Iain Meager

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.

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 cm(2)/(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.

Optimisation of diketopyrrolopyrrole:fullerene solar cell performance through control of polymer molecular weight and thermal annealing

Poly-thieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T) is a promising low bandgap donor polymer for organic solar cells. In this study we employ two different approaches to improve the device efficiency via optimisation of the morphology of the active layer: tuning of the molecular weight of the polymer and thermal annealing. In the former case, a higher molecular weight was found to yield a more intermixed morphology, resulting in enhanced exciton dissociation and charge separation, as confirmed by atomic force microscopy, and photoluminescence and transient absorption spectroscopies. In the later case, thermal annealing prior to metal electrode deposition increased the photon conversion efficiency to as high as 6.6%, with this enhanced efficiency being maintained even with prolonged annealing (240 hours at 80 °C). This enhancement in performance with thermal annealing was correlated with increased polymer crystallinity.

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%.

Alkyl side-chain branching point effects in thieno[3,4-c]pyrrole-4,6-dione copolymers

A novel and efficient route to thieno[3,4-c]pyrrole-4,6-dione (TPD) facilitating late-stage alkylation is presented. Four copolymers with alkylated bithiophene (2T) are synthesised, with different TPD N−alkyl side-chain branching points. The effect of steric bulk on solid-state properties is investigated (UV–VIS, differential scanning calorimetry, X-ray diffraction) and C1-branching found to increase crystallinity and solid-state packing in TPD-2T.

Effect of molecular weight on the vibronic structure of a diketopyrrolopyrrole polymer

Resonance Raman Spectroscopy (RRS) is employed in this study to examine the influence of molecular weight on the optical response of a diketopyrrolopyrrole polymer (DPP-TT-T) in solution. The vibronic structure observed for the ground state absorption of this polymer is found to vary with molecular weight and solvent. Resonance Raman Intensity Analysis (RRIA) revealed that the absorption spectra can be described by at least two dipole-allowed transitions and the vibronic structure variation is due to differing contributions from linear and curved segments of the polymer.