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International audience; Aluminum Phthalocyanine Chloride (AlPcCl) is generally known as donor material in organic photovoltaic cells (OPVs). Nevertheless, in the present study we have demonstrated a successful use of AlPcCl as acceptor material to replace fullerene (C-60) in a planar heterojunction OPVs based Alpha-Sexithiophene (alpha-6T) as donor material. For this purpose, we have elaborated two series of OPVs: alpha-6T/AlPcCl and alpha-6T/C-60 using thermal sublimation technique. After the optimization of the AlPcCl thickness we have compared both solar cells current density vs. voltage characteristics, we have shown that the use of AlPcCl as acceptor instead of C-60 induces to an increase in the open circuit voltage (V-oc) and the short current density (J(sc)) values by 18% and 23%, respectively, from 0.5 to 0.61 V and from 6.10 to 7.98 mA/cm(2), while, we have obtained a decrease in the shunt resistance which has dropped the fill factor (FF) from 58 to 51%. The V-oc and J(sc) increase allows to increase the power conversion efficiency by 28%. These findings have been supported by absorbance measurements and morphological studies. The V-oc and J(sc) increase have been discussed and assigned to the difference between the theoretical V-oc of both heterojunctions, and to the complimentary absorption of AlPcCl with alpha-6T. While, the FF deceases has been attributed to the low electron mobility of AlPcCl compared with C-60. These findings, brought in the spotlight the potential of the metal phthalocyanines, which can open up a wide range of possibilities to test other materials based like this family.

International audience; The exciton blocking layer (EBL) as an interfacial layer is extremely critical in determining the organic photo-voltaic cell (OPV) performances. Here, we studied inverted planar heterojunction solar cells PHJ-OPVs with the following configuration ITO/EBL/C60/CuPc/MoO3/Al. Upon the EBL functionality which can act as an exciton blocking layer and allows the electron collection at the cathode, we proposed the insertion of hybrid EBL con -sisted of KI/Alq(3) thin layer. The Alq(3) is known as an EBL due to its broad bandgap, whereas we found that when a thin layer of 1 nm of KI is introduced in ITO/Alq(3) interface, the KI decomposed during the thermal deposition, and only potassium interacts and diffuses in the Alq(3) layer, which effectively enhances the electrons collection at the ITO/C-60 interfaces leading to the improvement of open-circuit voltage (Voc), and device power conversion efficiency by 36% than the device using Alq(3) alone as EBL.

In this work, nanostructured TiO2 and poly-3,4-ethylenedioxythiophene (PEDOT) layers were electrochemically prepared over transparent electrodes. Morphological characterization evidenced the presence of nanostructures as planed with 50-nm-wide TiO2 rod formations followed by 30-nm-wide PEDOT wires. Different characterizations were made to the deposits, establishing their composition and optic properties of the deposits. Finally, photovoltaic cells were prepared using this modified electrode, proving that the presence of PEDOT nanowires in the cell achieves almost double the efficiency of its bulk analogue.

In the field of organic photovoltaic cells (OPVs), ternary planar heterojunction allows widening absorption range. Optimization of the energy levels at the organic interfaces is the first task to achieve in order to obtain OPVs with high efficiency. In ternary OPVs, carrier mobility, charge transfer and presence of energy transfer are determinant. Here, we compare the performances of different, binary (electron donor/fullerene) and ternary (electron donor/ambipolar material/fullerene) combinations of small organic molecules in planar heterojunction (PHJ) cells. We chose the different ED molecules in order to investigate the different possible band scheme alignments. The study shows that if the open circuit voltage (Voc) is limited by the energy levels of the outer layers, i.e. the difference between the lowest unoccupied molecular orbital (LUMO) of the electron acceptor and the highest occupied molecular orbital (HOMO) of the outer electron donor layer, the energy loss can be smaller than 0.2 eV. If efficient ternary OPVs need global energy offset higher than the exciton energy to separate the charges of the exciton, the distribution of this offset between ΔLUMO and ΔHOMO is not critical. The mobility value of the carriers in the intercalated layer is determinant in obtaining performance of OPVs. Overlap between the photoluminescence of ED and the optical absorption of EDA is primordial for good energy transfer. The highest OPV efficiency improvement is obtained with organic materials such as ED and EDA that have the same HOMO and similar hole mobility which give homogeneous thin layers.

We compare the performances of organic photovoltaic cells (OPVCs) based on binary and ternary planar heterojunctions. The organic active layers are pentathiophene (5T), subphthalocyanine (SubPc) and fullerene (C60). SubPc being an ambipolar material we used it either as electron acceptor in binary OPVCs or as central layer in ternary cells in order to increase the efficiency of OPVCs using 5T as electron donor. So, the different OPVC configurations were 5T/C60, 5T/SubPc and 5T/SubPc/C60. The effect of the different organic layer thicknesses on the device performances was studied. In order to understand the behavior of the different OPVC configurations, we proceeded with a morphological study. The influence of the high roughness of the 5T layer on the OPVCs performances is discussed. The best OPVCs performances are obtained with the binary structure 5T/SubPc. Its maximum efficiency corresponds to an increase of 50% compared to the OPVC based on the couple 5T/C60. External Quantum Efficiency measurements show that both layers participate to the current generation. The efficiency increase is mainly due to the increase of the open circuit voltage ([Formula: see text]). In the case of ternary OPVCs, [Formula: see text] is limited by the band structure of 5T and C60, moreover, the efficiency is also limited by the poor charge collection efficiency of the ternary structure and the series resistance of the three stacked organic layers.

