Pierre M. Beaujuge

Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications

Organic electronics are broadly anticipated to impact the development of flexible thin-film device technologies. Among these, solution-processable π-conjugated polymers and small molecules are proving particularly promising in field-effect transistors and bulk heterojunction solar cells. This Perspective analyzes some of the most exciting strategies recently suggested in the design and structural organization of π-functional materials for transistor and solar cell applications. Emphasis is placed on the interplay between molecular structure, self-assembling properties, nanoscale and mesoscale ordering, and device efficiency parameters. A critical look at the various approaches used to optimize both materials and device performance is provided to assist in the identification of new directions and further advances.

Synthetic Control of Structural Order in N-Alkylthieno[3,4-C]pyrrole-4,6-Dione-Based Polymers for Efficient Solar Cells

The correlation between the nature of alkyl substituents on N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD)-based polymers and solar cell device performance has been investigated. After adjusting device parameters, these TPD-based polymers used with PC(61)BM provided photovoltaic responses ranging from 4.0% to 6.8%, depending on the size and shape of the alkyl solubilizing groups. Further, we have correlated the effect of the alkyl groups on the structural order and orientation of the polymer backbone using grazing incidence X-ray scattering analysis, and we have demonstrated how fine-tuning of these parameters can improve the power conversion efficiency.

Side-Chain Tunability of Furan-Containing Low-Band-Gap Polymers Provides Control of Structural Order in Efficient Solar Cells

The solution-processability of conjugated polymers in organic solvents has classically been achieved by modulating the size and branching of alkyl substituents appended to the backbone. However, these substituents impact structural order and charge transport properties in thin-film devices. As a result, a trade-off must be found between material solubility and insulating alkyl content. It was recently shown that the substitution of furan for thiophene in the backbone of the polymer PDPP2FT significantly improves polymer solubility, allowing for the use of shorter branched side chains while maintaining high device efficiency. In this report, we use PDPP2FT to demonstrate that linear alkyl side chains can be used to promote thin-film nanostructural order. In particular, linear side chains are shown to shorten π-π stacking distances between backbones and increase the correlation lengths of both π-π stacking and lamellar spacing, leading to a substantial increase in the efficiency of bulk heterojunction solar cells.

Incorporation of Furan into Low Band-Gap Polymers for Efficient Solar Cells

The design, synthesis, and characterization of the first examples of furan-containing low band-gap polymers, PDPP2FT and PDPP3F, with substantial power conversion efficiencies in organic solar cells are reported. Inserting furan moieties in the backbone of the conjugated polymers enables the use of relatively small solubilizing side chains because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents. Bulk heterojunction solar cells fabricated from furan-containing polymers and PC(71)BM as the acceptor showed power conversion efficiencies reaching 5.0%.

The donor-acceptor approach allows a black-to-transmissive switching polymeric electrochrome.

In the context of the fast-growing demand for innovative high-performance display technologies, the perspective of manufacturing low-cost functional materials that can be easily processed over large areas or finely printed into individual pixels, while being mechanically deformable, has motivated the development of novel electronically active organic components fulfilling the requirements for flexible displays and portable applications. Among all technologies relying on a low-power stimulated optical change, non-emissive organic electrochromic devices (ECDs) offer the advantage of being operational under a wide range of viewing angles and lighting conditions spanning direct sunlight as desired for various applications including signage, information tags and electronic paper. Combining mechanical flexibility, high contrast ratios and fast response times, along with colour tunability through structural control, polymeric electrochromes constitute the most attractive organic electronics for tomorrows reflective/transmissive ECDs and displays. Although red, blue and most recently green electrochromic polymers (ECPs) required for additive primary colour space were investigated, attempts to make saturated black ECPs have not been reported, probably owing to the complexity of designing materials absorbing effectively over the whole visible spectrum. Here, we report on the use of the donor-acceptor approach to make the first neutral-state black polymeric electrochrome. Processable black-to-transmissive ECPs promise to affect the development of both reflective and transmissive ECDs by providing lower fabrication and processing costs through printing, spraying and coating methods, along with good scalability when compared with their traditional inorganic counterparts.

