Krishna Aryal

In Situ and Ex Situ Studies of Molybdenum Thin Films Deposited by rf and dc Magnetron Sputtering as a Back Contact for CIGS Solar Cells

In-situ RTSE was used to characterize the complex dielectric functions (ε) of rf and dc magnetron sputtered Mo thin films deposited onto soda-lime glass (SLG) substrates. Distinct features in the evolution of the dielectric functions with bulk layer thickness for rf and dc films are attributed to differences in void volume fraction and Drude free-electron relaxation time. Both rf and dc sputtered Mo thin films follow a similar trend of strain variation with argon pressure. The increased grain size, as well as smoother film morphology for the dc films compared to the rf films, were both confirmed by XRD and AFM measurements. It was also observed that increased deposition pressures lead to increased resistivities for both processes, with lower values for the dc films.

Real-Time, In-Line, and Mapping Spectroscopic Ellipsometry for Applications in Cu(In

In the scale-up of Cu(In1-xGax)Se2 (CIGS) solar cell processing for large-area photovoltaics technology, the challenge is to achieve optimum values of layer thicknesses, as well as CIGS Cu stoichiometry and alloy composition x within narrow ranges and simultaneously over large areas. As a result, contactless metrologies - those that provide such information in real-time or in-line process step by step, with the capabilities of large-area mapping - are of great interest in this technology. We have demonstrated high-speed multichannel spectroscopic ellipsometry (SE) in a number of modes for CIGS metrology, including 1) single-spot real-time SE monitoring of (In1-xGax)2Se3 as the first stage in multisource evaporation of three-stage CIGS; 2) control of Cu stoichiometry in the second and third stages of the process; 3) single-spot in situ SE analysis of alloy composition and grain size averaged through the thickness for the final CIGS film; 4) offline mapping of the CIGS thickness and composition over large areas, as well as mapping after each device fabrication step for correlation with local small area cell performance; 5) ex situ single-spot analysis of alloy composition profiles in CIGS and of completed solar cell stacks to extract thicknesses and properties of semiconductor and contact layers; and 6) predictive capability for quantum efficiency based on the results of SE multilayer analysis. With the future development of new instrumentation, the offline and ex situ capabilities in multilayer analysis and mapping will be possible in-line for both rigid and roll-to-roll flexible substrates.

_{{\bf 1}-{\bm x}}

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring and analysis of all three processing stages in the co-evaporation of copper indium-gallium diselenide (CuIn1-xGaxSe2; CIGS) for high efficiency photovoltaic devices. The first stage entails indium-gallium selenide (In1-xGax)2Se3 (IGS) deposition at a substrate temperature of 400°C on soda lime glass coated with opaque Mo. In this stage, an accurate deposition rate and the final IGS bulk and surface roughness layer thicknesses can be obtained. In the second stage, co-evaporation of Cu and Se converts the IGS film to CIGS at an elevated substrate temperature of 570°C. A bulk layer conversion model is justified and employed to analyze the second-stage RTSE data, resulting in steady-state IGS-to-CIGS thickness and volume fraction conversion rates. Near the end of the second stage, the formation of a Cu2-xSe layer on the CIGS surface can be tracked in terms of an effective thickness rate. The final Cu2-xSe effective thickness at the CIGS surface is obtained in a time interval spanning the end of the second stage to the beginning of the third. Finally, in the third stage, the Cu-rich CIGS/Cu2-xSe is converted to slightly Cu-poor CIGS by co-evaporation of In, Ga, and Se. In this stage, the thickness conversion rate, and the endpoint bulk and surface roughness layer thicknesses can be obtained. In the three stages, the thickness rates and final thicknesses yield information on the total elemental fluxes, and the roughness evolution yields information on grain growth and near-surface coalescence processes. Modeling of the dielectric functions in future studies is expected to yield compositional information and thus relative metallic fluxes. Variations in the RTSE-deduced information can yield insights into run-to-run irreproducibilities that influence the solar cell performance. The application of these capabilities in the fabrication of solar cells with thick (2.5 μm) and thin (0.3 μm) absorbers is demonstrated.

Ga

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.

_{\bm x}

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.

)Se

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.

_{\bf 2}

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.

Metrology

CuInSe2 (CIS) thin films have been shown to have extremely wide variability in resistivity, over 4 orders of magnitude. Highly conductive CIS offers many potential uses, including as a stable Cu-containing back contact for CdTe, or as a potential material for tunnel junctions in CIS devices. In an attempt to produce consistent, highly conductive, thin CIS films we introduced N 2 during the deposition process as well as implanting films with N separately. We were able to produce highly conductive films with the addition of N to the films. The change in electrical properties was not consistent when to N 2 gas was added during film growth.

Real time spectroscopic ellipsometry analysis of the three-stages of CuIn 1−x Ga x Se 2 co-evaporation

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.

Toward ultra thin CIGS solar cells

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.