Sub-bandgap States Research Articles

Continuous tuning of persistent luminescence wavelength by intermediate-phase engineering in inorganic crystals

Multicolor tuning of persistent luminescence has been extensively studied by deliberately integrating various luminescent units, known as activators or chromophores, into certain host compounds. However, it remains a formidable challenge to fine-tune the persistent luminescence spectra either in organic materials, such as small molecules, polymers, metal-organic complexes and carbon dots, or in doped inorganic crystals. Herein, we present a strategy to delicately control the persistent luminescence wavelength by engineering sub-bandgap donor-acceptor states in a series of single-phase Ca(Sr)ZnOS crystals. The persistent luminescence emission peak can be quasi-linearly tuned across a broad wavelength range (500–630 nm) as a function of Sr/Ca ratio, achieving a precision down to ~5 nm. Theoretical calculations reveal that the persistent luminescence wavelength fine-tuning stems from constantly lowered donor levels accompanying the modified band structure by Sr alloying. Besides, our experimental results show that these crystals exhibit a high initial luminance of 5.36 cd m−2 at 5 sec after charging and a maximum persistent luminescence duration of 6 h. The superior, color-tunable persistent luminescence enables a rapid, programable patterning technique for high-throughput optical encryption.

Lead-Free Cs2AgBiCl6 Double Perovskite: Experimental and Theoretical Insights into the Self-Trapping for Optoelectronic Applications.

Lead-free double perovskites (DPs) will emerge as viable and environmentally safe substitutes for Pb-halide perovskites, demonstrating stability and nontoxicity if their optoelectronic property is greatly improved. Doping has been experimentally validated as a powerful tool for enhancing optoelectronic properties and concurrently reducing the defect state density in DP materials. Fundamental understanding of the optical properties of DPs, particularly the self-trapped exciton (STEs) dynamics, plays a critical role in a range of optoelectronic applications. Our study investigates how Fe doping influences the structural and optical properties of Cs2AgBiCl6 DPs by understanding their STEs dynamics, which is currently lacking in the literature. A combined experimental-computational approach is employed to investigate the optoelectronic properties of pure and doped Cs2AgBiCl6 (Fe-Cs2AgBiCl6) perovskites. Successful incorporation of Fe3+ ions is confirmed by X-ray diffraction and Raman spectroscopy. Moreover, the Fe-Cs2AgBiCl6 DPs exhibit strong absorption from below 400 nm up to 700 nm, indicating sub-band gap state transitions originating from surface defects. Photoluminescence (PL) analysis demonstrates a significant enhancement in the PL intensity, attributed to an increased radiative recombination rate and higher STE density. The radiative kinetics and average lifetime are investigated by the time-resolved PL (TRPL) method; in addition, temperature-dependent PL measurements provide valuable insights into activation energy and exciton-phonon coupling strength. Our findings will not only deepen our understanding of charge carrier dynamics associated with STEs but also pave the way for the design of some promising perovskite materials for use in optoelectronics and photocatalysis.

How Effective Are Sub-Bandgap States in AgInS2 Quantum Dots for Electron Transfer?

How Effective Are Sub-Bandgap States in AgInS₂ Quantum Dots for Electron Transfer?

Plasmons Enhancing Sub-Bandgap Photoconductivity in TiO2 Nanoparticles Film.

The coupling between sub-bandgap defect states and surface plasmon resonances in Au nanoparticles and its effects on the photoconductivity performance of TiO2 are investigated in both the ultraviolet (UV) and visible spectrum. Incorporating a 2 nm gold nanoparticle layer in the photodetector device architecture creates additional trapping pathways, resulting in a faster current decay under UV illumination and a significant enhancement in the visible photocurrent of TiO2, with an 8-fold enhancement of the defects-related photocurrent. We show that hot electron injection (HEI) and plasmonic resonance energy transfer (PRET) jointly contribute to the observed photoconductivity enhancement. In addition to shedding light on the below-band-edge photoconductivity of TiO2, our work provides insight into new methods to probe and examine the surface defects of metal oxide semiconductors using plasmonic resonances.

