becoming a reality.

Recently, work at CREST has focused on the solution deposition of thin film absorbers, using metal chalcogenide precursors in thiol-amine solvents [2]. These precursors are ideal, since they contain no unwanted impurities (such as carbon, oxygen, and halides), and can be sprayed under atmospheric conditions to form continuous thin films, with precise composition control. After selenization, CIGS devices of 10% have been reported with this approach [3]. There is clear room for improvement however, and so it is important to develop a comprehensive understanding of the limiting processes happening during the film deposition, selenization and device fabrication stage.

With this in mind, the objectives of this project will be to utilise a range of advanced characterisation techniques for the development of solution processed CIGS solar cells at Loughborough. This will include a variety of temperature dependent electrical measurements (such as JVT and admittance techniques, thanks to the CDT capital equipment grant from 2014), as well as developing knowledge and understanding in less common techniques such as time resolved photoluminescence, rotating magnet Hall effect measurements, Jsc-Voc measurements, and multi spectral LBIC measurements, as well as standard material characterisation (such as XRD, Raman and SEM/TEM).

Liam Welch

Cohort 4