Elemental Evaporation Research Articles

Synthesis and formation mechanism of CuInSe2nanowires by one-step self-catalysed evaporation growth

High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.

Particle transport through an inductively coupled plasma torch: elemental droplet evaporation

The transport of copper droplets which are injected “on” or “off” axis is computationally studied.

Modified co-evaporation process for fabrication of 4 cm × 4 cm large area flexible CIGS thin film solar cells on polyimide substrate

A modified three-stage co-evaporation process has been studied in our work for the fabrication of large area Cu(In,Ga)Se2 (CIGS) thin-film solar cells on flexible polyimide substrates. According to our results, the open circuit voltage and fill factor are improved significantly by using the modified process. In order to quantitatively analyze the effects of the modified band-gap gradient in CIGS thin film, a modeling investigation is carried out by employing a simulation program, wxAMPS. The simulation indicates that the device improvement is not only attributed to the refined Ga gradient profile, but also to the better crystalline quality. The elemental evaporation rates and the content of Cu are found to be key factors for the large-area solar cell preparation. Finally, 16 cm2 CIGS thin-film solar cells on PI substrates are fabricated, and the highest cell efficiency has achieved 7%.

Structure and interface chemistry of MoO3 back contacts in Cu(In,Ga)Se2 thin film solar cells

Molybdenum oxide (MoO3) is considered as a possible primary back contact for Cu(InGa)Se2 thin film solar cells for its potential as a transparent back contact for superstrate and bifacial devices. MoO3 films were deposited on Mo or ITO-coated soda lime glass substrates by reactive rf sputtering in an ambient of Ar + O2 with O2/(O2 + Ar) = 35% on which Cu(In0.7Ga0.3)Se2 alloy absorber layers were deposited using multi-source elemental evaporation. Scanning Electron Microscopy studies showed uniform coverage of the as-deposited MoO3 layer and good adhesion was obtained in all cases. X-ray Photoelectron Spectroscopy depth profile analysis showed that MoSe2 was not formed at the Cu(InGa)Se2 interface with either the Mo-MoO3 or ITO-MoO3 back contacts. Determination of the valence band offsets showed that the MoO3 layer at the interface changes the energy band alignment with Cu(InGa)Se2, producing a primary contact with lower valence band offset than ITO. Cu(In,Ga)Se2 thin film solar cells prepared using an as-deposited Mo-MoO3 back contact yielded a best conversion efficiency of 14%, with VOC = 647 mV, JSC = 28.4 mA/cm2, and FF = 78.1%. Cells with ITO-MoO3 back contact showed a best efficiency of 12%, with VOC = 642 mV, JSC = 26.8 mA/cm2, and FF = 69.2%.

Role of Elemental Sublimation during Solution Heat Treatment of Ni-Based Superalloys

The role of elemental evaporation on the microstructural stability of blade surfaces has been investigated on solutioned and aged samples of Ni-based single-crystal superalloys. Evaporation of Ni and Cr at the casting surface during solution heat treatment leads to the formation of a Ni- and Cr-depleted layer at the surface. Nucleation and growth of γ′ phase occur within this layer through subsequent long-range diffusion of Re, Ta, and W between the γ′ layer and the substrate. Beyond a critical Ni and Cr loss, incipient melting initiates at the surface and principally γ′ and TCP phases are stabilized with de-stabilization of γ phase. Nucleation of TCP phases occurs at grain boundaries arising from cellular recrystallization during the ramp-up cycle. Therefore, on quenching, a range of microstructures are observed at the casting surface.

Effect of Rapid Thermal Processing on the structural and device properties of (Ag,Cu)(In,Ga)Se2 thin film solar cells

(AgwCu1−w)(In1−xGax)Se2 alloy absorber layers with w≈0.8 and x≈0.78 ratios were deposited using multi-source elemental evaporation and analyzed by glancing incidence X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The effect of Rapid Thermal Processing (RTP) on the structure and composition of the near-surface layers was investigated. It was found that the RTP doesn't have a noticeable effect on the grain size and roughness of the film surface. On the other hand, additional reflections of the films with 0.5≤Ga/(Ga+In)<1 and Ag/(Ag+Cu)>0.5, due to an ordered defect phase which is confined to the near-surface region of the film, are completely diminished after RTP with excess Se present. X-ray photoelectron spectroscopy results show that RTP with excess Se present altered the distribution of elements, rendering the film surface less group I deficient after that process. Consequently, atomic ratios were changed from the composition of I1III5VI8 compound to the composition close to that of I1III3VI5 ordered defect phase. Additionally, the RTP treatment, especially with excess Se, significantly enhanced solar cell efficiency performance due to an improvement in the open circuit voltage.