In this work, planar heterojunction organic photovoltaic cells (PHJ-OPVs) based on alpha-sexithiophene (α-6T) and fullerene active materials were investigated. We have studied the effect of the hole transporting layer (HTL) on the performances of the PHJ-OPVs. The studied HTLs are molybdenum trioxide (MoO3), cuprous iodide CuI and MoO3 coupled with CuI (MoO3/CuI). The PHJ-OPVs were fabricated using the thermal evaporation/sublimation technique. The experimental study shows that double HTL MoO3/CuI, using 1.5 nm of CuI thickness, improves significantly the efficiency of the PHJ-OPVs compared with those with single HTLs. This enhancement is due to the fact that MoO3 contributes to the optimization of the holes collection and CuI causes significant influence on the nucleation and the growth of α-6T. CuI has led to spectacular modification of the structural properties of α-6T. When α-6T is deposited onto ITO/MoO3, the α-6T layers are amorphous, while they are crystallized into monoclinic crystalline structure with a laying down molecular orientation when they are deposited onto ITO/MoO3/CuI. The use of the double HTL and the optimization of the α-6T deposition parameters have led to an important improvement in the α-6T light absorption, roughness and the holes mobility. These findings participated in the significant enhancement of the PHJ-OPVs performances and their life time.

In this study, Mo(S x O y ) thin films were deposited onto fluorine doped tin oxide (FTO) using pulsed electrochemical deposition method. It is shown by scanning electron microscopy, energy-dispersive spectroscopy and X-ray photoelectron spectroscopy that after water cleaning the deposited Mo(S x O y ) film corresponds to a hybrid layer MoS x :MoO 3 . This hybrid is used as anode buffer layer (ABL) in planar organic photovoltaic cells (OPVCs) based on the couple copper-phthalocyanine/fullerene. It is shown that it is necessary to proceed to a soft annealing-5 min at 150 °C- of the anode FTO/Mo(S x O y ) to clean the ABL surface in order to obtain efficient contact with the organic material. The OPVC with the optimum Mo(S x O y ) thickness, 12 nm, showed a power conversion efficiency, PCE = 1.41% under an illumination of AM1.5, which is 12% higher than that achieved with a simple MoO 3 ABL. This improvement is attributed to the fact that using a hybrid MoS 2 :MoO 3 ABL allows to combine the advantages of its both constituents. The MoS x blocks the electrons, while the high work function of MoO 3 induces a high hole extraction efficiency at the interface electron donor/anode.

In this work, two kinds of novel thiophene-aniline polymers, were studied through theoretical protocols in order to find new insights about the polymerization mechanism, the chemical structure and optical properties exhibited by this kind of conducting materials. The dual descriptor—a local reactivity descriptor derived from conceptual density functional theory— and the condensed electrophilic Fukui index were used to describe the most probable sites of polymerization. Frontier orbitals energies were calculated to predict the band gap of both polymeric films. All results demonstrated how theoretical protocols help in the understanding of chemical polymerization mechanism and in the prediction of relevant properties, which can accelerate the synthesis process of new monomers

Among ITO alternative, dielectric/metal/dielectric multilayer structures are one of the most often studied possible substituent. However, if their optical and electrical properties are systematically investigated it is not the same with regard to their mechanical properties. In the present manuscript we have studied the properties of ZnS/Cu/Ag/ZnS, ZnS/Cu/Ag/MO 3 (with M = Mo or W) structures. With a maximum transmission of 90% and a sheet resistance of 5 Ω/sq the optimum structure exhibits a figure of merit of 82 10 −3 Ω −1 when λ = 600 nm. Beyond these standard measures we proceeded to the study of the mechanical properties of the multilayer structures. The inner and outer bending tests show that the ZnS/Cu/Ag/ZnS (or MO 3 ) structures are more flexible than ITO, while their responses to scotch tests show that they exhibit a large adhesion to the substrate, glass or plastic. The scratching adhesion test puts in evidence that the adhesion to the substrate of the Ag layer is smaller than that of ZnS/Cu/Ag/ZnS, which is smaller than that of ITO. On the other hand, this test shows that the ZnS/Cu/Ag/ZnS (no cracks for L = 25 N) is less brittle than ITO (cracks L = 15N). Finally, when used as anode in organic solar cells, the structure ZnS/Cu/Ag/WO 3 allows achieving the best efficiency, similar to that obtained with ITO.

