Diketopyrrolopyrrole Polymer Research Articles

Inducing Elasticity through Oligo‐Siloxane Crosslinks for Intrinsically Stretchable Semiconducting Polymers

The promise of wearable and implantable devices has made stretchable organic semiconductors highly desirable. Though there are increasing attempts to design intrinsically stretchable conjugated polymers, their performance in terms of charge carrier mobility and maximum fracture strain is still lacking behind extrinsic approaches (i.e., buckling, Kirigami interconnects). Here, polymer crosslinking with flexible oligomers is applied as a strategy to reduce the tensile modulus and improve fracture strain, as well as fatigue resistance for a high mobility diketopyrrolopyrrole polymer. These polymers are crosslinked with siloxane oligomers to give stretchable films stable up to a strain ε = 150% and 500 strain‐and‐release cycles of 100% strain without the formation of nanocracks. Organic field‐effect transistors are prepared to assess the electrical properties of the crosslinked film under cyclic strain loading. An initial average mobility (μavg) of 0.66 cm2 V−1 s−1 is measured at 0% strain. A steady μavg above 0.40 cm2 V−1 s−1 is obtained in the direction perpendicular to the strain direction after 500 strain‐and‐release cycles of 20% strain. The μavg in the direction parallel to strain, however, is compromised due to the formation of wrinkles.

An effective way to reduce energy loss and enhance open-circuit voltage in polymer solar cells based on a diketopyrrolopyrrole polymer containing three regular alternating units

A planar DPP-based polymer containing three regular alternating units exhibits a good photovoltaic performance with a high PCE of 9.02% and a large Voc of 0.86 V.

The effect of branching in a semiconducting polymer on the efficiency of organic photovoltaic cells

The impact of branching in a diketopyrrolopyrrole polymer on the performance of polymer-fullerene photovoltaic cells is investigated. Compared to the linear polymer, the branched polymer affords a more finely dispersed fibrillar network in the photoactive layer and as a result a large enhancement of the photocurrent and power conversion efficiency.

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.

Dispersion of High‐Purity Semiconducting Arc‐Discharged Carbon Nanotubes Using Backbone Engineered Diketopyrrolopyrrole (DPP)‐Based Polymers

The isolation of semiconducting single‐walled carbon nanotubes (sc‐SWNTs) with ideal diameter and high purity is highly desired for high‐performance electronic devices. However, current sorting methods for large‐diameter sc‐SWNTs suffer from either low purity (<99%) or long processing time (>20 h). Here, a backbone‐engineering strategy is reported for the polymer used for sorting to improve the purity of sorted sc‐SWNTs. Six diketopyrrolopyrrole (DPP)‐based conjugated polymers are used to systematically investigate their sorting ability for sc‐SWNTs. It is found that incorporation of more thiophenes building blocks in the repeating units of DPP polymer backbone leads to increased selectivity and yield for sc‐SWNTs. The DPP polymers can disperse sc‐SWNTs with 1.4–1.6 nm in diameter and high purity of 99.6% by a processing time as short as 1 h. Furthermore, a scalable film coating method named “solution shearing” is used to fabricate SWNT network thin‐film transistors (TFTs). The TFT devices exhibit both high mobilities over 50 cm2 V−1 s−1 and high on/off ratios over 105, which are among the highest performance for solution‐processed SWNT network TFTs.

Ultra-flexible nonvolatile memory based on donor-acceptor diketopyrrolopyrrole polymer blends.

Flexible memory cell array based on high mobility donor-acceptor diketopyrrolopyrrole polymer has been demonstrated. The memory cell exhibits low read voltage, high cell-to-cell uniformity and good mechanical flexibility, and has reliable retention and endurance memory performance. The electrical properties of the memory devices are systematically investigated and modeled. Our results suggest that the polymer blends provide an important step towards high-density flexible nonvolatile memory devices.

Study of Vertical and Lateral Charge Transport Properties of DPP-Based Polymer/PC61BM Films Using Space Charge Limited Current (SCLC) and Field Effect Transistor Methods and their Effects on Photovoltaic Characteristics

We studied the vertical and lateral charge transport characteristics of a diketopyrrolopyrrole polymer donor (D)–PC61BM acceptor (A) system by measuring the space charge limited current (SCLC) mobility and field-effect mobility respectively. It was found that with an increase in annealing temperature, the SCLC hole mobility decreased for the pure polymer (PDBFBT) but increased for the PDBFBT:PC61BM blends, which could be explained by changes in the crystallinity and crystal orientation (edge-on versus face-on). The pure PDBFBT and most blend films showed the maximum field-effect hole mobility (µh) when annealed at 100°C, which then declined as the annealing temperature was further increased. Surprisingly, the D/A = 1/1 blend films annealed at high temperatures exhibited an abrupt increase in the field-effect µh. This unusual phenomenon was interpreted by the antiplasticization effect of PC61BM, which promoted the molecular organization of the polymer. The effect of annealing on the carrier mobility was further correlated with the performance of inverted organic solar cell devices with the PDBFBT:PC61BM blend (D/A = 1/3). Thermal annealing at high temperatures (>100°C) was found to obstruct electron transport and cause the device performance to significantly deteriorate.

