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In this work, we design and compare two intermediate layers (IMLs) for solution-processed tandem organic solar cells. The IMLs are made of PEDOT as the hole transport layer (HTL) coupled with either TiO x or ZnO nanoparticles as the electron transport layer (ETL). The IMLs are characterized with regard to the requirements usually found in the literature: process robustness, work function compatibilities, good optical transparency, good electrical conductivity, and finally, the effective presence of an ohmic contact between the ETL and the HTL. Specifically designed single and tandem cells are then made to compare the performances of the two IMLs. We show that the ohmic contact is the most important criterion, which has to be carefully probed in order to design a functional IML. We offer recommendations on how to properly investigate this property. We also show that the nature of the ETL/HTL contact cannot be predicted from the results of the materials characterization, particularly not from the work functions and neither from the conductivities. Finally, XPS and work function experiments suggest that the nature of the contact depends on phenomena arising at the interface between the HTL and the ETL. Specifically, a chemical interaction is observed between ZnO and PEDOT within the IML. We conclude that a ZnO-based IML is a better choice than a TiO x -based IML.

Tin sulfide (SnS), having a direct band gap of 1.3 eV, is a promising absorber material for solar energy conversion. We synthesized colloidal SnS nanocrystals with a size tuneable from 5 to 20 nm and low size dispersion. These nanocrystals can be processed as thin films using low-cost solution phase methods. They also offer the possibility of controlling the crystalline phase before deposition. With the goal to obtain dense and crack-free films of high conductivity, we used a layer-by-layer deposition technique. In the first step, the substrate was dipped in the nanocrystal colloidal solution (“ink”). Next, exchange of the nanocrystal surface ligands (oleylamine, trioctylphosphine, oleic acid) was carried out by dipping the substrate into a solution of small cross-linking molecules (1,4-benzenedithiol). This exchange enhances the electronic coupling and charge carrier mobilities by reducing the interparticle distance. At the same time it assures the immobilization of the nanocrystals to avoid their removal during subsequent depositions. The thickness of the nanocrystal thin films was controlled in a range of 100–250 nm by varying the number of the alternating nanocrystal deposition and ligand exchange steps. Scanning electron microscopy and atomic force microscopy investigations show that the obtained films are dense and homogeneous with a surface roughness as low as 3 to 4 nm root mean square. Using an inverted structure, the heterojunction of a SnS nanocrystals film with n-type ZnO nanocrystals shows a strongly increased current density under white light irradiation with respect to the dark.

Donor–acceptor alternated conjugated copolymers based on benzotriazole acceptor units and electron-donating units such as fluorene, carbazole and silyl fluorene, were synthesized through palladium catalyzed Suzuki cross-coupling polymerization. NMR and elemental analyses were achieved to confirm the structure of each copolymer. Their thermal, photophysical and electrochemical properties are also reported. The photovoltaic characteristics of these polymers determined from bulk-heterojunction photovoltaic cells are discussed.

A study of the optical and impedance behavior of optimized standard and inverted photovoltaic solar cells based on P3HT:PCBM active nano composites is presented. The standard cells sequence is ITO/HTL1/P3HT:PCBM/Ca/Al and the inverted cells one is ITO/ZnO/P3HT:PCBM/HTL2/Ag where HTL1 and HTL2 are Hole Transport Layers. Absorption and action spectra, together with I–V characteristics, are shown to be quite similar and lead to 4.05% and 3.90% energy conversions, for standard and inverted cells, respectively. Built-in potentials of 0.82–0.89 V and acceptor impurities concentrations of 1.6–2.4 1015 cm−3 are found through capacitance measurements. Impedance spectrometry shows the classical two-circle complex plan curves, one being related to the effective lifetime of charge carriers before recombination at low frequency, and the other one to the diffusion time of these carriers at high frequency. The shape of the curves is identical, showing the ohmic role of the ZnO layer. It is shown that overall resistances in the dark are higher for inverted cells as compared to standard ones, and that this feature is inverted under illumination, with a thousand-time decrease. Global mobilities are in the range 3.8–4.6 10−3 cm2 V−1 s−1, which is slightly higher as compared to the literature. Four different equivalent circuit models are tested on experimental results, and it is concluded that the classical RCPE model (or its Garcia–Belmonte variant) is suitable for this kind of cells.

Three new low-bandgap copolymers containing 3,6-linked carbazole units are synthesized and characterized using a combination of spectroscopic and electrochemical techniques and DFT calculations. The electron-withdrawing effect of the BTD units leads to a significant decrease of the optical bandgap from 2.59 eV for PBTC to 1.81 eV for PCDTBT, whereas pendant octyl chains on the thiophene ring lead to an increase of the gap to 2.05 eV for PCDOTBT due to steric hindrance. PBTC:PCBM photovoltaic devices exhibit a PCE of 0.35% with an EQE reaching 35% in the blue region of the solar spectrum whereas PCDTBT:PCBM blends are efficient over a wider spectral range due to the lower bandgap of PCDTBT.

