Graphite Box Research Articles

Cu2ZnSnS4 absorber layers obtained through sulphurization of metallic precursors: Graphite box versus sulphur flux

In this work we employed a hybrid method, combining RF-magnetron sputtering with evaporation, for the deposition of tailor made metallic precursors, with varying number of Zn/Sn/Cu (ZTC) periods and compared two approaches to sulphurization. Two series of samples with 1×, 2× and 4× ZTC periods have been prepared. One series of precursors was sulphurized in a tubular furnace directly exposed to a sulphur vapour and N2+5% H2 flux at a pressure of 5.0×10+4Pa. A second series of identical precursors was sulphurized in the same furnace but inside a graphite box where sulphur pellets have been evaporated again in the presence of N2+5% H2 and at the same pressure as for the sulphur flux experiments. The morphological and chemical analyses revealed a small grain structure but good average composition for all three films sulphurized in the graphite box. As for the three films sulphurized in sulphur flux grain growth was seen with the increase of the number of ZTC periods whilst, in terms of composition, they were slightly Zn poor. The films' crystal structure showed that Cu2ZnSnS4 is the dominant phase. However, in the case of the sulphur flux films SnS2 was also detected. Photoluminescence spectroscopy studies showed an asymmetric broad band emission which occurs in the range of 1–1.5eV. Clearly the radiative recombination efficiency is higher in the series of samples sulphurized in sulphur flux. We have found that sulphurization in sulphur flux leads to better film morphology than when the process is carried out in a graphite box in similar thermodynamic conditions. Solar cells have been prepared and characterized showing a correlation between improved film morphology and cell performance. The best cells achieved an efficiency of 2.4%.

Investigation of Se supply for the growth of Cu2ZnSn(SxSe1−x)4 (x ≈ 0.02–0.05) thin films for photovoltaics

In this work, the selenization growth of electrochemically deposited Cu2ZnSn(SxSe1−x)4 (CZTSSe) films with x≈0.02–0.05 was optimized by two groups of experiments in a graphite box. The selenization of CZTSSe is strongly dependent on the Se supply in the graphite box. Insufficient Se supply left the selenization incomplete. Higher Se supply to CZTSSe either by increasing the Se powder usage or by increasing the external pressure of nitrogen resulted in the degradation of CZTSSe films with lower degree of crystallinity. Our experimental results show that the refined usage amount of Se powder is 0.3g and the optimized external pressure of flowing nitrogen in the furnace is from 0.5Torr to 2Torr. The characterization of XRD, Raman and SEM confirmed the films obtained under the best conditions were well-developed CZTSSe films with compact faceted grains and good crystallinity. Additionally, the CZTSSe film grown using 2Torr of nitrogen pressure showed more orientation along (220/204).

Study of the chalcopyrite Cu(In,Al)Se 2 crystalline growth by selenization of different evaporated precursors ratios

Chalcopyrite CuInSe 2 (CIS) and Cu(In,Al)Se 2 (CIAS) thin films were prepared by a two-stage process onto soda-lime glass substrates. Different Cu/In and Al/(Al+In) thickness ratios have been evaporated in a vacuum chamber to be subsequently heated with elemental selenium in a quasi closed graphite box. Selenization temperatures have been varied from 150 °C to 530 °C to study the evolution of chalcopyrite growth from the metallic precursors. The results of spectrophotometry measurements, X-ray diffraction, energy dispersive analysis of X-ray and scanning electron microscopy have been interpreted for the samples with different precursors ratios and selenization temperatures, concluding that the proportion among the copper and indium precursors determines the way toward CIAS formation and crystallisation.

