Browsing by Author "Chung, Chih-Chun"

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A perspective of mesoscopic solar cells based on metal chalcogenide quantum dots and organometal-halide perovskites
(2013-10-01)
Rhee, Jae Hui
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Chung, Chih-Chun
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Diau, Eric Wei-Guang
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應用化學系
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應用化學系分子科學碩博班
We review two types of inorganic nanomaterials-metal chalcogenide quantum dots (QDs) and lead halide perovskites-that serve as prospective light harvesters in hybrid mesoscopic solar cells. Metal chalcogenide QDs are introduced in three parts: chalcogenides of cadmium (CdS, CdSe and CdTe), chalcogenides of lead (PbS and PbSe) and chalcogenides of antimony (Sb2S3 and Sb2Se3). The devices made using these chalcogenide QDs in a liquid-type electrolyte showed the best cell efficiencies, ranging from 3 to 6%. For solid-state QD-sensitized solar cells (QDSCs), the device performances were generally poor; only devices made of Sb2S3 and PbS QDs attained cell efficiencies approaching similar to 7%. In contrast, nanocrystalline lead halide perovskites have emerged since 2009 as potential photosensitizers in liquid-type sensitized TiO2 solar cells. In 2012, the efficiencies of the all-solid-state perovskite solar cells were enhanced to 9.7 and 10.9% using anodes of TiO2 and Al2O3 films, respectively, with 2,20,7,70-tetrakis-(N, N-di-p-methoxyphenylamine) 9,9'-spirobifluorene (spiro-OMeTAD) as a holetransporting material. In 2013, the performance of a TiO2 solar cell sensitized with lead iodide perovskite (CH3NH3PbI3) was optimized further to attain an overall power conversion efficiency eta = 15%, which is a new milestone for solar cells of this type having a device structure similar to that of a dye-sensitized solar cell.
Anomalous Charge-Extraction Behavior for Graphene-Oxide (GO) and Reduced Graphene-Oxide (rGO) Films as Efficient p-Contact Layers for High-Performance Perovskite Solar Cells
(2018-01-25)
Jokar, Efat
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Huang, Zhong Yi
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Narra, Sudhakar
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Wang, Chi-Yung
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Kattoor, Vidya
Reduced graphene oxides (rGO) are synthesized via reduction of GO with reducing agents as a hole-extraction layer for high-performance inverted planar heterojunction perovskite solar cells. The best efficiencies of power conversion (PCE) of these rGO cells exceed 16%, much greater than those made of GO and poly(3,4-ethenedioxythiophene):poly(styrenesulfonate) films. A flexible rGO device shows PCE 13.8% and maintains 70% of its initial performance over 150 bending cycles. It is found that the hole-extraction period is much smaller for the GO/methylammonium lead-iodide perovskite (PSK) film than for the other rGO/PSK films, which contradicts their device performances. Photoluminescence and transient photoelectric decays are measured and control experiments are performed to prove that the reduction of the oxygen-containing groups in GO significantly decreases the ability of hole extraction from PSK to rGO and also retards the charge recombination at the rGO/PSK interface. When the hole injection from PSK to GO occurs rapidly, hole propagation from GO to the indium-doped tin oxide (ITO) substrate becomes a bottleneck to overcome, which leads to a rapid charge recombination that decreases the performance of the GO device relative to the rGO device.
Fabrication of large-scale single-crystal bismuth telluride (Bi2Te3) nanosheet arrays by a single-step electrolysis process
(2014)
Tsai, Hung-Wei
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Wang, Tsang-Hsiu
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Chan, Tsung-Cheng
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Chen, Pei-Ju
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Chung, Chih-Chun
Nanolizing of thermoelectric materials is one approach to reduce the thermal conductivity and hence enhance the figure of merit. Bismuth telluride (Bi2Te3)-based materials have excellent figure of merit at room temperature. For device applications, precise control and rapid fabrication for the nanostructure of thermoelectric materials are essential issues. In the present study, we demonstrate a one-step electrolysis process to directly form Bi2Te3 nanosheet arrays (NSAs) on the surface of bulk Bi2Te3 with controllable spacing distance and depth by tuning the applied bias and duration. The single sheet of NSAs reveals that the average thickness and electrical resistivity of single crystalline Bi2Te3 in composition are 399.8 nm and 137.34 mu Omega m, respectively. The formation mechanism of NSAs has been proposed. A 1.12% efficiency of quantum dot-sensitized solar cells with Bi2Te3 NSAs for counter electrode has been demonstrated, indicating that Bi2Te3 NSAs from top-down processing with a high ratio of surface area to volume are a promising candidate for possible applications such as thermoelectrics, dye-sensitized solar cells (DSSCs), and lithium-ion batteries.
Femtosecond Excitonic Relaxation Dynamics of Perovskite on Mesoporous Films of Al2O3 and NiO Nanoparticles
(2014-08-25)
Hsu, Hung-Yu
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Wang, Chi-Yung
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Fathi, Amir
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Shiu, Jia-Wei
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Chung, Chih-Chun
The excitonic relaxation dynamics of perovskite adsorbed on mesoporous thin films of Al2O3 and NiO upon excitation at 450 nm were investigated with femtosecond optical gating of photoluminescence (PL) via up-conversion. The temporal profiles of emission observed in spectral region 670-810 nm were described satisfactorily with a composite consecutive kinetic model and three transient components representing one hot and two cold excitonic relaxations. All observed relaxation dynamics depend on the emission wavelength, showing a systematic time-amplitude correlation for all three components. When the NiO film was employed, we observed an extent of relaxation proceeding through the non-emissive surface state larger than through the direct electronic relaxation channel, which quenches the PL intensity more effectively than on the Al2O3 film. We conclude that perovskite is an effective hole carrier in a p-type electrode for NiO-based perovskite solar cells showing great performance.
