Photoluminescence properties of polycrystalline AgGaTe2

Due to its high absorption coefficient and close to optimal bandgap energy, AgGaTe₂ is a promising material for solar energy conversion. In order to avoid recombination losses, the study of the defect structure of solar cell materials is very important. This paper reports the results of photoluminescence experiments on polycrystalline AgGaTe₂. Two emission regions centred at 1.32 and 0.8 eV were found. The first region appears near the bandgap energy and comprises three bands that are identified by the theory of heavily doped semiconductors as the band-to-band (1.337 eV), the band-to-tail (1.317 eV) and the band-to-impurity (1.287 eV) recombination. The second deep PL region consists of two bands with the peak energies of 0.835 and 0.75 eV. Both these deep bands have rather low thermal activation energy; 18.5 and 20.8 meV, respectively. The possible origins of these bands are discussed.     

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS₂ thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS₂ films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS₂ thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.     

Optical properties of high-quality CuGaSe2 epitaxial layers examined by piezoelectric photoacoustic spectroscopy

The piezoelectric photoacoustic (PPA) signals for Cu-rich CuGaSe₂ (CGS) /GaAs (0 0 1) epitaxial layer (Cu/Ga=1.09–2.16) grown by molecular beam epitaxy (MBE) were successfully obtained at liquid-nitrogen temperature. The bandgap energies of CGS (A-band) decreased and GaAs was not almost changed with increasing the Cu/Ga ratios. This phenomenon was very similar to that of free exciton (FE) by photoluminescence (PL) and the lattice parameter c by X-ray diffraction (XRD) measurements.     

Photoluminescence studies of p-type chalcopyrite AgInS2:Sn

Chalcopyrite AgInS₂:Sn thin films were prepared by spray pyrolysis technique for a constant ratio of [Ag]/[In]=1.5 and different SnCl₄ concentrations (0.05, 0.1 and 0.2 ml) in the spray solution obtaining x=[SnCl₄]/[Ag]+[In]=0.01, 0.02, 0.04. All films were deposited at substrate temperature of 375 °C. The deposited film for which x=0.02 exhibited p-type conductivity, having band-gap energies of 1.87 and 2.01 eV. Photoluminescence (PL) studies reveal several PL bands located at 1.45, 1.68, 1.70, 1.80 and 1.88 eV at 10 K. Each one of these PL structures are related to different defects, the 1.45 eV emission is related to indium vacancies, 1.80 eV emission to interstitial silver (Ag_in) or Indium in sites of silver (Ag_In), whereas, the other emissions (1.70 and 1.88 eV) are related with a donor–acceptor pair recombination and free to bound transition, respectively, due to sulphur vacancies. Sn in excess modifies the emission bands located at 1.70 and 1.88 eV; we found that Sn reduces sulphur vacancies and PL spectra are dominated by acceptor impurities.     

Characterization of the defect levels in copper indium diselenide

High-resolution photoluminescence (PL) measurements were carried out at 10 K to identify the energy levels associated with the various defect states dominating the semiconductor CuInSe₂ (CIS). PL measurements were taken on the bare surfaces of both thin film and single-crystal (polished and cleaved) samples and through a (Cd,Zn)S window layer deposited by thermal co-evaporation onto the CIS absorber surface. A complete energy band diagram is proposed which identifies the origin of the 12 intrinsic defect states expected in this material. The effects of surface and heat treatments, used in device fabrication processing, on the existence and generation of defect states (deep and shallow) are identified and correlated with the device performance. The inferior single-crystal device performance is correlated with presence of a high density of process-generated radiative surface recombination states and trap levels.     

Effects of annealing on CuInSe2 films grown by molecular beam epitaxy

CuInSe₂ (CIS) thin films with a range of Cu/In ratios were grown by molecular beam epitaxy on GaAs (0 0 1) at substrate temperatures of T_s = 450–500°C and the effects of annealing under various atmospheres have been investigated. Photoluminescence spectra obtained from an ex-situ vacuum annealed CIS film at a temperature of T_A = 350°C showed a red-shift and a broadening of an emission peak (peak c) which originally appeared at 0.970 eV before annealing and the red-shifted peak c was found to consist of two overlapping peaks. The excitation power dependence of these overlapping peaks indicated the radiative recombination processes associated with the emissions to be a conduction band to acceptor transition (peak at 0.970 eV) and a transition due to donor-acceptor pairs (peak at 0.959 eV), indicating the formation of a shallow donor-type defect during the vacuum annealing process. The origin of this defect has tentatively been attributed to Se vacancies. On the other hand, the molar fraction of oxygen increased with increasing annealing temperature in dry-air. An epitaxially grown In₂O₃ phase was found both in Cu-rich and In-rich films annealed at T_A 350°C, which was not observed in the films annealed in Ar atmosphere. Thermodynamic calculations based on the Cu---In---Se---O---N system showed In₂O₃ to be the most stable phase in good agreement with the experimental results.     