Due to their light weight, flexibility and semi-transparency the organic photovoltaic cells play an important role for solar conversion photovoltaic (OPV). To achieve good performances, both donor and acceptor materials in OPVs need to have good extinction coefficients, high stabilities and good film morphologies. Since the donor plays a critical role as the absorber to solar photon flux, donor materials require wide optical absorption to match the solar spectrum. In this work the couple ED/EA in planar heterojunction was Tetracyano 4,4'-bis(9Hcarbazol-9-yl) biphenyl (TCC)/fullerene (C60). Optimum results are obtained when MoO3 alone is used as Hole Transporting Layer (HTL). The J/V characteristics do not exhibit S-shaped curves up to a TCC layer thickness of 15 nm, while they did when the HTL includes CuI. Theoretical study, complementary to the experimental study, shows that in the case of S-shaped curve the cell behaves as if it was made up of 2 diodes, one of which would be opposed to the flow of the photogenerated current. In the case of MoO3 HTL, i.e; without shaped curve, the optimum thickness is 13 nm, giving an efficiency η = 2.30% with V oc = 0.9 V, J sc = 5.17 mA/cm2 and FF = 49%.

Using three organic materials in the cascade configuration of organic photovoltaic cells (OPVs) broadens the absorption range of visible light, resulting in an increase in the short circuit current density (Jsc). Herein, we report for the first time the use of three organic molecules, pentacene, boron subphthalocyanine chloride (SubPc) and fullerene (C60). Upon comparison with the binary pentacene/C60 and SubPc/C60structures, the high Jsc value obtained for the ternary structure induces an increase in the OPV efficiency. This improvement is limited by the small open circuit voltage (Voc) value due to the low absolute value of the highest occupied molecular orbital of pentacene. Our experimental study confirmed that the Voc is ultimately limited by the energy levels of the outer layers in these cascade structures. Initial attempts to overcome this bottleneck were carried out using a variety of electron acceptors as an alternative to fullerene. However, increasing the Voc was detrimental to the current density, therefore the best OPVs remain those constructed using fullerene.

Ternary organic photovoltaic cells (OPVs) have been shown to be a promising approach to increase cells efficiency through broadening of their absorption range. Often, ternary cells are based on a blend of two donors and one acceptor, the efficiency of planar ternary heterojunction being limited by the resistance of the three stacked layers. Here, we show that by depositing the intercalated layer through a grid we are able to decrease significantly the series resistance of the device, while the use of two donors allows improving the short circuit current and the efficiency of the organic photovoltaic cells. Electrical, optical and morphological studies show the ternary cell behaves like parallel OPVs. The first donor layer consists in an AlPcCl film, the acceptor layer is a C60 film while the central layer consists in a laboratory made molecule called MD2. If the MD2 layer absorbs strongly the light and permits to the both types of carrier to diffuse, its carriers mobility is small. Therefore, the discontinuities, due to the grid, of the MD2 intercalated layer allow improving the cells efficiency over-passing the corresponding binary cells performances.

Studying the electrochemical characteristics is an important step for determining interactions between molecules and the chemical environment. Moreover, the electrochemical evaluation of dyes is highly needed to establish the behavior of electro-active chemical species inside dye-sensitized solar cells (DSSCs). Four compounds, M8-1, M8-2, M8-O1, and M8-O2 (with a common organic structure (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(phenylamino)-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid), are studied in two solvents, tetrahydrofuran (THF) and dimethylsulfoxide (DMSO). Among the studied compounds, M8-1 has highlighted characteristics compared with the others: its ground and excited states oxidation potential are the highest (1.14 and −1.22 V, respectively). Also, it shows the lowest energy gap between the excited state oxidation potential and the TiO2 conduction band. Relating to the substituent effect, the shorter the length, the higher the energetic difference in the electronic transition (M8-1 and 2). Comparing characteristics through quantum chemistry, the values obtained in DMSO are the most predictable. The injection energies signal that M8-1 is the best injector. The performances in solar cells are measured in three TiO2 materials: Degussa (D-TiO2), active opaque (A-TiO2), and transparent (T-TiO2). The IPCE results show the A > T > D average tendency, and the family of substituted alkyl has higher values than the alcoxyl one. Furthermore, in the first family the methyl substituent has a higher value than the ethyl one. M8-1 has the highest IPCE value, on average. In terms of efficiency, the alkyl substituted family again has higher values than the alcoxyl family. On average, the methyl substituent has a higher value than the ethyl one in both families. M8-1 has the highest efficiency value.

Realization of performing large area flexible organic photovoltaic cells needs highly conductive and transparent electrode. In the present manuscript we show that it is possible to improve the power conversion efficiency of organic solar cells deposited onto PET/ITO anode by improving the conductivity of the anode. When covered with a thin, 12 nm, metal bilayer, Cu/Ag, itself covered by a refractory metal oxide, MoO3 or WO3, it is possible to significantly decrease the sheet resistance of the ITO anode, while its transmission is only slightly reduced, which result in a figure of merit as high as 32 × 10−3 Ω−1. Moreover, the flexibility of these multilayer structures is better than that of ITO alone. In the case of PET substrates, when these optimized anodes are introduced in photovoltaic cells based on the planar heterojunction CuPc/C60, the power conversion efficiency of these cells is higher than that of the cells using ITO alone. This improvement becomes more significant and useful for larger areas devices that are needed for practical applications. The best results are obtained with ITO/Cu/Ag/WO3 anodes.