Visible-light photocatalytic activity of rare-earth-metal-doped TiO2: Experimental analysis and machine learning for virtual design

Modification with rare-earth (RE) metals has emerged as an important approach for enhancing the properties of titanium dioxide. Herein, the visible-light photocatalytic activity of RE-TiO2 (RE: Tm, Er, Nd, Dy, Lu, La, Ho, Pr, Tb, Sc, Ce, Yb, Sm, Gd, Y, or Eu) was investigated using a combination of computer simulations and experimental techniques. The RE-TiO2 photocatalysts were characterised using advanced experimental techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, luminescence, diffuse reflectance spectroscopy, and specific surface area measurements. Furthermore, a quantitative structure–activity relationship analysis was conducted by applying a partial least-squares machine learning algorithm. The Ho-TiO2 sample exhibited the highest photocatalytic activity under visible-light irradiation. Notably, the observed activity was not due to an up-conversion process, originating instead from slight bandgap narrowing caused by new sub-bandgap states from the RE 4 f levels.

Fractal microstructure of semiconducting Sb2S3 by surfactant-assisted spin coating and its advantage in optoelectronic properties

Antimony sulfide, Sb2S3 is a mixed band gap semiconductor having a great potential in photovoltaic applications. Its thin films and nanostructures are also a proven anode material in batteries and supercapacitors. This paper presents the synthesis of a unique fern-shaped fractal microstructure of Sb2S3 grown over bare glass substrate using a spin-coating assisted by Triton-X-100 surfactant. The FE-SEM images confirm the fractal formation spanning over hundreds of micrometers. The film exhibits a direct band gap of 1.62 eV. The photoluminescence spectra reveal a strong emission peak at 800 nm (1.55 eV) indicating radiative transitions from sub-band gap states. The surfactant, Triton-X-100, is found to be primarily responsible for forming the ‘Fern’ shaped fractals which have a fractal dimension of 1.84. Without the surfactant, a porous microstructure of Sb2S3 film is observed. The fractality is also associated with lower strain in the film leading to a reduced point defects which would otherwise act as potential recombination centers.

Direct laser printing of 3D optical imaging based on full-spectrum solar-absorption-enhanced perovskite-type oxides

The exploration of the solar light absorption by a material is of great use in photonics and optoelectronics. However, the fully exploiting of the solar spectrum of well-known wide-bandgap perovskite oxides (2.7–5 eV), remains very challenging. We show that the selective occupation of Bi3+ in the octahedrally coordinated Ba2+ is responsible for creating sub-bandgap states (as low as 0.6 eV), nearly 9 times lower than pristine bandgap (5.2 eV). We reveal that the role of the 6s6p excited state of Bi3+ is as a bridge to promote the dipole-allowed band-band transition for strong absorption. Furthermore, we demonstrate the double-sided laser printing of three-dimensional optical imaging based on the photothermal effect, enabling write-once-read-many information storage with ultrahigh stability and good data retention. Our results make Ba3–xGa2O6:xBi3+ a promising candidate for photovoltaics, ferroelectrics, photocatalysis, broadband photodetectors as well as other photonic and optoelectronic applications.

Dual charge-accepting engineering modified AgIn5S8/CdS quantum dots for efficient photocatalytic hydrogen evolution overall H2S splitting

Low charge carrier separation and transfer efficiency continues to be the major obstacle to harvest solar energy to split hydrogen sulfide (H2S) as hydrogen source for H2 production with value-added utilization for waste by-products. Herein, we designed a Type-II core/shell AgIn5S8/CdS (AIS/CdS) quantum dots (QDs) photocatalyst capped with short-chain inorganic sulfide ion (S2−) ligand with dual charge-accepting engineering to promote charge carrier extraction and faster photogenerated electron transfer. Transient absorption spectroscopy analysis demonstrates that the excited electrons are fast injected into CdS shell from AIS core within 1.7 ps, instead of transferring to sub-bandgap states. Consequently, the highest photocatalytic hydrogen evolution rate of AIS/CdS QDs with S2- ligand is 12.74 mmol g−1 h−1 that is more than four times of pristine AIS core. This work offers insightful guidance on the rational design of charge-accepting engineering QDs-based photocatalysts, thereby stimulating solar to hydrogen generation and resource utilization of pollutants.

