Compositional and optoelectronic properties of CIS and CIGS thin films formed by electrodeposition

CuInSe₂ (CIS) and Cu(In,Ga)Se₂ (CIGS) thin films were prepared by electrodeposition and processing. The influence of film deposition parameters such as bath composition, pH, deposition potential and material purity on film properties was studied. The structural, morphological, compositional and opto-electronic properties of electrodeposited and selenized CIS and CIGS thin films were characterized using various techniques. As-deposited as well as selenized films exhibited a compact or a granular morphology depending on the composition. The film stoichiometry was improved after selenization at 550°C in a tubular furnace. The films are formed with a mixed phase composition of CuInSe₂ and CuIn₂Se_3.5 ternary phases.     

Properties of ZnO thin films prepared by reactive evaporation

Non-doped ZnO thin films with suitable characteristics to be used as transport contact and as buffer layer in solar cells, were prepared by reactive evaporation. Through a parameter study, it was found that the main deposition parameters affect the optoelectrical properties of the ZnO films, being the oxygen partial pressure the parameter which most affects both, the transmittance and the conductivity. Actually, high-conductive ZnO films with blue transmittances greater than 80% are routinely prepared by using oxygen partial pressures greater than 0.2 mbar and evaporation temperatures of Zn about 540°C. AFM measurements revealed that the high values of blue transmittance obtained with ZnO film deposited at high O₂ – partial pressure are in part controlled by morphological effects. From Hall coefficient and conductivity measurements it was found that the conductivity is dominated by the resulting carrier concentration which is controlled by oxygen vacancies. The conditions to prepare in situ the double low ρ-ZnO/high ρ-ZnO bilayer structure regularly used in the fabrication of CdS-free, thin films solar cells, were also found through this study.     

Studies on growth of ZnO thin films by a novel chemical method

The zinc oxide (ZnO) thin films were deposited onto the glass substrates by a novel chemical method, which is based on the alternate dipping of substrate in an alkaline zinc nitrate solution and double-distilled water containing H₂O₂ (1%) at room temperature. The time duration for which the substrate is dipped in the precursor solution, plays an important role and it has been shown in this work that the time period for which a substrate is dipped in dilute H₂O₂ solution, which we referred as reaction period, affects significantly on the structural, surface morphological and optical properties. The structural analyses from X-ray diffraction (XRD) patterns displayed the formation of polycrystalline ZnO with no preferred orientations. The scanning electron micrographs (SEM) revealed the spongy and powdery clusters for low reaction period, which was converted to uniform spherical grains with increase in reaction period. The optical band gap was found to be declined from 3.28 to 3.12 eV with increase in reaction period.     

Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films ( 10 μm) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Å/s at the substrate temperature (T_s) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn⁺ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm², 1 cm²).     

The behaviour of Na implanted into Mo thin films during annealing

Na implants have been used to study diffusion of Na in rf diode sputtered Mo thin films used as back contacts for Cu(In,Ga)Se₂ solar cells. The samples were analysed with secondary ion mass spectrometry before and after vacuum anneals at 420°C and 550°C. In addition, X-ray photoelectron spectroscopy has been used for surface studies. The diffusion of Na within the Mo grains was found to be very slow as indicated by the unchanged shape and position of the implant peak after the anneal. An increased level of Na in the bulk of the Mo layer strongly suggests diffusion of Na out of the soda lime glass substrate into the Mo film. The oxygen content of the rf diode sputtered Mo films was 8 at% as found by Rutherford backscattering spectroscopy. It is suggested that Mo oxide phases are present in the grain boundaries and that these oxides, being intercalation hosts for Na, are responsible for the rapid diffusion and high solubility of Na in the sputter-deposited Mo films.     

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.     

Hydrogen absorption and optical properties of Pd/Mg thin films prepared by DC magnetron sputtering

Hydrogen storage in metallic thin films in the form of metal hydride have a great potential to solve the hydrogen storage challenges and also thin films offer an opportunity to grow new samples fast with novel structures. In the present work the ex situ study on structural, optical and hydrogen storage properties of Pd-capped Mg thin films have been investigated. The nano structured Pd-capped Mg thin films have been prepared by DC magnetron sputtering on glass substrate. The as deposited and hydrogenated samples have been characterized by XRD and FESEM. The content of hydrogen in thin films has measured by using Elastic Recoil Detection Analysis (ERDA) technique with 120 MeV₁₀₇ Ag⁺⁹ ions. The temperature dependent hydrogen contents in thin film samples have been estimated and the saturation of hydrogen absorption has been observed at 250 °C among all studied samples. In the optical reflectance spectra, Mg hydride samples have been observed partially transparent in comparison to as deposited Mg film.     

