Pipat Ruankham

Vertically aligned ZnO nanorods doped with lithium for polymer solar cells: defect related photovoltaic properties

The nanorod arrays of ZnO incorporated with lithium atoms show specific crystallinity, photoluminescence and absorption properties, which are promising for the improvement of photovoltaic performance of hybrid solar cells based on ZnO/poly(3-hexylthiophene). Li ions can be incorporated into ZnO crystals during the hydrothermal growth of the nanorods. The presence of Li in ZnO crystal was confirmed through X-ray diffraction analysis and by the photoluminescence spectra obtained. The difference in photovoltaic properties brought about by Li doping was determined from concentrations of the precursor solution. It was determined that appropriate Li doping improves both the short circuit current density (Jsc) and open circuit voltage (Voc). The quenching of photoluminescence of Li-doped ZnO nanorods/P3HT films indicates effective charge transfer at the interface due to oxygen-enrichment of the surface, corresponding to the enhancement of Jsc. The improvement of Voc was due to the suppression of the charge injection from the electrode brought about by the increase in barrier height at the ITO/ZnO interface as the work function of the ZnO nanorods was reduced after Li ion doping. However, further substitution of Li to Zn (LiZn) leads to increased reverse current densities of minority carriers decreasing Voc after the maximum value at 5 atom% incorporation. The maximum power conversion efficiency of 0.37% was obtained at 5 atom% doping, although improvements in photovoltaic performances through Li doping were still seen up to 20 atom% doping.

Realization of Interlinked ZnO Tetrapod Networks for UV Sensor and Room-Temperature Gas Sensor.

Here, interlinked ZnO tetrapod networks (ITN-ZnO) have been realized by using microwave-assisted thermal oxidation. With this simple and fast process, a nanostructured ZnO morphology having tetrapodlike features with leg-to-leg linking is obtained. The electrical and ethanol-sensing properties related to the morphology of ITN-ZnO compared with those of other ZnO morphologies have also been investigated. It has been found that ITN-ZnO unexpectedly exhibits superior electrical and gas-sensing properties in terms of providing pathways for electron transport to the electrode. A UV sensor and a room-temperature gas sensor with improved performance are achieved. Therefore, ITN-ZnO is an attractive morphology of ZnO that is applicable for many new applications because of its novel properties. The novel properties of ITN-ZnO are beneficial for electronic, photonic, optoelectronic, and sensing applications. ITN-ZnO may provide a means to improve the devices based on ITN-ZnO.

Effects of the morphology of nanostructured ZnO and interface modification on the device configuration and charge transport of ZnO/polymer hybrid solar cells

In an organic-based solar cell, the short exciton diffusion length of organic materials requires effective donor-acceptor heterojunction at the nanoscale. In this work, hybrid inorganic/polymer solar cells based on ZnO nanostructures and poly(3-hexylthiophene) (P3HT) are constructed to study the effects of ZnO morphologies and wettability of the surface on the P3HT infiltration ability and charge transport mechanisms. The P3HT infiltrates the ZnO nanorod (NR) more remarkably than ZnO nanoparticle (NP) substrates. Although surface modification with indoline D205 dye molecules improves the wettability (viz. enlarges the contact angle) of NP surface, the P3HT infiltration distance decreases in comparison with the pristine NP case. This leads to relatively low short-circuit current density (Jsc) of the NP devices in comparison with that of the NR devices, even though the surface area of NP layers is larger than that of NR ones. Moreover, surface modification with squaraine dye onto the NR surface shows more significant improvement in Jsc than the NP case. This is due to the well-aligned morphology of the NRs, which facilitates dye modification, P3HT infiltration, and charge transport processes. These indicate that the NRs are more qualified as electron accepting substrates and transport pathway in hybrid solar cells than NPs.

