Real-Time Optimization of Anti-Reflective Coatings for CIGS Solar Cells

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

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. Published by AIP Publishing

n-i-p Nanocrystalline Hydrogenated Silicon Solar Cells with RF-Magnetron Sputtered Absorbers

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

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1-xGaxSe2 Growth: Indium-Gallium Selenide Co-Evaporation

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 (In1-xGax)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

Linear and Nonlinear Optical constants of BiFeO_3

Using spectroscopic ellipsometry, the refractive index and absorption versus wavelength of the ferroelectric antiferromagnet Bismuth Ferrite, BiFeO_3 is reported. The material has a direct band-gap at 442 nm wavelength (2.81 eV). Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d_15/d_22 = 0.20 +/- 0.01, d_31/d_22 = 0.35 +/- 0.02, d_33/d_22 = -11.4 +/- 0.20 and |d_22| = 298.4 +/- 6.1 pm/V at a fundamental wavelength of 800 nm.Comment: 4 pages, 3 figure

Infrared optical properties: Hydrogen bonding and stability

FTIR spectroscopy is a versatile and non-destructive optical characterization method for many materials, including a-Si:H and nc-Si:H, and structural material properties can be derived with relative ease. The ratio of the FTIR absorption in the hydrogen-silicon stretching modes at 2090 and 2000 cm-1was correlated early in the history of a- Si:H solar cells to light-induced degradation. However, the stretching modes were predominantly attributed to the number of hydrogen atoms bonded to a silicon atom and only recently a more adequate model based on the a-Si:H nanostructure has been established, which accounts for the influence that vacancies and voids have on the material properties. Hydrogenated amorphous silicon stands out from other semiconductors by great tunability in a wide deposition parameter space. This allows for the synthesis of different layers with unique properties, and the IR absorptance spectra have proven to be useful as a tool to select the right materials for the right application: • For the archetypical application as a PV absorber layer, a-Si:H material is optimized for high mass density, low defect density, and a low microstructure factor. The combination of a moderately narrow bandgap with minimized light-induced degradation yields high-efficiency devices [10, 51, 83-87]. • Narrow-bandgap a-Si:H can be used as bottom-cell absorber in multi-junction solar cells, yielding high currents [14]. Alloying with Ge reduces the bandgap further. • Wide-bandgap a-Si:H can be used as top-cell absorber, yielding high voltages [14, 88-93]. Alloying with C or O widens the bandgap further. • Few nanometer thick a-Si:H layers are optimized for the surface passivation of crystalline silicon in heterojunction solar cells, in which case not only low microstructure material performs well, but also more porous a-Si:H can be suitable [16]. • Stress-controlled a-Si:H is required to grow thick a-Si:H for detector applications [94]. • For optical applications, a-Si:H can be used in waveguides [34, 35] and is also useful for programmable applications due to the tunability of the complex optical response [28, 32]. For the latter application, a-Si:H with a somewhat elevated microstructure factor seems to be preferred to realize a larger difference between two switchable values of the refractive index, owing to the more pronounced Staebler-Wronski effect in such a-Si:H material in comparison to the type of a-Si:H that is typically preferred as a PV absorber layer. • Porous a-Si:H can serve as solid matrix or reservoir to embed other materials such as lithium for battery applications [95]. When a-Si:H is utilized in each of these applications, the particular nanostructure, hydrogen content, and the way hydrogen is configured in the material all impact the final material and device functionality

Infrared Optical Properties: Hydrogen Bonding and Stability

FTIR spectroscopy is a versatile and non-destructive optical characterization method for many materials, including a-Si:H and nc-Si:H, and structural material properties can be derived with relative ease. The ratio of the FTIR absorption in the hydrogen-silicon stretching modes at 2090 and 2000 cm-1was correlated early in the history of a- Si:H solar cells to light-induced degradation. However, the stretching modes were predominantly attributed to the number of hydrogen atoms bonded to a silicon atom and only recently a more adequate model based on the a-Si:H nanostructure has been established, which accounts for the influence that vacancies and voids have on the material properties. Hydrogenated amorphous silicon stands out from other semiconductors by great tunability in a wide deposition parameter space. This allows for the synthesis of different layers with unique properties, and the IR absorptance spectra have proven to be useful as a tool to select the right materials for the right application: • For the archetypical application as a PV absorber layer, a-Si:H material is optimized for high mass density, low defect density, and a low microstructure factor. The combination of a moderately narrow bandgap with minimized light-induced degradation yields high-efficiency devices [10, 51, 83-87]. • Narrow-bandgap a-Si:H can be used as bottom-cell absorber in multi-junction solar cells, yielding high currents [14]. Alloying with Ge reduces the bandgap further. • Wide-bandgap a-Si:H can be used as top-cell absorber, yielding high voltages [14, 88-93]. Alloying with C or O widens the bandgap further. • Few nanometer thick a-Si:H layers are optimized for the surface passivation of crystalline silicon in heterojunction solar cells, in which case not only low microstructure material performs well, but also more porous a-Si:H can be suitable [16]. • Stress-controlled a-Si:H is required to grow thick a-Si:H for detector applications [94]. • For optical applications, a-Si:H can be used in waveguides [34, 35] and is also useful for programmable applications due to the tunability of the complex optical response [28, 32]. For the latter application, a-Si:H with a somewhat elevated microstructure factor seems to be preferred to realize a larger difference between two switchable values of the refractive index, owing to the more pronounced Staebler-Wronski effect in such a-Si:H material in comparison to the type of a-Si:H that is typically preferred as a PV absorber layer. • Porous a-Si:H can serve as solid matrix or reservoir to embed other materials such as lithium for battery applications [95]. When a-Si:H is utilized in each of these applications, the particular nanostructure, hydrogen content, and the way hydrogen is configured in the material all impact the final material and device functionality.</p