Conversion of Classical Light Emission from a Nanoparticle-Strained WSe2 Monolayer into Quantum Light Emission via Electron Beam Irradiation.

Solid-state single photon emitters (SPEs) within atomically thin transition metal dichalcogenides (TMDs) have recently attracted interest as scalable quantum light sources for quantum photonic technologies. Among TMDs, WSe2 monolayers (MLs) are promising for the deterministic fabrication and engineering of SPEs using local strain fields. The ability to reliably produce isolatable SPEs in WSe2 is currently impeded by the presence of numerous spectrally overlapping states that occur at strained locations. Here nanoparticle (NP) arrays with precisely defined positions and sizes are employed to deterministically create strain fields in WSe2 MLs, thus enabling the systematic investigation and control of SPE formation. Using this platform, electron beam irradiation at NP-strained locations transforms spectrally overlapped sub-bandgap emission states into isolatable, anti-bunched quantum emitters. The dependence of the emission spectra of WSe2 MLs as a function of strain magnitude and exposure time to electron beam irradiation is quantified and provides insight into the mechanism for SPE production. Excitons selectively funnel through strongly coupled sub-bandgap states introduced by electron beam irradiation, which suppresses spectrally overlapping emission pathways and leads to measurable anti-bunched behavior. The findings provide a strategy to generate isolatable SPEs in 2D materials with a well-defined energy range.

Real-time change of optical losses in chalcogenide waveguides induced by light illumination.

We prepared several GeGaSe waveguides with different chemical compositions and measured the change of optical losses induced by light illumination. Together with some experimental data in As2S3 and GeAsSe waveguides, the results showed that maximum change of the optical loss can be observed in the waveguides under bandgap light illumination. The chalcogenide waveguides with close to stoichiometric compositions have less homopolar bonds and less sub-bandgap states, and thus are preferential to have less photoinduced losses.

Covalently interconnected layers in g-C3N4: Toward high mechanical stability, catalytic efficiency and sustainability

The grim prospects for the industrial utilization of g-C3N4 nanosheets arise from multi-step processing resulting in low material yields and poor visible light response due to quantum confinement. Herein, we introduce a strategy for linking the adjacent layers of g-C3N4 covalently to realize a high surface area without excess mass loss in a one-step process by introducing diethylene glycol as a precursor that produces -(CH2)2-O-(CH2)2- linkers in-situ. Their presence increases interlayer spacing and introduces surface curvatures, discouraging the stacking of a larger number of layers to produce nanosheets with ∼3 times higher surface area. Interestingly, unlike other layered materials, the linkers also provide extraordinary mechanical stability against exfoliating forces. In addition, the process instills sub-bandgap states and a considerable visible light response at 500 nm to slow down the picosecond exciton recombination dynamics, resulting in ∼5 times enhancement in H2 generation efficiency from photocatalytic water-splitting over the bulk sample.

Alternate-Target Layer-By-Layer Pulsed Laser Deposition of Epitaxial BiVO4 Thin Films

