PbSe Films Research Articles

Optimizing the Physical Properties of CBD PbSe Thin Films for Photovoltaic Application

Nanocrystalline thin films of PbSe were deposited on glass substrates by simple chemical bath deposition technique at 60oC. The effects of the deposition time on the thickness as well as structural, morphological and optical properties of the PbSe thin films were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis absorption spectroscopy. Band gaps of the films were found in the ranges 1.7 to 2.3 eV. XRD spectra reveals that as prepared PbSe films are FCC (face centered cubic) structured with preferred orientation along (2 0 0) direction. The SEM micrograph illustrates the homogeneous surface morphology without any visible pinholes and cracks. The room temperature conductivity of the as-deposited PbSe films was found in the range 0.27x10-5Ω-1 cm-1 to 3.32x10-5 Ω-1cm-1. A set of three prototype Schottky solar cells with the structure ITO/PbSe/Ag has been fabricated by varying the thickness of PbSe layer from 230nm to 380nm. The cells exhibited power conversion efficiency in the range (0.26%-0.34%) under one sun illumination intensity (100mW/cm2).

Optimizing the Physical Properties of CBD PbSe Thin Films for Photovoltaic Application

Lead selenide (PbSe) is an important direct band gap IV-VI semiconductor material and have been attracting much interest among the researchers because of its interesting physical properties like high refractive index, narrow band gap (~0.27 eV), relatively large exciton Bohr radius (~46 nm), high mobility and high absorption coefficient (>105 Cm-1). PbSe thin films in polycrystalline and nanocrystalline form have significant technological importance as detectors of infrared radiation, infrared emitters, photovoltaic devices and thermoelectric devices. Due to its larger exciton Bohr radius, quantum confinement effect could be observed to a fairly bigger particle size. Various deposition techniques have been reported to deposit PbSe thin films that include electrochemical, thermal evaporation, pulsed laser deposition, molecular beam epitaxy and chemical bath deposition (CBD). But, chemically deposited PbSe thin film appears to be most suitable as it is easy to handle, cost effective and has wide industrial applications. Most of the reported works on the synthesis of PbSe thin film by CBD technique involves the use of complexing agents like Trisodium citrate (TSC), triethanolamine (TEA), 1, 2-ethanedithiol (EDT) etc. The rapid use of these complexing agents may cause some environment issues due to their toxicity. Therefore, development of a simple and reliable synthetic technique free of toxic complexing agent is of great importance. Nanocrystalline thin films of PbSe were deposited on glass substrates by simple chemical bath deposition technique at 60oC. The effects of the deposition time on the thickness as well as structural, morphological and optical properties of the PbSe thin films were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis absorption spectroscopy. Band gaps of the films were found in the ranges 1.7 to 2.3 eV. XRD spectra reveals that as prepared PbSe films are FCC (face centered cubic) structured with preferred orientation along (2 0 0) direction. The SEM micrograph illustrates the homogeneous surface morphology without any visible pinholes and cracks. The room temperature conductivity of the as-deposited PbSe films was found in the range 0.27 x 10-5Ω-1 cm-1 to 3.32 x 10-5 Ω-1 cm-1. A set of three prototype Schottky solar cells with the structure ITO/PbSe/Ag has been fabricated by varying the thickness of PbSe layer from 230nm to 380nm. The cells exhibited power conversion efficiency in the range (0.26%-0.34%) under one sun illumination intensity (100mW/cm2). PbSe thin films were grown by continuous dip coating technique by reacting Lead nitrate[Pb (NO3)2] and sodium selenosulfate [Na2SeSO3] under ambient pressure at 60oC. The synthetic technique is a modification of the method due to I. Grozdanov et al. [22]. A set of five PbSe thin films (S1, S2, S3, S4 and S5) were prepared during the deposition time 30, 40, 60, 90 and 120 min respectively. Results and discussion UV-Vis absorption spectroscopy The optical absorption spectra of the PbSe thin films (S1, S2, and S3) are presented in Fig. 1(a). It is observed that the shapes of the curves are similar in nature. XRD Analysis The X-ray diffraction patterns of the PbSe thin films are shown in Fig. 2(a) and (b) at various deposition periods. SEM Analysis The uniform pinhole-free surface morphology of PbSe thin films prepared for different deposition times are depicted in Fig. 3(a)-(c). Photovoltaic performance: Fig 4 shows the I-V characteristics of ITO/PbSe/Ag schottky solar cells and photovoltaic parameters are shown in Table 1. Conclusions PbSe thin films with grain sizes ~ 8.8-12.7 nm in diameter, free of pinholes or cracks have been synthesized by a simple CBD technique. The quality of the PbSe thin film in terms of crystallinity, microstrain, dislocation density and roughness etc. was improved by optimizing the deposition time. A set of three prototype Schottky solar cells with the structure ITO/PbSe/Ag has been fabricated by varying the thickness of PbSe layer from 230nm to 380nm and as-prepared cells exhibited power conversion efficiency in the range (0.26%-0.34%). Acknowledgments The authors D. Saikia , Pallabi Phukan acknowledges DeitY, New Delhi for the financial support under the Major research project to Sibsagar College, Joysagar, Assam, India [vide letter No. 12 (5)/2011-EMCD, Dated 01.02.2012] & SAIF, NEHU, Shillong and NEIST, Jorhat for providing the SEM and XRD results. The author R. Vaid acknowledges University Grants Commission (UGC) for providing financial assistance under Major research project (MRP- MAJOR-ELEC-2013-22797) under the 12th plan period. Figure 1

