Passivation of silicon wafers by Silicon Carbide (SiCx) thin film grown by sputtering

Silicon Carbide films for silicon solar cell application were deposited by means of RF sputtering process. Films were deposited from mixed Silicon – Graphite target onto silicon Cz wafers. Samples were characterized by Photo Conductance Decay (PCD) method to measure the effective lifetime. The thickness and refractive index of the films deposited were measured using a spectroscopic Ellipsometer. X-Ray Diffraction (XRD) was performed to measure the crystallinity of the samples. Results have indicated that the deposited films were mainly amorphous. The crystalline fraction was present in samples with a better passivation level. Results from PCD show that the effective lifetime improved up to 38 μs which corresponds to a Voc=641 mV. Deposition rates up to 30 nm/min were obtained for samples at 0.9 kW bias power

High rate deposition of thin film CdTe solar cells by pulsed dc magnetron sputtering

A new high rate deposition method has been used to fabricate thin film CdTe photovoltaic devices using pulsed dc magnetron sputtering. The devices have been deposited in superstrate configuration on to a commercial fluorine doped tin oxide transparent conductor on soda lime glass. The cadmium sulphide and cadmium telluride thin films were deposited from compound targets. The magnetrons were mounted vertically around a cylindrical chamber and the substrate carrier rotates so that the layers can be deposited sequentially. The substrates were held at 200ºC during deposition, a process condition previously found to minimize the stress in the coatings. Optimization of the process involved a number of parameters including control of pulse frequency, power and working gas pressure. The devices deposited using the process are exceptionally uniform enabling the CdTe absorber thickness to be reduced to ~1um. The asdeposited material is dense and columnar. The cadmium chloride treatment increases the grain size and removes planar defects. The microstructure of the films before and after activation has been characterized using a number of techniques including transmission electron microscopy, Energy Dispersive mapping and these measurements have been correlated to device performance. The deposition rate is much higher than can be obtained with radio-frequency sputtering and is comparable with methods currently used in thin film CdTe module manufacturing such as Vapour Transport Deposition and Close Space Sublimation

Testing of an anti-soiling coating for PV module cover glass

Soiling of solar module cover glass can significantly reduce the module power output. Coatings can be applied to the cover glass surface to reduce adhesion and make the surfaces easier to clean. These coatings should be resilient and resistant to environmental damage. A hydrophobic anti-soiling coating was exposed to a variety of environmental and abrasion stress tests. The hydrophobic performance of the coating was measured by monitoring the water contact angle and the water roll off angle after exposure to a range of environmental and mechanical stress tests. The coating was shown to be highly resistant to damp heat and thermal cycling. However, it was degraded by UV exposure and damaged during abrasion tests. The coating was also exposed to outdoor testing to compare the laboratory results with real performance degradation

The effect of temperature on resistive ZnO layers and the performance of thin film CdTe solar cells

The use of a highly resistive transparent (HRT) layer has been shown to increase the efficiency of thin film CdTe heterostructure solar cells incorporating a thin CdS layer. In this study ZnO HRT layers were deposited at different substrate temperatures on soda lime glass and on fluorine-doped tin oxide-coated glass to enable structural, optical and electrical characterization. The performance of equivalent films was tested within CdS/CdTe solar cells. The ZnO thickness was limited to 150 nm, whilst the substrate temperature was varied from 20 °C to 400 °C during deposition. X-ray diffraction patterns and transmission electron microscopy of the cross-sectional microstructure of completed devices showed that the growth of the ZnO is improved when the films are deposited at higher temperatures. Film resistivity was lowest at 100 °C and highest at 400 °C, ranging from 10− 2 Ω·cm to 0.33 Ω·cm. The high temperature deposited ZnO exhibits improved micro-structural growth and an improvement in device efficiency

Movement of cracked silicon solar cells during module temperature changes

Cracks in crystalline silicon solar cells can lead to substantial power loss. While the cells’ metal contacts can initially bridge these cracks and maintain electrical connections, the bridges are damaged by mechanical loads, including those due to temperature changes. We investigated the metallization bridges that form over cracks in encapsulated silicon solar cells. Microscopic characterization showed that the crack in the silicon can immediately propagate through the metal grid, but the grid can maintain electrical contact once the load is removed. We also quantified the movement of the cell fragments separated by a crack as a function of temperature. Cell fragments are free to move diagonally and to rotate, so the change in gap across the crack during a temperature change varies along the length of the crack. In one sample, we showed that a 10 ◦C temperature change, causing a 2 µm increase in the separation of cell fragments, was sufficient to cause a reversible electrical disconnection of metallization bridging a crack

