Dinesh Attygalle

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.

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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

The mapping capability of multichannel spectro-scopic ellipsometry (SE) has been demonstrated with examples from hydrogenated amorphous silicon (a-Si:H) and CdTe thin film photovoltaics (PV) technologies on glass. Maps as large as 40 x 80 cm2 have been obtained. For a-Si:H, maps of the bulk i-layer thickness and band gap as well as surface roughness layer thickness have been determined. For CdTe, a map of the CdS window layer thickness has been determined with the prospect of grain structure mapping. In both cases, maps of the thickness and properties of the underlying transparent conducting oxide (TCO) layers have been determined. These first results demonstrate the ability of mapping SE to guide scale-up of thin film PV deposition processes.

_{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.

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Real time spectroscopic ellipsometry (RTSE) has been developed to monitor the cassette roll-to-roll deposition of thin film Si:H n-i-p solar cells on flexible polymer substrates coated with a back-reflector (BR). The methodology is first demonstrated here in growth studies from nucleation to the final thickness for a magnetron sputtered ZnO layer on top of opaque Ag in the BR structure. The methodology is then extended to plasma-enhanced chemical vapor deposition (PECVD) of the i and p-layers in succession on the BR/n-layer stack. RTSE data collection is initiated before the plasma is ignited so that the nucleation of the layers can be observed. The film thickness increases with time until a steady state is reached, after which the bulk layer thickness at the monitoring point is constant with time. This occurs when the elapsed deposition time equals the time required for the moving substrate to travel from the leading edge of the deposition zone to the monitoring point. Although a constant substrate speed is selected such that the final film thickness is achieved in the time required to move through the entire deposition zone, this speed does not permit study of film growth that occurs after the substrate passes the monitoring point. To address this deficiency, the substrate speed is reduced only over an initial length of the roll such that the final film thickness of interest is reached at the monitoring point. In this way, RTSE can be used to analyze the entire layer on an initial length of the roll before its full length is coated. The goal of this work is to develop and verify optimum deposition procedures based on optical monitoring of thin film Si:H solar cell structures in roll-to-roll multi-chamber deposition.

_{2}

An understanding of the relationship between materials property and thin-film solar cell performance variations over large areas is of interest for evaluating the impact of macroscopic nonuniformities in scale-up from laboratory cells to production modules. In this study, we have spatially correlated the properties of the hydrogenated silicon (Si:H) i- and p-layers—as mapped over a 13 cm×13 cm substrate area—with device performance parameters from an array of a-Si:H based n-i-p dot cells. To evaluate materials and device nonuniformities, a 16 × 16 array of dot cells has been fabricated over the substrate area, and this same area has been mapped by spectroscopic ellipsometry (SE).Analysis of the SE data over the full area provides maps of i-layer thickness and band gap, p-layer thickness and band gap, and p-layer surface roughness thickness for the n-i-p solar cell structure. The mapped values adjacent to the devices have been correlated with photovoltaic (PV) device performance parameters. When sufficient nonuniformity exists, these correlations enable optimization based on specific values of the fundamental properties. Alternatively, if the optimum set of properties has been identified, the impact of deviations due to macroscopic uniformities can be evaluated.

Materials and Devices with Spectroscopic Ellipsometry

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.

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

Hydrogen generated by photoelectrochemical water splitting has drawn the attention of many alternative energy researchers as a promising way to fulfill the demand for a cleaner fuel. A triple junction hydrogenated amorphous silicon (a-Si:H) solar cell can produce a sufficient voltage (>1.23 eV) under sunlight for water splitting (1). With the aim of fabricating a hybrid photovoltaic-electrochemical system, our investigation was focused on developing a material which is transparent, electrically conducting, and corrosion resistive and can be made at temperatures less than 250 °C, and will run for thousands of hours continuously. These materials will then be coated on top of a-Si:H solar cell, and will work as the anode for a photoelectrochemical cell as well as the protecting layer for the a-Si:H layers underneath. Historically titania (titanium oxide, TiO2) and tungston oxide (WO3) were considered as good candidates for this purpose (2). However performance of a-Si based solar cells may degrade at temperatures above 250°C. Most commonly used oxide films, sputter deposited below this temperature limit, are found to be unstable in electrolyte. Cobalt oxide films made using different deposition parameters have consistently showed good corrosion resistance and high electrical conductivity.

Optical mapping of large area thin film solar cells

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.

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

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.