A. Mason

14.1% CuIn1 − x Ga x Se2‐Based Photovoltaic Cells from Electrodeposited Precursors

The authors have fabricated 14.1% efficient CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} (CIGS) based devices from electrodeposited precursors. As-deposited precursors are Cu-rich films and are polycrystalline in nature. Additional In, Ga, and Se were added to the precursor films by physical evaporation to adjust the final composition to CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2}. Addition of In and Ga and also selenization at high temperature are very crucial for obtaining high-efficiency devices. The X-ray analysis of the as-deposited precursor film indicates the presence of CIGS and Cu{sub 2}Se phases. The X-ray analysis of the film after adjusting the composition of the final film shows only the CIGS phase. The films/devices have been characterized by inductively coupled plasma spectrometry, Auger electron spectroscopy, X-ray diffraction, electron-probe microanalysis, current-voltage characteristics, capacitance-voltage, and spectral response.

12.3% Efficient CuIn1 − x Ga x Se2‐Based Device from Electrodeposited Precursor

Of the emerging materials for solar cell applications, CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} (CIGS) is a leading candidate and has received considerable attention in recent years. Copper-indium-gallium-selenium (Cu-In-Ga-Se) precursor thin films have been prepared by electrodeposition techniques on molybdenum substrates. The films have been characterized by inductively coupled plasma spectrometry, Auger electron spectroscopy, x-ray diffraction, electron probe microanalysis, current-voltage characteristics, spectral response, and electron-beam-induced current. Additional In or Cu, Ga, and Se have been added to the electrodeposited precursor film by physical evaporation to adjust the final composition to CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2}, and allowed to crystallize at 550 C. A ZnO/CdS/CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} device fabricated using electrodeposited Cu-In-Ga-Se precursor layers resulted in an efficiency of 12.3%.

Development of rf sputtered, Cu-doped ZnTe for use as a contact interface layer to p-CdTe

Cu-doped ZnTe films deposited by rf-magnetron sputtering have been analyzed with the intention to use this material as a contact interface in CdS/CdTe thin-film photovoltaic solar-cell devices. It is observed that unless careful attention is made to the pre-deposition conditioning of the ZnTe target, the electrical resistivity of thin films (∼70 nm) will be significantly higher than that measured on thicker films (∼1.0 μm). It is determined that N contamination of the target during substrate loading is likely responsible for the increased film resistivity. The effect of film composition on the electrical properties is further studied by analyzing films sputtered from targets containing various Cu concentrations. It is determined that, for targets fabricated from stoichiometric ZnTe and metallic Cu, the extent of Zn deficiency in the film is dependent on both sputtering conditions and the amount of metallic Cu in the target. It is observed that the carrier concentration of the film reaches a maximum value of ∼3 × 1020 cm−3 when the concentrations of Te and (Zn+Cu) are nearly equal. For the conditions used, this optimum film stoichiometry results when the concentration of metallic Cu in the target is ≈6 at.%.

Ge-related faceting and segregation during the growth of metastable (GaAs)1−x(Ge2)x alloy layers by metal–organic vapor-phase epitaxy

(GaAs)1−x(Ge2)x alloy layers, 0<x<0.22, have been grown by metal–organic vapor-phase epitaxy on vicinal (001) GaAs substrates. Transmission electron microscopy revealed pronounced phase separation in these layers, resulting in regions of GaAs-rich zinc-blende and Ge-rich diamond cubic material that appears to lead to substantial band-gap narrowing. For x=0.1 layers, the phase-separated microstructure consisted of intersecting sheets of Ge-rich material on {115}B planes surrounding cells of GaAs-rich material, with little evidence of antiphase boundaries. Atomic force microscopy revealed {115}B surface faceting associated with the phase separation.

OPTICAL PHONONS IN LASER-DEPOSITED CDSXTE1-X FILMS

We report the longitudinal optical (LO) phonon frequencies for thin films of the ternary alloy CdSxTe1−x and their composition dependence for the full range of x values from pure CdTe to pure CdS. Pulsed laser deposition was used to prepare the polycrystalline thin films including compositions well inside the miscibility gap. We find that this ternary system exhibits a “two-mode” behavior with CdS- and CdTe-like longitudinal optic phonon modes. The modified random-element isodisplacement model yields a good description of the composition dependence of the LO phonon frequencies.

CdS/CdTe thin-film solar cell with a zinc stannate buffer layer

This paper describes an improved CdS/CdTe polycrystalline thin-film solar-cell device structure that integrates a zinc stannate (Zn2SnO4 or ZTO) buffer layer between the transparent conductive oxide (TCO) layer and the CdS window layer. Zinc stannate films have a high bandgap, high transmittance, low absorptance, and low surface roughness. In addition, these films are chemically stable and exhibit higher resistivities that are roughly matched to that of the CdS window layer in the device structure. Preliminary device results have demonstrated that by integrating a ZTO buffer layer in both SnO2-based and Cd2SnO4 (CTO)-based CdS/CdTe devices, performance and reproducibility can be significantly enhanced.

