Ordered Vacancy Compound Research Articles

Effects of heavy alkali metals (Rb, Cs) post-deposition treatments on CuInGaSe2 using a spin-coating technique

Abstract This study used a controlled environment to explore the post-deposition treatment (PDT) effects on CuInGaSe2 (CIGSe2) semiconducting thin films using a non-vacuum spin-coating technique for doping the CIGSe layers with Cs and Rb. The structural characterization confirmed the successful deposition of chalcopyrite structures with no phases belonging to any alkali metals after the PDT, with crystallite sizes in the range between 40-67 nm, and with a slight change in the X Ray Diffraction (XRD) peak positions indicating a change from copper-rich to copper-poor phases. The morphological studies confirmed the increase in grain sizes after the PDT. The EDS chemical studies showed that there is a reduction in the copper content after PDT. The topographical studies showed a change in the surface morphology with modifications of the grain parameters. In addition, the electrical characterization showed a significant increase of the effective carrier mobility after the treatments, consistent with the grain size increase observed by both microscopic (SEM and AFM) studies. Raman characterization of the CIGSe2 films showed the A1 optical phonon mode of CIGS chalcopyrite structures and peaks at lower frequencies belonging to ordered vacancy compounds (OVCs). The deconvolution of the Raman spectroscopy broad peaks for the CIGSe2 films after their PDT confirmed the formation of alk-InSe2 OVC phases on top of the absorber layer.

Effect of Ordered Vacancy Compounds on the Carrier Collection in Wide‐Gap (Ag,Cu)(In,Ga)Se2 Solar Cells

This contribution studies the effect of ordered vacancy compounds (OVCs) on the minority carrier collection in wide‐gap (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells. For this purpose, three samples with different ([Ag]+[Cu])/([In]+[Ga]) (I/III) values were processed: 1) a very off‐stoichiometric absorber (I/III = 0.31), consisting of isolated chalcopyrite patches embedded in a major OVC bulk phase, 2) a moderately off‐stoichiometric absorber (I/III = 0.77) with OVC patches at the front and back interfaces and 3) a close‐stoichiometric absorber (I/III = 0.94) with only very few, isolated OVC patches. For each of these samples, synchrotron‐based X‐ray fluorescence (XRF) was measured on a nanoscale (55 nm resolution), while simultaneously recording the X‐ray beam induced current (XBIC). The results, complemented by transmission electron microscopy and electron‐beam induced current analyses, clearly indicate that the presence of the OVC phase reduces the carrier collection and thus the short‐circuit current density of off‐stoichiometric wide‐gap ACIGS solar cells.

Effect of vacancies on structural, electronic, elastic, and thermoelectric properties of CdIn[formula omitted]Se[formula omitted] defect chalcopyrite-type semiconductor: An ab-initio approach

In this work, we have investigated the detailed structural, electronic, elastic, mechanical and thermoelectric properties of pure (CdInSe2) and defect (CdIn2Se4) chalcopyrite-type semiconductors using the ab-initio approach within the density functional theory along with semi-classical Boltzmann transport theory. The structural properties reveal the pure structure is more distorted than the defect one. The electronic band structure infers that the pure structure is an n-type semiconductor while the defect one is an intrinsic semiconductor having a larger bandgap as compared to the former due to the cationic vacancy. Both compounds are direct bandgap semiconductors. From the calculation of elastic stiffness constants and elastic moduli, it is found that both the compounds are mechanically stable. The melting temperature of CdInSe2 and CdIn2Se4 are obtained as 831 K and 784 K, respectively. The electronic thermoelectric properties show a higher value of ZT = 0.86 for the ordered vacancy compound (CdIn2Se4) with a fixed hole concentration of 1020 cm−3 at 800K making it competent for the thermoelectric applications. The variation of various thermoelectric coefficients w.r.t. the chemical potential shows that in CdIn2Se4, the p-type doping is favorable to further enhance the thermoelectric behavior. The lattice thermal conductivity for the defect structure is found to be 3.78 W/mK at room temperature and a lowest value of 1.421 W/mK at 800K. The results demonstrate the defect CdIn2Se4 as a promising thermoelectric material.

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.

