The optical properties of AgInTe2 thin films

The optical absorption in electron-beam-evaporated AgInTe₂ thin films was studied in the energy range 0.5–2 eV. AgInTe₂ was found to be a direct gap semiconductor with a room temperature gap of 1.03±0.01 eV. Another direct transition observed at 1.04±0.01 eV was ascribed to an optical transition from the crystal-field-split valence band to the conduction band minimum. A third direct allowed transition from the spin-orbit-split valence band to the conduction band was identified at 1.77±0.03 eV. An estimate of the p-d hybridization of the uppermost valence bands yields a value of about 15%.     

Solvothermal synthesis of CuInS2 powders and CuInS2 thin films for solar cell application

In this study, a facile solvothermal method was developed to prepare CuInS₂ powders and CuInS₂ thin films. The CuInS₂ powders and CuInS₂ thin films were prepared by solvothermal route using the precursor of Copper (II) chloride, indium (III) nitrate, thiourea, oxalic acid, hexadecyl trimethyl ammonium bromide and ethanol. The morphology, crystallographic structure, chemical composition and optical band gap of CuInS₂ powders and CuInS₂ thin films were investigated using scanning electronic microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and UV–vis spectroscopy. The results reveal that both CuInS₂ powders and CuInS₂ thin films are in chalcopyrite phase. The CuInS₂ powders are mainly composed of flower-like microspheres. Both microstructure of the sphere surface and diameter of sphere are affected by indium nitrate concentration in precursors. The CuInS₂ thin films are composed of a large number of uniform flower-like nanosheets, and the nanosheets become smaller in size and denser in distribution density with increasing concentration of thiourea. The optical band gap is found to be 1.44 and 1.52 eV for CuInS₂ powders and CuInS₂ thin films, respectively. The deposition mechanism of the CuInS₂ is discussed.     

Locally resolved surface photo voltage spectroscopy on Zn-doped CuInS2 polycrystalline thin films

Incorporation of small amounts of Zinc (< 1 at.%) in polycrystalline CuInS₂ thin films for solar cells leads to an increased open circuit voltage. Here we investigate the optoelectronic effect of Zn doping by local surface photovoltage spectroscopy (SPS). SPS is measured using Kelvin probe force microscopy (KPFM) to obtain the surface photovoltage (SPV) and SPS with high lateral resolution, and thereby study the homogeneity of the doping. In our KPFM experimental setup, illumination is realized by a Xe arc lamp and monochromator in the visible spectrum range by means of an optical fiber into the UHV system of the KPFM.We compare CuInS₂ thin film samples with and without Zn doping. The pure CuInS₂ samples show a sharp onset of SPV at the band gap of 1.48 eV, whereas for Zn-doped CuInS₂ we observe a two step onset, with a steep increase of SPV at 1.48 eV. However, already below this band gap, we observe a slight SPV response, even down to about 1.40 eV. This indicates the presence of states in the band gap, likely resulting from disorder induced by the Zn-doping. The absence of lateral differences in the observed SPV spectra favors an explanation by Urbach-tails over the possible existence of a Zn foreign phase. These results are in agreement with transmission/absorption measurements.     

Photoconductivity in crystalline phthalocyanines

The wavelength, temperature, time and intensity dependence of photocurrent of metal-free phthalocyanine (H₂Pc) and copper phthalocyanine (CuPc) single crystals were investigated. The thermal activation energies in the dark are 0·5 and 0·6 eV for H₂Pc and CuPc respectively and the corresponding photo-thermal activation energies are 0·3 and 0·2 eV. An energy level scheme for single crystals of H₂Pc and CuPc is proposed which consists of two trapping levels and five narrow optically active valence bands. In H₂Pc (CuPc), one trapping level at 0·5 eV (0·6 eV) above the valence band edge to which the charge carriers are thermally excited in the dark; and the other trapping level is at 0·3 eV (0·2 eV) below the conduction band edge where all the optical transitions terminate. In H₂Pc(CuPc), the forbidden gap is 1·44 eV (1·34 eV) wide; the five valence bands are at the band edge, and 0·09 (0·22), 0·42 (0·63), 0·69 (0·90), 1·32 (2·17) eV below the band edge.     

