Hsin-Wei Chen

CoS Acicular Nanorod Arrays for the Counter Electrode of an Efficient Dye-Sensitized Solar Cell

One-dimensional cobalt sulfide (CoS) acicular nanorod arrays (ANRAs) were obtained on a fluorine-doped tin oxide (FTO) substrate by a two-step approach. First, Co(3)O(4) ANRAs were synthesized, and then they were converted to CoS ANRAs for various periods. The compositions of the films obtained after various conversion periods were verified by X-ray diffraction, UV-visible spectrophotometry, and X-ray photoelectron spectroscopy; their morphologies were examined at different periods by scanning electron microscopic and transmission electron microscopic images. Electrocatalytic abilities of the films toward I(-)/I(3)(-) were verified through cyclic voltammetry (CV) and Tafel polarization curves. Long-term stability of the films in I(-)/I(3)(-) electrolyte was studied by CV. The FTO substrates with CoS ANRAs were used as the counter electrodes for dye-sensitized solar cells; a maximum power conversion efficiency of 7.67% was achieved for a cell with CoS ANRAs, under 100 mW/cm(2), which is nearly the same as that of a cell with a sputtered Pt counter electrode (7.70%). Electrochemical impedance spectroscopy was used to substantiate the photovoltaic parameters.

Emergence of Hysteresis and Transient Ferroelectric Response in Organo-Lead Halide Perovskite Solar Cells.

Although there has been rapid progress in the efficiency of perovskite-based solar cells, hysteresis in the current-voltage performance is not yet completely understood. Owing to its complex structure, it is not easy to attribute the hysteretic behavior to any one of different components, such as the bulk of the perovskite or different heterojunction interfaces. Among organo-lead halide perovskites, methylammonium lead iodide perovskite (CH3NH3PbI3) is known to have a ferroelectric property. The present investigation reveals a strong correlation between transient ferroelectric polarization of CH3NH3PbI3 induced by an external bias in the dark and hysteresis enhancement in photovoltaic characteristics. Our results demonstrate that the reverse bias poling (-0.3 to -1.1 V) of CH3NH3PbI3 photovoltaic layers prior to the photocurrent-voltage measurement generates stronger hysteresis whose extent changes significantly by the cell architecture. The phenomenon is interpreted as the effect of remanent polarization in the perovskite film on the photocurrent, which is most enhanced in planar perovskite structures without mesoporous scaffolds.

Highly efficient dye-sensitized solar cell with a ZnO nanosheet-based photoanode

Zinc oxide (ZnO) nanosheets (ZnO-NS) were prepared for the photoanode of a dye-sensitized solar cell (DSSC), first by directly growing layered hydroxide zinc carbonate (LHZC) on an FTO substrate using a chemical bath deposition (CBD) method and then by transforming the LHZC into ZnO through pyrolysis at 300 °C. A light-to-electricity conversion efficiency (η) of 6.06% was achieved for the DSSC with ZnO-NS as its photoanode, under 100 mW cm−2 illumination, and this (η) was found to be much higher than that of the DSSC with ZnO nanoparticles (ZnO-NP) as the photoanode (2.92%). The far superior performance of the DSSC with ZnO-NS is essentially attributed to (i) higher effective electron diffusion coefficient of ZnO-NS (3.59 × 10−3 cm2s−1) than that of ZnO-NP (1.12 × 10−3 cm2s−1), and to (ii) higher dye loading on ZnO-NS (2.66 × 10−7 mol cm−2) than that on ZnO-NP (1.99 × 10−7 mol cm−2); this higher electron diffusion coefficient and dye-loading are attributed to the specific morphology of the ZnO-NS. A further improvement in the efficiency of the DSSC with ZnO-NS could be achieved through the electrophoretic deposition (EPD) of a very thin layer (3 μm) of titanium dioxide nanoparticles (TiO2-NPs of average size 14 nm) onto the ZnO-NS layer (12 μm). Notwithstanding a decrease in the effective electron diffusion coefficient (3.07 × 10−3 cm2s−1) in the TiO2-NP/ZnO-NS film, with reference to that in the ZnO-NS film (3.59 × 10−3 cm2s−1), a far higher cell efficiency was obtained in favor of the cell with TiO2-NP/ZnO-NS (7.07%), compared to that of the cell with bare ZnO-NS (6.06%); this enhancement in the η of the cell with TiO2-NP/ZnO-NS is ascribed to an increased dye-loading in favor of its cell (3.92 × 10−7 mol cm−2), with reference to that in the case of the cell with bare ZnO-NS (2.66 × 10−7 mol cm−2). As against the common ruthenium dyes, such as N3 and N-719, a metal-free dye, coded as D149, was used in this research. The efficiency achieved for the best DSSC in this work is the highest ever reported for a DSSC with ZnO as the main semiconductor material.