Degradation behavior of CIGS solar Cells: A parametric analysis

A parametric study of the effect of bulk and interface properties on device characteristics as a function of heat and light stress under open-circuit and short-circuit conditions is developed for Cu(In,Ga)(S,Se)2 solar cells using SCAPS-1D simulator. The variables and interdependencies modeled include: (1) conduction band offset (CBO) between buffer and absorber; (2) buffer ionized donor density; (3) CIGS shallow acceptor density; (4) CIGS deep acceptor density; (5) CIGS shallow donor density; (6) ionized acceptor concentration in the ordered vacancy compound (OVC) CIGS/buffer interface; and (7) back contact work-function. An increase in absorber shallow acceptor or a decrease in shallow donor density results in an increase in net carrier density and open-circuit voltage (VOC). A decrease in VOC is observed with an increase in the deep acceptors or neutral midgap defects in the CIGS due to higher charge carrier recombination. A change of CBO from spike to cliff condition is a dominant mechanism of decreasing VOC with an interface defect density of 1012 cm−2. The minimum depletion width is found to be specifically sensitive to CBO and interface defect density. An inflection in the capacitance–voltage curves (in reverse bias) is observed with simultaneous increase in bulk deep acceptor density and shallow donor density near the back contact.

Nanoscale Surface Analysis Reveals Origins of Enhanced Interface Passivation in RbF Post Deposition Treated CIGSe Solar Cells

AbstractAlkali post deposition treatments (PDTs) of Cu(In,Ga)Se2 absorbers have boosted the power conversion efficiency (PCE) of the solar cell devices in the last years. A detailed model explaining how the PDTs impact the optoelectronic properties at the nanoscale is still lacking. Here, via various scanning probe techniques, X‐Ray photo‐electron spectroscopy and high resolution secondary ion mass spectroscopy it is shown that the RbF PDT treatments lead to a one to one exchange of Rb with Cu at the surface. This exchange takes place in the naturally occurring Cu‐depleted CIGSe surface, known as the ordered vacancy compound. A detailed comparison between samples with different PDTs after various cleaning procedures furthermore highlights the necessity to perform all the measurements on NH4OH cleaning surfaces only. After NH4OH, no RbInSe2 phase could be detected at the surface anymore and the surface bandgap, as measured with scanning tunneling spectroscopy is only 1.7 eV. The findings demonstrate that the existence of a RbInSe2 phase is most likely not responsible for the recent improvements in power conversion efficiency for state of the art solar cells. The primary effect of the PDT treatment is a modification of the ordered vacancy compound, where Cu is exchanged with Rb.

Optical absorption of defect chalcopyrite and defect stannite ZnGa2Se4 under high pressure

Optical absorption measurements at high pressure have been performed in two phases of the ordered-vacancy compound (OVC) ZnGa2Se4: defect stannite (DS) and defect chalcopyrite (DC). The direct bandgap energy of both phases exhibits a non-linear pressure dependence with a blueshift up to 10 GPa and a redshift at higher pressures. We discuss the different behavior of both phases in these two pressure ranges in relation to the pressure-induced order-disorder processes taking place at cation sites. Measurements performed in both phases on downstroke after increasing pressure to 22 GPa show that the direct bandgap energy of the recovered samples at room pressure was 0.35 eV smaller than that of the original samples. These results evidence that different disordered phases are formed on decreasing pressure, depending on the cation disorder already present in the original samples. In particular, we attribute the recovered samples from the original DC and DS phases to disordered CuAu (DCA) and disordered zincblende (DZ) phases, respectively. The decrease of the direct bandgap energy and its pressure coefficient on increasing disorder in the four measured phases are explained. In summary, this combined experimental and theoretical work on two phases (DC and DS) of the same compound has allowed us to show that the optical properties of both phases show a similar behavior under compression because irreversible pressure-induced order-disorder processes occur in all adamantine OVCs irrespective of the initial crystalline structure.

Systematics of phase transitions induced by hydrostatic pressure in AIIBIII2 CVI4 ordered-vacancy compounds

Investigation of phase transitions related to changes in the crystalline structure of solid state materials represents an extremely important area of condensed mater physics, since they lead to reconfiguration of all the solid state physical properties, including electrical, optical, vibrational, elastic and mechanical properties. Hydrostatic pressure is one of the basic factors leading to the realization of phase transitions. Gaining knowledge of regularities and peculiarities of pressure induced phase transitions is important both from the fundamental and applicative point of view. In spite of multilateral investigation of this subject in elemental, binary and some ternary compounds, less attention has been paid to ordered-vacancy compounds. At the same time, possibilities to control the stoichiometric defects by applying hydrostatic pressure open new opportunities for defect engineering in such compounds. This review paper summarizes the results of investigations of phase transitions in ternary AIIBIII 2CVI 4 compounds, which represent a subclass of ordered-vacancy compounds. Investigations were performed by Raman spectroscopy, X-ray diffraction, and optical absorption spectroscopy in crystals with initial crystallographic structure from three categories, including tetragonal, spinel, and rhombohedral layered structures. The dynamics of pressure induced changes is described. The regularities and the systematics of phase transitions in this class of semiconductor materials are revealed.

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.