Tailoring Structure−Property Relationships in Dithienosilole−Benzothiadiazole Donor−Acceptor Copolymers

Four new DTS-BTD copolymers (P1-P4) differing by the concentration of electron-donating and -withdrawing substituents along the backbone have been synthesized and characterized by 2D-WAXS and in bottom-contact FETs. While all copolymers can self-assemble into lamellar superstructures, only P2 and P4 show a propensity to pi-stack. P4 exhibits a hole mobility as high as 0.02 cm(2) V(-1) s(-1) in excellent agreement with the close pi-stacking and lamellar distances found by structural analysis (0.36 and 1.84 nm, respectively) and absorbs homogenously across the entire visible spectrum as solar cell applications require.

Spray Processable Green to Highly Transmissive Electrochromics via Chemically Polymerizable Donor–Acceptor Heterocyclic Pentamers

Electrochromic polymers (ECPs) of tunable green: Two symmetrical donor-acceptor based oligomers were designed, synthesized and chemically polymerized affording solution-processable conjugated polymers of distinct green hues in their neutral state. The polyheterocyclic hybrids exhibited highly transmissive oxidized states, excellent optical contrasts both in the visible and in the near infrared, fast switching times and long-term redox switching stability as expected for practical ECP devices.

Spray‐Processable Blue‐to‐Highly Transmissive Switching Polymer Electrochromes via the Donor–Acceptor Approach

2010 WILEY-VCH Verlag Gm Relying on low-power stimulated optical changes, non-emissive electrochromic (EC) technologies can be operated under a wide range of viewing angles and lighting conditions (e.g., direct sunlight), making them especially desirable in the development of flexible-display-device applications including electronic papers (e-papers) and large-area information panels. With their applicability in scalable and cost-effective manufacturing processes spanning inkjetand flexo-printing, solution-processable p-conjugated polymers are finding the path to commercialization in several areas including solar power conversion and are now expected to be applied to various transmissive/reflective electrochromic device (ECD) configurations. However, for electrochromic polymers (ECPs) to compete with the more established electrophoretic technologies, their potential for color tunability should be confirmed by extending the design and synthesis of polymer electrochromes to a palette of derivatives possessing complementary color states, as well as a redox accessible transmissive state. The homopolymerization of dioxythiophenes (DOTs) and other electron-rich monomers with a high-lying highest occupied molecular orbital (HOMO) commonly produces narrow-bandgap materials absorbing in the red region of the visible spectrum, hence, exhibiting colors ranging from purple to dark blue. Nonetheless, it is worth noting that only a few are, at the same time, solution-processable (the vast majority being electropolymerized from unsubstituted monomers), cathodically coloring, and fast-switching analogues with long-term stability on repeated electrochemical switching. In parallel, the ‘‘donor– acceptor’’ (DA) approach, now frequently applied to macromolecular systems in the context of bandgap engineering, has recently been employed to achieve blue-to-clear switching ECPs incorporating low-lying lowest unoccupied molecular orbital (LUMO) moieties such as cyanovinylene and benzotriazole, thus, introducing the perspective of reducing the electron-rich character of the backbone, while retaining the low bandgap producing the desired blue color. In spite of these developments, designing neutral-state blue polymer electrochromes combining solution processability, high contrast ratios attainable in subsecond switching times, and long-term redox stability (over 10000 cycles) represents the next logical step towards truly useful EC materials. In recent work, we investigated the effect of varying the relative contribution of electron-rich and electron-deficient heterocycles composing the polymer repeat unit on the absorption spectrum of a series of well-defined p-conjugated polymers containing various DOTs and 2,1,3-benzothiadiazole (BTD) as the accepting component (see Fig. 1a P1–P3). These polymers were shown to possess a dual band of absorption in the visible spectrum consisting of a shortand a long-wavelength absorption band, which can be modulated in terms of their relative intensity, wavelength maxima, and absorption onsets as a function of the DA content in the main-chain. In particular, when the local minimum of absorption separating these two optical transitions was positioned in the 500–550-nm range, the resulting ECPs were found to exhibit the color green, a color-state historically difficult to achieve in the field of p-conjugated polymers. Herein, we demonstrate how the dual band of absorption found in macromolecular p-conjugated systems alternating electron-rich and -poor heterocycles can be tailored to open a broad window of transmission in the high-energy region of the visible spectrum, with the goal of producing neutral-state saturated blue ECPs. In this instance, the extent of electron-rich building units along the polymer backbone has been reduced to a point where the short-wavelength absorption band of the polymer is fully transferred into the UV to maximize the blue transmission, while the long-wavelength optical transition extends from the green region up to the near-IR (see Fig. 1b, P4a). Our approach is exemplified by a series of spray-processable cathodically coloring polymer electrochromes that switch to a highly transmissive oxidized state on electrochemical doping. The corresponding polymers are linear, strictly alternating in terms of their DA content (see Fig. 1c P4a–P4c), and possess oxidation potentials sufficiently high to provide ambient stability [in contrast with the majority of their all-donor counterparts including poly(3,4-ethylenedioxythiophene) (PEDOT)] but low enough to allow fast and reversible redox switching on repeated cycling. In addition, an ester-functionalized DA electrochrome (P4c) was achieved by solubilizing side chains that can be chemically removed in a postdeposition processing step to produce films insoluble in conventional organic solvents that retain their electroactivity. Our strategy offers perspectives for the fabrication of long-lived electrolyte-based switchable devices, as well as in the processing of vertically stacked ECDs with multiple electroactive layers.