Sol-gel법에 의한 박막태양전지용 CuInS2박막의 증착과 특성

박막 태양전지의 저가 고효율화를 실현하기 위해 넓은 면적의 기판 위에 코팅이 가능하며 진공의 유자가 필요 없기 때문에 장치가 간단하고 고순도의 균질한 박막을 얻을 수 있고 박막의 조성을 쉽게 조절할 수 있는 Sol-Gel법을 이용 하였다. Se보다 저가이며 독성이 없고 풍부한 원료인 S로 치환하여 사용하며 Cu/In비 값을 조절하고 tetragonal chalcopyrite <TEX>$CuInS_2$</TEX>의 열처리 온도에 따른 박막의 구조적, 광학적 특성에 미치는 변수들의 영향을 알아보았다. XRD pattern을 관찰한 결과 Cu/In비가 1.0일 때 <TEX>$2{\theta}=27.9^{\circ}$</TEX>에서 주피크가 가장 강하게 나타났으며 (112) 방향의 배향성을 가진 chalcopyrite상임을 확언 할 수 있었다. 열처리 온도가 증가할수록 (112) 면의 강도가 커지며 <TEX>$500^{\circ}C$</TEX>에서 열처리를 한 <TEX>$CuInS_2$</TEX> 박막은 tetragonal 구조의 화학량론적 <TEX>$CuInS_2$</TEX> 특징을 나타내고 본 실험의 샘플의 격자상수를 측정한 값이 a = 5.5032, c = 11.1064 <TEX>${\AA}$</TEX>이며 JCPDS(Joint Committee on Powder Diffraction Standards)에 보고된 데이터 a = 5.523, c = 11.14 <TEX>${\AA}$</TEX>과 거의 일치하였다. 광학적 특성을 알아보기 위해 측정한 광투과율은 가시광선 영역(380~770 nm)에서 전체적으로 30% 이하로 나타났다. <TEX>$CuInS_2$</TEX> thin films were prepared using a sol-gel spin-coating method. That makes large scale substrate coating, simple equipment, easy composition control available. The structural and optical properties of <TEX>$CuInS_2$</TEX> thin films that include less toxic materials (S) instead of Se, tetragonal chalcopyrite structure. Copper acetate monohydrate (<TEX>$Cu(CH_3COO)_2{\cdot}H2O$</TEX>) and indium acetate (<TEX>$In(CH_3COO)_3$</TEX>) were dissolved into 2-propanol and l-propanol, respectively. The two solutions were mixed into a starting solution. The solution was dropped onto glass substrate, rotated at 3000 rpm, and dried at <TEX>$300^{\circ}C$</TEX> for Cu-In as-grown films. The as-grown films were sulfurized inside a graphite container box and chalcopyrite phase of <TEX>$CuInS_2$</TEX> was observed. To determine the optical properties measured optical transmittance of visible light region (380~770 nm) were less than 30 % in the overall. The XRD pattern shows that main peak was observed at Cu/In ratio 1.0 and its orientation was (112). As annealing temperature increases, the intensity of (112) plane increases. The unit cell constant are a = 5.5032 and c = 11.1064 <TEX>$\AA$</TEX>, and this was well matched with JCPDS card. The optical transmittance of visible region was below than 30 %.

Characterization of chalcopyrite Cu(In,Al)Se2 thin films grown by selenization of evaporated precursors.

Chalcopyrite Cu(In,Al)Se2 (CIAS) thin films were prepared by a two-stage process onto soda-lime glass substrates. Different Cu/(Al+In) atomic ratios have been evaporated in a vacuum chamber to be subsequently heated with elemental selenium in a quasi closed graphite box at 530 °C. The results on the dependence of the crystallite size, preferential orientation, roughness, optical transmittance and aluminium incorporation as a function of the stoichiometry have been studied, concluding that a Cu/(Al+In) atomic proportion near the 1.07 value seems to be the best choice to favour crystalline CIAS formation without substantial CuSe secondary phases segregation.

DEPOSITION OF SINGLE-PHASE CuInSe2 THIN FILMS UNDER LOW VACUUM LEVEL BY A TWO-STAGE GROWTH TECHNIQUE

Single-phase CuInSe 2 films were grown by high vapor selenization of CuIn alloy precursors within a partially closed graphite box. The CuIn precursors were prepared using Cu x In y alloy targets with different composition rates under low vacuum level by a homemade sputtering system. The Cu and In composition rates of the used targets are 11:9, 10:10, and 9:11, respectively. The metallic precursor films were selenized using a two-step temperature profile, i.e. at 250°C and 400–500°C, respectively. The influence of the temperature at the second selenization step on the quality of the CIS absorbing layers was investigated. The CIS films were characterized by X-ray diffractometry, scanning electron microscopy, energy dispersive X-ray analysis, and Raman spectroscopy. The deposited CIS absorbers selenized at a high temperature of 500°C for 30 min exhibited a single-phase chalcopyrite structure with a preferential orientation in the (112) direction. These layers display uniform, large, and densely packed crystals with a grain size of about 3–5 μm. Cadmium sulfide buffer layer was manufactured by chemical bath deposition method. Bilayers ZnO / ZnO : Al were prepared by RF magnetron sputtering deposition. CIS solar cells with an efficiency of about 6.5% were produced without antireflective films. The method to fabricate CIS solar cells by a combination of the low vacuum sputtering deposition and the graphite box selenization process has provided a simple control process and shown a promising potential for developing high efficient and low-cost CuInSe 2 solar cells.

