Kai Wing Cheng

Poly(3-hexylthiophene):TiO2 nanocomposites for solar cell applications

The properties of organic/inorganic poly(3-hexylthiophene) (P3HT):TiO2 nanocomposite films and nanocomposite based solar cells as a function of TiO2 concentration and the solvent used for the film fabrication were studied. For low nanoparticle concentration (20?30%) the device performance was worse compared to pure P3HT, while for nanoparticle concentration of 50% and 60% significant improvements were obtained. P3HT photoluminescence quenching in 600?800?nm spectral region changes by a factor of two for the increase in TiO2 concentration from 20% to 60%, while the AM1 power conversion efficiency increases times. Photoluminescence quenching and solar cell efficiency were found to be strongly dependent not only on nanoparticle concentration but also on the solvent used for spin-coating. The changes in the film and device properties were explained by the change in the film morphology. For optimal fabrication conditions, external quantum efficiency up to 15% and AM1 power conversion efficiency of 0.42% were obtained.

Low-band-gap, sublimable rhenium(I) diimine complex for efficient bulk heterojunction photovoltaic devices

The use of chlorotricarbonyl rhenium(I) diimine complex as photosensitizing molecule in photovoltaic cells is demonstrated. Unlike other transition-metal-based sensitizers, the complex is sublimable and has low band gap. It also exhibits bipolar charge transport character with relatively high carrier mobilities on the order of 10−3 cm2 V−1 s−1. Multilayer heterojunction and bulk heterojunction devices with fullerene as the electron accepting molecule were prepared. For the bulk heterojunction devices, the fill factor and power conversion efficiency under simulated solar light illumination were 0.56 and 0.48%, respectively. Atomic force microscopic images showed that the complex dispersed evenly with fullerene molecules in solid state.

Fabrication of bulk heterojunction photovoltaic devices using sublimable rhenium diimine complexes as photosensitizers

A series of chlorotricarbonyl rhenium (I) bis(phenylimino)acenaphthene (Re-DIAN-X) complexes were used as the photosensitizers for photovoltaic cells. Unlike other transition-metal-based photovoltaic sensitizers that can only be prepared by solution method, these complexes are sublimable. Compared to other rhenium diimine complexes based on bipyridine or 1,4-diaza-1,3-butadiene ligands, these complexes have lower band gaps, which can be modified easily by changing the structure of the ligand. It allows the preparation of blend of metal complexes in order to broaden the sensitization region in UV-vis absorption spectrum. One of the complexes also shows bipolar charge transport character with relatively high charge carrier mobilities in the order of 10-3 cm2V-1s-1. Multilayer heterojunction and bulk heterojunction devices with fullerene as the electron accepting molecule were prepared. For the bulk heterojunction devices, the fill factor and power conversion efficiency under AM 1.5 simulated solar light illumination were 0.51 and 1.29 %, respectively. The effects of changing the Re-DIAN/C60 film thickness, Re-DIAN/C60 ratio and variation of ligand structures in the bulk heterojunction devices were studied. The amount of photosensitizer and electron transport molecules may strongly affect the balance between the photon absorption, exciton formation, dissociation, and charge transport processes. Atomic force microscopic images showed that the complex dispersed evenly with fullerene molecules in solid state.

Fabrication of photovoltaic cell from ruthenium-containing polymer using layer-by-layer polyelectrolytes adsorption technique

Multilayer photovoltaic devices were fabricated by the sequence adsorption of different polyelectrolytes. A ruthenium terpyridine complex containing poly(p-phenylenevinylene) was used as the polycation layer. This polymer has been shown to exhibit large photo-sensitivity due to the presence of the ruthenium complex, which has relatively long-lived excited state. This polymer absorbs strongly in the visible region at ca. 480 - 550 nm and it can act as the electron transporter. Sulfonated polyaniline was used as the hole-transporting polyanion layer. The ITO/(polyanion/polycation)n/Al devices were found to exhibit photovoltaic properties under the illumination of AM1 solar radiation. The short-circuit current Isc, open-circuit voltage Voc, and the fill factor FF were measured to be 14 μA/cm2, 0.84 V and 0.16 respectively. It was found that the power conversion efficiencies of the devices were dependent on the device thickness. This simple layer-by-layer self-assembly method allowed us to control the devices thickness accurately.