Se2 Thin Films Research Articles

Low temperature assisted phase transitions in thermally evaporated Cu2Se/Ga3Se2/In3Se2 multilayer thin film structure: Raman signatures of a pristine chalcopyrite CuInxGa1-xSe2 structure

Copper Indium Gallium Selenide (CIGS) thin film solar cell absorber layers are fabricated by the conventional co-evaporation of metal stack precursors followed by a selenization process at a high-temperature range. The article deals with an alternative approach to fabricate the CIGS layer by sequential evaporation of metal selenides followed by a low-temperature treatment (250 °C) during pre-and post-deposition processes. Our post annealing treatment on substrate heat treated film reported a first-ever pristine CuIn0.7Ga0.3Se2 thin film structure without selenization was reported at a very low temperature. A better microstructure with an enhanced grain growth, preferential (112) plane orientation, and Raman signatures of pristine chalcopyrite without a secondary Cu2-xSe phase at the surface was observed. Electron microscopy studies affirm the polycrystalline nature of film structure. Optical analyses have shown near-optimum values as of standard absorber layers. Crucial optical parameters such as refractive index, static and high-frequency dielectric constants were estimated by different relations. The values are concurrent with the semiconductor scale. These features in a low-temperature range optimistically land them as promising candidates in flexible electronics.

Fabrication and characterization of Se/WO3 heterojunctions designed as terahertz/gigahertz dielectric resonators

In this work, Selenium (Se) thin film substrates are coated with tungsten oxide thin layers. The Se/WO3 interfaces are fabricated using a vacuum deposition technique. The substrates displayed stable polycrystalline structure of hexagonal selenium when coated with amorphous WO3. The surface morphology revealed preferred growth of Se nanowires. The coating of Se with WO3 enhanced the light absorbability in the visible and infrared ranges of light spectrum. In addition, the Se/WO3 heterojunctions exhibited conduction and valence band offsets of values of 1.31 eV and 0.0.09 eV, respectively. The band offsets are sufficiently large to force quantum confinement at the interface. It was observed that Se/WO3 behaves as good dielectric resonator revealing resonance peaks in the infrared, visible and ultraviolet ranges of light. Moreover, computing the optical conductivity parameters for these dielectric oscillators revealed drift mobility and plasmon frequency values that are suitable for optical communications. Furthermore, as a terahertz resonator, the Se/WO3 dielectric media displayed terahertz cutoff frequency values in the range of 0.6–75 THz depending on the exciting photon energy. As practical application, when photo-excited with daylight light, the carbon contacted planner Se/WO3 interfaces displayed a light power dependent photosensitivity. The photosensitivity increases 38 times upon irradiation of 0.9 W. With these features, the Se/WO3 heterojunction devices can be nominated for applications in optical communications.

Smooth Cu electrodeposition for Cu(In, Ga)Se2 thin-film solar cells: Dendritic clusters elimination by Ag buffer layer

Smooth and compact Cu(In, Ga)Se2 (CIGSe) thin film is critical to prepare high-quality solar cells. However, the ”dendritic” morphology of Cu tends to form on Mo leading to a nonuniform Cu distribution during the electrodeposition process, which affects the uniformity of CIGSe and device performance. In this work, Ag was used as the underlying buffer layer and Cu was electrodeposited on Ag film instead of Mo film. By adjusting deposition parameters, smooth and compact Ag layer was obtained on the Mo surface. With the Ag buffer layer, the ”dendritic” Cu on Mo was successfully eliminated. The electrodeposited Ag/Cu/In/Ga metal precursors were annealed in Se-containing atmosphere at 450 °C∼550 °C for 15min to prepare (Ag, Cu)(In, Ga)Se2 (ACIGSe) absorbers. Compared with the CIGSe absorber, the crystallinity of the ACIGSe absorber was improved. The VOC, JSC, FF and efficiency of ACIGSe solar cells were improved compared with CIGSe solar cells. Ultimately, an 11.73% efficiency of ACIGSe thin-film solar cells was obtained.

