Tasnuva Ashrafee

Modeling of the effect of substrate temperature on Na diffusion through molybdenum films

In this study, Mo thin films were deposited on soda lime glass substrates at various temperatures using direct-current magnetron sputtering to observe the influence of substrate temperature (Tss) on Na diffusion. Tss was varied from room temperature to 200 °C. Structural analyses of the as deposited films were performed using scanning electron microscopy, atomic force microscopy and reflection measurements, while secondary ion mass spectroscopy measurements were carried out to obtain Na depth profile. Both theoretical and numerical models were used for simulating and understanding grain boundary diffusion mechanism for Na through Mo films as a function of substrate temperature.

Effect of substrate temperature on sputtered molybdenum film as a back contact for Cu(In,Ga)Se2 solar cells

Molybdenum (Mo) coated soda lime glass (SLG) is a commonly used substrate for Cu(In,Ga)Se2 (CIGS) solar cells as it also acts as the sodium (Na) source, which improves the efficiency of these devices. In this study, Mo thin films were deposited on SLG substrates using direct-current (DC) magnetron sputtering to observe the influence of substrate temperature on Na diffusion and films smoothness. The working gas (Ar) was maintained at 10 mTorr while substrate temperature was varied from room temperature (RT) to 200° C. In this study, Mo films were characterized using X-Ray Diffraction (XRD). Real time in-situ and ex-situ measurements by spectroscopic ellipsometry were also performed, allowing for the analysis of the growth processes as a function of temperature. Secondary ion mass spectroscopy (SIMS) analysis was carried out to obtain the Na depth profile in the Mo films. In addition, a grain boundary diffusion model was developed to reveal the Na diffusion mechanism in Mo films at various substrate temperatures.

Optical enhancement of ultra-thin CIGS solar cells using multi-layered antireflection coatings

The performance of ultra-thin CIGS solar cells can be greatly improved by the addition of multilayer anti-reflective coatings. They serve as excellent light traps in the red and near infra-red regions, thereby enhancing the total efficiency of the solar cell. In this study, multiple layer AR coatings were optimized on ultra-thin CIGS solar cells and a reduction in reflectance was observed. Different materials were explored in order to decrease the overall reflectance of the solar cell without increasing the complexity of the cell.

Comparative study of ZnS thin films deposited by CBD and ALD as a buffer layer for CIGS solar cell

Alternative deposition methods and materials are of interest for the fabrication of thin film solar cells since they offer potential enhancements for either low cost, high speed or high efficiency but also because they can help in better understanding the underlying physical and chemical processes that could lead to the next generation of solar cells. Structural and optical properties of ZnS and CdS films deposited by either chemical bath deposition (CBD) or atomic layer deposition (ALD) were studied. More specifically, ex-situ measurements by spectroscopic ellipsometry as well as transmission and reflection measurements were performed, allowing for the analysis of the growth processes as a function of deposition parameters. These measurements also allowed for a parameterization of the dielectric functions of ZnS and the evolution of its grain size and band gap as a function of thickness.

Phosphorization-synthesized virtual substrates for low-cost high efficiency III-V photovoltaics

The use of the low-cost vapor-liquid-solid (VLS) crystal growth method in the manufacturing of III-V solar cell substrates has the potential to provide a lightweight, flexible, and cheaper alternative to traditional epitaxial-based substrates typical of state-of-the-art power generation technology. In this work, the VLS method is used to produce high-quality poly-crystalline indium phosphide (InP) on lightweight flexible metal foils. This novel method is expanded upon by growing materials with unique lattice constants. Compositions of Inx Ga1-xP are explored to target the lattice constant (5.8 Å) identified as a promising candidate for surpassing 50% efficiency at 30 suns. X-ray diffraction results of preliminary trials verify the presence of InP and the absence of In confirming full phosphorization of In into InP. The photoluminescence spectra shows a correlation between the VLS grown InP sample and single crystal InP, both emitting at the InP bandedge of 1.337 eV.