A novel series of regioregular bithiophene disubstituted with a carbonitrile group as electron acceptor and a hexyl or hexyloxy chain as electron donor, have been synthesized. The position and the regioregularity of substituents on bithiophene have been controlled via a head-to-head (HH) or head-to-tail (HT) coupling reaction to give four monomers: the HH and the HT carbonitrilehexylbithiophene monomers and the HH and the HT carbonitrilehexyloxybithiophene monomers. Each of them lead, by chemical polymerization, to four polymers constituted by a polythiophene backbone containing alternating electron-donating and electron-withdrawing units directly connected to the polythiophene. Optical and electrochemical characterizations reveal that all these polymers have a low band gap and absorb at longer wavelengths in comparison with a poly(3-hexylthiophene) (P3HT) film. The planarity of the polythiophene backbone was increased by introduction of sulphur–oxygen (S–O) interactions between hexyloxy chains and thiophene units. In addition, the introduction of carbonitrile groups allows to stabilize the LUMO and the HOMO energy levels of polymers at around −3.10 eV for the LUMO and −5.0/−5.3 eV for the HOMO, which were lower than the ones of P3HT (−2.19 eV for the LUMO and ∼−4.9 eV for the HOMO). Bulk heterojunction photovoltaic devices were fabricated using blends of these polymers with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). Power conversion efficiency of 1.07% was obtained under solar illumination AM 1.5 (100 mW cm−2) from a device containing the polymer synthesized from HT 3-carbonitrile-4′-hexyloxyl-2,2′-bithiophene monomers and PCBM as the active layer.

International audience; The stereo-electronic control over bisadditions of conjugated polymers to fullerene (C60) is explored in the formation of alternating copolymers. The chemistry, resulting configuration, and properties of poly(3-hexylthiophene)- alt-C60 copolymers prepared by either classic pyrrolidine ring formation or an atom transfer radical addition are compared. Both routes result in controlled additions of polymers to C60. Extensive macromolecular modeling through PM6 methods indicate that there is no conjugation between P3HT and C60 in the systems studied. This along with 2D-NMR, AFM, and photovoltaic characterizations of the materials indicate the importance of the structure of the modified C60 and the nature of the linking group between C60 and P3HT segments in determining the morphology of the copolymers in the solid state. © 2009 Wiley Periodicals, Inc.

A series of four conjugated molecules consisting of a fluorenone central unit symmetrically coupled to different oligothiophene segments are conceptually designed and synthesized to provide new electroactive materials for application in photovoltaic devices. The combination of electron-donating oligothiophene building blocks with an electron-accepting fluorenone unit results in the emergence of a new band assigned to an intramolecular charge transfer transition that gives rise to the extension of the absorption spectral range of the resulting molecules. Detailed spectroscopic and voltammetric investigations show that all studied molecules have highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) level positions, which make them good candidates for the application as electron-donors in bulk-heterojunction photovoltaic cells, with (6,6)-phenyl-C61-butyric acid methyl ester (PCBM)-C60 as electron acceptor component. Moderate device performances, with power conversion efficiencies (PCEs) comprised between 0.3 and 0.6%, were obtained with rigid molecules, containing either the bridging units between the thiophene rings, i.e., (2,7-bis(4,4 0 -dioctyl-cyclopenta[2,1-b:3,4-b 0 ]dithiophen-2yl)-fluoren-9-one (SCPTF) and 2,7-bis(4-(dioctylmethylene)-cyclopenta[2,1-b:3,4-b 0 ]dithiophen-5-yl)-fluoren-9-one (MCPTF) or a vinylene unit 2,7-bis(5-[(E)-1,2-bis(3-octylthien-2-yl)ethylene])-fluoren-9-one (TVF), whereas with (2,7-bis-(3,3 000 -dioctyl[2,2 0 ;5 0 ,2 00 ;5 00 ,2 000 ]quaterthiophen-5-yl)-fluoren-9-one (QTF) PCE up to 1.2% (under AM 1.5 illumination, 100mW cm � 2 , active area 0.28cm 2 ) was obtained. The strong p-stacking interactions in the solid state for this oligomer leading to improved morphology could explain the good performances of QTF-based devices, which rank among the highest recorded for nonpolymeric materials. Consequently, fluorenone-based non-polymeric molecules constitute highly attractive materials for solution-processable solar cell applications.

