Jon Hollinger

Controlling Phase Separation and Optical Properties in Conjugated Polymers through Selenophene-Thiophene Copolymerization

Selenophene-thiophene block copolymers were synthesized and studied. The properties of these novel block copolymers are distinct from those of statistical copolymers prepared from the same monomers with a similar composition. Specifically, the block copolymers exhibit broad and red-shifted absorbance features and phase-separated domains in the solid state. Scanning transmission electron microscopy and topographic elemental mapping confirmed that the domains are either rich in selenophene or thiophene, indicating that the blocks of distinct heterocycles preferentially associate with one another in the solid state. This preference is surprising in view of the chemical similarities between repeat units. The overall results demonstrate a phase separation that is controlled by elemental differences. As a result of this phase separation, these novel conjugated block copolymers should find utility in a variety of studies and optoelectronics uses.

Selenophene-thiophene block copolymer solar cells with thermostable nanostructures.

The nanostructure morphology and electron donor performance of a poly(3-hexylselenophene)-block-poly(3-hexylthiophene) (P3HS-b-P3HT) copolymer was studied in a photovoltaic device with a [6,6]-phenyl C61 butyric acid methyl ester (PCBM) acceptor. P3HS-b-P3HT forms fiberlike nanostructures spontaneously, which leads to an initial optimal device performance. Furthermore the nanostructure morphology is not greatly affected by annealing, which leads to a device stability that outperforms P3HT, P3HS, or a P3HS/P3HT mixture under identical conditions. External quantum efficiency, hole mobility, and current-voltage measurements show that the block copolymer also outperforms a ternary blend that consists of a physical mixture of P3HS, P3HT, and PCBM with the same overall composition. Overall, the observation of optimal device performance and morphology without annealing as well as enhanced thermal stability demonstrates the advantage of fully conjugated diblock copolymers in nanostructured devices.

Polyselenophenes with distinct crystallization properties

The polythiophene derivative poly(3-hexyl)thiophene (P3HT) has become one of the most well studied organic materials due to its interesting and important chemical and physical properties. Two different crystal structures have been observed for P3HT, type-1 and type-2, however a pure type-2 structure has never been obtained. Herein, we investigate the crystal structure of polyselenophene analogs (P3HS), and discover that a pure type-2 phase is formed in low molecular weight P3HS (Mn = 5.9 kg mol−1). Wide-angle X-ray scattering shows that the type-2 phase is readily formed and stable at room temperature, which is very distinct from what is observed in control experiments with P3HT. Absorption spectra of P3HS films with the pure type-2 phase lack the typical shoulder peaks indicating that π–π stacking is relatively poor in the type-2 phase. Scanning transmission electron microscopy (STEM) images, however, show that large nanofibers are formed by type-2 crystallization thereby demonstrating the potential of P3HS to drive unique types of self-assembled structures through crystallization, and should motivate continued efforts on selenophene analogs of P3HT for a variety of studies and uses.

Statistical conjugated polymers comprising optoelectronically distinct units.

Poly(3-heptylselenophene)-stat-poly(3-hexylthiophene) is synthesized and characterized in terms of its crystallinity and performance in an organic photovoltaic (OPV) cell. Despite the random distribution of units along the polymer main chain, the material is semi-crystalline, as demonstrated by differential scanning calorimetry and wide-angle X-ray diffraction. Thin-film absorption suggests an increased compatibility than seen with 3-hexylselenophene monomer. Optoelectronic properties are an average of the two homopolymers, and OPV performance is enhanced by a broadened absorption profile and a favorable morphology.

Nanocrystal Self‐Assembly with Rod–Rod Block Copolymers

Two distinct morphologies of hexylselenophene-hexylthiophene rod-rod block copolymer films can be prepared depending on the molecular weight of the sample (see picture: left M(n) =12.9, right M(n) =3.9 kg mol(-1)). These polymers can be used to organize spherical CdSe nanocrystals (yellow) into either dispersed or aligned hierarchical structures. Scale bars: 200 nm.

