Se Diffusion Research Articles

Molecule Diffusion Behavior of Tritium and Selenium in Mongolia Clay Rock by Numerical Analysis of the Spatial and Temporal Variation

Clay rock in the Tamusu (TMS) area in the Inner Mongolia province is one candidate as a geological disposal site for highly radioactive waste in China. The diffusion behavior of HTO and Se(IV) in Tamusu clay rock was studied by through-diffusion (TD) experiments and numerical analysis to determine the spatial and temporal variation. A minimum error analysis was conducted to determine the HTO and Se(IV) diffusion coefficients in compacted TMS clay. The TD experimental results and numerical analysis showed that the diffusion of HTO and Se reached a steady state within 7 and 50 days, respectively, and the apparent diffusion coefficients (Da) decreased with the increases in the compacted density. In fact, there was retardation of Se diffusion in intact TMS clay rock. A two-site sorption model for Se was applied to simulate fast and slow sorption behavior quantitatively.

Plasma-Enhanced Atomic Layer Deposition of TiN Thin Films as an Effective Se Diffusion Barrier for CIGS Solar Cells.

Plasma-enhanced atomic layer deposition (PEALD) of TiN thin films were investigated as an effective Se diffusion barrier layer for Cu (In, Ga) Se2 (CIGS) solar cells. Before the deposition of TiN thin film on CIGS solar cells, a saturated growth rate of 0.67 Å/cycle was confirmed using tetrakis(dimethylamido)titanium (TDMAT) and N2 plasma at 200 °C. Then, a Mo (≈30 nm)/PEALD-TiN (≈5 nm)/Mo (≈600 nm) back contact stack was fabricated to investigate the effects of PEALD-TiN thin films on the Se diffusion. After the selenization process, it was revealed that ≈5 nm-thick TiN thin films can effectively block Se diffusion and that only the top Mo layer prepared on the TiN thin films reacted with Se to form a MoSe2 layer. Without the TiN diffusion barrier layer, however, Se continuously diffused along the grain boundaries of the entire Mo back contact electrode. Finally, the adoption of a TiN diffusion barrier layer improved the photovoltaic efficiency of the CIGS solar cell by approximately 10%.

Se diffusion in CdTe thin films for photovoltaics

Manipulating CdSeTe bandgrading to enhance photocurrent and carrier lifetime is an essential part of high-performance CdTe photovoltaics (PVs). In this work, we examine Se diffusion kinetics in single-crystal and polycrystalline CdTe during deposition, thermal annealing, and CdCl2 treatments. Se distributions are determined by dynamic secondary-ion-mass spectroscopy and Auger electron spectroscopy depth profiling and coupled with electron backscatter diffraction images of the crystalline structure. Effective bulk and grain boundary diffusion coefficients are determined by analytical models and discussed in the context of processing and film morphology. Se is found to diffuse in CdTe at much higher rates during CdCl2 treatments than with thermal processing alone. GB diffusion also occurs at a significantly faster rate than bulk diffusion. As a result of these two effects, the near interface bulk and GB Se diffusion during CdCl2 treatments dominates the bandgrading profiles in CdTe PVs.

Investigation on the properties of Cu 2 ZnSnSe 4 and Cu 2 ZnSn(S,Se) 4 absorber films prepared by magnetron sputtering technique using Zn and ZnS targets in precursor stacks

Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 absorber films were grown on soda lime glass and Mo-coated soda lime glass substrates by deposition of the precursor films via RF magnetron sputtering method in the stacking orders of Cu/Sn/Zn/Mo/SLG and Cu/Sn/ZnS/Mo/SLG using metallic Zn or binary sulfide ZnS targets and subsequently carrying out of selenization process. It was aimed to find out the effect of Zn or ZnS target types and the small amount of S originated from ZnS target material used in the deposition of precursor films on the structural, morphological, optical, and electrical characteristics of the films to be selenized. XRD and Raman spectroscopy analysis showed that the kesterite CZTSe structure was predominantly formed in both cases where Zn and ZnS targets were used. According to the EDX analysis, S content in the prepared film using ZnS target was only around 1.79 at%. This indicated that a considerable amount of S in the film was driven out during the selenization process. Scanning electron microscopy analysis revealed that ZnS target material contributed to the achievement of the absorber films with larger grain size. It was also determined that the thickness of the interfacial MoSe2 film between the absorber and Mo films decreased by using ZnS target in the precursor film. This was attributed to ZnS layer with a high melting point acting as a barrier layer over Mo film and retarding the diffusion of Se into the Mo film during the selenization process. In addition, the band gap energy values of the Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 films were found to be 1.18 and 1.28 eV, respectively.

Effect of sequential isochronal annealing on the structure and migration behaviour of selenium-ion implanted in glassy carbon

In a view to further study the diffusion properties of glassy carbon for its applicability as a diffusion barrier for fission products found in high-level nuclear waste, glassy carbon samples were implanted with 150 keV Se ions to a fluence of 1 × 1016 ion/cm2 at room temperature. Some of the implanted samples were subjected to sequential isochronal annealing at two temperature regimes (300–700 °C and 1000–1200 °C) for 5 h under vacuum. Raman spectroscopy was used to investigate the structural changes while Rutherford backscattering spectrometry (RBS) was used to monitor the migration of implanted Se. Implantation of Se amorphized glassy carbon resulting in an increase in density (from 1.42 to 2.1 gcm−3). No notable diffusion of Se in the glassy carbon occurred after annealing at the low-temperature regime (300–700 °C). Raman spectroscopy shows that limited recrystallization occurred after annealing at the low-temperature regime. Appreciable recrystallization coupled with some diffusion occurred at the high-temperature regime, 1000–1200 °C. The original structure of glassy carbon was not achieved after annealing at 1200 °C. Diffusion coefficients of 5.9 × 10−20 m2s−1 and 4.79 × 10−20 m2s−1 were obtained for Se in defective glassy carbon at 1000 and 1100 °C, respectively.

Microscopic Analysis of Interdiffusion and Void Formation in CdTe(1-x)Sex and CdTe Layers.

The use of CdSe layers has recently emerged as a route to improving CdTe photovoltaics through the formation of a CdTe(1–x)Sex (CST) phase. However, the extent of the Se diffusion and the influence it has on the CdTe grain structure has not been widely investigated. In this study, we used transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD) to investigate the impact of growing CdTe layers on three different window layer structures CdS, CdSe, and CdS/CdSe. We demonstrate that extensive intermixing occurs between CdS, CdSe, and CdTe layers resulting in large voids forming at the front interface, which will degrade device performance. The use of CdS/CdSe bilayer structures leads to the formation of a parasitic CdS(1–x)Sex phase. Following removal of CdS from the cell structure, effective CdTe and CdSe intermixing was achieved. However, the use of sputtered CdSe had limited success in producing Se grading in CST.

CZTSSe solar cell with an efficiency of 10.19% based on absorbers with homogeneous composition and structure using a novel two-step annealing process

Annealing CZTS precursors in Se-containing atmosphere to obtain large grain CZTSSe absorbers has proven to be a promising method for preparing high-efficiency solar cells. However, due to Se diffusion from surface to inside of the precursor film, inhomogeneous CZTSSe grain growth can be generally observed in the traditional selenization process. The undesired fine-grain layer forms beneath the top large-grain layer and harmful secondary ZnSe phase generates on CZTSSe/Mo interface on account of the insufficient selenization, thereby degrading the device performances. To derive high-quality CZTSSe absorbers, a novel two-step annealing process was performed. CZTS precursors were firstly annealed at a low temperature and then annealed at a high temperature. Compared with the traditional one-step process, there is a remarkable promotion of average device efficiency from 3.86% to 9.58% using the two-step process. The mechanism for improving device performance was proposed: The formation of densely packed large-grain layers on surface of the absorber was delayed under the low-temperature annealing stage, the Se spread to the inside of the absorber from the grain boundaries, then providing driving force for grain growth and suppressing the decomposition of CZTSSe in subsequent annealing process. Finally, CZTSSe solar cell with the highest efficiency of 10.19% was achieved based on the two-step process.

