Puja Pradhan

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}

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.

Ga

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.

_{x}

Real time spectroscopic ellipsometry (RT-SE) in the near-infrared to ultraviolet range, as well as pre-deposition and post-deposition mid-infrared spectroscopic ellipsometry (IR-SE) have been applied as probes of the formation of optical interfaces in sputter-deposited CdS/CdTe solar cell structures. Both optical probes are configured for reflection from the film side of the solar cell structure. One focus of this work is to assist in the development of optical models to be used for both on-line analysis and quantum efficiency modeling. Toward this goal, RT-SE during CdS deposition has provided information on (i) [transparent conducting oxide (TCO)]/CdS interface formation - the extent to which the TCO surface roughness is conformally covered by the depositing CdS film; (ii) CdS bulk layer growth, and (iii) CdS surface roughness evolution and the final roughness thickness, which influences interface formation with the overlying CdTe. Pre-deposition and post-deposition IR-SE has also proven valuable for exploring the TCO free electron characteristics and the CdS optical properties that determine their near-infrared absorption spectra. The TCO characteristics have been observed to change with the over-deposition of the semiconductor films.

)Se

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.

_{2}

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.

Materials and Devices with Spectroscopic Ellipsometry

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.

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

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1−xGax)2Se3 (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε1 − iε2, spectra. Here, RTSE has been used to obtain the (ε1, ε2) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε1, ε2) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε1, ε2) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε1, ε2) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control.

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

The dielectric functions of CuIn1-xGaxSe2 (CIGS) alloys as deduced by spectroscopic ellipsometry (SE) have been parameterized versus CIGS bandgap Eg and versus x. As a result, Eg can serve as a free parameter in regression analyses of SE data acquired on multilayer structures incorporating CIGS. This enables the determination of CIGS bandgap profiles in solar cell structures with SE measurement times <; 1 s. Such a capability further enables module scale mapping of Ga profiles with on-line SE instrumentation. This capability has been demonstrated by presenting maps of depth profile information on Ga composition for three stage co-evaporation of CIGS solar cells with thin absorbers. The Ga profiles have been simulated with two linear segments, and the compositions at the junction, at the minimum within the absorber, and at the back contact have been mapped. Spatial variations in x at the junction correlate with the cells open circuit voltage, supporting the validity of the methods.

Ga

CuIn1-xGaxSe2 (CIGS) solar cells fabricated with thin absorber layers were studied by spectroscopic ellipsometry to deduce their multilayer structures, which enable quantum efficiency (QE) simulations. For all cells with thin absorbers studied here, the measured QE spectra are reduced relative to those simulated assuming 100% carrier collection in the active CIGS-containing layers. Thus, the measured QE spectra can be understood in terms of both optical and carrier collection losses, the latter for electrons and holes generated near the Mo contacts. Collection losses are negligible for standard (2.2 μm) absorbers.