cited By 30; International audience; Highly regioregular poly[3-(4-octylphenylthiophene)] (POPT) was prepared to study the influence of phenyl groups as substituents on photovoltaic and thermal properties with respect to poly(3-hexylthiophene) (P3HT). The UV-visible absorption spectra of spin-coated films of POPT exhibited an extended absorption towards the near infrared. Differential scanning calorimetry showed the poor tendency of POPT to crystallise. In order to retain the extension in absorption and increase crystallinity, poly(3-hexylthiophene)-block-poly(3-tolylthiophene) (P3HT-b-P3TT) samples with varying ratios of P3HT and P3TT were prepared. Thermal fractionation by successive self-nucleation and annealing indicated that crystallites of P3HT were influenced by the P3TT block within the copolymers. Thermogravimetric analysis showed that thermal stabilities increased with increasing fractions of phenyl groups. Photovoltaic results of these materials blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) are presented and discussed. © 2007 Elsevier B.V. All rights reserved.

Various series of conjugated systems have been used as donor in hetero-junction solar cells. The results obtained with EDOT-containing π-conjugated oligomers show that besides their direct effect on the electronic properties of the conjugated chain, the number and position of the EDOT units control to a large extent the orientation of the molecules on the substrate and hence the performances of the derived solar cells. The characteristics of the cells based on oligothienylenevinylenes donors show that in this case structural control of orientation can be achieved by means of substitution of the conjugated structure by alkyl chains. Donors based on star-shaped oligothiophenes provide another example of control of horizontal molecular orientation by design. The photovoltaic characteristics of bi-layer hetero-junctions show that the combined effects of controlled molecular orientation and planarization of the structure by the use of a fused tri-thienobenzene central core lead to power conversion efficiencies approaching 1%. In order to avoid the need to control molecular orientation first examples of bulk hetero-junctions based on PCBM and three-dimensional conjugated systems with isotropic electronic properties have been realized. A power conversion efficiency of 0.30% has been obtained under 79 mW cm− 2 white light illumination. This result obtained with a 3D donor based on a terthiophene conjugated chain with very limited absorption of the visible spectrum demonstrates the large potentialities of this novel concept.

We describe in this article the quick optimization of bulk heterojunction organic solar cells based on poly-3-hexylthiophene and [6,6]-phenyl C61 butyric acid methylester blend. Best cells are found to be based on 1:1 in weight ratio and yield 3.6% power conversion efficiency under air-mass 1.5, 100 mW/cm2 illumination. On the optimized cells, ageing measurements have been realized to collect the absolute values of photovoltaic parameters along the time. On this study, accelerated lifetime has been calculated using the Arrhenius model for a first-order kinetic of degradation. According to the model, we can predict a long lifetime keeping more than 80% of the initial short-circuit current density after 1000 h of working.

We developed indium-tin-oxide/perylene diimide (or bathocuproine (BCP))/poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) and [6,6]-phenyl C60 butyric acid methyl ester (PCBM) blend/copper phthalocyanine (CuPc)/Au interpenetrated network polymer solar cells in order to improve air stability. The stability properties of the cells were characterized by current–voltage measurements under the influence of light and air. We achieved long lifetime solar cells which work at least 2 weeks under ambient air conditions without encapsulation. Solar energy conversion efficiency of the cells decrease 30% of the first day value at the end of 2 weeks. Photocurrent absorption properties of the devices were also investigated.

Organic field-effect transistors were fabricated with Quaterthiophene (4T) as the active material and a ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene) as the gate insulator. As-prepared devices showed normal p-type transistor operation. The ON and OFF-states could be written to the device by applying appropriate voltages to the gate with respect to short-circuited source and drain electrodes. The devices exhibited promising memory retention properties.

New series of C60-derivatized bithiophenic precursors with low oxidation potential have been synthesized using the thiolate deprotection chemistry. The analysis of the electropolymerization of these compounds shows that the use of two-site precursors leads to polymers combining enhanced conjugation length, faster switching time, and improved stability under redox cycling. The unsuccessful attempts to identify the optical signature of the reduced forms of the attached C60 by spectroelectrochemistry suggest that the PT backbone is unstable in the presence of the C60 anion radical. Preliminary photoelectrochemical experiments on films deposited on platinum electrode reveal a significant enhancement of the photocurrent for the C60-derivatized polymer when compared to a nonsubstituted reference polymer, indicating that these new materials are potentially useful for photovoltaic energy conversion.

