Optical Simulation of External Quantum Efficiency Spectra

Abstract

Applications of ex situ spectroscopic ellipsometry (SE) are presented for determination of the parameters that describe the dielectric function and structure of thin film solar cells. Complete optical models of solar cells developed using least squares regression analysis of the SE data enable external quantum efficiency (EQE) simulations for comparison with measurements. Through this comparison, it becomes possible to understand in detail the origins of optical and electronic collection and losses in thin film photovoltaics technologies and, as a result, the underlying performance limitations. Examples of this approach are presented for the three commercialized thin film technologies of hydrogenated amorphous silicon (a-Si:H), cadmium telluride (CdTe), and copper indium-gallium diselenide (CuIn1–xGaxSe2; CIGS). In the studies of a-Si:H solar cells, a comparison between the EQE simulation based on the SE model and the measured EQE suggests electrical losses from photo-generated carriers near the p/i and i/n interfaces, the latter caused by an i-layer thickness greater than the hole collection length. Also demonstrated here through comparisons of EQE measurements and simulation is enhanced carrier collection near the p/i interface when a protocrystalline Si:H i-layer of improved electrical quality is incorporated at the interface. For a CdS/CdTe heterojunction solar cell in the superstrate configuration, SE is performed through the glass, and simulations of the EQE spectra have been generated on the basis of comprehensive optical property and multilayer analysis by SE. In this case, observed deviations between simulated and measured EQE can assist in refining the optical model of the cell. Applying these methods, the optical losses that occur when photons with above-bandgap energies are not absorbed within the cell’s active layers can be distinguished from electronic losses that occur when electrons and holes photo-generated within these active layers are not collected. CIGS/CdS heterojunction solar cells incorporating both standard thickness and thin absorbers are also studied using SE. Data analysis is more challenging for CIGS because of the need to extract absorber layer Ga profiles for accurate optical 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. For cells with thin absorbers, however, the difference observed between the simulated and measured EQE can be assigned to losses via electron-hole recombination near the Mo back contact. When a probability profile for carrier collection is introduced into the EQE simulation, closer agreement between this simulation and the measurement is observed. In addition to a single spot capability of SE as presented in this study, a capability also exists for high resolution mapping of multilayer thicknesses and component layer characteristics that provide short-circuit current density predictions. The mapping capability is made possible due to the high speeds [<1 s per measurement of (ψ, Δ) spectra] of multichannel ellipsometers.