Fabrication of nano N-doped In2Ga2ZnO7 for photocatalytic hydrogen production under visible light

N-doped In₂Ga₂ZnO₇ photocatalysts were fabricated by solid state reaction route. All the prepared photocatalysts were successfully characterised by PXRD, optical absorption spectra, SEM, TEM, XPS, BET surface area and photoresponse studies. The formation of In₂Ga₂ZnO₇ was confirmed by the PXRD pattern. Optical absorption spectra showed that the visible light absorption of all the photocatalysts were enhanced by nitrogen doping. Among all the prepared photocatalysts, 1 wt% Pt loaded N-GaInZn-500 showed enhanced photocatalytic activity towards hydrogen evolution under visible light irradiation in presence of 10 vol% methanol solution as sacrificial agent. The excellent photocatalytic activity of N-GaInZn-500 is in agreement with N-content, bandgap energy, PL intensity and Surface area.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Surface ligand effects in MEH-PPV/TiO2 hybrid solar cells

TiO₂ nanorods (NRs) were synthesized by hydrolysis of titanium tetraisopropoxide (TTIP) using oleic acid (99%) as surfactant at low temperatures (80-100 °C) and are modified with different ligands: oleic acid (OLA), n-octyl-phosphonic acid (OPA) and thiophenol (TP) in order to investigate the effect of surface ligand on the excition dissociation and the charge transport in hybrid MEH-PPV/TiO₂ photovoltaic (PV) cells. The morphology and crystalline form of as-prepared TiO₂ NRs are examined by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and Raman spectrometer (RS). The FTIR analysis confirms all the ligands coordinated with the Ti center of TiO₂ NRs. The optical properties of the modified TiO₂ NRs are characterized by UV-vis absorption spectra and photoluminescence (PL) spectra. Thiophenol modified TiO₂ NRs quench the PL of MEH-PPV more effectively than OLA-TiO₂ NRs and OPA-TiO₂ NRs. The power conversion efficiency of hybrid PV cells from thiophenol modified TiO₂ NRs and MEH-PPV is the highest among the investigated TiO₂ NRs.     

Photoluminescence from photochemically etched silicon

The photoluminescence (PL) of photochemically etched silicon is studied. In the photochemical etching process, an n-type silicon wafer is immersed in an etchant solution of hydrofluoric acid (HF) and H₂O₂. A low-power visible laser (typically He–Ne) is used to illuminate the samples. The etching process occurs through the photogeneration of carriers. Although no electrodes are used in this etching method, the final samples show PL similar to electrochemically etched porous silicon. The samples were prepared using (1 0 0) n-type silicon with a resistivity of 1.0–5.5 Ω cm. An He–Ne laser with 20 mW of maximum power output was used and the spot radius (on the samples) was varied from 1 to 4 mm. A strong emission in the red-yellow optical region can be present in the final samples depending on the HF:H₂O₂ concentration ratio, etching time and laser intensity of the etching process. The PL spectra excited with the monochromated output of an Xe light source as excitation is studied. The peak wavelength of the PL intensity shifts to the blue region of the spectrum when increasing the laser intensity. Quantum confinement can explain this blue shifting if smaller silicon nanocrystallites are formed with higher laser intensity. The peak PL intensity also decreases when increasing the laser intensity, although a “threshold” condition must be reached to have measurable PL. Each sample also exhibits a shifting in peak PL wavelength when varying the PL excitation wavelength. The corresponding dependence and the variations of the PL intensity are studied. Other experimental conditions are discussed.     

MoS2-mesoporous LaFeO3 hybrid photocatalyst: Highly efficient visible-light driven photocatalyst

Perovskite type materials have high potential photocatalytic application towards both hydrogen energy generation and organic dye degradation due to their high stability and good reusability. Here, it is the first analysis of photocatalytic degradation of RhB and hydrogen energy evolution under visible light over MoS₂/LaFeO₃ nanocomposite. The physicochemical properties of the materials were characterized using a range of techniques such as XRD, TEM, XPS, FTIR, PL, photocurrent, etc. The optical properties of the nanocomposite show good absorption in UV-Vis spectra as compared to the bare LaFeO₃. In this study, MoS₂/LaFeO₃ nanocomposite was synthesized through single step in situ hydrothermal processes with a narrow bandgap, enhanced photocatalytic application under visible light. This novel MoS₂/LaFeO₃ nanocomposite is an efficient and promising photocatalyst for both hydrogen energy evolution and organic dye degradation.     