International audience; In standard organic solar cells, an electron transporting layer (ETL) is often inserted between the active layer and the cathode in order to achieve good performances. In this work, a zinc oxide (ZnO) ETL is used with an aluminum cathode, yielding functional fresh cells. However, upon storage in the dark and inert atmosphere, an S-shape quickly appears in the J−V curves, leading to a sharp decline of the solar conversion efficiency. By building cells with various ETL sequences, we show that the S-shape stems from a modification of the ZnO/Al interface. Further experiments using Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS) suggest that the source of this phenomenon is the appearance of an aluminum oxide layer at the ZnO/Al interface, which could result from a red−ox reaction between the two materials.

An organic solar cell based on a poly-3-hexylthiophene (P3HT): phenyl-C61-butyric acid (PCBM) bulk hetero-junction was directly coupled with molybdenum sulfide resulting in the design of a new type of photocathode for the production of hydrogen. Both the light-harvesting system and the catalyst were deposited by low-cost solution-processed methods, i.e. spin coating and spray coating respectively. Spray-coated MoS3 films are catalytically active in strongly acidic aqueous solutions with the best efficiencies for thicknesses of 40 to 90 nm. The photocathodes display photocurrents higher than reference samples, without catalyst or without coupling with a solar cell. Analysis by gas chromatography confirms the light-induced hydrogen evolution. The addition of titanium dioxide in the MoS3 film enhances electron transport and collection within thick films and therefore the performance of the photocathode.

The authors demonstrate the fabrication of multiscale electrodes (with lateral dimensions ranging from 10 μm to a few hundred of nanometers) on organic donor–acceptor photovoltaic thin films by mean of nanostencil lithography. The experimental setup allows the realization and in situ investigation of structures by combining nanostencil and atomic force microscopy (AFM) capabilities based on the use of tuning forks. Qplus AFM sensors with tungsten etched tips were used for near field imaging, and as microprobes to contact the deposited electrodes. The photovoltaic behavior was characterized by mean of current–voltage measurements in dark and under selective illumination. The influence of the electrode geometry on the electrical parameters is analyzed and the effect of in situ annealing is discussed in view of frequency modulation AFM images of the active layer surface morphology. This illustrates the powerfulness of nanostencil combined with Qplus AFM for local investigations of organic photovoltaic materials and in situ fabrication of devices.

Organic field-effect transistors were fabricated with pentacene as the active material and a ferroelectric copolymer poly(vinylidene fluoride–trifluoroethylene) as the gate insulator. As-prepared devices showed normal p-type transistor operation. The ON- and OFF-states could be written to the device by applying appropriate voltages to the gate with respect to short-circuited source and drain electrodes. The devices exhibited excellent memory retention properties.

Organic solar cells based on an interpenetrated network of conjugated polymer as donor and fullerene derivative as acceptor materials have a great potential for improving efficiency. We fabricated a device based on a composite of poly(2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylenevinylene and [6,6]-phenyl C60 butyric acid methyl ester. Surface treatment, insertion of interfacial layers, and improvement of the morphology of the active layer significantly increase the photovoltaic performances of the structure. We obtain an open circuit voltage of 0.87 V and short circuit current density of 8.4 mA/cm2 under 100 mW/cm2 air-mass 1.5 solar simulator illumination, yielding a 2.9% power conversion efficiency.

All-inorganic 12-nm CuInSe 2 nanocrystals, surface functionalized with metal chalcogenide complex ligands, were prepared via phase transfer ligand exchange in a biphasic (anhydrous hydrazine/hexane) mixture. I – V characteristics of thin films of these nanocrystals showed a four order of magnitude increase in current density as compared to nanocrystals capped with initial oleylamine ligands.

Narrow band gap tin(II) chalcogenide (SnS, SnSe, SnTe) nanocrystals are of high interest for optoelectronic applications such as thin film solar cells or photodetectors. However, charge transfer and charge transport processes strongly depend on nanocrystals' surface quality. Using (119)Sn-Mossbauer spectroscopy, which is the most sensitive tool for probing the Sn oxidation state, we show that SnS nanocrystals exhibit a Sn((IV))/Sn((II)) ratio of around 20:80 before and 40:60 after five minutes exposure to air. Regardless of the tin or sulfur precursors used, similar results are obtained using six different synthesis protocols. The Sn((IV)) content before air exposure arises from surface related SnS(2) and Sn(2)S(3) species as well as from surface Sn atoms bound to oleic acid ligands. The increase of the Sn((IV)) content upon air exposure results from surface oxidation. Full oxidation of the SnS nanocrystals without size change is achieved by annealing at 500 °C in air. With the goal to prevent surface oxidation, SnS nanocrystals are capped with a cadmium-phosphonate complex. A broad photoluminescence signal centered at 600 nm indicates successful capping, which however does not reduce the air sensitivity. Finally we demonstrate that SnSe nanocrystals exhibit a very similar behavior with a Sn((IV))/Sn((II)) ratio of 43:57 after air exposure. In the case of SnTe nanocrystals, the ratio of 55:45 is evidence of a more pronounced tendency for oxidation. These results demonstrate that prior to their use in optoelectronics further surface engineering of tin chalcogenide nanocrystals is required, which otherwise have to be stored and processed under inert atmosphere.