Effect of Annealing Process on the Structural, Morphological, Optical and Electrical Properties of the Ordered Vacancy Compound CuIn3Se5 Thin Films

Effect of Annealing Process on the Structural, Morphological, Optical and Electrical Properties of the Ordered Vacancy Compound CuIn3Se5 Thin Films

Evaluation of defect formation in chalcopyrite compounds under Cu-poor conditions by advanced structural and vibrational analyses

Cu-poor compositions are key for obtaining high efficiency Cu(In,Ga)Se2 (CIGSe) devices. These conditions lead to Cu deficit accommodation mechanisms like the formation of Cu-related defects in the CIGSe crystal structure or of ordered vacancy compound. However, the origin of the benefits of Cu-poor compositions is still under discussion since these mechanisms are difficult to detect and characterize. In this work, a high precision Raman spectroscopy and X-ray diffraction analysis on a compositionally-graded CISe sample ([Cu]/[In] ratios from 0.48 to 1.03) allows us to shed light on this topic by reporting the detection of a “defective chalcopyrite phase” in the slightly Cu-poor compositional regime that may play a critical role, rather than other previously discussed mechanisms, on device performance. This phase is mainly characterized by the formation of a specific defect in the CISe structure which also contributes to appearance of a Raman peak at 230 cm−1. Finally, the analysis of high efficiency (> 18%) CIGSe solar cells and the extrapolation of the results on defect formation obtained for CISe, suggest a possible impact of the defective chalcopyrite phase on the open circuit voltage of the devices, and opens a possible way to further investigate and develop the chalcopyrite based technologies.

A perspective on ordered vacancy compound and parent chalcopyrite thin film absorbers for photoelectrochemical water splitting

Chalcopyrites could fill the gap between the low-cost, poor-efficiency single junction metal oxide photoelectrochemical (PEC) water splitting cells and the high efficiency, yet costly III–V tandems. In this Perspective, we first review the key barriers that must be addressed by the community to enable economical chalcopyrite-based PEC water splitting. Then, we highlight how theoretical modeling can be used to identify promising ordered vacancy compound absorbers with improved energetics compared to their chalcopyrite parents. Finally, we describe how advanced spectroscopic analysis performed on chalcopyrite photocathodes after PEC testing uncovered a new passivation layer candidate for prolonged durability.

Performance Limitations of Wide‐Gap (Ag,Cu)(In,Ga)Se2 Thin‐Film Solar Cells

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells with bandgaps (Eg) > 1.40 eV is studied on a large sample set. It is confirmed that moving away in composition from ternary AgGaSe2 by simultaneous reduction in Ga and Ag content widens the chalcopyrite single‐phase region and thereby reduces the amount of ordered vacancy compounds (OVCs). As a consequence, a distortion in current−voltage characteristics, ascribed to OVCs at the back contact, can be successfully avoided. A clear anticorrelation between open‐circuit voltage (VOC) and short‐circuit current density (JSC) is detected with varying absorber stoichiometry, showing decreasing VOC and increasing JSC values for [I]/[III] > 0.9. Capacitance profiling reveals that the absorber doping gradually decreases toward stoichiometric composition, eventually leading to complete depletion. It is observed that only such fully depleted samples exhibit perfect carrier collection, evidencing a very low diffusion length in wide‐gap ACIGS films. The results indicate that OVCs at the surface play a minor or passive role for device performance. Finally, a solar cell with VOC = 0.916 V at Eg = 1.46 eV is measured, which is, to the best of our knowledge, the highest value reported for this bandgap to date.

Pressure-induced band anticrossing in two adamantine ordered-vacancy compounds: CdGa2S4 and HgGa2S4

This paper reports a joint experimental and theoretical study of the electronic band structure of two ordered-vacancy compounds with defect-chalcopyrite structure: CdGa2S4 and HgGa2S4. High-pressure optical-absorption experiments (up to around 17 GPa) combined with first-principles electronic band-structure calculations provide compelling evidence of strong nonlinear pressure dependence of the bandgap in both compounds. The nonlinear pressure dependence is well accounted for by the band anticrossing model that was previously established mostly for selenides with defect chalcopyrite structure. Therefore, our results on two sulfides with defect chalcopyrite structure under compression provide definitive evidence that the nonlinear pressure dependence of the direct bandgap is a common feature of adamantine ordered-vacancy compounds and does not depend on the type of anion.

