Effect of Chalcogen Atom Substitution on the Optoelectronic Properties in Cyclopentadithiophene Polymers

We report the synthesis and characterization of a series of cyclopentadithiophene polymers containing thiophene, selenophene, and tellurophene as comonomers. The effect of the chalcogen atom has been studied by a range of techniques, including thermal, optical, electrochemical, and computational. The results showed that by increasing the size of the chalcogen atom, the optical band gap is reduced mainly due to a downshift in the LUMO energy level. In addition, the larger size also increases the intermolecular heteroatom–heteroatom interactions facilitating the formation of polymer aggregates. This led to not only a stronger red-shifted band in the UV–vis absorption spectrum as well as raise in the HOMO energy level but also a potential solubility issue for higher molecular weight polymers containing particularly tellurophene units

Dithienosilolothiophene: A New Polyfused Donor for Organic Electronics

We report the synthesis of a novel pentacyclic donor moiety, dithienosilolothiophene, and its incorporation into low bandgap semiconducting polymers. The unique geometry of this new donor allowed attaching four solubilizing side chains on the same side of the fused ring system, thus ensuring sufficient solubility when incorporated into conjugated polymers while simultaneously reducing the steric hindrance between adjacent polymer chains. The optoelectronic properties of three new polymers comprising the novel pentacyclic donor were investigated and compared to structurally similar thieno­[3,2-<i>b</i>]­thienobis­(silolothiophene) polymers. Organic solar cells were fabricated in order to evaluate the new materials’ potential as donor polymers in bulk heterojunction solar cells and gain further insight into how the single-sided side-chain arrangement affects the active layer blend morphology

Benzotrithiophene Copolymers: Influence of Molecular Packing and Energy Levels on Charge Carrier Mobility

The planar benzotrithiophene unit (<b>BTT</b>) was incorporated into four different donor polymers, and by systematically changing the nature and positioning of the solubilizing alkyl side chains along the conjugated backbone, the polymers’ frontier energy levels and optoelectronic properties were controlled. Reducing the steric hindrance along the polymer backbone lead to strong interchain aggregation and highly ordered thin films, achieving hole mobilities of 0.04 cm²/(V s) in organic thin film transistors. In an attempt to increase the polymer’s processability and reduce chain aggregation, steric hindrance between alkyl side chains was exploited. As a result of the increased solubility, the film forming properties of the polymer could be improved, but at the cost of reduced hole mobilities in OFET devices, due to the lack of long-range order in the polymer films

Influence of Side-Chain Regiochemistry on the Transistor Performance of High-Mobility, All-Donor Polymers

Three novel polythiophene isomers are reported whereby the only difference in structure relates to the regiochemistry of the solubilizing side chains on the backbone. This is demonstrated to have a significant impact on the optoelectronic properties of the polymers and their propensity to aggregate in solution. These differences are rationalized on the basis of differences in backbone torsion. The polymer with the largest effective conjugation length is demonstrated to exhibit the highest field-effect mobility, with peak values up to 4.6 cm² V^–1 s^–1

Charge Separation in Intermixed Polymer:PC₇₀BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition

A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics <i>within</i> molecularly intermixed donor–acceptor domains versus the dynamics <i>between</i> phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC₇₀BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3–5 nm. At high fullerene compositions (≥67%), where the intermixed domains are 1–3 nm and the pure fullerene phases reach ∼4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase, making the fullerene domains accessible for electron escape

Improved Field-Effect Transistor Performance of a Benzotrithiophene Polymer through Ketal Cleavage in the Solid State

A benzotrithiophene polymer with a new thermally cleavable ketal substituent is reported. It is shown how this functional group can be used to facilitate solvent processing and, subsequently, how it can be removed by a thermal annealing process to generate a structurally ordered and crystalline thin film with significantly improved field-effect transistor properties

On the Energetic Dependence of Charge Separation in Low-Band-Gap Polymer/Fullerene Blends

The energetic driving force required to drive charge separation across donor/acceptor heterojunctions is a key consideration for organic optoelectronic devices. Herein we report a series of transient absorption and photocurrent experiments as a function of excitation wavelength and temperature for two low-band-gap polymer/fullerene blends to study the mechanism of charge separation at the donor/acceptor interface. For the blend that exhibits the smallest donor/acceptor LUMO energy level offset, the photocurrent quantum yield falls as the photon excitation energy is reduced toward the band gap, but the yield of bound, interfacial charge transfer states rises. This interplay between bound and free charge generation as a function of initial exciton energy provides key evidence for the role of excess energy in driving charge separation of direct relevance to the development of low-band-gap polymers for enhanced solar light harvesting

Photocurrent Enhancement from Diketopyrrolopyrrole Polymer Solar Cells through Alkyl-Chain Branching Point Manipulation

Systematically moving the alkyl-chain branching position away from the polymer backbone afforded two new thieno­[3,2-<i>b</i>]­thiophene–diketopyrrolopyrrole (DPPTT-T) polymers. When used as donor materials in polymer:fullerene solar cells, efficiencies exceeding 7% were achieved without the use of processing additives. The effect of the position of the alkyl-chain branching point on the thin-film morphology was investigated using X-ray scattering techniques and the effects on the photovoltaic and charge-transport properties were also studied. For both solar cell and transistor devices, moving the branching point further from the backbone was beneficial. This is the first time that this effect has been shown to improve solar cell performance. Strong evidence is presented for changes in microstructure across the series, which is most likely the cause for the photocurrent enhancement

Chalcogenophene Comonomer Comparison in Small Band Gap Diketopyrrolopyrrole-Based Conjugated Polymers for High-Performing Field-Effect Transistors and Organic Solar Cells

The design, synthesis, and characterization of a series of diketopyrrolopyrrole-based copolymers with different chalcogenophene comonomers (thiophene, selenophene, and tellurophene) for use in field-effect transistors and organic photovoltaic devices are reported. The effect of the heteroatom substitution on the optical, electrochemical, and photovoltaic properties and charge carrier mobilities of these polymers is discussed. The results indicate that by increasing the size of the chalcogen atom (S < Se < Te), polymer band gaps are narrowed mainly due to LUMO energy level stabilization. In addition, the larger heteroatomic size also increases intermolecular heteroatom–heteroatom interactions facilitating the formation of polymer aggregates leading to enhanced field-effect mobilities of 1.6 cm²/(V s). Bulk heterojunction solar cells based on the chalcogenophene polymer series blended with fullerene derivatives show good photovoltaic properties, with power conversion efficiencies ranging from 7.1–8.8%. A high photoresponse in the near-infrared (NIR) region with excellent photocurrents above 20 mA cm^–2 was achieved for all polymers, making these highly efficient low band gap polymers promising candidates for use in tandem solar cells

Benzocarborano[2,1‑<i>b</i>:3,4‑<i>b</i>′]dithiophene Containing Conjugated Polymers: Synthesis, Characterization, and Optoelectronic Properties

We report the stannylation of a benzocarborano­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene monomer and its polymerization by Stille polycondensation with solubilized cyclopentadithiophene and diketopyrrolopyrrole derivatives. The physical, material, and optoelectronic properties of the resultant conjugated copolymers are reported, demonstrating that benzocarboranodithiophene acts as a mildly electron-withdrawing monomer