Pb0.8Sn0.2Se thin films: synthesis, sensitization, and properties evolution

Pb0.8Sn0.2Se thin films: synthesis, sensitization, and properties evolution

Photovoltaic Solar Application Study of Cu0.5Zn0.5Se Thin Films by Chemical Bath Deposition Method

Inorganic ternary type materials are induced compound is worked as fundamental applications in transformation of the solar light energy into electrical energy. Copper zinc selenide thin films have been synthesized by chemical bath deposition method on to stainless steel plate. The configuration of fabricated cell is p-Cu0.5Zn0.5Se| NaOH(1M) + S(1M) + Na2S(1M) |C(graphite). The Photovoltaic cell characterization of the films is carried out by studying current–voltage characteristics in dark, capacitance–voltage in dark, barrier height measurements, power output, photoresponse and spectral response. The study shows that Cu0.5Zn0.5Se thin films are p-type conductivity. The junction ideality factor was found to be 2.93. The flat band potential was found to be -0.708V. The barrier height value was found to be 0.186 eV. The study of power output characteristic shows open circuit voltage, short circuit current, fill factor and efficiency were found to be 150 mV, 21 μA, 42.13%, and 0.63%, respectively. Photoresponse shows lighted ideality factor is 2.89. Spectral response shows the maximum current observed at 580 nm.

Significantly (00l)-textured Ag2Se thin films with excellent thermoelectric performance for flexible power applications

A flexible n-type Ag2Se thin film with a high power factor (PF) of 21.6 μW cm−1 K−2 and a ZT value over 0.6 was successfully prepared by a facile self-assembled growth method.

Performance optimization of single graded CIGS absorber and buffer layers for high efficiency: A numerical approach

We present a numerical simulation based study of single graded Cu(In,Ga)Se2 (Copper Indium Gallium Diselenide) thin film solar cell. In this work, initially a basic CIGS single graded cell structure is optimized in terms of thickness, band-gap and doping concentration. CdS is kept as the buffer layer, which is widely used for high efficiency CIGS solar cells. In the next step, CdS is replaced with ZnMgO as the buffer layer in order to exploit its greater photon absorption ability due to its higher band-gap which further enhances the cell efficiency. A thorough analysis is carried out on the solar cell parameters open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and quantum efficiency (η) of the photovoltaic cell structure. An intermediate layer of p-type MoS2 is inserted in between the single graded CIGS absorber layers. The objective is to limit the unintentional Ga inter diffusion and maintain the desired grading during the high temperature annealing for the absorber preparation. The power conversion efficiency of the bilayer device structure with Ga fraction x=(0.31) of the top absorber layer along with Ga fraction y=(0.25) of the bottom absorber layer exhibits an improved efficiency from 24.02% (CdS as the buffer layer) to 25.37% (ZnMgO as buffer layer). An excellent power efficiency of η = 26.78% is reported after adding the intermediate layer of p-type MoS2 and optimizing its thickness and the carrier concentration.

Investigation of nucleation and growth mechanism of selenium electrodeposited on ITO substrate

The present paper reports an investigation of the electrodeposition mechanism of selenium (Se) thin films from an aqueous solution containing selenite ion (HSeO3−) (2 mM) and nitric acid on indium tin oxide (ITO) substrate, with pH 3.2 at room temperature, by using the cyclic voltammetry (CV) and chronoamperometry (CA) methods. The (CV) showed that the electrodeposition reaction is a quasi-reversible controlled by selenite ions diffusion. The (CA) indicated that the selenium particles are electrodeposited following the instantaneous three-dimensional (3D) nucleation model with a diffusion-controlled process, and the average diffusion coefficient (D) of selenite ions was determined to be 2.860 10−6 cm2 s−1. The X-rays diffraction (XRD) showed that the selenium has a hexagonal crystal structure. Scanning electron microscopy (SEM) indicated that the selenium is distributed with uniform nuclei. The UV–Visible spectrophotometry revealed that the optical band gap (Eg) for the electrodeposited selenium is found to be 1.62 eV.

(BixSb1−x)2Se3 thin films for short wavelength infrared region solar cells

Close-spaced sublimation-synthesized (BixSb1−x)2Se3 thin films for short wavelength infrared solar cells.

