Thin-film Optoelectronic Devices Research Articles

Interfacial Heterojunction Enables High Efficient PbS Quantum Dot Solar Cells.

Colloidal quantum dots (CQDs) are promising optoelectronic materials for solution-processed thin film optoelectronic devices. However, the large surface area with abundant surface defects of CQDs and trap-assisted non-radiative recombination losses at the interface between CQDs and charge-transport layer limit their optoelectronic performance. To address this issue, an interface heterojunction strategy is proposed to protect the CQDs interface by incorporating a thin layer of polyethyleneimine (PEIE) to suppress trap-assisted non-radiative recombination losses. This thin layer not only acts as a protective barrier but also modulates carrier recombination and extraction dynamics by forming heterojunctions at the buried interface between CQDs and charge-transport layer, thereby enhancing the interface charge extraction efficiency. This enhancement is demonstrated by the shortened lifetime of carrier extraction from 0.72 to 0.46ps. As a result, the resultant PbS CQD solar cells achieve a power-conversion-efficiency (PCE) of 13.4% compared to 12.2% without the heterojunction.

Effect of traps on carrier transport in CdSe quantum dot thin films

The influence of trap effects on carrier transport characteristics in quantum dot (QD) thin films is the subject of study, aiming to provide a theoretical basis for the structural design and performance improvement of QD thin film optoelectronic devices. This study presents a specific mathematical description of capturing and releasing charges by traps, which includes the time-varying equation for captured charges. Utilizing the carrier hopping transport model, a system of partial differential equations is employed as the physical field, establishing hopping transport models that account for both shallow traps and a combination of shallow and deep traps. Simulations based on specific experimental samples reveal that the presence of traps introduces asymmetry in the diffusion motion of charge carriers, extending the duration of the photocurrent signal and resulting in the capture of charges, along with a reduction in the peak value of the current signal. The model also simulates carrier transport characteristics under the influence of repetitive light pulses, demonstrating distinct patterns in capturing and releasing charges for both shallow and deep traps.

Microstructural evolution during densification process of neodymium-doped indium zinc oxide ceramic targets and application for high mobility films

Oxide targets are crucial materials for transparent conducting oxide (TCO) thin films used in optoelectronic devices. In this study, neodymium-doped indium zinc oxide (NdIZO) ceramic targets were prepared using an advanced pressureless oxygen atmosphere sintering process. By optimizing the experimental conditions, the best sintering parameters were obtained, resulting in dense, fine-grained, and uniformly microstructured NdIZO targets. The influence mechanism of different Nd doping concentrations on the phase composition, microstructure evolution, and electrical properties of the targets during the process of oxide composite powder, green body shaping, and sintering densification was analyzed in detail. The results showed that the pretreatment of the composite powder improved its dispersibility, and sintering activity. The NdIZO-2 target with an atomic ratio of Nd:In:Zn = 0.02:1:1, fabricated under optimal conditions, exhibited high density of 99.8 %, low resistivity of 5.2 mΩ cm, and an average grain size of about 3.9 μm, thereby demonstrating the best comprehensive performance. Moreover, the sputtered films prepared using NdIZO-2 targets also showed excellent properties with transmittance (T) of 88.1 %, electron mobility (μe) of 31.5 cm2/Vs, and resistivity (ρ) of 7.5 × 10−3 Ω cm. This work provides theoretical support for the preparation of high-quality indium zinc oxide (IZO) ceramic targets and their application in high-performance optoelectronic thin film devices.

Film-Depth-Dependent Charge Mobilities in Organic Semiconductor Films

Carrier mobility is a crucial parameter characterizing the performance of organic semiconductors in thin film optoelectronic devices. The organic semiconductor film is usually not uniform along the film-perpendicular direction. This non-uniformity affects the carrier mobility at various depths within the film, thereby influencing device performance. Conventional methods for measuring mobility are basically not available to determine film-depth-dependent charge density or mobility. In this study, an easily accessed approach was proposed to investigate the film-depth-dependent electronic properties. Soft plasma etching was introduced to achieve incremental etching of the film, without causing damage to the underlying sublayers. For doped films with intrinsic charge, film-depth-dependent charge density, and film-depth-dependent mobility were extracted from the transfer characteristics evolution during the etching. For materials without intrinsic charge, film-depth-dependences of mobility could also be estimated under light illumination. The proposed method achieves a film-depth resolution of approximately 10 nm. It is revealed that for some model conjugated polymers, the mobility at the top surface is one order higher than that at the buried region of the transistor. This feature leads to unfavorable field-effect mobility. This work provides insights into the film-depth-dependent electronic properties and a strategy to optimize the electronic device performance.