Bismuth vanadate (BiVO4) has emerged as one of the highest performing metal oxide photoelectrodes for solar energy-to-fuels applications [1, 2]. This achievement has been largely attributed to the development of high-quality synthesis techniques. Specifically, epitaxial synthetic routes to producing lattice-matched, near single-crystalline quality BiVO4 with minimal extrinsic defects have been essential to understand the fundamental and PEC properties of BiVO4 that are inaccessible by conventional bulk synthesis techniques. In addition, epitaxial synthesis may also be employed as a strategy to further improve the PEC properties of BiVO4 through, for instance, altering the band structure and enhancing the carrier dynamics by strain engineering. Hence, the ability to produce high-quality single-phase epitaxial BiVO4 films is desirable.To date, molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and pulsed laser deposition (PLD) [3] have been used to fabricate epitaxial BiVO4 thin films, with PLD having the advantage of relatively simple experimental setup and material versatility. However, an oft-noted drawback in conventional PLD using single oxide targets is off-stoichiometry in the deposited films; this is especially so for BiVO4 which requires a Bi-rich compound target to achieve a stoichiometric BiVO4 film [4]. This problem is also expected in general for multinary oxides composed of elements with very different ablation properties. To alleviate the limitations of conventional PLD, we employ for the first time alternate-target layer-by-layer PLD as a more elegant approach to produce high quality epitaxial BiVO4 films. Briefly, constituent oxide targets are alternately ablated by an excimer laser (λ= 248 nm) to build the desired film one unit cell thick at a time. Stoichiometry is controlled by the number of shots corresponding to a specified laser fluence for the constituent oxide target.To demonstrate the method, we deposited epitaxial BiVO4 films onto (001)-oriented yttrium-stabilized zirconia (YSZ) using 4N-pure Bi2O3 and V2O5 targets. Films grown are single phase BiVO4 as shown by x-ray diffraction. Out-of-plane diffraction peaks indexed to BiVO4(00l) suggests epitaxy of the film onto the substrate; reciprocal space maps of the asymmetric BiVO4(208) and YSZ(204) peaks further confirm the BiVO4(001) || YSZ(001) epitaxial relationship (top figure). Rocking curves of the BiVO4 (004) peak (FWHM ~ 0.015-0.041) indicate high crystalline perfection of the BiVO4 film, almost approaching that of the YSZ substrate (FWHM ~ 0.010). Thickness dependent-rocking curve studies reveal that BiVO4 films are strained to the substrate for film thicknesses under 22 nm; above this critical thickness, film relaxation ensues. In turn, the resulting optoelectronic properties of the BiVO4 film is dictated by its relaxation state. The optical band gap narrows with film relaxation as observed with spectroscopic ellipsometry. Moreover, steady-state photoluminescence emission spectroscopy reveals a sub-bandgap state (A 2, bottom figure) associated with strained BiVO4 films, on top of a state consistent with band-to-band recombination (B1 ). A higher energy sub-bandgap state (A 1, bottom figure) develops as the film relaxes. The implications of the relaxation state on the charge carrier dynamics and photoelectrochemical properties of BiVO4 will be discussed. Figure: (top) reciprocal space maps of the asymmetric BiVO4(208) and YSZ(204) peaks; (bottom) photoluminescence emission spectra of strained and relaxed BiVO4 films.

Probing the Density of States of Organic and Perovskite Semiconductors By Energy-Resolved Electrochemical Impedance Spectroscopy

The performance of thin-film photovoltaic cells is closely related to the energy alignment of the absorber layers and charge collection layers used for constructing the device and is further influenced by the presence of energetic disorder and defects in the photoactive material. This especially holds for novel organic and perovskite semiconductors, where film processing conditions, such as solvents used, thermal or solvent vapor annealing, and aging of the active materials are known to have an influence on the device performance. Knowledge about the energy of the relevant frontier orbitals such as HOMO and LUMO for organic semiconductors,[1]and valence (VB) and conduction (CB) bands for perovskites is key to designing more efficient materials. In addition, the energetic disorder, such as broadening of the band edges and states that appear in the bandgap are strongly affecting solar cell efficiencies. Knowledge about the density of states close to and inside the bandgap is very relevant in this respect.In this work energy-resolved electrochemical impedance spectroscopy (ER-EIS) as advanced by Nádazdyet al.[2]is applied on various organic bulk-heterojunction and lead-halide perovskite semiconductors to map their energy landscape around and in the band gap. ER-EIS provides a direct electrochemical measurement of the HOMO/VB and LUMO/CB energies, but also resolves band tails and sub-bandgap states with a resolution of up to six orders of magnitude. This enables detection of subtle but important effects that are the consequence of layer processing conditions such as spin coat parameters, solvent combinations, thermal annealing time and temperature, and atmospheric exposure (vacuum, inert, air).We will present experiments that aim to quantify relative changes in the energy landscape due to varying processing conditions and assign physical meaning to these changes (e.g. morphology and (un)intentional doping). The systems under investigation are well-known donor-acceptor bulk-heterojunction (e.g., P3HT:PCBM and PM6:Y6) and triple-cation mixed-halide perovskites (CsFAMA). The results shed new light on largely unknown defect states in the bandgap of solution-processed semiconductors.[1] R. E. M. Willems, C. H. L. Weijtens, X. de Vries, R. Coehoorn, and R. A. J. Janssen, Relating frontier orbital energies from voltammetry and photoelectron spectroscopy to the open-circuit voltage of organic solar cells, Adv. Energy Mater. 2019, 9, 1803677.[2] V. Nádazdy, F. Schauer, and K. Gmucová, Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors, Appl. Phys. Lett. 2014, 105, 142109.