An Investigation of the Surface Properties of (Ag,Cu)(In,Ga)Se $_{\bf 2}$ Thin Films

(Ag,Cu)(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multisource elemental evaporation and analyzed by glancing incidence X-ray diffraction and energy dispersive X-ray spectroscopy. All films exhibit chalcopyrite reflections in the X-ray diffraction pattern and films with 0.5 ≤ Ga/(Ga+In) <; 1 and Ag/(Ag+Cu) >; 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy measurements show that all films studied have low (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios near the surface relative to the bulk composition, consistent with an ordered defect compound identified as (Ag,Cu)(In,Ga) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> . Additionally, the near-surface region of (Ag,Cu)(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film contains excess Ag near the surface.

Lattice vibrations of sequentially evaporated CuInSe2 by Raman microspectrometry

Copper Indium Diselenide (CuInSe2) thin films of different atomic concentrations were prepared using sequential elemental evaporation. Raman analysis has been carried out to determine the structure and the different vibration modes of CuInSe2 thin film. The analysis reveals that CuInSe2 films possess chalcopyrite structure with preferential orientation along (112) plane. Vibration modes such as A1, B1, B2 and E were observed in the Raman spectra. It has been observed that the intensity of the A1 mode is directly proportional to the crystallinity. Also it has been noted that, the intensity of all the vibration modes are varied with respect to the compositions. The Raman spectra of the CuInSe2 films were interpreted and incorporated with XRD and EDAX.

Scaleup of Cu(InGa)Se2Thin Film Coevaporative Physical Vapor Deposition Process, 1. Evaporation Source Model Development

Even though rapid advances have been made in improving Cu(InGa)Se2 thin-film-based solar cell efficiencies, the breakthroughs have been limited to the laboratory scale. Most commercially viable thin-film technologies reside at the premanufacturing development stage and scaleup has proven to be much more difficult than expected. Elemental in-line evaporation on flexible substrates in a roll-to-roll configuration is a commercially attractive process for the manufacture of large-area CuInSe2-based photovoltaics. At the University of Delaware’s Institute of Energy Conversion (IEC), such a process is being investigated, at the pilot scale, for a polyimide web substrate. The process works well for 6-in.-wide substrates and for short deposition runtimes. However, a commercially viable process is required to produce large-area, high-quality films at a much-higher throughput. Specifically, the desired film thickness (∼2 μm) and composition uniformity must be achieved continuously and reproducibly on large-area (12...

Characteristics of stacked CuInS 2 and CuGaS 2 layers as determined by the growth sequence

CuInS 2 and CuGaS 2 thin films have been prepared sequentially from elemental evaporation sources onto conventional soda lime glass substrates heated at 350 °C during the deposition process. The gradient in the structure and composition of the stacked layers has been investigated for the two possible growth sequences. Structural depth profiling and crystallographic phase analysis were performed by grazing incidence X-ray diffraction. The atomic distribution in the films depth was analyzed by X-ray photoelectron spectroscopy combined with sputter etching. Formation of the quaternary compound CuIn 1 − x Ga x S 2, with a high Ga content x > 0.80, has been detected with different distribution depending on the growth sequence.

