Thioalkyl-Substituted Benzothiadiazole Acceptors: Copolymerization with Carbazole Affords Polymers with Large Stokes Shifts and High Solar Cell Voltages

Copolymers of carbazole and 4,7-bis­(2-thienyl)-2,1,3-benzothiadiazole (dTBT) incorporating thioalkyl (−SR) and alkoxy (−OR) solubilizing groups on the 2,1,3-benzothiazdiazole (BT) unit are synthesized and compared. The introduction of −SR and −OR groups onto the BT unit of the polymer was found to have different effects on the electronic properties of the polymers as well as the conformation of the polymer backbone. Large conformational changes between the ground state (GS) and excited state (ES) geometries of the polymers with −SR groups led to very large Stokes shifts of up to 224 nm. The polymer with −OR groups was found to have approximately double the photovoltaic efficiency at ∼4% compared to the polymers with −SR groups (PCE ∼ 2%). However, polymers with −SR groups were found to give very high open circuit voltages (<i>V</i>_OC) of over 1 V. Changing the −SR chain length from ethyl to dodecyl was found to have little influence on the solar cell performance of the polymer or the magnitude of the Stokes shift

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

Effect of Fluorination on the Properties of a Donor–Acceptor Copolymer for Use in Photovoltaic Cells and Transistors

Two novel indacenodithiophene (IDT) based donor–acceptor conjugated polymers for use in organic field effect transistors and photovoltaic devices are synthesized and characterized. The effect of inclusion of two fluorine atoms on the acceptor portion of the polymer is thoroughly investigated via a range of techniques. The inductively withdrawing and mesomerically donating properties of the fluorine atoms result in a decrease of the highest occupied molecular orbital (HOMO), with little effect on the lowest unoccupied molecular orbital (LUMO) as demonstrated through density functional theory (DFT) analysis. Inclusion of fluorine atoms also leads to a potentially more planar backbone through inter and intrachain interactions. Use of the novel materials in organic field effect transistor (OFET) and organic photovoltaic (OPV) devices leads to high mobilities around 0.1 cm²/(V s) and solar cell efficiencies around 4.5%

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

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, <i>E</i>_opt,min ≡ min­{<i>E</i>_opt,donor; <i>E</i>_opt,acceptor}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage <i>V</i>_OC is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: <i>V</i>_OC ≲ <i>E</i>_opt,min/<i>e</i> – (0.66 ± 0.08)­eV

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

New Fused Bis-Thienobenzothienothiophene Copolymers and Their Use in Organic Solar Cells and Transistors

A new tetradodecyl-substituted DTBTBT donor unit is synthesized by a specific bis-annulation via Suzuki–Miyaura coupling and successfully incorporated into light absorbing electron donor copolymers for OPV and hole and electron transport OFET polymer devices. All copolymers (DTBTBT-<i>co</i>-benzothiadiazole (Bz), DTBTBT-<i>co</i>-thiophene (T) and DTBTBT-<i>co</i>-thienothiophene (TT)) show fully coplanar backbones and strong intermolecular interactions. The DTBTBT-Bz copolymer led to a deep HOMO level (−5.2 eV) and thus a large <i>V</i>_oc value of 0.92 V with PC₇₁BM as electron acceptor and a PCE of 3.7% with a <i>J</i>_sc of 6.78 mA/cm² could be obtained. A hole mobility of 0.1 cm²/(V s) has been observed for the highly coplanar and more crystalline DTBTBT-T copolymer

The Application of Y Series Acceptor-Based Double-Cable Polymers in Single-Material Organic Solar Cells

The development of efficient and stable organic photovoltaic (OPV) systems for commercial applications has long been a primary objective. While single-component material systems have demonstrated promising operational and thermal stability, their efficiency still lags behind that of multicomponent bulk heterojunction devices due to limitations in scarce building blocks, complex synthesis processes, and challenges in controlling morphology. In this work, we present a novel approach by introducing a fused-ring electron acceptor as a pendant segment, which offers new possibilities for the development of double-cable single-component copolymers. This innovative strategy not only broadens their spectral absorption but also simplifies their synthesis complexity. Through careful adjustment of molecular weight, we achieved an outstanding power conversion efficiency of 9.35% and a minimized energy loss of 0.517 eV, which is one of the best results reported for structure well-defined double-cable copolymer-based OPVs. Impressively, the designed double-cable polymers exhibit excellent photo, thermal, and mechanical stabilities, further highlighting their potential for practical applications

Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells

We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that are isostructural to poly­(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen–sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells