Maxwell M. Junda

Effects of oxygen partial pressure, deposition temperature, and annealing on the optical response of CdS:O thin films as studied by spectroscopic ellipsometry

Ex-situ spectroscopic ellipsometry measurements are made on radio frequency magnetron sputtered oxygenated cadmium sulfide (CdS:O) thin films. Films are deposited onto glass substrates at room temperature and at 270 °C with varying oxygen to total gas flow ratios in the sputtering ambient. Ellipsometric spectra from 0.74 to 5.89 eV are collected before and after annealing at 607 °C to simulate the thermal processes during close-space sublimation of overlying cadmium telluride in that solar cell configuration. Complex dielectric function (e = e1 + ie2) spectra are extracted for films as a function of oxygen gas flow ratio, deposition temperature, and post-deposition annealing using a parametric model accounting for critical point transitions and an Urbach tail for sub-band gap absorption. The results suggest an inverse relationship between degree of crystallinity and oxygen gas flow ratio, whereas annealing is shown to increase crystallinity in all samples. Direct band gap energies are determined from the ...

Spectroscopic Ellipsometry Applied in the Full p-i-n a-Si:H Solar Cell Device Configuration

Assessment of the performance of single-junction hydrogenated amorphous silicon (a-Si:H) p-i-n configuration solar cells has been developed with a combination of real-time spectroscopic ellipsometry (RTSE) and current-voltage (I-V) measurements. For each layer, RTSE measurements enabled the determination of thickness and optical properties in the form of the complex dielectric function (ε = ε1 + iε2) spectra. RTSE tracked changes in a as a function of depth and was used to extract profiles in the i-layer bandgap and crystallite fraction in the n-layer. Through mapping I-V characteristic measurements, spatial variations in device performance were determined. By comparing individual devices at the location of the RTSE beam spot, the influence of a and thickness for each layer on device performance was identified through simulations of quantum efficiency yielding the shortcircuit current. This study compares two devices prepared with different superstrate preheating processes and finds that the combination of RTSE and I-V measurements along with quantum efficiency simulations were able to identify plasma damage to the transparent conducting oxide as the likely cause for variation in device performance. This comparison serves as one example of how the optically obtained information, such as thickness and a for each layer, can be used to understand the final device performance.

Optical properties of borosilicate glass from 3.1 mm to 210 nm (0.4 meV to 5.89 eV) by spectroscopic ellipsometry

Borosilicate glass is of significant interest and utility to for a variety of applications, often for its robust ability to withstand significant thermal stresses resulting from a low coefficient of thermal expansion. Here, ellipsometric spectra of borosilicate glass from the millimeter wavelength range (terahertz) to the ultraviolet are collected from rotating compensator configuration ellipsometers. Specifically, spectra from three instruments each measuring a separate spectral range (0.4–4.1 meV; 0.035–0.75 eV; 0.75–5.89 eV) are modeled simultaneously. Continuous parameterization of the complex dielectric function (e = e1 + ie2) spectra from 0.4 meV to 5.89 eV describing the optical response of the glass is determined by modeling the measured spectra. Additionally, intensity-based transmittance measurement spanning 0.75–5.89 eV is used to determine the absorption coefficient at photon energies greater than 4 eV. Phonon absorption features are identified at low energies in the infrared and terahertz ranges.

Nanostructure Evolution of Magnetron Sputtered Hydrogenated Silicon Thin Films

Hydrogenated silicon (Si:H) thin films have been prepared by radio frequency (RF) magnetron sputtering. The effect of hydrogen gas concentration during sputtering on the resultant film structural and optical properties has been investigated by real time spectroscopic ellipsometry (RTSE) and grazing incidence x-ray diffraction (GIXRD). The analysis of in-situ RTSE data collected during sputter deposition tracks the evolution of surface roughness and film bulk layer thickness with time. Growth evolution diagrams depicting amorphous, nanocrystalline, and mixed-phase regions for low and high deposition rate Si:H are constructed and the effects of process parameter (hydrogen gas concentration, total pressure, and RF power) variations on the deposition rate have been qualified. Virtual interface analysis of RTSE data provides nanocrystalline volume fraction depth profiles in the mixed-phase growth regime. GIXRD measurements show the presence of (111) and (220) oriented crystallites. Vibrational mode absorption features from Si-Hn bonding configurations at 590, 640, 2000, and 2090 cm−1 are obtained by ex-situ infrared spectroscopic ellipsometry. Hydrogen incorporation decreases as films transition from amorphous to nanocrystalline phases with increasing hydrogen gas concentration during sputtering.Hydrogenated silicon (Si:H) thin films have been prepared by radio frequency (RF) magnetron sputtering. The effect of hydrogen gas concentration during sputtering on the resultant film structural and optical properties has been investigated by real time spectroscopic ellipsometry (RTSE) and grazing incidence x-ray diffraction (GIXRD). The analysis of in-situ RTSE data collected during sputter deposition tracks the evolution of surface roughness and film bulk layer thickness with time. Growth evolution diagrams depicting amorphous, nanocrystalline, and mixed-phase regions for low and high deposition rate Si:H are constructed and the effects of process parameter (hydrogen gas concentration, total pressure, and RF power) variations on the deposition rate have been qualified. Virtual interface analysis of RTSE data provides nanocrystalline volume fraction depth profiles in the mixed-phase growth regime. GIXRD m...

Spectroscopic Ellipsometry Studies of n-i-p Hydrogenated Amorphous Silicon Based Photovoltaic Devices

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.

Optical properties of soda lime float glass from 3 mm to 148 nm (0.41 meV to 8.38 eV) by spectroscopic ellipsometry

The optical properties of soda lime float glass (SLG) have been characterized over a wide spectral range from 0.41 meV to 8.38 eV using spectroscopic ellipsometry and from 0.74 to 4.80 eV by unpolarized transmittance spectroscopy to retain sensitivity to the onset of high photon energy absorption. In all, the raw measured spectra are collected using four separate instruments, each covering different portions of the full measured spectrum. The glass is modeled as two separate surface layers on either side of a bulk SLG slab to account for variations in surface properties arising from the float glass fabrication process. The measurements are sensitive to the thicknesses of each of these layers and to the optical properties of each, which are reported as both the complex dielectric function (e = e1 + ie2) and complex index of refraction (N = n + ik). Despite the measurements being collected on multiple instruments, the optical properties of each layer are represented as single, continuous parametric functions.

Optical properties and degradation monitoring of CH 3 NH 3 PbI 3

CH₃NH₃PbI₃ perovskite films for photovoltaics (PV) have been deposited by vapor deposition and solution processing. Spectroscopic ellipsometry from the near infrared to ultraviolet (0.75 to 5.89 eV) is used to study the optical response of CH₃NH₃PbI₃. In-situ spectroscopic ellipsometry is applied to monitor CH₃NH₃PbI₃ undergoing degradation in laboratory ambient conditions to study the degradation / decomposition of CH₃NH₃PbI₃ and evolution of unreacted and phase segregated materials (PbI₂, CH₃NH₃I) found at surface / ambient and substrate / film interfaces.

Investigation of doped a-Si 1−x C x :H as a novel back contact material for CdTe solar cells

Wide band-gap, p-type doped, hydrogenated amorphous silicon-carbon alloy (a-Si1-xCx:H:B) layers deposited by plasma enhanced chemical vapor deposition (PECVD) under conditions that yield efficient hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells have been applied as back contacts to sputter-deposited CdTe superstrate solar cells. We report a maximum observed Voc of 0.78 V and a best initial efficiency of ~ 7.7 % (relative to an ~ 12% standard cell baseline) without the introduction of Cu into the back contact region. We studied the stability of the best performing cells over a two year time period and found that although Voc is relatively stable, the series resistance of the cells increased significantly leading to fill-factor degradation. The role of hydrogen loss from the back contact layer via diffusion into the CdTe absorber is explored as a possible cause of this degradation. In related investigations, we have found that the effect of in-diffusing H on the CdTe solar cell performance is detrimental, as observed from a brief (~ 15 s) exposure of CdTe to a low power H2 plasma between the treatment by CdCl2 and the application of standard Cu/Au back contacts. This detrimental effect of H in the fabrication of the back contact layer from hydride gases, and the subsequent instability of the back contact itself, were found to be significant challenges encountered in this investigation.