Morphological and microstructural evolution of PbSe films grown on thermally oxidized Si (111) substrates by chemical bath deposition

Morphological and microstructural evolution of the PbSe films on the thermally oxidized Si (111) substrates during chemical bath deposition was studied by SEM, XRD and TEM. At the very early stage of deposition, sub-micron scale hemispherical particles are formed on the substrate and they grow with deposition time. The hemispherical particles consist of PbSe, PbSeO4, and PbO2 nanocrystals. Soon after appearance of hemispherical particles, small single crystalline PbSe cuboid particles begin to form mostly on the hemispherical particles and grow with deposition time. At the end of deposition, hemispherical particles are completely replaced by cuboid particles. The cluster mechanism seems to be working at the early stage of deposition, while the ion-by-ion mechanism dominates at the later stage of deposition. The classical heterogeneous nucleation theory is also needed to explain why hemispherical particles are nucleated in the initial stage, and cuboid particles are formed in the later stage of deposition.

Effect of iodine doping in the deposition solution and iodine vapor pressure in the sensitization treatment on the properties of PbSe films

Effect of iodine-doping in the deposition solution and iodine vapor pressure during the sensitization process on the morphological, microstructural, electrical, and optical properties of PbSe films was studied. Undoped and iodine-doped PbSe films of polycrystalline particles were coated on thermally oxidized silicon substrates by chemical bath deposition. The PbSe films were oxidized at 380°C for 30 min and then iodinated at different iodine vapor pressures at 380°C for 5 min. When the iodine vapor pressure was below 20 Pa, PbSeO3 was the main phase formed on the surface of PbSe microcrystals for both undoped and iodine-doped films. As the iodine vapor pressure was increased above 20 Pa, Pb3I2O2 and PbI2 phases were formed in both types of films and PbSeO3 disappeared in the undoped film. Only the iodine-doped films showed photo response. The sheet resistance and IR signal-to-noise ratio had maximum values at the iodine vapor pressure of 17.5 Pa in the iodine-doped film. The x-ray diffraction spectra, scanning electron microscopy morphologies, and EDS analyses of the sensitized PbSe films show that the main role of iodine in the sensitization is helping solid-state sintering of PbSe microcrystals which may lead to redistribution of oxygen atoms in the effective atomic sites.

Optical and structural properties of PbSe films obtained by ionic exchange of lead oxyhydroxicarbonate in a selenium-rongalite solution

In this work, starting from precursor films of Pb10O(OH)6(CO3)6, previously obtained by chemical bath deposition technique, we report the process to obtain PbSe films through ionic exchange. For this, after chemical deposition, several pieces of the precursor films were immersed in selenium ions solution in order to promote the transformation to PbSe films by ionic exchange. Instead using the conventional sodium selenium sulfite solution as the selenium ions solution, a new one is reported, which is based in rongalite (CH3NaO3S) and sodium citrate as reducing agents. Samples were removed after 30, 60 and 90min to determine the effects of ionic exchange time on the phase transformation process. The complete set of samples was analyzed by X-ray diffraction, scanning electron microscopy and optical absorption measurements. The XRD results show a gradual crystalline transformation from the precursor phase to cubic PbSe crystalline phase. The gradual phase transformation is accompanied by surface morphologic changes as shown by SEM images. From the optical absorption spectrum of the PbSe films, it was calculated the energy band gap, being around 0.34eV.

