Ordered Vacancy Compound Phase Research Articles

Optimization of Growth Parameters of the RF-Sputtered CuInGaSe2 Thin Films for Photovoltaic Applications

CuInGaSe2 (CIGS) thin films were deposited by RF sputtering using a single quaternary target. The effects of various sputtering parameters, such as substrate temperature, sputtering power, and gas flow rate, were studied systematically. The structural, morphological, compositional, and optical properties of the films were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), and UV–vis-NIR spectroscopy. The samples exhibited the chalcopyrite type tetragonal structure of CIGS, as confirmed by the XRD analysis. Raman spectroscopy represents the presence of Cu poor ordered vacancy compound (OVC) phase in all the present samples. The growth of the sample at a higher substrate temperature resulted in a higher crystalline nature with a suppressed OVC phase and energy bandgap of 1.18 eV. The deposition of CIGS at 160 W sputtering power favors growth towards (112) Bragg crystal plane, suppressing the (220)/(204) plane of CIGS, shows change in preferred orientation with a lower sputtering power at 80 and 120 W. The sample grown at a gas flow rate of 60 standard cubic centimeters per minute (SCCM) exhibited compact grain growth with marginally improved crystallinity. The sample grown at 160 W, 300 °C, and 60 SCCM showed better crystalline and morphological properties, and it can be used as an absorber layer for highly efficient CIGS thin-film solar cells.

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.

Insights into the Effects of RbF‐Post‐Deposition Treatments on the Absorber Surface of High Efficiency Cu(In,Ga)Se2 Solar Cells and Development of Analytical and Machine Learning Process Monitoring Methodologies Based on Combinatorial Analysis

AbstractThe latest record efficiencies of the Cu(In,Ga)Se2 (CIGSe) photovoltaic technology are driven by heavy alkali post‐deposition treatments (PDTs). Despite their positive effect, especially in the Voc of the devices, their underlying mechanisms are still under discussion. This work sheds light on this topic by performing a high statistics analysis on 620 high efficiency CIGSe solar cells submitted to four different PDT processes (different RbF source temperature) employing a combinatorial approach based on Raman and photoluminescence (PL) spectroscopies. This reveals with statistical confidence subtle differences in the spectroscopic data that relate to the redistribution of defects between the ordered vacancy compound (OVC) and the chalcopyrite phases at the absorber surface. In particular, there is an intertwined decrease of the OVC phase and increase of the so‐called “defective chalcopyrite phase.” Additionally, two industry‐compatible methodologies for the assessment of the RbF‐PDT process and prediction of the Voc of the final devices with a ±2% error and an efficacy of ≈95% are developed based on analytical and machine learning approaches. These results show that the combined Raman and PL spectroscopic techniques represent a powerful tool for the future development of the CIGSe technology at a research level and for more efficient industrial manufacturing.

Tuning Ga Grading in Selenized Cu(In,Ga)Se2 Solar Cells by Formation of Ordered Vacancy Compound

Severe Ga accumulation at the back contact is regarded as one of the major limiting factors for high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells fabricated by the sequential process. The Ga deficiency near the front surface of absorber leads to the reduction of open‐circuit voltage; however, controlling the Ga grading is quite challenging during the selenization process. Herein, the tuning of Ga profile is demonstrated by forming the ordered vacancy compound (OVC) phase at the beginning of selenization process under high Se partial pressure (PSe) with the precursor composed of a relatively high In content. The OVC phase formed with a columnar structure promotes Ga diffusion through Cu vacancies, resulting in Ga increase at the front side of CIGSe. However, high PSe also increases the thickness of MoSe2, significantly raising the series resistance. Thereby, a 5 nm TiN layer is deposited on top of Mo to suppress the formation of MoSe2. In addition, the extra Na‐contained precursor is added to increase the carrier concentration because the TiN layer also blocks the Na outdiffusion from the substrate. By controlling PSe, interlayer TiN thickness, and carrier concentration, a 16.04% record high efficiency of CIGSe solar cells (sulfur‐free) via elemental Se source is achieved.

Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells

AbstractSolution processing of Cu(In,Ga)Se2 (CIGS) absorber makes it cost‐competitive in the photovoltaic market. It is reported that copper‐poor ordered vacancy compound (OVC) is crucial for high performance CIGS solar cells. However, in solution process method, controllable formation of OVC is unavailable and limited research has been carried out. In this work, the controllable formation of the OVC phase on the CIGS surface is successful by controlling the selenization temperature and intentional variation of Cu/(In+Ga) stoichiometry in precursors for top layers and bulk layers deposition. The effects of OVC contents on the device performance are investigated. The CIGS thin film with OVC phase exhibits a lower valence band position. Meanwhile, the CIGS devices with optimized OVC content show decreased interface defects density and better carrier collection ability. The above advantages translate into a champion PCE of 16.39% for CIGS device with OVC phase, which is the champion performance among non‐hydrazine solution‐processed CIGS solar cells. The results demonstrate that the controllable formation of OVC phase approach should make a significant contribution to the efficiency promoting of solution processed CIGS solar cells.

