Effect of doping and heat treatment on the photoluminescence of CdS films deposited by spray pyrolysis

Large area thin films of n-type CdS were prepared by spray pyrolysis technique. The films were n-type doped by having [In/Cd] ion concentration ratio of 10⁻⁶, 10⁻⁵, 10⁻⁴, 10⁻³, and 10⁻² in the sprayed solution. These films were heat treated in N₂ atmosphere at 450°C for 1 h. The as-deposited undoped, doped, and heat treated were analyzed by photoluminescence (PL) at 5 K sample temperature. In general, the spectra displayed three main emission regions (green, yellow and red) with more than one band in each. The emission intensity is found to decrease with doping and the relative intensity of the bands is found to depend on the doping concentration level. The red band is only present in doped samples and its relative intensity is found to increase with doping. The effect of heat treatment in N₂ on the as-deposited undoped and the doped (10⁻⁴) samples on the relative intensity of the observed bands were compared and discussed. The results are compared with the electrical and morphological results and correlated with the probable changes in the concentration of shallow and deep radiative native defects and structural changes. These allow for better prediction of suitable doping and treatment conditions for good quality films.     

Controlled p-doping of pigment layers by cosublimation: Basic mechanisms and implications for their use in organic photovoltaic cells

We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The films are characterized in situ by temperature dependent Seebeck and conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states with increasing dopant concentration. We thus conclude that doping has the potential of both reducing the series resistance and increasing the photovoltage of organic solar cells. As a first step to exploit this potential, we present two different ways of preparing diodes with rectification ratios in excess of 10⁴ using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which work already in the strict absence of oxygen and are stable in air. To increase the efficiency of charge carrier generation in photovoltaic cells, we need to use photoactive donor–acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominally undoped layers.     

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.     

Photoelectrical properties of doped cadmium sulphide powders

Photosensitive powders of CdS were prepared with different concentrations of dopants. Doses of donors (Cl⁻) and acceptors (Cu²⁺, Ag⁺) varied from 0 to 41 and from 0 to 9 mg/g CdS, respectively. Reflectance, absorption coefficient and resistance dependence on illumination intensity and voltage at the wavelength of about λ=630 nm and photovoltage spectra in the range 450–900 nm were measured on layers prepared from the powders. The value of absorption coefficient grew with the increasing dopant concentrations; acceptors appeared more efficient than donors. Reflectance decreased with growing acceptor dose. Using the values of reflectance, absorption coefficient and resistance the corrected photovoltage, as the measure of the concentration photogenerated charge carriers, was calculated. The ratio (σ_IGB) of the corrected photovoltage and photocurrent was used as the criterion of intergrain barrier conductivity. All doped samples exhibited similar value of σ_IGB which was about three orders of magnitude lower than that of undoped sample.     

Spectroscopic study of Cu/Cu doubly doped and highly transmitting glasses for solar spectral transformation

This work investigates the formation of photoluminescence (PL) centres in high-transmission glasses (HTG) doped with Cu₂O and their capability to transform the solar spectrum by absorption/emission via upconversion, downconversion and Stokes-shifted PL into a more efficient spectrum for photovoltaic applications. Both the green PL Cu⁺ and the non-PL Cu²⁺ centres are formed in HTG, although their relative concentration depends on the thermal treatment and the presence of other codopants. Given that the absorption spectrum of Cu⁺ lies around the HTG band gap, measurement of the absorption coefficient α(λ) for these absorption bands is not easy due to corrections between the reflection coefficient and chromatic dispersion. We present a procedure, named two-thickness method, to extract the actual absorption coefficient for the spectrum of each formed centre. In addition it provides the relative Cu⁺/Cu²⁺ concentration as well as their absolute values. Analysis of the spectra also provides information on the absorption cross section, transition energy and bandwidth of each band, the knowledge of which is essential to check the suitability of such centres for photovoltaic applications in silicon solar cells.     

