Jenny E. Donaghey

Design of Semiconducting Indacenodithiophene Polymers for High Performance Transistors and Solar Cells

The prospect of using low cost, high throughput material deposition processes to fabricate organic circuitry and solar cells continues to drive research towards improving the performance of the semiconducting materials utilized in these devices. Conjugated aromatic polymers have emerged as a leading candidate semiconductor material class, due to their combination of their amenability to processing and reasonable electrical and optical performance. Challenges remain, however, to further improve the charge carrier mobility of the polymers for transistor applications and the power conversion efficiency for solar cells. This optimization requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology. In this Account, we describe an optimization process for a series of semiconducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. We demonstrate the effect of bridging atoms, alkyl chain functionalization, and co-repeating units on the morphology, molecular orbital energy levels, charge carrier mobility, and solar cell efficiencies. This conjugated unit is extremely versatile with a coplanar aromatic ring structure, and the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, and out of plane aliphatic chains present in both the carbon and silicon bridge promote solubility. This particular polymer conformation, however, typically suppresses long range organization and crystallinity, which had been shown to strongly influence charge transport. In many cases, polymers exhibited both high solubility and excellent charge transport properties, even where there was no observable evidence of polymer crystallinity. The optical bandgap of the polymers can be tuned by the combination of the donating power of the bridging unit and the electron withdrawing nature of co-repeat units, alternating along the polymer backbone. Using strong donors and acceptors, we could shift the absorption into the near infrared.

Pyrroloindacenodithiophene containing polymers for organic field effect transistors and organic photovoltaics

The synthesis of the novel electron-rich pyrroloindacenodithiophene (NIDT) unit is reported. Stille copolymerization of the distannylated NIDT unit, with the electron-deficient dibrominated benzothiadiazole (BT), difluorobenzothiadiazole (ffBT), thienopyrrolodione (TPD) and 1,1′-bithienopyrrolodione (biTPD) units afforded a series of low band gap semiconducting polymers. Initial testing shows promise for the use of these materials as p-type semiconductors in organic field effect transistors (OFETs) with mobilities as high as 0.07 cm2V−1s−1 being measured. These materials have also been tested as the donor polymer in polymer/fullerene bulk heterojunction organic photovoltaics (OPVs) giving maximum efficiencies of 2.5%.

Pyrroloindacenodithiophene polymers: the effect of molecular structure on OFET performance

The synthesis, characterization and testing of four novel pyrroloindacenodithiophene (NIDT) copolymers, in organic field effect transistor (OFET) devices, is reported. Copolymerization of thiophene, thienothiophene, thienopyrrolodione and diketopyrrolopyrrole with NIDT, to form alternating copolymers, is described. Two-dimensional Grazing Incidence Wide Angle X-ray Diffraction (2D GIWAXD) is used to probe the thin film molecular structure and, together with DFT studies, help to explain why the diketopyrrolopyrrole containing polymer outperforms the others in terms of hole transport, exhibiting an average mobility of 0.01 cm2 V−1 s−1.

Power conversion efficiency enhancement in diketopyrrolopyrrole based solar cells through polymer fractionation

Post polymerisation fractionation of diketopyrrolopyrrole based conjugated polymers through preparative gel permeation chromatography affords a varying range of molecular weight fractions with narrowed polydispersities. When used as the electron donor material in bulk heterojunction solar cells with both conventional and inverted architecture efficiency enhancements in excess of 50% are observed relative to non-fractionated material with the highest molecular weight fraction demonstrating a power conversion efficiency of 6.3%.

Compatibility of amorphous triarylamine copolymers with solution-processed hole injecting metal oxide bottom contacts

Triarylamine copolymers are p-type organic semiconducting materials which have been shown to have the crucial advantages of being air-stable, forming amorphous films (critical for device uniformity over large areas) and allowing the creation of high mobility transistors and highly efficient perovskite solar cells. A key area of recent technological progress has been the development of solution-processable metal oxides as charge injection layers in organic semiconducting devices. Here we report on the synthesis of a large ionization potential (IP = 5.65 eV) silafluorene bridged triarylamine copolymer poly(silafluorene-triarylamine) (PSiF-TAA), and compare its time-of-flight (TOF) bulk hole mobility to that of a fluorene bridged triarylamine copolymer poly(fluorene-triarylamine) (PF-TAA), (IP = 5.4 eV) and the homopolymer polytriarylamine (PTAA) (IP = 5.2 eV). Using these mobility values and current–voltage measurements, we then quantify the charge injection efficiency (χ) into these polymers from three ambiently-prepared solution processed hole-injecting contacts MoO3 (aqueous nanoparticle dispersion), V2O5 (sol–gel) and chlorinated indium tin oxide (Cl-ITO) (UV – solvent assisted), and compare them to the more conventional contacts ITO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Whilst hole injection into PTAA is relatively unaffected by the nature of the contact, injection into PF-TAA and PSiF-TAA is greatly improved by the use of MoO3 and Cl-ITO. Despite its similar mobility and larger ionization potential compared to the homopolymer, the highest injection efficiency is achieved for PF-TAA, indicating the role of chemical design in optimizing charge injection into organic semiconductor devices.