Thin Solid Films (v.518, #7)

In this work, the dispersion mechanisms affecting the electric transport in CuIn1−x Ga x Se2 (CIGS) thin films grown by a chemical reaction of the precursor species, which are evaporated sequentially in two and three-stage processes are analyzed. It was found, through conductivity and Hall coefficient measurements carried out as functions of temperature, that the electrical conductivity of the CuIn1−x Ga x Se2 films is affected by the transport of free carriers in extended states of the conduction band as well as for variable range hopping transport mechanisms, each one predominating in a different temperature range.
Keywords: CuIn1−x Ga x Se2; Thin films; Electric transport; Solar cells;

Non-destructive evaluation of carrier transport properties in CuInS2 and CuInSe2 thin films using photothermal deflection technique by Anita R. Warrier; K.G. Deepa; Tina Sebastian; C. Sudha Kartha; K.P. Vijayakumar (1767-1773).
Photothermal deflection technique (PTD) is a non-destructive tool for measuring the temperature distribution in and around a sample, due to various non-radiative decay processes occurring within the material. This tool was used to measure the carrier transport properties of CuInS2 and CuInSe2 thin films. Films with thickness < 1 μm were prepared with different Cu/In ratios to vary the electrical properties. The surface recombination velocity was least for Cu-rich films (5 × 105  cm/s for CuInS2, 1 × 103  cm/s for CuInSe2), while stoichiometric films exhibited high mobility (0.6 cm2/V s for CuInS2, 32 cm2/V s for CuInSe2) and high minority carrier lifetime (0.35 µs for CuInS2, 12 µs for CuInSe2).
Keywords: Photothermal; Thinfilm; Mobility; Carrier lifetime; Surface recombination velocity;

Optical properties of CdSe and CdO thin films electrochemically prepared by R. Henríquez; P. Grez; E. Muñoz; H. Gómez; J.A. Badán; R.E. Marotti; E.A. Dalchiele (1774-1778).
The present work comparatively studies the optical properties of CdO against CdSe samples potentiostatically electrodeposited onto fluorine-doped tin oxide/glasses (FTO). The films were prepared by electrochemical reduction processes in dimethyl sulfoxide (DMSO) solution. The optical properties are studied by transmittance measurements and diffuse reflectance spectra (DRS). Structural properties of the films were also studied by X-ray diffraction spectroscopy (XRD). For CdSe the direct bandgap energy obtained from both transmittance measurements and first derivative peak position of the DRS is 1.69 ± 0.01 eV. In the case of CdO both direct and indirect gaps were found. Careful deconvolution of the spectra allows assigning the direct bandgap energy at 2.52 eV. A first indirect bandgap appears at 2.03 eV, while it is possible to have a second one at 0.46 eV.
Keywords: Optical transmittance; Electrodeposition; Absorption coefficient; Bandgap;

Role of chlorine on the opto-electronic properties of β-In2S3 thin films by Angel Susan Cherian; Meril Mathew; C. Sudha Kartha; K.P. Vijayakumar (1779-1783).
Effect of chlorine doping on the opto-electronic properties of β-In2S3 thin film, deposited by spray pyrolysis technique is studied for the first time. Chlorine was incorporated in the spray solution, using HCl. Pristine sample prepared using In(NO3)3 and thiourea as the precursors showed very low photosensitivity. But upon adding optimum quantity of chlorine, the photosensitivity increased by 3 orders. X-ray analysis revealed that crystallinity was also increasing up to this optimum level of Cl concentration. It was also observed that samples with high photosensitivity were having higher band gap. The present study proved that doping with chlorine was beneficial as this could result in forming crystalline and photosensitive films of indium sulfide.
Keywords: β-In2S3; Spray pyrolysis; Chlorine doping;