Electrical and luminescent properties of CuGaSe2 crystals and thin films

CuGaSe₂ thin films with thicknesses of about 2 μm were prepared by flash and single source evaporation onto mica and (1 1 0)-oriented ZnSe substrates in the substrate temperature range 150–450°C. The obtained polycrystalline CuGaSe₂ films had the chalcopyrite structure with the predominant growth direction 2 2 1. Hall effect, conductivity and luminescence measurements have been carried out on CuGaSe₂ thin films and source materials: CuGaSe₂ single crystals grown by Bridgman technique and by chemical vapour transport using I₂ as transport agent. All films and crystals are p-type. Two acceptor levels with ionization energies E_A150–56 meV and E_A2130–150 meV have been identified as due to Ga vacancy and presence of Se atoms on interstitial sites respectively.     

Impacts of pulsed-laser assisted deposition on CIGS thin films and solar cells

We have proposed a novel laser-assisted-deposition (LAD) process for improving the crystalline quality of CIGS thin films and cell performance. The influences of laser power, Ga content in CIGS, substrate temperature, and photon energy of laser on CIGS thin films and solar cells have been investigated. In the LAD process a pulsed excimer laser and a pulsed Nd:YAG laser were irradiated onto the substrate surface during CIGS deposition by the three-stage process. The crystalline quality of CIGS thin films and cell performance, particularly open-circuit-voltage, improved by LAD process for all ranges of Ga content and at substrate temperature ranges of 400-550 °C. It was also found that the laser irradiation enhanced the diffusion of Ga into CIGS even at low substrate temperatures, which strongly affects the formation of double-graded bandgap. The PL decay time of LAD-CIGS solar cells was much longer than that of the fabricated by the three-stage process, which implies the reduced defects in CIGS absorber layer. The improved thin-film quality and cell performance became noticeable only when the laser wavelength was shorter than 266 nm (4.66 eV of photon energy). This result strongly suggests that the impacts of pulsed-laser irradiation are dominated by photon-energy rather than thermal-energy.     

CVT-growth of AgGaSe2 single crystals: Electrical and photoluminescence properties

Single crystals of AgGaSe₂ were grown by the Chemical Vapor Transport (CVT) -method using iodine as transport agent. Growth temperatures of 770°C and concentrations of the transport agent of 1.6–1.7 mg I₂/cm³ yielded compact single crystals with a size of up to 8 X 5 X 5 mm³ and a habitus dominated by the {112}-faces. The as-grown crystals were highly insulating (σ < 10⁻⁸(Ω cm)⁻¹). Annealing of the crystals in vacuum at 700°C resulted in n-type conductivity of 2 · 10⁻¹ (Ω cm)⁻¹ with a dominant peak in the photoluminescence spectra at 4 K associated with a donor level of 164 meV. Annealing in Se-atmosphere at 600°C lead to p-type conductivity of 6 · 10⁻⁶ (Ωcm)⁻¹ within a surface layer of the AgGaSe₂ single crystals. The corresponding photoluminescence spectra and the activation energy of the electrical conductivity (between 100 K and 300 K) suggest the presence of an acceptor (V_cation) with an activation energy of 60 meV and a donor (V_Se) with an activation energy of 100 meV.     

Photovoltaic properties of SnCl2Pc films and SnCl2Pc/pentacene heterostructures

The optical and photovoltaic properties of dichlorotin phthalocyanine (SnCl₂Pc) films and SnCl₂Pc/pentacene (Pn) heterostructures (HS) have been studied. Weak bands at 1.35, 1.52 and 2.05 eV have been found in absorption and modulated photoreflectance spectra of SnCl₂Pc films. These bands can be caused by the formation of charge transfer states. The low concentration of recombination centers of charge carriers has been formed on a free surface of SnCl₂Pc films. This concentration essentially decreases at air evacuation before vacuum deposition of a Pn layer. Therefore, interface with an insignificant recombination rate of charge carriers is formed for SnCl₂Pc/Pn HS.     

