Elemental Se Vapor Research Articles

Efficiency Boost of (Ag0.5,Cu0.5)(In1‐x,Gax)Se2 Thin Film Solar Cells by Using a Sequential Process: Effects of Ag‐Front Grading and Surface Phase Engineering

AbstractPost‐selenization‐fabricated (using elemental Se vapor) Cu(In,Ga)Se2 solar cell efficiency is limited by a low open‐circuit voltage, which is attributable to the Ga‐deficient surface and insufficient grain growth. In this study, a band‐grading structure is demonstrated by combining Ag‐front and Ga‐back grading in selenized (Ag,Cu)(In,Ga)Se2 (ACIGSe) absorbers with a properly designed precursor structure (Mo/CuGa/In/AgGa) and high Ag content ([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post‐selenization reveals that the precursor structure suppresses Ag2Se formation and promotes the ACIGSe phase formed at a low temperature with enhanced grain growth. A widened surface bandgap by Ag‐front grading substantially increases the open‐circuit voltage. Furthermore, Ag addition promotes ordered vacancy compound (OVC) formation on the front surface to enlarge the valence band offset, which in turn reduces interface recombination. Furthermore, the OVC phase also assists interface passivation. Promoting surface OVC phase by Ag addition is also validated by first‐principles calculations. Furthermore, the K‐doped CuGa precursor is used for a ACIGSe absorber to address the significantly reduced carrier density by the Ag addition. With a band‐grading structure and surface OVC phase, the superior device achieves an efficiency of > 19%, the highest efficiency by post‐selenization with an elemental Se source.

Band gap tuning of Cu(In,Ga)Se2 thin films by electrodeposition and their subsequent selenization using a rapid thermal annealing system

Band gap tuning of electrodeposited Cu(In1-x,Gax)Se2 thin films through varying In3+ and Ga3+ concentrations in the electrolytic bath is demonstrated. Cyclic voltammetry was used to determine best Cu(In1-x,Gax)Se2 deposition potentials at the different bath conditions. Band gap tuning of Cu(In1-x,Gax)Se2 was achieved, through incorporation of varying In3+ and Ga3+ levels during film growth, in the range of 1 to 1.4 eV, corresponding to Ga content 0 ≤ Ga/(In + Ga) ≤ 0.64. Deposited films were characterized by EDS/SEM, XRD, and Raman spectroscopy to determine chemical composition, morphology, and crystal structure. Results show that as-deposited Cu(In1-x,Gax)Se2 thin films are of low crystallinity, with Se and Cu-Se compounds present as a secondary phase. Selenization of electrodeposited Cu(In1-x,Gax)Se2 films was performed using a rapid thermal processing system at 550 °C in an overpressure reactive atmosphere of N2/H2 (96%:4%) and elemental Se vapor. Selenization treatment promoted recrystallization and elimination of secondary phases, resulting in an increase in grain size while maintaining film composition. Processed films with Ga/(In + Ga) = 0.35 were processed into devices, achieving 2.6% efficiency.

Comparison of antimony selenide thin films obtained by electrochemical deposition and selenization of a metal precursor

In this work, we present a study comparing two methods of Sb2Se3 deposition, namely electrochemical deposition and selenization of a thin-film metallic Sb – precursor deposited by magnetron sputtering. The Sb selenization process was carried out in the range of temperatures from 270 °C to 350 °C in an argon atmosphere enriched with elemental Se vapor. Electrochemical deposition of Sb2Se3 was performed in a three-electrode cell at constant potential. Following electrochemical deposition, the samples were annealed in argon at temperatures ranging from 270 °C to 350 °C. Characterization of the obtained thin films was performed using X-ray diffraction, Raman spectroscopy and optical transmission. Both deposition methods demonstrate successful emergence of the selenide phase, albeit with the presence of varying levels of antimony oxide. The possibility of preferential growth was observed, which is suggested as dependent on growth method and annealing temperature.