Influence of as-deposited conductive type on sensitization process of PbSe films

The as-grown n- and p-type polycrystalline PbSe thin films are fabricated by vapor phase deposition using substrate temperature regulation. The surface polycrystalline structures and photoelectric properties of n- and p-type polycrystalline PbSe films are provided. Surface composition of n-type-sensitized PbSe film has been analyzed according to X-ray photoelectron spectroscopy results. The oxygen roles in n- and p-type PbSe polycrystalline films during the sensitization process are studied experimentally, respectively. The dependence of sensitized photoelectric performance on the initial conductive state has been firstly observed and discussed, as we know presently. It is revealed that oxygen can trigger photo-response in the sensitization process for n-type PbSe film, but not for p-type. These discussions may be useful for understanding the sensitization mechanism of lead salt materials.

Structural, optical and electrical properties of PbS and PbSe quantum dot thin films

PbS and PbSe were prepared by hot injection method. The powders were used for preparing the corresponding films by using thermal evaporation technique. The structural, optical and electrical properties of PbS and PbSe thin films were investigated. The structural properties of PbS and PbSe were investigated by X-ray diffraction, transmission electron microscopy and energy dispersive X-ray techniques (EDX). PbS and PbSe films were found to have cubic rock salt structure. The particles size ranged from 1.32 to 2.26 nm for PbS and 1.28–2.48 nm for PbSe. EDX results showed that PbS films have rich sulphur content, while PbSe films have rich lead content. The optical constants (absorption coefficient and the refractive index) of the films were determined in the wavelength range 200–2500 nm. The optical energy band gap of PbS and PbSe films was determined as 3.25 and 2.20 eV, respectively. The refractive index, the optical dielectric constant and the ratio of charge carriers concentration to its effective mass were determined. The electrical resistivity, charge carriers concentration and carriers mobility of PbS at room temperature were determined as 0.55 Ω cm, 1.7 × 1016 cm−3 and 656 cm2 V−1 s−1, respectively, and for PbSe films they were determined as 0.4 Ω cm, 9 × 1015 cm−3 and 1735 cm2 V−1 s−1, respectively. These electrical parameters were investigated as a function of temperature.

Kinetics of hydrochemical deposition of PbSe films in the presence of ascorbic acid

The boundary conditions of the formation of PbSe and concomitant impurity phases, Pb(OH)2 and PbCN2, were determined by calculation of the ionic equilibria in the Pb2+–Na3C5Н6О7–CH4N2Se system. Complex kinetic studies of the PbSe deposition from an ammonia–citrate mixture containing ascorbic acid as antioxidant for selenourea were performed. PbSe films 250 to 425 nm thick were synthesized by the hydrochemical method based on the results obtained. The absence of impurities in the deposited samples was confirmed by X-ray diffraction analysis.

PbSe films by ion exchange of synthetic plumbonacrite layers immersed in a selenium ionic solution

Plumbonacrite is a lead compound with chemical formula Pb10(CO3)6(OH)6O, where several groups can be substituted by ion exchange in mild conditions. Plumbonacrite layers can be deposited by means of the chemical bath deposition technique. In this work it is studied the structural and morphological evolution of a plumbonacrite layer as a function of the immersion time in an aqueous solution containing Se−2 ions. The 1.39μm thick plumbonacrite layer was chemically deposited on a glass substrate and immersed in an aqueous solution with Se−2 ions for 10, 20, 30 and 50min. The as grown plumbonacrite layer as well as the immersed ones were analyzed by X-rays Diffraction, Scanning Electron Microscopy and Raman Spectroscopy measurements. The results show that the plumbonacrite layer is gradually converted to PbSe one by ion exchange process, where Se−2 ions substitute the groups of plumbonacrite.

Structural, optical, transient photoconductivity studies and low temperature thermoelectric power measurements on reactively evaporated lead selenide thin films

Thin film of PbSe has been prepared on ultrasonically cleaned glass substrate at a substrate temperature of 373 ± 5 K by reactive evaporation at a pressure of 10−5 mbar. The prepared PbSe film is characterized using X-ray diffraction, scanning electron microscopy, and UV–Vis–NIR spectrophotometer. The average particle size, number of crystallite per unit area and strain of a typical film are calculated and found as 38 nm, 3.098 × 1015 m−2 and 2.253 × 10−3 respectively. The room temperature electrical resistivity of the film is of the order of 0.16 Ω cm. The material possesses a direct allowed optical band gap of 2.02 eV and the absorption coefficient is found to be in the range 104–105 cm−1. The weak absorption below the absorption band edge is observed and tailing in the forbidden gap is found to follow Urbach tail rule. The width of the localized states in the band gap is estimated as 256 meV. The differential life time increases with increasing decay time and the decay curve of the photocurrent gives the localized energy states which act as trapping centre in the band gap of the material. The thermoelectric power measurements are done in the temperature range 10–293 K and the Fermi level is positioned at 0.039 eV above the valence band. The material possesses a Seebeck coefficient of about 137 μV K−1 and power factor of 0.12 × 10−4 Wm−1 K−2 near room temperature. The prepared films are of p-type in nature with activation energy of 0.063 eV.

