Shankar Karki

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.

Characterization and Analysis of Ultrathin CIGS Films and Solar Cells Deposited by 3-Stage Process

In view of the large-scale utilization of Cu(In,Ga)Se2 (CIGS) solar cells for photovoltaic application, it is of interest not only to enhance the conversion efficiency but also to reduce the thickness of the CIGS absorber layer in order to reduce the cost and improve the solar cell manufacturing throughput. In situ and real-time spectroscopic ellipsometry (RTSE) has been used conjointly with ex situ characterizations to understand the properties of ultrathin CIGS films. This enables monitoring the growth process, analyzing the optical properties of the CIGS films during deposition, and extracting composition, film thickness, grain size, and surface roughness which can be corroborated with ex situ measurements. The fabricated devices were characterized using current voltage and quantum efficiency measurements and modeled using a 1-dimensional solar cell device simulator. An analysis of the diode parameters indicates that the efficiency of the thinnest cells was restricted not only by limited light absorption, as expected, but also by a low fill factor and open-circuit voltage, explained by an increased series resistance, reverse saturation current, and diode quality factor, associated with an increased trap density.

Bandgap profiling in CIGS solar cells via valence electron energy-loss spectroscopy

A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn1-xGaxSe2 solar cell that possesses intentional Ga/(In + Ga) composition variation. The EELS-determined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., >100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices.A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn1-xGaxSe2 solar cell that possesses intentional Ga/(In + Ga) composition variation. The EELS-determined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., >100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconduc...

Detecting Sub Bandgap Energies in CIGS with Electron Energy-Loss Spectroscopy

Julia I. Deitz, Pran K. Paul, Shankar Karki, Sylvain Marsillac, Aaron R. Arehart, Tyler J. Grassman, and David W. McComb 1 Dept. of Materials Science & Engineering, The Ohio State University, Columbus, OH, 43210, USA. Dept. of Electrical & Computer Engineering, The Ohio State University, Columbus, OH, 43210, USA. Dept. of Electrical & Computer Engineering, Old Dominion University, Norfolk, VA, 23529, USA.

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.

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.