International audience; Among the numerous reduced bandgap polymers currently being developed, poly[3-(4-octylphenyl)thiophene)]s (POPT) may present attractive properties for organic solar cells due to its facile preparation and improved absorption with respect to poly(3-hexylthiophene). This article appraises methods of preparation, including the use of diphenyl ether as a reaction medium, and discusses the effects of variations in molar masses, from about 3200 to 65,000 g mol -1 and regioregularity on its optoelectronic properties. The photovoltaic properties of POPT with [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) in bulk heterojunction devices are also discussed in the light of morphological variations, as indicated by atomic force microscopy characterizations. With an initial screening of conditions, namely POPT:PCBM ratios and deposition solvent, a power conversion efficiency of 1.58% was obtained using a relatively high molar mass POPT sample.

Acceptor polymers for photovoltaic applications were synthesized by grafting fullerene C 60 onto polystyrene. The quality of the reaction was verified by various analytical techniques after each of the three steps of the reaction: nuclear magnetic resonance, infrared and UV–visible spectroscopies, and thermo-gravimetric analysis. In order to determine the optimal amount of C 60 , a series of polymers were prepared containing from 4 to 59 vol.% of fullerene. The optical (absorption, optical gap energy) and electrical (electron mobility, conductivity) properties have been measured. A percolation threshold at around 4 vol.% was identified for both conductivity and mobility measurements. This provides the lowest amount of C 60 required for solar cells applications.

We synthesise 7 nm CuInSe2 nanocrystals of low size and shape dispersion using copper(I) chloride, indium(III) chloride and selenourea as the precursors. The obtained nanocrystals are deposited on indium tin oxide (ITO) covered glass by means of layer-by-layer dip-coating. After each deposition step, a ligand exchange process with 1,2-ethanedithiol (EDT) is carried out. The new ligands interconnect the nanocrystals, which therefore become insoluble during subsequent dip-coating steps. Furthermore, the small EDT molecules replace the long insulating alkyl chains of the oleylamine stabilizing ligands used during the synthesis. I/V measurements of a 1 μm thick film sandwiched between the ITO substrate and an aluminium electrode show a current density of 1 μA/cm2 at 1 V in the dark, which increases to 16 μA/cm2 under illumination with white light.

We have synthesized a new series of donor-acceptor-donor (D-A-D) pi-conjugated molecules, consisting of fluorenone core end-capped with dendritic oligo(thiophene)s of increasing generation (abbreviated as FG0, FG1, and FG2). In view of the application of these new organic semiconductors in photovoltaic devices, we have explored their spectroscopic, redox, and structural properties. The thermal behaviour of the new organic semiconductors was investigated by differential scanning calorimetry and polarized-light optical microscopy. Liquid crystalline behaviour has been found in the case of FG1, corresponding to a smectic ordering with a triclinic symmetry (Sm(obl)) upon heating, as confirmed by variable temperature small-angle X-ray diffraction studies. In order to evaluate their photovoltaic performances, devices with an active area of 0.28 cm(2) were fabricated. Under AM1.5 simulated sunlight (100 mW cm(-2)) conditions, a device containing FG1/[70]PCBM blends showed a power conversion efficiency of ca. 0.8%.

This article describes a method to have a better knowledge of barrier performances needed for encapsulating materials, particularly in the case of organic solar cells devices. We have developed a high sensitivity permeameter which enables simultaneous measurements of water and oxygen permeation. Various polymers and inorganic coatings on polymer substrates have been measured. Experimental barrier parameters have been plotted considering the steady and transient states of permeation curves and compared to theoretical values. In addition, we have performed ageing experiments on encapsulated organic solar cells to establish a barrier requirement directly related to the device. Finally, we have performed such experiments using different cathode materials and encapsulating materials.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 659-664, Large area bulk heterojunction PV devices based on P3HT/PCBM have been realized on transparent plastic substrates, following an implementation procedure starting from small size individual cell on glass to end with 35 cm² modules on PET. The first step consisted in the optimization of elaboration parameters on individual cells of 0.28 cm² on PET starting from our standard procedure setup for glass substrates. PCE close to 4% with small dispersion on plastic substrates for cells of 0.28 cm² active area, compared to 5% on glass. Two module designs were then tested on both substrates of 5x5 cm²,varying the number of individual cells and the interconnection pattern. For glass module, with 12 individual cells on a 5x5 cm2 substrate we obtain PCE of 3.26 % (12.4 cm² active area). Larger modules with active area up to 35 cm2 exhibiting PCE of 2.8 % and open circuit voltage higher than 6V were also realized on glass, approaching the requirements for commercial electronic applications. On PET, record efficiencies of 2.8 % and 2.5% were obtained for active areas of 8.8 cm2 and 53 cm² respectively. The influence of the geometric parameters of the individual cells and their type of connection (parallel or series) on the module characteristics is also discussed.