Synthesis of Regioregular and Random Boron-Fused Azomethine Conjugated Polymers for Film Morphology Control.

Regioregular and random conjugated polymers based on a boron-fused azomethine unit were synthesized by Sonogashira-Hagihara cross coupling reaction. Although these polymers exhibited similar optical properties in the solution states, a distinct difference was observed in the aggregation forming ability in the film states; scanning electron microscope (SEM) observation indicated the existence of fiber-like aggregates in the spin-coated film of the regioregular polymer, while regiorandom polymer showed no aggregate in the film state. Accordingly, the UV-vis absorption spectrum of the regioregular polymer showed an increased shoulder peak due to the aggregate formation, whereas the random one showed no change. Furthermore, an absolute fluorescence quantum efficiency of the regioregular polymer was enhanced in response to the aggregate disassembly via thermal annealing treatment. In this study, we demonstrate that controlling regioregularity of the conjugated polymers can induce the different morphological structures and thermal-responsive behaviors. These findings could be beneficial for the design strategy and potential applications of thin-film optoelectronic devices with stimuli-responsive properties.

Improving photoluminescence properties and reducing recombination of CsPbBr3 perovskite through lithium doping.

This study investigates the impact of lithium doping on the structural and photophysical properties of spin-coated CsPbBr3 perovskite thin films. The deposited films display a pristine structure, preferentially growing along the (220) direction, and exhibit high-quality green photoluminescence at around 530 nm. The doping leads to an improvement in the optical properties of the films, as evidenced by a stronger photoluminescence (PL) intensity compared to undoped CsPbBr3, particularly at temperatures below 200 K. The increase in PL intensity suggests a decrease in defects and surface passivation. Additionally, the decrease in the power-law exponent β from 1.6 to 1.0 indicates a reduction in non-radiative recombination, likely due to trap states filling with free electrons induced by the doping. Overall, doping with lithium reduces non-radiative recombination, fills trap states, and reduces band tail/activation energy, leading to improved optoelectronic properties of the films. This investigation provides insights into the photophysical properties of the Li-CsPbBr3 absorber layer and the recombination mechanism, and helps to unravel new methods for the development of high-stability, high-performance perovskite thin-film solar cells and optoelectronic devices.

Optical Properties of PS-MgO Thin Films

In this study, optical properties of thin films made of different volume ratios of polysulfone (PS) and magnesium oxide (MgO) thin film deposited on germania (GeO2 ) were studied as a function of wavelength in the range 0.3 – 1.1 μm using Matlab. Refractive indices, extinction coefficients, and absorption coefficients were investigated. The transmittance spectra of the PS-MgO thin films of different thicknesses deposited on the GeO2 substrate were also examined. The films were found to show an increase in transmittance with an increase in the MgO percentage the thin film. However, the absorbance of the film was found to be high in the ultraviolet. The results give good reason for the applications of MgO thin films in optoelectronic devices.

Luminescence of In(III)Cl-etioporphyrin-I.

The luminescent and photophysical properties of the etioporphyrin-I complex with indium(III) chloride, InCl-EtioP-I were experimentally studied at room and liquid nitrogen temperatures in pure and mixed toluene solutions. At 77 K, in a 1:2 mixture of toluene with diethyl ether, the quantum yield of phosphorescence reaches 10.2%, while the duration of phosphorescence is 17 ms. At these conditions, the ratio of phosphorescence-to-fluorescence integral intensities is equal to 26.1, which is the highest for complexes of this type. At 298 K, the quantum yield of the singlet oxygen generation is maximal in pure toluene (81%). Quantum-chemical calculations of absorption and fluorescence spectra at temperatures of 77 K and 298 K qualitatively coincide with the experimental data. The InCl-EtioP-I compound will further be used as a photoresponsive material in thin-film optoelectronic devices.