An efficient approach for improving the photoelectrical conversion efficiency (PCE) of the bulk heterojunction (BHJ) solar cells, based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acidmethyl ester (PC61BM), by incorporating PbSe nanorods decorated with graphene (G) into their active layer has been reported for the first time. Pristine PbSe and PbSe:G composites (with different amount of graphene) are synthesized via hydrothermal process and the formation mechanism is explained. The systematic investigation indicates that the crystallite size of PbSe:G increases with increasing graphene content. The PCE of the classical BHJ solar cells based on P3HT:PC61BM is improved from 2.32 up to 2.57% by the incorporation of pristine PbSe. It is also enhanced by the incorporation of PbSe:G up to certain composition of graphene in which a maximum PCE value of 5.16% is achieved. The external quantum efficiency of the BHJ solar cells is also investigated. The photovoltaic parameters are discussed based on the morphology variation detected by scanning electron microscope and atomic force microscope of the active layers together with their UV–VIS absorption measurements.

International audience; The present manuscript is dedicated to In free transparent conductive and flexible heterostructure electrodes. The new electrodes correspond to dielectric/metal/dielectic (D/M/D) multilayer structures deposited under vacuum. In the present work, after a fast review of the general properties of D/M/D electrodes using Ag as metal, we then develop the study of D/Cu/D multilayer structures. We propose to substitute Cu to Ag, because it is far cheaper. However, Cu having tendency to diffuse into many dielectrics it is difficult to obtain stable electrodes. We show that using Cu:alloys, i.e. Cu:Ni and Cu:Al, as metal it is possible to decrease significantly the Cu ions diffusion and to increase significantly the stability of the multilayer electrodes with Cu. Finally, we show that when these electrodes are used as anode in organic photovoltaic cells, they can allow achieving efficiency similar to that obtained with ITO.

Indium tin oxide (ITO) is the most common transparent conductive material used in industrial processes. It has many advantages, but also some disadvantages: Indium is scarce and ITO deposition techniques are aggressive for organic materials, making it difficult to use it as top electrode in organic devices. Moreover its ceramic structure limits its application in flexible devices. Among the possible new In free transparent conductive electrode, dielectric/metal/dielectric multilayer structures such as WO3/M/WO3 appear very promising. However, silver, which is the metal the more often used is expensive. Therefore it would be very profitable if copper, which is abundant on earth, could be substituted for silver. However the stability with time of the structure using Cu is questionable due to the high Cu diffusivity. In the present manuscript we improve significantly the lifetime of the structures using the alloy Cu:Al when a thin silver layer (2 nm) is introduced between the WO3 bottom layer and the Cu:Al. It is shown that the Cu atom mobility is significantly decreased by the presence of Al of the alloy and of Ag which appears to diffuse into the metal layer forming an eutectic with Cu.

International audience; Recently, significant progress in the field of organic photovoltaic cells was obtained by substituting new electron acceptor molecules to the fullerene, which was attributed to the fact that the fullerene absorption is quite small. Nevertheless, we demonstrate in the present work that, in the case of inverted cells, i.e. when the transparent bottom electrode is used as cathode, the contribution of fullerene to the Jsc short-circuit current of the cells, if not dominant, is not negligible; and that mainly in the short wavelength spectral range. The experimental results are confirmed by an optical simulation. Due to this significant contribution to Jsc, the light transmission of the transparent electrode towards the UV-part of the spectrum is crucial for inverse cell performances. When a transparent conductive electrode based on an alternative dielectric/metal/dielectric structure is substituted to ITO, such as ZnS/Ag/TiO 2 , the study allows to obtain promising results, although there is a loss of performance due to the decrease of transmission of TiO 2 below 400 nm in wavelength.

In the near future many optoelectronic devices should be flexible therefore it is necessary to look for a new transparent electrode, ITO, the most often used transparent electrode, being brittle. Oxide/Metal/Oxide structures were shown to be highly promising as substituent to ITO. However, the metal the most often used, Ag, is quite expensive. Cu is a possible substituent to Ag, but it diffuses strongly in the oxide layers. In order to stabilize the Cu alloy, we use Cu alloys: Cu:Ni, Cu:Al. We show that using such alloys allows increasing the electrode stability. The introduction of a thin, 2nm, Ag layer, further improves the electrode stability. Finally we show that WO3/Cu:M/WO3 electrodes can be used successfully as anode in organic solar cells.

Organic solar cells are based on a couple of organic materials, one is called electron donor and the other electron acceptor. Usually the acceptor is the fullerene. However if it is an efficient electron conductor, it absorbs weakly the light and its contribution to the current is small. Therefore, in the present work we study couples where the acceptor is SubPc, AlPcCl, DTPMM or DBP. These molecules have been chosen as electron acceptor because their band structure is compatible with such use. The study shows that the band offset value between the electron donor and the electron acceptor must be higher than the exciton energy, whatever the distribution between the bands. Moreover, it is shown that, if a sufficient band offset value is necessary, it is also necessary that the molecule proposed as electron acceptor must have a sufficient electron mobility, i.e., the molecule must be ambipolar. On the other hand the electron donor layer must be smooth and must exhibit a sufficient hole mobility.