Improvement of Power Conversion Efficiency in Organic Photovoltaics by Slow Cooling in Annealing Treatment

We investigated the effect of cooling rate in the annealing treatment of bulk heterojunction organic photovoltaic devices using blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the active layer. Slow cooling after thermal annealing altered the surface morphology and optical properties of the film as compared with those of fast cooling, which resulted in higher power conversion efficiency (PCE). The incident photon-to-current conversion efficiency (IPCE) was also improved in the case of slow cooling. The highest PCE, 3.4% under air mass 1.5 simulated solar illumination (100 mW/cm2), was achieved.

Improved performance of hybrid ZnO/polymer solar cell via construction of hierarchical nanostructures and surface modification of ZnO

Photovoltaic performance of hybrid ZnO/polymer solar cell is enhanced through the addition of ZnO nanoparticles (NPs) onto ZnO nanorod arrays (NRs), and surface modification with squaraine (Sq). The cluster of NPs at the end of NRs is formed during spin coating process but poly(3-hexylthiophene) (P3HT), as electron donor, is able to infiltrate into the rod-to-rod space, as confirmed by zero-loss energy-filtered transmission electron microscopy (zero-loss EFTEM) observation. Due to the increase in ZnO/P3HT interface area, the NP-coated NR device shows power conversion efficiency (PCE) of 0.49%, which is higher than that of the pristine ZnO NRs/P3HT one (0.26%). Further improvement is achieved by the adsorption of Sq onto the NP-coated ZnO NR surfaces. After the dye-modification, the P3HT infiltration is poorer than the unmodified one. This leads to the ZnO NRs/Sq/air interface, where exciton separation does not occur. However, due to the large surface area of NPs and the light absorption in near IR region of Sq, the PCE increased to 1.37% with relatively higher short circuit current density as compared with the case without the NPs and Sq.

Surface Modification of Porous Photoelectrode Using Etching Process for Efficiency Enhancement of ZnO Dye-Sensitized Solar Cells

Surface modification of porous ZnO photoelectrode using one- and two-step etching process is investigated for enhancing power conversion efficiency of ZnO dye-sensitized solar cells. ZnO films are modified by the diluted NH4OH solutions for one-step etching process and used as photoelectrode of dye-sensitized solar cells. Rough porous films are observed after one-step etching process. The fabricated cells based on the optimized one-step etched films show a significant increase in short-circuit current density. The short-circuit current density is directly changed with amount of dye adsorption, which is related to specific surface area. The etched films exhibit higher specific surface area over two times than nonetched films. Thus, the large specific surface area is the key success for increasing amount of dye adsorption. Internal electrochemical property of fabricated cells is also improved, indicating that chemical surface of ZnO films is modified in the same time. The DSSCs fabricated on two-step etched films with NH4OH and mixed acid HCl : HNO3 show the maximum power conversion efficiency of 2.26%. Moreover, fill factor is also increased due to better redox process because of the formation of fine porous structure during the etching process. Therefore, these results implied that the roles of etching processes are improving specific surface area and fine porous formation which can provide better dye adsorption and redox process for dye-sensitized solar cell application.

Influence of surface modification with D205 dye on charge dynamics of hybrid ZnO nanorods/polymer solar cells

ABSTRACT Influence of surface modification with D205 dye on charge dynamics of hybrid solar cells based on ZnO nanorods and poly(3-hexylthiophene) or P3HT was investigated. Electrochemical impedance spectroscopy analysis reveals that ZnO nanorods/D205/P3HT device shows longer effective lifetime (less charge recombination) in comparison with ZnO nanorods/P3HT one. Moreover, transit time of electrons travelling to the electrode for the D205-modified device is shorter than that of the unmodified one. These results indicate that the adsorption of D205 dye promotes efficient charge collection, leading to the improvement in fill factor. The surface modification efficiently enhances charge dynamics of hybrid ZnO/P3HT devices.

