M. Morsli

MoO3 surface passivation of the transparent anode in organic solar cells using ultrathin films

An original surface passivation technique of indium tin oxide (ITO) used as anode in organic solar cells is proposed. We demonstrate that a thin MoO3 film (3.5±1 nm) at the interface ITO/organic donor allows improving significantly the devices’ performances. The devices are based on the multiheterojunction structure copper phthalocyanine (CuPc)/fullerene (C60)/aluminum tris(8-hydroxyquinoline) (Alq3). The deposition of MoO3 onto ITO improves the charge transfer from CuPc to ITO. The enhancement in the hole collection efficiency in the presence of an oxide layer can be explained in terms of the reduction in the effective barrier against hole transfer from CuPc into the ITO anode. The contact ITO/MoO3/CuPc behaves like a metal-insulator-semiconductor (MIS) structure, which allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc. It is shown that the optimum MoO3 thickness corresponds to a compromise between an optimum ITO c...

Improvement of organic solar cell performances using a zinc oxide anode coated by an ultrathin metallic layer

The authors have achieved an efficient organic solar cell based on copper phthalocyanine (CuPc) layer as donor and fullerene (C60) as acceptor. The aluminum doped zinc oxide (ZnO:Al) instead of indium tin oxide (ITO) is used as the anode. An ultrathin gold film is introduced among ZnO:Al, transparent conductor oxide, and the CuPc donor layer. We show that the power conversion efficiency of this cell is enhanced by one order of magnitude compared to that achieved with a ZnO anode without ultrathin gold film. Therefore, the power conversion efficiency of this cell is comparable to that with an ITO anode.

Electrical properties of polyaniline–polystyrene blends above the percolation threshold

Blends of conductive polymers with classical ones can exhibit good mechanical properties and good electrical conductivity and deserve great attention for application in electronic industrial technology. Conductive polyaniline solutions have been chemically prepared using bis(2-ethyl hexyl)hydrogen phosphate (DiOHP) as the dopant chemical species. The codissolution method leads to conductive polyaniline–polystyrene (PANI–PSt) composites with good mechanical properties. The dependence of electrical conductivity on the volume fraction of PANI in the blend is found to be characteristic of a percolation system. Electrical conductivity and thermoelectric power measurements are interpreted on the basis of hopping mechanisms between polaronic clusters.

Effect of the thickness of the MoO3 layers on optical properties of MoO3/Ag/MoO3 multilayer structures

The electrical and optical properties of MoO3/Ag/MoO3 multilayer structures have been studied using the Ag deposition rate and layer thicknesses as parameters. When the silver film is deposited at 0.20 nm/s rate, the silver layer thickness necessary to achieve the percolation threshold of the resistivity ρ towards conductive structures is 10 nm. Below 10 nm, the films are semiconductor and above the films are conductors. In the present work, the variation of the thicknesses of top and bottom MoO3 layers is shown to strongly modify the optical properties of the multilayer structures. By using a Ag thickness of 10 nm, we demonstrate an increasing of the transmittance of the MoO3/Ag/MoO3 structures by optimizing the MoO3 layers thicknesses. When the MoO3 bottom layer is 20 nm thick, and the MoO3 top layer is 35 nm, the maximum transmission is 86% at the wavelength of 465 nm, while the averaged transmission in the visible range (350 nm-800 nm) is 70%. The best measured conductivity, σ = 1.1 × 105 (Ω cm)-1, corresponds also to this MoO3 (20 nm)/Ag (10 nm)/MoO3 (35 nm) structure. A good qualitative agreement between the theoretical calculations of the variation of the optical transmittance and reflectance of the MoO3/Ag/MoO3 structures is also highlighted.

Comparison of polycarbazole obtained by oxidation of carbazole either in solution or in thin film form

Abstract A comparative characterization of polycarbazole obtained either by electrochemical oxidation of carbazole in solution or vacuum evaporated carbazole films is described. The first method starts with conducting electrodes in an electrolyte solution containing the carbazole monomer. In the second, the oligomer source consists of thin films deposited under vacuum on conducting SnO 2 coated glass substrate. The polymerization is corroborated by the red shift in the threshold absorption edge and the appearance of features in the visible-to-near ultraviolet (UV) domain. After oxidation, thin films obtained by the second method are amorphous while some crystallites with carbazole structure are present in the other. The electrical conductivity and the spin density of the polycarbazole obtained by oxidation of predeposited thin films are higher and thermogravimetric measurements show that it is more stable. These results could be attributed to a longer averaged chain length in agreement with X-ray diffraction spectra which shows that probably some monomers and/or oligomers are effectively present in the polycarbazole obtained from carbazole in solution.

