Laboratory of Organic Matter Physics

We have examined the effect of the addition of graphene nanoflakes (GNs) to improve the field-effect mobility of poly(3-hexylthiophene) (P3HT)-based field-effect transistors (FETs). We observe that the FET mobility increases up to 0.02 cm²/Vs at GNs concentration of 0.06 mg/mL. The remarkable increase in FET mobility can be ascribed to the incorporation of highly conducting and highly ordered graphene flakes, which acts like conducting bridges between the P3HT molecules. With further increasing the concentration of GNs, the mobility and I_on/I_off ratio starts to decrease, due to mismatching of the energy levels of graphene and P3HT. The effect is further evidenced from the time-of-flight photocurrent (TOFP) measurements, in which the transit time (t_tr) of the charge carriers are shifted to shorter times in blended layers compared to pristine P3HT. Further, we have analyzed time-resolved photocurrent variation on these samples in terms of hopping transport model using Monte-Carlo (MC) simulations within the framework of Gaussian disorder. These studies reveal that the addition of GNs causes improvement in mobility and reduces the extremely slow carriers and uniform arrival of the charge carriers. More here.

We have published the results of a study of the effect of 1-pyrenesulfonicacid sodium salt (1-PSA), tetracyanoethylene (TCNE) and tetrafluoro- tetracyanoquinodimethane (F4-TCNQ) on charge transport properties of reduced graphene oxide (RGO) is examined by measuring the transfer characteristics of field-effect transistors and co-planar time-of-flight photocurrent technique. Evidence of p-type doping and a reduction of mobility of electrons in RGO upon deposition of these materials is observed. Time-resolved photocurrent measurements show a reduction in elec- tron mobility even at submonolayer coverage of these materials. The variation of transit time with different coverages reveals that electron mobility decreases with increasing the surface coverage of 1-PSA, TCNE and F4- TCNQ to a certain extent, while at higher coverage the electron mobility is slightly recovered. All three molecules show the same trend in charge carrier mobility variation with coverage, but with different magnitude. Among all three molecules, 1-PSA acts as weak electron acceptor compared to TCNE and F4-TCNQ. The additional fluorine moieties in F4-TCNQ provides excellent electron withdrawing capability compared to TCNE. The experimental results are consistent with the density functional theory calculations. Click here for the full publication.

We have prepared a review of recent advances in characterisation of charge carrier transport in organic semiconductor layers by time-of-flight photocurrent measurements, with the emphasis on the measurements of the samples with co-planar electrodes. These samples comprised an organic semiconductor layer whose thickness is on the order of a μm or less, and thus mimic the structures of organic thin film transistors. In the review we emphasise the importance of considering spatial variation of electric field in these, essentially two-dimensional structures, in interpretation of photocurrent transients. We review the experimental details of this type of measurements and give examples that demonstrate exceptional sensitivity of the method to minute concentration of electrically active defects in the organic semiconductors as well as the capability of probing charge transport along the channels of different mobility that reside in the same sample. Click here to see the review.

Our group was involved in a study of transient photocurrent responses of self‐standing nanostructures. These devices are of fundamental interest in materials science and nanoscience and are widely used in (opto‐)electronic and photonic devices as well as in micro‐electromechanical systems. To date, large‐area and self‐standing nanoelectrode arrays assembled on flexible substrates have not been reported. Here the fabrication of a hollow nanomesh scaffold on glass and plastic substrates with a large surface area over 1 mm² and ultralow leakage current density (≈1–10 pA mm⁻² @ 2 V) across the empty scaffold is demonstrated. Thanks to the continuous sub‐micrometer space formed in between the nanomesh and the bottom electrode, highly crystalline and dendritic domains of 6,13‐bis(triisopropylsilylethinyl)pentacene growing within the hollow cavity can be observed. The high degree of order at the supramolecular level leads to efficient charge and exciton transport; the photovoltaic detector supported on flexible polyethylene terephthalate substrates exhibits an ultrafast photoresponse time as short as 8 ns and a signal‐to‐noise ratio approaching 10⁵. Such a hollow scaffold holds great potential as a novel device architecture toward flexible (opto‐)electronic applications based on self‐assembled micro/nanocrystals. Published in Advanced Materials.

