Xiangxin Liu

Development of a semitransparent CdMgTe/CdS top cell for applications in tandem solar cells

The band gap of Cd1−xMgxTe can be easily tuned from 1.48 to 3.5 eV (x = 0 to 1), and hence this material is a potential candidate for developing the wide band gap top cell in tandem solar cells. In this paper, we present an all-sputtered Cd1−xMgxTe/CdS top cell developed on commercial SnO2:F-coated (Tec-7) glass substrates with sheet resistance of ~7Ω/. The Cd1−xMgxTe (x = 0.05, 0.21, 0.26) films were deposited by RF sputtering at deposition temperatures in the range 250–300 °C. The films were characterized for structure, morphology and optical properties. Raman spectroscopic studies showed that the lattice perturbations increase with Mg content in the film. We have explored cadmium chloride vapor annealing of Cd1−xMgxTe thin films, and found that the film composition has a significant influence on stability. Films with a lesser amount of Mg show better stability during vapor chloride treatments at 387 °C. Longer annealing durations resulted in a decrease in band gap of the films. Recrystallization due to the vapor chloride treatments were studied using transmittance, XRD, AFM and Raman spectroscopy. Prototype devices of the configuration Tec-7/CdS/Cd0.95Mg0.05Te/metal show promising photovoltaic characteristics such as 630 mV open circuit voltage and 17 mAcm−2 short circuit current.

Effects of postdeposition treatments on surfaces of CdTe/CdS solar cells

Soft x-ray spectroscopy has been used to follow the effects of postdeposition steps (CdCl2 activation and back contact treatment) on surfaces and interfaces in CdTe-based superstrate solar cells. We find that the CdCl2 activation drives sulfur atoms from the CdS layer toward the back contact but not to its surface. Using atomic force microscopy, we find that both treatments strongly influence the morphology of the Au/Cu back contact. The spectroscopic results, in contrast, suggest that CdCl2 activation exhibits a larger impact on the surface composition and chemical structure of the interfaces involved in CdTe solar cells.

Influence of substrate bias and post-deposition Cl treatment on CdTe film grown by RF magnetron sputtering for solar cells

CdTe thin films were grown by RF magnetron sputtering at deposition pressures of 1.5–4 Pa and substrate potentials of 8.8, 0, −8.2, −18.9, −28.3, −38.1, −48.0, and −98.7 V. All as-grown CdTe thin films had undergone CdCl2 treatment in dry air. The deposition pressure had a large influence on the crystalline quality, morphology, and grain size. At 1.5–3 Pa, CdTe thin films with good crystalline quality and column morphology were successfully obtained. The substrate potential also influenced the phase composition, grain size, morphology, microstress, and roughness of the obtained CdTe thin films. The phase composition, morphology, grain size, microstress, and crystalline quality changed significantly after CdCl2 annealing treatment. For CdTe grown at 3 Pa, the grain size reached a maximum when the Cl treatment time was 20 min; for CdTe thin films deposited at 2 Pa, grain size was the greatest when the Cl treatment time was 43 min. The maximum grain size corresponded to the best performance of the CdTe solar cell. CdTe thin films grown at different substrate potentials were subjected to Cl treatment at 400 °C for 43 min. The internal structure was observed using scanning electron microscopy of sample cross-sections sliced by a focused ion beam. After Cl treatment, voids were seen at the CdTe grain boundaries, and at the interfaces CdS/F:SnO2 and CdS/CdTe. The CdTe film grown at a substrate potential of −18.9 V produced the best solar cell, with device parameters of η = 12.78%, Voc = 779 mV, Jsc = 22.91 mA cm−2, and FF = 71.62%.

Migration and oxidation of sulfur at the back contact in CdTe cells

In a cross-sectional X-ray energy dispersive spectroscopy mapping study of sputtered CdTe cells, sulfur was found to migrate and accumulate near the back contact. X-ray emission spectra of chloride-treated CdS/CdTe films confirm the accumulation of sulfur at the back surface. A part of the surface sulfur is found to be oxidized in the form of sulfate when Cu is present at the back contact.

A ferroelectric–semiconductor-coupled solar cell with tunable photovoltage

In this work, we proposed and febricated a device with a symmetric sandwich structure as: “ITO/ ferroelectric-PV layer/ITO” to study the feasibility of nanodipole photovoltaic devices. The ferroelectric-PV layer is composed of CdS nanodipole particles and CdTe absorber. It is fabricated by extract CdS piezoelectric nanoparticles from CdSTe mixed film due to phase segregation. Some convincing evidences were found to confirm the mechanism of nanodipoles. First, an analogous electric hysteresis of photovoltage was observed in such devices. Second, piezoresponse was observed on grain boundaries, which is believed to contribute; significantly to the hysteresis behavior of photovoltage. Such hysteresis feature of photovoltage can not be explained by classical junctional device theory.

