Drew E. Swanson

Improved CdTe Solar-Cell Performance by Plasma Cleaning the TCO Layer

A hollow-cathode plasma-cleaning source, designed for uniformity, was added to the load-lock region of an existing single-vacuum CdTe-cell fabrication system. This plasma source cleans the transparent-conductive-oxide layer of the cell prior to the deposition of the CdS and CdTe layers. This plasma exposure enables both thinner CdS layers and enhanced cell voltage. The net result is a reduction in CdS thickness by approximately 20 nm, while maintaining the same cell voltage or, equivalently, an increase in voltage of as much as 80 mV for the same thickness of CdS. Maps that are generated by electroluminescence and light-beam-induced current show modest uniformity improvement with plasma-cleaning treatment.

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...

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 ...

Plasma cleaning of TCO surfaces prior to CdS/CdTe deposition

We describe a hollow cathode plasma formed within a heated pocket deposition (HPD) source covered with a transparent conductive oxide (TCO)-coated substrate. Super-hydrophilic behavior and little change in TCO transmission of TCO substrates exposed to the plasma are observed. The “plasma cleaner” source was placed in-line with the CSU advanced deposition system (ARDS), and CdS and CdTe films were deposited on both plasma and conventionally cleaned substrates. Plasma-cleaned substrates result in improved cadmium sulfide films without pinholes after plasma exposure times as short as 5 sec. Cells formed with plasma cleaned substrates display improved VOC, fill factor, and efficiency.

The effect of a post-activation annealing treatment on thin film cdte device performance

The cadmium chloride activation treatment of cadmium telluride solar cells is essential for producing high efficiency devices. The treatment has many effects but the most significant is the complete removal of stacking faults in the cadmium telluride grains and the diffusion of Chlorine along the grain boundaries of the device. Chlorine decorates all cadmium telluride and cadmium sulphide grain boundaries and also builds up along the CdTe/CdS junction.. This paper reveals that by annealing devices to temperatures of 400°C to 480 °C for times ranging from 30 to 600 seconds in moderate vacuum results in the re-appearance of stacking faults and the removal of Choline from the grain boundaries. STEM analysis confirms the re-appearance of the stacking faults and SIMS and EDX confirm the removal of chlorine from the grain boundaries. This directly corresponds to a lowering in cell efficiency. The study provides further evidence that CdCl2 diffusion and certain microstructural defects directly affect the performance of cadmium telluride photovoltaic devices.

Effect of the cadmium chloride treatment on RF sputtered Cd0.6Zn0.4Te films for application in multijunction solar cells

Single phase Cd0.6Zn0.4Te (CdZnTe) films of 1 μm thickness were deposited by radio frequency planar magnetron sputter deposition on commercial soda lime glass samples coated with fluorine-doped tin oxide and cadmium sulphide (CdS). The stack was then treated with cadmium chloride (CdCl2) at different temperatures using a constant treatment time. The effect of the CdCl2 treatment was studied using optical, materials, and electrical characterization of the samples and compared with the as-deposited CdZnTe film with the same stack configuration. The band gap deduced from Tauc plots on the as-deposited CdZnTe thin film was 1.72 eV. The deposited film had good crystalline quality with a preferred orientation along the {111} plane. After the CdCl2 treatment, the absorption edge shifted toward longer wavelength region and new peaks corresponding to cadmium telluride (CdTe) emerged in the x-ray diffraction pattern. This suggested loss of zinc after the CdCl2 treatment. The cross sectional transmission electron mi...

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.

Progress and challenges with CdTe cell efficiency

CdTe solar-cell efficiency at Colorado State has now been independently verified above 18% on commercial glass. The parameters for the highest-efficiency cell are 863 mV for VOC, 26.8 mA/cm2 for JSC, and 79.2% for fill-factor, combining for 18.3% efficiency. The cell features that allowed the increases include higher-temperature pre-heating before CdTe deposition, a Te layer to facilitate the back contact, MgZnO for the buffer layer, and an anti-reflective coating. The current and fill-factor have achieved large fractions of their ideal values for the CdTe band gap, but as is typical for CdTe cells, the voltage has not. Two general strategies to address the voltage deficit are described. One of these, electron reflection with fully-depleted CdTe, has achieved voltage increases of 60 mV with 1-μm CdTe and higher with sub-micron absorbers.

Effects of oxygen plasma treatment on the performance of CdTe thin-film solar cells

CdTe solar cell devices are a promising technology, but they still have room for improvement before the theoretical efficiency of 30% is reached. In this study, we fabricate CdTe devices using close space sublimation to deposit CdTe and chemical bath deposition for the CdS layer. We explore the effects of oxygen plasma treatment on both the F-doped SnO2 (FTO) layer of commercial glass substrates and the CdS layer before depositing the CdTe absorber layer. While plasma cleaning the FTO results in a marked improvement in conversion, treating the CdS layer further improves performance. X-ray diffraction and scanning electron microscopy show no distinct changes in morphology on the CdS layer as a result of the plasma treatment. It is anticipated that plasma treatment induces a drastic reduction in the amount of organics on the substrates and modifies the chemistry of the surface of CdS layers; which may account for the observed performance improvements.

Development of plasma enhanced closed space sublimation for the deposition of CdS:O in CdTe solar cells

A hollow-cathode plasma-enhanced close space sublimation (PECSS) source was utilized to modify the CdS window layer material as it was being deposited for CdTe Solar cell fabrication. This was done by integrating PECSS into the CSU inline CdS/CdTe-cell fabricating system and by sublimating the CdS semiconductor material through a plasma discharge. To date oxygenated CdS (CdS:O) cells have been grown by sublimating CdS through a PECSS source operated on oxygen. Data are presented showing that PECSS CdS:O films have increased the band gap of the window layer therefore reducing absorption loss, increasing cell current, and improving efficiency by 1.2%.