Kai-Yin Cheung

Polymer solar cells based on very narrow-bandgap polyplatinynes with photocurrents extended into the near-infrared region

The synthesis, characterization and photophysics of some solution-processable intensely coloured polyplatinynes functionalized with the thienopyrazine-thiophene hybrid spacer and their model molecular complexes are described. These metallated polymers possess extremely low bandgaps of 1.47-1.50 eV, which extend towards the near-infrared (NIR) range of the solar spectrum, and represent the lowest optical bandgaps ever reported for metallopolyynes in the literature. Both polymers can be used to fabricate efficient solar cells with power conversion efficiencies (PCEs) of up to 0.63% under air mass (AM1.5) simulated solar illumination, and the possibility of covering the 600-900 nm solar-radiation range to harvest photocurrent has been demonstrated. The influence of the thienyl core as well as its substituent group, on the optical and photovoltaic behavior of these metallopolymers was investigated in detail. The power dependencies of the solar cell parameters (including the short-circuit current density, open-circuit voltage, fill-factor and PCE) were also studied. The present work offers an attractive avenue towards conjugated materials with broad solar absorptions and demonstrates the potential of metallopolyynes for both visible and NIR light power generation.

Effect of fabrication processes on bulk heterojunctions (BHJ) photovoltaic device performance

Organic photovoltaic (OPV) devices have attracted much interest in recent decades because they have a great potential for low cost solar cells. Among different kinds of organic solar cells, conjugated polymer/fullerene bulk heterojunction (BHJ) solar cells have exhibited improvements in the power conversion efficiency (PCE) in recent years. The performance of BHJ solar cells is highly dependent on different fabrication processes. To address this issue, we focus on the dependence of different photovoltaic parameters on the fabrication methods. BHJ solar cells fabricated using platinum metallopolyyne (P1) with a low band gap of 1.85 eV as an electron donor and phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have been studied. The fabrication parameters, such as ratios of P1 to PCBM, solvents used, thickness of the active layers and top contact materials, have been systematically investigated. Blend ratio and solvent used had most significant influence on photovoltaic performance with several times higher efficiency of the best condition compared to the worst condition. They affected all photovoltaic parameters [open circuit voltage (Voc), short circuit current density (Jsc) and the fill factor (FF)]. Top contact materials affected the Voc and the FF, while thickness of the active layer mainly affected the Jsc and FF. The influence of different fabrication conditions on photovoltaic performance has been discussed.

Influence of the Film Thickness on the Optical and Electrical Properties of ITO

Indium tin oxide (ITO) is widely used for opto-electronic products such as organic light-emitting diodes, organic photovoltaic devices and liquid crystal displays due to its high transparency and electrical conductivity. Since there is a trade-off between the conductivity and transparency of ITO, it is necessary to optimize performances of opto-electronic products by balancing the sheet resistance and transmittance. Both sheet resistance and transmittance are affected by a number of factors such as working temperature, working pressure, oxygen-to-argon ratio during the fabricating process, and thickness. In our study, ITO thin films were deposited on glass substrates by dc sputtering. Effects of ITO with different thicknesses, sheet resistances, and transmission spectra on the performance of bulk heterojunction photovoltaic devices were investigated.

Titania nanotube array based photovoltaic cells

It has been shown that dye sensitized solar cells (DSSCs) based on porous titanium dioxide (titania) layers have efficiencies exceeding 10%. Although porous structure has the advantage of large surface area for light harvesting, electron transport through the random nanoparticle network forming a porous film results in electron mobilities which are two orders of magnitude lower compared to the single crystal materials. Therefore, considerable efforts have been made to fabricate DSSC based on one dimensional nanostructures, such as nanowires or nanotubes. Titania nanotube arrays are typically made by anodization of titanium, followed by annealing to improve crystallinity. In this work, we investigated the influence of annealing temperature and annealing atmosphere on the crystal structure, the electron transport, and the solar cell performance of titania nanotube arrays. The titania nanotube arrays were prepared from electrochemically anodized titanium foils and their morphology and crystal structure were characterized by scanning electron microscopy and transmission electron microscopy. The crystal phases and the compositions of nanotube arrays were further investigated by X-ray diffraction for different annealing temperatures and X-ray photoelectron spectroscopy for different annealing atmospheres. For optimal annealing conditions, the short circuit current density of 4.27 mA/cm2 and power conversion efficiency of 1.30% could be achieved under AM 1.5 simulated solar irradiation for 2 μm long nanotubes.