In this study we were trying to answer the question why the best laboratory size CuIn1-xGaxSe2-based (CIGSe) solar cells have been achieved at about 30% Ga, whereas the optimal band gap requires much higher Ga content (around 50%). Also for the first time ever we examined the effects of 4 keV Ar+ irradiation on optical properties of Cu2ZnSnSe4 (CZTSe) thin films. Photoluminescence (PL) measurements were performed on samples of CIGSe single crystals with x=0, 0.05, 0.10, 0.25, 0.50, 0.75, 1 and on samples of CZTSe thin films before and after Ar+ irradiation.The impact of variations in excitation intensity and temperature on the nature of the PL bands was investigated. For most CIGSe samples band-tail (BT) recombination coupled with band-band (BB) transition were identified as being responsible for the obtained PL peaks. The calculated band tails are deeper in CuGaSe2 than in CuInSe2, explaining why an increase in Ga content x > 0.3 does not improve the efficiency of solar cell.Also band-impurity (BI) transition and donor acceptor pair (DAP) recombination were discovered. For the first time ever excitons in the CIGSe samples with 5% and 10% Ga content were clearly observed and identified. In the CZTSe samples BI recombination was established as the main source of PL. Ar+ irradiation of CZTSe produced significant changes in the material: reduced PL intensity and creation of tail defect complexes supported by an increase in the average band tail depth γ from 25 to 30 meV as well as increase in the activation energy from 75 to 88 meV.The composition of the CIGSe and CZTSe samples was investigated using energy dispersive x-ray (EDX) analysis and wavelength dispersive x-ray (WDX) technique, respectively. According to the microscopy results, calculated cation ratios proved the nature of established recombinations.
|Date of Award||1 Nov 2017|
- University Of Strathclyde
|Supervisor||Robert Martin (Supervisor) & Maxim Fedorov (Supervisor)|