Controlled growth of dibenzotetraphenylperiflanthene thin films by varying substrate temperature for photovoltaic applications

We investigate the growth of a novel organic photovoltaic material, dibenzotetraphenylperiflanthene (DBP) thin film by ultra-high-vacuum evaporation. It is shown that the surface morphology and crystalline order of DBP thin films are greatly changed by growth temperature. Especially, at 90 °C, the DBP exhibits a unique orientated growth with long well-directional grain structures. The power conversion efficiency of the device using DBP grown at 90 °C is enhanced by a factor of 61% relative to that of the device using DBP grown at room temperature due to the improvements in photocurrent, open-circuit voltage and fill factor. The results indicate the feasibility of the controlled growth of DBP thin films, which plays a crucial role in improving the performances of photovoltaic devices.     

Enhanced UV-sensitivity of vis-responsive anatase thin films fabricated by using precursor solutions involving Ti complexes

Fabrication of vis-responsive anatase thin films with enhanced UV-sensitivity was attained on an ITO pre-coated glass substrate by applying two precursor solutions involving Ti complexes of oxalic acid and EDTA. The transparent and crack-free thin films were characterized by XRD, XPS, UV–vis and FE-SEM observation. The highest sensitivity to UV light of the vis-responsive film, whose photocatalytic activity was measured by the decomposition rate of methylene blue, was four times as compared with that formed by a sol–gel method under the same conditions. The vis-responsive films showed a characteristic absorption band at around 480 nm.     

Properties of a-SiO:H films prepared by RF glow discharge

Hydrogenated amorphous silicon oxygen alloy (a-SiO:H) films have been prepared by RF plasma enhanced chemical vapour deposition from (SiH₄+CO₂+H₂) gas mixture. Films have been characterized, in detail, by electrical, optical as well as structural studies. The effect of the oxygen incorporation into the Si-network was studied by controlling various deposition parameters e.g., CO₂ to SiH₄ flow ratio, H₂ dilution of the plasma, total flow rate of the reacting gases, RF power applied to the electrodes, working gas pressure in the plasma chamber and the substrate temperature. Optical gap of the films increased due to the incorporation of O, and a lowering in photoconductivity with optical gap widening was monitored. Increasing polyhydrogenation at higher O-content resulted in a rise in defect density. O-incorporation into the Si-network increased the light-induced degradation in photoconductivity.     

Hydrogen absorption by metallic thin films detected by optical transmittance measurements

The hydrogen absorption by bilayers of Pd/Nb and Pd/Ti, grown on glass substrates, was studied by measuring changes in optical transmittance and reflectance in the visible range (wavelengths between 400 nm and 1000 nm) of the films at hydrogen pressures between 3.99 × 10² and 4.65 × 10⁴ Pa. The electrical resistance of the films was also measured during absorption to correlate with the optical data. All the films were grown by a controlled sputtering technique in high vacuum. Pd films ranging in thickness between 4 nm and 45 nm were also characterized when the films were exposed to a hydrogen pressure. The resistance and transmittance of all the Pd samples increased with the uptake of hydrogen until saturation occurred. For Pd/Ti bilayers, fast uptake of hydrogen was deduced from a transmittance increase, indicating hydrogen absorption in the Ti layer. In the case of the Pd/Nb bilayer, a decrease in transmittance was observed, indicating that hydrogen was not absorbed in the Nb layer. The transmittance decrease could be explained by a reduction of Nb native oxide by the hydrogen at the surface.     

Photoelectrochemical investigations on electrochemically deposited CdSe and Fe-doped CdSe thin films

Thin films of CdSe and Fe-doped CdSe (Fe:CdSe) were deposited onto stainless steel substrates by electrodeposition technique. The photoelectrochemical investigations have been carried out using the cell configurations CdSe/1 M (Na₂S–S–NaOH)/C and Fe:CdSe/1 M (Na₂S–S–NaOH)/C for studying the current–voltage (I–V) characteristics in dark and under illumination, photovoltaic output, spectral response, photovoltaic rise and decay characteristics. The studies reveal that films are n-type conductivity. The junction quality factor in light (n_l), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. After Fe doping, efficiency and FF of PEC solar cell is found to be improved from 0.34% and 31.12 to 1.80% and 35.78, respectively.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Large area CIGS2 thin film solar cells on foils: nucleus of a pilot plant

In earlier research, conversion efficiency of 10.4% (AM1.5) and 9.9% (AM0) has been achieved on small area CuIn_xGa_1−xS₂ (CIGS2) solar cell on 127 μm thick stainless steel substrate. The area of research is mainly focused on studying CIGS2 thin films as solar cell absorber material and growing high efficiency cells on ultralightweight and flexible metallic foils such as 127 μm thick stainless steel and SiO₂ coated 25 μm thick Ti foils. This paper presents the scaling up process of CIGS2 thin film substrate from 2.5 × 2.5 cm² to 10 × 10 cm². Initial scaling up efforts focused on achieving uniform thickness and stress-free films. Process of scaling up consisted of refurbishment of selenization/sulfurization furnace, design and fabrication of scrubber and enlargement of new CdS deposition setup. The scaling up from 2.5 × 2.5 cm² to 10 × 10 cm² substrate size has laid the foundation for PV Materials Lab of Florida Solar Energy Center becoming the nucleus of a pilot plant.     