All-sputtered CdS/CdTe solar cells on polyimide

We have made CdS/CdTe solar cells on polyimides from different sources. These cells were fabricated using sputtered zinc oxide doped with aluminum as the transparent conducting oxide. The CdS and CdTe were also deposited by sputtering. In this paper we report properties of the sputtered AZO and its changes with CdCl2 heat treatments. We have achieved a cell efficiency of 10.5% at air mass 1.5G illumination.

Electron backscatter diffraction and photoluminescence of sputtered CdTe thin films

Electron backscatter diffraction (EBSD) has been used to characterize the grain size, grain boundary structure, and texture of sputtered CdTe at varying deposition pressures before and after CdCl2 treatment in order to correlate performance with film microstructure. It is known that twin boundaries may have different electrical properties than high-angle grain boundaries and in this work we have included the effects of twin boundaries. We found better correlation of solar cell device performance to the twin-corrected grain size than to the standard grain size. In addition, we have correlated the photoluminescence (PL) spectra with device performance and with the EBSD results. We find that sputtering at 18 mTorr yields the highest efficiency, largest twin-corrected grain size and the strongest PL.

CdTe cell stability vs. CdS thickness

We have studied the stability in air of sputtered CdTe/CdS cells with a focus on the thickness of the CdS layer and the substrate used during cell fabrication. The stability of cell efficiencies under light soak is compared for CdTe cells on both HRT-coated Pilkington TEC 15 glass and standard TEC15 glass for a range of seven different CdS layer thicknesses from 0 to 230 nm. Little difference in stability can be found between cells with or without the HRT layer for all CdS thicknesses. The buffer layer helps to produce high initial performance and uniformity of cells by maintaining high VOC for very thin CdS layers. But there is no indication that the buffer layer improves the stability of unencapsulated, sputtered cells under one-sun light soak at open circuit. Devices with the thicker CdS layers show better stability under one-sun light soak in air.

Development of wide band gap Cd1-XMgXTe/CdS top cells for tandem devices

In order to achieve advances in CdTe technology, we are continuing our efforts in developing wide band gap ternary alloy films based on CdTe. This approach will help in harnessing the already existing CdTe technology to develop high efficiency tandem devices. The band gap of Cd1-xMgxTe can be easily tuned from 1.48 to 1.8 eV (x=0 to 0.17). Close match of the lattice constant of MgTe with CdTe and the apparent complete miscibility of MgTe in CdTe and the rapid increase in band gap with Mg content gives the flexibility to prepare material with appropriate band gap for current- matching in a tandem solar cell. In this paper development of Cd1-xMgxTe films by vacuum co-evaporation and prototype Cd1-xMgxTe/CdS solar cells are reported. The co-evaporated films on large area substrates exhibited high spatial uniformity. We have used different experimental techniques such as XRD, Raman, PL, UV-VIS spectroscopy, and opto-electronic tools for studying the effect of Mg incorporation in to the lattice of CdTe. Evidences of lattice perturbations due to the incorporation of Mg were observed in XRD and Raman data, however no phase segregation was observed in the XRD spectra. We are continuing our investigation of the effect of post deposition chloride treatment and annealing on the band gap stability of the Cd1-xMgxTe films. Films chloride treated with MgCl2 exhibited better stability compared with those films annealed in CdCl2 vapor. However, our preliminary data on the CdS/Cd1-xMgxTe devices show that CdCl2 vapor treatment is beneficial for attaining better efficiency. Prototype devices of the configuration Tec7/CdS/Cd1-xMgxTe/Cu-Au was developed with Cd1-xMgxTe films with different x values. Efficiency varies in the range 2 to 4.6% depending on the band gap of the absorber.

Twin boundaries enhanced current transport in 14.4%-efficient CdTe solar cells by RF sputtering

In this paper, we are reporting a radio-frequency (rf) sputtered CdTe solar cell on soda lime glass (SLG) with 14.4% efficiency. {111} Σ3 twin boundaries (TBs) are found in this solar cell and are directly found to be electrically beneficial to the device performance, as confirmed by conductive atomic force microscopy. TBs are observed to be preferred paths for carrier transport. As indicated by scanning kelvin probe microscopy, there is potential difference between grain interior and TBs.