Showing papers by "Yeng Ming Lam published in 2012"

Evolution pathway of CIGSe nanocrystals for solar cell applications

Abstract: CuInxGa1–xSe2 nanocrystals synthesized via the hot injection route have been used to make thin film solar cells with high power conversion efficiency Thus, CuInxGa1–xSe2 nanocrystals have the potential to provide a low cost and high efficiency solution to harvest solar energy Stoichiometry control of these nanocrystals offers the possibility of tuning the band gap of this material It is important to understand how the composition of quaternary CuInxGa1–xSe2 nanocrystals evolves to control the stoichiometry of this compound We report a systematic study of the growth and evolution pathways of quaternary CuIn05Ga05Se2 nanocrystals in a hot coordination solvent The reaction starts by the formation of a mixture of binary and ternary nanocrystals, which transforms subsequently to CuIn05Ga05Se2 nanocrystals These binary and ternary compounds dissolve in the course of the reaction, so as to provide the molecular precursor for monophasic CuIn05Ga05Se2 nanocrystals to form Here, we study the growth seq

Light scattering enhancement from sub-micrometer cavities in the photoanode for dye-sensitized solar cells

Abstract: A simple cell architecture that achieves enhanced light harvesting with less dye quantity while simultaneously improving the power conversion efficiency (PCE) of dye-sensitized solar cells is presented. Polystyrene (PS) spheres of sub-micrometer size were incorporated into the titanium dioxide paste resulting in photoanodes with bimodal pore size distribution. Scanning electron micrographs of TiO2 films revealed a mixture of mesoporous and macroporous morphology in which sub-micrometer cavities created by the combustion of PS increased the light scattering, enhancing the optical path length and hence the harvesting of photons by the sensitizer. The amount of dye uptake by these films is lower than that of standard films because the high porosity reduces the total surface area. Even with lower dye adsorption, the photovoltaic performance has been maintained and even improved, mainly due to better open circuit voltage and higher fill factor. Overall, better light harvesting has helped to maintain the efficiency of dye-sensitized solar cells while saving up to 30% of dye loading and replacing the conventional 400 nm scattering layer with voids. By employing these photoanodes, an efficiency of 6.9% was achieved in ionic liquid based dye sensitized solar cells.

From benzobisthiadiazole, thiadiazoloquinoxaline to pyrazinoquinoxaline based polymers: effects of aromatic substituents on the performance of organic photovoltaics

Abstract: Here we report the syntheses of low bandgap polymers based on benzobisthiadiazole (BBT), thiadiazoloquinoxaline (TQ) and pyrazinoquinoxaline (PQ) core structures with different aromatic substituents. The effects of changing core structures from BBT to PQ and also substituents from biphenyl and bithienyl on the photophysical, electrochemical and morphology of the polymers were studied. These polymers were incorporated into solar cell devices as donors, with PC[71]BM as acceptors, and their device performances were correlated with their properties. It was found that the effect of these structural changes has significant consequences on the overall device performances.

Controlled synthesis of CdE (E = S, Se and Te) nanowires

Abstract: This work focused on a catalyst-free solution-based method to synthesize single-crystal CdE (E = S, Se and Te) nanowires. Using the hot coordinating solvents method, we have successfully synthesized high aspect ratio CdE nanowires. In this paper, we present our very recent results on the synthesis of CdTe nanowires and summarize our understanding of the effect of reaction parameters on the growth of CdE nanowires. The reaction parameters include ligands for Cd-complexes and E-complexes, ligand-to-Cd mole ratio, Cd-to-E mole ratio, precursor concentration, reaction temperature and the injection process. We propose the optimum conditions for the growth of CdE nanocrystals with a large aspect ratio. Possible growth mechanisms were also investigated using time-dependent studies. Furthermore, a Raman study shows a higher concentration of tellurium on the surface of CdTe nanowires. This is understandable because the free energy of Te is smaller than that of CdTe and thus Te crystals can easily form during the synthesis. Our high aspect-ratio nanowires have good dispersibility and exhibit huge potential applications in areas such as solution processed photovoltaic cells and transistors.

Understanding polycarbazole-based polymer:CdSe hybrid solar cells

Abstract: We report for the first time the fabrication and characterization of organic-inorganic bulk heterojunction (BHJ) hybrid solar cells made of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and pyridine-capped CdSe nanorods. By optimizing both CdSe loading and active layer film thickness, the power conversion efficiencies (PCEs) of PCDTBT:CdSe hybrid solar cells were able to reach 2%, with PCDTBT:CdSe devices displaying an open-circuit voltage (V(OC )) that is 35% higher than P3HT:CdSe devices due to the deeper HOMO level of PCDTBT polymer. The performance of PCDTBT:CdSe devices is limited by its morphology and also its lower LUMO energy offset compared to P3HT:CdSe devices. Hence, the performance of PCDTBT:CdSe solar cells could be further improved by modifying the morphology of the films and also by including an interlayer to generate a built-in voltage to encourage exciton dissociation. Our results suggest that PCDTBT could be a viable alternative to P3HT as an electron donor in hybrid BHJ solar cells for high photovoltage application.