Thin-film CdS (300–400 nm) was deposited onto p-GaAs with low-temperature pulsed-laser deposition (PLD) using 532 nm emission of a Nd:YAG laser (6 ns, 10 Hz). The ablation threshold takes place at a fluence of 0.64 J/cm2 and the deposition rate reaches its maximum at 2.68 J/cm2, while further fluence increase caused a deposition rate drop due to plume shielding. X-ray investigations illustrated that the CdS film texture is composed of nano-sized crystallites (10–30 nm) embedded in an amorphous matrix. Energy dispersive analysis of X-ray and electron probe microanalysis revealed almost stoichiometric composition. Alternating photocurrent spectroscopy showed that the CdS/GaAs sample exhibits intrinsic room-temperature responsivity, which might be useful for specific optoelectronic interconnects. The work emphasizes versatility and straightforwardness of PLD to form operative devices based on hetero-pairing.
Keywords: Pulsed-laser deposition; Thin-film Cds; Photocurrent; Cds/GaAs heterostructures; Deposition rate;

Chalcopyrites are important contenders among thin-film solar cells due to their direct band gap and higher absorption coefficient. Copper–Indium–Gallium Sulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of ~ 1.5 eV. Record efficiency of 11.99% has been achieved on a 2.7 μm CIGS2 film prepared by sulfurization at the Florida Solar Energy Center (FSEC) PV Materials Lab. In this work, photovoltaic performance analysis has been carried out for a 1.5 μm absorber prepared under similar conditions as that of a 2.7 μm thick absorber sample. It was observed that there is an increase in diode factor and reverse saturation current density when the absorber thickness was decreased. The diode factor increased from 1.69 to 2.18 and reverse saturation current density increased from 1.04 × 10− 10  mA/cm2 to 1.78 × 10− 8  mA/cm2. This can be attributed to a decrease in the grain size when the absorber thickness is decreased. It was also observed that there is an improvement in the shunt resistance. Improvement in shunt resistance can be attributed to optimized value of i:ZnO for lower absorber thickness and less shunting paths due to a smoother absorber.
Keywords: CIGS2; Absorber thickness; Thin-film solar cells;

CdS thin films doped with metal-organic salts using chemical bath deposition by J. Santos Cruz; R. Castanedo Pérez; G. Torres Delgado; O. Zelaya Angel (1791-1795).
CdS thin films doped with metal-organic salts were grown on glass substrates at 90 °C by the chemical bath deposition technique. Metal-organic salts such as zinc acetate, chromium acetylacetonate, ammonium fluoride, aluminum nitrate, tin acetate and indium acetate were used. The chemical bath was prepared with cadmium acetate, ammonium acetate, thiourea and ammonium hydroxide. In the case of un-doped films, the S/Cd ratio was varied by changing the thiourea in the range 1–12. The best optical, structural and electrical properties were found for S/Cd = 2. The doped films were prepared by always keeping the ratio S/Cd constant at 2. The band gap (E g) of doped and un-doped films was evaluated from transmittance spectra, where films with lower sulfur concentration exhibited higher E g. X-ray analysis showed that both un-doped and doped films were polycrystalline with preferential orientation along the (111) direction and with the zincblende structure in all cases. The dark electrical results showed that CdS doped with Zn (1 at.%) exhibited the lowest resistivity values of 10 Ω cm.
Keywords: Chemical bath deposition; Semiconductor; Thin films; Cadmium sulfide;

CV calculations in CdS/CdTe thin films solar cells by F.L. Castillo-Alvarado; J.A. Inoue-Chávez; O. Vigil-Galán; E. Sánchez-Meza; E. López-Chávez; G. Contreras-Puente (1796-1798).
Polycrystalline thin film CdS/CdTe heterojunction solar cells are important candidates for large scale photovoltaic applications. In this work we use a CV (capacitance vs. voltage) theoretical method for the determination of the interface charge density σ and band discontinuity ΔEv of the CdS/CdTe heterojunction. The methodology is based on three cardinal equations: i) line up of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi level (when voltage is applied), ii) charge neutrality and iii) the total capacitance of the heterostructure. We used CdS/CdTe solar cells, grown in our laboratory by the chemical bath deposition (for CdS film) and the close space vapor transport (for CdTe film) techniques. The interface parameters σ, and ΔEv are determined from CV fitting between the calculated and the measured curve. The methodology presented in this study is general and can be applied to semiconductor–semiconductor and semimetal–semiconductor heterojunctions.
Keywords: CdS/CdTe thin films; CV matching; Solar cells; Semiconductors;

