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The effects of the CdCl2passivation treatment on thin-film CdTe photovoltaic films and devices have been extensively studied. Recently, with an addition of CdSeTe layer at the front of the absorber layer, device conversion efficiencies in excess of 19% have been demonstrated. The effects of the CdCl2passivation treatment for devices using CdSeTe has not been studied previously. This is the first reported study of the effect of the treatment on the microstructure of the CdSeTe /CdTe absorber. The device efficiency is < 1% for the as-deposited device but this is dramatically increased by the CdCl2treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl2passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults, increase in grain size and reorientation of grains. The CdCl2treatment leads to grading of the absorber CdSeTe/CdTe films by diffusion of Se between the CdSeTe and CdTe regions. Chlorine decorates the CdSeTe and CdTe grain boundaries leading to their passivation. Direct evidence for these effects is presented using STEM and Energy Dispersive X-ray Analysis (EDX) on device cross-sections prepared using focused ion beam etching. The grading of the Se in the device is quantified using EDX line scans. The comparison of CdSeTe/CdTe device microstructure and composition before and after the CdCl2treatment provides insights into the important effects of the process and points the way to further improvements that can be made.

Magnesium zinc oxide (MZO, MgxZn1-xO) is a leading emitter for CdTe-based solar cells due to its transparency and the ability to tune its conduction band offset with the absorber. Devices employing alloyed cadmium selenide telluride (CST, CdSeyTe1-y) absorbers achieved high efficiency (>19%) using MZO deposited by reactive sputtering over a broad composition range (3.68–3.92 eV, x: 0.20–0.35). Minimal differences in implied and measured open circuit voltage indicate that the contacts are well passivated and highly selective across the spectrum of MZO employed. Device performance insensitivity to MZO composition, which is not observed in CdTe devices, is attributed to the formation of an oxygenated interface layer. Se volatility creates a group VI deficiency at the interface that drives O migration from the MZO into the absorber. This introduces conductivity in the emitter not present in its as-deposited state, contributing to the exceptional performance observed. It is shown that the quality of device passivation depends on the oxidation state of the as-deposited MZO such that intelligent control and management of the reactive sputtering process is required.

Distortions in the current–voltage curves of CdTe-based thin-film solar cells with MgZnO emitter layers are not uncommon. Even when such distortions are very subtle with standard sunlight, they can be greatly accentuated by the removal of short-wavelength photons with optical filters, and the dark-curve turn-on voltage can be noticeably increased. The strong suggestion is that good J–V curves with full-spectrum light often rely on photogeneration of electrons from trap states in the MgZnO emitter. When the carrier concentration of the CdTe absorber is increased with external doping, there is a corresponding need for a larger MgZnO carrier concentration. Based on the assumption that the emitter carrier concentration needs to be comparable to that of the absorber, Ga-doping of the MgZnO was shown to very nearly eliminate the large filtered-light distortions and is arguably superior to MgZnO that relies on photogeneration. In addition, it was shown that photogeneration and recovery in MgZnO is relatively slow, so that without Ga-doping, the cell efficiency improves gradually for most of an hour in the light, and the turn-on voltage of the dark curve increases on a similar time scale when the light is removed.

CdTe thin film solar cells have demonstrated efficiencies over 20%, but CdTe has a somewhat higher band gap than optimal for single-junction terrestrial solar-cell power generation. A reduction in the band gap could therefore result in an overall improvement in performance. To reduce the band gap, selenium was alloyed with CdTe using a novel co-sublimation extension of the close-space-sublimation process. Co-sublimated layers of CdSeTe with various selenium concentrations were characterized for optical absorption and atomic concentrations, as well as to track changes in their morphology and crystallinity. The lower band-gap CdSeTe films were then incorporated into the front of CdTe cells. This two-layer band-gap structure demonstrated higher current collection and increased quantum efficiency at longer wavelengths.

Resistive oxide materials play an important role in the front contact of CdTe solar cells. The high-resistance transparent (HRT) or “buffer” layer has been used extensively in CdTe thin-film photovoltaics to enable a reduction in CdS thickness while maintaining near-maximum device voltage and fill factor. SnO2- and ZnO-based alloys were tested as HRT layers on a fluorine-doped tin oxide transparent conducting oxide. SnO2-based alloy HRT layers were deposited via atmospheric pressure chemical vapor deposition (APCVD). Alloying ZnO with MgO to create MgxZn1−xO (MZO) via radio-frequency sputter deposition was explored as a way to reduce the electron affinity of ZnO HRT layers. To fully understand the behavior of these materials, many devices were fabricated with either no CdS layer, a sublimated CdS layer, or a sputtered, oxygenated CdS layer. MZO layers resulted in high open-circuit voltage and device efficiency even with the complete elimination of the CdS layer. In both HRT systems, controlling electron affinity to optimize front contact band alignment is an important consideration. Band measurements using photoelectron spectroscopy and synchrotron techniques correlate band alignment measurements with efficiency parameters in the design of HRT and CdS layers.

Magnesium zinc oxide (MZO) is a promising front contact material for CdTe solar cells. Due to its higher band gap than traditional CdS, MZO can reduce parasitic absorption to significantly increase short-circuit current density while also providing a benefit of conduction band offset tuning through Mg:Zn ratio optimization. MZO has been successfully implemented into CdTe devices, however its stability has been of 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. Here we report on MZO thin films deposited by reactive co-sputtering. The reactively co-sputtered MZO thin films have encouraging stability, show no significant variation in work function of the surface over a period of 6 months, as measured by Kelvin probe. Energy conversion efficiencies of around 16% have been achieved both with and without presence of oxygen in device processing ambients across multiple research facilities. These efficiencies should be possible to increase further by tuning of the thin film deposition and device processing parameters, especially through optimization of the back contact.