Metal/metal sulfide functionalized single-walled carbon nanotubes: FTO-free counter electrodes for dye sensitized solar cells.

Abstract: The use of single-walled carbon nanotubes (CNT) thin films to replace conventional fluorine-doped tin oxide (FTO) and both FTO and platinum (Pt) as the counter electrode in dye sensitized solar cells (DSSC) requires surface modification due to high sheet resistance and charge transfer resistance. In this paper, we report a simple, solution-based method of preparing FTO-free counter electrodes based on metal (Pt) or metal sulfide (Co8.4S8, Ni3S2) nanoparticles/CNT composite films to improve device performance. Based on electrochemical studies, the relative catalytic activity of the composite films was Pt > Co8.4S8 > Ni3S2. We achieved a maximum efficiency of 3.76% for the device with an FTO-free counter electrode (Pt/CNT). The device with an FTO- and Pt-free (CoS/CNT) counter electrode gives 3.13% efficiency.

Carrier Dynamics in Polymer Nanofiber:Fullerene Solar Cells

Abstract: Organic photovoltaic (OPV) devices fabricated with P3HT nanofiber (NF) networks typically exhibit poorer device performance compared to their nanoscale phase separated P3HT:PCBM counterparts despite possessing superior light harvesting properties and high in-plane charge mobility. Herein, we investigate the charge generation and recombination dynamics in P3HT-NF:PCBM blend films using transient absorption spectroscopy (TAS) spanning a wide temporal range over 7 orders of magnitude (i.e., from 100 fs to 1 μs), which are correlated with device performance studies. TAS reveals a more efficient charge generation and polaron formation rate in the NF samples as compared to the control samples at the onset which persists up to ∼2 ns. However, within the nanoseconds to microseconds time scale, there is a significant amount of nongeminate recombination in the NF system. We attribute this to the poor interfibrillar charge transport between the NFs, which tend to align parallel to the electrodes, thereby causing cha...

Conjugated polymers based on dicarboxylic imide-substituted isothianaphthene and their applications in solar cells

Abstract: Four new polymers containing a benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide (DIITN) unit including the homopolymer and three donor–acceptor copolymers were designed, synthesized, and characterized. For these copolymers, DIITN unit with low bandgap was selected as an electron acceptor, whereas 5,5′-(2,7-bisthiophen-2-yl)-9-(2-decyltetradecyl)-9H-carbazole), 5,5′-(3,3′-di-n-octylsilylene-2,2′-bithiophene), and 5,5′-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-yl) were chosen as the electron donor units to tune the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of the copolymers for better light harvesting. These polymers exhibit extended absorption in the visible and near-infrared range and are soluble in common organic solvents. The relative low lying HOMO of these polymers promises good air stability and high open-circuit voltage (Voc) for photovoltaic application. Bulk heterojunction solar cells were fabricated by blending the copolymers with [6,6]-phenyl-C61-butyric acid methyl ester or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The best power conversion efficiency of 1.6% was achieved under simulated sunlight AM 1.5G (100 mW/cm2) from solar cells containing 20 wt % of the fluorene copolymer poly[5,5′-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-yl)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide)] and 80 wt % of PC71BM with a high open-circuit voltage (Voc) of 0.84 V. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

New 3D supramolecular Zn(II)-coordinated self-assembled organic networks

Abstract: New 3D supramolecular networks S1 and S2 were prepared by Zn(II) coordination of the tetraphenylmethane-based p-type and n-type molecules bearing four terpyridine ligands. XRD and BET results indicate they are relatively amorphous and non-porous with a high degree of interpenetration within the networks. These could be disassembled by adding more Zn(II) ions and re-assembled to form extended 3D networks S3–6 by inserting linear n-type or p-type linking units. BET data suggests that these expanded networks are more porous than the original networks S1–2, but the low porosity and surface area suggest a high degree of interpenetration remains within the expanded networks. The optical properties of these materials were compared to the linear polymers P1–3 made by Zn(II)-mediated assembly of the same linear linking units. The emission spectra of both the 3-D and 1-D cases with the same linking unit matched each other, confirming the incorporation of the linker units into the expanded assemblies. This shows that metal–ligand mediated self-assembly can be used to make two component systems in which the optical properties can be tuned by selection of the units. The assembly was also performed in the presence of CdSe nanocrystals to form nanocomposites.

Cu-zn-sn-s/se thin film and methods of forming the same

Abstract: A method for forming a copper-zinc-tin-X (Cu-Zn-Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se), is provided. The method comprises a) forming a dispersion comprising Cu p X(1 ≤ p ≤ 2) nanoparticles, ZnX nanoparticles and SnX q (1 ≤ q ≤ 2) nanoparticles in a dispersing agent; b) depositing the dispersion on a substrate to form a precursor thin film; and c) heating the substrate comprising the precursor thin film in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn-Sn-X thin film. A Cu-Zn-Sn-X thin film formed by the method, and a device comprising the Cu-Zn-Sn-X thin film is also provided. Photoresponse data of a Cu-Zn-Sn-X thin film formed according to an embodiment is also provided.