Amino acid 2-aminopropanoic CH3CH(NH2)COOH crystals: materials for photo- and acoustoinduced optoelectronic applications

Photo-induced treatment of l-alanine single crystals grown by slow evaporation method at an ambient temperature was performed using a 25 ps Nd:YAG pulsed laser in the presence of an external acoustic filed. The changes of the absorption were studied for the wavelength 265 nm near the energy band gap edge at acoustical power density varying within 4–6 W/cm². The observed absorption changes indicate that the external optical electric field strengths and acoustical power densities may be efficient parameters for the characterization of photo-optical and acousto-optical treatment of the samples. From the X-ray diffraction data we have optimized the atomic positions assuming that force on the atoms is around 1 mRy/au. These are used to calculate the electronic structure and the chemical bonding for the amino acid l-alanine single crystals. The calculated electronic band structure and densities of states confirms the experimental results that this compound possesses a relatively large energy band gap. The upper valence band has its maximum at the Z point of the Brillouin zone while the conduction band minimum is located at Γ point in the zone center, resulting in an indirect energy band gap. The electronic energy gap is equal to 4.19 eV within a framework of the used local density approximation and 4.54 eV with the Engel–Vosko generalized gradient approximation as the exchange correlation potential. This is in an agreement with our experimentally measured energy band gap ~4.67 eV. The existence of O-p character in the upper valence band has a significant consequence for the optical band gap. From our calculated electron charge density distribution, we obtain a space electron charge density distribution in the average unit cell of the crystal. The chemical bonding features of l-alanine amino acid were analyzed.     

Preparation and optical properties of LiInS2 thin films

LiInS₂ films with an orthorhombic (wurtzite-like) structure were prepared by rapid evaporation onto glass and (111)-oriented silicon substrates. Various characterization techniques such as X-ray analysis, Rutherford backscattering analysis and scanning electron microscopy were used to evaluate the quality of the films. Single-phase films were obtained using a source containing excess sulphur $\text{(}\text{[S]}\text{[LiInS}{}_2\text{]}\text{= 0.5}$ by weight) at a substrate temperature of 620 K. The films were annealed at 570 K for 40 min. The optical absorption in LiInS₂ thin films was studied in the photon energy range 2.5–3.5 eV. LiInS₂ was found to posses a direct band gap with a gap energy of 3.170±0.005 eV at room temperature.     

CuInS2 thin films obtained by spray pyrolysis for photovoltaic applications

Copper indium disulphide, CuInS₂, is a promising absorber material for thin film photovoltaic which has recently attracted considerable attention due to its suitability to reach high efficiency solar cells by using low-cost techniques. In this work CuInS₂ thin films have been deposited by chemical spray pyrolysis onto glass substrates at ambient atmosphere, using different composition solutions at various substrate temperatures. Structural, chemical composition and optical properties of CIS films were analysed by X-ray diffraction, energy dispersive X-ray spectroscopy and optical spectroscopy. Sprayed CIS films are polycrystalline with a chalcopyrite structure with a preferential orientation along the <112> direction and no remains of oxides were found after spraying in suitable conditions. X-ray microanalysis shows that a chemical composition near to stochiometry can be obtained. An optical gap of about 1.51 eV was found for sprayed CIS thin films.     

Band offset of SnS solar cell structure measured by X-ray photoelectron spectroscopy

The energy band offset at the heterointerface is one of the most important properties of semiconductor heterostructures, particularly in solar photovoltaic devices. Band discontinuities of CdS/SnS and SnS/SnO₂ heterointerfaces were measured by X-ray photoelectron spectroscopy and capacitance-voltage measurements. The valence band offsets were determined to be approximately 1.5 eV for CdS/SnS and 3.5 eV for SnS/SnO₂ interfaces whereas the conduction band discontinuities for these junctions were respectively found to be 0.4 eV and 1.0 eV. Using these values and the energy band gaps of the corresponding layers, the energy band diagram was developed and it was considered to be a TYPE-II heterostructure. The Fermi level was found to be much closer to the valence band maximum for SnS, whereas it appeared in the upper half of the band gap for both CdS and SnO₂.     

Growth behavior and optical properties of N-doped Cu2O films

N-doped Cu₂O films are deposited by sputtering a CuO target in the mixture of Ar and N₂. The structures zand optical properties have been studied for the films deposited at different temperatures. It is found that N-doping can suppress the formation of CuO phase in the films. The films are highly (100) textured at low temperatures and gradually change to be highly (111) textured at the temperature of 500 °C. With the analysis of (111) and (100) grain sizes, the surface free energy and grain size of critical nuclei are suggested to dominate the film texture. The analysis of the atomic force microscopy shows that the film growth can be attributed to the surface-diffusion-dominated growth. The forbidden rule of band gap transition is found disabled in the N-doped Cu₂O films, which can be attributed to the occupation of 2p electrons of nitrogen at the top of valence band. The optical band gap energy is determined to be 2.52 ± 0.03 eV for the films deposited at different temperatures.     

