Synthesis of Diketopyrrolopyrrole Containing Copolymers: A Study of Their Optical and Photovoltaic Properties

Synthesis and Emergent Photophysical Properties of Diketopyrrolopyrrole-Based Supramolecular Self-Assembly

Diketopyrrolopyrrole (DPP)-based molecular semiconductors exhibit intriguing optical and charge transport properties. Herein, we rationally design a series of electronically identical but structurally distinct Hamilton receptor (HR)-based supramolecular assembly of DPP. The HR endows supramolecular assemblies via hydrogen bonding with enhanced structural ordering and excitonic couplings. The mechanism of supramolecular self-assembly was probed by diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) and solid-state IR spectroscopy studies. We investigated the morphology of self-assembly, photophysical and electrochemical properties and compared them with the identical DPP molecular structures without HRs. The microstructure of self-assembly was probed with atomic force microscopy in thin films. Subsequently, the influence of solid-state packing was studied by single-crystal X-ray diffraction. The single-crystal structure of HR-TDPP-C20 reveals slipped stack arrangements between the two neighboring chromophores with π-π stacking distance and slip angle of 3.55 Å and 35.4°, respectively. Notably, the slight torsional angle of 1° between thiophene and lactam rings and small π-π stacking distance suggest a significant intermolecular coupling between thiophene (D) and lactam (A) rings. This intramolecular coupling between two π-π chromophore stacks manifests in their optical properties. In this manuscript, we report rational design and synthesis of supramolecular self-assembly of DPP with a collection of compelling structural and optical properties.

Recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells

Organic semiconductor materials, especially donor-acceptor (D-A) polymers, have been increasingly applied in organic optoelectronic devices, such as organic field-effect transistors (OFETs) and organic solar cells (OSCs). Plenty of high-performance OFETs and OSCs have been achieved based on varieties of structurally modified D-A polymers. As the basic building block of D-A polymers, acceptor moieties have drawn much attention. Among the numerous types, lactam- and imide-functionalized electron-deficient building blocks have been widely investigated. In this review, the structural evolution of lactam- or imide-containing acceptors (for instance, diketopyrrolopyrrole, isoindigo, naphthalene diimide, and perylene diimide) is covered and their representative polymers applied in OFETs and OSCs are also discussed, with a focus on the effect of varied structurally modified acceptor moieties on the physicochemical and photoelectrical properties of polymers. Additionally, this review discusses the current issues that need to be settled down and the further development of new types of acceptors. It is hoped that this review could help design new electron-deficient building blocks, find a more valid method to modify already reported acceptor units, and achieve high-performance semiconductor materials eventually.

Reduced Energy Loss in Non-Fullerene Organic Solar Cells with Isomeric Donor Polymers Containing Thiazole π-Spacers

Large energy loss is one of the key factors that limit the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we report reduced energy losses of OSCs via introducing thiazole π-spacers with different orientations to replace the thiophene π-spacers of the prototype polymer PBDB-T. The newly formed thiazole-containing isomeric polymers, PBDBTz-2 and PBDBTz-5, exhibited blue-shifted absorption and deeper lying energy levels compared to PBDB-T. When blended with IT-4F, the two polymers realized PCEs of 10.4% for PBDBTz-2 and 9.6% for PBDBTz-5, respectively, which were higher than that of PBDB-T (PCE = 9.3%). More critically, considerable open-circuit voltage (V_oc) enhancements were achieved by PBDBTz-2 and PBDBTz-5, which were 0.14 and 0.21 V higher than that of PBDB-T. A detailed analysis showed that the reduced energy loss resulted from the lower radiative recombination below the band gap and nonradiative recombination loss. This study demonstrated that the introduction of thiazole π-spacers with different orientations is effective to reduce the energy losses of OSCs, which provided valuable inspirations for the development of new conjugated polymers to the efficiency breakthrough of OSCs in future.

The synthesis and properties of a new class of π-expanded diketopyrrolopyrrole analogs and conjugated polymers

A new class of π-expanded diketopyrrolopyrrole analogs has been incorporated into novel alternating conjugated polymers with very low band gaps.

Two-acceptor one-donor random terpolymers comprising thiophene- and phenyl-capped diketopyrrolopyrrole for organic photovoltaics

A series of random terpolymers comprising two electron deficient phenyl (PDPP) and thiophene (ThDPP)-capped diketopyrrolopyrrole (DPP) in conjugation with the electron-donating thiophene moiety are synthesised using Stille coupling.