3-D point defect density distributions in thin film Cu(In,Ga)Se2 measured by atom probe tomography

This paper will discuss the findings of an ordered vacancy compound (OVC) phase that exists deep into the bulk of a high-efficiency Cu(In,Ga)Se2 (CIGS) absorber that is shown to be a result of many ordered defect pairs of 2VCu + (In,Ga)Cu as determined by atom probe tomography (APT). To date, literature has shown that absorbers grown with the three-step process exhibit the OVC Cu(In,Ga)3Se5 (135 phase) only within the first few nanometers from the CdS/CIGSe interface and at grain boundaries. In this contribution, we have found a small volume (100 nm × 100 nm × 300 nm) of an OVC phase to exist about 400 nm into the absorber. We show through concentration and density profiles that the concentration change from the stoichiometric Cu(In,Ga)Se2 to the OVC is indeed a result of many ordered defect pairs. We use this volume to perform point defect density distributions to give unique insight to the band structure at the nanoscale.

Microstructure characterization of the soda-lime-glass/copper–indium–gallium–selenium interface in Cu-poor Cu(In,Ga)Se2 thin films

Abstract The microstructure characteristics of the soda-lime-glass/Cu(In,Ga)Se 2 (SLG/CIGS) interface in Cu-poor CIGS films are investigated by transmission electron microscopy and selected area electronic diffraction (SAED). The SAED patterns show very sharp and strong spots, indicating the main structure of CIGS chalcopyrite. Small dispersed crystals with size distribution from 2 to 5 nm seem to be embedded in amorphous matrix, and additional spots indicate the presence of an ordered vacancy compound (OVC). This observation is consistent with the Raman results, and the OVC phase with the nanoclusters exists in the CIGS matrix, instead of layer structure. Lattice distortion results in local changes in contrast. Some pseudo-disordered structure is observed, however, the structure is actually the chalcopyrite CIGS structure. 180° rotation twins are also observed at the SLG/CIGS interface. Lattice distortion is widely observed at the interface of the Cu-poor CIGS films, and the extra spots could be caused by different lattice orientations.

Raman scattering analysis of electrodeposited Cu(In,Ga)Se2 solar cells: Impact of ordered vacancy compounds on cell efficiency

This work reports the detailed Raman scattering analysis of Cu-poor Cu(In,Ga)Se2 (CIGS) electrodeposited solar cells using different excitation wavelengths. The systematic assessment of cells fabricated with Cu-poor absorbers that were synthesized with different Cu contents has allowed identifying the existence of a quasi-resonant excitation of a Raman peak characteristic of an Ordered Vacancy Compound (OVC) secondary phase when using a 785 nm excitation wavelength. The enhanced sensitivity of the spectra measured with these conditions to the presence of the OVC phase provides with a suitable tool for the non destructive assessment on the occurrence of this Cu-poor secondary phase in the surface region of the CIGS absorbers from measurements performed on finished cells. The correlation between the Raman scattering data and the optoelectronic parameters of the devices shows the existence of an optimum OVC content range leading to devices with highest open circuit voltage and efficiency. These data provide with a clear experimental evidence on the impact of the OVC phases on the device efficiency.

Cu(In, Ga)Se2 thin films prepared from stacked precursors by post-selenization process

The copper–indium–gallium metallic precursors were fabricated corresponding to the sequence CuGa/CuIn/CuGa/soda-lime-glass by sequential direct current magnetron sputtering. The as-sputtered precursors were comprised of Cu11In9, Cu16In9, In, Cu9Ga4, and CuGa2 phases, which may be closely correlated to the deposition sequences of multi-layered metallic precursors. Cu(In, Ga)Se2 (CIGS) absorber films were prepared from the stacked precursors by post-selenization process with solid Se powder, and the morphological, structural, and compositional properties were investigated. The as-selenized CIGS film exhibited a smooth, compact, and densely packed morphology with well-defined and faceted crystal grains. The film was Cu-deficient and had a low Ga content. X-ray diffractometer results indicated the formation of single-chalcopyrite structure CIGS absorber layers. Depth-resolved Raman patterns showed the formation of a dominant CIGS phase in the as-selenized layer, and an ordered vacancy compound phase like Cu(In, Ga)3Se5 at the surface and inner region. With increasing the sputtering time, the full width at half maximum of the chalcopyrite A1 Raman peak increased. Band broadening can be interpreted as a result of a higher density of defects within the chalcopyrite CIGS phase. The A1 peak shifts with increasing sputtering depths were not apparent, which was related to a uniform distribution of Ga in the CIGS thin film.

