Robert W. Collins

Flux calibrations in multi-source thermal co-evaporation of thin films have been developed based on real-time spectroscopic ellipsometry (RTSE) measurements. This methodology has been applied to fabricate CuInSe₂ (CIS) thin film photovoltaic (PV) absorbers, as an illustrative example, and their properties as functions of deposition rate have been studied. In this example, multiple Cu layers are deposited step-wise onto the same Si wafer substrate at different Cu evaporation source temperatures (T_Cu). Multiple In₂Se₃ layers are deposited similarly at different In source temperatures (T_In). Using RTSE, the Cu and In₂Se₃ deposition rates are determined as functions of T_Cu and T_In. These rates, denoted R_eff, are measured in terms of effective thickness which is the volume per planar substrate area and accounts for surface roughness variations with deposition time. By assuming that all incident metal atoms are incorporated into the films and that the atomic concentrations in the deposited material components are the same as in single crystals, initial estimates of the Cu and In atom fluxes can be made versus T_Cu and T_In. Applying these estimates to the co-evaporation of a set of CIS films from individual Cu, In, and Se sources, atomic concentration corrections can be assigned to the Cu and In₂Se₃ calibration films. The corrections enable generation of a novel calibration diagram predicting the atomic ratio y = [Cu]/[In] and rate R_eff within the T_Cu-T_In plane. Using this diagram, optimization of the CIS properties as a PV absorber can be achieved versus both y and R_eff.

Zinc oxide (ZnO) nanoparticles have shown great potential because of their versatile and promising applications in different fields, including solar cells. Various methods of synthesizing ZnO materials have been reported. In this work, controlled synthesis of ZnO nanoparticles was achieved via a simple, cost-effective, and facile synthetic method. Using transmittance spectra and film thickness of ZnO, the optical band gap energies were calculated. For as-synthesized and annealed ZnO films, the bandgap energies were found to be 3.40 eV and 3.30 eV, respectively. The nature of the optical transition indicates that the material is a direct bandgap semiconductor. Spectroscopic ellipsometry (SE) analysis was used to extract dielectric functions where the onset of optical absorption of ZnO was observed at lower photon energy due to annealing of the nanoparticle film. Similarly, X-ray diffraction (XRD) and scanning electron microscopy (SEM) data revealed that the material is pure and crystalline in nature, with the average crystallite size of ~9 nm.

Motivated by their utility in CdTe-based thin film photovoltaics (PV) devices, an investigation of thin films of the magnesium-zinc oxide (Mg_xZn_1−xO or MZO) alloy system was undertaken applying spectroscopic ellipsometry (SE). Dominant wurtzite phase MZO thin films with Mg contents in the range 0 ≤ x ≤ 0.42 were deposited on room temperature soda lime glass (SLG) substrates by magnetron co-sputtering of MgO and ZnO targets followed by annealing. The complex dielectric functions ε of these films were determined and parameterized over the photon energy range from 0.73 to 6.5 eV using an analytical model consisting of two critical point (CP) oscillators. The CP parameters in this model are expressed as polynomial functions of the best fitting lowest CP energy or bandgap E₀ = E_g, which in turn is a quadratic function of x. As functions of x, both the lowest energy CP broadening and the Urbach parameter show minima for x ~ 0.3, which corresponds to a bandgap of 3.65 eV. As a result, it is concluded that for this composition and bandgap, the MZO exhibits either a minimum concentration of defects in the bulk of the crystallites or a maximum in the grain size, an observation consistent with measured X-ray diffraction line broadenings. The parametric expression for ε developed here is expected to be useful in future mapping and through-the-glass SE analyses of partial and complete PV device structures incorporating MZO.

A new method combining in-situ real-time spectroscopic ellipsometry and optical modeling to optimize the thickness of an anti-reflective (AR) coating for Cu(In,Ga)Se₂ (CIGS) solar cells is described and applied directly to fabricate devices. The model is based on transfer matrix theory with input from the accurate measurement of complex dielectric function spectra and thickness of each layer in the solar cell by spectroscopic ellipsometry. The AR coating thickness is optimized in real time to optically enhance device performance with varying thickness and properties of the constituent layers. Among the parameters studied, we notably demonstrate how changes in thickness of the CIGS absorber layer, buffer layers, and transparent contact layer of higher performance solar cells affect the optimized AR coating thickness. An increase in the device performance of up to 6% with the optimized AR layer is demonstrated, emphasizing the importance of designing the AR coating based on the properties of the device structure.

