Spectroscopic ellipsometry: metrology for photovoltaics from the nanoscale to gigawatts

Abstract

Non-destructive, non-invasive measurement and monitoring tools, such as spectroscopic ellipsometry (SE), are needed at all scales of thin film photovoltaic (PV) technology development -from the nanometer scale that describes the electronic and physical structure of materials fabricated in research laboratories to the gigawatt scale that requires the large-area uniformity of materials made in mass production. In the research laboratory, real time SE during materials fabrication has provided insights into the structural and electronic property evolution, and dielectric functions of the thin films. Here we will present such results for the growth of thin film PV materials of various compositions and properties, including those used in the demanding I-III-VI 2 solar cell technology. In the PV manufacturing plant, mapping SE can be used to evaluate the uniformity of thickness and properties of the layers in full scale PV panels and even in completed modules. Here we will present representative results for the uniformity of CdS, the window layer in CdTe PV technology, which reached 1.4 GW annual production in 2010.

Determining the Complex Refractive Index of Neat CdSe/CdS Quantum Dot Films

“…By measuring the absorbance spectra of dilute solutions of QD nanocrystals using a UV–vis spectrometer, we can obtain the intrinsic refractive index of isolated QD particles in a manner similar to those reported elsewhere. , In bulk materials, the absorption coefficient, α, is defined as where k is the imaginary portion of the refractive index of the nanocrystal. However, in a colloidal dispersion, the electric field within the inclusion will change on the basis of the refractive index of its surroundings.…”

“…Here, E is energy, and ε ∞ is the value of ε Im (∞). By using a gradient minimization process, the discrete form of the KK relation, and fixing the imaginary permittivity values to those of bulk CdSe at wavelengths shorter than 350 nm, we can obtain the real and imaginary values of the permittivity through an iterative process adapted from those used elsewhere. , Further details about the iterative process can be found in the Supporting Information. The resulting intrinsic permittivity values for our CdSe QDs after seven iterative steps (steps shown in Figure S1) are shown in Figure b along with values of bulk CdSe.…”

“…15 By measuring the absorbance spectra of dilute solutions of QD nanocrystals using a UV−vis spectrometer, we can obtain the intrinsic refractive index of isolated QD particles in a manner similar to those reported elsewhere. 7,8 In bulk materials, the absorption coefficient, α, is defined as…”

“…When the complex refractive index of a QD solid is known, a variety of nanophotonic designs can be more accurately modeled and understood. Applications include the design of QD lasers and gain media, the prediction of photovoltaic device performance including calculations of exciton generation rates and quantum efficiencies, , nanophotonic designs for absorption enhancement or light guiding, − and interference engineering for improved photovoltaics and color filters …”

Spectroscopic ellipsometry for active nano- and meta-materials

“…As shown in Figure 2C, the Δ spectrum is strongly affected when an ultrathin overlayer is grown. The thickness dependence of the dielectric properties of ultrathin films may have several different origins such as confinementinduced effects in smooth films [5,101] or a discontinuous nanostructure of the film. The latter effect is frequently encountered in the first stages of Volmer Weber-like growth where isolated NPs first grow on the substrate until percolation is reached and a continuous film finally forms.…”