Puruswottam Aryal

Large-Area Compositional Mapping of Cu(In

A spectroscopic ellipsometry (SE) capability having the potential to scan production-scale areas at high speed has been developed and successfully applied to map the alloy composition of copper-indium-gallium-diselenide (CuIn<inf>1−x</inf>Ga<inf>x</inf>Se<inf>2</inf>: CIGS) thin films. This technique not only generates a compositional map but simultaneously provides maps of the more typical SE-determined properties as well, including bulk layer and surface roughness layer thicknesses. As a result, the methodology is suitable for characterization in online production-scale applications. In order to develop the mapping capability, CIGS films having different molar Ga contents x and fixed copper stoichiometry were deposited and measured in situ by SE in order to extract the complex dielectric functions (ε = ε<inf>1</inf>+iε<inf>2</inf>) of these films. For mathematical interpolation between the available alloy contents, the (ε<inf>1</inf>, ε<inf>2</inf>) spectra were parameterized using an oscillator sum. Best-fitting equations were obtained that relate each oscillator parameter to the Ga content x, as determined by energy dispersive X-ray analysis. This approach reduces the number of fitting parameters for (ε<inf>1</inf>, ε<inf>2</inf>) from several to just one: the Ga content x. Because (ε<inf>1</inf>, ε<inf>2</inf>) is now represented by this single parameter, the chances of parameter correlations during fitting are reduced, enabling production-scale compositional mapping of chalcopyrite films by SE.

_{1-x}

Through-the-glass and film side spectroscopic ellipsometry (SE) are being developed as in situ, on-line, and off-line mapping tools for large area thin film photovoltaics. Given that such instrumentation allows one to extract thicknesses, as well as parameterized optical functions versus wavelength, there exists the possibility to utilize this information further to predict the optical quantum efficiency (QE) and optical losses, the latter including the reflectance and inactive layer absorbances. By spatially resolving this information, one can gain a better understanding of the origin of performance differences between small area cells and large area modules. We have demonstrated these techniques for thin film hydrogenated amorphous silicon (a-Si:H) and Cu(In1−xGax)Se2 solar cell structures. For solar cells on glass superstrates, film-side SE can be supplemented with through-the-glass SE, which helps to increase the sensitivity of the analysis to the critical transparent conducting oxide and window layer properties. A comparison of predicted and experimental QE can reveal optical and electronic losses and light trapping gains.

Ga

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.

_{x}

Real-time spectroscopic ellipsometry (RTSE) has been applied for in situ monitoring and control of thin-film copper-indium-gallium-diselenide, i.e., Cu(In1-xGax)Se2 (CIGS), deposition by high vacuum coevaporation in the three-stage process used for efficient photovoltaic devices. Initial studies have been performed on a ~0.7-μm CIGS layer deposited on crystal silicon to minimize surface roughness and to develop an accurate structural/ optical model of the Cu-poor-to-Cu-rich and Cu-rich-to-Cu-poor transitions that define the ends of the second (II) and third (III) stages of growth, respectively.With a better understanding of the surface achieved through this model, correlations can be made between the surface state and the unprocessed RTSE data {ψ(t), Δ(t)}. During deposition in the solar cell configuration with 2- μm-thick CIGS on a Mo-coated glass substrate, indications of the Cu poor-to-rich and Cu rich-to-poor transitions appear clearly in {ψ(t), Δ(t)}, enabling direct control of stage II and III transitions. The transition times deduced optically are in good agreement with those identified from the film/substrate emissivity by tracking the substrate heater power. It is clear, however, that RTSE can provide higher sensitivity to these transitions and is, therefore, suitable for improved control of three-stage CIGS deposition.

)Se

We have advanced the technique of through-the-glass spectroscopic ellipsometry (SE) toward the nondestructive, non-invasive analysis of large area coated glass plates and completed solar modules in the superstrate configuration. The focus of this work involves reducing the effects of artifacts due to changes in the polarization state of light as it traverses the glass to the film side. By including the effects of (i) strain in the glass, (ii) differences in soda lime glass optical properties at the tin side versus the film side, and (iii) possible collection of both tin side and film side reflections, the accuracy in the determination of film properties in through-the-glass measurements can be improved. For example, measurements of the index of refraction spectra of the uncoated film side glass using a through-the-glass method agree with direct measurements from the uncoated film side to within ±0.004 over the full spectral range of through-the-glass measurements (∼300 to 1600 nm).

_{2}

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.

Materials and Devices with Spectroscopic Ellipsometry

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.

Quantum efficiency simulations from on-line compatible mapping of thin-film solar cells

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.

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

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.

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

Spectroscopic ellipsometry (SE) from the ultraviolet (UV) to mid-infrared (IR) has been applied to analyze thin film solar cell structures deposited on transparent conducting oxide (TCO) coated glass substrates. Two structures were studied here, chosen from two different thin film photovoltaic (PV) technologies, a hydrogenated amorphous silicon (a-Si:H) p-i-n and a CdS/CdTe heterojunction, both without back contact processing. The mid-IR capability was used to study TCO free carrier absorption in the actual solar cell device configuration, which was further analyzed to extract free carrier properties. In addition, network vibrational absorption bands due to the wagging and stretching modes of hydrides in a-Si:H were also measurable in the device configuration. These results can be used to characterize properties such as H content, its bonding configurations, and amorphous/crystalline content. By combining film side and glass side measurements in the UV-visible range, the ability to obtain structural parameters of multilayer devices can be enhanced. The associated optical property determinations yield insights into disorder in amorphous films and grain structure and strain in micro/polycrystalline films.