Michael Sommer

Crystalline-crystalline donor-acceptor block copolymers.

One of the challenging issues in designing new materials for organic electronics, such as photovoltaics or field-effect transistors, is that film-forming properties must be combined with functional and well-defined nanostructured morphologies to fulfill the complex requirements of light absorption, charge separation, and charge transport in confined geometries. Morphological control on the nanoscale is also required to tune the interface between the functional domains and to ensure long-term stability of such devices. Block copolymers can meet these requirements, as they undergo phase segregation into versatile equilibrium microdomains such as cylinders, gyroids, or lamellae, thus offering the possibility to control the type, size, and orientation of microstructure in the device. We have demonstrated the chain of control on all length scales—from molecular through mesoscopic to macroscopic—using the principle of selfassembly of donor–acceptor block copolymers for photovoltaic (PV) applications. A polymerizable perylene bisimide derivative was used as electron acceptor and crystalline microdomains in an amorphous poly(triaryl amine) matrix (donor) were observed as a result of strong p–p interactions between adjacent perylene bisimide moieties. Block copolymers that contain conjugated donor segments and electron-acceptor segments have also been presented, but microphase separation was not reported. The design of the Grignard metathesis polymerization (GRIM) of poly(3-hexylthiophene), P3HT, and the in situ introduction of defined end groups has stimulated the synthesis of P3HT block copolymers. Recently, nitroxidemediated radical polymerization (NMRP) starting from P3HT macroinitiators was demonstrated. Herein we report on the first synthesis of crystalline– crystalline donor–acceptor block copolymers with P3HT as donor block and poly(perylene bisimide acrylate) (PPerAcr) as acceptor block (Figure 1). The crystallinity of both blocks not only can give rise to rich phase behavior in the bulk of these materials, but is also advantageous for charge-carrier mobility in the domains. We show that all important prerequisites for PV applications such as a high optical density, photoluminescence quenching in film, and microphase separation can be achieved in one molecule at the same time. A set of donor–acceptor block copolymers poly(3-hexylthiophene)-block-poly(perylene bisimide acrylate) (P3HTblock-PPerAcr) was prepared in only two steps (Scheme 1). In the first step, we used a modified procedure of Yokozawa et al. starting from the thiophene derivative 1. In the last stage of polymerization, quenching of the active P3HT chain end in situ with 2 yields well-defined and narrowly distributed macroinitiator P3HT-I (3 ; polydispersity index (PDI) 1.1, Mn,SEC = 8900 gmol ). The introduction of 2 at the end of the P3HT block was verified by H NMR spectroscopy, which showed 84% of the chains to be functionalized. However, this is outweighed by the simple and straightforward one-pot procedure for 3. Nonfunctionalized P3HT and the resulting block copolymers with perylene bisimide acrylate can be separated in a later purification process which is necessary anyway. In the second step, 3 was used to polymerize perylene bisimide acrylate (PerAcr, 4). To obtain a series of block copolymers with different segment lengths of 4, the reaction time and [3]/[4] ratio were varied. All other reaction parameters, including the solvent, 0.2 equivalents TIPNO (5), and 5 mol% styrene (with respect to 4), were kept constant. Similar reaction conditions were already applied in earlier polymerizations of PerAcr. Addition of a small amount of styrene as comonomer results in improved control of the polymerization of PerAcr, whereby selfinitiation of styrene leading to possible homopolymerization of PerAcr 4 is not observed. Also, incorporation of a few styrene units into the chain does not affect the chargetransport properties of PPerAcr negatively. In this manner, Figure 1. Poly(3-hexylthiophene)-block-poly(perylene bisimide acrylate), P3HT-block-PPerAcr. The first block consists of a rigid-rod poly(3hexylthiophene) (blue); the second segment is built up of a flexible polyacrylate backbone with pendant side chains of crystalline perylene bisimides (red).

Kumada Catalyst-Transfer Polycondensation: Mechanism, Opportunities, and Challenges

Kumada catalyst-transfer polycondensation (KCTP) is a new but rapidly developing method with great potential for the preparation of well-defined conjugated polymers (CPs). The recently discovered chain-growth mechanism is unique among the various transition metal-catalyzed polycondensations, and has thus attracted much attention among researchers. Most progress is found in the areas of mechanism and external initiation via new initiators, but also the number of monomers other than thiophene that can be polymerized is steadily increasing. Accordingly, the variety of CP chain architectures is increasing as well, and a considerable contribution of KCTP toward more efficient materials can be expected in the future. This review critically focuses on very recent progress in the synthesis of CPs and the mechanism of KCTP, and is finally aimed at providing a comprehensive picture of this exciting polymerization method.

Solvent Additive Control of Morphology and Crystallization in Semiconducting Polymer Blends

4-bromoanisole is used as a very versatile processing additive to control the phase separation and phase purity of organic photovoltaic devices. Polymer-polymer blends based on P3HT as donor and a wide range of acceptor materials (F8TBT, PCDTBT,…︁) are investigated. The additive promotes the aggregation of P3HT which improves the morphology for both initially mixed and demixed blends.

Ternary photovoltaic blends incorporating an all-conjugated donor-acceptor diblock copolymer.

We present a new fully conjugated diblock copolymer, P3HT-b-PFTBTT, containing donor and acceptor blocks with suitably positioned energy levels for use in a solar cell. This is the first block copolymer to be based on an existing high-performance polymer:polymer blend. We observe phase separation of the blocks and self-assembly behavior. In ternary blends with the respective homopolymers the diblock copolymer introduces lateral nanostructure without restricting P3HT crystallization in the charge transport direction, resulting in standing lamellae. By adding the diblock to the homopolymer blend as a compatibilizer, we prevent phase separation at elevated temperatures and benefit from a dramatic increase in P3HT ordering, allowing us to demonstrate polymer blend photovoltaics where the nanostructure is thermodynamically, rather than kinetically, controlled.