Charge-carrier mobility has been investigated by time-of-flight (TOF) transient photocurrent in a lateral transport configuration in highly crystalline thin films of 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C₈-BTBT) grown by a zone-casting alignment technique. High TOF mobility has been revealed that it is consistent with the delocalized nature of the charge transport in this material, yet it featured a positive temperature dependence at ?≥295K. Moreover, the mobility was surprisingly found to decrease with electric field in the high-temperature region. These observations are not compatible with the conventional band-transport mechanism. We have elaborated an analytic model based on effective-medium approximation to rationalize the puzzling findings. The model considers the delocalized charge transport within the energy landscape formed by long-range transport band-edge variations in imperfect organic crystalline materials and accounts for the field-dependent effective dimensionality of charge transport percolative paths. The results of the model calculations are found to be in good agreement with experimental data. The work was published in Chemical Papers.

Our group has been involved in a collaborative approach to study non-fullerene organic solar cells. Recent advances in the development of non-fullerene acceptors have increased the power conversion efficiency of organic solar cells to approximately 13%. Fullerene-derivatives and non-fullerene acceptors possess distinctively different structural, optical and electronic properties, which also change the requirements on the polymer donor in non-fullerene organic solar cells. Therefore, in this study, the effect of the molecular weight of the conjugated polymer on the photovoltaic performance, charge carrier mobility, crystallization properties, film morphology, and non-geminate recombination dynamics is systematically investigated in polymer:small molecule organic solar cells using the low bandgap polymer PTB7-Th as the donor and the non-fullerene indacenodithiophene-based small molecule O-IDTBR as the acceptor. Among the examined polymer samples (50–300 kDa), high molecular weights of PTB7-Th (with an optimum molecular weight of 200 kDa) are advantageous to achieve high efficiencies up to 10%, which can be correlated with an increased crystallinity, an improved field-effect hole mobility (1.05 × 10⁻² cm² V⁻¹ s⁻¹), lower charge carrier trapping and a reduced activation energy of charge transport (98 meV). Bias-assisted charge extraction and transient photovoltage measurements reveal higher carrier concentrations (10¹⁶ cm⁻³) and long lifetimes (4.5 μs) as well as lower non-geminate recombination rate constants in the corresponding devices, supporting the high photocurrents (ca. 15.2 mA cm⁻²) and fill factors (>60%). Work was published in Journal of Materials Chemistry A.

We used time-of-flight photocurrent measurements to determine the role of grain boundaries in charge carrier transport in thin layers of methyl ammonium lead iodide (CH₃NH₃PbI₃). The measurement results were compared to Kinetic Monte Carlo simulations, based on a transport model, which disentangles the transport within crystallites and hopping across grain boundaries. The observed mobilities of electrons are in the order ∼2.5 × 10⁻¹ cm²V⁻¹s⁻¹. The hopping across grains is modeled with an Arrhenius-type probability rate, characterized by activation energy (E_a). It was found that the E_a estimated from the slope of a mobility-temperature dependence is in the range of ∼56–70 meV. The factors contributing to E_a are shunting pathways and the grain-size variations including energy level misalignments at the grain boundaries. These results represent a step toward a design of novel windowless organic-inorganic perovskite solar cells. More is published in Materials Chemistry and Physics.

Our group was involved in collaboration to investigate fast photocurrent transients of crystalline dioctyl‐3,4,9,10‐perylenedicarboximide nanowires and 6,13‐bis(triisopropylsilylethynyl) pentacene microplates , which were integrated into a vertical‐yet‐open asymmetrical heterojunction for the realization of a high‐performance organic photovoltaic detector, which shows fast photoresponse, ultrahigh signal‐to‐noise ratio, and high sensitivity to weak light. More of this interesting research can be found in Advanced Materials.

We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time. More can be found in Organic Electronics.

Comparison of photoconductivity, optical absorption and theoretical calculation of coranulenne. From RSC Adv.

We have been measuring photoconductivity spectra in non-crystalline corannulene thin layers and compared them to optical absorption in solution phase and thin films. The unexpected enhanced photoconductivity is correlated with GW–BSE theoretical predictions of corannulene gas-phase excitonic spectra. Theoretical analysis reveals a consistent contribution involving transitions to Super Atomic Molecular Orbitals (SAMOs), a unique set of diffuse orbitals typical of curved conjugated constructs. Results suggest SAMO population via direct photoexcitation as a potential mechanism towards exploiting these diffuse orbitals as conducting channels in suitably assembled quantum nanostructures or solids. Results are published in RSC Advances.