Zhenggang Huang

The Influence of Polymer Purification on Photovoltaic Device Performance of a Series of Indacenodithiophene Donor Polymers

A series of low bandgap indacenodithiophene polymers is purified by recycling SEC in order to isolate narrow polydispersity fractions. This additional purification step is found to have a significant beneficial influence on the solar cell performance and the reasons for this performance increase are investigated.

Performance enhancement of fullerene-based solar cells by light processing.

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC60BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC60BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC60BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.

Photovoltaic and field effect transistor performance of selenophene and thiophene diketopyrrolopyrrole co-polymers with dithienothiophene

Here we report the synthesis of two new alternating co-polymers of thiophene and selenophene flanked diketopyrrolopyrrole with dithienothiophene. We find that the inclusion of the rigid fused dithienothiophene co-monomer affords semi-crystalline polymers, that exhibit promising ambipolar field effect transistor performance, with hole mobilities up to 0.23 cm2 V−1 s−1. The selenophene containing co-polymer exhibits a reduced band gap compared to the thiophene co-polymer, as a result of stabilisation of the polymer LUMO and destabilisation of the HOMO. The thiophene co-polymer exhibits promising solar cell performance in blends with PC71BM, with device efficiencies up to 5.1%.

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/cm(2)) 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.

Synthesis of novel thieno[3,2-b]thienobis(silolothiophene) based low bandgap polymers for organic photovoltaics

Thieno[3,2-b]thienobis(silolothiophene), a new electron rich hexacyclic monomer has been synthesized and incorporated into three novel donor-acceptor low-bandgap polymers. By carefully choosing the acceptor co-monomer, the energy levels of the polymers could be modulated and high power conversion efficiencies of 5.52% were reached in OPV devices.

BPTs: thiophene-flanked benzodipyrrolidone conjugated polymers for ambipolar organic transistors

A series of novel thiophene-flanked benzodipyrrolidone (BPT)-based alternating copolymers are synthesised, their optical and electrical properties evaluated. The BPT unit promotes a conjugated, planar polymer backbone, with a low bandgap, primarily due to low lying LUMO energy levels. Copolymerisation with thiophene exhibits well balanced ambipolar organic field-effect transistor performance, with electron and hole mobilities 0.1 and 0.2 cm(2) V(-1) s(-1), respectively.

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%.

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.

Dihydropyrroloindoledione-based copolymers for organic electronics

A series of four dihydropyrroloindoledione-based organic semi-conducting polymers are examined for performance in transistor and photovoltaic cell devices. The dihydropyrroloindoledione unit was alternately copolymerized with phenyl, thiophene and bithiophene comonomers, and the resultant polymers exhibit broad absorption, low-bandgaps and deep energy levels, with charge carrier mobilities approaching 0.1 cm2 V−1 s−1. Solar cells processed in a printing friendly solvent (m-xylene) exhibited >2% PCE with a high fill-factor of 0.62 and Voc of 0.75 V.

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%.