Scott E. Watkins

Synthesis, photophysical, and device properties of novel dendrimers based on a fluorene-hexabenzocoronene (FHBC) core.

The synthesis of easily functionalized and highly soluble fluorene-containing hexabenzocoronenes (FHBC) has been achieved in high yield at a gram scale. Conjugated triarylamine oligomers were coupled to the FHBC cores via Buchwald-Hartwig coupling, and the photophysical properties of resulting dendritic materials were examined by ultrafast laser spectroscopic techniques. Efficient quenching of the triarylamine oligomer fluorescence was observed paving the way for the inclusion of these materials in bulk heterojunction solar cells. In preliminary studies, solar cell devices with external quantum efficiencies above 5% have been fabricated.

Differentially pumped spray deposition as a rapid screening tool for organic and perovskite solar cells

We report a spray deposition technique as a screening tool for solution processed solar cells. A dual-feed spray nozzle is introduced to deposit donor and acceptor materials separately and to form blended films on substrates in situ. Using a differential pump system with a motorised spray nozzle, the effect of film thickness, solution flow rates and the blend ratio of donor and acceptor materials on device performance can be found in a single experiment. Using this method, polymer solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61 butyric acid methyl ester (PC61BM) are fabricated with numerous combinations of thicknesses and blend ratios. Results obtained from this technique show that the optimum ratio of materials is consistent with previously reported values confirming this technique is a very useful and effective screening method. This high throughput screening method is also used in a single-feed configuration. In the single-feed mode, methylammonium iodide solution is deposited on lead iodide films to create a photoactive layer of perovskite solar cells. Devices featuring a perovskite layer fabricated by this spray process demonstrated a power conversion efficiencies of up to 7.9%.

Blue-to-green electrophosphorescence of iridium-based cyclometallated materials

The photo- and electroluminescence properties of a series of novel, heteroleptic, mer-cyclometallated iridium complexes have been fine-tuned from green to blue by changing the substituents on the pyridyl ring of the phenylpyridyl ligand. The X-ray crystal structures of two Ir-based triazolyl complexes are reported.

Direct correlation of charge transfer absorption with molecular donor:acceptor interfacial area via photothermal deflection spectroscopy

Here we show that the charge transfer (CT) absorption signal in bulk-heterojunction solar cell blends, measured by photothermal deflection spectroscopy, is directly proportional to the density of molecular donor:acceptor interfaces. Since the optical transitions from the ground state to the interfacial CT state are weakly allowed at photon energies below the optical gap of both the donor and acceptor, we can exploit the use of this sensitive linear absorption spectroscopy for such quantification. Moreover, we determine the absolute molar extinction coefficient of the CT transition for an archetypical polymer:fullerene interface. The latter is ∼100 times lower than the extinction coefficient of the donor chromophore involved, allowing us to experimentally estimate the transition dipole moment as 0.3 D and the electronic coupling between the ground and CT states to be on the order of 30 meV.

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.

Low band gap dithienogermolodithiophene copolymers with tunable acceptors and side-chains for organic solar cells

We report the synthesis and characterisation of five new donor–acceptor type co-polymers based on a fused dithienogermolodithiophene unit for use in photovoltaic devices. The influence of three electron deficient co-monomers, as well as the length and variety of the solubilising side-chains, on the physical and optoelectronic properties of the polymers is reported. The number and variety of alkyl side-chains is found to have a significant impact on the polymer aggregation and film morphology, with larger and more bulky side-chains leading to improved solubility and molecular weight. The influence of these properties upon the performance of bulk heterojunction solar cells is shown.

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.

Optically monitored spray coating system for the controlled deposition of the photoactive layer in organic solar cells

A spray deposition process equipped with an in situ optical thickness monitoring system has been developed to fabricate the photoactive layer of solar cells. Film thickness is monitored by a photodiode–LED couple after each deposition cycle. Using optimized conditions, the thickness of the spray deposited photoactive films can be tuned to increase linearly with the number of deposition cycles over a wide range of deposition conditions. After instrument calibration, optimization of the active layer thickness can be accomplished by simply setting the desired absorbance of the film. The simple process outlined here may be used for the rapid optimization of thin film photovoltaic devices. As proof of this, we fabricate a poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester as well as a P3HT and indene-C60 bis-adduct organic solar cells, which achieve a champion power conversion efficiency of 4.2%.

Incorporation of benzocarborane into conjugated polymer systems: synthesis, characterisation and optoelectronic properties

We present the novel 1,4-difunctionalisation of benzocarborane with organometallic groups suitable for cross-coupling and its subsequent insertion for the first time into conjugated polymer backbones. Copolymers with solubilised cyclopentadithiophene and diketopyrrolopyrrole derivatives were prepared by Stille polymerisation in good molecular weight. The physical, material and optoelectronic properties of the resulting polymers were investigated, demonstrating that benzocarborane acts similarly to a stabilised, electron-deficient cis-diene linker. We also report the first polymer field effect transistors incorporating a benzocarborane in the backbone.

Enhanced photovoltaic performance of nanocrystalline CdTe/ZnO solar cells using sol-gel ZnO and positive bias treatment

The effect of doping and porosity of the n-type ZnO layer on the performance of solution-processed, sintered p-CdTe/n-ZnO nanocrystal photovoltaic (PV) devices is investigated. Amorphous sol-gel ZnO is found to be the best candidate with overall energy conversion efficiencies above 8% obtained if the ZnO is also indium doped. We demonstrate that when such PV devices are left under forward bias (in dark or light), the device efficiency values are raised to at least 9.8%, due to a substantially increased open-circuit voltage and fill-factor. This drastic enhancement is attributed to improved band alignment at the ITO/CdTe interface. The forward-bias treatment is slowly reversed over a period of days to weeks on standing under open circuit conditions, but is readily restored with further voltage treatment. The moderate processing conditions and high efficiency of such devices demonstrate that nanocrystal-based systems are a promising technology for photovoltaics.