The Interface between FTO and the TiO2 Compact Layer Can Be One of the Origins to Hysteresis in Planar Heterojunction Perovskite Solar Cells.

Organometal halide perovskite solar cells have shown rapid rise in power conversion efficiency, and therefore, they have gained enormous attention in the past few years. However, hysteretic photovoltaic characteristics, found in these solid-state devices, have been a major problem. Although it is being proposed that the ferroelectric property of perovskite causes hysteresis in the device, we observed hysteresis in a device made of nonferroelectric PbI2 as a light absorber. This result evidently supports the fact that ferroelectric property cannot be the sole reason for hysteresis. The present study investigates the roles of some key interfaces in a planar heterojunction perovskite (CH3NH3PbI(3-x)Cl(x)) solar cell that can potentially cause hysteresis. The results confirm that the interface between fluorine doped tin oxide (FTO) substrate and the TiO2 compact layer has a definite contribution to hysteresis. Although this interface is one of the origins to hysteresis, we think that other interfaces, especially the interface of the TiO2 compact layer with perovskite, can also play major roles. Nevertheless, the results indicate that hysteresis in such devices can be reduced/eliminated by changing the interlayer between FTO and perovskite.

A novel polymer gel electrolyte for highly efficient dye-sensitized solar cells

A structurally interconnected block copolymer was facilely prepared by the oligomerization of poly(oxyethylene)-segmented diamine and 4,4′-oxydiphthalic anhydride, followed by a late-stage curing to generate amide-imide cross-linked gels. The gel structure, with multiple functionalities including poly(oxyethylene) segments, amido-acid linkers, amine termini, and amide cross-linker was characterized by Fourier transform infrared spectroscopy. The gel-like copolymer was used to absorb a liquid electrolyte; formation of 3D interconnected nanochannels, as could be observed by field emission scanning electronic microscopy has confirmed this absorption of the liquid electrolyte by the copolymer. This elastomeric copolymer was used as the matrix of a polymer gel electrolyte (PGE) for a dye-sensitized solar cell (DSSC), which shows extremely high photovoltaic performance (soaking for 1 h in the electrolyte). In particular, the PGE containing 76.8 wt% of the liquid electrolyte renders a power conversion efficiency of 9.48% for its DSSC, with a short-circuit photocurrent density of 19.50 mA cm−2, an open-circuit voltage of 0.76 V, and a fill factor of 0.64. The outstanding performance of the gel-state DSSC, superior to that (8.84%) of the DSSC with the liquid electrolyte, is mainly ascribed to the suppression of the back electron transfer through the PGE. Electrochemical impedance spectra, and dark current measurements were used to substantiate the explanations of the photovoltaic parameters.

A highly efficient dye-sensitized solar cell with a platinum nanoflowers counter electrode

This study applied the pulse reversal electrodeposition (PRE) technique to deposit a platinum film having a nanoflowers (PtNFs) structure onto an indium tin oxide (ITO) glass. The physical characteristics and electro-catalytic abilities of the PtNF-CEs were analyzed by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) patterns, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). A double layer theory and a crystal facet formation mechanism are used to explain the catalytic abilities of the PtNFs. Scanning electron microscopy (SEM) images depict a dramatic transformation in the surface structure of the Pt clusters. The ITO glass with the PtNFs was used as the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The DSSC assembled with the as-prepared PtNF-CE exhibits a high power conversion efficiency (η) of 7.74%, while the cell with an additional thin (2 nm) sputtered layer of platinum on the PtNF film shows much higher η of 8.13%, both at 1 sun conditions. The performances of the DSSCs are further substantiated by the data from electrochemical impedance spectroscopy (EIS) and UV-Vis reflectance spectra.