Investigation of Se(IV) diffusion in compacted Tamusu clay by capillary method

For the first time in laboratory, diffusion parameters of Se(IV) in the clay of Chinese Tamusu region(TMS clay) against compacted density, ionic strength and pH value were obtained by capillary method. Diffusion process can be Simplified by using one-dimensional semi-infinite medium diffusion model. The results show a good similarity with the international literature and meet the internationally recognized range of 10−9–10−12. This work will provide data support and theoretical references for the pre-selection of clay rock sites in China’s high-level radioactive waste deep geological repository.

Rear‐Passivated Ultrathin Cu(In,Ga)Se2 Films by Al2O3 Nanostructures Using Glancing Angle Deposition Toward Photovoltaic Devices with Enhanced Efficiency

AbstractIn this work, for the first time, the addition of aluminum oxide nanostructures (Al2O3 NSs) grown by glancing angle deposition (GLAD) is investigated on an ultrathin Cu(In,Ga)Se2 device (400 nm) fabricated using a sequential process, i.e., post‐selenization of the metallic precursor layer. The most striking observation to emerge from this study is the alleviation of phase separation after adding the Al2O3 NSs with improved Se diffusion into the non‐uniformed metallic precursor due to the surface roughness resulting from the Al2O3 NSs. In addition, the raised Na concentration at the rear surface can be attributed to the increased diffusion of Na ion facilitated by Al2O3 NSs. The coverage and thickness of the Al2O3 NSs significantly affects the cell performance because of an increase in shunt resistance associated with the formation of Na2SeX and phase separation. The passivation effect attributed to the Al2O3 NSs is well studied using the bias‐EQE measurement and J–V characteristics under dark and illuminated conditions. With the optimization of the Al2O3 NSs, the remarkable enhancement in the cell performance occurs, exhibiting a power conversion efficiency increase from 2.83% to 5.33%, demonstrating a promising method for improving ultrathin Cu(In,Ga)Se2 devices, and providing significant opportunities for further applications.

Effect of Se diffusion and the role of a thin CdS buffer layer in the performance of a CdSe/CdTe solar cell

Abstract In this work the beneficial aspects of substituting CdS, the traditional heterojunction partner to CdTe, with a bi-layer of CdSe/CdS is studied and the results are discussed. The high short-circuit current in a CdTe/(CdSe/CdS) solar cell is mainly due to contribution from two factors; enhanced blue-response due to the reduction in CdTe band gap which a result of Se diffusion into CdTe film forming the ternary CdTe1-xSex at the interfacial region, and the use of a thinner CdSe layer which was possible due to the presence of a thin CdS film. A thin layer of CdS between indium-tin-oxide (ITO) and CdSe is essential to serve as a buffer layer minimizing the shunt paths at the heterojunction interface. A 10% increase in short-circuit current is observed when the CdS-alone heterojunction layer is substituted with a CdSe/CdS bi-layer. The SIMS depth profile analysis indicate that in devices using a CdSe/CdS bi-layer, the Cu diffusion from back contact into the CdTe bulk is limited significantly compared to a CdS-only device. The net carrier concentration across the absorber layer is not uniform and showed the characteristic U-shape with respect to depletion layer width. The apparent increase in carrier concentration near the back contact region is related to the Cu doping. The shallow vs. sharp curving of the NA-ND profile of the CdS-only and CdSe/CdS bi-layer devices is explained on the basis of the regulated Cu diffusion from the back contact into CdTe, which is a consequence of Se diffusion.

