Browsing by Author "Lo, Chen-Fu"

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A strategy to design highly efficient porphyrin sensitizers for dye-sensitized solar cells
(2011)
Chang, Yu-Cheng
;
Wang, Chin-Li
;
Pan, Tsung-Yu
;
Hong, Shang-Hao
;
Lan, Chi-Ming
We designed highly efficient porphyrin sensitizers with two phenyl groups at meso-positions of the macrocycle bearing two ortho-substituted long alkoxyl chains for dye-sensitized solar cells; the ortho-substituted devices exhibit significantly enhanced photovoltaic performances with the best porphyrin, LD14, showing J(SC) = 19.167 mA cm(-2), V(OC) = 0.736 V, FF = 0.711, and overall power conversion efficiency eta = 10.17%.
Design and Characterization of Novel Porphyrins with Oligo(phenylethylnyl) Links of Varied Length for Dye-Sensitized Solar Cells: Synthesis and Optical, Electrochemical, and Photovoltaic Investigation
(2009-01-15)
Lin, Ching-Yao
;
Lo, Chen-Fu
;
Luo, Liyang
;
Lu, Hsueh-Pei
;
Hung, Chen-Shiung
Novel zinc porphyrins with 1-4 pi-conjugated phenylethyinyl (PE) units (labeled PE1-PE4) as a link of controlled length were synthesized for fundamental tests and applications as a dye-serisitized solar cell (DSSC). The UV-visible spectra of the solution samples show clear absorption patterns of the PE groups in a region 300-400 nm, consistent with results calculated with density-functional theory. Cyclic voltammograms of PE1-PE4 in tetrahydrofuran show similar electrochemical potentials for each compound. Ferntosecond fluorescence up-conversion of solution samples and of porphyrin-sensitized TiO(2) films was measured with excitation at 420 or 430 nm and emission at 460, 470, 620, and 680 nm. When these porphyrins were fabricated into DSSC devices, the efficiency of power conversion of these devices decreased systematically with increasing length of the link: 2.5 +/- 0.2% (PE1), 2.0 +/- 0.1% (PE2), 0.78 +/- 0.09% (PE3), 0.25 +/- 0.02% (PE4). This great photovoltaic degradation from PE1 to PE4 is not interpretable according to the rate of electron injection independent of length; other factors, including electron transfer from the semiconductor back to the porphyrin cation or the electrolyte, must be considered to account for the observed dependence of photovoltaic performance on length.
Effects of aggregation and electron injection on photovoltaic performance of porphyrin-based solar cells with oligo(phenylethynyl) links inside TiO(2) and Al(2)O(3) nanotube arrays
(2010-01-01)
Luo, Liyang
;
Lin, Chia-Jung
;
Tsai, Chiau-Yiag
;
Wu, Hui-Ping
;
Li, Lu-Lin
Porphyrins with phenylethynyl links of varied length (PE1-PE4) were sensitized on vertically oriented, anodic titanium-oxide (ATO) nanotube arrays for application as dye-sensitized solar cells (DSSC). The efficiency of power conversion decreased systematically from the dye with a short link to the dye with a long link. We measured the efficiency of conversion of incident photons to current (IPCE), the photocurrent decay of the devices, and steady-state and time-resolved fluorescence spectra of the thin-film samples to understand how the cell performance depends on the length of the link. Measurements of femtosecond fluorescence confirmed that the efficiency of electron injection depended on length because of dye aggregation that significantly increased the rate of aggregate-induced energy transfer for porphyrins with a long link. The rate of electron injection depended on the length of the link with an attenuation factor beta similar to 0.1 angstrom(-1). Resonant energy transfer (RET) kinetics of porphyrins sensitized on anodic aluminium-oxide (AAO) nanotube arrays were performed with picosecond time-correlated single-photon counting and four molecular densities for each porphyrin. The kinetic data of PE1 and PE2 are described satisfactorily according to a Forster model, whereas those of PE3 and PE4 conform to a Dexter formula. A formation of clusters is proposed to rationalize the observed density-dependent kinetics for the RET of porphyrins on semiconductor films.