Exploration of sub-bandgap states in 2D halide perovskite single-crystal photodetector

Greater stability of low-dimensional halide perovskites as opposed to their three-dimensional counterparts, alongside their high extinction coefficient and thus excellent emission properties, have made them popular candidates for optoelectronic applications. Topological edges are found in two-dimensional perovskites that show distinct electronic properties. In this work, using Kelvin Probe Force Microscopy, performed on butylammonium lead bromide (BA2PbBr4) single crystals with optical bandgap of ~413 nm, we elucidate the electronic response of the edges and their potential impact on photodetector devices. We show that the charge-carriers are accumulated at the edges, increasing with the edge height. Wavelength-dependent surface photovoltage (SPV) measurements reveal that multiple sub-bandgap states exist in BA2PbBr4. As the edge height increases, the SPV amplitude at the edges reduces slightly more as compared to the adjacent regions, known as terraces, indicating relatively less reduction in band-bending at the surface due possibly to increased de-population of electrons from sub-bandgap states in the upper bandgap half. The existence of sub-bandgap states is further confirmed by the observation of below-bandgap emission (absorption) peaks characterised by spectral photoluminescence and photothermal deflection spectroscopy measurements. Finally, we fabricated a photodetector using a millimetre size BA2PbBr4 single crystal. Noticeable broadband photodetection response was observed in the sub-bandgap regions under green and red illumination, which is attributed to the existence of sub-bandgap states. Our observations suggest edge-height dependence of charge-carrier behaviour in BA2PbBr4 single crystals, a potential pathway that can be exploited for efficient broadband photodetector fabrication.

Unraveling the Origin of Dark Current in Organic Bulk Heterojunction Photodiodes for Achieving High Near-Infrared Detectivity

Organic infrared materials are attractive due to their high absorption coefficients and facile tuning of the absorption spectrum beyond that of silicon. Although bulk heterojunctions (BHJs) are widely used in organic photodetectors (OPDs), achieving a high signal-to-noise ratio in the near-infrared spectrum is often challenging due to the inherently large dark currents in low-bandgap polymers and their blending morphology effect. Herein, we investigate the origin of dark currents in near-infrared OPDs and reveal a strategy to improve the detectivity in terms of two aspects: (1) donor–acceptor blending morphology and (2) effects of carrier injection from outer electrodes. To do so, a series of random terpolymers were synthesized in a similar band structure and their blending morphology with the PC71BM acceptor was gradually controlled. As the phase separation was smaller, the dark current gradually reduced while the responsivity increased, leading to a higher detectivity. Complex charge-transporting paths formed under the fine percolating network account for the dark current reduction, while the increased heterojunction area facilitates the dissociation of the photogenerated excitons. From our thermal admittance spectroscopy, deeper sub-bandgap states were found under the smaller phase separation, further contributing to the dark current reduction. Second, the carrier injection effect was revealed by inserting a charge-blocking layer at the active layer/electrode interface. While manipulating both parameters could yield a high near-infrared detectivity up to 1012 Jones at −0.5 V, the blending morphology effect turns out to be more dominant over the carrier injection effect in suppressing the dark current and achieving higher detectivity in BHJ OPDs.