Poly(3-octylthiophene) (P3OT) was synthesized by direct oxidation of 3-octylthiophene with FeCl3 as oxidant. Molecular weight of P3OT polymer was measured by size exclusion chromatography. Homogeneous poly(3-octylthiophene) (P3OT) and polystyrene (PS) composite films have been synthesized by spin-coating technique from toluene with different polymer concentrations. The doped films were obtained by immersion for 30 s in a 0.3 M ferric chloride (FeCl3) solution in nitromethane. A classical percolation phenomenon was observed in the electrical properties of these blends, it was smaller than 5% of P3OT in the blend. Surface topographical changes were studied by atomic force microscopy (AFM). AFM images of the composite films revealed surface morphology variation as a function of different P3OT concentration in PS, phase segregation was observed, and PS is shown to segregate to the surface of the films. The higher PS solubility, in comparison with the P3OT solubility, in toluene resulted in PS/P3OT bilayers. The films exhibited pit and island like topography, the pit size changed with the polymer concentration. Optical absorption properties of the polymeric films were analyzed in pristine and doped state. In doped state, the bipolaronic bands at 0.5 and 1.6 eV are shown in a 4% conductive polymer in the PS/P3OT film. Finally thermogravimetric analysis was also made on the simple and composite polymers.
Keywords: Poly(3-octylthiophene); Polystyrene; Chemical synthesis; Optical properties; Morphology;

Synthesis, structure and photoelectrochemical properties of single crystalline silicon nanowire arrays by E.A. Dalchiele; F. Martín; D. Leinen; R.E. Marotti; J.R. Ramos-Barrado (1804-1808).
In the present work, n-type silicon nanowire (n-SiNW) arrays have been synthesized by self-assembly electroless metal deposition (EMD) nanoelectrochemistry. The synthesized n-SiNW arrays have been submitted to scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and optical studies. Initial probes of the solar device conversion properties and the photovoltaic parameters such as short-circuit current, open-circuit potential, and fill factor of the n-SiNW arrays have been explored using a liquid-junction in a photoelectrochemical (PEC) system under white light. Moreover, a direct comparison between the PEC performance of a polished n-Si(100) and the synthesized n-SiNW array photoelectrodes has been done. The PEC performance was significantly enhanced on the n-SiNWs photoelectrodes compared with that on polished n-Si(100).
Keywords: Silicon; Nanowires; Photoelectrochemistry; Photovoltaic solar cell;

Characteristics of gold/cadmium sulfide nanowire Schottky diodes by Sai Guduru; Vijay P. Singh; Suresh Rajaputra; Shounak Mishra; Raghu Mangu; Ingrid St. Omer (1809-1814).
Schottky diode junctions were formed between nanowires of cadmium sulfide and nanowires of gold, through sequential cathodic electrodeposition into the pores of anodized aluminum oxide (AAO) templates. Lengths of CdS and Au nanowires were 100–500 nm and 300–400 nm respectively, while the diameter was 30 nm, each. Analysis of Schottky diodes yielded an effective reverse saturation current (J o), of 0.32 mA/cm2 and an effective diode ideality factor (A) of 8.1 in the dark. Corresponding values under one sun illumination were, J o  = 0.92 mA/cm2 and A  = 10.0. Dominant junction current mechanisms are thought to be tunneling and/or interface state recombination.
Keywords: Nanowire; Schottky diode; Anodic aluminum oxide; Cadmium sulfide; Electro-deposition;

