Pavel S. Kobyakov

Single vacuum chamber with multiple close space sublimation sources to fabricate CdTe solar cells

Photovoltaic technologies have shown efficiencies of over 40%, however, manufacturing costs have prevented a more significant energy market penetration. To bridge the gap between the high efficiency technology and low cost manufacturing, a research and development tool and process was built and tested. This fully automated single vacuum photovoltaic manufacturing tool utilizes multiple inline close space sublimation (CSS) sources with automated substrate control. This maintains the proven scalability of the CSS technology and CSS source design but with the added versatility of independent substrate motion. This combination of a scalable deposition technology with increased cell fabrication flexibility has allowed for high efficiency cells to be manufactured and studied. The single vacuum system is capable of fabricating a 3.1 × 3.6 in. substrate every 45 min with a cell efficiency of 12% with a standard deviation of 0.6% as measured over 36 months. The substrate is generally scribed into 25 small area dev...

Sublimation of Mg onto CdS/CdTe films fabricated by advanced deposition system

One way to improve the efficiency of CdS/CdTe solar cells is to incorporate complex CdTe alloys into the device architecture. While formation of wider bandgap Cd1−xMgxTe has been demonstrated with sputtering and co-evaporation, use of sublimation techniques for large areas is yet to be investigated. Using a sublimation technique suitable for large area processing, we deposited Mg thin films onto TEC10 and TEC10/CdS/CdTe structures. XPS depth profiling and ellipsometry show that the Mg and the underlying CdTe layer intermixed substantially, formed an alloyed film, and incorporated oxides into the film. For non-CdCl2 treated CdS/CdTe devices coated with Mg, a 120 mV gain in Voc was realized by utilizing a Br2/methanol etching process prior to metallization. Nonetheless, additional measures are needed to effectively implement this complex alloy into a high efficiency device structure. Use of statistical process optimization and high vacuum systems are suggested to improve the film quality.

Deposition and characterization of Cd1−xMgxTe thin films grown by a novel cosublimation method

Photovoltaic cells utilizing the CdS/CdTe structure have improved substantially in the past few years. Despite the recent advances, the efficiency of CdS/CdTe cells is still significantly below their Shockley–Queisser limit. CdTe based ternary alloy thin films, such as Cd1−xMgxTe (CMT), could be used to improve efficiency of CdS/CdTe photovoltaic cells. Higher band gap Cd1−xMgxTe films can be the absorber in top cells of a tandem structure or an electron reflector layer in CdS/CdTe cells. A novel cosublimation method to deposit CMT thin films has been developed. This method can deposit CMT films of band gaps ranging from 1.5 to 2.3 eV. The cosublimation method is fast, repeatable, and scalable for large areas, making it suitable for implementing into large-scale manufacturing. Characterization of as-deposited CMT films, with x varying from 0 to 0.35, reveals a linear relationship between Mg content measured by energy dispersive x-ray spectroscopy and the optical band gap. Glancing angle x-ray diffraction ...

Continuous in-line processing of CdS/CdTe devices: Process control using XRF and efficient heating

A simple, quick, and non-destructive way to measure deposited Cu, used for back contact formation, and thickness of CdS and CdTe layers in the device is desirable for manufacturing process control. A simple X-ray Fluorescence (XRF) system was investigated for these measurements. Experiments show that 5 min. polychromatic XRF spectra can be used to measure CdTe and CdS thickness, and accurate 30 sec. measurements are possible. Statistical integrity of Cu XRF measurements was investigated. XRF measurements are able to differentiate Cu concentrations within 10% of nominal, making it useful for process control. A description of a new, improved in-line R&D deposition system is also presented. Thermal modeling results show the new NiCr wire based heating system will reach desired operating temperatures while providing opportunities for improved temperature uniformity and energy usage.

Progress towards a CdS/CdTe solar cell implementing An Electron Reflector

An Electron Reflector (ER) structure in CdS/CdTe photovoltaic cells has potential to increase the cells open circuit voltage. Progress towards implementing a Cd1-xMgxTe thin film as an electron reflector is presented, including deposition method, film characterization, investigation of passivation, and initial results of ER structures.

Photoemission study of CdTe surfaces after low-energy ion treatments

We present a study of low-energy ion surface cleaning treatments and their impact on the surface electronic structure of an air-exposed CdTe thin film treated with CdCl2. In order to determine the electronic structure using surface-sensitive photoemission, surfaces need to be free of contaminants. This is achieved by subsequent low-energy ion treatment steps, carefully monitoring the chemical and electronic surface structure. We present data on the valence band maximum (VBM), and core-level binding energies, that suggest that neither preferential sputtering occurs nor metallic states are formed using our cleaning procedure. For a clean CdTe surface, the VBM is determined to be (0.8 ± 0.1) eV below the Fermi energy.

Growth and characterization of Cd 1−x Mg x Te thin films for possible application in high-efficiency solar cells

Expanded band gap ternary alloys, such as Cd1-xMgxTe, could be beneficial for formation of high-efficiency CdTe solar cells structures, such as multi-junction and electron reflector devices. Cd1-xMgxTe thin films were grown by side-by-side co-evaporation from CdTe and Mg precursors. Optical measurements reveal increased band gap with higher Mg incorporation and lateral band gap grading across the substrate. SEM imaging denotes a grain size decrease with Mg incorporation. XPS analysis indicates Mg directly replaces Cd in the film. TEC10/CdS/Cd1-xMgxTe structures with and without CdCl2 treatment demonstrate photovoltaic diode behavior similar to typical CdS/CdTe devices. LBIC and QE measurements register grading consistent with band gap grading of the film. Although successful, refinement of Cd1-xMgxTe thin film co-evaporation is needed to improve spatial uniformity for large area deposition.

Effect of microstructure on the performance of stressed cadmium telluride photovoltaic (PV) devices

Our group has demonstrated a commercially viable, continuous, in-line process to fabricate PV devices with efficiencies of 13% on a low iron soda-lime glass (3„×3„) with anti-reflection coatings. The process has been extended to large area devices (16„×16„ substrate size). In this paper we present our results on the microstructural features of PV devices subjected to stress conditions. The results help to gain insight into the stability and lifetime of CdTe PV modules.