Michelle N. Sestak

Effect of reduced dimensionality on the optical band gap of SrTiO3

The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1–5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases.

Spectroscopic ellipsometry studies of thin film CdTe and CdS: From dielectric functions to solar cell structures

Real time, in situ, and ex situ spectroscopic ellipsometry (SE) methods have been applied in systematic studies of the structure and optical properties of polycrystalline CdTe and CdS thin films deposited by rf magnetron sputtering. The goal of this ongoing research is to establish a basic understanding of the relationships between the physical and optical properties, including how the void fraction, grain size, strain, and temperature of measurement affect the complex dielectric function. This goal has been achieved through recent in situ and real time SE determinations of the structural evolution and dielectric functions of CdTe, CdS, and related alloys, in conjunction with in-depth analyses of the dielectric functions and their critical points. With a collection of parameterized dielectric functions and parameters linked to the physical properties, ex situ SE analysis of solar cell structures becomes possible for extracting not only thicknesses, but other useful physical properties as well.

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

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.

Optical mapping of large area thin film solar cells

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.

Through-the-glass spectroscopic ellipsometry of CdTe solar cells

Ex-situ spectroscopic ellipsometry (SE) has been advanced for the determination of the optical structure of CdTe solar cells on transparent conducting oxide (TCO) coated glass superstrates. SE measurements directly through the top glass are performed using a method in which the reflection from the glass/film-stack interface is collected whereas the reflection from the ambient/glass interface is blocked. The approach applies reference dielectric functions for the CdS and CdTe deduced from real time SE and for the TCO components from the coated glass before solar cell fabrication. A step-by-step fitting procedure identifies the most important thicknesses and compositional parameters for optimum methodology of characterization. This methodology has potential applications for quantum efficiency predictions, large area mapping, and on-line monitoring. SIMS analysis has been performed in an attempt to corroborate step-by-step analysis of through-the-glass SE data. SIMS and SE are in reasonable accord, and this supports SE as a viable non-invasive method for solar cell analysis at a single point or in a mapping mode.

Application of real time spectroscopic ellipsometry for analysis of roll-to-roll fabrication of Si:H solar cells on polymer substrates

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.

Through-the-glass spectroscopic ellipsometry of superstrate solar cells and large area panels

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).

Optics of CdS/CdTe Thin-Film Photovoltaics

Optical reflection probes based on high-speed multichannel spectroscopic ellipsometry (SE) have been applied at different stages in the development and evaluation of CdS/CdTe thin-film photovoltaics (PV). Real-time SE during materials fabrication has provided insights into the nucleation, coalescence, and structural evolution of CdS and CdTe films. The deduced optical properties of the materials have provided insights into material density, stress, and excited carrier mean free path or defect density. The optical properties have also served as a database for analyzing complete PV stacks by probing through the glass. Uses of the database include CdTe materials analysis by Br2/methanol etching for depth profiling of devices before and after CdCl2 treatment, as well as large-area PV plate and module analysis by off-line mapping with future on-line capabilities.

Comparison of Al/ZnO and Ag/ZnO interfaces of back-reflectors for thin film Si∶H photovoltaics

Real time spectroscopic ellipsometry (RTSE) and exsitu normal incidence reflectance and scattering spectroscopies have been applied to analyze the optical characteristics of Al/ZnO back-reflector (BR) structures used in thin film Si∶H photovoltaics. The results of this study have been compared with those of previous detailed studies of Ag/ZnO structures. The structures explored here are relevant to the substrate/BR/n-i-p solar cell configuration and consist of an opaque Al film having a controllable thickness of surface roughness, followed by up to ∼3000 Å of ZnO. All Al and ZnO films have been prepared by rf magnetron sputtering, the ZnO under a standardized set of conditions at room temperature. The thickness of the final roughness thickness on Al has been varied by adjusting its deposition conditions in order to investigate the effects of Al film roughness on interface formation and Al/ZnO interface optical properties. From a comparison of the complex dielectric functions ε = ε1 + iε2 of the Al/ZnO and Ag/ZnO interface layers, we find that the localized plasmon feature is observed at higher energies for the Al/ZnO than for the Ag/ZnO interface, as would be expected based on the higher bulk plasma energy of Al. As a result, the primary loss mechanism for Al/ZnO is dissipation, not through localized plasmon modes as in Ag, but rather through intraband and interband absorption intrinsic to the Al. General approaches for reducing losses for both Al/ZnO and Ag/ZnO structures are discussed.

Optical metrology of thin film solar cells from 0.2 to 30 µm

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.