Physical and sensing properties of ZnO:F:Al thin films deposited by sol–gel by A. Maldonado; S. Tirado-Guerra; J.M. Cázares; M. de la L. Olvera (1815-1820).
Fluorine and aluminum-doped zinc oxide thin films, ZnO:F:Al, were prepared on soda–lime glass substrates by the sol–gel method and repeated dip-coating. The effect of the solution ageing and film thickness on the physical characteristics of the films was studied. Two ageing times, namely, two and seven days, and three different thicknesses, in the order of 220, 330, and 520 nm, were the main variables used in this work. As-deposited ZnO:F:Al films showed a high electrical resistivity, however after a vacuum thermal treatment, it was registered a significant decrease. Structural, optical, and morphological characterizations were carried out in vacuum-annealed films. The X-ray diffraction (XRD) patterns revealed that both as-deposited and vacuum-annealed ZnO:F:Al thin films were polycrystalline with a hexagonal wurtzite-type structure with a well-defined (002) diffraction peak, irrespective of the ageing time of the starting solution. The (002) peak shows a proportional increase with the thickness magnitude. An average crystallite size of about 20 nm was estimated using the well-known Scherrer's formula. From the surface morphological study it was observed that the grain size is almost independent of the ageing time of the starting solution, and the film thickness. Films presented an average optical transmittance in the visible range (400–700 nm) in the order of 90%, as well as a band gap of 3.3 eV. The gas-sensing properties of ZnO:F:Al thin films in an atmosphere containing different concentrations of carbon monoxide, and at different operation temperatures were probed. The highest sensitivity registered was of the order of 93%.
Keywords: Zinc oxide; Sol–gel; Thin films; Transparent electrodes; Gas sensor;

Synthesis of AgInSnS4 thin films by adding tin (Sn) into the chalcopyrite structure of AgInS2 using spray pyrolysis by M. Ortega-López; V.M. Sánchez-Reséndiz; J.J. Cayente-Romero; Y. Matsumoto; M.L. Albor-Aguilera; E. Barrera-Calva (1821-1824).
AgInSnS4 thin films were prepared by adding a tin salt to the starting solution used for preparing chalcopyrite AgInS2 thin films by spray pyrolysis The AgInSnS4 films were grown at substrate temperatures in the 300–400 °C range, using an alcoholic solution comprised of silver acetate, indium chloride, tin chloride and thiourea. The tin chloride content in the starting solution was gradually varied in terms of the molar ratio x  = [Sn]/([S] + [Ag]) from 0 to 0.5 to obtain Sn-doped chalcopyrite AgInS2 (x  < 0.2) and spinel-like AgInSnS4 (x  = 0.2–0.4). X-ray diffraction studies indicated that AgInSnS4 has a cubic spinel-like structure with lattice parameter of 10.77 A. All AgInSnS4 thin films exhibited p-type conduction, and their room temperature conductivity ranged from 10− 1 to 10− 2  S/cm. The conductivity versus 1/T plots for this material showed an Arrhenius-like behavior, from which two activation energies of E a1  = 0.23–0.40 eV and E a2  = 0.07–0.20 eV were determined. These results suggest that the grain boundary scattering and the ionization of shallow acceptors dominate the charge carrier transport in the sprayed AgInSnS4 thin films. The AgInSnS4 absorption spectrum revealed an energy gap around E g  = 1.89 eV, which was associated to direct-allowed transitions. To our knowledge, the quaternary compound has been prepared for the first time using spray pyrolysis.
Keywords: Spray pyrolysis; AgInSnS4; Optical and electrical properties;