The influence of different locations of sputter guns on the morphological and structural properties of Cu–In–Ga precursors and Cu(In,Ga)Se2 thin films

The influence of two different locations of sputter guns on the morphological and structural properties of Cu–In–Ga precursors and Cu(In,Ga)Se2 (CIGS) thin films was investigated. All the precursors contained cauliflower-like nodules, whereas smaller subnodules were observed on the background. All the precursors revealed apparent three-layered structures, and voids were observed at the CIGS/SLG interface of Sets 1 and 2 films rather than Set 3 film. EDS results indicated that all CIGS thin films were Cu-deficient. Based on the grazing incidence X-ray diffraction (GIXRD) patterns, as-selenized films showed peaks corresponding to the chalcopyrite-type CIGS structure. Depth-resolved Raman spectra showed the formation of a dominant CIGS phase inside the films for all the as-selenized samples investigated, and of an ordered vacancy compound (OVC) phase like Cu(In,Ga)3Se5 or Cu(In,Ga)2Se3.5 at the surface and/or CIGS/SLG interface region of Sets 2 and 3 films. No evidence was obtained on the presence of an OVC phase in Set 1 CIGS film, which may be speculated that long-time annealing is contributed to suppress the growth of OVC phases. The results of the present work suggest that the metallic precursors deposited with the upright-location sputter gun might be more appropriate to prepare CIGS thin films than those sputtered with the titled-location gun.

Recombination in Cu(In,Ga)Se2 thin-film solar cells containing ordered vacancy compound phases

We investigated the transport and photovoltaic properties of Cu-deficient Cu(In1−xGax)Se2 (CIGS) thin-film solar cells containing ordered vacancy compound (OVC) layers. Raman spectra clearly revealed that the CIGS thin films with lower Cu concentrations contained larger volumes of OVC layers. The temperature-dependent inverse ideality factor showed that the CIGS film containing more (less) OVC layers exhibited tunneling-mediated bulk (interface)-dominated recombination. The capacitance–voltage characteristics and admittance spectra showed that the CIGS cells containing more OVC layers had more uniform carrier concentration near the junction and less interfacial trap states compared with cells with less OVC layers. These results suggested that the Cu-deficiency and the resulting OVC layer formation reduced the interfacial defect density and suppressed the interface recombination processes of the CIGS solar cells.

Raman scattering analysis of Cu-poor Cu(In,Ga)Se 2 cells fabricated on polyimide substrates: Effect of Na content on microstructure and phase structure

This work reports the Raman scattering surface and in-depth resolved analysis of Cu-poor Cu(In,Ga)Se 2 (CIGS) grown on polyimide substrates. In order to study the effect of Na on the formation and microstructure of the CIGS and the corresponding Cu-poor ordered vacancy compound (OVC) phases, a NaF precursor layer with different thicknesses was deposited on the Mo-coated substrates before growing of the samples. The Raman spectroscopy data are correlated with the analysis of the samples by Auger electron spectroscopy and scanning electron microscopy. These data corroborate the significant role of Na on the inhibition of Ga–In interdiffusion and on the formation of the MoSe 2 interfacial phase at the back region of the layers. Presence of Na also leads to an enhancement in the formation of the chalcopyrite CIGS phase and a decrease in the occurrence of the dominant OVC phase at the surface region. This study confirms the strong dependence of the microstructure and phase distribution in CIGS absorber layers on the Na available during their growth.

In-depth resolved Raman scattering analysis of secondary phases in Cu-poor CuInSe2 based thin films

Raman scattering analysis of Cu-poor CuInSe2 layers shows the coexistence of the ordered vacancy compound (OVC), CuAu–CuInSe2 and chalcopyrite (CH) CuInSe2 phases as function of the Cu/In content ratio x. In-depth resolved measurements from layers with x≤0.57 show a strong inhibition in the relative intensity of the CH-CuInSe2 mode at the back region. Micro-Raman spectra directly measured at different regions from the layers with 0.66≤x≤0.71 also suggest a higher content of the OVC phase at this back region. These data suggest an enhancement in the formation of OVC at this region in the layers.

Epitaxial Growth and Characterization of the Ordered Vacancy Compound CuIn3Se5 on GaAs (100) Fabricated by Molecular Beam Epitaxy

ABSTRACTEpitaxial growth of the ordered vacancy compound (OVC) CuIn3Se5 has been achieved on GaAs (100) by molecular beam epitaxy (MBE) from Cu2Se and In2Se3 sources. Electron probe microanalysis and X-ray diffraction have confirmed the composition for the 1-3-5 OVC phase and that the film is single crystal Culn3Se5 (100). Transmission electron microscopy (TEM) characterization of the material also showed it to be single crystalline. Structural defects in the layer consisted mainly of stacking faults. Photoluminescence (PL) measurements performed at 7.5 K indicate that the bandgap is 1.28 eV. Raman spectra reveal a strong polarized peak at 152 cm−1, which is believed to arise from the totally symmetric vibration of the Se atoms in the lattice. Atomic force microscopy reveals faceting in a preferred (100) orientation.