Characterization of thin CdTe solar cells with a CdSeTe front layer

ABSTRACTThin CdTe photovoltaic device efficiencies show significant improvement with the incorporation of a CdSeTe alloy layer deposited between a MgZnO emitter and CdTe absorber. CdTe and CdSeTe/CdTe devices fabricated by close-space sublimation with a total absorber thickness of 1.5 µm are studied using microscopy measurements and show minimal diffusion of Se into the CdTe. Current loss analysis shows that the CdSeTe layer is the primary absorber in the CdSeTe/CdTe structure, and fill factor loss analysis shows that ideality-factor reduction is the dominant mechanism of fill factor loss. Improvement in the CdSeTe/CdTe absorber quality compared to CdTe is also reflected in spectral and time-resolved photoluminescence measurements. Current density vs. voltage measurements show an increase in current density of up to 2 mA/cm2 with the addition of CdSeTe due to a band gap shift from 1.5 to 1.42 eV for CdTe and CdSeTe/CdTe absorbers respectively. Voltage deficit is lower with the incorporation of the CdSeTe layer, corroborated by improved electroluminescence intensity. The addition of CdSeTe into CdTe device structures has increased device efficiencies from 14.7% to 15.6% for absorbers with a total thickness less than two microns.

Diffusion of Re(VII) and Se (IV) in compacted GMZ bentonite in the presence of Bacillus spp.

The effect of Bacillus spp. on Re(VII) and Se(IV) diffusion in compacted GMZ bentonite was investigated by a through-diffusion method. Bacillus spp. had no significance effect on the effective diffusion coefficient (De) and the accessible porosity (eacc) of Re(VII), whereas a decrease of De and an increase of distribution coefficient (Kd) were observed for Se(IV). Moreover, Archie’s law and an exponential decay equation were employed to fit the De value of Re(VII) and Se(IV).

N-type vapor diffusion for the fabrication of GaSb thermophotovoltaic cells to increase the quantum efficiency in the long wavelength range

N-type vapor diffusions in p-GaSb wafers is investigated using the group-VI elements S, Se and Te as diffusion sources. The group-VI elements are found difficult to diffuse in bare p-GaSb wafers because they react with Ga atoms to form compounds, thus hindering the diffusion. Deposition of a SiO layer on the p-GaSb surface is found to be crucial for realizing n-type diffusion. With this SiO coating layer, the concentration of group-VI atoms decreased to a low value limiting the reaction with Ga atoms. Se atoms are found to be good n-type diffusion sources. GaSb thermophotovoltaic cells with n-on-p structures are fabricated using Se diffusion. Compared with a traditional GaSb cell with a p-on-n structure, the cell with the n-on-p structure had a higher quantum efficiency at wavelengths beyond 1000 nm. The output power density was also 1.42 times higher compared to the traditional cell when measured with a 1300 °C - SiN ceramic IR emitter.

Improvement of Short-Circuit Current Density in GaInP Solar Cells Grown by Dynamic Hydride Vapor Phase Epitaxy

We demonstrate GaInP solar cells grown by dynamic hydride vapor phase epitaxy with improved efficiency through elimination of nonradiative recombination centers. We redesigned our reactor to eliminate unintentional impurities, which led to a reduction of nearly 30× in dark current. This improvement led to a significantly improved open-circuit voltage of 1.41 V and a band gap-voltage offset of 0.47 V in a device with a rear reflector, achieved without front surface passivation. We demonstrate that the limiting factor toward achieving higher efficiency is control over the emitter thickness. Recombination is high in the unpassivated emitter, but losses can be minimized by designing it to be thin. We show that Se diffusion from the GaAs contact layer into the base is responsible for unintentional thickening of the emitter. Use of reduced contact doping reduces the penetration of Se into the p-GaInP base layer. With improved control of Se diffusion, we achieve 13.0 mA/cm2 and 15.2% certified efficiency in a single-junction GaInP cell structure that is optically equivalent to the top cell in a GaInP/GaAs tandem.