In this study, organic photovoltaic cells based on planar heterojunctions using small-molecules were fabricated. Two cell configurations were used, planar heterojunctions based on a classical electron donor/electron acceptor couple and planar heterojunctions using three active layers, one donor, one acceptor and a central ambipolar layer. The donor is a thiophene derivative called BSTV, the acceptor is the fullerene while the ambipolar material is a phthalocyanine dye, the SubPc. After studying the performances of cells based on the couples SubPc/C60, BSTV/C60 and BSTV/SubPc, we explore the dependence of the short circuit current and the external quantum efficiency on the SubPc interlayer thickness in cells with three active layers. By comparison with the optimum couple SubPc/C60 the ternary structure BSTV/SubPc/C60 exhibits higher short circuit current but smaller fill factor. Nevertheless, for an optimum interlayer thickness of 12.5 nm, the ternary structure allows an improvement of the cell efficiency of 11% by comparison with the highest result obtained with the best couple. As shown by optical measurements, long range Foster energy transfer from the larger band gap donor, BSTV, to the smaller band gap, SubPc, is possible. Therefore, the increase of Jsc is attributed to two-step exciton dissociation process. Excitons produced in the outer BSTV layer are transferred to the SubPc central layer and then dissociated at the interface.

A new series of two-dimensional statistical conjugated polymers based on aniline and 9,9-dihexylfluorene as donor units and benzo- or naphtho-quinoxaline/thiadiazole derivatives as acceptor moieties, possessing PANI segments as side chains, were designed and synthesized. To investigate the effects of the perpendicular PANI branches on the properties of the main chain, the optical, electrochemical, morphological and electroluminescence properties were studied. The 2D materials tend to possess lower molecular weights and to absorb and to emit light red-shifted compared to the trunk 1D-polymers, in the yellow-red region of the visible spectrum. The 1D- and 2D-conjugated polymers present optical band gaps ranging from 2.15–2.55 eV, HOMO energy levels between −5.37 and −5.60 eV and LUMO energy levels between −3.02 and −3.29 eV. OLED devices based on these copolymers were fabricated. Although the performances were far from optimal due to the high turn-on voltages for which electroluminescence phenomena occur, a maximum luminescence of 55,100 cd/m2 together with a current density of 65 mA/cm2 at 18.5 V were recorded for a 2D-copolymer, PAFC6TBQ-PANI.

A new branched sexithienylene vinylene oligomer, (E)-bis-1,2-(5,5″-dimethyl-(2,2′:3′,2″-terthiophene)vinylene (BSTV), was synthesized, characterized, and used as the electron donor in a planar heterojunction organic photovoltaic cell (OPVC). The OPVC utilized fullerene (C60) as the electron acceptor, ITO-coated glass as the anode, and aluminum as the cathode. Hybrid electrode buffer layers of MoO3/CuI on the anode side and of Alq3/Ca on the cathode side were used. This shows the effectiveness of the bilayer Alq3/Ca as a cathode buffer layer. The thickness of the Ca layer and its effect on the performance and lifetime of the OPVCs were studied. It was found that an Alq3/Ca structure with 6 nm of Alq3 and 3 nm of Ca has an efficiency (η) of 2.28%, while a device without Ca has an efficiency of only 1.47%. Combining an optimized thickness of the new donor BSTV (22 nm) together with the bilayer cathode buffer layer, a device having an open-circuit voltage, Voc of 0.84 V, a short-circuit current, Jsc of 3.60 mA cm−2, and a fill factor, FF of 50% was achieved. However, the efficiency of the OPV with Ca decreases rapidly during the first hours of air exposure, resulting in device performance that is similar to a device fabricated without Ca. After this initial degradation, device performance for both types of OPV evolves similarly with continued air exposure.

A successful approach to improve organic photovoltaic cell performance is the introduction of a thin layer called the electron-blocking layer (EBL) at the interface organic material/cathode. Here, the effect of the EBL is discussed through the use of different multiheterojunction organic solar cells configurations. It is shown that, whatever the configuration used, classical or inverted cells, the EBL allows to significantly improve the solar cells performances. The optimum thickness of this layer is, whatever the solar cell configuration, 8-9 nm. Both cell families permit achieving high short circuit current through this quite thick insulating layer. This invalidates the justification, proposed in the case of classical solar cells, based on Al diffusion into the exciton-blocking layer, since there is no metal deposition onto the exciton blocking layer in the case of inverted structure. Different hypothesis explaining the high electron current through the 8-9 nm of the large band gap EBL are proposed.

Dielectric/Metal/Dielectric structures can be used as substituent to transparent conductive electrodes. The dielectric used is often a transition metal oxide such as MoO 3-x and the metal is Ag. In the present work we propose to substitute Cu to Ag. The difficulty with Cu is its high diffusion rate into MoO 3-x . In order to prevent this negative effect we used Cu:Ni alloy as metal layer. If using such alloy is efficient to reduce Cu diffusion, it works well only with WO 3-x and not with MoO 3-x . We show that after deposition of the alloy only 0.5 at% of Ni is present in Cu films. This small atomic concentration makes it possible to preserve the electrical and optical properties of the metal films but limits its control of Cu diffusion in time. Therefore it is necessary to use an oxide, here WO 3-x , which also limits the diffusion of metals. By adding these two effects due to the alloy and the oxide it is possible to form Dielectric/Metal/Dielectric structures with quite stable properties. These WO 3- x /Cu:Ni/WO 3- x structures can be used as anodes in organic photovoltaic cells. The different behaviors of the structures according to whether they use WO 3-x or MoO 3-x are discussed in terms of thin film porosity.

