Benjamin Grévin

Imaging the Carrier Photogeneration in Nanoscale Phase Segregated Organic Heterojunctions by Kelvin Probe Force Microscopy

In this work, we spatially resolve by Kelvin probe force microscopy (KPFM) under ultrahigh vacuum (UHV) the surface photovoltage in high-efficiency nanoscale phase segregated photovoltaic blends of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester. The spatial resolution achieved represents a 10-fold improvement over previous KPFM reports on organic solar cells. By combining the damping contrast to the topographic data in noncontact atomic force microscopy under UHV, surface morphologies of the interpenetrated networks are clearly revealed. We show how the lateral resolution in KPFM can be significantly enhanced by optimizing the damping signal, allowing a direct visualization of the carrier generation at the donor-acceptor interfaces and their transport through the percolation pathways in the nanometer range. Henceforth, high-resolution KPFM has the potential to become a routine characterization tool for organic and hybrid photovoltaics.

Multi-scale scanning tunneling microscopy imaging of self-organized regioregular poly(3-hexylthiophene) films

Two-dimensional self-organized poly(3-hexylthiophene) films on highly oriented pyrolytic graphite have been probed at the solid/substrate interface by scanning tunneling microscopy (STM). Structural morphology and typical polymer conformations are visualized and discussed from mesoscopic to nanoscopic scales, including mesoscopic assembly of polycrystals, crystalline monodomain orientations and sizes, grain boundaries, chain folds, and other conformational features. STM estimation of the average chain length is in remarkably good agreement with that derived from size-exclusion chromatography. The multiscale analysis supports a picture where heterogeneities exist at different length scales.

Fast Responding Exhaled-Breath Sensors Using WO3 Hemitubes Functionalized by Graphene-Based Electronic Sensitizers for Diagnosis of Diseases

Diagnostic sensing device using exhaled breath of human have critical advantages due to the noninvasive diagnosis and high potential for portable device with simple analysis process. Here, we report ultrafast as well as highly sensitive bumpy WO3 hemitube nanostructure assisted by O2 plasma surface modification with functionalization of graphene-based material for the detection of acetone (CH3COCH3) and hydrogen sulfide (H2S) which are biomarkers for the diagnosis of diabetes and halitosis, respectively. 0.1 wt % graphene oxide (GO)- and 0.1 wt % thin layered graphite (GR)- WO3 hemitube composites showed response times of 11.5 ± 2.5 s and 13.5 ± 3.4 s to 1 ppm acetone as well as 12.5 ± 1.9 s and 10.0 ± 1.6 s to 1 ppm of H2S, respectively. In addition, low limits of detection (LOD) of 100 ppb (Rair/Rgas = 1.7 for acetone and Rair/Rgas = 3.3 for H2S at 300 °C) were achieved. The superior sensing properties were ascribed to the electronic sensitization of graphene based materials by modulating space charged layers at the interfaces between n-type WO3 hemitubes and p-type graphene based materials, as identified by Kelvin Probe Force Microscopy (KPFM). Rapid response and superior sensitivity of the proposed sensing materials following cyclic thermal aging demonstrates good potential for real-time exhaled breath diagnosis of diseases.

Fluorenone core donor–acceptor–donor π-conjugated molecules end-capped with dendritic oligo(thiophene)s: synthesis, liquid crystalline behaviour, and photovoltaic applications

We have synthesized a new series of donor–acceptor–donor (D–A–D) π-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 (Smobl) 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 cm2 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%.

High-Resolution Kelvin Probe Force Microscopy Imaging of Interface Dipoles and Photogenerated Charges in Organic Donor–Acceptor Photovoltaic Blends

We present noncontact atomic force microscopy and Kelvin probe force microscopy studies of nanophase segregated photovoltaic blends based on an oligothiophene-fluorenone oligomer and [6,6]-phenyl C70 butyric acid methyl ester. We carried out a complete analysis of the influence of the tip-surface interaction regime on the topographic, in-dark contact potential and surface photovoltage contrasts. It is demonstrated that an optimal lateral resolution is achieved for all channels below the onset of a contrast in the damping images. With the support of electrostatic simulations, it is shown that in-dark contact potential difference contrasts above subsurface acceptor clusters are consistent with an uneven distribution of permanent charges at the donor-acceptor interfaces. A remarkable dependence of the surface photovoltage magnitude with respect to the tip-surface distance is evidenced and attributed to a local enhancement of the electromagnetic field at the tip apex.

Scanning tunneling spectroscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite

We report on a hybrid scheme to perform efficient and accurate simulations of scanning tunneling spectroscopy (STS) of molecules weakly bonded to surfaces. Calculations are based on a tight binding (TB) technique, including a self-consistent calculation of the electronic structure of the molecule, to predict STS conductance spectra. The use of a local basis makes our model easily applicable to systems with several hundreds of atoms. We performed first-principles density-functional calculations to extract the geometrical and electronic properties of the system. In this way, we can include, in the TB scheme, the effects of structural relaxation upon adsorption on the electronic structure of the molecule. This approach is applied to the study of regioregular poly(3-dodecylthiophene) polymer chains adsorbed on highly oriented pyrolytic graphite. Results of spectroscopic calculations are discussed and compared with recently obtained experimental data.

