Solar Cells最新文献

High-resolution photoluminescence (PL) measurements were carried out at 10 K to identify the energy levels associated with the various defect states dominating the semiconductor CuInSe₂ (CIS). PL measurements were taken on the bare surfaces of both thin film and single-crystal (polished and cleaved) samples and through a (Cd,Zn)S window layer deposited by thermal co-evaporation onto the CIS absorber surface. A complete energy band diagram is proposed which identifies the origin of the 12 intrinsic defect states expected in this material. The effects of surface and heat treatments, used in device fabrication processing, on the existence and generation of defect states (deep and shallow) are identified and correlated with the device performance. The inferior single-crystal device performance is correlated with presence of a high density of process-generated radiative surface recombination states and trap levels.

Measurements of transport properties and microstructure of hydrogenated amorphous silicon-germanium alloys are reported. Emphasis is placed on the effects of hydrogen dilution of the source gas. The transport properties include photoconductivity, ambipolar diffusion length; the structure was determined from small-angle X-ray scattering.

Fluorine-doped ZnO films were deposited on soda lime glass by atmospheric pressure chemical vapor deposition at temperatures from 350 to 470 °C by using diethyl zinc, ethanol and hexafluoropropene as precursors. The deposited films typically contained about 0.1 to about 1.0 at.% fluorine with conductivities up to 2500 ω⁻¹ cm⁻¹. The free electron concentrations determined from Hall coefficient measurements were up to $\text{5 × 10}{}^{20}\text{cm}{}^{\mathrm{-3}}$ and the mobilities were between 10 and 40 cm² V⁻¹ s⁻¹. The films with very low sheet resistances of 5 ω/□ were found to have visible absorption of only 3% and transmittance up to 90% in the visible and reflectance of about 85% in the infrared. The film roughness was controlled by the deposition temperature and by introducing a small amount of water vapor. The rough films were used as substrates for amorphous silicon solar cells with very high quantum efficiency (up to 90%).

In this paper we discuss the phase behavior and microstructure of sequentially evaporated indium-on-copper layer stacks deposited onto molybdenum-coated glass substrates. It was determined that both equilibrium and non-equilibrium phases can exist depending on processing. Indium evaporated onto copper, without intentional substrate heating and no subsequent annealing, resulted in the formation of the CuIn (JCPDS card 35-1100) alloy phase and an unreported f.c.c. phase of undetermined composition. Auger spectrometry and X-ray diffraction strongly suggested this to be a new CuIn alloy. Indium evaporated onto copper with substrate heating at 200 °C followed by uncontrolled cooling to room temperature strongly favored the formation of Cu₁₁In₉ in addition to the two phases previously mentioned. The lack of elemental indium in both cases, the presence of which is expected from phase diagram analysis, was attributed to kinetic limitations. Annealing of Cu/In layer stacks for 1 h at 200 °C immediately following indium deposition promoted the formation of elemental indium and Cu₁₁In₉ while simultaneously reducing the concentration of both CuIn and the new f.c.c. alloy phase. The implications of these observations are discussed with regard to future selenization experiments.

By analyzing the internal recombination losses in state-of-the-art GaAs solar cells, we demonstrate that such cells are approaching a material limit efficiency. Two new designs for III–V cells are then proposed and analyzed; both approaches make use of thin epitaxial layers. The first approach is to make use of light-trapping techniques to reduce volume recombination losses while maintaining the short-circuit current. Projections show that light trapping might add 2%–3% in efficiency. A second approach is to trap photons emitted during radiative recombination in order to enhance the bulk lifetime. With this approach, a 2% gain in efficiency is projected. These new design approaches will require the development of techniques for producing and processing thin epitaxial layers of GaAs, but they promise substantial gains in the efficiency of single-junction cells.

Detailed measurements were made on CuInSe₂ solar cells following thermal anneals in air at progressively higher temperatures. For evaporated cells there is an initial improvement in many of the individual measured parameters. The common explanation is a decrease in compensating defect states. After higher temperature anneals, however, compensating state densities show a dramatic rise, resulting in large forward currents and small photocurrents, which effectively terminate the photovoltaic response.