Strategy for synthesizing mesoporous NiO polyhedra with empty nanovoids via oxidation of NiSe polyhedra by nanoscale Kirkendall diffusion and their superior lithium-ion storage performance

A novel strategy for the synthesis of mesoporous metal oxide polyhedra with empty nanovoids by introducing nanoscale Kirkendall diffusion into a spray pyrolysis process is suggested. In this study, NiSe polyhedra with high index facets synthesized by one-pot spray pyrolysis were transformed into mesoporous NiO polyhedra with empty nanovoids by a facile oxidation process at 500 °C through nanoscale Kirkendall diffusion. Details of the formation mechanism of these uniquely structured NiO polyhedra were also studied. In the initial oxidation step, a thin NiO layer was formed on the surface of the NiSe polyhedra. The fast diffusion of Ni cations out through the NiO layer and the melting of the remaining metalloid Se formed a porous Se layer. The formation of a mesoporous NiO layer then enabled the penetration of oxygen gas by gas phase diffusion. The diffusion of metalloid Se into the mesopores of NiO and its reaction with oxygen gas formed SeO2 nanocrystals which then evaporated into the gas phase. The step-by-step oxidation of the NiSe crystals therefore resulted in mesoporous NiO polyhedra with empty nanovoids. When used as an anode material in a lithium ion battery, mesoporous NiO polyhedra with empty nanovoids showed superior cycling and rate performance compared to anodes fabricated from dense-structured NiSe polyhedra and commercial NiO nanopowders. The mesoporous NiO polyhedra showed a high discharge capacity of 1072 mA h g−1 after 200 cycles at a current density of 1.0 A g−1 and they showed a high reversible capacity of 453 mA h g−1, even at a high current density of 15.0 A g−1.

Low temperature incorporation of selenium in Cu2ZnSnS4: Diffusion and nucleation

Band gap grading of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells can be achieved by varying the Sr = [S]/([S] + [Se]) ratio in the absorber layer with depth. One approach is a two-step annealing process where the absorber is first sulfurized to Cu2ZnSnS4 (CZTS) followed by selenization to form CZTSSe. However, once nucleation of CZTSSe initiates, the rapid interchange of S and Se limits the control over the Sr ratio with depth. Here, we have studied incorporation of Se into CZTS and observed the behavior of Se below and up to the nucleation temperature of CZTSSe. Se diffusion at 337 and 360 °C is dominated by grain boundary diffusion while some increase of Se is also seen in the region from 100 to 800 nm from the surface. After selenization at 409 °C, recrystallization is observed and CZTSSe grains are formed. The recrystallization is more rapid for a smaller average grain size and is facilitated by diffusion of Na from the back contact. The grain boundary diffusion is identified with secondary ion mass spectrometry measurements by measuring the accumulation in the CZTS/Mo interface for three samples with different average grain size.

Effect of CdCl2 passivation treatment on microstructure and performance of CdSeTe/CdTe thin-film photovoltaic devices

The effects of the CdCl2 passivation treatment on thin-film CdTe photovoltaic films and devices have been extensively studied. Recently, with an addition of CdSeTe layer at the front of the absorber layer, device conversion efficiencies in excess of 19% have been demonstrated. The effects of the CdCl2 passivation treatment for devices using CdSeTe has not been studied previously. This is the first reported study of the effect of the treatment on the microstructure of the CdSeTe /CdTe absorber. The device efficiency is < 1% for the as-deposited device but this is dramatically increased by the CdCl2 treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl2 passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults, increase in grain size and reorientation of grains. The CdCl2 treatment leads to grading of the absorber CdSeTe/CdTe films by diffusion of Se between the CdSeTe and CdTe regions. Chlorine decorates the CdSeTe and CdTe grain boundaries leading to their passivation. Direct evidence for these effects is presented using STEM and Energy Dispersive X-ray Analysis (EDX) on device cross-sections prepared using focused ion beam etching. The grading of the Se in the device is quantified using EDX line scans. The comparison of CdSeTe/CdTe device microstructure and composition before and after the CdCl2 treatment provides insights into the important effects of the process and points the way to further improvements that can be made.