The influence of mobile impurities on photoelectric properties of PbSe-based multiphase photosensitive structures

The PbSe-based photoelectric sensor technology is considered. The polycrystalline PbSe films are used for the production. The influence of mobile impurities in a material on the mechanism of the photovoltaic effect is revealed and investigated. The experimental results of investigation of the photovoltaic effect are discussed.

Effect of bath temperature on PbSe thin films prepared by chemical synthesis

Lead selenide (PbSe) thin films were deposited using chemical bath deposition techniques by varying bath temperatures (70–90°C) keeping deposition time and other parameters constant. The influence of the synthesis/bath temperature on structural and thermoelectric properties of as-deposited PbSe nanostructured film was investigated using x-ray diffraction, scanning electron microscope, Raman spectroscopy and thermo-emf measurement. The improvement of crystallinity of the PbSe films was studied using x-ray diffraction and Raman scattering. The structural parameters, such as the lattice constant (a), crystallite size (D), dislocation density (δ) and microstrain (ε) were evaluated from XRD spectra. The average crystallite size, as calculated from Scherrer's formula, increased from 22.45 to 24.87nm as the bath temperatures was varied from 70°C to 90°C. The dislocation density and microstrain were found to vary inversely with the crystallite size, whereas the lattice constant was found to slightly different with an increase in crystallite size. The scanning electron microscopy observation shows that with an increase in bath temperature, the grain size increases and they agglomerates. At the elevated bath temperatures individual particles agglomerate and lead to higher grain density and film crystallinity. The thermo-emf measurements have shown n-type conductivity in the as deposited films.

Discrimination of Conductive Surface Electron States by Laser Terahertz Radiation in PbSe—A Base for Pb$_{1-x}$Sn$_x$Se Topological Crystalline Insulators

We report on the influence of oxidation on features of surface electron states in PbSe films—materials belonging to the family of topological crystalline insulators Pb $_{1-x}$ Sn $_x$ Se. These states are detected through observation of the photoelectromagnetic effect induced by terahertz laser pulses. It is demonstrated that highly conductive surface electron states in PbSe are inherent to the semiconductor itself and are not related to the material oxidation. This allows excluding surface states induced by oxidation as a reason for high surface conductivity in topological crystalline insulators based on Pb $_{1-x}$ Sn $_x$ Se.

Incubation of PbSe Thin Films in a Tin(II) Salt Aqueous Solution: Modification and Ion-Exchange Reactions

Topochemical ion-exchange reactions between solid micro- and nanostructured metal chalcogenides and aqueous salt solutions are generally used for formation of composite structures based on initial metal chalcogenides and products of their ion-exchange transformation. However, ion exchange has promises as a route to obtaining both composites and solid solutions based on the initial and the end chalcogenide phases. With the help of the ion-exchange technique, single-phase films of Pb1–xSnxSe substitutional solid solutions with a tin content up to ∼2 at.%, which are promising for mid- and long-wavelength infrared radiation (IR) optoelectronics, have been obtained at the interface between PbSe polycrystalline thin films and SnCl2 aqueous solutions containing sodium citrate. It has been shown that the pH value and temperature of the reaction system play an important role in the ion-exchange process. Incubation of lead selenide (PbSe) films in a tin(II) salt aqueous solution also leads to their modification with oxygen-containing tin compounds to a depth of ∼3 nm. Differences in the film structure, such as changes in the coherent scattering region sizes and orientation of crystallites along the [220] direction, which arise during the contact with citrate-containing SnCl2 solutions, have also been revealed. For the first time, an idea of the existence of a relatively wide reaction zone of an intragranular topochemical ion-exchange reaction in an aqueous solution, within which substitutional solid solutions can form in micro- and nanostructured systems, has been set forth.

Dependence of the Structural, Electrical and Optical Properties of PbSe Nanomaterial Prepared by Hydrothermal Method on the Polyethylene Glycol Content

PbSe was prepared by hydrothermal method using 20, 60, 80, 100 mg contents of polyethylene glycol (PEG) as a surfactant. PbSe films with a thickness of about 120 nm and constant rate of evaporation were prepared on glass and quartz substrates using thermal evaporation technique for studying their electrical and optical properties, respectively. The prepared films were identified by X-ray diffraction, transmission electron microscope and energy dispersive X-rays. The variations of resistivity, carrier concentration and carrier mobility of the films with temperature and PEG content were investigated. The optical properties of the films were studied by using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 500–2500 nm. The optical band gap of the films was calculated and it is found to increase with increasing PEG content.