New π-conjugated alternating copolymers containing thienylene and fluorenone units, namely poly[(5,5‴-dioctyl-[2,2′;5′,2″;5″,2‴]quaterthiophene)-alt-(2,7-fluoren-9-one)] (PQTF8), as well as its analogue without fluorenone groups (PQT8), have been synthesized. Absorption studies carried out both in solution and in thin films indicate that the presence of fluorenone chromophores in PQTF8 leads to a significant extension of the absorption spectrum in the visible range as compared to PQT8. The redox properties of both polymers, in particular their LUMO and HOMO levels, have been characterized by cyclic voltammetry and have been found suitable for potential use of these systems in organic solar cells. Finally, these materials have been tested as donor components in bulk-heterojunction-type photovoltaic cells using PCBM as an electron acceptor. We demonstrate a strong effect of the polymer's molecular weight on crucial cell parameters, such as the short-circuit current density Jsc, and therefore on the overall cell efficiency. This effect is particularly pronounced for PQTF8-based cells leading to power conversion efficiencies up to 1.5% for the highest molecular weights.

Among the class of conjugated polymers, polythiophenes and in particular 3-alkyl-substituted thiophenes seem to focus all the attention in the domain of photovoltaic conversion. At CEA, we are working on the optimization of bulk heterojunction solar cells made of poly-3-hexylthiophene (P3HT)and [6,6]-phenyl C 61 butyric acid methylester (PCBM) blend. First we will describe the influence of the ratio of P3HT and PCBM blend on the efficiency of the resulting bulk heterojunction solar cells. Best cells based on 1:1 in weight ratio yield 3.6 % power conversion efficiency under air-mass 1.5, 100 mW/cm 2 illumination. Then we will compare the efficiency and lifetime of different cells by changing the nature and thickness of cathode (Aluminum or Calcium/Silver). On the optimized cells, we have proceeded to ageing and accelerated lifetime measurements on devices with Ca/Ag cathode. It shows that the current densities decrease less than 3 % and that efficiency is still higher than 1.7 %, after 400 hours under AM 1.5, 100 mW/cm 2 illuminations and at high temperature (60°C).

Though being much less efficient than silicon cells, organic solar cells exhibit a unique combination of interesting properties: low cost, flexibility, and the possibility of large surface coverage. Large progresses have been made over the last years using MDMO-PPV (Poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) reaching efficiencies of 2.9% and recently efficiencies over 3%, using poly(3-hexyl thiophene). A great deal of research however has still to be invested to improve the current state of the art. Among the main key-points to be addressed are namely the stability and lifetime of such devices. We are currently working on bulk heterojunction solar cells made from MDMO-PPV and PCBM (methano-fullerene[6,6]-phenyl C61-butyric acid methyl ester). Different batches of MDMO-PPV, originating from different synthesis modes (classical "Gilch" synthesis and "Sulphinyl" synthesis led by IMEC-IMOMEC) have been tested. Evolution of the power efficiency following continuous illumination (AM1.5, 80 mW.cm-2) was characterized under controlled atmosphere of nitrogen. In parallel, photodegradation studies are also investigated and electrical modeling is under way in order to get a better understanding of the relations between photochemical and electrical parameters of the diode that can be deduced from I/V curves.

The nonvolatile transistor memory element is an interesting topic in organic electronics. In this case a memory cell consists of only one device where the stored information is written as a gate insulator polarization by a gate voltage pulse and read by the channel conductance control with channel voltage pulse without destruction of the stored information. Therefore such transistor could be the base of non-volatile non-destructively readable computer memory of extremely high density. Also devices with polarizable gate dielectrics can function more effectively in certain circuits. The effective threshold voltage V t can be brought very close to zero, for applications where the available gate voltage is limited. Resonant and adaptive circuits can be tuned insitu by polarizing the gates. Poly(vinylidene fluoride), PVDF and its copolymer with trifluoroethylene P(VDF-TrFE) are among the best known and most widely used ferroelectric polymers. In this manuscript, we report new results of an organic FET, fabricated with pentacene as the active material and P(VDF-TrFE) as the gate insulator. Application of a writing voltage of -50 V for short duration results in significant change in the threshold voltage and remarkable increase in the drain current. The memory effect is retained over a period of 20 hours.