Mapping the Energetics of Defect States in Cu2ZnSnS4 films and the Impact of Sb Doping.

The sub-bandgap levels associated with defect states in Cu2ZnSnS4 (CZTS) thin films are investigated by correlating the temperature dependence of the absorber photoluminescence (PL) with the device admittance spectroscopy. CZTS thin films are prepared by thermolysis of molecular precursors incorporating chloride salts of the cations and thiourea. Na and Sb are introduced as dopants in the precursor layers to assess their impact on Cu/Zn and Sn site disorder, respectively. Systematic analysis of PL spectra as a function of excitation power and temperature show that radiative recombination is dominated by quasi-donor–acceptor pairs (QDAP) with a maximum between 1.03 and 1.18 eV. It is noteworthy that Sb doping leads to a transition from localized to delocalized QDAP. The activation energies obtained associated with QDAP emission closely correlate with the activation energies of the admittance responses in a temperature range between 150 K and room temperature in films with or without added dopants. Admittance data of CZTS films with no added dopants also have a strong contribution from a deeper state associated with Sn disorder. The ensemble of PL and admittance data, in addition to energy-filtered photoemission of electron microscopy (EF-PEEM), shows a detailed picture of the distribution of sub-bandgap states in CZTS and the impact of doping on their energetics and device performance.

The Deep Eutectic Solvent Precipitation Synthesis of Metastable Zn4V2O9.

A precipitation method involving a deep eutectic solvent (DES)─a mixture of hydrogen bond donor and acceptor─is used to synthesize a ternary metal oxide. Without toxic reagents, precipitates consisting of Zn3(OH)2V2O7·nH2O and Zn5(OH)6(CO3)2 are obtained by simply introducing deionized H2O to the DES solution containing dissolved ZnO and V2O5. Manipulation of the synthetic conditions demonstrates high tunability in the size/morphology of the two-dimensional nanosheets precipitated during the dynamic equilibrium process. According to differential scanning calorimetry and high-temperature powder X-ray diffraction, Zn3V2O8 and ZnO obtained by the annealing of the precipitate are intermediates in the reaction pathway toward metastable Zn4V2O9. Intimate mixing of the metal precursors achieved by the precipitation method allows access to the metastable zinc-rich vanadate with unusually rapid heat treatment. The UV-vis and surface photovoltage spectra reveal the presence of sub-band gap states, stemming from the reduced vanadium (V4+) center. Photoelectrochemical measurements confirm weak photoanodic currents for water and methanol oxidation. For the first time, this work shows the synthesis of a metastable oxide with the DES-precipitation route and provides insight into the structure-property relationship of the zinc-rich vanadate.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells.

The charge carrier dynamics in organic solar cells and organic-inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

The influence of surface roughness on the presence of polymorphs and defect states in P3HT layers

Despite extensive studies on the archetypal poly(3-hexylthiophene-2,5-diyl) (P3HT) over the past decade, a concurrent investigation of the surface roughness influence on polymorphism, density of states (DOS), photoluminescence (PL), and hole mobility is missing. We report on the effect of substrate roughness on the optical and electrical properties of P3HT thin films deposited on smooth c-Si (roughness < 1 nm) and rough ITO (roughness ∼ 6 nm) substrates. Grazing-incidence wide-angle X-ray scattering and the energy-resolved electrochemical impedance spectroscopy were used to correlate the two polymorphs with the defect DOS in P3HT thin film grown on ITO substrate. The Jablonski diagram reconstructed from PL spectra elucidates the PL bleaching associated with sub-bandgap defect states above the highest occupied molecular orbital found in the case of the rough ITO substrate. A decrease in the PL intensity of the 0–1 transition caused by this extra band of defect states is presumably due to a non-radiative transition from the excited to the ground state or reduction of transition probability. The observed sub-band gap states have no effect on hole mobility. However, the continuum of gap states, which increases with decreasing layer thickness on both substrates, lowers the hole mobility by one order.

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

We report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10−4 A (a photo-to-dark current ratio of ~ 107) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10−7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC.