Browsing by Author "Lee, Chia-Hua"

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Efficient electron transport in tetrapod-like ZnO metal-free dye-sensitized solar cells
(2009)
Chiu, Wei-Hao
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Lee, Chia-Hua
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Cheng, Hsin-Ming
;
Lin, Hsiu-Fen
;
Liao, Shih-Chieh
The tetrapod-like ZnO nanopowders are employed to construct an efficient electron transport network as the photoanode of the dye-sensitized solar cells (DSCs). Due to the high extinction coefficient of the metal-free D149 dye offering a high photocurrent through the ZnO network, the best performance of DSCs based on a 42 mu m terapod-like ZnO film showed a high energy conversion efficiency of 4.9% with a high short-circuit photocurrent density of 12.3 mA cm(-2), an open-circuit photovoltage of 0.6 V, and a fill factor of 0.65 under AM 1.5 irradiation. Highly efficient electron transport may also be ascribed by a long effective electron diffusion length of 46 mu m determined from the electrochemical impedance spectroscopy which is consistent with thickness dependent J(SC) measurements.
Enhanced performance of dye-sensitized solar cells by an Al(2)O(3) charge-recombination barrier formed by low-temperature atomic layer deposition
(2009-01-01)
Lin, Ching
;
Tsai, Feng-Yu
;
Lee, Min-Hsueh
;
Lee, Chia-Hua
;
Tien, Ta-Chang
Al(2)O(3) films are deposited conformally and uniformly by atomic layer deposition (ALD) at 150 degrees C throughout the surface of the nanoporous TiO(2) electrode of dye-sensitized solar cells (DSSCs) to serve as charge recombination barriers (CRB). The self-limiting film growth of ALD enables detailed analysis of the thickness dependence of the CRB effects on the DSSCs, revealing the optimal Al(2)O(3) CRB thickness to be that produced by 1 ALD cycle (nominal thickness - 0.1 nm), at which the CRB increases the power conversion efficiency (PCE) of the DSSCs by 14% (to up to 7.8% PCE). Above 1 cycle, the ALD films excessively raise the Fermi level of the TiO(2) electrode surface, as determined by ultraviolet photoelectron spectroscopy (UPS), so as to block electron injection at the dye-to-TiO(2) heterojunction and to cause significant degradation in the DSSC performance. Instead of its nominal 0.1 nm thickness, the 1-cycle ALD film has an effective CRB thickness of 0.3 nm because of steric hindrance among the ALD precursor molecules and the dye molecules, as analyzed by a graphical model. The optimal thickness of the ALD Al(2)O(3) CRB is considerably thinner than the 0.9-2.5 nm reported for Al(2)O(3) CRB formed by conventional sol-gel processes, but it should better reflect the true value, considering the better infiltrating capability and finer thickness resolution of the ALD films. The low temperature and fine thickness control of the ALD process will broaden the utility of CRB.
Enhanced performance of dye-sensitized solar cells by an Al2O3 charge-recombination barrier formed by low-temperature atomic layer deposition
(2009-01-01)
Lin, Ching
;
Tsai, Feng-Yu
;
Lee, Min-Hsueh
;
Lee, Chia-Hua
;
Tien, Ta-Chang
Al2O3 films are deposited conformally and uniformly by atomic layer deposition (ALD) at 150 degrees C throughout the surface of the nanoporous TiO2 electrode of dye-sensitized solar cells (DSSCs) to serve as charge recombination barriers (CRB). The self-limiting film growth of ALD enables detailed analysis of the thickness dependence of the CRB effects on the DSSCs, revealing the optimal Al2O3 CRB thickness to be that produced by 1 ALD cycle (nominal thickness - 0.1 nm), at which the CRB increases the power conversion efficiency (PCE) of the DSSCs by 14% (to up to 7.8% PCE). Above 1 cycle, the ALD films excessively raise the Fermi level of the TiO2 electrode surface, as determined by ultraviolet photoelectron spectroscopy (UPS), so as to block electron injection at the dye-to-TiO2 heterojunction and to cause significant degradation in the DSSC performance. Instead of its nominal 0.1 nm thickness, the 1-cycle ALD film has an effective CRB thickness of 0.3 nm because of steric hindrance among the ALD precursor molecules and the dye molecules, as analyzed by a graphical model. The optimal thickness of the ALD Al2O3 CRB is considerably thinner than the 0.9-2.5 nm reported for Al2O3 CRB formed by conventional sol-gel processes, but it should better reflect the true value, considering the better infiltrating capability and finer thickness resolution of the ALD films. The low temperature and fine thickness control of the ALD process will broaden the utility of CRB.