Adjustment of the selenium amount provided during formation of CuInSe2 thin films from the metallic precursors

AbstractCuInSe2 (CIS) thin films were grown by selenization of sequentially evaporated Cu and In metallic precursors. The adjustment of the Selenium amount provided during the selenization process within a partially closed graphite box for film thicknesses between 0.5–2.5 μm was performed. We obtained CuInSe2 films with chalcopyrite structure and preferential orientation along the (112) direction plane. XRD patterns showed intense and narrow peaks that indicate high crystallinity corresponding to a Se amount between 120–180 mg. Direct band gap energy between 0.90–0.97 eV, depending on the Se amount and film thickness, and a high absorption coefficient (∼105 cm–1) were found. A slight improvement in the surface morphology for the optimum Se amount aforementioned was observed. The measured film resistivities varied from 0.1 Ω cm to 40 Ω cm. Thus, the structural, optical and morphological characteristics were dependent of the Se amount provided during the selenization process and after fitting of this selenium, an optimization of the CIS thin films properties and a saving of the Se consumption were achieved. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

CuIn 1 − xAl xSe 2 thin films obtained by selenization of evaporated metallic precursor layers

CuIn 1 − x Al x Se 2 (CIAS) thin films were grown by a two stage process. Cu, In and Al layers were sequentially evaporated and subsequently heated with elemental selenium in a quasi-closed graphite box. Different x values (0 ≤ x ≤ 0.6) were obtained by varying the Al and In precursor layers thicknesses. Selenization conditions such as Se amount provided during the selenization process were adjusted in order to optimize the film properties. Polycrystalline CuIn 1 − x Al x Se 2 thin films with chalcopyrite structure were obtained. Referred to CuInSe 2 thin films the lattice parameters, the (112) orientation and the average crystallite size decreased and the band gap energy increased with increasing Al content. To optimize structural properties of the CIAS films a higher Se amount was required as the x value increased. The incorporation of Al changed the thin film morphology towards smaller grain sizes and less compact structures.

Oxidation effect on densification of CuInS 2 absorber layer by paste coating

Copper indium disulfide (CuInS 2) absorber layers were prepared by using a paste coating method. CuInS 2 and CuS were chosen as precursor powders. Ethylcellulose and phosphate ester were used respectively as a resin and a dispersant. Dihydroterpineol was used as a solvent. The paste was distributed on an Mo-coated soda–lime glass substrate by screen-printing method. It was difficult to obtain the densification of the CuS-doped CuInS 2 layer with only sulfurization in a graphite container box. However, when an oxidation process was applied to the CuS-doped CuInS 2 layer before the sulfurization process, quite dense CuInS 2 layers were obtained. Even though the chalcopyrite phase was nearly broke down with the oxidation, the phase was restructured with the sulfurization.

CuInS 2 thin films deposited by sol–gel spin-coating method

CuInS 2 thin films were prepared using a sol–gel spin-coating method. Copper acetate monohydrate (Cu(CH 3COO) 2·H 2O) and indium acetate (In(CH 3COO) 3) were dissolved into 2-propanol and 1-propanol, respectively. The two solutions were mixed into a starting solution. The solution was dropped onto glass substrate, rotated at 1500 rpm, and dried at 300 °C for Cu–In as-grown films. The as-grown films were sulfurized inside a graphite container box. A clear chalcopyrite phase was observed without a secondary phase. Surface roughness of the films sulfurized at 500 °C was 19.1 nm. A Raman spectra measurement confirmed that no Cu–S or In–S compounds were created in the thin films.