Tuning the energy gap of conjugated polymer zwitterions for efficient interlayers and solar cells

ABSTRACTNarrow band gap conjugated polymer zwitterions (CPZs) were synthesized by Suzuki polymerization and characterized to understand their electronic properties and utility as cathode modification layers in solar cells. The polymers were prepared from diketopyrrolopyrrole (DPP) and iso‐indigo monomers containing sulfobetaine (SB) pendant groups, benefiting from an ion‐rich aqueous phase in the polymerizations. UV–vis absorption spectroscopy revealed the optical energy gap value for the CPZs, ranging from 1.7 to 1.2 eV. Ultraviolet photoelectron spectroscopy of the CPZs as thin layers on Ag metal showed that the pendent zwitterions impart an interfacial dipole (Δ) to the metal and a work function reduction of ∼0.9 eV. OPVs fabricated using a conventional bulk heterojunction (BHJ) device architecture of ITO/PEDOT:PSS/(PTB7:PC71BM)/CPZ/Ag led to dramatic improvements in power conversion efficiency (PCE) values relative to devices having bare Ag cathodes (PCE < 2% for bare Ag vs. 6.7–7.7% for CPZ/Ag). The benzothiadiazole (BT)/DPP polymer denoted as PT2BTDPPSB gave an optimal PCE of 7.7% in a conventional BHJ OPV device architecture fabricated on a Ag cathode. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 327–336

Influence of Chemical Structure on the Charge Transfer State Spectrum of a Polymer:Fullerene Complex

Charge transfer (CT) state properties at the donor:acceptor interface in organic photovoltaic materials are widely regarded as crucial in determining the charge separation efficiency of organic photovoltaic devices. In this work, we use time dependent density functional theory (TDDFT) with B3LYP/6-31g* to study the influence of chemical structure on excitation energies, oscillator strengths, and electronic structure. We vertically excite states in a series of polymer:fullerene blends, modeling each blend as an oligomer:fullerene pair in vacuo. Our method reproduces experimentally observed trends in CT state energy with chemical structure as measured by electroluminescence. For oligothiophene:PCBM (PCBM = [6,6]-phenyl C61-butyric acid methyl ester), we find that Coulomb binding tends to reduce in higher excited CT states. In the case of several isoindigo and diketopyrrolopyrrole (DPP) based donors, we find that the first excited state of the pair lies close in energy to the first singlet of the oligomers, and low excited states have hybrid, or incomplete charge transfer, character. The natures and energies of these states are dependent on the fullerene position. We discuss the effect of thiophene substitution in a DPP polymer on charge generation in terms of the calculated CT state properties and rationalize the observed charge separation efficiency of corresponding experimental systems in terms of these calculations.

Polymer Solar Cells with Diketopyrrolopyrrole Conjugated Polymers as the Electron Donor and Electron Acceptor

A new class of diketopyrrolopyrrole conjugated acceptor polymer incorporating thiazoles with low-lying energy levels, high electron mobility, and broad absorption to the near infrared region provides a power conversion efficiency of 2.9% in solar cells with a second diketopyrrolo-pyrrole polymer as the donor.

A comparative study of diketopyrrolopyrrole and isoindigo based polymers for organic photovoltaic applications

Two new donor/acceptor π-conjugated polymers with functional dyes, either diketopyrrolopyrrole or isoindigo as the electron accepting units and 5,10-bis(2,3-didecylthiophen-5-yl)-naphtho[1,2-b:5,6-b′]difuran as an electron donating units have been synthesized. The photo-physical, electrochemical and photovoltaic properties of the new polymers have been studied and compared. With the same donor unit, the diketopyrrolopyrrole based polymer showed low bandgap and broad absorption extended into near IR region. The isoindigo based polymer showed little absorption in near IR region. However, the isoindigo based polymer showed more efficient absorption than diketopyrrolopyrrole based polymer in the most of visible region (from 300 to 690 nm) in solution. The isoindigo based polymer showed lower HOMO and LUMO energy levels than diketopyrrolopyrrole based polymer. The isoindigo polymer based solar cell devices consistently showed higher open circuit voltages than the diketopyrrolopyrrole polymer based devices.

A phenothiazine–diketopyrrolopyrrole polymer: Synthesis and photovoltaic applications

A phenothiazine–diketopyrrolopyrrole polymer (PPT–DPP) with a thiophene spacer was synthesized with a number-average molecular weight of 17600. The new polymer exhibited an optical band-gap of 1.63eV and a highest occupied molecular orbital energy level of −5.20eV. Photovoltaic devices were fabricated using the polymer and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) with the configuration ITO/PEDOT:PSS/PPT–DPP:PC71BM/LiF/Al. The polymer-based devices exhibited power conversion efficiencies as high as 1.8%. Based on atomic force microscopy data, the device performance was related to the morphology of the active layers.

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-b]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.

Nanoscale phase domain structure and associated device performance of organic solar cells based on a diketopyrrolopyrrole polymer

We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform : dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor–acceptor interfacial area and resulting greater capacity for charge carrier generation.