Ultrafast bridge planarization in donor-π-acceptor copolymers drives intramolecular charge transfer

Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

Diketopyrrolopyrrole Polymers with Thienyl and Thiazolyl Linkers for Application in Field-Effect Transistors and Polymer Solar Cells

Conjugated polymers consisting of diketopyrrolopyrrole (DPP) units have been successfully applied in field-effect transistors (FETs) and polymer solar cells (PSCs), while most of the DPP polymers were designed as symmetric structures containing identical aromatic linkers. In this manuscript, we design a new asymmetric DPP polymer with varied aromatic linkers in the backbone for application in FETs and PSCs. The designation provides the chance to finely adjust the energy levels of conjugated polymers so as to influence the device performance. The asymmetric polymer exhibits highly crystalline properties, high hole mobilities of 3.05 cm² V^-1 s^-1 in FETs, and a high efficiency of 5.9% in PSCs with spectra response from 300 to 850 nm. Morphology investigation demonstrates that the asymmetric polymer has a large crystal domain in blended thin films, indicating that the solar cell performance can be further enhanced by optimizing the microphase separation. The study reveals that the asymmetric design via adjusting the aromatic linkers in DPP polymers is a useful route toward flexible electronic devices.

Bi-diketopyrrolopyrrole (Bi-DPP) as a novel electron accepting compound in low band gap π-conjugated donor-acceptor copolymers/oligomers

The synthesis and characterization of a novel 2,5-diketopyrrolo[3,4-c]pyrrole(DPP)-based accepting building block with the scheme DPP-neutral small linker-DPP (Bi-DPP) is presented, which was utilized as electron accepting moiety for low band gap π-conjugated donor–acceptor copolymers as well as for a donor–acceptor small molecule. The electron accepting moiety Bi-DPP was prepared via a novel synthetic pathway by building up two DPP moieties step by step simultaneously starting from a neutral phenyl core unit. Characterization of the synthesized oligomeric and polymeric materials via cyclic voltammetry afford LUMO energy levels from −3.49 to −3.59 eV as well as HOMO energy levels from −5.07 to −5.34 eV resulting in low energy band gaps from 1.52 to 1.81 eV. Spin coating of the prepared donor–acceptor oligomers/polymers resulted in well-defined films. Moreover, UV–vis measurements of the investigated donor–acceptor systems showed a broad absorption over the whole visible region. It is demonstrated that Bi-DPP as an electron accepting moiety in donor–acceptor systems offer potential properties for organic solar cell devices.

Diketopyrrolopyrrole Polymers for Organic Solar Cells

Conjugated polymers have been extensively studied for application in organic solar cells. In designing new polymers, particular attention has been given to tuning the absorption spectrum, molecular energy levels, crystallinity, and charge carrier mobility to enhance performance. As a result, the power conversion efficiencies (PCEs) of solar cells based on conjugated polymers as electron donor and fullerene derivatives as electron acceptor have exceeded 10% in single-junction and 11% in multijunction devices. Despite these efforts, it is notoriously difficult to establish thorough structure-property relationships that will be required to further optimize existing high-performance polymers to their intrinsic limits. In this Account, we highlight progress on the development and our understanding of diketopyrrolopyrrole (DPP) based conjugated polymers for polymer solar cells. The DPP moiety is strongly electron withdrawing and its polar nature enhances the tendency of DPP-based polymers to crystallize. As a result, DPP-based conjugated polymers often exhibit an advantageously broad and tunable optical absorption, up to 1000 nm, and high mobilities for holes and electrons, which can result in high photocurrents and good fill factors in solar cells. Here we focus on the structural modifications applied to DPP polymers and rationalize and explain the relationships between chemical structure and organic photovoltaic performance. The DPP polymers can be tuned via their aromatic substituents, their alkyl side chains, and the nature of the π-conjugated segment linking the units along the polymer chain. We show that these building blocks work together in determining the molecular conformation, the optical properties, the charge carrier mobility, and the solubility of the polymer. We identify the latter as a decisive parameter for DPP-based organic solar cells because it regulates the diameter of the semicrystalline DPP polymer fibers that form in the photovoltaic blends with fullerenes via solution processing. The width of these fibers and the photon energy loss, defined as the energy difference between optical band gap and open-circuit voltage, together govern to a large extent the quantum efficiency for charge generation in these blends and thereby the power conversion efficiency of the photovoltaic devices. Lowering the photon energy loss and maintaining a high quantum yield for charge generation is identified as a major pathway to enhance the performance of organic solar cells. This can be achieved by controlling the structural purity of the materials and further control over morphology formation. We hope that this Account contributes to improved design strategies of DPP polymers that are required to realize new breakthroughs in organic solar cell performance in the future.

Synthesis of fluorinated diphenyl-diketopyrrolopyrrole derivatives as new building blocks for conjugated copolymers

A series of conjugated copolymers with an identical backbone (PBDT-DPP) but different fluorination numbers and positions were synthesized, and the impact of fluorine atoms on the structure–property-device performance was comprehensively investigated.

Designing interchain and intrachain properties of conjugated polymers for latent optical information encoding

Based on systematic conjugated polymers design, we demonstrate latent information encoding which reveals and conceals hidden information upon polymers' aggregation/deaggregation.

