Sebastian Albert-Seifried

High-performance ambipolar diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymer field-effect transistors with balanced hole and electron mobilities.

Ambipolar OFETs with balanced hole and electron field-effect mobilities both exceeding 1 cm(2) V(-1) s(-1) are achieved based on a single-solution-processed conjugated polymer, DPPT-TT, upon careful optimization of the device architecture, charge injection, and polymer processing. Such high-performance OFETs are promising for applications in ambipolar devices and integrated circuits, as well as model systems for fundamental studies.

Effect of Annealing on P3HT:PCBM Charge Transfer and Nanoscale Morphology Probed by Ultrafast Spectroscopy

We employ sub-picosecond TA spectroscopy on operating P3HT:PCBM devices to probe the effect of annealing on charge transfer dynamics and nanoscale morphology. Our measurement configuration allows us to remove the effect of high excitation densities that would otherwise dominate. Charge transfer in pristine P3HT:PCBM devices proceeds on a sub-picosecond time scale, implying molecular level intermixing and explaining the more localized character of excitons and charges. In annealed devices, annealing results in diffusion-limited charge generation with a half-life time of approximately 3 ps, complete only after 30 ps. This is the result of exclusion of PCBM molecules and ordering of P3HT domains and is correlated with improved photovoltaic efficiency. We are able to use the spectra and dynamics of optical excitations themselves to interpret blend morphologies on the appropriate time- and length scales of photoinduced charge generation.

Exciton Fission and Charge Generation via Triplet Excitons in Pentacene/C60 Bilayers

Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers

Adv. Mater. 2010, 22, 2371–2375 2010 WILEY-VCH Verlag G Field-effect transistors (FETs) based on conjugated polymers and small molecules have been of extensive fundamental and practical interest for more than two decades. In terms of fundamental charge transport properties organic semiconductors have been recently shown to be intrinsically ambipolar, i.e., able to accumulate and transport both holes and electrons within the same material under suitable biasing conditions and device configurations. The discovery of the intrinsic ambipolar charge transport properties in common semiconducting polymers was made possible by the understanding of the crucial role played by electronegative trapping groups in the dielectric, such as hydroxyl groups on the surface of a SiO2 gate dielectric. [10] Ambipolar charge transport is not only of fundamental, but also of practical interest as it enables the realization of novel device architectures such as complementary-like voltage inverters with a single organic semiconductor as well as ambipolar light-emitting field-effect transistors (LFETs). Here we report the general observation of ambipolar charge transport characteristics in a series of regioregular polyselenophene-based polymers. Compared to the well-studied polythiophenes, which appear among the most promising solution processable organic semiconductors, polyselenophenes were recently developed as analogue systems providing several advantages over their predecessors. The highest occupied molecular orbital (HOMO) of polythiophenes has little contribution from the sulfur heteroatom, whereas the lowest unoccupied molecular orbital (LUMO) has significant electron density on the heteroatom. Polyselenophenes were initially developed as promising alternatives to polythiophenes for solar cell applications, mainly because of their reduced optical band gaps and their enhanced photostability due to the lower lying LUMO. For FET applications we expect the hole transport to be similar to that of polythiophenes, while the lower lying LUMO in polyselenophenes should result in improved electron transport due to enhanced electron injection from metal electrodes and lower susceptibility of electrons to trap states and oxidation. The regioregular polyselenophenes investigated in this work were: (1) poly(3,300-di-n-alkylterselenophene) (PSSS) of three different alkyl side-chains, namely PSSS-C10, PSSS-C8, and PSSS-C6; and (2) poly(3-octyl)selenophene (P3OS) (Fig. 1). We employed identical top-gate, bottom contact (TGBC) configurations with gold source-drain electrodes for all polymers. For ambipolar FETs the TGBC device configuration offers several advantages over a bottom-gate/bottom-contact (BGBC) configuration: (i) the freedom to select different gate dielectrics to minimize irreversible charge trapping at the semiconductordielectric interface and to act as encapsulation for the FET channel, and (ii) a lower contact resistance due to reduction of current-crowding effects. PSSS is the selenium analogue of the previously reported poly(3,300-dialkylterthiophene) (PTT) with a ‘‘spaced-out’’ distribution of the alkyl side-chains along the polymer backbone. 28] PTT was reported to readily self-assemble into a threedimensional lamellar p-stacking arrangement with an ‘‘edge-on’’

Improved Exciton Dissociation at Semiconducting Polymer:ZnO Donor:Acceptor Interfaces via Nitrogen Doping of ZnO.

Exciton dissociation at the zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) interface as a function of nitrogen doping of the zinc oxide, which decreases the electron concentration from approximately 1019 cm−3 to 1017 cm−3, is reported. Exciton dissociation and device photocurrent are strongly improved with nitrogen doping. This improved dissociation of excitons in the conjugated polymer is found to result from enhanced light-induced de-trapping of electrons from the surface of the nitrogen-doped ZnO. The ability to improve the surface properties of ZnO by introducing a simple nitrogen dopant has general applicability.

Nanosecond intersystem crossing times in fullerene acceptors: implications for organic photovoltaic diodes.

Triplet-exciton formation through intersystem crossing of photogenerated singlet excitons in fullerene acceptors can compete with charge generation in organic photovoltaic diodes. This article reports the intersystem crossing timescale (τISC ) of the most commonly used fullerene acceptors, PC60 BM and PC70 BM, in solutions and in spin-coated films. These times are on the nanosecond timescale, and are longer than the characteristic times for charge generation (τd ).

Measurement of thermal modulation of optical absorption in pump-probe spectroscopy of semiconducting polymers

We analyze the transient absorption spectra of regioregular poly(3-hexylthiophene) films and find substantial response due to heating of the sample by the picosecond pump-pulse. The increased transmission at the onset of the absorption spectrum, caused by thermal blueshifting of the absorption, produces responses that have often been misinterpreted as arising from electronic excited states. We demonstrate that the use of a high thermal conductivity substrate such as sapphire allows thermal effects to be conveniently separated from electronic excited states.

Sequential energy and electron transfer in polyisocyanopeptide-based multichromophoric arrays.

We report on the synthesis and detailed photo-physical investigation of four model chromophore side chain polyisocyanopeptides: two homopolymers of platinum-porphyrin functionalized polyisocyanopeptides (Pt-porphyrin-PIC) and perylene-bis(dicarboximide) functionalized polyisocyanopeptides (PDI-PIC), and two statistical copolymers with different ratios of Pt-porphyrin and PDI molecules attached to a rigid, helical polyisocyanopeptide backbone. (1)H NMR and circular dichroism measurements confirm that our model compounds retain a chiral architecture in the presence of the chromophores. The combination of Pt-porphyrin and PDI chromophores allows charge- and/or energy transfer to happen. We observe the excitation and relaxation pathways for selective excitation of the Pt-porphyrin and PDI chromophores. Studies of photoluminescence and transient absorption on nanosecond and picosecond scales upon excitation of Pt-porphyrin chromophores in our multichromophoric assemblies show similar photophysical features to those of the Pt-porphyrin monomers. In contrast, excitation of perylene chromophores results in a series of energy and charge transfer processes with the Pt-porphyrin group and forms additional charge-transfer states, which behave as an intermediate state that facilitates electronic coupling in these multichromophoric systems.