Rapid thermal annealing process for Se thin-film solar cells.

Recently, selenium (Se) has regained interest as a possible wide-bandgap photovoltaic material for silicon-based tandem applications. However, the easy sublimation of Se below the melting point (220 °C) brings challenges for high-quality Se thin films. Herein, we design a rapid thermal annealing (RTA) method to balance the contradiction between the sublimation and crystallization of Se thin films. Through optimizing the annealing temperature, a high-quality Se thin film is obtained with a large grain size (∼1 μm) and preferred [003] orientation during the RTA process. Then, an optimized efficiency of 3.22% is achieved in a ZnO/Se heterojunction solar cell. This study provides a new guide to obtain high-quality Se thin film by RTA and the method can be extended to other materials with high saturated vapor pressure.

Surface/Interface Effects by Alkali Postdeposition Treatments of (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells.

Ag alloying and the introduction of alkali elements through a postdeposition treatment are two approaches to improve the performance of Cu(In,Ga)Se2 (CIGS) thin film solar cells. In particular, a postdeposition treatment of an alkali metal fluoride of the absorber has shown a beneficial effect on the solar cells performance due to an increase in the open circuit voltage (VOC) for both (Ag,Cu)(In,Ga)Se2 (ACIGS) and CIGS based solar cells. Several reasons have been suggested for the improved VOC in CIGS solar cells including absorber surface and interface effects. Less works investigated how the applied postdeposition treatment influences the ACIGS absorber surface and interface properties and the subsequent buffer layer growth. In this work we employed hard X-ray photoelectron spectroscopy to study the chemical and electronic properties at the real functional interface between a CdS buffer and ACIGS absorbers that have been exposed to different alkali metal fluoride treatments during preparation. All samples show an enhanced Ag content at the CdS/ACIGS interface as compared to ACIGS bulk-like composition, and it is also shown that this enhanced Ag content anticorrelates with Ga content. The results indicate that the absorber composition at the near-surface region changes depending on the applied alkali postdeposition treatment. The Cu and Ga decrease and the Ag increase are stronger for the RbF treatment as compared to the CsF treatment, which correlates with the observed device characteristics. This suggests that a selective alkali postdeposition treatment could change the ACIGS absorber surface composition, which can influence the solar cell behavior.

Fabrication of Buffer-Window Layer System for Cu(In, Ga)Se2 Thin Film Devices by Chemical Bath Deposition and Sol–Gel Methods

Recently, nontoxic fabrication for Cu(In, Ga)Se2 thin films gained increasing popularity. In this work, chemical bath deposition and Sol–gel methods were utilized in the fabrication of an environmentally buffer-window (ZnS-ZnO/ZnO:Al) layer system for Cu(In, Ga)Se2 thin film devices. The influences of conditions like annealing temperature, coating times, and Al-doping amount were investigated. The surface morphology and optical properties illustrated a compact and uniform ZnS film with a band-gap of 3.78, which can be a good alternative to CdS film. Resistivity of 0.35 Ω·cm for the ZnO/ZnO:Al film was achieved by the ideal condition of 9 coating times and 5 Al-doping amounts. A cross-sectional view of the complete Cu(In, Ga)Se2 thin film was obtained, and the structure status was analyzed.

Effect of Se‐Free Annealing on Cesium Fluoride‐Treated Cu(In,Ga)Se2 Thin Films and Corresponding Solar Cells

The effect of selenium‐free annealing on cesium fluoride (CsF)‐treated Cu(In,Ga)Se2 (CIGS) thin films is investigated and their solar cell performance is evaluated. Annealing of CsF‐treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained, because of an increased buffer/CIGS interface recombination as a consequence of re‐evaporation of the alkali‐containing layer from the surface region upon annealing. On the other hand, CsF‐treated CIGS thin films without any annealing after the post‐deposition treatment (PDT) preserve the alkali‐containing layer at the surface, thereby leading to an improved buffer/CIGS interface. Consequently, the open‐circuit voltage (V OC) and fill factor improve significantly in these devices. The Urbach energies of both annealed and nonannealed solar cells are calculated from external quantum efficiency measurement to understand their impact on V OC and V OC deficit. No correlation between the V OC deficit and the Urbach energies is observed.