Two-dimensional tellurium-based diodes for RF applications

The research of two-dimensional (2D) Tellurium (Te) or tellurene is thriving to address current challenges in emerging thin-film electronic and optoelectronic devices. However, the study of 2D-Te-based devices for high-frequency applications is still lacking in the literature. This work presents a comprehensive study of two types of radio frequency (RF) diodes based on 2D-Te flakes and exploits their distinct properties in two RF applications. First, a metal-insulator-semiconductor (MIS) structure is employed as a nonlinear device in a passive RF mixer, where the achieved conversion loss at 2.5 GHz and 5 GHz is as low as 24 dB and 29 dB, respectively. Then, a metal-semiconductor (MS) diode is tested as a zero-bias millimeter-wave power detector and reaches an outstanding linear-in-dB dynamic range over 40 dB, while having voltage responsivities as high as 257 V ⋅ W−1 at 1 GHz (up to 1 V detected output voltage) and 47 V ⋅ W−1 at 2.5 GHz (up to 0.26 V detected output voltage). These results show superior performance compared to other 2D material-based devices in a much more mature technological phase. Thus, the authors believe that this work demonstrates the potential of 2D-Te as a promising material for devices in emerging high-frequency electronics.

Two-Dimensional Van Der Waals Thin Film and Device.

In the rapidly evolving field of thin-film electronics, the emergence of large-area flexible and wearable devices has been a significant milestone. Although organic semiconductor thin films, which can be manufactured through solution processing, have been identified, their utility is often undermined by their poor stability and low carrier mobility under ambient conditions. However, inorganic nanomaterials can be solution-processed and demonstrate outstanding intrinsic properties and structural stability. In particular, a series of two-dimensional (2D) nanosheet/nanoparticle materials have been shown to form stable colloids in their respective solvents. However, the integration of these 2D nanomaterials into continuous large-area thin with precise control of layer thickness and lattice orientation still remains a significant challenge. This review paper undertakes a detailed analysis of van der Waals thin films, derived from 2D materials, in the advancement of thin-film electronics and optoelectronic devices. The superior intrinsic properties and structural stability of inorganic nanomaterials are highlighted, which can be solution-processed and underscor the importance of solution-based processing, establishing it as a cornerstone strategy for scalable electronic and optoelectronic applications. A comprehensive exploration of the challenges and opportunities associated with the utilization of 2D materials for the next generation of thin-film electronics and optoelectronic devices is presented.

Graphene Oxide: Key to Efficient Charge Extraction and Suppression of Polaronic Transport in Hybrids with Poly (3-hexylthiophene) Nanoparticles.

Nanoparticles (NPs) of conjugated polymers in intimate contact with sheets of graphene oxide (GO) constitute a promising class of water-dispersible nanohybrid materials of increased interest for the design of sustainable and improved optoelectronic thin-film devices, revealing properties exclusively pre-established upon their liquid-phase synthesis. In this context, we report for the first time the preparation of a P3HTNPs-GO nanohybrid employing a miniemulsion synthesis approach, whereby GO sheets dispersed in the aqueous phase serve as a surfactant. We show that this process uniquely favors a quinoid-like conformation of the P3HT chains of the resulting NPs well located onto individual GO sheets. The accompanied change in the electronic behavior of these P3HTNPs, consistently confirmed by the photoluminescence and Raman response of the hybrid in the liquid and solid states, respectively, as well as by the properties of the surface potential of isolated individual P3HTNPs-GO nano-objects, facilitates unprecedented charge transfer interactions between the two constituents. While the electrochemical performance of nanohybrid films is featured by fast charge transfer processes, compared to those taking place in pure P3HTNPs films, the loss of electrochromic effects in P3HTNPs-GO films additionally indicates the unusual suppression of polaronic charge transport processes typically encountered in P3HT. Thus, the established interface interactions in the P3HTNPs-GO hybrid enable a direct and highly efficient charge extraction channel via GO sheets. These findings are of relevance for the sustainable design of novel high-performance optoelectronic device structures based on water-dispersible conjugated polymer nanoparticles.

Preparation of a CeO2 ball-core structure and its application in quantum dot sensitized solar cells.