CuInS2 and CuGaS2 thin films grown by modulated flux deposition with various Cu contents

AbstractCopper indium sulphide and copper gallium sulphide thin films have been prepared using a novel modulated flux deposition process from elemental evaporation sources that were adjusted to give different cation (Cu/In or Cu/Ga) ratios ranging from 0.6 to 1.4. The crystalline structure and optical properties of the samples, as determined by X‐ray diffraction and spectrophotometry, have been analysed as a function of the overall composition. Also, the atomic distribution in the film depth has been obtained by combining X‐ray photoelectron spectroscopy and Ar+ sputter etching. An increase in copper content leads to an increase of the average crystallite size and a decrease of the structural stress, and an increase of the absorption coefficient at low energies. These effects are more pronounced for the copper gallium sulphide than for the copper indium sulphide samples for the preparation conditions used. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Meyer-Neldel rule and the influence of entropy on capture cross-section determination in Cu(In,Ga)Se2

We use deep-level transient spectroscopy (DLTS) to investigate the electronic properties of ZnO∕CdS∕CuxIn1−yGaySe2∕Mo∕soda-lime glass thin-film solar cells. We deposited films with different x and y values using elemental evaporation. Devices made from these films exhibit more than 40 hole and electron traps, with activation energies ranging from 0.07to1.17eV. Standard DLTS analysis (which assumes that entropy changes can be neglected) gives apparent capture cross sections that vary by 18 orders of magnitude over this energy range. All our data show that the charge-carrier emission rate obeys the Meyer-Neldel rule (MNR) with an isokinetic temperature of 340±30K. By including the MNR relation in detailed balance, we show that there is a single cross section for all the traps.

Application of Advanced Microstructural and Microchemical Microscopy Techniques to Chalcopyrite Solar Cells.

AbstractAnalytical TEM techniques have been used to characterize the structure and chemistry of chalcopyrite solar cell devices produced by elemental evaporation using a bi-layer process at Ts=400°C and Ts=550°C. Dislocations, stacking faults, twins and voids exist in both materials but at a reduced density at the higher deposition temperature. The higher processing temperature also improved the density and structure of the grain boundaries and created a less rough surface. The CdS layer coats the surface conformally, although the roughness impacts the structure. CdS fills the grooves in the surface and infiltrates lower density grain boundaries to significant depths. The chemistry of the grain boundaries does not appear to be significantly impacted by the presence of the Cd and S.

Analysis of Cu(InGa)Se2Alloy Film Optical Properties and the Effect of Cu Off-Stoichiometry

AbstractVariable angle spectroscopic ellipsometry has been used to characterize Cu(InGa)Se2 thin films as a function of relative Ga content and to study the effects of Cu off-stoichiometry. Uniform Cu(InGa)Se2films were deposited on Mo-coated soda lime glass substrates by elemental evaporation with a wide range of relative Cu and Ga concentrations. Optical constants of Cu(InGa)Se2were determined over the energy range of 0.75–C4.6 eV for films with 0 ≤ Ga/(In+Ga) ≤ 1 and used to determine electronic transition energies. Further, the changes in the optical constants and electronic transitions as a function of Cu off-stoichiometry were determined in Cu-In-Ga-Se films with Cu atomic concentration varying from 10 to 25 % and Ga/(In+Ga) = 0.3. Films with Cu in the range 16–24 % are expected to contain 2 phases so an effective medium approximation is used to model the data. This enables the relative volume fractions of the two phases, and hence composition, to be determined. Two distinctive features are observed in the optical spectra as the Cu concentration decreases. First, the fundamental bandgaps are shifted to higher energies. Second, the critical point features at higher energies become broader suggesting degradation of the crystalline quality of the material.

Growth of CuInS2 thin films by sulphurisation of Cu–In alloys

Abstract The structural, electrical and optical properties of copper–indium alloys sulphurised in H 2 S atmosphere have been studied by varying the thermal cycle of the sulphurisation process. Cu–In alloy prepared by elemental evaporation of copper and indium was used as the precursor for sulphurisation. The chalcopyrite CuInS 2 phase was found at sulphurisation temperature as low as 250°C and single phase at sulphurisation temperature 350°C. At low sulphurisation temperature different binary phases like CuS, Cu 2 S, InS and In 6 S 7 were found. Short sulphurisation time also results in secondary binary phases. The optimum sulphurisation temperature was 350°C for three hours, which resulted in single-phase p-type chalcopyrite CuInS 2 films with a band gap of 1.45 eV. The dependence of processing parameters and the Cu/In ratio of the starting precursors on the electrical, optical and structural properties have also been studied.

