Properties of the Back Contact Interface for Non-Vacuum Deposited Precursor-Based Cu(In,Ga)Se₂ Solar Cells

In this work two possibilities for steps on the way towards cost-efficient solar cells are presented. Non-vacuum processes for the absorber fabrication are implemented and an idea how thinner absorber layers can still show relatively high conversion efficiencies is substantiated.
In order to investigate the interface properties of absorbers built with non-vacuum processes a basic non-vacuum selenisation process has been developed initially. The selenisation set-up has been designed and built up and the corresponding process has been optimized.
As a next step the molybdenum selenide formation and the influence of the changed selenium partial pressure and the precursor layers on it has been investigated: Both the molybdenum fabrication process (especially sputter pressure) and the selenisation (especially substrate temperature) were found to influence the MoSe2 formation. For sputtered precursors MoSe2 was formed during the selenisation. For doctor bladed precursors made from metal salts with ethylcellulose no molybdenum selenide formation has been observed. During the formation of the CIGS absorber layer a carbon layer was left between absorber and back contact which protected the molybdenum. ... mehr
Finally the influence of this carbon layer on the solar cell properties has been investigated. For thin absorber layers an additional thin carbon layer between back contact and absorber material was beneficial. Especially very thin layers showed significantly higher open circuit voltages. Most likely a lower recombination at the back contact is a key factor for the higher voltages in samples with a carbon layer at the back contact. Simulations with the Solar Cell Capacity Simulator (SCAPS) back up this assumption.