Optimization of multi-layered anti-reflective coatings for ultra-thin Cu (In, Ga)Se 2 solar cells

Multi-layer anti-reflective coatings serve as excellent light traps in the red and the near infra-red regions, thereby enhances the performance of ultra-thin CIGS solar cells. In this study, the properties of different materials are explored and the scope of the layers to be used as multiple layer AR coatings are studied. A greater reduction in reflectance was observed for ultra-thin CIGS solar cells with an optimized structure. Different combinations of AR coatings were explored in order to decrease the overall reflectance of the solar cell without increasing the complexity of the cell. There was almost an increase of 8% in the short circuit density of the device with the optimized multi-layer AR structure.Multi-layer anti-reflective coatings serve as excellent light traps in the red and the near infra-red regions, thereby enhances the performance of ultra-thin CIGS solar cells. In this study, the properties of different materials are explored and the scope of the layers to be used as multiple layer AR coatings are studied. A greater reduction in reflectance was observed for ultra-thin CIGS solar cells with an optimized structure. Different combinations of AR coatings were explored in order to decrease the overall reflectance of the solar cell without increasing the complexity of the cell. There was almost an increase of 8% in the short circuit density of the device with the optimized multi-layer AR structure.

Effect of annealing on molybdenum films used as a back contact for Cu(In, Ga)Se 2 solar cells

Mo films were deposited on SLG substrates by sputtering to observe the influence of substrate temperature (Tss) and post-deposition annealing on film structure and alkali diffusion. The Mo films were characterized by XRD and AFM, indicating an increase of the grain size and a decrease of the roughness with Tss. After annealing, the samples deposited at 100°C show smaller grain size and higher surface roughness. Secondary ion mass spectroscopy (SIMS) analysis were also performed, and indicated a decrease of the Na and K with increased Tss, with the smallest intensity for the samples deposited at 100°C. A 3-D numerical model was used for simulating the grain boundary diffusion.Mo films were deposited on SLG substrates by sputtering to observe the influence of substrate temperature (Tss) and post-deposition annealing on film structure and alkali diffusion. The Mo films were characterized by XRD and AFM, indicating an increase of the grain size and a decrease of the roughness with Tss. After annealing, the samples deposited at 100°C show smaller grain size and higher surface roughness. Secondary ion mass spectroscopy (SIMS) analysis were also performed, and indicated a decrease of the Na and K with increased Tss, with the smallest intensity for the samples deposited at 100°C. A 3-D numerical model was used for simulating the grain boundary diffusion.

Real-time optimization of anti-reflective coatings for CIGS solar cells

In this paper, we describe a model allowing for the optimization of the thickness of the anti-reflective (AR) coating for Cu (In1-xGax) Se2 (CIGS) solar cells. This model is based on an optical model developed based on transfer matrix theory as well as accurate measurement of the dielectric function and thickness of each layer in the stack by in-situ spectroscopic ellipsometry. The thickness of the AR coating was then optimized in real time to optically enhance the performance of the device for various device configurations, including CIGS thickness and AZO thickness. The influence of the substrate temperature on the properties of the AR coating and the device was also studied.

Direct MOCVD epitaxial growth of III-V alloys on Mo for low cost PV applications

In order to reduce the cost of manufacturing III-V solar cells, an investigation using the direct epitaxial growth process of III-V alloys on metal foils is performed with a particular effort to make III-V devices practical outside of specialty applications. The samples studied here consist of Indium Phosphide and Indium Arsenide thin film deposition. In this research, direct MOCVD epitaxial growth of III-V alloys, specifically, InP and InAs, on molybdenum foils is studied in order to develop a means of creating a virtual substrate based on a specific lattice constant. Lattice constants of 5.94 Å and 6.04 Å are reported.

Characterization of Cd 1−x Zn x S buffer layers deposited by CBD for CIGS solar cell application

Alternative materials are of interest for the fabrication of thin film solar cells since they could offer potential enhancements for high efficiency devices. Cd1−xZnxS is such a material. In this preliminary study, structural and optical properties of films deposited by chemical bath deposition (CBD) for the two main compositions, ZnS and CdS, were studied. More specifically, ex-situ measurements by spectroscopic ellipsometry were performed, allowing for the analysis of the dielectric functions. GIXRD measurements were also used to assess the crystal structure for both films, indicating mixed phases for CdS, versus a single phase for ZnS, with larger grains for the CdS films.