Structural and optical properties of CuIn 1− xAl xSe 2 thin films prepared by four-source elemental evaporation

CuIn 1− x Al x Se 2 (CIASe) thin films with x=0.25, 0.5 and 0.65 were prepared by four-source elemental evaporation. The structural and optical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive analysis, and optical transmission. The results showed that these films contain chalcopyrite structure with preferred orientation along (112) direction. The morphology, grain distribution and composition of CIASe films were studied and compared for different Al content. The optical studies revealed that the films were highly absorbing and the energy band gap calculated from transmission spectra for x=0.25, 0.5 and 0.65 were 1.2, 1.51 and 1.73 eV, respectively. The variation of Al content in the CIASe composition offered a very effective change in the optical band gap.

The critical pressure and impeding pressure of Al evaporation during induction skull melting processing of TiAl

In this article, two key pressures, the critical pressure Pcrit and the impeding pressure Pimpe of elemental evaporation, are defined and studied based on the calculation relationship between the evaporation loss rate Nm and the chamber pressure P during melting of titanium-aluminum alloys. When the chamber pressure is below Pcrit or above Pimpe, Nm tends to the maximum or the minimum value and remains almost unchanged. However, if Pcrit<P<Pimpe, with the increase of the chamber pressure, Nm declines sharply. A method has been put forward to calculate Pcrit and Pimpe of Al evaporation in a Ti-XAl (at pct) (X=25 to 50) melt. The calculation result shows that Pcrit or Pimpe is a second-order function of the molar percentage of Al and the melt temperature.

High-efficiency solar cells based on Cu(InAl)Se2 thin films

A Cu(InAl)Se2 solar cell with 16.9% efficiency is demonstrated using a Cu(InAl)Se2 thin film deposited by four-source elemental evaporation and a device structure of glass/Mo/Cu(InAl)Se2/CdS/ZnO/indium tin oxide/(Ni/Algrid)/MgF2. A key to high efficiency is improved adhesion between the Cu(InAl)Se2 and the Mo back contact layer, provided by a 5-nm-thick Ga interlayer, which enabled the Cu(InAl)Se2 to be deposited at a 530 °C substrate temperature. Film and device properties are compared to Cu(InGa)Se2 with the same band gap of 1.16 eV. The solar cells have similar behavior, with performance limited by recombination through trap states in the space charge region in the Cu(InAl)Se2 or Cu(InGa)Se2 layer.

CuIn 1−x Al x Se 2 thin films and solar cells

CuIn 1−x Al x Se 2 thin films are investigated for their application as the absorber layer material for high efficiency solar cells. Single-phase CuIn1−xAlxSe2 films were deposited by four source elemental evaporation with a composition range of 0⩽x⩽0.6. All these films demonstrate a normalized subband gap transmission &amp;gt;85% with 2 μm film thickness. Band gaps obtained from spectroscopic ellipsometry show an increase with the Al content in the CuIn1−xAlxSe2 film with a bowing parameter of 0.62. The structural properties investigated using x-ray diffraction measurements show a decrease in lattice spacing as the Al content increases. Devices with efficiencies greater than 10% are fabricated on CuIn1−xAlxSe2 material over a wide range of Al composition. The best device demonstrated 11% efficiency, and the open circuit voltage increases to 0.73 V.

Effect Of Grain Size, Morphology and Deposition Temperature on Cu(InGa)Se2 Solar Cells

ABSTRACTCu(InGa)Se2 films have been deposited by multisource elemental evaporation at substrate temperatures from 550 to 400°C to determine the effect of temperature on grain size and morphology and on solar cell performance. Films were deposited with different flux profiles to compare cases where a copper selenide phase is formed during different stages of growth or not at all. The grain size and surface area of the films are determined by analysis of atomic force microscope images. With the Cu-rich growth, the mean lateral grain area decreases from 1.8 to 0.3 μm2 as substrate temperature decreases. Device efficiency decreases with the lower temperature but cannot be simply attributed to changes in grain size, surface area, or the availability of sodium which diffused from the glass substrate. Instead the decrease in open circuit voltage and fill factor are attributed to a greater density of intra-grain trap states in the Cu(InGa)Se2.