Organic solar cells based on interpenetrated network of conjugated polymer as donor and fullerene derivative as acceptor materials have a great potential for improvement of the efficiency. We fabricated a device based on a composite of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) and [6,6]-phenyl C60 butyric acid methyl ester (PCBM). Surface treatment, insertion of interfacial layers, and improvement of the morphology of the active layer significantly increase the photovoltaic performances of the structure. We obtained an open circuit voltage of 0.87 V and short circuit current density of -8.4 mA/cm2 under 100 mW/cm2 AM 1.5 solar simulator illumination, yielding a 2.9 % power conversion efficiency. Two others fullerene derivatives were synthesized in order to get a stronger acceptor. The electrical and optical properties of devices made with MEH-PPV and these acceptors are investigated.

[structure: see text] In four new dendrimers terminated by 12 electroactive tetrathiafulvalenyl substituents, the tridimensional character of the inter- and intradendrimeric charge and electron transfer, and hence of the electroconductivity, is evidenced by examination of the electronic spectra of their corresponding neutral state and cation radical, dication, and mixed-valence salts, including a closed-shell anion.

Two types of conjugated polymers were prepared with the goal to blend them with rod-like CdSe nanocrystals. The polymers of the first type were synthesized through copolymerization of 3-octylthiophene and 3-methylene-ethylcarboxylate-thiophene to give polythiophene with solubilizing alkyl groups and methylene ester functional groups (PE series). Post-polymerization hydrolysis of the ester type polymers yielded acid-type ones (PA series). Photoluminescence (PL) quenching in these polymers induced by their titration with nanocrystals solution was chosen as a measure of the polymer-nanocrystal interactions. PL of polyacids turned out to be more efficiently quenched as compared to the case of polymers with ester groups which was interpreted as an indication of better electronic communication between the hybrid components. Infrared (IR) spectroscopy confirmed efficient coordination of the carboxylic groups to CdSe. Voltammetric studies combined with UV-vis spectroelectrochemistry enabled the determination of energy levels alignment of the molecular composite components which turned out to be of staggered type-appropriate for photovoltaic applications. The obtained blends of polyacids with CdSe nanocrystals, when studied by transmission electron microscopy (TEM), revealed the presence of an interpenetrating network in which nanorods were homogeneously distributed within the polymer matrix without any indication of agglomerates formation both on the film surface and in the cross-section. Blends with polymers containing ester groups were less homogeneous which could be explained by weaker polymer-nanocrystals interactions. Photovoltaic cells based on these hybrid materials are also discussed.

Regioregular poly(3-hexylthiophene) containing one diaminopyrimidine side group per ten repeat units (P3HT-co-P3(ODAP)HT) can form molecular composites with 1-(6-mercaptohexyl)thymine capped CdSe nanocrystals (CdSe(MHT)) via hydrogen bonds directed molecular recognition. Here we report complementary spectroscopic, electrochemical and spectroelectrochemical investigations of both the functionalized poly(thiophene) and its composite with the nanocrystals, the latter being fabricated using the layer-by-layer (LbL) deposition technique. UV-Vis-NIR and Raman spectroelectrochemical investigations unequivocally show that the onset of the first anodic peak in the cyclic voltammogram of the copolymer can be attributed to the oxidation of the pi-conjugated backbone in the polymer chains. For this reason, it is possible to determine the width and the position of its band gap (corresponding to the pi-pi* transition) by UV-Vis spectroscopy combined with cyclic voltammetry. These studies show that the polymer exhibits a slightly larger band gap with the HOMO level insignificantly lower in energy (by 0.03 eV) as compared to the case of regioregular poly(3-hexylthiophene) of comparable degree of polymerization. Hydrogen bond interactions of the polymer with CdSe(MHT) in the molecular composite result in a hypsochromic shift of the band corresponding to the pi-pi* transition from 504 nm to 488 nm. This can be taken as a spectroscopic manifestation of the conformational changes induced by shortening of the conjugation length. The observed spectral modifications are consistent with electrochemically determined lowering of the polymer HOMO level (from -4.91 eV in the pure polymer to -4.99 eV in the composite). Cyclic voltammetry studies supported by spectroelectrochemistry also show that the redox stability of CdSe(MHT) in the molecular composite with P3HT-co-P3(ODAP)HT is lower than that determined for stearate-capped nanocrystals. Their irreversible oxidation starts at E = +0.7 V vs. Ag/0.1 M Ag(+)i.e. at potentials by ca. 0.3 V lower than the oxidation of stearate stabilized CdSe nanocrystals of the same size. We show that-despite these modifications-the alignment of the HOMO and LUMO levels of the composite components remains appropriate for its use in hybrid solar cells, which is demonstrated by the photovoltaic effect observed for the LbL-processed composite sandwiched between two electrodes.

