MoO3/CuI hybrid buffer layer for the optimization of organic solar cells based on a donor-acceptor triphenylamine

We investigate the effect of anode buffer layers (ABLs) on the performances of multi-layer heterojunction solar cells with thienylenevinylene-triphenylamine with peripheral dicyanovinylene groups (TDCV-TPA) as donor material and fullerene C-60 as acceptor. The deposition of a CuI layer between the ITO anode and the electron donor significantly improves the short-circuit current density (J(sc)) and fill factor (FF) but reduces the open-circuit voltage (V-oc). On the other hand, a MoO3 buffer layer increases the V-oc but leads to limited J(sc) and FF values, thus reducing power conversion efficiency (PCE). In this context, we show that the use of a hybrid anode buffer layer MoO3/CuI leads to a considerable improvement of the cells performances and a PCE of 2.50% has been achieved. These results are discussed on the basis of the dual function of MoO3 and CuI. While both of them reduce the hole injection barrier, CuI improves the conductivity of the organic film through an improvement of molecular order while MoO3 prevents leakage current through the diode. Finally the results of a cursory study of the ageing process provide further support to this interpretation of the effects of the various buffer layers. (C) 2012 Elsevier B.V. All rights reserved

Study of CuI thin films properties for application as anode buffer layer in organic solar cells

After chemico-physical characterization of CuI thin films, the structures indium tin oxide (ITO) /CuI are systematically studied. We show that the morphology of the 3 nm thick CuI film depends on its deposition rate. To obtain smooth homogeneous CuI film, it is necessary to depose it at 0.005 nm/s. After optimization of the deposition conditions of CuI, it is shown that it behaves like a template for the organic layer. For instance, when the organic film is copper-phthalocyanine, the molecules which are usually perpendicular to the plane of the substrate lie parallel to it when deposited onto CuI. In a same way, when the electron donor is a prophyrin derivative, CuI allows to double the power conversion efficiency of the cells based on the heterojunction porphyrin/C-60. When CuI is used as anode buffer layer, it increases systematically the short circuit current, the open circuit voltage, thus the efficiency of the organic solar cell. These effects are related, not only to the improvement of the band matching between the ITO and the electron donor, but also to the templating effect of the CuI. Moreover, we show that the beneficial effect of CuI. is effective, not only with ITO, but also with fluorine doped tin oxide

Morphological and physicochemical properties of dip-coated poly {(2,5-diyl pyrrole) [4-nitrobenzylidène]} (PPNB) thin films: towards photovoltaic applications

A new material: conjugated poly {(2,5-diyl pyrrole) [4-nitrobenzylidène]}, that we called (PPNB), has been synthesized and characterized. The cyclic voltammetry has been used in order to estimate first oxidation (Ep) and reduction (En) potentials of our polymer. These values have been assigned, respectively, to the position of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and determination of the energy band gap which have been estimated to be 6.16, 3.89 and 2.27 eV respectively. Energy levels values of the HOMO and LUMO of the PPNB polymeric donor material were evaluated and the results are compatible with an electron transfer to C60 within an eventual junction, such values show that PPNB could be probed for applications in organic solar cells as donor material. PPNB Thin films have been deposited by dip-coating technique from Dichloromethane solvent with different polymer concentrations, and a dipping speed of 3.0 cm/min. For morphological characterization of the films scanning electron microscopy (SEM) was carried out. The samples, when observed by SEM, reveals that the films deposited are less dense, uniform. Cross-sectional SEM micrographs PPNB films show that thickness of the layers is homogeneous and has value of 35–40 nm. Optical characteristics of the polymer thin films were studied using UV-vis spectroscopy; absorption of wide range of wavelengths from 350 to 700 nm was observed. The optical band gap energy ranges between 1.9 eV and 1.94 eV. Based on these analyzes we realized heterojunction organic solar cells with the structure: ITO/Au/PPNB/C60/BCP/Al, the cells had a photovoltaique effect after J-V measuring, however the efficiency of photo generation under AM1.5 illumination was weak (about 0.02%) and needs to be improved

Schiff base polymer based on triphenylaminemoieties in themain chain. Characterization and studies in solar cells

Polytriphenylamine (PTPA), a Schiff base polymer containing triphenylamine (TPA) segments and whose monomer contains triphenylamine and thiophene end groups, was synthesized. The monomer structure enabled the polymerization to be performed under conditions similar to those of thiophene. Oxidative coupling using FeCl3 as oxidizing agent in anhydrous CHCl3 medium was employed for the polymer synthesis. Scanning electron microscopy, fluorescence spectroscopy, and cyclic voltammetry were used to characterize the polymer. PTPA exhibited high thermal stability with a mass loss of 13.3 % at 546.5 °C. The fluorescence spectrum showed emission at 300–550 nm and the optical band gap was found to be 2.6 eV. It was also established that PTPA forms complexeswith Lewis acids, e.g. MoO3 and CuI. Its absorption bandwidened and extended up to the near-IR. Itwas seen that PTPA is rich in π-electrons and thus can act as electron donor. The value of the Highest Occupied Molecular Orbital (HOMO) was−5.35 eV indicating its potential application in optoelectronic devices. An attempt was also made to investigate the photovoltaic potential of PTPA. Organic photovoltaic devices with various buffer layer structures, namely ITO/CuI/PTPA/C60/BCP/Al, ITO/MoO3/PTPA/C60/BCP/Al, and ITO/MoO3/CuI/PTPA/C60/BCP/Al, where ITO stands for indium tin oxide and BCP for bathocuproine, were utilized for the studies. Power conversion efficiency of these devices ranged between 0.21 and 0.43% under simulated AM 1.5 illumination (100mWcm−2). This result proved that polymers containing TPA in the main chain hold promising properties that would allow their use in photovoltaic devices