Inverted planar solar cells based on perovskite/graphene oxide hybrid composites
(2017-07-21)
Chung, Chih-Chun
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Narra, Sudhakar
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Jokar, Efat
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Wu, Hui-Ping
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Diau, Eric Wei-Guang
Mixing graphene oxide (GO) with CH3NH3PbI3 (PSK) in varied proportions (0.025, 0.05 and 0.075 mg mL(-1)) and using GO nanosheets as a p-type electrode for devices with a layer-by-layer thin-film configuration ITO/GO/PSK: GO/PCBM/Ag produced hybrid composite perovskite solar cells. The efficiency of power conversion (PCE) of the device prepared with GO in PSK solution (0.05 mg mL(-1)) attained 15.2%, which is greater than that of a conventional planar heterojunction device (PCE 12.3%) fabricated using pristine PSK. In the homogeneous PSK: GO active layers, PSK acted as a hole donor and GO as a hole acceptor. The optimum GO concentration (0.05 mg mL(-1)) increased the charge mobility (investigated using Hall effect measurements), enhanced the charge separation (investigated using photoluminescence decays) and retarded the charge recombination (investigated using electrochemical impedance spectra), consistent with the trend of corresponding device performances. Our results thus indicate that the hybrid PSK: GO layer increased the interfacial contact between the donor and acceptor of holes to balance the charge mobility and improved the photovoltaic performance with excellent reproducibility and stability.
Phosphonic Acid and Lithium Salt as Effective p-Dopants to Oxidize Spiro-OMeTAD for Mesoscopic Sb2S3 Solar Cells
(2017-08-31)
Chung, Chih-Chun
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Tsai, Ting -Wei
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Wu, Hui -Ping
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Diau, Eric Wei-Guang
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應用化學系
1-Decyl phosphonic acid (DPA) proved an effective p-doping additive to oxidize spiro-OMeTAD as a hole-transporting material (HTM) for nanostructured Sb2S3 extremely thin absorber (ETA) solar cells. DPA had the effect of enhancing the hole conductivities of spiro-OMeTAD in the presence of lithium bis(trifluoromethyl- sulfonyl)-imide (LiTFSI) to significantly improve the device performance. Transient photoelectric measurements and electrochemical impedance spectra of devices fabricated with varied additive conditions provided information about the charge- transport and recombination kinetics and hole conductivities for Sb2S3 devices with additives DPA and LiTFSI that exhibited superior performance. Thirty-five identical devices were fabricated to show the excellent reproducibility and long-term stability in air; the best Sb2S3 ETA device attained efficiency of power conversion 6.0%, which is twice that of a device using other typical additives (LiTFSI and 4-tert-butyl-pyridine) for spiro-OMeTAD serving as HTM.
Simultaneous Observation of an Intraband Transition and Distinct Transient Species in the Infrared Region for Perovskite Solar Cells
(2016-07-07)
Narra, Sudhakar
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Chung, Chih-Chun
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Diau, Eric Wei-Guang
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Shigeto, Shinsuke
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應用化學系
Solar cells based on organometal-halide perovskites such as CH3NH3PbI3 have emerged as a promising next generation photovoltaic system, but the underlying photo physics and photochemistry remain to be established because of the limited availability of methods to implement the simultaneous and direct measurement of various charge carriers and ions that play a crucial role in the operating device. We used nanosecond time-resolved infrared (IR) spectroscopy to investigate, with high molecular specificity, distinct transient species that are formed in perovskite solar cells after photoexcitation. In CH3NH3PbI3 planar-heterojuction solar cells, we simultaneously observed infrared spectral signatures that are associated with an intraband transition of conduction-band electrons, Fano resonance, and the spiro-OMeTAD cation having an exceptionally short lifetime of 1.0 mu s (at similar to 4485 cm(-1)). The present results show that the time resolved IR method offers a unique capability to elucidate these important transients in perovskite solar cells and their dynamic interplay in a comprehensive manner.
Well-Organized Mesoporous TiO2 Photoanode by Using Amphiphilic Graft Copolymer for Efficient Perovskite Solar Cells
(2016-05-12)
Chung, Chih-Chun
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Lee, Chang Soo
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Jokar, Efat
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Kim, Jong Hak
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Diau, Eric Wei-Guang
The control of the thickness and porosity of a mesoporous TiO2 layer is important to improve the photovoltaic performance of perovskite solar cells. We produced organized mesoporous TiO2 (om-TiO2) layers using a low-cost amphiphilic graft copolymer, poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM), as a sacrificial template. This simple but effective synthetic approach generates highly mesoporous and well-organized TiO2 nanostructures with interconnected and size-tunable features. Specifically, the average pore size increased with the amount of hydrophobic PVC main chain in the graft copolymer, which acted as the pore forming agent. Perovskite layers were prepared on top of an om-TiO2 layer according to a two-step sequential deposition: after coating the PbI2, solution in dimethylformamide (DMF) on an om-TiO2 substrate, the substrate was prewetted in isopropyl alcohol (IPA) solvent before immersing into a CH3NH3I/IPA solution. This prewetting treatment not only improves the yields of conversion from PbI2, to CH3NH3PbI3, but also increases the size of perovskite crystals with cuboid morphology. On varying the pore size and the film thickness of the om-TiO2 layer, the device performance attained 11.9% of power conversion efficiency (PCE) at pore size 70 nm and film thickness 300 nm. We measured extracted charge densities and decays of transient photovoltage to understand the kinetics of charge recombination in relation to the corresponding device performance.