Multifoil UCx target for the SPES project --An update

The target system is one of the key issues for the facilities aimed at the production of neutron-rich radioactive ion beams. In the framework of the SPES project (Study for the Production of Exotic Species), the possibility of using a target configuration with a proton beam (40MeV, 0.2mA) directly impinging on multiple uranium carbide disks is investigated. The 238U fission fragments constitute the source for the exotic beams and for this purpose the disks are placed inside a graphite box at 2000 °C. The target is split into several thin disks in order to allow the cooling of the system by thermal radiation. In this way about ∼ 1013 fissions s -1 are obtained with a relative simple system and with relative low costs. Further steps have been performed compared to previous publications and now all the main parameters of the system have been analysed by means of calculation codes: the fission rates and the fission fragment distribution; the power deposition and the thermal analysis; the thermo-mechanical behaviours of the disks; the effusive and diffusive extraction release properties of the target.

Optical and electrical properties of CuIn 1− xGa xSe 2 thin films obtained by selenization of sequentially evaporated metallic layers

The aim of this work was to study the dependence of the optical, electrical and structural properties of CuIn 1− x Ga x Se 2 (CIGS) thin films on different parameters such as Cu/(In+Ga) and Ga/(In+Ga) ratios and film thicknesses. CIGS thin films have been deposited in a two-stage process. In the first stage, the metallic precursors were evaporated on glass substrates following the In/Ga/Cu/In sequence. The selenization, named the second stage, was performed inside a partially closed graphite box. This process was realized at 500 °C in vacuum with elemental selenium vapour. The obtained polycrystalline CIGS films showed the chalcopyrite structure with predominant growth in the (1 1 2) direction. The band gap energy was E g=0.93–1.58 eV, being higher for higher Ga/(In+Ga) and near stoichiometric Cu/(In+Ga) atomic ratios. The electrical conductivity σ, measured in the 40–370 K range, was found to increase with Cu content and to present a conductivity activation energy of 0.034 eV between 240 and 370 K.

Structural studies on SnS films grown by a two-stage process

Thin films of SnS have been grown by a novel two-stage process. It involves sulphurisation of thin metallic tin precursor layers in a vacuum furnace using a graphite box. The precursor layers were annealed at different temperatures in the range of 100–400 °C in excess ambient sulphur. The layers, annealed at temperatures 300–350 °C, were polycrystalline with a strong {111} preferred orientation. These layers had orthorhombic crystal structure with a grain size of 100 nm and exhibited stoichiometric composition. The precursor layers, reactively annealed at temperatures lower than 300 °C, had secondary phases such as SnS 2 and Sn 2S 3 with a grain size of 50 nm. However, the Sn layers, annealed at higher temperature >350 °C, had an additional phase of SnS 2 along with the SnS phase and showed deviation from stoichiometry with a deficiency of sulphur. The results were analysed systematically to develop the growth process.

CuInTe 2 thin films synthesized by graphite box annealing of In/Cu/Te stacked elemental layers

CuInTe 2 films were synthesized by graphite box annealing of In/Cu/Te stacked elemental layers (SEL) deposited by evaporation onto glass substrates. The films were polycrystalline with preferential orientation in the (1 1 2) direction. Lattice constants ( a=0.62 nm, c=1.24 nm) were determined from the X-ray diffraction analysis. The optical band gap, determined from transmittance spectra, varied within 0.91–0.95 eV with variation of Cu/In ratio within 0.55–1.97, while the grain size varied between 0.35 and 0.51 μm. The electrical conductivity, Hall mobility and photoluminescence of CuInTe 2 films with different Cu/In and Te/(Cu+In) ratios were studied to ascertain the carrier transport mechanism and nature of defect states in the films.

CuIn(S xSe 1− x) 2 films prepared by graphite box annealing of In/Cu stacked elemental layers

CuIn(S x Se 1− x ) 2 films were synthesized by sulphurization of In/Cu stacked elemental layers deposited onto glass and Mo-coated glass substrates followed by selenization by graphite box annealing. The films, thus synthesized, were characterized by measuring electrical, optical and microstructural properties. The microstructure and hence the physical properties of the films depended critically on the amount of sulphur incorporation. Nature of charge carriers depended on Cu/In, (S+Se)/(Cu+In) and S/(S+Se) ratios while their concentrations varied between 10 16 and 10 19 cm −3. Grain boundary scattering effects were critically studied by measuring the electrical conductivity ( σ) and Hall mobility ( μ) simultaneously on the same sample. Optical transmittance studies indicated the band gap to vary within 0.98–1.40 eV with x values. The photoluminescence spectra, recorded at 80 K were dominated by the excitonic peak located within 1.40–1.16 eV followed by a small peak within ∼0.96–0.98 eV arising due to transition from conduction band to neutral acceptor ( V Cu) or exciton bound to ionized acceptor (Cu In) states.

