George Dewey Cody

Optical characterization of amorphous silicon hydride films

Abstract The performance of solar cells can be approached from the view of optical limitations (the adsorbed solar photon) or electrical limitations (the collected carriers). In this paper we focus on the optical behavior of a-SiHx films and through these data define the intrinsic optical limitations on the performance of a-SiHx. We present proven techniques for the extraction of the optical constants for thin films of a-SiHx on quartz over the absorption range 10−1–106 cm−1. For the range 104–106 cm−1 we define an optical gap EG from a plot of (αE) 1 2 against E for films of comparable thickness. We show the utility of this parameter for guiding materials research and we discuss its theoretical significance. Below EG we utilized bothoptical and photoconductivity data to establish a sharp exponential edge. For a-SiHx the edge has an activation energy that can be sample dependent and for our films it ranges from 0.05 to 0.10. The device implications of this exponential edge are discussed.

Optical reflectance and transmission of a textured surface

Abstract The reflectivity of a solar thermal or solar electric device is a key parameter in efficiency. In the recent solar device literature, highly “textured” surfaces have been shown to reduce the reflectivity appreciably. The theoretical model used to describe this phenomenon is light trapping by multiple reflections. Surface roughness has also been considered by others through statistical scattering theory. The range of validity of either model is limited to a scale of texture larger than the wavelength of the light. For the micron scaled texture which is of interest, however, both approaches fall into the category of approximate solutions to approximate models of the surface. We approached the problem differently. We obtained the effects of texture on reflectivity and transmission through an exact calculation of a boundary layer whose complex dielectric constant is an appropriate average of the bulk dielectric constant of the material and air. The calculations were made for arbitrary angles of incidence, polarization and wavelength, as well as for arbitrary spatial variation of the dielectric constant through the boundary layer. We developed the spatial variation through effective medium models for a discontinuous surface layer. Finally, we compared the computer calculation with an exact analytic treatment for normal incidence, as well as with experimental reflectivities on several textured surfaces.

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Abstract Optical absorption below the mobility gap of a -SiH x :P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.

Optical absorption above the optical gap of amorphous silicon hydride

From transmission and reflection measurements on glow discharge prepared aue5f8Si:Hx(x≈0.09−0.19) films we obtain the imaginary part of the dielectric constant, ϵ2 in the range 1.4 to 3.8 eV. We compare this data with the prediction of a model first introduced by Tauc for optical transitions, with momentum conservation relaxed, between the valence (occupied) and conduction (unoccupied) bands of an amorphous semiconductor. The agreement with the Tauc model is good, but is greatly improved if the constant momentum matrix element of that is replaced by a constant dipole matrix element. This substitution, which is physically plausible for the high energy optical transition considered, completely removes a spurious thickness dependence of the derived optical gap. We obtain from the modified Tauc model, an optical gap for aue5f8Si:Hx(x≈0.09−0.19) of 1.64 eV with a constant dipole matrix element of 6.7 A.

Exponential absorption edge in hydrogenated α-Si films

Abstract Optical and photoelectric measurements demonstrate that hydrogenated amorphous silicon prepared by glow discharge decomposition of silane has an exponential optical absorption edge, over the photon energy range 1.4–1.8 eV with a slope of 0.05–0.08 eV. Evidence is presented that the photogeneration efficiency is unity at room temperature and independent of electric field (102−104 V/cm−1) and photon energy (1.2–2.2 eV).

The influence of carrier generation and collection on short-circuit currents in amorphous silicon solar cells

Abstract A detailed study of air mass one (AM1) short-circuit currents and of the carrier collection efficiencies was carried out on discharge-produced hydrogenated amorphous silicon (a-SiHx) films. The techniques used to correlate these currents accurately and quantitatively with the intrinsic properties of the different a-SiHx films are described. Results are presented which illustrate the techniques and the various characteristics found for the collection of photogenerated carriers in the a-SiHx films. The dependence of these characteristics on both the optical and the electronic properties of the a-SiHx films as well as on the different processes determining the carrier collection are discussed. The methods used in correlating the AM1 short-circuit currents with the generation and collection of photogenerated carriers are presented. This includes the characterization of photogenerated carriers in terms of a carrier collection width Xc and the optical absorption of the a-SiHx. The importance of Xc, which is independent of photon energies, in correlating the collection of photogenerated carriers with other measurements on the a-SiHx electronic properties and in the evaluation of AM1 short-circuit currents in different films is indicated.

Comment on the optical absorption edge in a-Si:H

Abstract The optical absorption edge of undoped amorphous silicon hydride has been measured using optical transmission, photoconductivity, and photothermal deflection spectroscopy. The results obtained by these techniques agree in the exponential edge region. An apparent inconsistency pointed out by Redfield between the optical absorption edge and the valence band tail density of states as measured by drift mobility is attributed to the non-exponential behavior of the absorption edge above α∼10 3 cm -1 .

Inhomogeneous surfaces as selective solar absorbers

Abstract An outstanding problem facing solar thermal collectors is the development of efficient, cheap and long-lived selective solar absorbing surfaces. Among the best products currently available are complicated inhomogeneous mixtures of metal and dielectric particles optimized from serendipitous discoveries rather than premediated constructions. We have utilized a computer code which uses mean field approximations and can accurately describe the reflectance of inhomogeneous surfaces. This approach gave us the freedom to use realistic materials parameters and surface topologies and arbitrary angles of incidence in our calculations. As a result of calculations on a large number of specific surface configurations and material combinations, we found that the optical properties of such surfaces are largely independent of the details of their topologies and can be described with only two parameters: surface depth, D, and correlation length, 2a. A main conclusion of the paper is that the impression of a rough surface on an otherwise shiny metal leads to a sharply defined low reflection window between λII≈20A and λ≈6D if λII

Capture cross-section and density of deep gap states in a−SiHx schottky barrier structures

Abstract Recombination of charge carriers in a −SiH x Schottky barriers with density of states near mid-gap ranging from 2.8×10 15 −7×10 16 cm -1 eV -1 is attributed to recombination centers with hole capture cross-section of 1.3×10 -15 cm 2 .

Hydrogenated amorphous silicon solar cells

Abstract A simple description of the operation of the hydrogenated amorphous silicon (a-SiHx) pin solar cell is given and general guidelines for increasing the efficiency are established. The use of heterostructures in which the n and p layers have larger band gaps than the intrinsic (i) layer helps to reduce losses in efficiency due to optical absorption in the doped layers and back diffusion of carriers across the n-i and p-i interfaces. The density of gap states of the intrinsic layer is inferred from measurements of optical absorption, photoconductivity, drift mobility, collection efficiency and capacitance. Based on the density of gap states of the best intrinsic material available, an upper theoretical limit of 17% is estimated for the solar cell efficiency. Materials improvements required to achieve experimentally a 10% efficient a-SiHx solar cell are discussed.