Evolution of the optical transitions in Al x Ga1 −  x As/GaAs quantum well structures grown on GaAs buffers with different surface treatments by molecular beam epitaxy by C. Mejía-García; A. Caballero-Rosas; M. López-López; A. Winter; H. Pascher; J.L. López-López (1825-1829).
Al0.3Ga0.7As/GaAs Quantum Well structures were grown by molecular beam epitaxy (MBE) on a 500 nm thick GaAs buffer layer subjected to the following surface processes: a) in-situ Cl2 etching at 70 °C and 200 °C, b) air-exposure for 30 min. The characteristics of these samples were compared to those of a continuously grown sample with no processing (control sample). We obtained the quantum wells energy transitions using photoreflectance spectroscopy as a function of the temperature (8–300 K), in the range of 1.2 to 2.1 eV. The sample etched at 200 °C shows a larger intensity of the quantum well peaks in comparison to the others samples. We studied the temperature dependence of the excitonic energies in the quantum wells (QWs) as well as in GaAs using three different models; the first one proposed by Varshni [4], the second one by Viña et al. [5], and the third one by Pässler and Oelgart [6]. The Pässler model presents the best fitting to the experimental data.
Keywords: Al x Ga1 −  x As/GaAs; Molecular beam epitaxy; Photoreflectance; Optical transitions;

We present a spectroscopic ellipsometry study of silicon nitride based antireflection films deposited on chemically textured multi- and monocrystalline silicon wafers. The ellipsometric parameters were measured from the near infrared to the ultra violet spectral region. We report the effective thickness and complex index of refraction parameters of the antireflection films from all studied surfaces, regardless of their microscopic morphology. We report on a method to make ellipsometric measurements of the effective optical constants and thickness parameters of thin films deposited on alkaline etched (100)-oriented monocrystalline silicon. The effect of the texture on the complex index of refraction can be described within an effective medium approximation approach. The optical properties are consistent with those obtained from a series of reference films deposited on flat silicon surfaces.
Keywords: Ellipsometry; Solar cells; Antireflective coating; Texture;

Optical properties of silver sulphide thin films formed on evaporated Ag by a simple sulphurization method by E. Barrera-Calva; M. Ortega-López; A. Avila-García; Y. Matsumoto-Kwabara (1835-1838).
Silver sulphide (Ag2S) thin films were grown on the surface of silver films (Ag) deposited on glass substrate by using a simple chemical sulphurization method. According to X-ray diffraction analysis, the Ag2S thin films display low intensity peaks at 34.48°, 36.56°, and 44.28°, corresponding to diffraction from (100), (112) and (103) planes of the acanthite phase (monoclinic). A model of the type Ag2S/Ag/glass was deduced from spectroscopic ellipsometric measurements. Also, the optical constants (n, k) of the system were determined. Furthermore, the optical properties as solar selective absorber for collector applications were assessed. The optical reflectance of the Ag2S/Ag thin film systems exhibits the expected behavior for an ideal selective absorber, showing a low reflectance in the wavelength range below 2 µm and a high reflectance for wavelengths higher than that value. An absorptance about 70% and an emittance about 3% or less were calculated for several samples.
Keywords: Silver sulphide thin films; Optical properties; Solar selective tandems;

A special class of transparent conducting oxides (TCO) with high mobility of > 65 cm2  V− 1  s− 1 allows film resistivity in the low 10− 4  Ω cm range and a high transparency of > 80% over a wide spectrum, from 300 nm to beyond 1500 nm. This exceptional coincidence of desirable optical and electrical properties provides opportunities to improve the performance of opto-electronic devices and opens possibilities for new applications. Strategies to attain high mobility (HM) TCO materials as well as the current status of such materials based on indium and cadmium containing oxides are presented. Various concepts used to understand the underlying mechanisms for high mobility in HMTCO films are discussed. Examples of HMTCO layers used as transparent electrodes in thin film solar cells are used to illustrate possible improvements in solar cell performance. Finally, challenges and prospects for further development of HMTCO materials are discussed.
Keywords: Transparent conducting oxide; High mobility; Solar cells;