The use of indium-free transparent conductive electrodes is of great interest for organic optoelectronic devices. Among the possible replacements for ITO, dielectric/metal/dielectric (D/M/D) multilayer structures have already proven to be quite efficient. One issue with organic devices is their lifetime, which depends not only on the organic molecules used but also on the electrodes. Therefore we study the variation, with elapsed time, of the electrical and optical properties of different D/M/D structures, with M = Ag or Cu/Ag. Six years after realization, it has been shown that if some structures retained an acceptable conductivity, some others became non-conductive. For a sample which remains conductive, in the case of a PET/MoO3/Ag/MoO3 multilayer structure, the sheet resistance changes from 5 Ω/sq–17 Ω/sq after six years. This evolution can be compared to that of a PET/ITO electrode that varies from 25 Ω/sq–900 Ω/sq after six years. It means that not only are the PET/MoO3/Ag/MoO3 multilayer structures more flexible than PET/ITO, but they can also be more stable. Nevertheless, if some PET/MoO3/Ag/MoO3 multilayer structures are quite stable, some others are not. This possible degradation appears to be caused primarily by the physical agglomeration of Ag, which can result in Ag film disruption. This Ag diffusion seems to be caused by humidity-induced degradation in these Ag-based D/M/D structures. Initially, defects begin to grow at a 'nucleus', usually a microscopic particle (or pinhole, etc), and then they spread radially outward to form a nearly circular pattern. For a critical density of such defects, the structure becomes non-conductive. Moreover the effect of humidity promotes Ag electrochemical reactions that produce Ag+ ions and enhances surface diffusivity with AgCl formation.

The study of new dipolar A-π-D molecules, which have an acceptor (A) and donor (D) charge joined by a conjugate bridge, have been an attention focus in the recent years due their different properties. In the current work, a molecular system has been modified in order to compare the effect on properties, such as quantum yield. Thus, two series were generated (alkyl- and alkoxy-substituted) to determine if molecules with tertiary asymmetric amines change their optical properties and whether quantum yield is affected. The different products have been characterized by several techniques such as UV–Vis spectrophotometry, elemental analysis, NMR, FT-IR, mass spectroscopy and fluorescence spectroscopy. Furthermore, their behavior in eight organic solvents, dichloromethane, tetrahydrofuran, ethyl acetate, 1,4-dioxane, acetone, acetonitrile, dimethylformamide and dimethylsulfoxide were experimentally and theoretically studied. The quantum yields were higher for the alkyl-substituted series. Theoretically, the dihedral angles formed between the tertiary amine and carbonyl group moieties have a correlation with quantum yield values, helping to explain why they are higher in non-polar solvents. Consequently, the maximum quantum yield was obtained with (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(methyl(phenyl)amino)-9H-fluoren-2-yl) vinyl)thiophen-2-yl)acrylic acid (M8-1) in 1,4-dioxane, reaching 98.8%.

We report the electrosynthesis of a novel semiconductor polymer based on alkyl vinylthiophene derivative in the presence of an ionic liquid (IL). The polymerization was performed under galvanostatic conditions and the polymer was studied as potential donor component of a multilayer heterojunction organic solar cell (OSC). The monomer used was (E)-1,2-di-(3-octyl-2-thienyl) vinylene (OTV) and the IL used for the electropolymerization was 1-octyl-3-methylimidazole hexafluorophosphate C8mimPF6. Optical properties, stability and morphology of the polymer were analyzed using FT-IR, UV-vis, Raman and XPS spectroscopy. Voltammetry analysis and scanning electron microscopy (SEM-EDX) were also performed on the polymer. The OSC assembled with the polymer of OTV was used as electro donor and C60 as acceptor. Molybdenum trioxide (MoO3) and bathocuproine (BCP) were used as buffer layer between anode and cathode respectively. I-V curves, in the dark and under AM 1.5 solar simulator were performed to measure its efficiency. publishedVersion

We have deposited transparent p-type semiconductive NiO thin films by reactive HiPIMS which appeared to be a powerful method to produce thin films exhibiting gradients of chemical compositions and opto-electronic properties. For a fixed amount of oxygen in the discharge (9%), the influence of the pulse duration was investigated. The position of the valence band with respect to the Fermi level was evaluated by X-ray Photoelectron Spectroscopy (XPS), for two different pulse durations, 15 and 30 μs. We have then investigated the dependence of optical properties of NiO films using spectroscopic ellipsometry (1.5–5.0 eV range). Refractive index n, extinction coefficient k, and gap energy of the NiO films were determined with a refractive index gradient decreasing along the film growth direction.

The open circuit voltage (Voc) of organic photovoltaic cells (OPVs) is an important parameter in terms of OPV performance. In the present work, we check that its value depends on the energy difference between the Lowest Unoccupied Molecular Orbital of the electron acceptor (LUMOA) and the Highest Occupied Molecular Orbital of the donor (HOMOD). The electron acceptor is the fullerene, while the electron acceptors are used as parameter. The results show that Voc increases with the value of Δ(LUMOA–HOMOD). However, for some molecules, this increase is not linear, which shows that other parameters are also determinant.