Enhanced Solid-State Order and Field-Effect Hole Mobility through Control of Nanoscale Polymer Aggregation

Efficient charge carrier transport in organic field-effect transistors (OFETs) often requires thin films that display long-range order and close π-π packing that is oriented in-plane with the substrate. Although some polymers have achieved high field-effect mobility with such solid-state properties, there are currently few general strategies for controlling the orientation of π-stacking within polymer films. In order to probe structural effects on polymer-packing alignment, furan-containing diketopyrrolopyrrole (DPP) polymers with similar optoelectronic properties were synthesized with either linear hexadecyl or branched 2-butyloctyl side chains. Differences in polymer solubility were observed and attributed to variation in side-chain shape and polymer backbone curvature. Averaged field-effect hole mobilities of the polymers range from 0.19 to 1.82 cm(2)/V·s, where PDPP3F-C16 is the least soluble polymer and provides the highest maximum mobility of 2.25 cm(2)/V·s. Analysis of the films by AFM and GIXD reveal that less soluble polymers with linear side chains exhibit larger crystalline domains, pack considerably more closely, and align with a greater preference for in-plane π-π packing. Characterization of the polymer solutions prior to spin-coating shows a correlation between early onset nanoscale aggregation and the formation of films with highly oriented in-plane π-stacking. This effect is further observed when nonsolvent is added to PDPP3F-BO solutions to induce aggregation, which results in films with increased nanostructural order, in-plane π-π orientation, and field-effect hole mobilities. Since nearly all π-conjugated materials may be coaxed to aggregate, this strategy for enhancing solid-state properties and OFET performance has applicability to a wide variety of organic electronic materials.

A Mechanistic Understanding of Processing Additive‐Induced Efficiency Enhancement in Bulk Heterojunction Organic Solar Cells

The addition of processing additives is a widely used approach to increase power conversion efficiencies for many organic solar cells. We present how additives change the polymer conformation in the casting solution leading to a more intermixed phase-segregated network structure of the active layer which in turn results in a 5-fold enhancement in efficiency.