Na incorporation in Mo and CuInSe2 from production processes

Results of characterization of thin films of Mo deposited by DC magnetron sputtering on soda-lime glass (Mo/SLG) and CuInSe₂ (CIS) on Mo/SLG are presented. The primary objective of the work was to clarify the factors determining the concentration of Na in commercial-grade CIS. Mo films were deposited by three laboratories manufacturing CIS thin film solar cells. Analysis was by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffraction. Changes in Mo deposition parameters in general affected the Na level but there was no obvious link to any single Mo deposition parameter. Oxygen content directly affected the Na level. The Na behavior was not obviously connected to film preferred orientation. Selenization of the Mo layers was also examined. Elemental Se vapor was found to produce significantly less selenization than H₂Se. The amount of selenization was also strongly dependent upon Mo deposition conditions, although a specific source of the change in reaction rate was not found. Na distributions in the CIS deposited on the Mo were not limited by the diffusivity of the Na. The Na concentration in the CIS was increased by annealing the Mo films both with and without intentionally added Na. The Na level in the CIS appears to be set more by the CIS deposition process than by the Na concentration in the Mo so long as the Mo contains sufficient Na to saturate the available sites in the CIS.     

Impurities and defects in multicrystalline silicon for solar cells: low-temperature photoluminescence investigations

The low-temperature photoluminescence (PL) measurements (down to 4.2 K) were employed for the investigations of the defects and impurities in multicrystalline silicon (mc-Si) samples grown by block-casting method. The optical properties of as-grown, irradiated by gamma-rays, heat and hydrogen plasma treated samples were studied. It was found that carbon and oxygen as the residual impurity atoms are responsible for the formation of the zero-phonon PL lines with 0.9355 eV (T line) and 0.9652 eV (I line) after heat treatments at about 350–550°C. The appearance of PL lines with the energies of 0.9697 eV (A line) and 0.7894 eV (C line) after a gamma-rays irradiation can be attributed to the formation of carbon- and oxygen-related centers, respectively. The comparison of the PL properties of the mc-Si samples with the mono-crystalline one is performed. It is shown that the main peculiarities of the low-temperature PL spectra of mc-Si can be explained both by the influence of residual impurities and the residual strains in this material.     

Aliphatic–aromatic polyimide blends for H2 separation

In this work the synthesis of a semi aliphatic BTDA-DAH polyimide and their blends with BTDA-ODA and BTDA-DDS polyimides was carried out in order to improve the H₂ permselective properties of polyimides. The syntheses were made using the well-known two steps method and the silylation method. The prepared films were characterized by FTIR, DSC, thermal stability and fluorescence spectroscopy. Intercatenary distances (d-spacing) and gas separation properties were also investigated. PI blend membranes presented only one glass transition temperature (Tg) intermediate between those of the neat polyimides. Fluorescence spectra were a useful tool to recognize electron-donor and electron-acceptor interactions indicating intermolecular charge-transfer complex (CTC) formation which were confirmed by UV–Vis absorptions. As a result, a decrease in the intercatenary distances and a shift for both IR and fluorescence bands of polyimide blends were measured. PI blend membranes showed a permeability decrease with respect to the neat ones, while the selectivity increased according to X-ray diffraction results. To analyze the polyimide blend permselectivities, H₂/CH₄, H₂/CO₂, H₂/O₂ and H₂/N₂ systems were chosen. As a result, H₂/CH₄ separation factor of PI blends was among the highest reported by other authors using traditional membrane materials.     

Defect physics of the ordered defect compound CuIn3Se5

The activation energies of acceptor and donor defect states in the ordered defect compound CuIn₃Se₅ are calculated by using a simple model based on the effective-mass theory for single, double and triple point defect centers. It is found that the values of these energies for both shallow and moderately deep levels are in reasonable agreement with those determined from experimental data. From the analysis of the results, a tentative assignment of the origins of these centers has been made.     

Responsivity properties of ZnTe/GaAs heterostructures formed with pulsed-laser deposition

The paper deals with the optoelectronic properties of the technologically appealing ZnTe/GaAs hetero-pairing formed with nanosecond infrared pulsed-laser deposition. Depending on the applied bias polarity of only 0.5 V, the spectral shape of the direct current responsivity is either a narrow (0.29 meV) peak at 1.378 eV or a broad response starting at the GaAs bandgap. The spectra of the alternating responsivity did not show such a drastic bias dependence. The results are interpreted by the peculiar absorption properties at the ZnTe/GaAs interface.