Effects of aggregation and electron injection on photovoltaic performance of porphyrin-based solar cells with oligo(phenylethynyl) links inside TiO2 and Al2O3 nanotube arrays
(2010-01-01)
Luo, Liyang
;
Lin, Chia-Jung
;
Tsai, Chiau-Yiag
;
Wu, Hui-Ping
;
Li, Lu-Lin
Porphyrins with phenylethynyl links of varied length (PE1-PE4) were sensitized on vertically oriented, anodic titanium-oxide (ATO) nanotube arrays for application as dye-sensitized solar cells (DSSC). The efficiency of power conversion decreased systematically from the dye with a short link to the dye with a long link. We measured the efficiency of conversion of incident photons to current (IPCE), the photocurrent decay of the devices, and steady-state and time-resolved fluorescence spectra of the thin-film samples to understand how the cell performance depends on the length of the link. Measurements of femtosecond fluorescence confirmed that the efficiency of electron injection depended on length because of dye aggregation that significantly increased the rate of aggregate-induced energy transfer for porphyrins with a long link. The rate of electron injection depended on the length of the link with an attenuation factor beta similar to 0.1 angstrom(-1). Resonant energy transfer (RET) kinetics of porphyrins sensitized on anodic aluminium-oxide (AAO) nanotube arrays were performed with picosecond time-correlated single-photon counting and four molecular densities for each porphyrin. The kinetic data of PE1 and PE2 are described satisfactorily according to a Forster model, whereas those of PE3 and PE4 conform to a Dexter formula. A formation of clusters is proposed to rationalize the observed density-dependent kinetics for the RET of porphyrins on semiconductor films.
Effects of potential shift and efficiency of charge collection on nanotube-based porphyrin-sensitized solar cells with conjugated links of varied length
(2010)
Luo, Liyang
;
Lin, Chia-Jung
;
Hung, Chen-Shiung
;
Lo, Chen-Fu
;
Lin, Ching-Yao
For dye-sensitized solar-cell devices fabricated from porphyrin sensitizers with links of varied length (PE1-PE4) adsorbed on anodic titanium-oxide nanotube arrays, we measured induced photocurrent and photovoltage decays under constant bias illumination; the evaluated efficiencies of charge collection of the devices show a systematic trend PE4 > PE3 > PE2 > PE1 at a large short-circuit current, implying that a long link would improve the charge separation if the electrons were effectively injected into the semiconductor.
Enhanced light harvesting with pi-conjugated cyclic aromatic hydrocarbons for porphyrin-sensitized solar cells
(2011-05-01)
Wang, Chin-Li
;
Chang, Yu-Cheng
;
Lan, Chi-Ming
;
Lo, Chen-Fu
;
Diau, Eric Wei-Guang
Zinc porphyrins in a series bearing a phenylethynyl, naphthalenylethynyl, anthracenylethynyl, phenanthrenylethynyl or pyrenylethynyl substituent, denoted LD1, LD2, LD3a, LD3p or LD4, respectively, were prepared as photosensitizers for dye-sensitized solar cells. The overall efficiencies of the corresponding devices show a trend LD4 > LD3p > LD2 > LD3a > LD1. Significantly, LD4 features J(SC)/mA cm(-2) 19.627, V(OC)/V = 0.711, and FF = 0.721, giving an efficiency eta = 10.06% of power conversion. This value is superior to that of a N719-based solar cell fabricated under similar experimental conditions. The remarkable performance of the LD4 cell is rationalized to be due to the broader and more red-shifted spectral feature that makes the IPCE spectrum to cover broadly across the entire visible region, 400-800 nm.