Effect of sodium diffusion on the properties of CIGS solar absorbers prepared using elemental Se in a two-step process

The influence of Na diffusion from various glass substrates during a high-temperature slenization process on the microstructure and morphology of two-step formed CIGS absorber layers is investigated. In order to minimise the CIGS absorber formation time, elemental Se vapour is used to prepare the CIGS absorber. The grain sizes of the CIGS films are found to increase with increasing sodium in the glass substrates (extra clear glass, soda-lime glass, borosilicate glass). TiN and SiN thin films are used as diffusion barrier layers inserted between the glass substrate and the Mo rear conatct to tune the Na diffusion from the soda-lime glass. The interdiffusion between the In-rich CuInSe2 surface layer and the Ga-rich CuGaSe2 layer is promoted by the barrier layer, leading to larger CIGS grains. Efforts are also taken to understand the differences in Na diffusion (from the glass substrates) and their effects on the MoSe2 intermediate layer formation during the high-temperature CIGS absorber formation processes. We find that excess amounts of Na and Se are essential for the MoSe2 growth. The excessive Na in the form of Na2Sex at the CIGS/Mo interface works as a Se source and catalyses the MoSe2 formation. The Se flow in the two-step CIGS formation process must be sufficiently high to obtain high-efficiency CIGS solar cells.

Adjusting the Ga grading during fast atmospheric processing of Cu(In,Ga)Se2 solar cell absorber layers using elemental selenium vapor

AbstractWe study the sequential fabrication of Cu(In,Ga)Se2 (CIGSe) absorber layers by using an atmospheric pressure selenization with a process duration of only a few minutes and the utilization of elemental selenium vapor from independent Se sources. This technology could proof to be an industrially relevant technology for the fabrication of thin‐film solar cells. Controlling the amount of Se provided during the selenization of metal precursors is shown to be an effective measure to adjust the Ga in‐depth distribution. A reduced Se supply for CIGSe formation leads to a more homogeneous Ga distribution within the absorber. The underlying growth dynamics is investigated by interrupting the selenization at different times. At first, CIGSe formation occurs in accordance with previously suggested growth paths and Ga segregates at the Mo back contact. Between 520 and 580 °C, the growth dynamics differs distinctly, and In and Ga distribute far more uniformly within the absorber depth. We also studied the impact of the precursor architecture. The best performing precursor in terms of efficiency of the respective solar cells was a multilayer with 22 In/CuGa/In triple layers. Simple bilayers stacks lead to films of higher roughness and correlated shunting. By optimizing the precursor architecture and the Ga in‐depth distribution in the CIGSe layer, a conversion efficiency of up to 15.5% (active area) could be achieved. To our knowledge, this is the highest reported efficiency for sulfur free CIGSe‐based solar cells utilizing fast (few minutes) atmospheric processes and elemental Se vapor. Copyright © 2017 John Wiley & Sons, Ltd.

Morphological, Structural, and Optical Properties of Single-Phase Cu(In,Ga)Se2Thin Films from the Selenization of Thermally Evaporated InSe/Cu/GaSe Precursors

The relatively small band gap values (~1 eV) of CuInSe2thin films limit the conversion efficiencies of completed CuInSe2/CdS/ZnO solar cell devices. In the case of traditional two-stage growth techniques, limited success has been achieved to homogeneously increase the band gap by substituting indium with gallium. In this study, thermal evaporation of InSe/Cu/Gase precursors was exposed to an elemental Se vapour under defined conditions. This technique produced large-grained, single-phase Cu(In,Ga)Se2thin films with a high degree of in-depth compositional uniformity. The selenization temperature, ramp time, reaction period, and the effusion cell temperature with respect to the Cu(In,Ga)Se2films were optimized in this study. The homogeneous incorporation of Ga into CuInSe2led to a systematic shift in the lattice spacing parameters and band gap of the absorber films. Under optimized conditions, gallium in cooperation resulted only in a marginal decrease in the grain size, X-ray diffraction studies confirmed single-phase Cu(In,Ga)Se2material, and X-ray photoluminescence spectroscopy in-depth profiling revealed a uniform distribution of the elements through the entire depth of the alloy. From these studies optimum selenization conditions were determined for the deposition of homogeneous Cu(In,Ga)Se2thin films with optimum band gap values between 1.01 and 1.21 eV.

