Lu-Yin Lin

Recent progress in organic sensitizers for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) are fabricated using low-cost materials and a simple fabrication process; these advantages make them attractive candidates for research on next generation solar cells. In this type of solar cell, dye-sensitized metal oxide electrodes play an important role for achieving high performance since the porous metal oxide films provide large specific surface area for dye loading and the dye molecule possesses broad absorption covering the visible region or even part of the near-infrared (NIR). Recently, metal-free sensitizers have made great progress and become the most potential alternatives. This review mainly focuses on recent progress in metal-free sensitizers for applications in DSSCs. Besides, we also briefly report DSSCs with near-infrared (NIR) organic sensitizers, which provide the possibility to extend the absorption threshold of the sensitizers in the NIR region. Finally, special consideration has been paid to panchromatic engineering, co-sensitization, a key technique to achieve whole light absorption for improving the performance of DSSCs.

A composite catalytic film of PEDOT:PSS/TiN–NPs on a flexible counter-electrode substrate for a dye-sensitized solar cell

A composite film of PEDOT:PSS/TiN–NPs, containing poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and titanium nitride nanoparticles (TiN–NPs), was deposited on a Ti foil by a doctor blade technique. Various weight percentages of TiN–NPs (5, 10, 20, 30 wt%) were used to prepare different composite films. This Ti foil with the composite film was used as the flexible counter-electrode (CE) for a dye-sensitized solar cell (DSSC). Performances of the DSSCs with the platinum-free CEs containing PEDOT:PSS/TiN–NPs with various contents of TiN–NPs were investigated. After the optimization of composition and thickness of the composite film PEDOT:PSS/TiN–NPs, a light-to-electricity conversion efficiency (η) of 6.67% was achieved for the pertinent DSSC, using our synthesized CYC-B1 dye, which was found to be higher than that of a cell with a sputtered-Pt film on its CE (6.57%). The homogeneous nature of the composite film PEDOT:PSS/TiN–NPs, the uniform distribution of TiN–NPs in its polymer matrix, and the large electrochemical surface area of the composite film are seen to be the factors for the best performance of the pertinent DSSC. Scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDX) were used to characterize the films. The high efficiency of the cell with PEDOT:PSS/TiN–NPs is explained by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and incident photon-to-current conversion efficiency (IPCE) curves.

A novel core–shell multi-walled carbon nanotube@graphene oxide nanoribbon heterostructure as a potential supercapacitor material

A novel core–shell heterostructure with multi-walled carbon nanotubes as the core and graphene oxide nanoribbons as the shell (MWCNT@GONR), fabricated by the facile unzipping of MWCNTs with the help of microwave energy, was used as a supercapacitor (SC) electrode material. Graphene nanopowder (GNP) and multi-walled carbon nanotubes (MWCNTs) have also been applied as SC materials for comparison. A smooth surface and a tube-like structure are found for the GNP and MWCNTs, respectively, while for the MWCNT@GONR material, graphene oxide sheet structures are observed on both sides of central nanotube cores that retain their tube-like structure. The specific capacitance is much better for the SC electrode with the MWCNT@GONR (252.4 F g−1) compared to the SC electrodes with commercial MWCNTs (39.7 F g−1) and GNP (19.8 F g−1), as determined using cyclic voltammetry (CV) at a scan rate of 50 mV s−1, which is due to the defective edges of the nanostructures in the former. The SC electrode with the MWCNT@GONR also exhibits good stability and capacitance retention even after 1000 cycles of galvanostatic charge–discharge testing, indicating its potential as a SC material. CV, galvanostatic charge–discharge (GC/D) and electrochemical impedance spectroscopy (EIS) were applied to analyze the SC performance.

All-solid-state dye-sensitized solar cells incorporating SWCNTs and crystal growth inhibitor

A solid organic ionic crystal, 1-ethyl-3-methylimidazolium iodide (EMII), was employed as a charge transfer intermediate (CTI) to fabricate all-solid-state dye-sensitized solar cells (DSSCs). In addition, single wall carbon nanotubes (SWCNTs) were incorporated into the CTI as the extended electron transfer materials (EETM), which can reduce charge diffusion length and serve simultaneously as catalyst for the electrochemical reduction of I3−. An all-solid-state DSSC with this hybrid SWCNT-EMII achieved a higher cell efficiency (1.88%), as compared to that containing bare EMII (0.41%). To further improve the cell efficiency, we utilized 1-methyl-3-propylimidazolium iodine (PMII), which acts simultaneously as a co-charge transfer intermediate and crystal growth inhibitor. The binary CTI (EMII mix with PMII) is in the form of solid as the weight percentage of PMII reaches 60%, at which a smoother surface morphology for the binary CTI is observed. The highest cell efficiency (3.49%) was obtained using a hybrid SWCNT-binary CTI. At-rest durability of the DSSC with the hybrid SWCNT-binary CTI was also studied and found to be far superior to that of a cell with an organic solvent electrolyte. Electrochemical impedance spectroscopy (EIS) and laser-induced photo-voltage transient were used to substantiate the results.

Solvent-Annealing-Induced Self-Organization of Poly(3-hexylthiophene), a High-Performance Electrochromic Material

We have systematically studied the self-organization of poly(3-hexylthiophene) (P3HT), an electrochromic material, upon control of the solvent evaporation rate. We characterized these polymer films using atomic force microscopy and X-ray diffraction measurements. Well-ordered P3HT structures were developed after solvent annealing; these highly crystalline structures exhibited enhanced electrochromic contrast and reduced resistance within the film, leading to larger coloration efficiencies and faster switching times. The optical contrast (Delta%T), coloration efficiency, and switching time of the P3HT films increased from 54.2%, 182.6 cm(2) C(-1), and 5.3 s, respectively, prior to solvent annealing to 64.8%, 293.5 cm(2) C(-1), and 3.2 s, respectively, after application of the solvent-annealing conditions.

