Grace Rajan

Toward ultra thin CIGS solar cells

In this paper, we present our results on the fabrication of solar cells down to thicknesses of 0.5 μm, and how real time and in situ analysis by spectroscopic ellipsometry (SE) can help in (i) understanding the results of the devices; and (ii) modeling the growth and properties of the CIGS solar cell. These in situ and real time measurements are correlated with ex situ structural measurements of the films such as XRD and AFM; broad spectral range optical measurements of the films and devices such as T&R, variable angle SE; and device specific measurements such as I-V and QE measurements.

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.

Solar cells with thin Cu(In1-xGax)Se 2 absorbers: Optical analysis and quantum efficiency simulations

We present in-depth quantum efficiency analyses of of Cu(In,Ga)Se2 (CIGS) solar cells. Ex-situ spectroscopic ellipsometry (SE) analysis is applied to partially and fully completed solar cells with standard thickness and thin CIGS absorbers. Optical properties and multilayer structural data are deduced and used to predict the maximum obtainable quantum efficiency spectra and short-circuit current densities (Jsc). We validate optical model development and the resulting quantum efficiency (QE) simulations with experimental results for CIGS solar cells incorporating standard 2.2 μm thick absorbers. We find that both the bulk CIGS layer and the CdS-CIGS interface layer serve as active layer components and together contribute 100% of the photo-generated current. Thus, essentially all photo-generated carriers are collected from these layers. Solar cells with thin absorbers were also fabricated and efficiencies of 13.2% at 0.73 μm CIGS thickness, 10.1% at 0.50 μm and 8.0% at 0.36 μm were obtained. Although Jsc is expected to decrease with decreasing absorber thickness due to reduced optical collection, modeling results suggest that electronic losses are also occurring upon thinning the absorber, ranging from ~ 1.3 to 1.9 mA/cm2 for cells with CIGS thicknesses from 0.73 to 0.36 μm, respectively.

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.

Real time analysis of ultra-thin CIGS thin film deposition

Thin films of Cu(In,Ga)Se2 with various copper contents as functions of the copper and gallium contents were deposited by co-evaporation onto thermally oxidized silicon wafer (100). In-situ Real Time Spectroscopic Ellipsometry (RTSE) is used to understand the effect of the Ga/(In+Ga) ratio and the Cu atomic % on the growth and optical properties of ultra -thin CIGS films. We have demonstrated that RTSE can be used effectively to identify the growth process and to distinguish the effects of copper from those of gallium on the surface roughness evolution and dielectric functions.

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.

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.