Abstract
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.
Highlights
Development of thin film technologies based on amorphous silicon and germanium, including photovoltaic (PV) devices, involves understanding of material electrical and optical properties [1,2,3,4].It is essential to measure, monitor, and control the thickness, structure, phase, and composition of solar cell component layers in the same configuration used in manufacturing, especially for devices processed over large areas
The first layers deposited for n-i-p configuration amorphous silicon (a-Si):H solar cells comprise the zinc oxide (ZnO)/Ag back reflector (BR)
real time spectroscopic ellipsometry (RTSE) and IR-spectroscopic ellipsometry (SE) have been demonstrated as a useful metrology technique for characterization of plasma enhanced chemical vapor deposition (PECVD) Si:H layers and components of the BR structure used in n-i-p a-Si:H solar cells
Summary
Development of thin film technologies based on amorphous silicon and germanium, including photovoltaic (PV) devices, involves understanding of material electrical and optical properties [1,2,3,4]. Dahal et al reported growth evolution diagrams for intrinsic and p-type Si:H deposited on unoptimized n-layer/ZnO/Ag back reflector (BR) coated PEN in n-i-p configuration PV devices [15] Overall these results and analysis procedures developed here are applicable to more directly relating properties of layers in the device configuration, as obtained by non-destructive measurements, with variations in device performance. When combined with Σσ-minimization approaches for structurally graded Si:H, VIA yields ε for both the nc-Si:H and a-Si:H components as well as the time and bulk layer thickness dependence of component material fractions in the overlayer of material accumulated between each pseudo-substrate and subsequent data set pair These techniques are used to provide guidance for the deposition and in situ characterization of a-Si:H, nc-Si:H, and mixed-phase (a+nc)-Si:H layers during growth in device structures. In addition to information on each of these materials in the n-i-p device structure, the structural and optical properties derived here can be applied in the future analysis of ex situ SE in either single spot [36] or mapping configurations
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