Husnu Emrah Unalan

Conducting and transparent single-wall carbon nanotube electrodes for polymer-fullerene solar cells

We describe the use of single-wall carbon nanotube (SWNT) thin films as transparent and conducting electrodes for hole collection in poly(hexyl)thiophene-[6-6]phenyl-C61-butyric acid methyl ester (P3HT-PCBM) organic photovoltaics. We report a power conversion efficiency of 1%, with a fill factor of 0.3 and a short-circuit current of 6.5mA∕cm2 under 100mW∕cm2 polychromatic white light illumination measured in air. These values are comparatively higher than reference cells of similar thickness made on indium tin oxide (ITO) glass substrates. This is attributed to the three-dimensional nature of the interface between the SWNTs and the P3HT-PCBM nanocomposite. Our results indicate that solution processed SWNT thin films are a viable alternative to ITO for photovoltaic devices, eliminating an expensive vacuum deposition step in the fabrication of organic solar cells.

Nanomaterial-enhanced all-solid flexible zinc--carbon batteries.

Solid-state and flexible zinc carbon (or Leclanche) batteries are fabricated using a combination of functional nanostructured materials for optimum performance. Flexible carbon nanofiber mats obtained by electrospinning are used as a current collector and cathode support for the batteries. The cathode layer consists of manganese oxide particles combined with single-walled carbon nanotubes for improved conductivity. A polyethylene oxide layer containing titanium oxide nanoparticles forms the electrolyte layer, and a thin zinc foil is used as the anode. The battery is shown to retain its performance under mechanically stressed conditions. The results show that the above configuration can achieve solid-state mechanical flexibility and increased shelf life with little sacrifice in performance.

Photoelectrochemical cell using dye sensitized zinc oxide nanowires grown on carbon fibers

Zinc oxide (ZnO) nanowires (NWs) grown on carbon fibers using a vapor transport and condensation approach are used as the cathode of a photoelectrochemical cell. The carbon fibers were obtained by electrospray deposition and take the form of a flexible carbon fabric. The ZnO NW on carbon fiber anode is combined with a “black dye” photoabsorber, an electrolyte, and a platinum (Pt) counterelectrode to complete the cell. The results show that ZnO NW and carbon fibers can be used for photoinduced charge separation/charge transport and current collection, respectively, in a photoelectrochemical cell.

Effect of electroless etching parameters on the growth and reflection properties of silicon nanowires

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated over large areas using an electroless etching (EE) method, which involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. A detailed parametric study determining the relationship between nanowire morphology and time, temperature, solution concentration and starting wafer characteristics (doping type, resistivity, crystallographic orientation) is presented. The as-fabricated Si NW arrays were analyzed by field emission scanning electron microscope (FE-SEM) and a linear dependency of nanowire length to both temperature and time was obtained and the change in the growth rate of Si NWs at increased etching durations was shown. Furthermore, the effects of EE parameters on the optical reflectivity of the Si NWs were investigated in this study. Reflectivity measurements show that the 42.8% reflectivity of the starting silicon wafer drops to 1.3%, recorded for 10 µm long Si NW arrays. The remarkable decrease in optical reflectivity indicates that Si NWs have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.

Flexible organic photovoltaics from zinc oxide nanowires grown on transparent and conducting single walled carbon nanotube thin films

The fabrication of flexible organic photovoltaics (OPVs) which utilize transparent and conducting single walled carbon nanotube (SWNT) thin films as current collecting electrodes on plastic substrates in zinc oxide nanowire (ZnO NW)/poly(3-hexylthiophene) (P3HT) bulk heterojunction photovoltaic devices is reported. The bulk heterojunctions for exciton dissociation are created by directly growing ZnO nanowires from solution on the SWNT electrodes and spin coating the P3HT polymer. A maximum OPV power conversion efficiency of ∼0.6% was achieved. Our results indicate that nanotube–nanowire hybrids fabricated via solution based methods are promising for optoelectronic and energy harvesting devices.

