Lionel Hirsch

Thermal Stabilisation of Polymer–Fullerene Bulk Heterojunction Morphology for Efficient Photovoltaic Solar Cells

A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.

Self-assembly of supramolecular fullerene ribbons via hydrogen-bonding interactions and their impact on fullerene electronic interactions and charge carrier mobility.

The anisotropy of the electronic interactions between fullerenes in crystalline solids was examined using a confocal fluorescence microscope by probing the polarization of the fluorescence emission arising from fullerene excimer-like emitting states. Crystals of C(60) obtained by vacuum-sublimation or from chloroform solution exhibited no or little polarization (p = 0 or 0.11, respectively), as expected from the high symmetry of the C(60) fcc lattice or the low degree of anisotropy induced by included solvent molecules. The use of hydrogen-bonding to supramolecularly control interfullerene electronic interactions was explored using a fullerene derivative (1) combining a solubilizing 3,4-di-tert-butylbenzene group and a barbituric acid hydrogen-bonding (H-B) moiety. The crystal structure of 1 establishes the existence of fullerene H-B tapes along which interfullerene electronic interactions are expected to be large. In agreement with this, we observe very strong polarization of the fullerene excimer-like emission (p = 0.78), indicative of a high degree of anisotropy in the fullerene interactions. The charge-carrier mobility of 1 as determined from OFET devices was found to be lower than that of C(60) (1.2 x 10(-4) vs 1.2 x 10(-2) cm(2)/s V), which is rationalized on the basis of the reduced dimensionality of 1 as a wire-like semiconductor and variations in the morphology of the device active layer revealed by AFM measurements.

Electrical characterization of InGaN/GaN light emitting diodes grown by molecular beam epitaxy

We studied the electrical behavior of multiple InGaN/GaN quantum well based light emitting diodes grown by molecular beam epitaxy and we determined three different domains of current-voltage dependence. We then described the charge carrier transport mechanisms for these three domains. The first domain, corresponding to leakage currents (V 3.5 V), the current is limited by the p-GaN zone. In this zone, the...

Low-Temperature UV Processing of Nanoporous SnO2 Layers for Dye-Sensitized Solar Cells

Connection of SnO₂ particles by simple UV irradiation in air yielded cassiterite SnO₂ porous films at low temperature. XPS, FTIR, and TGA-MS data revealed that the UV treatment has actually removed most of the organics present in the precursor SnO₂ colloid and gave more hydroxylated materials than calcination at high temperature. As electrodes for dye-sensitized solar cells (DSCs), the N3-modified 1-5 μm thick SnO₂ films showed excellent photovoltaic responses with overall power conversion efficiency reaching 2.27% under AM1.5G illumination (100 mW cm⁻²). These performances outperformed those of similar layers calcined at 450 °C mostly due to higher V(oc) and FF. These findings were rationalized in terms of slower recombination rates for the UV-processed films on the basis of dark current analysis, photovoltage decay, and electrical impedance spectroscopy studies.

GaInN GaN multiple-quantum-well light-emitting diodes grown by molecular beam epitaxy

GaInN and GaN were grown by molecular beam epitaxy on c-plane sapphire using NH3. 9 K photoluminescence performed on both GaInN thin layers and GaInN/GaN multiple-quantum wells (MQWs) exhibits narrow emission (∼50 meV linewidths). Transmission electron microscopy images show sharp GaInN/GaN interfaces and homogeneous GaInN layers. Strong indium surface segregation is also evidenced. Light-emitting diodes were fabricated from 5×GaInN (25 A)/GaN (35 A) MQW heterostructures. The 300 K electroluminescence yields blue light at 440 nm.

Size and shape fine-tuning of SnO2 nanoparticles for highly efficient and stable dye-sensitized solar cells

Innovative solution routes led to two types of tin dioxide nanocrystals, i.e. 10–15 nm spheroid cassiterite nanoparticles and 50–150 nm anisotropic cassiterite particles showing octahedral facets. Nanoporous SnO2 electrodes of various architectures (mono- or bilayered) were then processed by the screen-printing method using suitable combinations of these SnO2 particles; the final texture, composition and morphology of the photoanodes obtained depending upon the nature of the post-treatment (with or without TiCl4). After sensitization by the ruthenium dye N719, ATR-FTIR studies revealed that chemisorption of the dye onto porous cassiterite SnO2 layers took place through a bridging coordination mode. As-prepared dye-sensitized photoanodes, when embedded in DSC devices containing a liquid electrolyte, led to a record overall power conversion efficiency (PCE) of 3.2% for pure SnO2 composed of both kinds of particles and to very promising PCE above 4% for photoanodes post-treated with TiCl4. The remarkable photovoltaic performances of the photoanodes including both kinds of particles, associated or not with a TiCl4 post-treatment, were due to improved Voc and FF, and were related to: (i) lower charge transfer resistance at the SnO2–N719–electrolyte interface; (ii) onset of dark current occurring at higher potential; (iii) enhanced electron lifetimes as determined by transient Voc decay measurements. Finally, the most striking feature of this study concerns the improvement of the power conversion efficiency upon aging under ambient conditions and the amazing long-term stability of DSCs fabricated from different SnO2-based photoanodes since standard devices built from N719 dye and I3−/I− electrolytes usually show fast decrease of efficiency.

MoO3 Thickness, Thermal Annealing and Solvent Annealing Effects on Inverted and Direct Polymer Photovoltaic Solar Cells

Several parameters of the fabrication process of inverted polymer bulk heterojunction solar cells based on titanium oxide as an electron selective layer and molybdenum oxide as a hole selective layer were tested in order to achieve efficient organic photovoltaic solar cells. Thermal annealing treatment is a common process to achieve optimum morphology, but it proved to be damageable for the performance of this kind of inverted solar cells. We demonstrate using Auger analysis combined with argon etching that diffusion of species occurs from the MoO3/Ag top layers into the active layer upon thermal annealing. In order to achieve efficient devices, the morphology of the bulk heterojunction was then manipulated using the solvent annealing technique as an alternative to thermal annealing. The influence of the MoO3 thickness was studied on inverted, as well as direct, structure. It appeared that only 1 nm-thick MoO3 is enough to exhibit highly efficient devices (PCE = 3.8%) and that increasing the thickness up to 15 nm does not change the device performance.

Towards an understanding of light activation processes in titanium oxide based inverted organic solar cells

The light activation phenomenon in inverted P3HT:PCBM bulk heterojunction organic solar cells based on titanium oxide sublayer (TiOx) is characterized by fast acquisition of current-voltage (J-V) curves under light bias as function of time. TiOx layers were thermally treated under inert atmosphere at different temperatures prior active layer deposition and for every device an activation time was extracted. It is shown that the higher the TiOx annealing temperature, the faster the activation. The improvement of the overall device performances is also observed for devices with TiOx layers baked above 100 °C. The evolution of the characteristic of the organic semiconductors (OSC) device, from dielectric to diode, is attributed to the increase of TiOx conductivity by three orders of magnitude upon white light illumination. Additionally, devices based on baked TiOx present higher conductivity than those based on unbaked TiOx which would explain the gain in performances and the short activation time of the OSC....

Long-Term Stable Organic Photodetectors with Ultra Low Dark Currents for High Detectivity Applications

Printed organic photodetectors can transform plastic, paper or glass into smart surfaces. This innovative technology is now growing exponentially due to the strong demand in human-machine interfaces. To date, only niche markets are targeted since organic sensors still present reduced performances in comparison with their inorganic counterparts. Here we demonstrate that it is possible to engineer a state-of-the-art organic photodetector approaching the performances of Si-based photodiodes in terms of dark current, responsivity and detectivity. Only three solution-processed layers and two low-temperature annealing steps are needed to achieve the performance that is significantly better than most of the organic photodetectors reported so far. We also perform a long-term ageing study. Lifetimes of over 14,000 hours under continuous operation are more than promising and demonstrate that organic photodetectors can reach a competitive level of stability for successful commercialization of this new and promising technology.

Interfacial thermal degradation in inverted organic solar cells

The efficiency of organic photovoltaic (OPV) solar cells is constantly improving; however, the lifetime of the devices still requires significant improvement if the potential of OPV is to be realised. In this study, several series of inverted OPV were fabricated and thermally aged in the dark in an inert atmosphere. It was demonstrated that all of the devices undergo short circuit current-driven degradation, which is assigned to morphology changes in the active layer. In addition, a previously unreported, open circuit voltage-driven degradation mechanism was observed that is highly material specific and interfacial in origin. This mechanism was specifically observed in devices containing MoO3 and silver as hole transporting layers and electrode materials, respectively. Devices with this combination were among the worst performing devices with respect to thermal ageing. The physical origins of this mechanism were explored by Rutherford backscattering spectrometry and atomic force microscopy and an increase in roughness with thermal ageing was observed that may be partially responsible for the ageing mechanism.