Femtosecond Transient Absorption of Zinc Porphyrins with Oligo(phenylethylnyl) Linkers in Solution and on TiO(2) Films
(2009-07-09)
Chang, Chih-Wei
;
Luo, Liyang
;
Chou, Chung-Kung
;
Lo, Chen-Fu
;
Lin, Ching-Yao
We measured femtosecond transient absorption of dye solutions and TiO(2) films sensitized with two zinc porphyrins (PE1 and PE4) to investigate the interfacial dynamics of electron transfer in relation to the dependence of cell performance on the length of the linker (Lin, C.-Y.; Lo, C.-F.; Luo, L.; Lu, H.-P.; Hung, C.-S.; Diau, E. W.-G. J. Phys. Chem. C 2009, 113, 755-764). For both porphyrins adsorbed on TiO(2) films with S(1) excitation (lambda(ex) = 620 nm), the transient absorption kinetics probed at 630 nm and 4.9 mu m are complementary to each other because only the excited-state and ground-state species were observed at 630 nm whereas only the charge-separation intermediates were observed at 4.9 mu m. The transient of the PE1/TiO(2) film shows a single exponential decay with a large offset, whereas the transient of the PE4/TiO(2) film displays a biexponential decay with a small offset. Because of the existence of an additional ps component, the amount of free delocalized electrons (offset signal) in PE4/TiO(2) is smaller than that in PEI/TiO(2). Our results imply that back electron transfer at the TiO(2)/dye interface is an important factor to be considered to account for the cell performance of a device.
Femtosecond Transient Absorption of Zinc Porphyrins with Oligo(phenylethylnyl) Linkers in Solution and on TiO2 Films
(2009-07-09)
Chang, Chih-Wei
;
Luo, Liyang
;
Chou, Chung-Kung
;
Lo, Chen-Fu
;
Lin, Ching-Yao
We measured femtosecond transient absorption of dye solutions and TiO2 films sensitized with two zinc porphyrins (PE1 and PE4) to investigate the interfacial dynamics of electron transfer in relation to the dependence of cell performance on the length of the linker (Lin, C.-Y.; Lo, C.-F.; Luo, L.; Lu, H.-P.; Hung, C.-S.; Diau, E. W.-G. J. Phys. Chem. C 2009, 113, 755-764). For both porphyrins adsorbed on TiO2 films with S-1 excitation (lambda(ex) = 620 nm), the transient absorption kinetics probed at 630 nm and 4.9 mu m are complementary to each other because only the excited-state and ground-state species were observed at 630 nm whereas only the charge-separation intermediates were observed at 4.9 mu m. The transient of the PE1/TiO2 film shows a single exponential decay with a large offset, whereas the transient of the PE4/TiO2 film displays a biexponential decay with a small offset. Because of the existence of an additional ps component, the amount of free delocalized electrons (offset signal) in PE4/TiO2 is smaller than that in PEI/TiO2. Our results imply that back electron transfer at the TiO2/dye interface is an important factor to be considered to account for the cell performance of a device.
Photoactivation Studies of Zinc Porphyrin-Myoglobin System and Its Application for Light-Chemical Energy Conversion
(2011)
Chang, Chin-Hao
;
Hu, Yi-Ting
;
Lo, Chen-Fu
;
Luo, Liyang
;
Lin, Hung-Ming
An artificial zinc porphyrin-myoglobin- based photo-chemical energy conversion system, consisting of ZnPP-Mb or ZnPE1-Mb as a photosensitizer, NADP(+) as an electron acceptor, and triethanolamine as an electron donor, has been constructed to mimic photosystem I. The photoirradiated product is able to reduce a single-electron acceptor protein cytochrome c, but cannot catalyze the two-electron reduction of acetaldehyde by alcohol dehydrogenase, thus demonstrating a single electron transfer mechanism. Furthermore, the artificial system can bifunctionally promote oxidoredox reactions, depending on the presence or absence of a sacrificial electron donor, thus suggesting its potential application in electrochemical regeneration steps involved in chemical transformation and/or energy conversion.