Evidence for the Rapid Conversion of Primary Photoexcitations to Triplet States in Seleno- and Telluro- Analogues of Poly(3-hexylthiophene)

Broadband pump-probe spectroscopy is used to examine the ultrafast photophysics of the π-conjugated polymers poly(3-hexylselenophene) (P3HS) and poly(3-hexyltellurophene) (P3HTe) in solution. An excited-state absorption feature that we attribute to a transition in the triplet manifold appears on the picosecond time scale. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations support this assignment. The formation of triplets is consistent with significant fluorescence quenching observed in solutions of the neat polymers. Triplet formation occurs in ~26 and ~1.8 ps (upper limit) for P3HS and P3HTe, respectively. The successive decrease in fluorescence quantum efficiency and triplet formation time are consistent with intersystem crossing facilitated by the heavier selenium and tellurium atoms. These results strongly suggest that primary photoexcitations are rapidly converted into triplet states in P3HS and P3HTe.

Influence of selenophene–thiophene phase separation on solar cell performance

We compare the morphology and solar cell device performance of selenophene–thiophene copolymers that have the same degree of polymerization and composition, and differ only in their sequence (statistical vs. block copolymers). P3HS-b-P3HT spontaneously undergoes phase separation and P3HS-s-P3HT does not. P3HS-b-P3HT performs best when the intrinsic self-assembled nanostructure is the most perturbed. P3HS-s-P3HT does not undergo intrinsic phase separation, and vapor annealing can be used to optimize the polymer:fullerene morphology, where better nanostructure is well correlated with the best device. While the block structure provides the best stability, the statistical structure is a valuable method to balance the advantage of different monomers while precluding large-scale polymer self-assembly that is a strong intrinsic property of block structure.

‘Blocky ’ donor–acceptor polymers containing selenophene, benzodithiophene and thienothiophene for improved molecular ordering

Controlling the phase-separation behavior and achieving an ideal morphology has turned into one of the most important challenges in the field of polymer electronics. In this study we report a straightforward route to ‘blocky’ copolymers that incorporates selenophene into a benzodithiophene (BDT)–thienothiophene (TT) donor–acceptor system for improved molecular ordering. The blocky structure preserves the optical properties of the parent polymers, which is different than an analogue employing purely statistical sequence. Peak force quantitative nanomechanical mode atomic force microscopy reveals a more ordered network-like morphology in blocky polymer:PC71BM films. However the photovoltaic properties of blocky polymers are still lower than the physical mixtures of the two parent polymers. This blocky copolymer approach can be applied to many other polymerization methods to prepare many new types of blocky D–A polymers. As such, it could be a new tool for tuning the polymer crystallinity, and eventually achieving controllable solid-state morphology for polymer electronic applications.

Impact of N-substitution of a carbazole unit on molecular packing and charge transport of DPP–carbazole copolymers

Two diketopyrrolopyrrole (DPP)–carbazole (Cz) based π-conjugated copolymers, PDBTCz-H (P1) and PDBTCz-Me (P2), were designed and synthesized to study the effects of N-substitution of the carbazole unit on the molecular ordering, main chain conjugation, and charge transport properties of these polymers. It was found that the existence of hydrogen bonding interaction between the N–H group in the carbazole unit and the CO group in the DPP unit has a significant impact on the UV absorption, crystallinity, thin film morphology, as well as charge transport characteristics of P1. The hydrogen bonding is a very competitive force with the π–π stacking interaction, leading to the more twisted backbone structure and poorer molecular ordering of P1 in the solid state. Although the crystallinity of the P1 thin films could be somewhat improved by thermal annealing, the polymer main chains of P1 remain rather twisted and less conjugated in comparison with P2. The poorer main chain conjugation of P1 caused by the hydrogen bonding led to a dramatic drop in charge transport performance in organic thin film transistors (OTFTs). The highest hole mobility achieved for P1 is 8.9 × 10−3 cm2 V−1 s−1, which is almost two orders of magnitude lower than that of P2 (0.53 cm2 V−1 s−1).

Synthesis and Network‐Like Self‐Assembly of Porphyrin‐Polyselenophene Complexes

A hexagonal network structure fabricated by self-assembly of a branched conjugated polymer with a porphyrin core and P3HT or P3HS arms is presented (see picture). Polymer symmetry is very important to the network structure formation probably due to the different viscosities in linear and branched polymers.