Abdel-Rahman Ibdah

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

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.

_{\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.

)Se

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.

_{\bf 2}

In this study, we have optimized the anti-reflective coating for Cu(In1-xGax)Se2 (CIGS) absorbers. The structure of the CIGS solar device has been intensively monitored using in-situ spectroscopic ellipsometry, and the thickness has been optimized to optically enhance the performance of the device. Various simulation techniques have been employed to optimize the thickness of the AR coating and this modeling has been correlated with experimental results to obtain the highest efficiency for the device. The overall efficiency has increased approximately by 5% with the new optimized AR coating and the device parameters for the best cells are reported.

Metrology

In this study, we have developed top side multilayer stacks as anti-reflective coatings to serve as light traps in the red and near infrared where enhanced absorption is required for solar cells with ultra-thin Cu(In1-xGax)Se2 (CIGS) absorbers. Multiple AR layers were deposited on the solar cells and large increases in short circuit current density were observed. We have explored various materials in order to reduce overall AR coating thickness (and thus complexity) for ease of manufacturing.