Optical properties of copper-doped Cd3P2

The effects of copper-doping on the optical properties of Cd₃P₂ have been investigated. A greatly enhanced low-temperature photoconductive response at energies below the band gap provides evidence of deep copper impurity levels (≈0.1 eV above the valence band edge). The presence of electrically active copper impurities which bring about a high degree of compensation is believed to be responsible for the sharp increase in the absorption constant at energies less than the band gap as well as for the pronounced reduction in the intensity of photoluminescence. The index of refraction was found to be 3.8 for both as-grown and copper-doped Cd₃P₂.     

Temperature dependence of photocapacitance spectrum of CIGS thin-film solar cell

Deep levels in Cu(In_1 − x,Ga_x)Se₂ (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x, from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.     

Optical and thermal band gap energy of chemically deposited bismuth(III) selenide thin films

The band gap energy of bismuth(III) selenide in thin-film form was determined using the optical and thermal methods. The optical band gap energy of 0.35 eV was calculated on the basis of the recorded optical spectra in the near-infrared region, within the framework of a parabolic approximation for the dispersion relation, using the equations which arise from Fermi’s golden rule for electronic transitions from valence to conduction band. From the temperature dependence of the dark electrical resistance of the bismuth(III) selenide thin films in the region of intrinsic and extrinsic conduction, a thermal band gap energy of 0.37 eV and an ionization energy of the donor impurity level of 0.13 eV were calculated. The thermal, as well as the optical band gap energy are in excellent agreement with a literature value for bulk bismuth(III) selenide. On the basis of these data, several conclusions on the film microstructure (nanocrystalline versus glassy) are derived and also an estimation of the higher bound to the Bohr’s excitonic radius for bulk Bi₂Se₃ is given.     

Space-charge-limited currents and carrier trapping in plasma-polymerized malononitrile films

The dark electrical conductivity of plasma-polymerized malononitrile (PPMa) films in an Ag/PPMa/Ag sandwich structure was investigated over the temperature range 300–525 K at a reduced pressure of 10^-5 Torr. The room temperature current-voltage characteristics indicate space-charge-limited currents. The results for the conductivity as a function of inverse temperature are in accordance with the band model proposed by Barbe and Westgate, and the thermal energy gap between the top of the valence band and the bottom of the conduction band was determined to be 0.86±0.01 eV. At temperatures below about 380±5 K the conduction process is consistent with the presence of an electron trapping level situated 0.34±0.05 eV below the conduction band edge with a density of (5.02±0.05) × 10¹⁶ cm^-3. It is assumed that above about 435±5 K the conduction process is intrinsic. Experiments on the chemisorption of oxygen suggest that electrons are the majority carriers in PPMa films.     

Near-infrared absorption of CuGaTe2 and CuGaSe2 thin films

Near-infrared absorption spectra of p-type CuGaTe₂ and CuGaSe₂ thin films deposited by thermal evaporation were measured at room temperature. Two structures were found in the spectra in the photon energy range from hv = 0.4–1.5 eV. One of them is the free carrier absorption below 0.6 eV in which the absorption coefficient increases as the third power of the wavelength. Another structure seems from carrier concentration dependences to be associated with an absorption band at hv 0.95 eV for CuGaTe₂ and 0.75 eV for CuGaSe₂ due to transition from a lower-lying valence band to an upper one. From the optical absorption data also, the ionization energy of the acceptor level was found to be 190 meV for CuGaTe₂ and 280 meV for CuGaSe₂.     