Preparation and Photovoltaic Characterization of Free-Base and Metallo Carboxyphenylethynyl Porphyrins for Dye-Sensitized Solar Cells
(2010-10-01)
Lin, Ching-Yao
;
Lo, Chen-Fu
;
Hsieh, Min-Hsu
;
Hsu, Shun-Ju
;
Lu, Hsueh-Pei
Free-base, manganese, iron, cobalt, nickel, copper and zinc 5-(4-carboxy-phenylethyny1)-10,20-biphenylporphines are synthesized to study the impact of central metal ions to the photovoltaic performance of the solar cells.
Preparation and Spectral, Electrochemical, and Photovoltaic Properties of Acene-Modified Zinc Porphyrins
(2010-01-14)
Lin, Ching-Yao
;
Wang, Yu-Chien
;
Hsu, Shun-Ju
;
Lo, Chen-Fu
;
Diau, Eric Wei-Guang
A series of acene-modified zinc porphyrins (benzene to pentacene, denoted as LAC-1 to LAC-5) were prepared to study their absorption spectra, electrochemical properties, and photovoltaic properties. For the absorption spectral changes in THF, porphyrin B bands are red-shifted and broadened from 449 to 501 nm for LAC-I to LAC-3, showing the effect of additional pi-conjugation. In contrast, the B bands of LAC-4 and LAC-5 are blue-shifted. In addition, the tetracenyl group of LAC-4 gives rise to absorption bands in between B and Q bands. On the other hand, the Q bands of LAC-1 to LAC-5 are systematically broadened and red-shifted from 629 to 751 nm. By comparison, the absorption bands of LAC porphyrins on TiO(2) films are broadened and slightly shifted. Fluorescence emission maxima of LAC porphyrins in THF are also systematically red-shifted from LAC-1 to LAC-5. Cyclic voltammetry experiments in THF/TBAP show that the first reductions are systematically positive-shifted from -1.16 to -0.85 V vs SCE for LAC-1 to LAC-5, indicating the effect of increasing pi-conjugation. As for the performance of DSSCs using LAC porphyrins, the overall efficiency are LAC-1 (2.95%), LAC-2 (3.31%), LAC-3 (5.44%), LAC-4 (2.82%), and LAC-5 (0.10%). Overall efficiency of a LAC-3-sensitized solar cell is nearly twice of that of a LAC-1-sensitized solar cell and is about 81% overall efficiency of N719-sensitized solar cells under the same experimental conditions. The conversion efficiency of incident photons to current (IPCE) experiments shows that the broadened absorption bands of LAC-3 effectively minimizes the gap between B and Q bands, contributing to the improved DSSC performance. The very poor performance of LAC-5 is suggested to be caused by rapid nonradiative relaxation of the molecule in the singlet excited state.
Tuning Spectral and Electrochemical Properties of Porphyrin-Sensitized Solar Cells
(2010-07-15)
Lo, Chen-Fu
;
Hsu, Shun-Ju
;
Wang, Chin-Li
;
Cheng, Yi-Hui
;
Lu, Hsueh-Pei
Phenylethynyl-substituted porphyrin (PEI) sensitizers bearing a nitro, cyano, methoxy, or dimethylamino phenylethynyl substituent were prepared to examine the electron-donating or -withdrawing effects of dyes on the photovoltaic performance of the corresponding dye-sensitized solar cells. The overall efficiencies of power conversion of the devices show a systematic trend Me(2)N-PE1 > MeO-PE1 > CN-PE1 > NO(2)-PE1, for which Me(2)N-PE1 has a device performance about 90% of that of a N719-sensitized solar cell under the same experimental conditions. The superior performance of Me(2)N-PE1 is attributed to the effective electron-donating property of the dye that exhibits broadened and red-shifted spectral features. According to frontier orbitals based on DFT calculations, the electrons are effectively injected from the dye to TiO(2) for Me(2)N-PE1 and MeO-PE1 upon excitation, but that driving force reverses for NO(2)-PE1. Electrochemical tests indicate that both LUMO and HOMO levels show a systematic trend Me(2)N-PE1 > MeO-PE1 > CN-PE1 > NO(2)-PE1, consistent with the trend of variation of the short-circuit currents in this series of sensitizers.