Rapid Fabrication of Cu(In,Ga)Se$_{2}$ Thin Films by the Two-Step Selenization Process

Two-step processes currently used for the industrial Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) module production require a long process time of several hours for the CIGS absorber formation. In this paper, we are studying the reaction pathway to rapid selenization of stacked metal precursors in elemental Se vapor. The objective is to understand the reaction kinetics to find the best precursor structure and the optimal selenization conditions to form high-quality CIGS films with proper Ga depth profiles. In addition to stacked metal precursors, the effect of the use of Se-containing precursors was also examined. As expected, the stacking order of the metal precursors influences the properties of the resulting CIGS absorbers. The Cu amount deposited for the precursor formation critically affected the final film and cell properties, as well. We also found that the formation of CIGS films with large grain sizes and flat Ga depth profiles was possible even for [Cu]/([In] + [Ga]) <; 1 conditions with the use of particular precursor structures and selenization conditions. The results suggest that the selenization reaction pathway can be dictated with the precursor structure, and further improvements are expected by controlling reaction kinetics with precursor structure modification.

Structural and optical properties of the Cu2ZnSnSe4 thin films grown by nano-ink coating and selenization

Quaternary semiconductor Cu2ZnSnSe4 (CZTSe) is a very promising alternative to semiconductors based on indium (In) and gallium (Ga) as solar absorber material due to its direct band gap, inherent high absorption coefficient (>104 cm−1) and abundance of cheap elements zinc (Zn) and tin (Sn). In this study, high quality CZTSe thin films were successfully synthesized by a green and low-cost solution based non-vacuum method, which involves spin coating non-toxic solvent-based CZTSe nano-inks onto Mo coated soda lime glass substrates followed by selenization with elemental Se vapor. The effect of selenization temperature on structural, morphological, compositional and optical properties of CZTSe films are investigated using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and photoluminescence spectroscopy. XRD and Raman analysis indicates that a tetragonal stannite-type structured CZTSe is formed. Depend on the selenization temperature, the dense and compact films with grain sizes from 200 nm (500 °C) up to about 1 μm (580 °C) are obtained. EDS measurement indicates that the composition ratios of the prepared CZTSe films are copper-poor and zinc-rich nature. The CZTSe films are p-type conductivity confirmed by a hot point probe method. Photoluminescence spectrum shows slightly asymmetric narrow bands with a maximum of intensity at 0.92 eV. The dependence of the photoluminescence on the excitation temperature reveals a decrease in the intensity of the photoluminescence bands. An absorption coefficient exceeding 104 cm−1 and the band gap energy about 0.87 eV of the studied films are determined by an absorption spectroscopy.

Cu 2ZnSnSe 4 films by selenization of Sn–Zn–Cu sequential films

This paper deals with the formation of Cu 2ZnSnSe 4 (CZTS) in the process of selenization of metal precursor layers in elemental selenium vapour. Metallic precursors were sequentially evaported from Sn, Zn and Cu sources. Precursor Sn–Zn–Cu films have a “mesa-like” structure and consist mainly of Cu 5Zn 8 and Cu 6Sn 5 phases. It was confirmed that the formation of different binary copper selenides is the dominating process of selenization in elemental Se vapour at temperatures up to 300 °C. The formation of kesterite CZTS films begins at 300 °C and dominates at higher temperatures, always resulting in multiphase films that consist of high-quality Cu 2ZnSnSe 4 crystals and of a separate phase of ZnSe.

