In this work the authors have studied the advantages of using electrochemically deposited molybdenum oxide as a buffer layer in an organic bilayer heterojunction solar cell arrangement. Furthermore, it has been probed that electrochemistry provides an alternative low cost, reproducible and less laborious method to prepare thin layered deposits. The precursor solution is composed by a concentrated molybdic acid solution in a sulphuric media in order to ensure the obtainment of low reduced molybdenum species. Therefore, by means of potentiostatic techniques, ITO/molybdenum oxide transparent anodes were tested for the photovoltaic device showing improved surface properties. XDR and AFM techniques were used to characterize the morphology of the deposits. The films with optimum thickness (5 nm) are amorphous. XPS analysis indicates that the best results in solar cell performance are in hand with a heterogeneous composition of the molybdenum oxide film presenting MoV and MoVI as predominant species. The MoO3 films deposited by cyclic voltammetry are not as homogeneous as those deposited by potentiostatic technique and only MoVI species are present. These differences may justify the different behaviour of the solar cells using these different buffer layers. Only buffer layers deposited by potentiostatic technique allow improving the cells performances in the same way than those achieved by evaporation.

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In2O3 (ITO), ZnO, SnO2 are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO3 (45nm)/Ag (x nm)/MoO3 (37.5nm), with x=5–15nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10nm the films are semiconductor, from 10nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10nm. A structure MoO3 (45nm)/Ag (10nm)/MoO3 (37.5nm) resulted with a resistivity of 8×10−5Ωcm and a transmittance, at around 600nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C60/Alq3/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3nm) MoO3 layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO3/Ag/MoO3/CuPc/C60/Alq3/Al and ITO/MoO3/CuPc/C60/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.

Polycarbazole (PCz) thin films have been synthesized by electrochemistry. The influence of the deposition technique, potentiostatic or potentiodynamic, on the properties of the PCz films has been studied using the deposition time and the potential maximum as parameters. The PCz films have been characterized by thermal analysis, X-ray photoelectron spectroscopy (XPS), near ultra violet, visible and infra red absorption, photoluminescence (PL) measurements, scanning electron microscopy and microprobe analysis. Whatever the technique used, polymerized carbazole (Cz) films are deposited onto tin oxide coated glass. It is shown that potential maximum of 1400 mV or more induces some Cz nucleus destruction with N-N bonds formation. It is also shown by different techniques such as XPS, visible and IR absorption, microanalysis, PL, that the oxido-reduction reaction is reversible. Moreover, after dedoping treatment, nearly pure PCz film can be obtained, which allows obtaining PCz depoped films with good PL properties. (C) 2007 Wiley Periodicals, Inc.

Polycarbazole (PCz) thin films have been deposited by electro-oxidation of carbazole in LiCIO, + anhydrous acetonitrile onto SnO2 coated glass substrates, by potentiostatic method and the nucleation and growth mechanism (NGM) were studied. The obtained current time transients (j-t) were fitted using a mathematical equation with three contributions: instantaneous nucleation with two-dimensional (IN2D) or three-dimensional (IN3D) growth, and also a progressive nucleation with three dimensional (PN3D) growth. At the beginning, the IN2D contribution is predominant but, quickly the IN3D processes become more important. At a deposition time t >= 17 s the IN3D corresponds to 80 % of the total current. The visualization by scanning electron microscopy of the surface morphology of the PCz films is in agreement with the NGM proposed by the mathematical method. The film covers very rapidly the SnO2 under layer related to the 2D process, even if some heterogeneities randomly distributed in the films, issued from the 3D processes, are also visible. After two minutes of deposition, the roughness of the whole surface of the films corroborates the 3D processes domination. Therefore, the information directly obtained from the (j-t) transients is a suitable and very useful tool to predict the working conditions in order to control the type of morphology of the film prepared by electropolymerization.

Al or Sn doped ZnO films were deposited by spray pyrolysis using aqueous solutions. The films were deposited on either indium tin oxide coated or bare glass substrates. ZnCl2, AlCl3 and SnCl2 were used as precursors. The effect of ZnCl2 molar concentration (0.1–0.3M) and doping percentage (2–4% AlCl3 or SnCl2) have been investigated. The main goal of this work being to grow porous ZnO thin films, small temperature substrates (200–300°C) have been used during the spray pyrolysis deposition. It is shown that, if the X-ray diffraction patterns correspond to ZnO, the films deposited onto bare glass substrate are only partly crystallized while those deposited onto ITO coated glass substrate exhibit better crystallization. The homogeneity of the films decreases when the molar concentration of the precursor increases, while the grain size and the porosity decrease when the Al doping increases. The optical study shows that band tails are present in the absorption spectrum of the films deposited onto bare glass substrate, which is typical of disordered materials. Even after annealing 4h at 400°C, the longitudinal resistivity of the films is quite high. This result is attributed to the grain boundary effect and the porosity of the films. Effectively, the presence of an important reflection in the IR region in samples annealed testifies of a high free-carriers density in the ZnO crystallites. Finally it is shown that when deposited in the same electrochemical conditions, the transmission of a polymer film onto the rough sprayed ZnO is smaller than that onto smooth sputtered ZnO.

Polycarbazole (PCZ) thin films have been electrochemically synthesized on SnO2 coated glass substrates. The electrolyte used was the anhydric LiClO4. It is shown that, after optimization of the potential scanning domain, PCZ films are systematically obtained. The physical and morphological properties of films are described. The films obtained with LiClO4 as electrolyte are homogeneous and their coverage efficiency on the SnO2 underlayer is very high. Films are exempt of pinholes, cracks and other morphological defaults as shown by scanning electron microscopy. Cyclic voltammograms are reversible, attesting the high quality of the structural properties of the films. Moreover these films are photoluminescent. After deposition of an aluminum upper layer, the structures SnO2/PCZ/Al behave like diodes with a forward polarization when the SnO2 electrode is positively biased. Also these structures exhibit electroluminescent properties.

Poly(tetrabromo-p-phenylenediselenide) (PBrPDSe) has been doped by IBr, H2SO3 and CH3COOH acids. The samples have been studied by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). Conductivity measurements have also been performed on pressed pellet samples. It has been shown by XPS and ESR that, after doping, positive charges are localized on Se atoms. The conductivity of the acid-doped PBrPDSe exhibits an increase by about four orders of magnitude. However, the limit of 10(-7) Ohm (-1) cm(-1) appears difficult to overcome. This saturation effect could be attributed not only to charge localization on Se atoms but also to steric hindrance related to the substituent introduced on the backbone of the polymer. (C) 2000 John Wiley & Sons, Inc.