Effect of Gallium Interlayer in ZnO and Al-doped ZnO Thin Films

For applications in the field of optoelectronics, zinc oxide (ZnO) based transparent conducting oxide thin films such as aluminium-doped zinc oxide (AZO) have recently received much attention. It is one of the most promising alternative materials to the widely used indium tin oxide (ITO or tin-doped indium oxide) and fluorine-doped tin oxide (FTO). In this work, the ZnO and AZO base thin films were prepared by radio frequency (rf) magnetron sputtering technique using ZnO and AZO (1at%Al) ceramic target. The Ga interlayer films, added in between base thin films (ZnO and AZO), were deposited by evaporation technique. Both ZnO/Ga/ZnO and AZO/Ga/AZO multilayer thin film structures were grown on glass substrates. Then the obtained multilayer structures were annealed in argon (Ar) ambient at 400°C for 1 hr. The morphology, qualitative and quantitative elemental analysis, crystal structure, electrical properties, and optical properties of the multilayer thin films were characterized by field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), a four-point probe method and UV-Vis spectroscopy. The results revealed that after annealing treatment, the average transmittance of the both multilayer thin films in visible region was improved due to a decrease in surface roughness resulting in an increase in crystalline surface. Furthermore, the results revealed lower sheet resistance in the AZO/Ga/AZO relative to pure ZnO multilayer base thin films. The significance of the electrical properties of the AZO/Ga/AZO films can be achieved due to local structure of Al site in ZnO and partial diffusion of Ga atoms into the base thin film layers. This indicates that the Ga interlayer in both ZnO/Ga/ZnO and AZO/Ga/AZO multilayer thin films can be used to further improve electrical conductivity without degrading optical transmission.

Synthesis and Characterization of MgO by Microwave-Assisted Thermal Oxidation for Dye-Sensitized Solar Cells

Magnesium oxide (MgO) nanostructures were synthesized by microwave-assisted thermal oxidation at various amount of activated carbon additive. The MgO nanostructures were characterized by scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffractrometry (XRD) and UV-Visible spectroscopy, respectivly. It was observed that, the obtained MgO have nanocube shape. The MgO nanostructures were applied as a blocking layer in ZnO dye-sensitized solar cells (DSSC). The photovoltage, photocurrent, and power conversion efficiency characteristics of DSSCs were measured under illumination of simulated sunlight obtained from a solar simulator with the radiant power of 100 mW/cm2. The DSSCs with MgO layer exhibited higher current density, open circuit voltage and photoconversion efficiency than those without MgO layer The optimum power conversion efficiency (PCE) was 2.49 % with short circuit current (Jsc) of 6.61 mA/cm2, the open circuit voltage (Voc) of 0.66 V and the fill factor (FF) of 0.59, respectively.

Ag–In–Zn–S quantum dots for hybrid organic–inorganic solar cells

Quantum dots of (AgIn)xZn2(1−x)S2 (x = 0.6, 0.8, and 1.0) capped by oleylamine were prepared and applied for hybrid organic–inorganic solar cells consisting of glass–indium–tin-oxide/ZnO/(AgIn)xZn2(1−x)S2/poly(3-hexylthiophene)/MoO3/Ag. The short-circuit current density (Jsc) and open-circuit voltage (Voc) of the hybrid solar cells were measured, and we found a low power conversion efficiency (PCE) below 0.1%. From the incident photon-to-current efficiency (IPCE) profiles of the hybrid devices, there is no marked photocurrent generation from 350 to 700 nm, which is ascribed to the absorption region of (AgIn)xZn2(1−x)S2. To improve the photovoltaic performance, ligand substitution from oleylamine to pyridine was performed. The PCE of the hybrid cell using the pyridine-capped (AgIn)xZn2(1−x)S2 was improved twofold in terms of both Jsc and Voc as compared with that of the oleylamine-capped one. In particular, from the IPCE measurements, a remarkable (more than doubled) enhancement of photocurrent generation from 400 to 450 nm was observed with the pyridine-substituted nanoparticles.