Deep defect determination by the constant photocurrent method (CPM) in annealed or light soaked amorphous hydrogenated silicon (a-Si:H)

We present systematic measurements of CPM on two independent series of slightly phosphorous and boron doped films. For “n-type” samples of both series, the CPM deep defect absorption is proportional to the square root of the gas dopant ratio. For these samples we discuss the influence of Fermi level on the CPM spectra. For slightly “p-type” samples, CPM deep defect absorption as evaluated by CPM becomes higher than the corresponding PDS-values. This fundamental problem can be traced back to the violation of two basic conditions necessary for a correct evaluation of the absorption from CPM measurements: (1) the power law exponent γ (Rose factor) of the photoconductivity must be spectrally independent, and (2) the generation rate G, which corresponds to the CPM photocurrent, also has to be spectrally independent. Further, we compare the annealed and the “saturated” light soaked states of selected slightly doped samples and an undoped sample: the variations in the CPM deep defect absorption and in photoconductivity due to light-soaking are discussed.     

Effect of molecular aggregation by thermal treatment on photovoltaic properties of MEH-PPV: Fullerene-based solar cells

The effect of a thermal annealing treatment at various temperatures on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and buckminsterfullerene (C₆₀) composites was investigated. With the increase in temperature, three stages were observed: from 60 to 120 °C, the photovoltaic performance of the composite devices became better with the rising temperature; from 120 to 180 °C, the photovoltaic performance decreased to a minimum; but, at about 200 °C, a sharp rise occurred, followed by a gradual decline. The obvious red-shift of the photoluminescence (PL) spectra and the new PL peak and shoulder at about 620 and 660 nm, respectively, indicated the formation of molecular aggregation and the lengthened and the ordered conjugated segments. It was suggested that the crystallization of C₆₀ after thermal annealing at 200 °C, which matched the nano-sized ordered domains of MEH-PPV led to a sharp enhancement in photovoltaic performance.     

The structure and photocatalytic studies of N-doped TiO2 films prepared by radio frequency reactive magnetron sputtering

N-doped TiO₂ films were prepared by a radio frequency reactive magnetron sputtering (RF-MS) deposition method from an undoped TiO₂ target in a mixture of Ar/N₂ atmosphere on heated quartz glass substrates. The structures and properties of the N-doped were studied by XRD, Raman, XPS, TEM, ultraviolet (UV)-vis and PL spectroscopy. By analyzing the structures and photocatalytic activities of undoped and N-doped TiO₂ films under ultraviolet and visible light irradiation, the probable photocatalytic mechanism of N-doped TiO₂ films was investigated. Because many oxygen defects are caused in films by nitrogen doping, it is presumed that nitrogen doping and oxygen defect induced the formation of new states closed to the valence band and conduction band, respectively. The cooperation of nitrogen and oxygen defects leads to a significant narrowing of the band gap and greatly improves the absorption in the visible light region. It is found that the degradation efficiencies of N-doped TiO₂ films greatly decreased under ultraviolet irradiation, but slowly improved under visible light irradiation, compared with the undoped TiO₂ film. It is suggested that the N-doped TiO₂ films are formed for the nitrogen to occupy oxygen defect sites directly. The doped nitrogen ions and oxygen defects act as recombination centers that reduce the lifetime of photo-induced electrons and holes, thereby resulting in the decrease of photocatalytic activity under ultraviolet light illumination.     

Spectrally dependent photocurrent generation in aggregated MEH-PPV:PPDI donor–acceptor blends

Films of MEH-PPV and PPDI blends with weight ratio 1:2 have been prepared by spin-coating and annealing between 0 and 60 min at 95 °C. The films were characterized by absorption spectroscopy and atomic force microscopy (AFM). The main emphasis has been on the photon conversion efficiency in the photovoltaic cells as a function of excitation wavelength and applied voltage/electric field. Site selective excitation at wavelengths at which either non-aggregated bulk PPDI or dimers/aggregates of PPDI absorb prove that (i) the rate-limiting process for power conversion is the field-assisted escape of optically generated geminate electron–hole pairs from their mutual coulombic potential and (ii) the photogeneration yield depends on the donor–acceptor topology. A significant difference of the yield has been noted when alkoxy-pendent groups in MEH-PPV are replaced by phenyl-alkoxy groups.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

