The effect of annealing pressure and time on the crystallinity of CZTSe

In this study, the effect of annealing pressure and time on the homogeneity and the formation of the CZTSe structure was investigated. The deposition of the CZTSe coating was carried out using an electroplating method. The morphology and crystal structure of the coating was investigated using SEM-EDS, XRD and Raman spectroscopy. CZTSe films with optimised crystallinity and uniformity were obtained using an annealing process performed at 10 Torr for 1 hour. The use of lower pressures increases the crystallinity and the purity of the CZTSe film and decreases the density of secondary phases and the annealing time providing an additional benefit

Optimisation of the CZTSe thin film composition obtained by a sequential electrodeposition process

The influence of the elemental composition of CuZnSn coatings deposited by electrodeposition on the formation of the Cu2ZnSnSe4 crystal structure following a selenisation process was investigated using X-ray diffraction, scanning electron microscopy and Raman spectroscopy. This study showed that the optimum alloy ratio to achieve the Cu2ZnSnSe4 crystal without impurity phases was in the range of 0.37–0.50 for Cu/(Zn+Sn) and 2.5–4 for Zn/Sn

Cupric oxide-based p-type transparent conductors

This study examines the impact of doping on the resistivity of sputtered cupric oxide (CuO), and investigates the effects of co-sputtering CuO with tin dioxide (SnO2). It was found that films sputtered from a 2 at. % sodium-doped target have resistivities of four orders of magnitude lower than equivalent undoped films. Addition of oxygen was found to reduce the resistivity further. The best films were found to have resistivities of 4.3x10-2 Ω.cm. Co-sputtering with SnO2 was found to increase the band gap significantly, although it also caused an increase in the resistivity. All mixed oxide films were both amorphous and p-type

Aluminium-doped zinc oxide deposited by ultrasonic spray pyrolysis for thin film solar cell applications

Aluminium-doped zinc oxide (AZO) thin films were deposited on glass substrates by ultrasonic spray pyrolysis from metal salt precursors. The electrical and optical properties were investigated as a function of the deposition parameters and the optimum conditions were defined. The thin film exhibits approximately 80% transparency and a resistivity in the order of 2 x 10(-2) Ωcm. The electrical properties can be improved further with post-deposition annealing in vacuum, or with the increase in thickness which causes insignificant transmission losses. AZO nanoparticles can be used as a seed layer and affect the optical properties of the material. The optimized process results in good quality AZO films for their application as the transparent conductive oxide (TCO) layer in thin film solar cells

Optical optimization of high resistance transparent layers in thin film cadmium telluride solar cells

Thin film photovoltaic devices are multilayer opto-electrical structures in which light interference occurs. Light reflection at the interfaces and absorption within the window layers reduces transmission and, ultimately, the conversion efficiency of photovoltaic devices. Optical reflection losses can be reduced by adjusting the layer thicknesses to achieve destructive interference within the structure of the cell. The light transmission to the CdTe absorber of a CdS/CdTe cell on a fluorine doped tin oxide transparent conductor has been modeled using the transfer matrix method. The interference effect in the CdS layer and high resistance transparent buffer layers (SnO2 and ZnO) has been investigated. The modeling shows that due to relatively high absorption within the SnO2 layer, there are modest benefits to engineering anti-reflection interference in the stack. However, a ZnO buffer layer has limited absorption and interference can be exploited to provide useful anti-reflection effects. Optical modeling and optimization shows that for a 50 nm CdS layer, a maximum transmission of 78.5% is possible using ZnO as a buffer layer at 58 nm thickness, and 78.0% for a SnO2 buffer layer at a thickness of 48 nm

High temperature stability of broadband Anti-Reflection coatings on soda lime glass for solar modules