A pomegranate-like nanosphere structure of CeO2 was prepared by a simple one-step hydrothermal method, and the novel CeO2 structure was printed on a TiO2 film to form a scattering layer, which constituted the composite structure of a photoanode in a thin film optoelectronic device. Then ZnCuInSe quantum dots, a ZnS passivation layer and a CuS counter electrode were prepared, and these parts were assembled into quantum dot-sensitized solar cells. By changing the thickness of the scattering layer film in the photoanode, the quantum dots were adsorbed on TiO2 more effectively. By using the special ball-core structure, light was scattered so that the photoanode used the light many times, which effectively increased the efficiency of photoelectron production. After electrochemical testing of the device, it was found that the photoconversion efficiencies of the TiO2 transparent layer and the CeO2 scattering layer composite photoanode were greater than that of a single TiO2 photoanode without a scattering layer. The results showed that when the thickness of the scattered layer was 10 ± 1 μm, the highest photoelectric conversion efficiency (PCE) was obtained, which was 20% higher than that seen with TiO2 alone.

Wavelength dependence of nonlinear optical susceptibility of ZnSe nanocrystalline film

The present work reports the nonlinear optical susceptibility of nanocrystalline ZnSe thin films with different thicknesses, measured using third-order harmonic technology at irradiation wavelengths λ of 1200–1900 nm. The results show that the third-order nonlinear susceptibility χ(3) of ZnSe thin films is a function of the pump wavelength and film thickness. The χ(3) values for the thinner 202-nm film generally decreased with the redshift of the wavelength and were approximately 10−20 m2/V2 at λ ≤ 1500 nm and 10−21 m2/V2 at λ > 1500 nm. The χ(3) values for the thicker 1021-nm film did not follow a decreasing trend, but instead exhibited local peaks at 1350, 1500 and 1750 nm, with a maximum value of approximately 2.4 × 10−19 m2/V2 at λ = 1750 nm. Within 1200–1900 nm, the χ(3) of the thicker film was always greater than that of the thinner film. In addition, a weak second-order harmonic at wavelengths within 1100–1300 nm was observed for the 1021 nm ZnSe thin film. The large difference in the nonlinear optical properties between the thinner and thicker film samples is conjectured to be caused by the increased density of geometric defects and the gradual formation of local anisotropy with increasing film thickness. Nonlinear optical coefficient measurements of ZnSe thin films with different thicknesses over a broad spectrum are important for the application of these films in nonlinear thin-film optoelectronic devices.

Simple thermal vapor deposition process for and characterization of n-type indium oxysulfide thin films

The search continues for alternative nontoxic n-type electron transport layers in optoelectronic thin-film devices. Indium oxysulfide, In2(O,S)3, represents one promising material for this application, especially when paired with chalcogenide absorber layers. The ternary nature of the composition allows for electrical conductivity and optical bandgap tuning by tailoring the sulfur to oxygen ratio in the oxysulfide alloy. However, thin films of In2(O,S)3 are typically deposited only by chemical bath deposition or plasma-enhanced atomic layer deposition. We report deposition of thin films of In2(O,S)3 in a custom-built thermal reactor using only water vapor and hydrogen sulfide as the coreactants. This advance is enabled by the use of a recently reported, highly reactive indium formamidinate precursor. As shown by x-ray photoelectron spectroscopy, the composition can be tuned from pure In2O3 to pure In2S3 by varying the ratio of cycles employing water or hydrogen sulfide. The oxygen to the sulfur ratio in the film can be controlled by altering the dose sequence, although films typically contain more sulfur than would be expected naively from the percentage of hydrogen sulfide doses in the deposition recipe. Rutherford backscattering spectrometry confirms the composition is sulfur-rich relative to the dosing ratio. Structural characterization indicates films are relatively amorphous in nature. Electrically, these films offer reasonably constant electron mobility at different O:S ratios, with an electron concentration tunable over 4 orders of magnitude. These oxysulfide films possess a higher indirect bandgap than their oxygen-free indium sulfide counterparts, indicating higher transmittance to blue light. These indium oxysulfide films may be suitable candidates for electron transport layers in thin-film solar cells where their wider bandgap might result in higher optical transparency and thus short circuit current density, while the tunability of their conduction band offset with an absorber layer may result in higher open circuit voltage.