International audience; MoO3/Ag/MoO3(MAM) multilayer structures were deposited by vacuum evaporation on polyethylene terephthalate (PET) substrate. We demonstrate that, as in the case of glass substrate, the sheet resistance of such structures depends significantly on the Ag film deposition rate. When it is deposited between 0.2 and 0.4 nm/s, an Ag thickness of 11 nm allows achieving sheet resistance of 13 Ω/sq and an averaged transmission of 74%. A study of the influence of the PET substrate on the optimum MoO3 thicknesses was done. A good qualitative agreement between the theoretical calculations of the variation of the optical transmittance of the MoO3/Ag/MoO3 structures is obtained. The optimum MAM structures MoO3 (17.5 nm)/Ag (11 nm)/MoO3 (35 nm) has a factor of merit FM = 4.21 10− 3 (Ω/sq)− 1. Proven by the scotch test the MAM structures exhibit a good adhesion to the PET substrates. The MAM structures were also submitted to bending tests. For outer bending, the samples exhibit no variation of their resistance value, while for inner bending there is a small increase of the resistance of the MAM structures. However this increasing is smaller than that exhibited by Indium Tin Oxide. When the PET/MAM structures are used as anode in organic photovoltaic cells, it is shown that the need to use thicker Ag films inside the multilayer and to cover the MAM with Au to obtain promising Current density vs Voltage characteristics is due to the heating of the PET substrate during the deposition process.

Depending on their resistivity and their transmittance, the thin films of transparent conductive oxide (TCO) are widely used in optoelectronic devices. In2O3:Sn (ITO) is the most widely used TCO in optoelectronic devices. As indium is scarce and ITO is limited in flexibility due to its ceramic structure, many studies have been dedicated to new transparent conductive electrodes. This review article presents the state-of-the-art concerning the dielectric/metal/dielectric structures and their application as transparent electrodes in organic photovoltaic cells (OPVCs). First, TCO/Ag/TCO structures were created to achieve higher conductivity than ITO films. Then others dielectrics have been used such as transition-metal oxides (WO3, MoO3, V2O5, etc.), ZnS, etc. Such structures exhibit excellent flexibility, high conductivity, and good transparency. They can be deposited onto substrates at room temperature by simple evaporation under vacuum. Moreover, it is possible to manage the anode work function through the choice of the dielectric, which can allow them to be used as cathodes or anodes and as intermediate electrodes in tandem solar cells. The properties of the dielectric/metal/dielectric (D/M/D) structures depend on the thickness of the different layers. The threshold thickness value of the metal film is usually around 10 nm, where the structures change from an insulating state to a highly conductive state. This is attributed to the percolation of conducting metal paths. The transmittance of the films increases when the metal thickness increases up to the percolation thickness, while further increase induces a decrease in transmittance. Finally, the nature and the thickness of the dielectric layers can be chosen as a function of the device properties requested, which is illustrated through different examples of OPVCs.

Organic photovoltaic cells using polymer belonging to the aniline-heteroaryl family as electron donor have been achieved by thermal evaporation. We show that the properties of the polymer film, morphology, molecule order and conductivity depend strongly on the bottom anode buffer layer. While cells without anode buffer layer or with MoO3 or CuI anode buffer layer have been probed, we show that CuI allows improving strongly the cells efficiency through an improvement of the morphology and conductivity of the polymer film. This shows that although it is necessary a good band matching at the interface, this is not sufficient, because the templating effect of CuI on the polymer film is primordial for photovoltaic cells improvement.

International audience; Transparent conductive oxide/metal/oxide, where the oxide is MoO3 and the metal is Cu, is realized and characterized. The films are deposited by simple joule effect. It is shown that relatively thick Cu films are necessary for achieving conductive structures, what implies a weak transmission of the light. Such large thicknesses are necessary because Cu diffuses strongly into the MoO3 films. We show that the Cu diffusion can be strongly limited by sandwiching the Cu layer between two Al ultra-thin films (1.4 nm). The best structures are glass/MoO3 (20 nm)/Al (1.4 nm)/Cu (18 nm)/Al (1.4 nm)/MoO3 (35 nm). They exhibit a transmission of 70% at 590 nm and a resistivity of 5.0 * 10− 4 Ω cm. A first attempt shows that such structures can be used as anode in organic photovoltaic cells.

While they have different electronic properties n-type MoO3 and p-type NiO are very efficient as buffer layers between the ITO anode and the organic electron donor in organic photovoltaic cells. While it is admitted that MoO3 is n-type, its band structure is still under study. Here, the band alignment at the interface of an ITO/MoO3 heterojunction is studied by X-ray photoelectron spectroscopy (XPS). The same study is realized on the structure ITO/NiO, NiO being a p-type semiconductor. The measurements have been performed on samples obtained under the same experimental conditions as those used to achieve organic photovoltaic cells. The MoO3 (NiO) upper layer was 3 nm thick. The semidirect XPS technique used to measure the band offsets allows us to estimate the band discontinuities at the interface ITO/MoO3: ΔEv = 0.50 eV and ΔEc = 0.90 eV, while at the interface ITO/NiO we have ΔEv = −2.10 eV and ΔEc = −1.90 eV. Therefore, n-type MoO3 and p-type NiO, which are both very efficient anode buffer layers (ABLs), exhibit different band structure at the contact with ITO. However, the measurement, by means of a Kelvin probe, of the work functions of the structures ITO/NiO and ITO/MoO3, shows that they are close and significantly higher than that of ITO alone.