Deposition of cupric oxide thin films by spin coating

Cupric oxide thin films were deposited onto soda lime glass by spin coating and subsequent annealing of copper nitrate dissolved in a glycerol–water solvent. It was found that the solution consistently gave reproducible films with good adhesion on glass. A range of band gaps were estimated between 0.8 and 1.17 eV, showing that this material has potential as a photoabsorber. Resistivity was successfully reduced from 1.47×105 to 7.02 Ω cm by doping the films with sodium. Dopant concentrations of 1 at-% gave the lowest resistivity, showing that the ideal doping is 1% or less. Film structure was found to improve with an increase in annealing time from 10 min to 1 h, although this did not have any noticeable effect on either the electrical or optical properties of the films

Investigation of localized phase changes using high resolution electron back-scatter diffraction in thin film cadmium telluride photovoltaic material with high lattice defect densities

This study focuses on the microstructural and crystallographic characteristics of cadmium telluride thin film photovoltaics using the novel characterization technique of transmission electron back-scatter diffraction (T-EBSD). Taking advantage of the increase in resolution of transmission electron back-scatter diffraction capabilities, identification of localized changes of phase within the cadmium telluride grains have been detected. T-EBSD of the cadmium telluridegrains show areas containing very high defect densities indexed to the hexagonal phase whereas the rest of the grain is indexed to the cubic phase, showing that the high densities of defects alters the stacking formation enough to causes a localized change of phase, forming two different phases within the same grain

Measurement of thin film interfacial surface roughness by coherence scanning interferometry

Coherence Scanning Interferometry (CSI), which is also referred to as scanning white light interferometry, is a well-established optical method used to measure the surface roughness and topography with sub-nanometer precision. One of the challenges CSI has faced is extracting the interfacial topographies of a thin film assembly, where the thin film layers are deposited on a substrate, and each interface has its own defined roughness. What makes this analysis difficult is that the peaks of the interference signal are too close to each other to be separately identified. The Helical Complex Field (HCF) function is a topographically defined helix modulated by the electrical field reflectance, originally conceived for the measurement of thin film thickness. In this paper, we verify a new technique, which uses a first order Taylor expansion of the HCF function to determine the interfacial topographies at each pixel, so avoiding a heavy computation. The method is demonstrated on the surfaces of Silicon wafers using deposited Silica and Zirconia oxide thin films as test examples. These measurements show a reasonable agreement with those obtained by conventional CSI measurement of the bare Silicon wafer substrates

Cl diffusion in CdTe solar cells activated by gaseous CHClF2 atmosphere

The activation treatment is consider of dramatic importance for CdTe solar cells. The most effective activation processes are based on chlorine compounds. However it is still not clear how Cl diffusion depends on the chlorination method. In this paper we activated CdTe solar cells by gaseous CHClF2 and investigated the Cl presence by TEM and XPS. Preliminary results show clear presence of Cl trapped at the CdS/CdTe interface. Thanks to the flexibility of the gaseous activation treatment, further samples will be prepared in order to force larger quantity of Cl into the absorber and they will be similarly analyzed. TEM will be used to investigate Cl presence at grain boundaries and XPS to determine the compounds formed by Cl

Effect of varying process parameters on CdTe thin film device performance and its relationship to film microstructure

The performance of CdTe thin film photovoltaic devices are sensitive to process parameters. In this study, efforts are made to further understand the effects of process parameters like process temperature and variation in cadmium chloride passivation treatment on CdTe films deposited using a sublimation based deposition system. The effects on film microstructure are studied using advanced microstructural characterization methods like TEM, SEM, EDS and SIMS while electrical performance is studied using various electrical measurements such as current density vs. voltage and electroluminescence. The aim of this study is to provide new insight into the understanding of relationship between fabrication process, device performance and thin film microstructure