Hydrophenazine-linked two-dimensional ladder-type crystalline fused aromatic network with high charge transport

•A ladder-type hydrophenazine-linked 2D fused aromatic network (HP-FAN) was realized •The HP-FAN has an inherent semiconducting band gap along with a parallel flat band •The HP-FAN has high intrinsic electrical conductivity without doping •The HP-FAN exhibits a remarkable performance in electronic devices with high mobility Since the advent of graphene, the development of crystalline two-dimensional (2D) organic materials with semiconducting features has been extensively explored for their potential optoelectronic applications. Despite extensive progress in this field, it is still challenging to realize laterally extended organic materials with high electrical transport properties. Here, we report a 2D ladder-type fused aromatic network (FAN) in which backbones are composed of hydrophenazine (HP) linkage (designated HP-FAN). Consequently, its 2D extended delocalization of π-molecular orbitals imparts a semiconducting band gap and facilitates fast intra-chain charge transport. The as-prepared HP-FAN exhibits semiconducting features with calculated and experimental band gaps of approximately 1.44 and 1.54 eV, respectively, with an unusual flat band. The HP-FAN thin flakes, isolated by polydimethylsiloxane stamping, exhibit remarkable performance in a p-type field-effect transistor (FET) and a Hall effect device. Given its laterally extended ladder-type π-conjugated structure, the HP-FAN has extensive potential for applications in thin-film optoelectronic devices. Since the advent of graphene, the development of crystalline two-dimensional (2D) organic materials with semiconducting features has been extensively explored for their potential optoelectronic applications. Despite extensive progress in this field, it is still challenging to realize laterally extended organic materials with high electrical transport properties. Here, we report a 2D ladder-type fused aromatic network (FAN) in which backbones are composed of hydrophenazine (HP) linkage (designated HP-FAN). Consequently, its 2D extended delocalization of π-molecular orbitals imparts a semiconducting band gap and facilitates fast intra-chain charge transport. The as-prepared HP-FAN exhibits semiconducting features with calculated and experimental band gaps of approximately 1.44 and 1.54 eV, respectively, with an unusual flat band. The HP-FAN thin flakes, isolated by polydimethylsiloxane stamping, exhibit remarkable performance in a p-type field-effect transistor (FET) and a Hall effect device. Given its laterally extended ladder-type π-conjugated structure, the HP-FAN has extensive potential for applications in thin-film optoelectronic devices.

Improving the photovoltaic performance for PbS QD thin film solar cells through interface engineering

Interfaces exist between functional layers inside thin film optoelectronic devices, and it is very important to minimize the energy loss when electrons move across the interfaces to improve the photovoltaic performance. For PbS quantum dots (QDs) solar cells with the classical n-i-p device architecture, it is particularly challenging to tune the electron transfer process due to limited material choices for each functional layer. Here, we introduce materials to tune the electron transfer across the three interfaces inside the PbS-QD solar cell: (1) the interface between the ZnO electron transport layer and the n-type iodide capped PbS QD layer (PbS-I QD layer), (2) the interface between the n-type PbS-I layer and the p-type 1,2-ethanedithiol (EDT) treated PbS QD layer (PbS-EDT QD layer), (3) the interface between the PbS-EDT layer and the Au electrode. After passivating the ZnO layer through APTES treating; tuning the band alignment through varying the QD size of PbS -EDT QD layer and a carbazole layer to tune the hole transport process, a power conversion efficiency of 9.23% (Voc of 0.62 V) under simulated AM1.5 sunlight is demonstrated for PbS QD solar cells. Our results highlights the profound influence of interface engineering on the electron transfer inside the PbS QD solar cells, exemplified by its impact on the photovoltaic performance of PbS QD devices.

Influence of 8 MeV Electron Irradiation on the Properties of ZnO Nanocrystalline Thin Films for Optoelectronic Devices in the High Radiation Environment

Herein, we present the influence of 8 MeV electron beam irradiation on the structural, optical and electrical properties of sol-gel derived spin-coated ZnO films. The electron irradiation doses varied from 0 kGy to 10 kGy. We observed significant changes in the morphology, optical and electrical properties of the films upon irradiation. Structural analysis of the films revealed the preferential and dominant c-axis orientation of the crystallites along the (002) plane. The preferential growth of the films was also supported by the values of texture coefficient. Atomic Force Microscopy studies reveal the linear relationship between the surface roughness, grain size and irradiation dose. The average transmittance of the films decreased from 87% to 66% after irradiation. A slight red-shift of the optical band gap values was observed when the absorbed dose was increased from 0 kGy to 10 kGy. The Photoluminescence measurement of the films showed that the intensity of the defect-related peak increases with the electron dose. The electrical resistivity of the films decreased after irradiation and a minimum resistivity of 7.566 × 10−2 Ω m was obtained from the film irradiated with a 5 kGy electron dose. To find the performance and quality of the prepared film, the Figure of Merit was calculated and was found that the film irradiated with a 5 kGy electron dose shows the maximum value. The obtained results show that electron irradiation is a potential alternative approach to improve the performance of the ZnO films for optoelectronic devices in a high radiation environment.

