Solar Cells最新文献

Amorphous silicon samples with an intentionally modulated impurity content at concentrations below 1 at.% were grown by the glow discharge deposition technique. The impurity profiles were determined by secondary-ion mass spectroscopy (SIMS) measurements and the spatial distributions of deep defects were determined by junction capacitance profiling measurements before and after light-induced degradation. For the case of intentionally added carbon, we found a significant enhancement in the concentration of light induced defects in regions with higher impurity levels such that a carbon level of 0.5 at.% leads to an additional $\text{(1–2) × 10}{}^{16}\text{cm}{}^{\mathrm{-3}}$ increase in defects after light soaking.

Solar cell and film analyses indicate that electron mobility in amorphous hydrogenated silicon-germanium decreases with increasing hydrogen $\text{C}{}_{\mathrm{<mtext>H</mtext>}}$ and germanium $\text{C}{}_{\mathrm{<mtext>Ge</mtext>}}$ contents. The hole mobility-lifetime product μτ is less dependent on germanium content than the electron μτ product. Thin (less than 1000 Å) graded band gap alloy solar cells were prepared by photochemical vapor deposition with greater than 5% efficiency (at air mass 1.5) and 40% quantum efficiency at 800 nm. Unalloyed a-Si:H with $\text{C}{}_{\mathrm{<mtext>H</mtext>}}$ values of 7% and 11% having similar annealed state dangling bond densities was prepared by photochemical vapor deposition. Under light exposure or high temperature current injection, high $\text{C}{}_{\mathrm{<mtext>H</mtext>}}$ materials were markedly less stable.

The relationship between transport research and solar cell models for hydrogenated amorphous silicon is reviewed. It is argued that a complete program of steady-state photoconductivity, ambipolar diffusion length, and photoconductivity response time measurements is required to support modeling; the present knowledge of these measurements in electronic quality a-Si:H is summarized. A qualitative discrepancy between trap distributions used for steady-state transport models and estimated from transient photocurrent measurements is discussed.

This paper presents a brief review of the research being carried out under the crystalline silicon materials research program. The primary emphasis of this program is the development of processes for post-growth quality enhancement of low-cost silicon substrates. The various electronic mechanisms that can be exploited for material quality enhancement are summarized. Major research results of the current program and some important aspects of future research are identified.

The two-stage process was used to prepare thin films of Cd(Zn)Te and CuInSe₂. The technique involves first depositing the elemental components of the compound onto a substrate in the form of thin stacked layers and then reacting these elemental components to obtain a thin film of the desired compound. While CdTe films grown on thin CdS layers have uniform stoichiometries and sharp interfaces with the underlying CdS layers, CdZnTe films deposited onto similar substrates give rise to diffused CdZnTeCdS interfaces because of the reactive nature of zinc. In CuInSe₂ processing, the nature of the reacted compound film strongly depends on the nature of the CuIn layers. CdS/CuInSe₂ device efficiencies are also influenced by the method of deposition for the CdS window layers.

Rapid thermal processing of precursor small-grained (less than or equal to 1 μm) CuInSe₂ thin films can result in enhanced grain growth on the order of 100 μm under certain processing conditions. Crystal growth was accompanied by large increases (×10) in the X-ray diffraction intensity of the (112) peak, indicating near-perfect crystal growth in the [221] direction. Single-crystal quality was verified by selected-area electron channeling. However, at present, this effect occurs in conjuction with excessive void and pinhole formation between single-crystal platelets. These voids are often decorated with relatively selenium-deficient phases.

Hydrogen-induced metastability is analysed by a density of states distribution for bonded hydrogen, which described the alternative hydrogen sites and the effects of disorder. The metastability is explained by the redistribution of hydrogen following illumination, annealing or a shift of the Fermi energy. Hydrogen and defect equilibria are related to the hydrogen diffusion and the chemical potential. An estimate of the hydrogen density of states distribution, based on the weak bond model, is shown to be consistent with the measured defect density. Irreversible metastable changes are related to structural reconstructions which change the shape of the distribution, while the reversible changes correspond to a redistribution of hydrogen within an approximately constant density of states.

Polycrystalline cadmium sulfide-cadmium telluride heterojunction solar cells were fabricated for the first time using a laser-driven physical vapor deposition method. An XeCl excimer laser was used to deposit both of the II–VI semiconductor layers in a single vacuum chamber from pressed powder targets. Results are presented from optical absorption, Raman scattering, X-ray diffraction, and electrical characterization of the films. Solar cells were fabricated by deposition onto SnO₂-coated glass with top contacts produced by gold evaporation. Device performance was evaluated from the spectral quantum efficiency and current-voltage measurements in the dark and with air mass 1.5 solar illumination.

A novel vertical stagnation flow organometallic vapor phase epitaxy reactor was designed and fabricated for the growth of GaAs and AlGaAs for solar cell applications. The reactor had an inverted configuration to eliminate recirculation problems. The susceptor and gas inlet nozzle were closely spaced (about 1 cm) in order to achieve improvements in deposition efficiency, layer uniformity and abruptness of interfaces. A specially designed water-cooled inlet nozzle was used to maintain the nozzle surface at relatively low temperatures under all operating conditions. A computer model was formulated to study the various thermal processes in this reactor. The model used rigorous thermal boundary conditions which included thermal radiation effects. Simulated and experimental nozzle temperatures were compared for different susceptor temperatures, susceptor-nozzle distances, gas flow rates and reactor pressures. The maximum nozzle temperature was about 100 °C, which is sufficiently low to prevent premature decomposition of the reactants on its surface.

Multijunction solar cell modules based on a-Si:H and its alloys have been developed. Triple junction devices have been modeled to quantify parasitic optical losses. A laser patterning method for modules has been developed and 98% active/aperture areas demonstrated. An ad hoc module of light-induced stability is developed which suggests much improved stability is to be expected in the triple junction devices; this is verified by experimental data. Triple junction 939.6 cm² a-Si:H/a-Si:H/a-SiGe:H modules tested at SERI have demonstrated an aperture area efficiency of 9.27%.