Swift heavy ion irradiation induced microstructural transformation in selenium thin films

The structural, morphological and optical properties of selenium thin films are important for the application in electronic and optoelectronic devices. In the present work, the effect of annealing in vacuum at different temperatures (130 °C, 160 °C and 200 °C) and swift heavy ion irradiation (Ag9+ ions with energy of 120 MeV) at different fluences (5 × 1011, 1 × 1012, 5 × 1012 and 1 × 1013 ions/cm2) on various properties of thermally deposited Se thin films has been investigated. The phase transition from amorphous to crystalline (t-Se) phase has been achieved with annealing followed by ion irradiation. Raman spectral analysis also confirms the trigonal phase of selenium in annealed and ion irradiated thin films. The morphology of films is found to be dependent on annealing temperature and ion fluence. The growth of nano-rods is observed in the pre annealed (130 °C), irradiated (5 × 1011 ions/cm2) selenium thin film. Optical properties also have been found to be tailored with annealing temperature and ion fluence.

Research Progress of Alkali Doped Cu(In,Ga)Se2 Thin Film Solar Cells

Cu(In,Ga)Se2 (CIGS) thin film solar cell has the advantages of high efficiency, good working stability and low manufacturing cost, which is the most promising thin film solar cell. Currently, the efficiency of CIGS thin film cell has reached 23.35% by doping alkali elements. This review summarized the current status of doping alkali elements on flexible CIGS thin film solar cells with a focus on recent advancements intended for higher efficiency and novel applications. First, the structure of CIGS thin film cell was introduced. According to the structural characteristics of CIGS cells, different doping methods of alkali metals were summarized. Then, the recent developments and trends of research in doping methods of alkali elements within the last years were reviewed, and the effect of different alkali elements on CIGS efficiency was emphasized. Finally, the challenges and opportunities of alkali doping CIGS solar cell were prospected.

Impact of substrate temperature during NaF and KF post-deposition treatments on chemical and optoelectronic properties of alkali-free Cu(In,Ga)Se2 thin film solar cell absorbers

The substrate temperature TPDT during the alkali fluoride post-deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers is a critical process parameter. So far, TPDT is optimized empirically for a particular alkali distribution, and literature only reports on the effects of high TPDT (300–350 °C). To better understand the influence of TPDT and to close the gap for TPDT below 300 °C, a temperature series from 100 to 315 °C was studied. Furthermore, most studies on KF PDTs are performed with alkali-containing soda-lime glass substrates, in which the electrical effects of alkalis from the substrate and the PDT on the final device cannot be separated. To circumvent this obstacle, we use alkali-free substrates in our study and compare electrical and chemical effects of NaF and KF PDTs at different TPDT using current-density-voltage and capacitance-voltage measurements, as well as time-of-flight secondary-ion mass spectrometry and ultraviolet photoelectron spectroscopy. The alkali concentrations are quantified as a function of TPDT and discussed in relation to the respective cell parameters. Both, NaF and KF PDTs, improve the cell efficiency significantly, with the highest conversion efficiency of 16.3% obtained for a NaF PDT at TPDT = 315 °C. A given TPDT leads to a larger K concentration (after KF PDT) than Na concentration (after NaF PDT). For TPDT above 150 °C, similar conversion efficiencies are achieved for NaF and KF PDTs. In contrast, the KF PDT results in a significant decrease of the conversion efficiency at TPDT = 105 °C, likely caused by the formation of a Cu-Se secondary phase at the CIGSe surface, which is accompanied by a shift of the valence band maximum towards the Fermi energy.