Reflections from glass surfaces reduce the efficiency of photovoltaic devices. Reflections can be reduced using a broadband Multi-layer Anti-Reflection (MAR) coating. For thin film CdTe modules, the glass is also the substrate. Manufacturers would prefer to use pre-MAR coated glass, so it is essential to establish if the MAR coating can withstand the module production process conditions. Thin film CdTe module fabrication requires temperatures up to ~500°C. Crazing may occur due to mismatch of the thermal expansion coefficients between the glass and the coating materials. The resilience of MAR coatings on soda lime glass, Eagle 2000™ Glass, and NSG TECTM 7 has been tested by exposure to increasing temperatures up to 800°C to establish the point of failure. SEM imaging and reflection measurements were used to observe the damage caused. Surprisingly, the MAR coating is unaffected up to a temperature of 590°C on soda lime glass substrates and up to 800°C on Eagle Glass. This provides confidence that thin film CdTe module manufacturers can use existing processes with pre-MAR coated glass

Thin film thickness measurements using Scanning White Light Interferometry

Scanning White Light Interferometry is a well-established technique for providing accurate surface roughness measurements and three dimensional topographical images. Here we report on the use of a variant of Scanning White Light Interferometry called coherence correlation interferometry which is now capable of providing accurate thickness measurements from transparent and semi-transparent thin films with thickness below 1 μm. This capability will have many important applications which include measurements on optical coatings, displays, semiconductor devices, transparent conducting oxides and thin film photovoltaics. In this paper we report measurements of thin film thickness made using coherence correlation interferometry on a variety of materials including metal-oxides (Nb2O5 and ZrO2), a metal-nitride (SiNx:H), a carbon-nitride (SiCxNy:H) and indium tin oxide, a transparent conducting oxide. The measurements are compared with those obtained using spectroscopic ellipsometry and in all cases excellent correlation is obtained between the techniques. A key advantage of this capability is the combination of thin film thickness and surface roughness and other three-dimensional metrology measurements from the same sample area

Interfacial surface roughness determination by coherence scanning interferometry using noise compensation

The capability of coherence scanning interferometry has been extended recently to include the determination of the interfacial surface roughness between a thin film and a substrate when the surface perturbations are less than ∼10  nm∼10  nm in magnitude. The technique relies on introducing a first-order approximation to the helical complex field (HCF) function. This approximation of the HCF function enables a least-squares optimization to be carried out in every pixel of the scanned area to determine the heights of the substrate and/or the film layers in a multilayer stack. The method is fast but its implementation assumes that the noise variance in the frequency domain is statistically the same over the scanned area of the sample. This results in reconstructed surfaces that contain statistical fluctuations. In this paper we present an alternative least-squares optimization method, which takes into account the distribution of the noise variance-covariance in the frequency domain. The method is tested using results from a simulator and these show a significant improvement in the quality of the reconstructed surfaces

Ultrasonically sprayed aluminium-doped zinc oxide layers for transparent conductive oxide in thin film solar cells

Aluminium-doped zinc oxide (AZO) thin films have been deposited on glass substrates using ultrasonic spray deposition whilst varying the operational parameters. Electrical and optical properties were investigated as a function of the deposition parameters and the optimum conditions were defined. The optimum parameters result to films with a sheet resistance of ~6kΩ/□ when they are as deposited and to ~1kΩ/□ for annealed films in vacuum. The resulting transparency ranges from 80 to 85% for optimum films

Combinatorial study of Sn-Ti-W-O transparent conducting oxide thin films for photovoltaic applications

a combinatorial study of transparent conducting oxide thin films based on SnO2–TiO2-WO3 phase space is reported. These multinary oxide films were fabricated by magnetron reactive co-sputtering of tin monoxide (SnO), titanium (Ti) and tungsten (W) targets. SnO2–TiO2-WO3 film compositions with Ti/Sn ratio (0.02 – 0.12) and W/(Ti+Sn) ratio (0.02 – 0.25) were explored. The effect of oxygen partial pressure on composition, structure and optical properties was evaluated. High optical transparency above 80% across the visible spectrum was obtained for sputtered ternary SnO2-TiO2 oxide films for oxygen partial pressure >19.4%. A positive correlation between optical bandgap and Ti/Sn ratio was observed. However, optical properties deteriorated as Ti-content increased in the as-deposited SnO2-TiO2-WO3 films. All studied as-deposited SnO2-TiO2-WO3 thin films were found to be highly resistive. X-ray diffraction data indicated no long-range structural order