Growth of Cu(In&lt;sub&gt;1-x&lt;/sub&gt;Al&lt;sub&gt;x&lt;/sub&gt;)Se&lt;sub&gt;2&lt;/sub&gt; Thin Films by Atmospheric Pressure Selenization of Sputtered Precursors

Cu(In1-xAlx)Se2 films were prepared using a two-stage process of sputtering and selenization. Stacked elemental layer precursors of Cu, In and Al were deposited onto corning glass substrates by RF magnetron sputtering. Precursors with different Cu/(In+Al) and In/Al ratio were selenized using elemental Se-vapor at atmospheric pressure in a commercial tube furnace under constant argon gas flow. Films with good adhesion to the substrate were grown successfully. All of the films show strong (112) and (220)/(204) CIS peaks. Addition of Al, at expense of In, shifts the peaks towards higher 2θ. This paper explores the possibility to use sputtering deposition and selenization process to grow Cu(InAl)Se2 thin films for solar cells applications.

Influence of the stacking order on structural features of the Cu‐In‐Ga‐Se precursors for formation of Cu(In,Ga)Se 2 thin films prepared by thermal reaction of InSe/Cu/GaSe alloys to elemental Se vapor and diethylselenide gas

A novel partway for the fabrication of copper-indium (gallium) diselenide has been developed. This two-stage process consists of the formation of Cu-In-(Ga)-Se precursors and subsequent selenization to form CuIn(Ga)Se2. In this work, we have investigated and compared the possible interactions in Cu-In-Ga-Se systems, using sequentially stacked precursors premixed with Se, in order to get a better understanding of the Cu(In,Ga)Se2 thin film formation. Comparison of these SEM micrographs clearly revealed that the surface morphologies and hence surface roughness of the resulting Cu(In,Ga)Se2 absorber films were significantly influenced by the structure of the precursor films prior to selenization. XRD analyses revealed the presence of a graded CuIn1-xGaxSe structure, irrespective of the stacking order during the precursor formation step for samples selenized using elemental Se vapor. It was established that distinct from the case of using Se vapor, a single-phase Cu(In,Ga)Se2 films were obtained by diethylselenide (DESe) selenization from Cu-In-Ga metal precursors premixed Se irrespective of the stacking order during the precursor formation step. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and optical properties of homogeneous Cu(In,Ga)Se2 thin films prepared by thermal reaction of InSe/Cu/GaSe alloys with elemental Se vapour

In this study, thin films of Cu(In,Ga)Se2 were prepared by the controlled reaction of thermally evaporated InSe/Cu/GaSe precursors with elemental Se vapour in vacuum. We indicate that this classical two-step growth process can be utilized to produce homogeneous single-phase chalcopyrite absorber films with superior structural properties. X-ray diffraction studies indicated that the interplanar spacing d(112) decreases linearly with an increase in the Ga/[In + Ga] atomic ratio due to homogeneous incorporation of gallium into the chalcopyrite lattice. Optical studies revealed the expected systematic increase in the band gap with increasing gallium concentration, once again confirming the monophasic nature of the alloys.

Preparation and structural properties of Culn (Se,S)2 thin films prepared by the thermal diffusion of sulphur into CuInSe2

SYNOPSIS In this study, Copper Indium Selenide (CuInSe2) thin films were prepared by the thermal evaporation of InSe/Cu/InSe precursors, followed by a selenisation step in elemental Se vapour. This technique produced large-grained, single-phase CuInSe2 thin films with a high degree of below surface compositional uniformity. The samples were subsequently sulphurised by the controlled solid-state thermal diffusion of sulphur into the CuInSe2 thin films. The sulphurisation temperature, reaction period and position of the elemental sulphur source with respect to the CuInSe2 films were optimised in this study. Under optimised conditions, sulphurisation resulted only in a marginal increase in the surface roughness, XRD (X-ray diffraction) studies confirmed single-phase Culn(Se,S)2 material and XRF (X-ray in-depth) profiling revealed uniform Cu/In atomic number ratios and a systematic increase in the sulphur concentration towards the top surface region of the samples. The increase in band gap of the absorber layers after sulphurisation was also confirmed by an increase in the open-circuit voltage of completed solar cell devices.