Nanocrystalline hydrogenated silicon (nc-Si:H) substrate configuration n-i-p solar cells have been fabricated on soda lime glass substrates with active absorber layers prepared by plasma enhanced chemical vapor deposition (PECVD) and radio frequency magnetron sputtering. The cells with nanocrystalline PECVD absorbers and an untextured back reflector serve as a baseline for comparison and have power conversion efficiency near 6%. By comparison, cells with sputtered absorbers achieved efficiencies of about 1%. Simulations of external quantum efficiency (EQE) are compared to experimental EQE to determine a carrier collection probability gradient with depth for the device with the sputtered i-layer absorber. This incomplete collection of carriers generated in the absorber is most pronounced in material near the n/i interface and is attributed to breaking vacuum between deposition of layers for the sputtered absorbers, possible low electronic quality of the nc-Si:H sputtered absorber, and damage at the n/i interface by over-deposition of the sputtered i-layer during device fabrication.

Electrical transport properties [carrier concentration (N), mobility (μ), effective mass (m*), resistivity (ρ), and scattering time (τ)] are deduced from free carrier optical absorption using noncontacting optical Hall effect measurements. Here, we use this technique to determine N, μ, and m* for semiconductor absorber materials for photovoltaic (PV) devices. As case studies, PV component materials including thin-film methylammonium lead iodide, cadmium telluride, and copper indium diselenide as well as silicon wafers are characterized. Optical Hall effect measurements involve collecting long wavelength ellipsometric spectra under the application of an external magnetic field to determine N and μ for these thin films, and N, μ, and m* for the silicon wafers. ρ and τ are also determined for each sample from N, μ, and m* using the Drude model. In all cases, an additional transport parameter measurement is accessible relative to standard optical measurements alone even for the PV absorber materials having low free carrier absorption. As the technique is noncontacting, it could be very useful for the determination of transport properties of layers in complex multilayer stacks such as those in thin film and wafer-based PV devices.

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 (In_1−xGa_x)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.

Optimization of thin film photovoltaics (PV) relies on characterizing the optoelectronic and structural properties of each layer and correlating these properties with device performance. Growth evolution diagrams have been used to guide production of materials with good optoelectronic properties in the full hydrogenated amorphous silicon (a-Si:H) PV device configuration. The nucleation and evolution of crystallites forming from the amorphous phase were studied using in situ near-infrared to ultraviolet spectroscopic ellipsometry during growth of films prepared as a function of hydrogen to reactive gas flow ratio R = [H₂]/[SiH₄]. In conjunction with higher photon energy measurements, the presence and relative absorption strength of silicon-hydrogen infrared modes were measured by infrared extended ellipsometry measurements to gain insight into chemical bonding. Structural and optical models have been developed for the back reflector (BR) structure consisting of sputtered undoped zinc oxide (ZnO) on top of silver (Ag) coated glass substrates. Characterization of the free-carrier absorption properties in Ag and the ZnO + Ag interface as well as phonon modes in ZnO were also studied by spectroscopic ellipsometry. Measurements ranging from 0.04 to 5 eV were used to extract layer thicknesses, composition, and optical response in the form of complex dielectric function spectra (ε = ε₁ + iε₂) for Ag, ZnO, the ZnO + Ag interface, and undoped a-Si:H layer in a substrate n-i-p a-Si:H based PV device structure.

Spectroscopic ellipsometry (SE) is used to study the growth evolution of the bottom cell p-type doped and intrinsic hydrogenated silicon (Si:H) layers in p-i-n amorphous/nanocrystalline Si:H (a-Si:H/nc-Si:H) tandem photovoltaic (PV) devices. SE data collected in situ during the growth enables the identification of crystallite coalescence transitions in the surface roughness evolution, as well as the variations in optical response in the form of the complex dielectric function (ε = ε1 + iε₂). The latter indicates relative amounts of the a-Si:H and nc-Si:H components during mixed-phase Si:H growth. A growth evolution diagram for the i-layer in the tandem PV device configuration is constructed. Device properties are related to p- and i-layer structure and can be understood on the basis of the growth evolution diagram.