ГИДРОТЕРМАЛЬДЫ ӘДІСПЕН МЫРЫШ ОКСИДІНІҢНАНОСТЕРЖЕНЬДЕРІН АЛУ

Thin films of zinc oxide (ZnO) are very promising for thin-film transistors, solar panels, sensors and optoelectronic devices. The article presents the structural properties and morphology of zinc oxide (ZnO) obtained by hydrothermal synthesis. ZnO nanofilms (as a seed layer) were deposited by the sol-gel method on the glass surface of STO. The results of the experiment showed that the concentration of chemical reagents in the process of sol-gel transformation has a strong effect on the properties of ZnO films. Samples of ZnO nanorods on the surface of the nanofilm were obtained by hydrothermal method at a solution temperature of about 97 ° C for 3 hours.After firing the synthesized ZnO nanorods, the electrical resistance increased several times. The chemical composition and morphology of the obtained nanorods arrays were characterized by scanning electron microscopy and X-ray diffraction. As a result of the study, arrays of ZnO nanoparticles showed a highly improved response to visible light. The results of the article can be used in other combinations of metal oxides, and also offer many interesting opportunities for further development of high-performance photoelectrochemical devices for solar energy conversion. The results of scientific research are the most important material data for alternative energy and can be used by industry specialists in the creation of photoelectrochemical systems for generating hydrogen fuel with the formation of active photanode layers of water decomposition systems with zinc oxide.

Engineering organic–inorganic perovskite planar heterojunction for efficient carbon dots based light-emitting diodes

When it comes to building high-efficiency thin-film optoelectronic devices, we are constantly striving to improve the efficiency of charge transport and injection. Device performance is hampered by the low mobility and injection ability of organic charge transporting materials that are routinely used. In this paper, we show that instead of using organics as a hole transporting layer, metal halide perovskite can be used to fabricate high-efficiency carbon dots-based light-emitting diodes for the first time. The organic light-emitting layer and the underlying perovskite layer combine to form an organic–inorganic perovskite planar heterojunction, and the sufficient contact at the junction takes advantage of the high charge mobility of perovskite, facilitating the hole transportation and injection. Moreover, the interaction between perovskite and the organic emitting layer can be engineered via manipulating the halogenic component, thickness, surface morphology, etc., contributing to the device optimization and the understanding of the carrier kinetics in this unique organic–inorganic hybrid optoelectronic device. Our work comprehensively evaluates the full potentials of metal halide perovskite as a hole transporting layer by uncovering the positive effect on hole transportation and injection. As a consequence, our findings open up new avenues for the development of efficient carbon dot-based light-emitting diodes.

Structure and optical properties of spray deposited Cu–Mn–O thin films for optoelectronic devices

Nanostructured copper manganese oxide (CMO) thin films were prepared by sol-gel method and deposited on glass substrate by spray pyrolysis technique. Different Cu acetate/Mn acetate molar ratios were used to study the effect of Cu acetate on the structure, morphological and optical properties of the deposited films. The prepared films were annealed at 500 °C in air. A polycrystalline structure and mixed oxide phases of copper manganese oxide were confirmed by X-ray diffraction, (XRD). High resolution transmission electron microscopy, (HRTEM), reveals homogenous and very fine grainy structures of the prepared films with a particle size range from 0.5 nm to 2 nm. The optical measurements showed that 93% is maximum optical transmission and the optical band gap of the prepared films was found to be influenced by increasing Cu acetate ratios from 40% to 60% indicating a blue shift. The energy band gap correlated with the allowed direct transition is found to increase from 3.21 eV to 3.58 eV with increasing Cu acetate while the refractive index decreases. The optical investigation by ultraviolet, visible light, and near-infrared, (UV–VIS–NIR), showed a clear absorption peak at 4.28 eV which attributed to the excitonic nature.