Synthesis, optoelectronic properties and photovoltaic performances of wide band-gap copolymers based on dibenzosilole and quinoxaline units, rivals to P3HT

Three π-conjugated alternating copolymers, based on dibenzosilole as an electron-rich unit and fluorinated or non-fluorinated quinoxaline as an electron-withdrawing unit, connected through thiazole or thiophene moieties, have been synthesized, fully characterized and applied as donors in polymer solar cells (PSCs). The three copolymers, namely PDBS-TQx, PDBS-TQxF and PDBS-TzQx, belong to the wide band-gap semiconductor materials family, and they show an absorption edge in the visible region close to 650 nm. In order to tune the position of the polymer energy levels, and in particular to decrease their HOMO energy level, we compare the use of a thiazole spacer sandwiching the electron-deficient moiety as an alternative way to the popular backbone fluorination. PSCs based on a blend of PDBS-TQx and [6,6]-phenyl-C71-butyric acid methylester (PC71BM) as an active layer have shown the best device performances with a maximum power conversion efficiency (PCE) of 5.14% for the active area of 0.28 cm2 (under standard illumination of AM 1.5G, 1000 W m−2). Interestingly this polymer outperforms P3HT:(PC61BM) solar cells used as a reference material in this work. In addition to the thorough characterization data, including among other spectroscopy techniques, XRD, OFET, AFM and nc-AFM, we discuss in detail the relationship between the chemical structures of the three polymers, their optoelectronic properties, the phase separation in blends with PC71BM and their photovoltaic performances.

Qplus AFM driven nanostencil

We describe the development of a novel setup, in which large stencils with suspended silicon nitride membranes are combined with atomic force microscopy (AFM) regulation by using tuning forks. This system offers the possibility to perform separate AFM and nanostencil operations, as well as combined modes when using stencil chips with integrated tips. The flexibility and performances are demonstrated through a series of examples, including wide AFM scans in closed loop mode, probe positioning repeatability of a few tens of nanometer, simultaneous evaporation of large (several hundred of micron square) and nanoscopic metals and fullerene patterns in static, multistep, and dynamic modes. This approach paves the way for further developments, as it fully combines the advantages of conventional stenciling with the ones of an AFM driven shadow mask.

Multimodal Kelvin Probe Force Microscopy Investigations of a Photovoltaic WSe2/MoS2 Type-II Interface

Atomically thin transition-metal dichalcogenides (TMDC) have become a new platform for the development of next-generation optoelectronic and light-harvesting devices. Here, we report a Kelvin probe force microscopy (KPFM) investigation carried out on a type-II photovoltaic heterojunction based on WSe2 monolayer flakes and a bilayer MoS2 film stacked in vertical configuration on a Si/SiO2 substrate. Band offset characterized by a significant interfacial dipole is pointed out at the WSe2/MoS2 vertical junction. The photocarrier generation process and phototransport are studied by applying a differential technique allowing to map directly two-dimensional images of the surface photovoltage (SPV) over the vertical heterojunctions (vHJ) and in its immediate vicinity. Differential SPV reveals the impact of chemical defects on the photocarrier generation and that negative charges diffuse in the MoS2 a few hundreds of nanometers away from the vHJ. The analysis of the SPV data confirms unambiguously that light absorption results in the generation of free charge carriers that do not remain coulomb-bound at the type-II interface. A truly quantitative determination of the electron-hole (e-h) quasi-Fermi levels splitting (i.e., the open-circuit voltage) is achieved by measuring the differential vacuum-level shift over the WSe2 flakes and the MoS2 layer. The dependence of the energy-level splitting as a function of the optical power reveals that Shockley-Read-Hall processes significantly contribute to the interlayer recombination dynamics. Finally, a newly developed time-resolved mode of the KPFM is applied to map the SPV decay time constants. The time-resolved SPV images reveal the dynamics of delayed recombination processes originating from photocarriers trapping at the SiO2/TMDC interfaces.

On the Photo‐Induced Charge‐Carrier Generation within Monolayers of Self‐Assembled Organic Donor–Acceptor Dyads

By means of STM and nc-AFM the self-assembly of a new donor-acceptor (DA) dyad molecule on highly oriented pyrolytic graphite is identified and compared to molecular simulations. Kelvin probe force microscopy (KPFM) measurements clearly show the photovoltaic activity of this model system under illumination. The optoelectronic properties and the local morphology of the DA dyad assembly are simultaneously probed by KPFM down to the level of one molecular monolayers.