Formation of Branched ZnO Nanowires from Solvothermal Method and Dye-Sensitized Solar Cells Applications
(2008-10-01)
Cheng, Hsin-Ming
;
Chiu, Wei-Hao
;
Lee, Chia-Hua
;
Tsai, Song-Yeu
;
Hsieh, Wen-Feng
Branched ZnO nanowires have been fabricated on conductive glass substrates via a solvothermal method for dye-sensitized solar cells (DSCs). The 1D branched nanostructures can afford a direct conduction pathway instead of interparticle hops while using nanoparticles. Furthermore, the short-circuit current density and the energy conversion efficiency of the branched ZnO nanowire DSCs are 4.27 mA/cm(2) and 1.51%,which are twice as high as the bare ZnO nanowire ones. The improvement was a consequence of the enlargement of the internal surface area within the photoelectrode and allowed us to achieve higher dye adsorption to significantly enhance the performance of the DSCs.
Improved performance of flexible dye-sensitized solar cells by introducing an interfacial layer on Ti substrates
(2011)
Lee, Chia-Hua
;
Chiu, Wei-Hao
;
Lee, Kun-Mu
;
Hsieh, Wen-Feng
;
Wu, Jenn-Ming
The recombination reaction of injected electrons with triiodide ion in the electrolyte limits the efficiency of dye-sensitized solar cells (DSSCs). This study reports the preparation of a sponge-like and conformal TiO(2) underlayer by hydrogen peroxide oxidation of Ti foil. This underlayer serves as a charge recombination barrier layer at the nanocrystalline TiO(2)/substrate interface, and suppresses the recombination reaction. This sponge-like TiO(2) underlayer increases the electrical contact area between the Ti substrate and nanocrystalline TiO(2), helping nanocrystalline TiO(2) attach to the Ti substrate. This study compares the performance of DSSCs that were subjected to different Ti surface treatments. Electrochemical impedance spectroscopy results confirm that the proposed sponge-like TiO(2) underlayer increased the open-current voltage (V(OC)) and fill factor (FF) due to prolonged electron life time (tau(eff)), and minimized resistance at the TiO(2)/Ti interface (R(CT)). With an optimal thickness of nanocrystalline TiO(2) and concentration of I(2), we achieved a conversion efficiency of 6.75% for a back-illuminated DSSC.
Interfacial energy levels and related properties of atomic-layer-deposited Al(2)O(3) films on nanoporous TiO(2) electrodes of dye-sensitized solar cells
(2009-07-29)
Tien, Ta-Chang
;
Pan, Fu-Ming
;
Wang, Lih-Ping
;
Lee, Chia-Hua
;
Tung, Yung-Liang
Low-temperature (similar to 150 degrees C), atomic-layer-deposited Al(2)O(3) films on nanoporous TiO(2) electrodes of dye-sensitized solar cells (DSSCs) were investigated using electron spectroscopy. The power conversion efficiency (PCE) of the DSSCs was increased from 5.7% to 6.5%, an improvement of 14%, with one monolayer of Al(2)O(3) with a thickness of similar to 0.2 nm. The formation of Ti-O-Al(OH)(2) and interfacial dipole layers exhibited a strong influence on the work function of the Al(2)O(3) over-layers, while the thicker Al(2)O(3) over-layers caused the values of valence band maximum and band gap to approach the values associated with pure Al(2)O(3). A work function difference (Delta Phi(A-T)) of 0.4 eV and a recombination barrier height (epsilon(RB)) of 0.1 eV were associated with the highest PCE achieved by the first monolayer of the Al(2)O(3) layer. Thicker Al(2)O(3) over-layers, however, caused significant reduction of PCE with negative Delta Phi(T-A) and increased interfacial energy barrier height (*epsilon(IB)) between the N719 dyes and TiO(2) electrodes. It was concluded that the PCE of the DSSCs may correlate with Delta Phi(A-T), epsilon(RB), and *epsilon(IB) resulting from various thicknesses of the Al(2)O(3) over-layers and that interfacial reactions, such as the formation of Ti-O-Al(OH)(2) and dipole layers, play an important role in determining the interfacial energy levels required to achieve optimal performance of dye-sensitized TiO(2) solar cells.