Nickel oxide thin films were deposited by Direct Current magnetron reactive sputtering from Ni target onto SnO2:F conductive glass substrates. The process was carried out without intentional heating, in an argon/oxygen gas mixture with various oxygen contents and discharge currents. The polycrystalline NiO thin films were deposited with controlled growth of the structure along [111] and [200] crystallographic directions for chosen conditions. Morphology of as-deposited films was found to depend on the preferentially oriented NiO crystals. Moreover, on the basis of discharge voltage as a function of the O2 partial pressure for a constant discharge current, we present here the method to estimate the deposition conditions allowing us to achieve the desired preferential growth of transparent p-type semiconductor NiO, by Direct Current magnetron reactive sputtering.

In 2 O 3 thin films (100 nm thick) have been deposited by reactive evaporation of indium, in an oxygen partial atmosphere. Conductive ( σ = 3.5 × 10 3 S/cm) and transparent films are obtained using the following experimental conditions: oxygen partial pressure = 1 × 10 −1 Pa, substrate temperature = 300 °C and deposition rate = 0.02 nm/s. Layers of this In 2 O 3 thick of 5 nm have been introduced in AZO/In 2 O 3 and FTO/In 2 O 3 multilayer anode structures. The performances of organic photovoltaic cells, based on the couple CuPc/C 60 , are studied using the anode as parameter. In addition to these bilayers, other structures have been used as anode: AZO, FTO, AZO/In 2 O 3 /MoO 3 , FTO/In 2 O 3 /MoO 3 and FTO/MoO 3 . It is shown that the use of the In 2 O 3 film in the bilayer structures improves significantly the cell performances. However the open circuit voltage is quite small while better efficiencies are achieved when MoO 3 is present. These results are discussed in the light of surface roughness and surface work function of the different anodes.

International audience; Perylene derivatives, that behave as liquid crystal and might be used aselectron acceptors, and poly(tetrabromo-p-phenylenediselenide) (PTBrPDSe) weresynthesized with the purpose of using the polymer as buffer layer in solar cells.It was demonstrated that perylene compounds of N,N0-diheptyl-3,4,9,10-perylentetracarboxyldiimide(PTCDI-C7) and N,N0-diundecyl-3,4,9,10-perylentetracarboxyldiimide(PTCDI-C11) enabled obtaining photovoltaic effect when coupled with copperphthalocyanine (CuPc). The power conversion efficiency of the cells prepared fromthese perylenes is similar, whatever the x value. However this efficiency is smallerthan the one achieved when the couple CuPc/C60 (fullerene) is used. More precisely,the best efficiency was obtained when a PTBrPDSe/Au buffer layer is introducedbetween the ITO anode and the CuPc. It was established that the presence of the thinPTBrPDSe layer allows improving the shunt resistance and consequently the cellsperformance.

International audience; ZnO thin films were deposited on glass, ITO (In2O3; Sn) and on ZnO:Al coated glass by spray pyrolysis. The substrates were heated at 350 °C. Structural characterization by X-ray diffraction (XRD) measurements shows that films crystallize in hexagonal structure with a preferential orientation along (0 0 2) direction. XRD peak-shift analysis revealed that films deposited on glass substrate (-0.173) were compressive, however, films deposited onto ITO (0.691) and on ZnO:Al (0.345) were tensile. Scanning electron microscopies (SEM) show that the morphologies of surface are porous in the form of nanopillars. The transmittance spectra indicated that the films of ZnO/ITO/glass and ZnO/ZnO:Al/glass exhibit a transmittance around 80% in the visible region. An empirical relationship modeled by theoretical numerical models has been presented for estimating refractive indices (n) relative to energy gap. All models indicate that the refractive index deceases with increasing energy band gap (Eg).

International audience; The effect of ZnO buffer layer on the performances of inverted multilayers organic solar cells has been studied. ZnO:Al conductive films and insulating ZnO films deposited by spin coating have been probed. The ZnO buffer layer has been introduced between the ITO cathode and the Organic acceptor. The cells are based on the multilayer junctions bathocuproine/fullerene/copper phthalocyanine. The organic photovoltaic cells performances improvement depends of the ZnO layer introduced. ZnO:Al conductive layers decreases the potential barrier at the interface cathode/organic. This allows decreasing the series resistance which improves significantly the cell efficiency. Insulating ZnO increases only slightly the solar cells performance by increasing significantly the shunt resistance and therefore the open circuit voltage of the cells.

The energy conversion efficiency of organic solar cells based on simple electron donor/electron acceptor heterojunction has been studied as a function of the interface anode/electron donor. The anode is fluorine doped tin oxide film. It is shown that the solar cell efficiency increases when a metal ultra thin layer is introduced between the anode and the electron donor. Experimental and theoretical studies show that the work function value of the metal is one of the main factors which allow improving the organic solar cells performances, via better hole collection efficiency. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)