Effect of deposition time on lead selenide thermoelectric thin films prepared by chemical bath deposition technique

A series of lead selenide (PbSe) films was deposited at constant bath temperature with various deposition time (3–5h) using simple chemical bath deposition techniques, to study the effect of deposition time on its structural and thermoelectric properties. The as-deposited film was analyzed through X-ray diffraction, SEM, Energy dispersive X-ray analysis, Raman spectroscopy and Seebeck coefficient measurement. The improvement of crystallinity of the PbSe films was studied using X-ray diffraction and Raman scattering. The structural parameters, such as the lattice constant (a), crystallite size (D), dislocation density (δ) and microstrain (ε) were evaluated from the XRD spectra. Average crystallite size was calculated from Scherrer׳s formula and it was found to be increased from 19.65 to 23.97nm as the deposition time was varied from 3h to 5h. The dislocation density and microstrain were found to vary inversely with the crystallite size, whereas the lattice constant increases with an increase in crystallite size. SEM images show that the morphology of particles strongly depends on the deposition time. The possible growth mechanism for the variation in the morphology is discussed. The thermoelectric measurements have shown n-type conductivity in “as deposited films” and the magnitude of Seebeck coefficient is found to be increasing with an increase in deposition time.

Influence of substrate temperature on structural, morphological and electrical properties of PbSe film deposited by radio frequency sputtering

PbSe films were prepared by radio frequency magnetron sputtering from PbSe slices target under different substrate temperatures (from room temperature to 300°C). The effect of substrate temperature on structural properties of PbSe thin film was investigated. The surface morphology and the crystal structure of film were determined using field emission scanning electron microscopy and X-ray diffractometry, respectively. It was found that the grain shape changed with substrate temperature. When the substrate temperature was below 250°C, most of the crystal grains were spherical in shape. For temperatures above 250°C, the grains transformed to triangle or prismatic ones. Meanwhile, with increasing substrate temperature, the preferential orientation of the film changed from (200) to (220). To figure out the intrinsic mechanisms for this behavior, the texture coefficient, as well as the comparison between surface energy and elastic strain energy was performed. At lower temperature, the film growth was determined by surface energy, which was replaced by strain energy at higher temperature. Therefore, the diversity of crystal structure and morphology of the films at different substrate temperatures occurred. Moreover, the electrical properties of the p-type PbSe films are also quite dependent on substrate temperature. With substrate temperature increased, the electrical resistivity decreased from 1.88 to 0.14Ωcm, while the carrier concentration increased from 1.74×1018 to 4.08×1019cm−3 as the mobility was enhanced from 0.54 to 2.21cm2/Vs.

Impact of thickness on crystal structure and optical properties for thermally evaporated PbSe thin films

Nanocrystalline lead selenide (PbSe) thin films with varied thickness were deposited on glass substrates by thermal evaporation technique. Physical and surface properties of PbSe films were examined by XRD, AFM, UV–Vis spectra. It was found that the structural and optical properties of the PbSe films were strongly dependent on thickness. On one hand, the particles were uniformly distributed in the surface for the thinner samples. On the other hand, however, the particles began to aggregate and worm-like structures appeared for the thicker ones (93 nm and 127 nm). The absorption band-edge was red shifted with increasing thickness. The optical band gap ranging 2.04–2.30 eV with thickness was observed. This could be attributed to the increasing of grain size and the improvement of film uniformity.

Morphological Change of a PbSe film grown by Chemical Bath Deposition during Sensitization

ABSTRACTA PbSe film was grown by chemical bath deposition on a thermally oxidized Si (111) substrate. Morphological change of the PbSe film during sensitization under the oxygen and iodine atmospheres was studied by SEM. The as-grown polycrystalline PbSe film consists of clusters of about 200nm in diameter. By the oxidation treatment for 30 min at 380°C, the clusters became joined together. On the other hand, recrystallization of new PbSe crystals with faceted surfaces occurred during the iodination treatment under an iodine plus nitrogen atmosphere at 380°C for different durations. This morphological change during the sensitization treatment might affect the electro-optical properties of the PbSe film.

Термосенсибилизация нанострукурированных пленок PbSe

The mode and conditions of thermal sensitization in the temperature range 648-698K lead selenide films deposited from ethylendiamine acetate system were determined. The effect of heat treatment on the morphology and properties of photosensitive chemically deposited films of PbSe was shown. It is concluded that the results obtained by hydrochemical synthesis of nanostructed highly sensitive to infrared range radiation layer of lead selenide provide performance on the level of best foreign analogues.