Incorporation of CdSe layers into CdTe thin film solar cells

Incorporation of CdSe layers into CdTe thin film solar cells has recently emerged as a route to improve cell performance. It has been suggested that the formation of lower band gap CdTe(1-x)Se(x) phases following Se diffusion induces bandgap grading which may increase the carrier lifetime and thereby open circuit voltage. In this study we investigate the impact of CdSe incorporation on CdTe solar cell performance. We demonstrate that the standard CdS/CdTe device architecture is incompatible with Se incorporation, owing to large optical losses. An alternative cell structure with an oxide partner layer replacing the CdS with SnO2/CdSe/CdTe is developed, leading to cell efficiencies of > 13.5%. The differences in processing required for effective selenium incorporation are investigated with performance improvements resulting from additional post-growth annealing. Finally, other oxides such as TiO2, ZnO and FTO are demonstrated to be unsuitable partner layers but highlight that the choice of partner layer is key to further improving the performance.

Polycrystalline CdSeTe/CdTe Absorber Cells With 28 mA/cm2 Short-Circuit Current

An 800 nm CdSeTe layer was added to the CdTe absorber used in high-efficiency CdTe cells to increase the current and produce an increase in efficiency. The CdSeTe layer employed had a band-gap near 1.41 eV, compared with 1.5 eV for CdTe. This lower band-gap enabled a current density increase from approximately 26 to over 28 mA/cm2. The open-circuit voltage obtained in the high-efficiency CdTe-only device was maintained and the fill-factor remained close to 80%. Improving the short-circuit current density and maintaining the open-circuit voltage lead to device efficiency over 19%. External quantum efficiency implied that about half the current was generated in the CdSeTe layer and half in the CdTe. Cross-sectional STEM and EDS showed good grain structure throughout. Diffusion of Se into the CdTe layer was observed. This is the highest efficiency polycrystalline CdTe photovoltaic device demonstrated by a university or national laboratory.

Tailoring Mo(S,Se)2 structure for high efficient Cu2ZnSn(S,Se)4 solar cells

The direction of Se-Mo-Se sheets to the Mo film is crucial for the thickness of Mo(S,Se)2 and the electrical conductivity, so as the back contact of Cu2ZnSn(S,Se)4 solar cells. In this study, the preferred orientation of Mo(S,Se)2 film changes from (100) peak to (103) peak, i.e. from perpendicular to the substrate to tilted to the substrate for Se-Mo-Se sheets as the roughness decrease of Mo back contact layer. The Se vapor can easily diffuse through the channels between Se-Mo-Se sheets when the formed Mo(S,Se)2 layer is (100) peak preferred with Se-Mo-Se sheets perpendicular to the substrate, and thus excessively thick Mo(S,Se)2 will be formed. Whereas, the Se-Mo-Se sheets are tilted to substrate when the preferred orientation of Mo(S,Se)2 is (103) peak. The tilted Se-Mo-Se sheets can act as a natural Se diffusion barrier to suppress the Se vapor diffusion through the already formed Mo(S,Se)2 layer to further selenize the remaining Mo film, and also can provide a good electrical conductivity. As a result, the thickness of Mo(S,Se)2 sharply decreased from 1500nm to 200nm with the surface morphology change of Mo back contact, which resulting in the decrease of series resistance of Cu2ZnSn(S,Se)4 solar cells from 2.94Ωcm2 to 0.49Ωcm2, and the increase of conversion efficiency of Cu2ZnSn(S,Se)4 solar cells from 6.98% to 9.04%.