High-efficiency Cd/sub 2/SnO/sub 4//Zn/sub 2/SnO/sub 4//Zn/sub x/Cd/sub 1-x/S/CdS/CdTe polycrystalline thin-film solar cells

CdTe-based thin-film solar cells have been limited to the conventional SnO/sub 2//CdS/CdTe device structure. In this paper, we report a modified device structure consisting of Cd/sub 2/SnO/sub 4//Zn/sub 2/SnO/sub 4//Zn/sub x/Cd/sub 1-x/S/CdS/CdTe layers, that yields improved performance and reproducibility. Cadmium stannate (Cd/sub 2/SnO/sub 4/, or CTO) transparent conductive oxide (TCO) films have several significant advantages over conventional SnO/sub 2/ films. CTO-based CdTe cells have approximately 1 mA/cm/sup 2/ higher J/sub sc/ than SnO/sub 2/-based CdTe cells. Integrating zinc stannate (Zn/sub 2/SnO/sub 4/, or ZTO) into the device as a buffer layer helps maintain high V/sub oc/ and fill factor when reducing CdS thickness to improve J/sub sc/. XPS and SIMS results show substantial interdiffusion between the CdS and ZTO layers. This feature can be used to optimize device performance and reproducibility. We have fabricated a Cd/sub 2/SnO/sub 4//Zn/sub 2/SnO/sub 4//Zn/sub x/Cd/sub 1-x/S/CdS/CdTe cell with an NREL-confirmed total-area efficiency of 15.8%.

Techniques for measuring the composition of hydrogenated amorphous silicon–germanium alloys

Abstract We grow hydrogenated amorphous silicon–germanium alloys by the hot-wire chemical vapor deposition (HWCVD) technique at deposition rates between 0.5 and 1.4 nm per second. We prepared a set of these alloys to determine the concentrations of the alloying elements as measured by various techniques. This set consists of samples throughout the range of germanium alloying from 0% (a-Si:H) to 100% (a-Ge:H). We find that by making the appropriate calibrations and corrections, our compositional measurements agree between the various techniques. Nuclear reaction analysis (NRA), Fourier transform infrared spectroscopy (FTIR)and secondary ion mass spectrometry (SIMS) all yield similar hydrogen contents, within ±20% for each sample. Electron probe micro-analysis (EPMA) and SIMS yield silicon and germanium contents within ±7% of each other with results being confirmed by Rutherford backscattering (RBS). EPMA oxygen measurements are affected by oxidized surface layers, thus these data show larger O concentrations than those measured by SIMS.

The Influence of W Filament Alloying on the Electronic Properties of Hwcvd Deposited a-Si:H Films

In depositing a-Si:H by HWCVD using W filaments, one issue common to this technique is that of filament lifetime. When using undiluted silane as the source gas, a buildup of silicon at the colder ends of the filament is routinely observed (thickening), and it is here that filament breakage usually occurs. Less well understood is the effect of filament alloy formation on a-Si:H electronic properties. In this work we combine ambipolar diffusion length (SSPG) measurements on consecutively deposited a-Si:H films with SEM surface topography and sputter (SP) Auger depth profiling of the filament Si/(W+Si) composition to track film electronic properties as a function of the Si buildup on short filaments entirely exposed to the growing film surface. We find that with increasing exposure time of the filament to silane, appreciable Si exists everywhere along the filament length, even in the non-thickened central regions. We discuss the effect of this alloying on the film deposition rate and electronic properties, and suggest that the nature of the filament surface must be carefully considered when optimizing a- Si:H film electronic properties. Finally, we discuss possible ways to minimize this alloying by post deposition treatments, which include different filament ‘run’ temperatures.

Effect of hydrogen dilution on the properties and bonding in plasma‐deposited silicon nitride

The effects of hydrogen dilution on the properties and structure of silicon nitride films deposited by plasma‐enhanced chemical vapor deposition from NH3/SiH4 mixtures were studied. The addition of relatively small amounts of hydrogen at a fixed NH3/SiH4 ratio increased the excess Si in the film with a corresponding increase in the Si—H/N—H bonding ratio. At higher dilution [H2/(NH3+SiH4)] the films became more stoichiometric with significant changes in the hydrogen bonding. Decreases in the etch rate and refractive index with increasing hydrogen flow are discussed in terms of the changes in bonding structure and were found to be well correlated to changes in the Si—N bond density.