Influence of metal plasma ion implantation on photo-sensitivity of anatase TiO2 thin films

Nano-scale TiO₂ thin films were synthesized by using sol-gel and spin-coating techniques on glass substrates for photo-catalytic applications. The Ti(IV) butoxide-based TiO₂ thin films were optimized for transforming into the high-purity crystalline anatase phase when calcined at 500 °C. To further enhance the photo-catalysis sensitivity of TiO₂ thin films for use in visible light environments, a metal plasma ion implantation process was implemented to modify the band gap electron configuration of Ti. Various transition metal atoms such as Ni, Cu, V, and Fe were ionized and accelerated at 20 keV to impinge on the surface of TiO₂ substrates at a dosage of 5 × 10¹⁵ ions/cm². ESCA analysis confirmed the binding energy shift of Ti by 0.8-1.2 eV, which accounted for the increased effective positive charge of Ti, resulting in more effective electron trapping capability and, thus, the electron-hole pair separation. In addition, the absorption spectroscopy demonstrated that optical absorption in the visible light regime occurred in specimens implanted with transition metal ions, likely due to the formation of extra impurity energy levels within the original TiO₂ band gap energy structure. Among all tested implant materials, the band gap energy of TiO₂ was effectively reduced by Cu and Fe ion implantation by 0.9-1.0 eV, which was sufficient enough to excite valence electrons over the band gap in visible light environments. The feasibility of the metal-doped TiO₂ thin films for effective applications under visible light irradiation was further confirmed by using super-hydrophilicity contact-angle measurement.     

Composition-dependence of photoconduction band gap in cadmium zinc phosphate glasses

Photoconduction in cadmium zinc phosphate glasses of various compositions has been measured in the spectral energy range 1.5 to 6.2 eV. With the P₂O₅ percentage remaining fixed at 60 mol %, CdO and ZnO contents have been varied between 0 and 40 mol %. Photocurrents start at about 3.85 eV and rise sharply above 4.5 eV. The optical band gap obtained by the extrapolation of the linear region of the photoresponse curve shows a slow decrease with an increase in the applied field. The optical band gap at a particular applied field decreases with increasing ZnO content. The field-dependence of the optical band gap may be explained in terms of an energy band scheme for non-crystalline solids. The composition-dependence may be due some structural changes in the glasses.     

Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

Metal-free phthalocyanine (H₂Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H₂Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H₂Pc is in good agreement, thereby indicating that H₂Pc is thermally stable even at relatively higher temperature as semiconductor.     

Experimental and first-principles theoretical studies on Ag-doped cuprous oxide as photocathode in photoelectrochemical splitting of water

Nanostructured thin films of undoped and Ag-doped cuprous oxide were deposited on indium tin oxide-coated glass substrate using simple spray pyrolysis method for their use as photocathode in photoelectrochemical (PEC) cell for solar energy based water splitting. Combination of experiments and first-principles density functional theory based calculations was used to determine and understand the effect of Ag substitution on electronic structure and PEC performance. Thin films were characterized using XRD, FE-SEM, UV–Vis spectroscopy and PEC measurements. The results of DFT calculations show that the top of valence band and bottom of conduction band of undoped Cu₂O lie at Г point of brillouin zone, respectively, suggesting that pure Cu₂O is a direct band gap material. Minimal changes appear in the band gap and band gap energies in the Ag-doped Cu₂O system, keeping it still a direct band gap material. A defect band appearance can be seen between ?4 and ?5 eV in the valence band consisting mainly of Ag 4d states and can be explained by a stronger interaction between the Ag 4d and O 2p, due to the larger Ag size. Ag-doped samples exhibit improved conductivity and fourfold increase in photocurrent density with respect to undoped samples.     

Structural and optical characterization of Sn incorporation in CuInS2 thin films grown by vacuum evaporation method

Structural and optical properties of non-doped and Sn-doped CuInS₂ thin films grown by double source thermal evaporation method were studied. Sn deposition time is taken between 0 and 5 min. The films were annealed at 250 °C for 2 h in vacuum after evaporation. The X-ray diffraction spectra indicated that polycrystalline CuInS₂ films were successfully obtained after annealing and no Sn binary or ternary phases are observed for the Sn time depositions less or equal to 5 min. The Sn-doped samples after annealing have bandgap energy of 1.45–1.49 eV. Furthermore, we found that the Sn-doped CuInS₂ thin films exhibit N-type conductivity after annealing.     

Optical properties of β-FeSi2 single crystals grown from solutions

We studied the optical absorption, polarized reflectance (PR) and photoluminescence (PL) of β-FeSi₂ single crystals grown from solution. In low-absorption measurements, we found a phonon emission and absorption structure, which suggests an indirect transition. The exciton energy gap of 0.814 eV was determined from the absorption spectrum at 3.5 K. We also found a direct transition with the gap energy of 0.939 eV. PR measurements for Ea, Eb and Ec revealed the anisotropy of reflectivity of β-FeSi₂. We observed the PL with a peak wavelength of about 1.56 μm at 20 K.