Patrick Pingel

Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC(70)BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC(70)BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors

Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.

Effect of molecular p-doping on hole density and mobility in poly(3-hexylthiophene)

Employing impedance spectroscopy, we have studied the hole density, conductivity, and mobility of poly(3-hexylthiophene), P3HT, doped with the strong molecular acceptor tetrafluorotetracyanoquinodimethane, F4TCNQ. We find that the hole density increases linearly with the F4TCNQ concentration. Furthermore, the hole mobility is decreased upon doping at low-to-medium doping level, which is rationalized by an analytic model of carrier mobility in doped organic semiconductors [V. I. Arkhipov, E. V. Emelianova, P. Heremans, and H. Bassler, Phys. Rev. B 72, 235202 (2005)]. We infer that the presence of ionized F4TCNQ molecules in the P3HT layer increases energetic disorder, which diminishes the carrier mobility.

Hole-transporting side-chain polystyrenes based on TCTA with tuned glass transition and optimized electronic properties

The development of crosslinkable materials for the fabrication of solution processable OLEDs presents challenges, especially regarding the adjustment of the glass transition (Tg), which has a significant influence on crosslinking kinetics and device life-time. Crosslinkable hole transport materials based on poly(N,N-bis(4-(9H-carbazol-9-yl)phenyl)-4-vinylaniline) (poly-TCTA) with covalently attached plasticizers for Tg control and azide functionalities for azide-alkyne crosslinking are presented. These polymers have an optimal Tg of around 150 °C and show superior crosslinking performances and solution resistibilities. Incorporation of electron-pushing alkoxides to the pendant groups combines the Tg adjustment approach with a systematic tuning of the HOMO level from −5.7 to −5.3 eV. All presented polymers have good charge transport and injection properties and are ideal for applications in phosphorescent OLEDs due to their high triplet energies (>2.8 eV). The new crosslinkable poly-TCTA-based materials are applied as hole-transport layers (HTLs) in fully solution-processed OLEDs. An improvement of the device performance is demonstrated for OLEDs with additional crosslinked HTL.

Orthogonal Solution-Processable Electron Transport Layers Based on Phenylpyridine Side-Chain Polystyrenes

This article reports the synthesis and characterization of a series of polystyrenes containing phenylpyridine moieties as side chains. Methanol solubility of these polymers is induced if the relative pyridine content of the overall aromatic units of the side chains is larger than 0.5. This allows for orthogonal processing of multilayered organic light emitting diode (OLED) stacks fabricated from solutions. The polymers show high thermal stability due to their glass-transition temperatures ranging from 136 up to 247 °C. High triplet energies of up to 2.8 eV are obtained by combination of the side-chain aromatic rings in the meta position. The use of the methanol soluble side-chain polymers as an electron transport layer (ETL) is demonstrated in an orthogonally processed three-layer green-emitting OLED stack. When depositing the ETL from methanol, redissolution of the underlying emission layer does not occur.

High triplet energy electron transport side-chain polystyrenes containing dimesitylboron and tetraphenylsilane for solution processed OLEDs

A series of polystyrenes was developed as electron transport materials (ETMs) for solution processed organic light emitting diodes (OLEDs), containing dimesitylboryl (BMes2) and tetraphenylsilanyl in their side-chains. The synthesis and characterisation of the polymers is reported. High triplet energy up to 2.95 eV was obtained for ETMs containing BMes2 and tetraphenylsilanyl. High glass transition temperatures over 180 °C were detected for the series due to the polymer approach, ensuring high thermal stability. The role of both molecular motifs was evaluated in Ir based green OLEDs with all organic layers processed from solution. Similar current efficacies around 30 cd A−1 were measured for the electron-transport polymers, most likely due to the confinement of the LUMO delocalization into the BMes2 unit, leading to similar charge mobility. These results show that incorporation of BMes2 into polymers is an adequate approach for further development of wet-process materials.