The electrode-active layer interface of organic photovoltaic cells, a critical point in the development of organic devices, was studied by the energy dispersive x-ray reflectivity (EDXR) technique applied in situ. An EDXR-based protocol allowing discrimination between the possible mechanisms that produce the aging process at the interface was established. The study detects photoinduced oxidation of the electrode at the buried interface, to which fading of the device performances could be attributed. This conclusion was further confirmed by results obtained on a new cell, of selectively modified architecture, whose performances turned out to be stable in time.

Tetrahedral conjugated systems involving four conjugated oligothiophene chains fixed onto a central silicon node (1, 2) have been synthesized and used as donor materials in hetero-junction solar cells. Bilayer solar cells have been realized by thermal evaporation of compounds 1 and 2 as donors and N,N′-bis-tridecylperylenedicarboxyimide as an acceptor. Comparison of the performances of these devices to those of a reference system based on dihexylterthienyl (H3T) shows that despite comparable effective conjugation lengths, the 3D compounds 1 and 2 lead to a power conversion efficiency four–five times higher, suggesting better absorption of the incident light and better hole transport properties. Whereas fabrication of bulk hetero-junction with H3T was prevented by the lack of film forming properties, a prototype bulk hetero-junction based on compound 2 as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor has been realized. A short-circuit current density of 1.13 mA cm−2 and a power conversion efficiency of 0.30% has been measured under AM 1.5 simulated solar irradiation at 80 mW cm−2.

A submicrometric periodical patterning of the surface of an organic solar cell is investigated towards optimisation of the photovoltaic conversion efficiency. Patterning is achieved using a technique based on photoinduced mass transport in azopolymer films.

In order to reach commercialization, organic photovoltaic solar cells need to reach efficiencies above 10 % and achieve lifetimes of several thousands of hours. Tandem solar cells are a way of improving the efficiencies. The objectives of this work are therefore to study the fabrication process, the operation and the ageing of organic tandem solar cells. First, single solar cells based on the active material PCDTBT are used as model to investigate the factors governing the cells efficiencies. Using characterization techniques such as impedance spectroscopy, the roles played by each layer of the cells are identified. Based on these results, a protocol to make series tandem cells is developed. Each of its steps is dedicated to treating a key aspect of tandem cells : choice of complementary absorbing polymers, design of the intermediate layer (IML) and thickness optimization. The functioning of the IML is subjected to a particular attention. To optimize the thicknesses, optical phenomena are numerically simulated. The prediction thus made are then compared to experimental results. Finally, the ageing of single and tandem cells is investigated on time spans ranging from several dozens to several thousands of hours. It is shown that the device degradation can be linked to poor ageing of the interface layers, while the active layer stays stable. Organic tandem cells are promising candidates to reach both high efficiencies and long lifetime., Pour être viable économiquement, les cellules photovoltaïques organiques doivent dépasser 10 % de rendement et atteindre plusieurs milliers d'heure de durée de vie. Les cellules tandems constituent une voie probante d'augmentation des rendements. Ce travail de thèse a pour objectifs l'élaboration de cellules organiques tandems puis l'étude de leur fonctionnement et de leur vieillissement. Dans un premier temps, les paramètres gouvernant le rendement de cellules photovoltaïques organiques sont examinés sur le modèle de cellules simples à base de PCDTBT. Des caractérisations s'appuyant sur la spectroscopie d'impédance mettent en valeur le rôle joué par chacune des couches constitutives de la cellule. Dans un deuxième temps, un protocole de fabrication des cellules tandems connectées en série est établi. Celui-ci traite successivement les trois points critiques pour l'efficacité des tandems : choix des polymères aux absorptions complémentaires, puis mise au point de la couche intermédiaire (CI) et enfin optimisation des épaisseurs. Une attention particulière est portée au fonctionnement de la CI. Pour optimiser les épaisseurs, un programme de simulation des phénomènes optiques est élaboré et ses prédictions comparées aux résultats expérimentaux. Enfin, le vieillissement des cellules simples et tandems est étudié sur des temps de plusieurs centaines ou milliers d'heures. Les diminutions de performances observées sont liées à la dégradation des couches d'interface tandis que la couche active semble stable. Les cellules tandems organiques présentent un fort potentiel pour l'obtention de rendements élevés et stables dans le temps.