Small molecules organic photovoltaic devices based on the planar heterojunction porphyrin derivates/fullerene

In this paper, we studied of photoelectric properties of multilayer organic photovoltaic cells (OPV cells). The active organic layers consisted of a planar heterojunction between a layer of meso-tetrakis(5-bromo-2-thienyl)porphyrin), (TBrTP) as electron donor (ED) and a layer fullerene molecules. The TBrTP allows achieving OPVCs exhibiting promising efficiencies when the ABL is the MoO3/CuI DABL

Effect of MoO3 in the cathode buffer layer on the behaviour of layered organic solar cells

The behaviour of small-molecule organic solar cells based on coper-phthalocyanine/fullerene with different cathode buffer layer is investigated as a function of air exposure duration. We present the study of the effect of MoO3 on the properties of photovoltaic solar cells (OPVCs) when it is introduced in the cathode buffer layer (CBL). Photovoltaic performances were measured as a function of time of air exposure. During the first days of air exposure the efficiency of the OPVCs with MoO3 in their CBL increases significantly, while it decreases immediately after air exposure in the case of reference OPVCs, i.e. without MoO3 in the CBL. Nevertheless, the lifetime of the OPVCs with MoO3 in their CBL is around 60 days, while it is only 10 days in the case of reference OPVCs. The initial increase of the OPVC with MoO3 in their CBL is attributed to the slow decrease of the work function of MoO3 due to progressive contamination. Then, the progressive degradation of the OPVCs efficiency is due water vapour and oxygen contamination of the organic layers. The use of a double CBL, Alq3/MoO3, allows to interrupt the growth of pinholes, defects and increases the path of permeating gas. Also it can prevent the contamination of the organic layer by Al. All this results in significant increase of the lifetime of the OPVCs

Sintesis y propiedades fotovoltaicas de oligomero hexatienilenovinileno ralificado en celdas solares organicas con heterounion planar

A novel branched sexithienylene vinylene oligomer, (E)-Bis-1,2-(5,5´´-Dimethyl-(2,2´:3´,2´´- terthiophene)vinylene has been used as electron donor in a planar heterojunction Organic Photovoltaic cell (OPV). The electron acceptor was the fullerene (C60) and the cathode was an aluminum film, while ITO glass was used as anode. A hybrid anode buffer layer of MoO3 and CuI was used between the ITO glass and the electrodonor. The effect of the thickness of the branched sexithienylene vinylene film, thickness and deposit rate of CuI on the solar cells performances was studied. In the present work, it has been found that MoO3/CuI structure with 1.5 nm of CuI, deposited at 0.005 nm/s and the optimum branched sexithienylene vinylene thickness of 22 nm, efficiency (η) of 1.47 % could be achieved. This branched sixithienylene vinylene oligomer emerges as a promising electron donor compound which can be further be investigated in different combination of buffer layers and structures of OPV

Importance des Couches Tampons Anodiques dans les Cellules Photovoltaïques Organiques.

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Cellules photovoltaïques organiques planes basées sur l’hétérojonction donneur d’électrons/C60, importance de la valeur du HOMO du donneur d’électrons.

Les performances des cellules planes basées sur l’hétérojonction Donneur d’Electrons(DE)/Fullerène sont étudiées en fonction de la nature du donneur d’électrons. Les cellules sont de type : ITO/MoO3/CuI/DE/C60/Alq3/Al, avec De = CuPc ou SubPc. Il est montré que le rendement des cellules dépend de la nature du donneur d’électrons. Plus spécifiquement, le paramètre dont la variation est la plus significative est la tension de circuit ouvert Voc. Celle-ci passe de 0.53 V dans le cas du CuPc à  1.06 V dans le cas du SubPc. Elle  a donc doublé, alors que le courant de court circuit Jsc augmente de 37% et que le facteur de forme FF diminue légèrement. Sachant que le Voc dépend de la différence d’énergie entre le LUMO de l’accepteur d’électrons, ici le C60, et le HOMO du donneur, on peut constater (tableau1) que la différence de valeur entre les HOMOs des deux donneurs d’électrons utilisés se retrouve intégralement au niveau de la valeur du Voc. Ceci montre l’intérêt de travailler avec des donneurs d’électrons dont le HOMO est, en valeur absolue, le plus grand possible

Use of Cu–Ag bi-layer films in oxide/metal/oxide transparent electrodes to widen their spectra of transmittance

Original ZnO/Cu/Ag/MoO3 multilayer structures were deposited under vacuum. The optical transmittance spectrum of these structures is significantly broadened by using a double layer as metal interlayer. While the thickness of Ag was 6 nm, that of Cu was used as parameter. The highest averaged transmittance, 88% between 400 and 700 nm is obtained with the structure ZnO (20 nm)/ Cu (3 nm)/Ag (6 nm)/ MoO3. However, a better factor of merit is achieved, ΦM=16×10−3, when the Cu thickness is 4 nm, making that these innovative ZnO/Cu/Ag/MoO3 structures are very promising for use as substitute to ITO electrodes