On the Role of Single Regiodefects and Polydispersity in Regioregular Poly(3-hexylthiophene): Defect Distribution, Synthesis of Defect-Free Chains, and a Simple Model for the Determination of Crystallinity

Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl(2) as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ∼70%. For Ni(dppp)Cl(2)-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.

Self-Assembly of Semiconductor Organogelator Nanowires for Photoinduced Charge Separation

We investigated an innovative concept of general validity based on an organogel/polymer system to generate donor-acceptor nanostructures suitable for charge generation and charge transport. An electron conducting (acceptor) perylene bisimide organogelator forms nanowires in suitable solvents during gelation process. This phenomenon was utilized for its self-assembly in an amorphous hole conducting (donor) polymer matrix to realize an interpenetrating donor-acceptor interface with inherent morphological stability. The self-assembly and interface generation were carried out either stepwise or in a single-step. Morphology of the donor-acceptor network in thin films obtained via both routes were studied by a combination of scanning electron microscopy and atomic force microscopy. Additionally, photoinduced charge separation and charge transport in these systems were tested in organic solar cells. Fabrication steps of multilayer organogel/polymer photovoltaic devices were optimized with respect to morphology and surface roughness by introducing additional smoothening layers and charge injection/blocking layers. An inverted cell geometry was used here in which electrons are collected at the bottom electrode and holes at the top electrode. The simultaneous preparation of the interface exhibits almost 3-fold improvement in device characteristics compared to the successive method. The device characteristics under AM1.5 spectral conditions and 100 mW/cm(2) for the simultaneous preparation route are short circuit current J(sc) = 0.28 mA cm(-2), open circuit voltage V(OC) = 390 mV, fill factor FF = 38%, and a power conversion efficiency eta = 0.041%.

Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation.

A highly efficient, simple, and environmentally friendly protocol for the synthesis of an alternating naphthalene diimide bithiophene copolymer (PNDIT2) via direct arylation polycondensation (DAP) is presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative yield using aromatic solvents. Most critical is the suppression of two major termination reactions of NDIBr end groups: nucleophilic substitution and solvent end-capping by aromatic solvents via C-H activation. In situ solvent end-capping can be used to control MW by varying monomer concentration, whereby end-capping is efficient and MW is low for low concentration and vice versa. Reducing C-H reactivity of the solvent at optimized conditions further increases MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy, which reveals PNDIT2 chains to be fully linear and alternating. This is further confirmed by investigating the optical and thermal properties as a function of MW, which saturate at Mn ≈ 20 kDa, in agreement with controls made by Stille coupling. Field-effect transistor (FET) electron mobilities μsat up to 3 cm(2)/(V·s) are measured using off-center spin-coating, with FET devices made from DAP PNDIT2 exhibiting better reproducibility compared to Stille controls.

Chain-growth polymerization of unusual anion-radical monomers based on naphthalene diimide: a new route to well-defined n-type conjugated copolymers.

Strongly electron-deficient (n-type) main-chain π-conjugated polymers are commonly prepared via well-established step-growth polycondensation protocols which enable limited control over polymerization. Here we demonstrate that activated Zn and electron-deficient brominated thiophene-naphthalene diimide oligomers form anion-radical complexes instead of conventional Zn-organic derivatives. These highly unusual zinc complexes undergo Ni-catalyzed chain-growth polymerization leading to n-type conjugated polymers with controlled molecular weight, relatively narrow polydispersities, and specific end-functions.

Donor-acceptor block copolymers for photovoltaic applications

Extensive research activities in polymer synthesis and device engineering have been devoted to the development of donor–acceptor (D–A) bulk heterojunction solar cells in the last years. In such devices, several photophysical processes occur all of which have to be optimized for efficient operation. First, excitons created upon light absorption need to reach the D/A interface within their exciton diffusion length (10– 20 nm), where they may dissociate into holes and electrons. Subsequent charge transport and finally charge collection at the electrodes can occur, given that co-continous pathways of donor and acceptor domains are provided. Owing to the small exciton diffusion lengths and the required optical absorption length of 100–200 nm, vertically aligned pathways with a high aspect ratio of either phase should percolate through the film. The morphologies resulting from this ideal situation resemble those of vertically oriented microphase separated block copolymer thin films, and hence suggest the importance of D–A block copolymers for organic photovoltaics. Furthermore, the covalent bond between the donor and acceptor blocks is not only desired to improve morphology control, but also to enhance long term stability of the device. The potential of block copolymers with electronic functionality to microphase separate into well-defined microstructures with several tens of nanometers in size thus addresses the morphological requirements mentioned above. This article gives an overview of donor–acceptor block copolymers and summarises recent developments of this field.

n-type organic field effect transistors from perylene bisimide block copolymers and homopolymers

We present organic field effect transistors (OFETs) based on solution-processable n-type polymers containing perylene bisimide as pendant groups. The OFET characteristics of a diblock copolymer consisting of polystyrene and poly(perylene acrylate) (PPerAcr) blocks and a PPerAcr homopolymer are compared. Thermal annealing improves the OFET performance by two to three orders of magnitude, which can be attributed to the improved order and interface properties in the transport layer, arising from the better alignment of the perylene bisimide moieties. Both polymers show excellent n-type performances with electron carrier mobility of 1.2×10−3cm2∕Vs and low threshold voltages of 4–7V.