Investigations on the sulphur incorporation in sputtered Cu, In multilayer precursors

High quality CuInS 2 layers have been grown using a two step process. Multiple Cu, In bilayers were deposited by RF magnetron sputtering on Mo-coated glass substrates followed by sulphurisation using elemental sulphur in a closed graphite box at different temperatures up to 550°C. The layers were characterised to study their structure and composition using XRD, SEM and EDAX. More sulphur was incorporated at lower annealing temperatures with sulphur content higher than 50 at.%, while the Cu/In ratio ranges between 0.9 to 1.1. As the annealing temperature increases beyond 400°C, the composition of sulphur decreases to 46 at.% with an incorporation of Mo in the layers. The secondary phases observed at different annealing temperatures were identified and reported. Polycrystalline, single phase and densely packed CuInS 2 layers with (1 1 2) preferred orientation were obtained for annealing temperatures of about 350°C.

In situ RBS analysis of CuInSe2

CuInSe2 thin films were prepared by sequential evaporation of the constituent elements onto Mo-coated glass substrates and the layers were subsequently selenized in a closed graphite box at 400°C. The films were characterised by X-ray diffraction (XRD) and in situ, real-time Rutherford backscattering spectrometry (RBS) analysis to monitor the progress of the reaction as the elemental layers interdiffused. For the Cu/In/Se structure, XRD performed after various anneals revealed that the Cu–In layer mixed with the Se layer to form a number of binary phases. For Cu/Se/In it was found that the Cu and Se had intermixed at room temperature. In both the structures investigated the Cu–Se and In–Se binary phases formed below 200°C and upon annealing above this temperature interdiffused to form chalcopyrite CuInSe2.

Synthesis of CuInS 2 films by sulphurization of Cu/In stacked elemental layers

CuInS 2 films were synthesized by sulphurization of In/Cu stacked elemental layers (SEL) deposited onto glass and Mo-coated glass substrates by graphite box annealing at temperatures of 620–880 K. The films thus synthesized were characterized by measuring electrical, optical, microstructural and photoluminescence properties. The microstructure and hence the physical properties of the films depended critically on the amount of sulphur incorporation. Nature of charge carriers depended on both Cu/In and S/(Cu+In) ratio and the carrier concentration varied between 10 14 and 10 18 cm −3. Grain boundary scattering effects were critically studied by measuring the electrical conductivity and Hall mobility simultaneously on the same sample. The shape of the grains depended critically on the sulphur content. The PL spectra were dominated by the excitonic peak ∼788 nm followed by another peak at ∼892 nm which may be ascribed to the DA transition.

PHOTOLUMINESCENCE STUDIES IN CuInxGa1-xSe2 FILMS PREPARED BY GRAPHITE BOX ANNEALING OF In/Ga/Cu/Se STACKED ELEMENTAL LAYERS

CuIn x Ga 1-x Se 2 films were synthesized by graphite box annealing of In/Ga/Cu/Se stacked elemental layers deposited by thermal evaporation onto sodalime glass substrates. Photoluminescence (PL) studies were carried out in near stoichiometric and copper-rich films with Ga/(In+Ga) ratios varying between 0.24–0.29. The PL spectra were dominated by peaks at ~1.19 eV and 0.87 eV with a shoulder at ~0.84 eV. There was also a peak around 1.3 eV which could be assigned to the absorption from the split-off band. The transitions at ~1.19 eV and 0.87 eV were due to excitonic transition and donor-acceptor pair formation respectively.

CuInSe2 films produced by graphite box annealing of multilayer precursors

Copper indium diselenide films were prepared by graphite box annealing of stacked elemental layer (SEL) precursors. Effects of annealing temperature and ambient atmosphere on the microstructural, optical and electrical properties were studied critically. PL measurements were also carried out to find information on the defect levels. From the above studies, the optimum condition for the deposition of high quality CuInSe2 films by this technique was determined.