A solid-state dye-sensitized solar cell based on a novel ionic liquid gel and ZnO nanoparticles on a flexible polymer substrate

This paper describes a new strategy to make a full solid-state, flexible, dye-sensitized solar cell (DSSC) based on novel ionic liquid gel, organic dye, ZnO nanoparticles and carbon nanotube (CNT) thin film stamped onto a polyethylene terephthalate (PET) substrate. The CNTs serve both as the charge collector and as scaffolds for the growth of ZnO nanoparticles, where the black dye molecules are anchored. It opens up the possibility of developing a continuous roll to roll processing for THE mass production of DSSCs.

Investigation of single-walled carbon nanotube growth parameters using alcohol catalytic chemical vapour deposition

A detailed parametric study of single-walled carbon nanotubes (SWNTs) synthesized in powder form and on substrates using the alcohol catalytic chemical vapour deposition (ACCVD) method is reported. As-grown SWNTs were analysed using transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy and UV-vis-NIR spectroscopy to obtain structural and electronic information. We found that nucleation and growth of SWNTs occurs within seconds after introduction of the alcohol vapour and that high quality SWNTs with a narrow diameter distribution without amorphous carbon can be grown using Co acetate catalyst doped with Fe acetate above 750 degrees C. Defective multiwalled nanotubes were observed at lower temperatures, with the optimum temperature being 850 degrees C. These and other results reported in this paper allow the basis for optimizing the ACCVD process for the synthesis of large numbers of SWNTs.

Transparent and flexible supercapacitors with single walled carbon nanotube thin film electrodes.

We describe a simple process for the fabrication of transparent and flexible, solid-state supercapacitors. Symmetric electrodes made up of binder-free single walled carbon nanotube (SWCNT) thin films were deposited onto polydimethylsiloxane substrates by vacuum filtration followed by a stamping method, and solid-state supercapacitor devices were assembled using a gel electrolyte. An optical transmittance of 82% was found for 0.02 mg of SWCNTs, and a specific capacitance of 22.2 F/g was obtained. The power density can reach to 41.5 kW · kg(-1) and shows good capacity retention (94%) upon cycling over 500 times. Fabricated supercapacitors will be relevant for the realization of transparent and flexible devices with energy storage capabilities, displays and touch screens in particular.

Silicon nanowire - poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) heterojunction solar cells

Radial heterojunctions are known to exhibit magnificent anti-reflectivity and enhanced carrier collectivity due to short carrier diffusion distances. In this work, silicon nanowire-poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) radial heterojunction solar cells are presented. Both layers of the heterojunction are fabricated using simple and cost-effective methods. Radial heterojunctions showed remarkable improvements in solar cell characteristics compared to planar heterojunctions, fabricated under the same conditions. The highest solar cell efficiency of 5.30% is obtained. The cells exhibit external quantum efficiency of 77% at 500 nm wavelength and harvest light over the entire 300-1200 nm spectral bandwidth. The effect of nanowire length on device performance is also determined.

Zinc oxide nanowire networks for macroelectronic devices

Zinc oxide (ZnO) nanowire networks have been proposed as an alternative to organic and amorphous semiconductors for plastic electronics. Although the mobility of the ZnO networks is lower than that of individual nanowires, they offer the advantages of high transparency and flexibility. A major drawback of using individual nanowires in nano or microelectronic applications is the lack of a manufacturable process to precisely assemble nanowires into small devices. The use of ZnO networks avoid this issue for relatively large area macroelectronic devices since the devices exhibit the average properties of a large number of random individual nanowires. In this work, we have deposited uniform ZnO thin films using an easy, scalable, stamping method and characterized their electronic and optoelectronic properties. We have also demonstrated the use of ZnO networks as an active material in thin film transistors where mobility values in excess of 20 cm2/Vs has been achieved. The results presented here simply reveal the potential use of inorganic nanowires for optoelectronic devices.