Laboratory of Organic Matter Physics

Our group was involved in the charge transport study of non-fullerene acceptors in organic photovoltaics. Recent success in this field also entails a change in the requirements to the polymer donor in terms of optical and morphological properties leading to a demand for novel conjugated polymers. In the published paper it is reported on the synthesis of a 1,4-bis-(thiophene-2-yl)-pyrrolopyridazinedione based copolymer with 2-ethylhexyl substituents on the pyrrolopyridazinedione moiety. A 2D conjugated benzodithiophene (BDT) was chosen as comonomer. The resulting copolymer T-EHPPD-T-EHBDT showed a molecular weight of 10.2 kDa, an optical band gap of 1.79 eV, a hole mobility of 1.8 × 10⁻⁴ cm² V⁻¹ s⁻¹ and a preferred face-on orientation with regard to the substrate. The comparably wide band gap as well as the determined energy levels (HOMO: −5.47 eV, LUMO: −3.68 eV) match well with the narrow band gap non-fullerene acceptor ITIC-F, which was used as the acceptor phase in the bulk heterojunction absorber layers in the investigated solar cells. The solar cells, prepared in inverted architecture, revealed power conversion efficiencies up to 7.4% using a donor:acceptor ratio of 1 : 1 in the absorber layer. The work is published in https://doi.org/10.1039/D0NJ04573J.

A joint study revealed that in bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level. More here.

We have been involved in a project NanoElMem of novel polymer-based nanocomposite anion-exchange membranes (AEMs) with improved features for direct alkaline fuel cell applications. AEMs based on chitosan (CS), magnesium hydroxide (Mg(OH)₂), and graphene oxide (GO) with benzyltrimethylammonium chloride (BTMAC) as the hydroxide conductor were fabricated by a solvent casting method. To impart better mechanical properties and suppressed swelling, the enzymatic cross-linking with dodecyl 3,4,5-trihydroxybenzoate having C-10 alkyl chain was employed. The structure and surface morphology, KOH uptake and swelling ratio, ethanol permeability, mechanical property, ionic conductivity, cell performance, and stability of AEMs were investigated. The as-obtained AEMs showed improved hydroxide conductivity compared with previously reported CS AEMs. The highest value for hydroxide conductivity, 142.5 ± 4.0 mS cm^–1 at 40 °C, was achieved for the CS + Mg(OH)₂+ GO + BTMAC AEMs with an ethanol permeability value of 6.17 × 10^–7 ± 1.17 × 10^–7 cm² s^–1 in spite of its relative high KOH uptake (1.43 g KOH/g membrane). The highest peak power density value of 72.7 mW cm^–2 was obtained at 209 mA cm^–2 when the pristine CS + Mg(OH)₂ AEM was used as the polymer electrolyte membrane in the direct alkaline ethanol fuel cell at 80 °C. This is the highest reported power density value for CS-based membranes. More here.M

We presented a study on the charge transport in a composite of liquid‐exfoliated graphene nanoparticles (GNPs) and a polythiophene semiconducting polymer. While the former component is highly conducting, although it consists of isolated nanostructures, the latter offers an efficient charge transport path between the individual GNPs within the film, overall yielding enhanced charge transport properties of the resulting bi‐component system. The electrical characteristics of the composite layers were investigated by means of measurements of time‐of‐flight photoconductivity and transconductance in field‐effect transistors. In order to analyze both phenomena separately, charge density and charge mobility contributions to the conductivity were singled out. With the increasing GNP concentration, the charge mobility was found to increase, thereby reducing the time spent by the carriers on the polymer chains. In addition, for GNP loading above 0.2 % (wt.), an increase of free charge density was observed that highlights an additional key role played by doping. Variable‐range hopping model of a mixed two‐ and three‐dimensional transport is explained using temperature dependence of mobility and free charge density. The temperature variation of free charge density was related to the electron transfer from polythiophene to GNP, with an energy barrier of 24 meV. More here.

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.