Interfacial energy levels and related properties of atomic-layer-deposited Al2O3 films on nanoporous TiO2 electrodes of dye-sensitized solar cells
(2009-07-29)
Tien, Ta-Chang
;
Pan, Fu-Ming
;
Wang, Lih-Ping
;
Lee, Chia-Hua
;
Tung, Yung-Liang
Low-temperature (similar to 150 degrees C), atomic-layer-deposited Al2O3 films on nanoporous TiO2 electrodes of dye-sensitized solar cells (DSSCs) were investigated using electron spectroscopy. The power conversion efficiency (PCE) of the DSSCs was increased from 5.7% to 6.5%, an improvement of 14%, with one monolayer of Al2O3 with a thickness of similar to 0.2 nm. The formation of Ti-O-Al(OH)(2) and interfacial dipole layers exhibited a strong influence on the work function of the Al2O3 over-layers, while the thicker Al2O3 over-layers caused the values of valence band maximum and band gap to approach the values associated with pure Al2O3. A work function difference (Delta Phi(A-T)) of 0.4 eV and a recombination barrier height (epsilon(RB)) of 0.1 eV were associated with the highest PCE achieved by the first monolayer of the Al2O3 layer. Thicker Al2O3 over-layers, however, caused significant reduction of PCE with negative Delta Phi(T-A) and increased interfacial energy barrier height (*epsilon(IB)) between the N719 dyes and TiO2 electrodes. It was concluded that the PCE of the DSSCs may correlate with Delta Phi(A-T), epsilon(RB), and *epsilon(IB) resulting from various thicknesses of the Al2O3 over-layers and that interfacial reactions, such as the formation of Ti-O-Al(OH)(2) and dipole layers, play an important role in determining the interfacial energy levels required to achieve optimal performance of dye-sensitized TiO2 solar cells.
Photoconductive Enhancement of Single-Layer Tin Oxide-Coated ZnO Nanowires
(2010)
Lin, Chin-Ching
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Chen, Yu-Wei
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Chiang, Mei-Ching
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Lee, Chia-Hua
;
Tung, Yung-Liang
In this study, zinc oxide nanowires (ZnO NWs) coated with crystalline tin oxide (SnO(2)) by an ultrasonic spray pyrolysis were investigated. High resolution transmission electron microscopy analysis confirms that a continuous single SnO(2) layer of about 10 nm has been coated on the ZnO NWs. Photoconductivity measurements revealed that the ZnO NWs coated with the single uniform SnO(2) layer had a higher photoconductivity ratio by about a factor of 8 compared to the pristine ZnO NWs, attributed to the multi-photoexcited electrons. Under a steady radiation of UV light (lambda=325 nm), a substantial increase in the steady-state photocurrent is noticeable, suggesting that extra photoinduced charges are transferred from SnO(2) to ZnO NWs.
The influence of tetrapod-like ZnO morphology and electrolytes on energy conversion efficiency of dye-sensitized solar cells
(2010-12-01)
Lee, Chia-Hua
;
Chiu, Wei-Hao
;
Lee, Kun-Mu
;
Yen, Wen-Hsiang
;
Lin, Hsiu-Fen
Tetrapod-like ZnO nanostructures prepared by dc plasma technology were used as photoelectrodes in dye-sensitized solar cells (DSSCs) Each of the tetrapod-like ZnO possesses four extended arms that offer improved electron transport properties Tetrapod-like ZnO with short (S-ZnO) and long arms (L-ZnO) were synthesized by controlling the plasma gas flow and the input power Between these two tetrapod-like ZnO nanopowders the DSSCs using S-ZnO showed higher energy conversion efficiency than using L-ZnO This is due to the resulting increase in dye adsorption and enhanced short-circuit current density using S-ZnO Electrochemical Impedance spectroscopy (EIS) shows that the properties of electron transport of S-ZnO are superior to that of the L-ZnO We investigated the effect of the redox electrolytes (12) and the additives (Lil and TBP) on the performance of the DSSCs by intensity-modulated photovoltage spectroscopy and EIS (C) 2010 Elsevier Ltd All rights reserved