MoO3/Ag/MoO3 anode in organic photovoltaic cells: Influence of the presence of a CuI buffer layer between the anode and the electron donor

MoO3/Ag/MoO3 (MAM) multilayer structures (layers thickness 20 nm/10 nm/35 nm) are used as anode in CuPc/C60/Alq3/Al organic photovoltaic cells. The averaged transmittance (400 nm-800 nm) of these MoO3/Ag/MoO3 multilayer structures is 70% ± 2% and their sheet resistance is 3.5 ± 1.0 Ω/sq. When these multilayer structures are used as anode, the power conversion efficiency of the MoO3/Ag/MoO3/CuPc/C60/Alq3/Al cells is around 1%, this efficiency is increased of 50% when a thin CuI film (3 nm) is introduced at the interface between the anode and the organic film. This improvement is attributed to the templating effect of CuI on the CuPc molecules.

Aging of electrical conductivity in conducting polymer films based on polyaniline

The aim of this work is to present an electrical study of the aging of conducting polyaniline–polystyrene (PANl–PSt) blends. Results concerning time dependence of the electrical conductivity of some samples artificially aged at different temperatures are presented. It is shown that the electrical conductivity decreases with time with a rate depending principally on the nature of the dopant. When aged at the same temperature, similar degradation kinetics are observed as well for doped polyaniline as for PANl–PSt blend. A study of the electrical conductivity variations versus temperature is presented for samples which have been aged at the same temperature but for different durations. Experimental results are discussed in the frame of the heterogeneous picture already proposed for nonaged samples.

XPS study of conducting polypyrrole-poly(vinyl alcohol) composites

Abstract Conductive polypyrrole-poly(vinyl alcohol) composites have been chemically prepared by exposing poly(vinyl alcohol) (PVA) films containing FeCl 3 to vapours of pyrrole. The obtained films have been characterized by photoelectron spectroscopy analysis (XPS), optical absorption, scanning electron microscopy and electron microprobe analysis. XPS measurements show that Fe 3+ in PVA is reduced to Fe 2+ during the polymerization reaction, confirmed also by optical measurements. A detailed analysis of the films with a scanning electron microscope equipped with an electron microprobe shows that PVAPPyFeCl 3 compounds are homogeneous composites. It is also shown by XPS that covalently bonded chlorine is presumably present in the films and diffuses with time towards the surface of the sample. This chlorine release has been checked by electron microprobe analysis.

X‐ray photoelectron spectroscopy of conducting polyaniline and polyaniline–polystyrene blends

Conductive polyaniline solutions were chemically prepared using bis (2-ethylhexyl) hydrogen phosphate (DiOHP) as the dopant chemical species. The codissolution method leads to conductive polyaniline–polystyrene (PANI–PSt) composites with good mechanical properties. The electronic structure of both conducting PANI films and PANI–PSt blends was investigated by X-ray photoelectron spectroscopy, which allowed one to quantify the proportion of benzenoid amine, quinoid imine, and protonated units. Blending polyaniline with PSt does not involve important modifications in the polymer electronic structure.

Influence of the electrochemical conditions on the properties of polymerized carbazole

Polycarbazole (PCZ) was obtained by electrochemical oxidation of carbazole monomers. Two potentials were used: the oxidation potential of the monomer 1700 mV and an over oxidation potential 2500 mV. The carbazole monomers were supplied either in solution in the electrolyte or in the form of pre-deposited thin film evaporated on a working electrode. It is shown that if PCZ is systematically obtained its electrical and morphological properties are strongly dependent on the experimental conditions. The stability of over-oxidized PCZ films corresponds to chain reticulation beyond the first oxidation potential value, with the loss of flexibility of the chains and steric hindrance, which decreases the carrier mobility and, therefore, its conductivity. The polymers obtained at the oxidation of carbazole have conductivity corresponding to higher carrier mobility. These polymers are conjugated. Moreover, in the case of preevaporated carbazole thin film the coverage of the SnO2 is very high and the films are homogenous.