Influence of GaSe deposition temperature on the structural properties and in-depth compositional features of two-step grown Cu(In,Ga)Se 2 thin films

In this study, Cu(In,Ga)Se2 thin films were prepared by a classical two-stage growth process, which involved the selenization of thermally evaporated InSe/Cu/GaSe precursors. During the precursor-formation step the InSe and Cu were always deposited at 200 °C, while the GaSe layers were deposited at temperatures between 200 °C and 400 °C. The respective precursors were simultaneously selenized under identical conditions in elemental Se vapor. In cases where the GaSe layers were deposited at low temperatures around 200 °C, X-ray fluorescence (XRF) analysis revealed a large variation in element concentration with sample depth after selenization. In correspondence, X-ray diffraction (XRD) studies revealed the presence of separate CuInSe2 and CuGaSe2 phases in these specific samples. Optimum structural properties were obtained when the GaSe films were deposited at 300 °C, followed by selenization. In general, these films were uniform and dense and XRD studies revealed single-phase Cu(In,Ga)Se2 material. Even more importantly, XRF analysis revealed a remarkable improvement in in-depth compositional uniformity when the GaSe films were deposited at or above 300 °C. An increase in GaSe deposition temperature to 400 °C, however, resulted in a deterioration in the structural features of the Cu(In,Ga)Se2 thin films. In contradiction with other reports, these results indicated that the in-depth composition uniformity and especially the Ga diffusion profile in two-step grown Cu(In,Ga)Se2 thin films can be controlled. The crucial factor influencing the depth profile of these films is the GaSe deposition temperature during the precursor formation step.

Material properties of CuIn(Se, S)2 thin films prepared by the thermal diffusion of sulfur into CuInSe2

In this paper, we demonstrate that device quality chalcopyrite thin films can be produced by a simple and reproducible processing technology. CuInSe2 thin films were prepared by the thermal evaporation of InSe/Cu/InSe precursors, followed by a selenization step in elemental Se vapour. This technique produced large-grained, single-phase CuInSe2 thin films with a high degree of in-depth compositional uniformity. The samples were subsequently sulfurized by the controlled solid-state thermal diffusion of sulfur into the CuInSe2 thin films. The sulfurization temperature, reaction period and position of the elemental sulfur source with respect to the CuInSe2 films were optimized in this study. Under optimized conditions, sulfurization resulted only in a marginal increase in the surface roughness, x-ray diffraction studies confirmed single-phase CuIn(Se, S)2 material and x-ray fluorescence in-depth profiling revealed uniform Cu/In atomic ratios and a systematic increase in the sulfur concentration towards the top surface region of the samples. The increase in bandgap of the absorber layers after sulfurization was also confirmed by an increase in the open-circuit voltage of completed solar cell devices.

Comparison of Material Properties of CuInSe2 Films Produced by Reaction of Metallic Alloys to H2Se/Ar and Elemental Se Vapour

In this study, CuInSe2 thin films were produced by the reaction of identical precursors to elemental Se vapour and H2Se/Ar gas. In order to compare the respective growth processes, the reaction temperature was varied between 300°C and 600°C, while all other parameters were kept constant. For a given set of experimental parameters, significant differences were observed in terms of crystalline quality and in-depth compositional uniformity. At low selenization temperatures below 400°C, the reaction process was incomplete, irrespective of the selenization process considered. Reaction of precursors to H2Se/Ar at temperatures above 400°C (for 40 min), produced fully reacted films with uniform and dense surface morphologies and a high degree of in-depth compositional uniformity. Under similar conditions, Se vapour treated samples had nonuniform surface morphologies with an associated relatively large variation in the Cu/In atomic ratio and Se concentration through the depth of the samples. This comparative study identifies the important differences between the two reactive processes and the advantages in using H2Se/Ar as a chalcogenide source.