Building molecular-design insights for controlling both the intrachain and the interchain properties of conjugated polymers (CPs) is essential to determine their characteristics and to optimize their performance in applications. However, most CP designs have focused on the conjugated main chain to control the intrachain properties, while the design of side chains is usually used to render CPs soluble, even though the side chains critically affect the interchain packing. Here, we present a straightforward and effective design strategy for modifying the optical and electrochemical properties of diketopyrrolopyrrole-based CPs by controlling both the intrachain and interchain properties in a single system. The synthesized polymers, P1, P2 and P3, show almost identical optical absorption spectra in solution, manifesting essentially the same intrachain properties of the three CPs having restricted effective conjugation along the main chain. However, the absorption spectra of CP films are gradually tuned by controlling the interchain packing through the side-chain design. Based on the tailored optical properties, we demonstrate the encoding of latent optical information utilizing the CPs as security inks on a silica substrate, which reveals and conceals hidden information upon the reversible aggregation/deaggregation of CPs.

Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors

Since the report of the first diketopyrrolopyrrole (DPP)-based polymer semiconductor, such polymers have received considerable attention as a promising candidate for high-performance polymer semiconductors in organic thin-film transistors (OTFTs). This Progress Report summarizes the advances in the molecular design of high-mobility DPP-based polymers reported in the last few years, especially focusing on the molecular design of these polymers in respect of tuning the backbone and side chains, and discussing the influences of structural modification of the backbone and side chains on the properties and device performance of corresponding DPP-based polymers. This provides insights for the development of new and high-mobility polymer semiconductors.

A new diketopyrrolopyrrole-based probe for sensitive and selective detection of sulfite in aqueous solution

A new probe was synthesized by incorporating an α,β-unsaturated ketone to a diketopyrrolopyrrole fluorophore. The probe had exhibited a selective and sensitive response to the sulfite against other thirteen anions and biothiols (Cys, Hcy and GSH), through the nucleophilic addition of sulfite to the alkene of probe with the detection limit of 0.1 μM in HEPES (10 mM, pH 7.4) THF/H2O (1:1, v/v). Meanwhile, it could be easily observed that the probe for sulfite changed from pink to colorless by the naked eye, and from pink to blue under UV lamp after the sulfite was added for 20 min. The NMR and Mass spectral analysis demonstrated the expected addition of sulfite to the C=C bonds.

Pyridine-bridged diketopyrrolopyrrole conjugated polymers for field-effect transistors and polymer solar cells

Five different pyridine-bridged diketopyrrolopyrrole-based polymers with variable energy levels were applied in organic field-effect transistors and polymer solar cells.

Record high electron mobility of 6.3 cm² V⁻¹ s⁻¹ achieved for polymer semiconductors using a new building block

A new electron acceptor building block, 3,6-di(pyridin-2-yl)pyrrolo[3,4-c ]pyrrole-1,4(2H ,5H)-dione (DBPy), is used to construct a donor-acceptor polymer, PDBPyBT. This polymer exhibits a strong self-assembly capability, to form highly crystalline and oriented thin films with a short π-π stacking distance of 0.36 nm. PDBPyBT shows ambipolar charge-transport performance in organic thin-film transistors, reaching a record high electron-mobility value of 6.30 cm(2) V(-1) s(-1).

Recent advances in the development of semiconducting DPP-containing polymers for transistor applications

This progress report summarizes the numerous DPP-containing polymers recently developed for field-effect transistor applications including diphenyl-DPP and dithienyl-DPP-based polymers as the most commonly reported materials, but also difuranyl-DPP, diselenophenyl-DPP and dithienothienyl-DPP-containing polymers. We discuss the hole and electron mobilities that were reported in relation to structural properties such as alkyl substitution patterns, polymer molecular weights and solid state packing, as well as electronic properties including HOMO and LUMO energy levels. We moreover consider important aspects of ambipolar charge transport and highlight fundamental structure-property relations such as the relationships between the thin film morphologies and the charge carrier mobilities observed for DPP-containing polymers.

Electrical conduction and dielectric relaxation in p-type PVA/CuI polymer composite

PVA/CuI polymer composite samples have been prepared and subjected to characterizations using FT-IR spectroscopy, DSC analysis, ac spectroscopy and dc conduction. The FT-IR spectral analysis shows remarkable variation of the absorption peak positions whereas DSC illustrates a little decrease of both glass transition temperature, Tg , and crystallization fraction, χ, with increasing CuI concentration. An increase of dc conductivity for PVA/CuI nano composite by increasing CuI concentration is recoded up to 15 wt%, besides it obeys Arhenuis plot with an activation energy in the range 0.54-1.32 eV. The frequency dependence of ac conductivity showed power law with an exponent 0.33 < s < 0.69 which predicts hopping conduction mechanism. The frequency dependence of both dielectric permittivity and dielectric loss obeys Debye dispersion relations in wide range of temperatures and frequency. Significant values of dipole relaxation time obtained which are thermally activated with activation energies in the range 0.33-0.87 eV. A significant value of hopping distance in the range 3.4-1.2 nm is estimated in agreement with the value of Bohr radius of the exciton.