Fabrication and characterization of plastic-based flexible dye-sensitized solar cells consisting of crystalline mesoporous titania nanoparticles as photoanodes

We fabricated a highly efficient (with a solar-to-electricity conversion efficiency (η) of 5.51%), plastic-based, flexible dye-sensitized solar cell (DSSC). The photoanode was made of a crystalline mesoporous TiO2 film using a low-temperature electrophoretic deposition (EPD) process and compression treatment. The crystalline mesoporous TiO2 film was composed of secondary mesoporous TiO2 nanoparticles (MTNs, ca. 260 nm in size), synthesized by an aggregation of primary TiO2 nanocrystallites. In contrast to commercial TiO2 nanoparticles (i.e., P90, ca. 15.9 nm in size) that are widely used in DSSCs, the synthesized MTN-based film exhibited a higher surface area and porosity that increased dye adsorption, promoted effective electron transport, and enhanced light scattering, as evidenced by analysis of reflectance and absorbance spectra, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). We also optimized the compression pressure for MTN- and P90-based DSSCs in order to achieve maximum efficiency. We further systematically investigated the cause for the enhanced conversion efficiency of MTN-based DSSCs by measuring incident photon-to-electron conversion efficiency (IPCE) curves and electrochemical impedance spectra (EIS). IPCE results explained the increase in the short-circuit photocurrent density (JSC) for MTN-based DSSCs, and EIS results indicated that MTN-based DSSCs exhibited a larger diffusion coefficient (Deff), longer effective diffusion length (Ln), longer electron lifetime (τe), and lower charge transfer resistance (Rk), resulting in a higher power conversion efficiency. The MTN-based DSSC fabricated in the study showed great potential for application in plastic-based DSSCs using room temperature procedures.

The mechanism of toluene-assisted crystallization of organic–inorganic perovskites for highly efficient solar cells

We investigate the influence of solvent drenching in hybrid organic–inorganic perovskite (CH3NH3PbX) crystallization process with a non-solvent, toluene, during film fabrication process. We use three different precursor compositions, CH3NH3I (MAI):PbI2, 3MAI:PbI2 and 3MAI:PbCl2 to unravel the crystallization mechanism with toluene drenching. The mixed halide precursor (3MAI:PbCl2) results in the highest quality films with the toluene treatment, including high surface coverage, large grains, long PL lifetimes and high photoluminescence quantum efficiency (PLQE). The neat halide-based precursors (MAI:PbI2 and 3MAI:PbI2) with the treatment have increased photo-physical properties (PL lifetime and PLQE) and a surface coverage, but slightly decrease grain size in the film, while the 3MAI:PbI2 precursor has still formed numerous pinholes in the film. Mechanistically, we visually observe that the toluene drenching accelerates the nucleation at early stage of crystallization in 3MAI:PbCl2 precursor. X-ray diffraction pattern in this stage confirms the formation of both MAPbI3 and MAPbCl3, nucleation. During the crystallization process MAPbCl3 is transformed into MAPbI3 phase by the anion exchange. Toluene treatment strongly affects the ratio of MAPbI3 and MAPbCl3, nucleation and hence plays a critical role in deciding the final film morphology, their optoelectronic properties and hence their device performances.

Planar Heterojunction Perovskite Solar Cells Incorporating Metal–Organic Framework Nanocrystals

Zr-based porphyrin metal-organic framework (MOF-525) nanocrystals with a crystal size of about 140 nm are synthesized and incorporated into perovskite solar cells. The morphology and crystallinity of the perovskite thin film are enhanced since the micropores of MOF-525 allow the crystallization of perovskite to occur inside; this observation results in a higher cell efficiency of the obtained MOF/perovskite solar cell.

Plastic based dye-sensitized solar cells using Co9S8 acicular nanotube arrays as the counter electrode

Novel one-dimensional (1D) Co9S8 acicular nanotube arrays (ANTAs) are fabricated on a conducting plastic substrate by a two-step approach. Layered cobalt carbonate hydroxide (Co(CO3)0.5(OH)x·11H2O, LCCH) acicular nanorod arrays (ANRAs) are fabricated on a conducting plastic substrate by chemical bath deposition (CBD), followed by a simple ionic-exchange process to convert the LCCH ANRAs into Co9S8 ANTAs. The compositions of the films are verified by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) mapping; their morphologies are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The films of Co9S8 ANTAs, obtained after various periods of the CBD, are analyzed by cyclic voltammetry (CV) measurements and Tafel polarization curves, and the Co9S8 ANTAs obtained from 150 min of CBD show higher electrocatalytic ability towards the I−/I3− reaction than sputtered Pt. In addition, the long-term stability of the Co9S8 ANTAs film in I−/I3− electrolyte was tested by CV. The plastic based Co9S8 ANTAs electrodes are used as the counter electrodes (CEs) of flexible dye-sensitized solar cells (DSSCs), and a high power conversion efficiency of 5.47% is achieved, which is comparable to that of the DSSC using sputtered Pt (5.62%). Therefore, the Co9S8 ANTAs are proposed to be a reliable material to replace Pt as plastic based CEs of DSSCs.