Na incorporation in Mo and CuInSe 2 from production processes

Results of characterization of thin films of Mo deposited by DC magnetron sputtering on soda-lime glass (Mo/SLG) and CuInSe 2 (CIS) on Mo/SLG are presented. The primary objective of the work was to clarify the factors determining the concentration of Na in commercial-grade CIS. Mo films were deposited by three laboratories manufacturing CIS thin film solar cells. Analysis was by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffraction. Changes in Mo deposition parameters in general affected the Na level but there was no obvious link to any single Mo deposition parameter. Oxygen content directly affected the Na level. The Na behavior was not obviously connected to film preferred orientation. Selenization of the Mo layers was also examined. Elemental Se vapor was found to produce significantly less selenization than H 2Se. The amount of selenization was also strongly dependent upon Mo deposition conditions, although a specific source of the change in reaction rate was not found. Na distributions in the CIS deposited on the Mo were not limited by the diffusivity of the Na. The Na concentration in the CIS was increased by annealing the Mo films both with and without intentionally added Na. The Na level in the CIS appears to be set more by the CIS deposition process than by the Na concentration in the Mo so long as the Mo contains sufficient Na to saturate the available sites in the CIS.

Reaction Pathways to CuInSe2 Formation from Electrodeposited Precursors

thin films have been obtained from different precursors prepared by direct or sequential electrodeposition processes. The nature of the as‐deposited layers and the evolution of the films during the heat‐treatment in an inert (vacuum) or a reactive (elemental Se vapor) atmosphere have been studied by x‐ray diffraction and x‐ray photoelectron spectroscopy analysis. The chemistry of the different phase transformations occurring as a function of the annealing temperature has been examined, and possible reaction pathways for the formation of are presented. The results show that high crystalline chalcopyrite films with the desired composition can be obtained after annealing either direct or sequentially electrodeposited precursors at 400°C. An improvement in film quality can be gained by using an electrodeposited Cu layer as growth surface for the formation. If elemental Se is also added during the heat‐treatment, then a higher recrystallization of the films is observed.

Development of High-Efficiency CuInxGa1-xSe2 Thin-Film Solar Cells by Selenization with Elemental Se Vapor in Vacuum

Achievements in the development of CuInSe2 (CIS) and CuInx Ga1-x Se2 (CIGS). thin-film absorbers with efficiency over 12% by the two-stage method are reported as follows: 12.3% efficiency with CIS, 13.3% efficiency with CIGS (Ga/III ratio=0.25) and 10.1% efficiency with CIGS (Ga/III ratio=0.40, graded band-gap structure). These results clearly indicate that the simple two-stage method developed in this study has high capability to fabricate high-quality CIS and CIGS thin-film absorbers. The Se content in the precursor layer and the Se beam flux intensity during the selenization stage are key factors that must be controlled precisely in order to achieve a high efficiency of over 15%. It is reported, for the first time, that the spontaneous formation of a graded band-gap structure is possible by changing Se and Ga contents in the precursor layer, which should be applicable to the preparation of a high-quality CIS or CIGS thin-film absorber. The formation chemistry models of CIS and CIGS thin films related to this study are proposed.

Selenization of Cu and In thin films for the preparation of selenide photo-absorber layers in solar cells using Se vapour source

Abstract Gas phase selenization of vacuum deposited Cu and In thin films employing an elemental Se vapour source is demonstrated as an essential first step in the search for optimized process parameters for the formation of single phase CuInSe2 materials suitable for solar cell applications. The selenization was accomplished in Se vapour, derived from an elemental Se source, held at 240–260°C. This source was placed in a flow of nitrogen gas at 500 Torr to transport the Se vapour to the metal films. The selenization reaction readily occurs at Cu and In films kept at 340–400°C. Lower selenization temperatures invariably lead to the formation of Cu and In selenides with well defined crystalline microstructures. Hexagonal CuSe with an excess of Se in the matrix is the equilibrium growth phase, while the cubic Cu2−xSe phase evolves under conditions of excess Se flux. Selenization of the In films consistently led to the formation of the β-form of hexagonal In2Se3. At high selenization temperatures (400°C), while the β-form still emerges as the major component, traces of the α-form of In2Se3 are also detected. Detailed X-ray diffraction, electron probe analysis and microstructure data are presented.