Preparation and photo-induced charge transfer of the composites based on 3D structural CdS nanocrystals and MEH-PPV

We report the synthesis of 3D structural CdS nanocrystals by a simple biomolecule-assisted hydrothermal process. The CdS nanocrystals are composed of many branched nanorods with the diameter of about 50 nm, and the length of about 250 nm. The phase and crystallographic properties are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD). The composites based on CdS nanocrystals and poly[2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) have been prepared by spin-coating of the mixture in the common solvent. The optical properties of the composites are investigated using ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectroscopies. A significant fluorescence quenching of MEH-PPV in the composites is observed at high CdS nanocrystals/MEH-PPV ratios, indicating that the photo-induced charge transfer occurred due to the energy level offset between the donor MEH-PPV and the acceptor CdS nanocrystals. The obvious photovoltaic behavior of the solar cell made from this composite further demonstrates the mentioned photo-induced charge transfer process.     

Low-temperature deposition of amorphous silicon solar cells

We develop amorphous silicon (a-Si:H)-based solar cells by plasma-enhanced chemical vapor deposition (PECVD) at deposition temperatures of T_s=75°C and 100°C, compatible with low-cost plastic substrates. The structural and electronic properties of low-temperature standard PECVD a-Si:H, both doped and undoped, prevent the photovoltaic application of this material. In this paper, we demonstrate how to achieve device-quality a-Si:H even at low deposition temperatures. In the first part, we show the dependence of structural and carrier transport properties on the deposition temperature. The sub-band gap absorption coefficient and the Urbach energy increase when the deposition temperature declines from T_s=150°C to 50°C, the conductivity of doped layers and mobility-lifetime product of intrinsic a-Si:H drop drastically. Therefore, in the second part we investigate the impact of increasing hydrogen dilution of the feedstock gases on the properties of low-temperature a-Si:H. We restore n-type a-Si : H device-quality conductivity while the p-type a-Si:H conductivity is still inferior. For undoped layers, we depict the hole diffusion length, the mobility-lifetime product for electrons, the Urbach energy, and sub-band gap absorption coefficient as a function of the hydrogen dilution ratio. We incorporate these optimized materials in solar cell structures of single and multilayer design and record initial efficiencies of η=6.0% at a deposition temperature of T_s=100°C, and η=3.8% at T_s=75°C. For prospective opaque polymer substrates we develop, in addition to our conventional pin cells, devices in nip design with similar performance.     

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.     

Glow discharge deposited a-Si:H,Al thin films

The modification of the electrical and optical properties of a-Si : H by the incorporation of aluminium has been studied. The a-Si : H,Al films were obtained by means of dc glow discharge decomposition of gas silane and simultaneous evaporation of alumunium using the same discharge conditions and varying only the evaporation rate of Al. The study of the films properties includes V-NIR transmission spectroscopy, temperature dependence of dark conductivity and steady state photoconductivity. From these measurements an important variation of both optical gap and dark conductivity has been observed as the aluminium content in the films increases. Electrical conduction probably takes place, in the analyzed temperature range (300–450 K), through extended states of the valence band. It has also been observed that the normalized photoconductivity of the slightly Al doped films is three or four orders of magnitude lower than that of undoped a-Si : H film.     

Structure of PECVD Si:H films for solar cell applications

The structure of undoped SiH films and solar cells deposited under different hydrogen concentration and substrate temperatures were studied. The characterization techniques used were XRD, Raman spectroscopy, TEM, optical absorption, and hydrogen effusion. The high concentration films were amorphous in the as-deposited state but crystallized upon annealing at 700°C. Middle and low concentration films were nanocrystalline (nc) and remained nc up to 800°C annealing. A theoretical explanation is given for the stability of these films. Such films, on glass substrates, had optical absorption spectra close to those of amorphous material. The solar cell samples, showed some nc morphology in all-concentration states.     