Cu (In x Ga1-x )Se2 Thin Films Mechanical and Structural Properties

Due to Cu (In x Ga1-x )Se2 (CIGS) thin films are used as an absorbing material in solar cells because their direct bandgap overlaps the red edge of the solar spectrum and they have a relatively high absorption coefficient (α~105 cm−1) [1]. The efficiency of CIGS solar cells reached 22.9% for laboratory scale devices and up to 15.7% for commercial modules [2], which is the highest conversion efficiency among all commercial polycrystalline solar cells [3]. Structural/nanomechanical properties of Cu(In x Ga1-x )Se2 films deposited on Si(100) by one-step radio frequency magnetron sputtering are discussed. The films were sputtered at different substrate temperatures (25-500 °C). X-Ray diffraction revealed dominant diffraction peak of (112). Surface morphology was examined by field emission scanning electron microscopy and atomic force microscopy. The film composition was examined by electron probe microanalysis. The Cu/(In+Ga) and Ga/(In+Ga) ratios were 0.9 and 0.3, respectively. Nanoindentation, in conjunction with the scanning electron microscopy, was conducted and revealed that the films suffered severe delamination type fracture due to poor adhesion to the Si substrate. The hardness of the films was not influenced by the substrate temperature increase except for the film grown at 500 °C. The hardness results were largely affected at depths larger than the film thickness due to poor adhesion of the film. At deep indents, the hardness results agreed closely with the Si substrate hardness.

Defect assisted nonlinear absorption and optical limiting in amorphous TlGaS2(1-x)Se2(x) (0 ⩽ x ⩽ 1) thin films

In an attempt to investigate the effect of band gap energy and defect states on nonlinear absorption, 80 nm thick amorphous TlGaS2(1-x)Se2(x) thin films with different compositions (0 ⩽ x ⩽ 1) were deposited by vacuum evaporation method. Band gap energies and Urbach energies were obtained from linear absorption spectra. The change in composition led to 0.24 eV increase in band gap energy and 0.32 eV increase in Urbach energy. A theoretical model incorporating one photon absorption, two photon absorption, free carrier absorption and saturation of each absorption process was applied to derive the transmittance in open aperture Z-scan data. The values of nonlinear absorption coefficient and saturation intensity threshold were obtained from the model and two different nonlinear absorption coefficient variation behaviors were observed with increasing Se composition (x) in amorphous thin films based on the opposite contributions of decreasing band gap energies and decreasing Urbach energies on nonlinear absorption.

Unified potential fluctuations model for photoluminescence spectra at room temperature—Cu(In,Ga)Se2 thin films

Room temperature photoluminescence (PL) is a powerful technique to study the properties of semiconductors. However, the interpretation of the data can be cumbersome when non-ideal band edge absorption takes place, as is the case in the presence of potential fluctuations. In this study, PL measurements are modeled to quantify potential fluctuations in Cu(In,Ga)Se2 (CIGS) absorber layers for photovoltaic applications. Previous models have attributed these variations to either bandgap fluctuations (BGFs) or electrostatic fluctuations (EFs). In reality, these two phenomena happen simultaneously and, therefore, affect the PL together. For this, the unified potential fluctuation (UPF) model is introduced. This model incorporates the effect of both types of fluctuations on the absorptance of the material and subsequently the PL spectra. The UPF model is successfully used to fit both single- and three-stage co-evaporated ultrathin (around 500 nm) CIGS samples, showing a clear improvement with respect to the previous BGF and EF models. Some PL measurements show possible interference distortions for which an interference function is used to simultaneously correct the PL spectra of a sample measured with several laser excitation intensities. All the models used in this work are bundled into a user-friendly, open-source Python program.

Effect of cesium fluoride treatment in bifacial Cu(In1-x,Gax)Se2 solar cell

Bifacial Cu(In1-x,Gax)Se2 (CIGS) thin film solar cells were fabricated on titanium-doped indium oxide (ITiO) coated soda-lime glass substrate, and the effect of cesium fluoride (CsF) treatment on solar cell performance was investigated. CsF treatment improved the photovoltaic properties of CIGS/ITiO-based solar cells not only under front-side illumination but also under rear-side illumination. The quantum efficiency for rear-side illumination improved significantly in CsF-treated CIGS solar cells. As a result, the bifaciality improved in the CsF-treated CIGS solar cell. ITiO exhibited good contact with the CIGS thin film, regardless of Cs incorporation. In addition, the effect of heat-light soaking treatment was also investigated in CIGS/ITiO-based solar cells.