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Development of II-VI ternary alloys for CdTe-based solar cells
Samoilenko, Yegor Yurevich
Samoilenko, Yegor Yurevich
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2020
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CdTe has emerged as the leading commercial thin film photovoltaic technology. Recent advancements in photoconversion efficiency were achieved through the introduction of alloyed CdSeyTe1-y (CST) absorber, as well as replacement of conventional CdS window layer with more transparent alternatives such as MgxZn1-xO (MZO). These II-VI ternary alloys offer a wide range flexibility of adjusting several important parameters of the CdTe-based solar cells, such as band gap and conduction band alignment. The focus of this thesis can be divided into two parts: development of Cd1-xZnxTe (CZT) absorbers and combinatorial discovery of MZO emitters.CZT alloys form at the back of the device stack during back contact activation by interdiffusion between CdTe and ZnTe. ZnTe doped with Cu is commonly used as a buffer layer at the back of the device stack to facilitate creation of an ohmic contact with CdTe absorber. In addition, Cu diffuses into CdTe and provides p-type doping for the absorber. The role of Cu during back contact activation was studied in this thesis. By depositing bilayers of CdTe and ZnTe:Cu with varying Cu loadings and subjecting them to short annealing steps it was shown that Cu is a critical flux agent that induces interdiffusion, recrystallization, and grain growth in a matter of minutes at studied temperatures of 320 oC and above. Cu was also shown to scavenge excess Te in ZnTe to form CuxTe clusters. CZT alloys are also of interest as absorber due to the full miscibility of CdTe and ZnTe that enables a tunable band gap from 1.5 to 2.26 eV. In particular, CZT alloys with a band gap of ~1.7 eV can potentially be used as a top cell in a tandem device. However, conventional device processing, specifically the CdCl2 activation step, results in the loss of Zn and the formation of stacking faults. We explored the use of molecular Cl2 to improve stability of CZT alloys during device processing. In comparison to conventional CdCl2, the use of molecular Cl2 offers an advantage of independent control of temperature and Cl2 concentration. Cl2 concentration of 15 ppm was identified to eliminate Zn loss at temperatures equivalent to conventional CdCl2 treatment, i.e. 400-450 oC, however, photoluminescence measurements revealed minimal increase in the signal, suggesting incomplete activation of the absorber material. Cross-sectional transmission electron microscopy revealed accumulation of chlorine on top of the absorber but no chlorine was seen inside the bulk of the absorber or at the grain boundaries, which is most likely the reason for the poor activation of the absorber.MZO emitters are significantly better than conventional CdS due to their transparency and tunable conduction band alignment with CdTe or CST absorber. MZO is most commonly deposited by sputtering using pre-formed ceramic targets of fixed composition in an Ar ambient. This limits one to discrete compositions and is expensive. In addition, the stability of MZO has been a concern. The MZO stability issue has been attributed to the presence of oxygen in the CdTe device processing ambient, leading to double-diode behavior (S-kink) in the current density-voltage curves. Reactive co-sputtering technique from elemental Zn and Mg targets in Ar:O2 ambient developed in this work offers an alternative to conventionally prepared MZO. Reactive co-sputtering produced high quality, robust MZO films with promising stability and resilience to processing conditions. This stability of reactively co-sputtered MZO is attributed to be due to low concentrations of oxygen vacancies in the films, which was confirmed by electrical and Kelvin probe measurements. The “ideal” MZO composition depends on the specific architecture and processing employed. The combinatorial approach enabled rapid identification of optimal composition, as evidenced by achievement of high performing devices (~16%) across multiple research facilities both with and without oxygen in device processing ambients.
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