Temperature effect on the electrical properties of undoped and vanadium-doped ZnTe thin films

Vanadium-doped ZnTe films of composition 0 to 10 wt% V, were prepared onto glass substrate by e-beam evaporation of the element in vacuum at 8×10⁻⁴ Pa. The effects of various deposition conditions on the electrical properties of the films have been studied in detail. The deposition rate of the ZnTe films was at about 2.05 nm s⁻¹. X-ray diffraction (XRD) study shows that the as-deposited ZnTe films are amorphous in nature.The effects of temperature on the electrical properties of the ZnTe and ZnTe:V films were studied in details. The heating and cooling cycles of the samples are reversible in the investigated temperature range after successive heat treatments in air. Thickness dependence of electrical conductivity is well in conformity with the Fuchs–Sondheimer theory. Temperature dependence of electrical conductivity shows a semiconducting behavior with a spectrum of activation energy. The value of activation energy for undoped ZnTe films do agree well with earlier reported values. Dopant vanadium concentration increases the conductivity of the samples. The composition and thickness dependence of the activation energy as well as thermoelectric power studies were done in the 300–413 K temperature range. The results of d.c. conductivity and thermopower obey an activated conduction mechanism. The thermopower of undoped ZnTe films indicates a p-type conductivity. Thermopower results of ZnTe:V films also suggest that the simultaneous bipolar conduction of both carriers take place.     

Metal contact properties of poly3-octylthiophene thin films

Chemically synthesized undoped poly3-octylthiophene (P3OT) was deposited as thin films on conducting glass substrates by drop casting. Doping state of P3OT was obtained by adding FeCl₃ into the original P3OT solution. A qualitative explanation of the electrical contact behavior of the P3OT films was given by a band diagram made from optical absorbance spectra and electrochemical current–voltage curves of the polymeric films. Gold contacts on both undoped and doped P3OT films give an ohmic contact. Silver/P3OT contact shows a rectifying behavior; the forward current is 500 times the reverse current at 0.5 V. Aluminum also forms rectifying contact with the two types of P3OT films, although the experimental rectification ratio is lower than that of the silver. The I–V curves of rectifying contacts were analyzed with Schottky equation and different diode parameters were obtained.     

Temperature dependence of the conductivity in large-grained boron-doped ZnO films

The temperature dependence of the conductivity is investigated as a function of boron doping in large-grained, degenerate polycrystalline ZnO films prepared by low-pressure chemical vapor deposition. Carrier transport in undoped and lightly doped films is mainly controlled by the grain boundary; field emission through grain boundaries limits the conductivity below 90 K, while thermally activated thermoionic-field emission leads to an increase in the conductivity with the temperature near room temperature. In contrast, carrier transport in highly doped films is mainly governed by intra-grain scattering, which does not depend on the temperature for degenerate electron gases, limits the mobility below 120 K, whereas a metallic behavior (decrease in conductivity with increasing temperature) is observed at room temperature, which is linked to the ionized impurity scattering. The transition between the “semiconductor”-like and metallic-like behavior at room temperature takes place for a film with carrier concentration between 6×10¹⁹ and 9×10¹⁹ cm⁻³.     

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.     

Properties of p-type amorphous silicon carbide window layers prepared using boron trifluoride

One set (A) of undoped and three sets (B, C and D) of doped hydrogenated amorphous silicon carbide samples have been made in the framework of a research plan for obtaining high quality p-type window layers by radiofrequency glow discharge of silane-based gas mixtures. The samples of sets A and B were made using different RF-power-density to mass-flow ratios for various methane percentages in the gas mixture. The best carbon incorporation in the amorphous silicon lattice was obtained at the highest RF-power density. The properties of sets C and D, prepared using different RF-power densities and silane and methane propertions have been analysed as functions of the concentration of borom trifluoride with respect to silane. In both cases, the optical gap E_G, after a slight initial decrease, remains at a value of approximately 2.1 eV without quenching in the dopiong ranges covered. The best conductivity obtained is 2 × 10⁻⁷ (μ cm)⁻¹. IR spectra allow to associate these features with the structural quality of the films.