Optical simulation of external quantum efficiency spectra of CuIn1−Ga Se2 solar cells from spectroscopic ellipsometry inputs

Abstract

Applications of in-situ and ex-situ spectroscopic ellipsometry (SE) are presented for the development of parametric expressions that define the real and imaginary parts (ε 1 , ε 2) of the complex dielectric function spectra of thin film solar cell components. These spectra can then be utilized to analyze the structure of complete thin film solar cells. Optical and structural/compositional models of complete solar cells developed through least squares regression analysis of the SE data acquired for the complete cells enable simulations of external quantum efficiency (EQE) without the need for variable parameters. Such simulations can be compared directly with EQE measurements. From these comparisons, it becomes possible to understand in detail the origins of optical and electronic gains and losses in thin film photovoltaics (PV) technologies and, as a result, the underlying performance limitations. In fact, optical losses that occur when above-bandgap photons are not absorbed in the active layers can be distinguished from electronic losses when electron-hole pairs generated in the active layers are not collected. This overall methodology has been applied to copper indium-gallium diselenide (CuIn 1 −x Ga x Se 2 ; CIGS) solar cells, a key commercialized thin film PV technology. CIGS solar cells with both standard thickness (> 2 μm) and thin (< 1 μm) absorber layers are studied by applying SE to obtain inputs for EQE simulations and enabling comparisons of simulated and measured EQE spectra. SE data analysis is challenging for CIGS material components and solar cells because of the need to develop an appropriate (ε 1 , ε 2) database for the CIGS alloys and to extract absorber layer Ga profiles for accurate structural/compositional models. For cells with standard thickness absorbers, excellent agreement is found between the simulated and measured EQE, the latter under the assumption of 100% collection from the active layers, which include the CIGS bulk and CIGS/CdS heterojunction interface layers. For cells with thin absorbers, however, an observed difference between the simulated and measured EQE can be attributed to losses via carrier recombination within a ∼0.15 μm thickness of CIGS adjacent to the Mo back contact. By introducing a carrier collection probability profile into the simulation, much closer agreement is obtained between the simulated and measured EQE. In addition to the single spot capability demonstrated in this study, ex-situ SE can be applied as well to generate high resolution maps of thin film multilayer structure, component layer properties and their profiles, as well as short-circuit current density predictions. Such mapping is possible due to the high measurement speed of < 1 s per (ψ,) spectra achievable by the multichannel ellipsometer.

Irradiance and temperature considerations in the design and deployment of high annual energy yield perovskite/CIGS tandems

“…The external quantum efficiencies (EQEs) were determined by using the complex index of refraction data for the device materials layers within a transfer matrix formalism 31,41 while taking the internal quantum efficiencies to be unity and zero for light absorbed in the photoactive and photoinactive portions of the devices, respectively. Complex index of refraction data were collected from the literature 30 42,43 allowing modeling of bottom cells with bandgaps of 1.00, 1.08, 1.16, and 1.24 eV, respectively. The complex index of refraction data for remaining material were collected from literature.…”

“…The complex index of refraction data for remaining material were collected from literature. 42,44,45 Data for CdS, Mo, ZnO layers were taken from ref. 43 and those for ITO and MgF 2 were taken from ref.…”

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

“…The thinner 1 µm thick sputtered i -layer is described as a bilayer of the same material properties although the crystal silicon component of the i -layer adjacent to the p -layer is 0.1 µm thick, while the effective medium approximation of crystalline silicon and a-Si:H near the n -layer remains the same thickness and volume fraction as for the PECVD i -layers. The sputtered i -layer is divided into ten 0.1 µm thick sublayers to introduce a collection probability profile to further improve agreement between simulated and measured EQE [45]. The collection probability profile is determined by fitting a variable fraction of carriers collected for each sublayer in a least squares regression to match the measured EQE.…”

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

“…A single Tauc–Lorentz oscillator and single CP oscillator with resonance energy 3.09 eV are used along with and a Drude contribution to model the complex dielectric functions of the AZO layer. The detailed analyses for all these materials are given in previous papers [ 6 , 7 , 10 , 11 , 12 , 13 ]. Experimental ellipsometric spectra in terms of ψ and ∆ are shown in Figure 1 along with the best fit obtained by a least squares regression for data collected from a specific complete CIGS solar cell device.…”

“…As more research is being implemented to push the limitations in energy conversion of the devices, improvements are made in the device structure and process parameters to optimize the device efficiency, such as with thinner cadmium sulfide (CdS) heterojunction partner layers or aluminum doped zinc oxide (AZO) window layers, or to reduce process cost, such as through thinning the CIGS absorber layer. Various studies have been performed to optically simulate the external quantum efficiency (QE) spectra of high-efficiency CIGS-based solar cells incorporating the effects of variation in the compositional profile of CIGS and thicknesses of various solar cell component layers [ 5 , 6 , 7 ]. However, for all these alterations, the role and importance of the AR coating are always overlooked.…”