Studies on electrochromic properties of nickel oxide thin films prepared by spray pyrolysis technique

Electrochromic nickel oxide thin films were prepared by using a simple and inexpensive spray pyrolysis technique (SPT) onto fluorine-doped tin oxide (FTO) coated glass substrates from nickel chloride solution. Transparent NiO-thin films were obtained at a substrate temperature 350°C. The films were cubic NiO with preferred orientation in the (1 1 1) direction. Infrared spectroscopy results show presence of free hydroxyl ion and water in nickel oxide thin films. The electrochromic properties of the thin films were studied in an aqueous alkaline electrolyte (0.1 M KOH) using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. The films exhibit anodic electrochromism, changing colour from transparent to black. The colouration efficiency at 630 nm was calculated to be 37 cm²/C.     

低温制备硅薄膜的晶粒尺寸研究

利用等离子体增强（PECVD）法和快速热处理（RTP）法分别在玻璃衬底上制备了硅薄膜.用光谱测试仪器,对2种方法制备的硅薄膜进行了研究.结果显示,各种制备参数对增大薄膜晶粒尺寸存在一个最佳点;总的来说,RTP法制备的晶粒尺寸大于PECVD中制备的晶粒尺寸.     

Improvement of Ga distribution and enhancement of grain growth of CuInGaSe2 by incorporating a thin CuGa layer on the single CuInGa precursor

The growth of grain size of CuInGaSe₂ and the Ga distribution in the thin film CuInGaSe₂ solar cell devices fabricated using a sputtering CuInGa ternary target have been studied. It was observed, adding a thin CuGa layer on top of the surface of CuInGa ternary precursor would increase the Ga concentration, and thus the energy gap in the space-charge region after selenization. As a result, the open circuit voltage (V_oc) of the device was increased by 15%. The SEM and XRD studies further show that the addition of a CuGa layer enhanced the growth of grain size of CuInGaSe₂ during selenization and increased the conversion efficiency of the solar cell devices by 27% (from 6.3% to 8%).     

Preparation and properties of thin Pt-Ir films deposited by dc magnetron co-sputtering

A series of Pt-Ir thin films envisaged for application as fuel cell cathodic catalysts are deposited by dc co-sputtering from pure metal targets. To achieve different metal ratios, the sputtering power applied on the iridium target (P_Ir) is varied in the range 0-100 W at constant power of the Pt target (P_Pt). The influence of the sputtering power on the film composition, morphology, and surface structure is analysed by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The catalytic activity towards oxygen reduction reaction (ORR) is evaluated in sulphuric acid solutions applying the methods of cyclic voltammetry and potentiodynamic polarization curves. The performed morphological and electrochemical investigations reveal that catalytic efficiency of the co-sputtered Pt-Ir films is superior compared to pure Pt. The ORR is most intensive on the sample deposited at power ratio P_Pt:P_Ir = 100:30 W containing 11 at.% Ir that has also the most developed active surface. The ORR current density for this film achieved at 0.825 V in acid solution (4.1 mA cm⁻²) is about 6 times higher than for pure Pt (0.67 mA cm⁻²). The improved activity of the thin co-sputtered Pt-Ir over Pt allows for essential reduction of the catalyst loading at preserved performance.     

The effect of precursor aging on the morphology and electrochromic performance of electrodeposited tungsten oxide films

Tungsten oxide films that have larger effective surface area or extensive grain boundaries tend to be more suitable for use in electrochromic devices. We propose in this paper a simple methodology for increasing the roughness and thus the effective surface area of WO₃ films. This method is based on the tendency of the peroxytungstate precursor to form large aggregates within its solution with time. To this aim, a systematic study of the precursor aging effect on the resulting WO₃ film properties was conducted. It was established that with increasing aging time of the precursor solution, more and larger aggregates are formed, which are then deposited on the film surface. The deposition of the aggregates causes the formation of large cracks on the film surface, thereby increasing its effective area.An optimum of the precursor aging time was found to be around 80 h. Films prepared with such an aged solution were found to have the highest Li⁺ diffusion coefficient and voltammetrically intercalated charge density per unit film thickness. It was also observed that the coloration efficiency of films prepared using the aforementioned method was higher than that of equivalent electron-gun deposited films throughout the visible spectrum and especially in the near infrared. The enhanced properties of these films indicate their improved electrochromic performance, which is mainly due to their increased surface area.     

Hydrogen effect on the optical and electrical properties of metal oxide and nitride thin films

Effect of hydrogen introduction into some metal oxide and nitride thin films was examined. Modification of electrical and optical properties by hydrogen introduction was examined on SnO₂${}_2$, Sn–GeO₂${}_2$, Ge–CdO₂${}_2$, ZnO. Electrical resistance could be reduced in some films without loosing their transparence to the visible light. Hydrogen introduction to Cu₃${}_3$N changed the film an electrical conductor. Films of AlN and TiN were rather insensitive to the hydrogen introduction; the hydrogen effect was only observed when the nitrides contain some imperfection in the compounds.