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%

Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole

We present the synthesis and characterization of a series of encapsulated diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials display extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Both the absorption and emission spectra show clearer, more defined features compared to their naked counterparts demonstrating the suppression of inter and intramolecular aggregation. We find that the encapsulation results in decreased energetic disorder and a dramatic increase in backbone colinearity as evidenced by scanning tunnelling microscopy. This study paves the way for diketopyrrolopyrrole to be used in emissive solid state applications and demonstrates a novel method to reduce structural disorder in conjugated polymers

Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole

We present the synthesis and characterization of a series of encapsulated diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials display extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Both the absorption and emission spectra show clearer, more defined features compared to their naked counterparts demonstrating the suppression of inter and intramolecular aggregation. We find that the encapsulation results in decreased energetic disorder and a dramatic increase in backbone colinearity as evidenced by scanning tunnelling microscopy. This study paves the way for diketopyrrolopyrrole to be used in emissive solid state applications and demonstrates a novel method to reduce structural disorder in conjugated polymers

Morphological Stability and Performance of Polymer–Fullerene Solar Cells under Thermal Stress: The Impact of Photoinduced PC₆₀BM Oligomerization

We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC₆₀BM to photo-oligomerize, which is capable of both stabilizing the polymer:PC₆₀BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC₆₀BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC₆₀BM crystallization within the blend films are investigated as a function of substrate and temperature. PC₆₀BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm²) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC₆₀BM aggregation is observed on smaller (≈100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC₆₀BM oligomers in blend films is also investigated and the activation energy of the fullerene–fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC₆₀BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC₇₀BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC₆₀BM micro- and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC₆₀BM oligomerization

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

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

A solution-processable near-infrared thermally activated delayed fluorescent dye with a fused aromatic acceptor and aggregation induced emission behavior

The unique synergy of properties offered by an efficient and processable near-infrared thermally activated delayed fluorescent (NIR TADF) dye could be transformative across research fields. Here, a solution-processable NIR TADF material is demonstrated (CAT-TPE). Good solubility is achieved through the use of a new tetraphenylethylene (TPE)-based triphenylamine electron donor. TADF is confirmed through variable temperature time-resolved measurements at a peak photoluminescence (PL) wavelength of 842 nm in a solution-processed film. An OLED with good roll-off characteristics for a solution-processed NIR TADF device is reported with electroluminescence λmax > 700 nm. CAT-TPE also demonstrates classic aggregation induced emission (AIE) behavior, being more emissive when aggregated than in solution with all PL > 700 nm. This work opens the door to the considerably enhanced structural diversity of solution-processable NIR TADF and will inform the design of future high efficiency AIE NIR TADF materials

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