Synthesis and Crystal Structure of the Ordered Vacancy Compound Cu3In5Se9

This work focuses on the preparation and structural characterization of the semiconductor Cu3In5Se9, an important member of ordered vacancy compounds family, belonging to the semiconductor system I3-III5--VI9, where denotes the cation vacancy which is included in the formula to maintain the same number of cations and anions sites. This material was synthesized by the Bridgman-Stockbarger technique, and its structure was refined from powder X-ray diffraction data using the Rietveld method. Cu3In5Se9 crystallizes with tetragonal symmetry in the space group P2c (Nº 112), with a = 5.7657(1) Å, c = 11.5353(4) Å, V = 383.47(2) Å3. This ternary compound consists of a three-dimensional arrangement of distorted CuSe4 and InSe4 tetrahedral connected by common faces. In this structure, each Se atom is coordinated by four cations located at the corners of a slightly distorted tetrahedron, and each cation is tetrahedrally bonded to four anions. Cu3In5Se9 is related to the p-type CuInSe2 and n-type CuIn3Se5 semiconductor compounds, which are being used in the preparation of high-efficiency solar cells. DO: http://dx.doi.org/10.17807/orbital.v13i3.1560

Origin of the Improved Performance of Cu(In,Ga)(S,Se)2 Solar Cells by Postdeposition Treatments: Effect of Band Offsets

Postdeposition treatment (PDT) with alkali metals has profoundly improved the performance of $\mathrm{Cu}(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})(\mathrm{S},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Se}{)}_{2}$ solar cells. Several mechanisms have been proposed to explain the improved performance, but the exact origin is still under debate. Here, using first-principles calculations, we demonstrate that alkali metals tend to accumulate at the interface, which significantly improves the band offset between the absorber and the alkali-metal-doped ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga}){\mathrm{Se}}_{2}$ buffer layer. Our results show that, as the fraction of $\mathrm{Na}$ increases, the valence-band maximum (VBM) of the alloy layer decreases, while the conduction-band maximum (CBM) increases. The drop of the VBM introduces a hole barrier, repelling holes away from the interface, and a moderately higher CBM forms a benign spikelike conduction-band offset (CBO) against the absorber layer independent of the $\mathrm{Ga}$ concentration. Both effects are beneficial for the solar-cell efficiency and exhibit extra advantages over the ordered vacancy compound (OVC), which commonly exists before the PDT, because, although the OVC can introduce the hole barrier, it also introduces a detrimental clifflike CBO, especially at a high $\mathrm{Ga}$ concentration. Such understandings of band offsets introduced by PDT can further be extended to other alkali metals, such as $\mathrm{K}$, $\mathrm{Rb}$, and $\mathrm{Cs}$, and the design principles can also be extended to other types of thin-film solar cells.

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.

False metals, real insulators, and degenerate gapped metals

This paper deals with a significant family of compounds predicted by simplistic electronic structure theory to be metals but are, in fact, insulators. This false metallic state has been traditionally attributed in the literature to reflect the absence of proper treatment of electron-electron correlation (“Mott insulators”) whereas, in fact, even mean-field like density functional theory describes the insulating phase correctly if the restrictions posed on the simplistic theory are avoided. Such unwarranted restrictions included different forms of disallowing symmetry breaking described in this article. As the science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an “internal band gap” between the principal band edges. The significance of this electronic configuration is that it might be unstable toward the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator. The analogous process also exists for degenerate but gapped metals with the Fermi level inside the valence band, where the energy gain is defined by transfer of electrons from the donor level to the unoccupied part of the valence band. We focus here on the fact that numerous electronic structure methodologies have overlooked some physical factors that could stabilize the insulating alternative, predicting instead false metals that do not really exist (note that this is in general not a physical phase transition, but a correction of a previous error in theory that led to a false prediction of a metal). Such errors include: (i) ignoring spin symmetry breaking, such as disallowing magnetic spin ordering in CuBi2O4 or disallowing the formation of polymorphous spin networks in paramagnetic LaTiO3 and YTiO3; (ii) ignoring structural symmetry breaking, e.g., not enabling energy-lowering bond disproportionation (Li-doped TiO2, SrBiO3, and rare-earth nickelates), or not exploring pseudo-Jahn–Teller-like distortions in LaMnO3, or disallowing spontaneous formation of ordered vacancy compounds in Ba4As3 and Ag3Al22O34; and (iii) ignoring spin–orbit coupling forcing false metallic states in CaIrO3 and Sr2IrO4. The distinction between false metals vs real insulators is important because (a) predicting theoretically that a given compound is metal even though it is found to be an insulator often creates the temptation to invoke high order novel physical effects (such as correlation in d-electron Mott insulators) to explain what was in effect caused by a more mundane artifact in a lower-level mean-field band theory, (b) recent prediction of exotic physical effects such as topological semimetals were unfortunately based on the above compounds that were misconstrued by theory to be metal, but are now recognized to be stable insulators not hosting exotic effects, and (c) practical technological applications based on stable degenerate but gapped metals such as transparent conductors or electrides for catalysis must rely on the systematically correct and reliable theoretical classification of metals vs insulators.