Bilayers of a slightly copper-rich composition of copper and indium deposited (in that order) at room temperature by vacuum evaporation, electroplating, and r.f. sputtering, were analyzed by MeV ⁴He backscattering spectrometry, scanning electron microscopy and X-ray diffraction before and after thermal annealing at 400 °C in vacuum. The surface morphology of as-deposited samples is roughest for evaporated samples and smoothest for samples deposited by sputtering with low r.f. power. All as-deposited samples contain copper and indium phases and the metastable CuIn compound. After 1 h at 400 °C, this metastable phase and that of copper disappear and are replaced by some copper-rich compound (Cu₉In₄, Cu₇In₄) in all cases. Simultaneously, the surface morphology smooths out considerably for the initially rough samples. Bilayers of various Cu/In compositions were also prepared. The results are consistent with the absence of stable compounds outside the 26–38 at.% indium range, as indicated by published phase diagrams.

Amorphous silicon and silicon carbon alloy thin films (a-Si:H, a-SiC:H) were deposited by electron cyclotron resonance (ECR) microwave plasmas using SiH₄, CH₄ and hydrogen gas mixtures. The ECR-deposited, photosensitive a-Si:H films show a light conductivity of $\text{5 × 10}{}^{\mathrm{-5}}\text{Ω}{}^{\mathrm{-1}}\text{cm}{}^{\mathrm{-1}}$, and a light-to-dark conductivity ratio of about $\text{2 × 10}{}^6$. Optical bandgaps of ECR-deposited a-Si:H films are in the range 1.75–1.85 eV. The integrated defect density in the mobility gap of the photosensitive a-Si:H film was determined by junction capacitance measurements to be $\text{1.6 × 10}{}^{16}\text{cm}{}^{\mathrm{-3}}$. A new type of conductive a-Si:H film, which may contain a microcrystalline phase, was also deposited by ECR PECVD. The p-type dopedECR-deposited, conductive a-Si:H films show a conductivity of $\text{5 × 10}{}^{\mathrm{-2}}\text{Ω}{}^{\mathrm{-1}}\text{cm}{}^{\mathrm{-1}}$.

The ECR-deposited a-SiC:H films show slightly higher optical bandgaps than those of r.f.-deposited a-SiC:H films. Hydrogen dilution in the ECR plasma with a dilution ratio up to 5 shows no significant effect on the optical bandgap of a-SiC:H films. The deposition rate of a-SiC:H films is found to be strongly dependent on the ECR magnetic field and the hydrogen dilution. The hydrogen dilution effect on the deposition rate indicates that the etching in ECR hydrogen plasmas plays an important role in the deposition of a-SiC:H films. The two-phase nature of ECR-deposited, microcrystalline silicon carbon films (μ-SiC:H) is shown by X-ray diffraction to consist of 1000 Å microcrystallites of α-SiC and amorphous network structures.

The activation treatments necessary to produce photoactive screen-printed pyrite electrodes for photoelectrochemical applications are described. In particular, air (340 °C), hydrogen (200 °C) and air-hydrogen (340-200 °C) treatments have been selected. Surface and bulk characterization of the electrodes have been carried out by X-ray photoelectron spectroscopy, X-ray diffraction and Mössbauer spectroscopy. Diffuse reflectance spectroscopy and photoelectrochemical tests in I⁻/I₃⁻ solutions allowed us to ascertain the optical absorption and solar energy conversion properties of the differently activated samples. The best performing electrode is the air-hydrogen activated electrode (η = 5.52%) which shows an optimal combination of the optical absorption characteristics and semiconductor-electrolyte charge transfer properties. The results have been discussed on the basis of the electronic structure of the compounds involved in the interfacial chemistry.

Endoreversible processes are a special class of irreversible processes: the irreversibilities are all located in the transport of heat from the heat sources to the heat engine and from the heat engine to the heat sinks. It has been demonstrated before that photothermal solar energy conversion can be modeled as an endoreversible process. In the present paper, we investigate if a solar cell can equally be considered as an endoreversible engine.

We will demonstrate that in fact a photovoltaic converter is somewhat more complicated than a photothermal converter. This forces us to introduce a more general model of endoreversible engines, where thermodynamic reservoirs are not only characterized by a temperature, but also by a chemical potential, and where not only energy, but also matter is exchanged between reservoirs.