Dye‐Sensitized Solar Cells with Reduced Graphene Oxide as the Counter Electrode Prepared by a Green Photothermal Reduction Process

Highly conductive reduced graphene oxide (rGO) with good electrocatalytic ability for reducing triiodide ions (I3(-)) is a promising catalyst for the counter electrode (CE) of dye-sensitized solar cells (DSSCs). However, hazardous chemical reducing agents or energy-consuming thermal treatments are required for preparing rGO from graphene oxide (GO). Therefore, it is necessary to find other effective and green reduction processes for the preparation of rGO and to fabricate rGO-based DSSCs. In this study, GO was prepared using a modified Hummers method from graphite powder, and further reduced to rGO through a photothermal reduction process (to give P-rGO). P-rGO shows better electrocatalytic ability due mainly to its high standard heterogeneous rate constant for I3(-) reduction and in part to its considerable electrochemical surface area. The corresponding DSSC shows a higher cell efficiency (η) of 7.62% than that of the cell with a GO-based CE (η=0.03%). When the low-temperature photothermal reduction process is applied to all-flexible plastic DSSCs, the DSSC with a P-rGO CE shows an η of 4.16%.

Dye-sensitized solar cells with low-cost catalytic films of polymer-loaded carbon black on their counter electrode

Dye-sensitized solar cells (DSSCs), consisting of counter electrodes (CEs) with composite films made of carbon black (CB) and conducting polymers such as polypyrrole (PPy) and polyaniline (PANI), were fabricated. A slurry was first prepared with 99 wt% of conducting polymer-loaded CB using the requisite 1 wt% of poly(oxyethylene)-segmented imide (POEM) as the dispersant in ethanol. The conducting polymer-loaded CB was a commercial product with a composition of about 80 wt% of CB and 20 wt% of conducting polymer. The POEM dispersant was required to generate a homogeneous dispersion of the carbon material in the ethanolic solution. The dispersive ability of POEM for CB was demonstrated both visually and through UV–vis absorption spectra and the dynamic light scattering method (DLS). The slurries with PPy/CB and PANI/CB were coated on FTO glasses to prepare CEs for DSSCs. The DSSCs with CEs containing PPy/CB and PANI/CB have shown power conversion efficiencies (η) of 5.85 ± 0.23% and 6.77 ± 0.13%, respectively, while the cells with CEs containing bare CB and bare platinum have shown ηs of 5.35% and 7.24 ± 0.11%, respectively. The dispersive function of POEM for CB to impart high electrocatalytic abilities on the corresponding CEs was responsible for the better performance of the cells for both PPy/CB and PANI/CB than that of the cell with bare CB. Hydrogen-1 nuclear magnetic resonance (1H-NMR), gel permeation chromatography (GPC), scanning electronic microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), incident photo-to-current conversion efficiency (IPCE) spectra, Tafel polarization plots, and rotating disk electrode measurements (RDE) were used to substantiate the explanations.

Improved exchange reaction in an ionic liquid electrolyte of a quasi-solid-state dye-sensitized solar cell by using 15-crown-5-functionalized MWCNT

Nanocomposite, 15-crown-5-functionalized multi-wall carbon nanotubes (denoted as MWCNT-15-C-5) were synthesized and used as an additive along with the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) in the electrolyte of a dye-sensitized solar cell (DSSC); the pertinent quasi-solid-state DSSC showed a far superior photovoltaic performance than that of a cell with bare EMIBF4 or with MWCNT-added EMIBF4 (MWCNT/EMIBF4). The heterocyclic structure of the crown ether, 15-C-5, provides its cavities to capture the lithium ions (Li+) in a DSSC, thereby facilitating the dissolution of Li+ and I− in the electrolyte of the cell. This further contributes to an improvement in the exchange reaction of I−/I3− in the electrolyte with EMIBF4. Consequently, the values of short-circuit current density (JSC) and power-conversion efficiency (η) of the DSSC with both EMIBF4 and MWCNT-15-C-5 in its electrolyte showed an increase from 3.23 ± 0.30 to 5.53 ± 0.38 mA cm−2 and from 1.52 ± 0.04 to 2.11 ± 0.10%, respectively, with reference to the values of a DSSC with bare EMIBF4. Moreover, the at-rest durability of this quasi-solid-state DSSC was found to be unfailing for a period of 1200 h at 100 mW cm−2 illumination. Explanations are substantiated with Raman spectra, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS).

Bacteraemia due to ciprofloxacin-resistant Salmonella enterica serotype Choleraesuis in adult patients at a university hospital in Taiwan, 1996-2004

Eighty-one adult patients with Salmonella enterica serotype Choleraesuis (S. Choleraesuis) bacteraemia treated at a university hospital from 1996 to 2004 were evaluated. Multivariate analysis with a logistic regression model was used to characterize risk factors for primary bacteraemia and mycotic aneurysm and to determine the association of clinical characteristics of patients based on ciprofloxacin susceptibility of the causative organism. The incidence per 100,000 discharges was 0.76 in 1996 and 3.9 in 2004. The overall rate of ciprofloxacin resistance among these isolates was 59% (87 isolates) and the annual rate increased with time from 0% prior to 2000 to 80% in 2004. Among these patients, 48 (59%) had primary bacteraemia and 13 (16%) had secondary bacteraemia with mycotic aneurysm. Seventy (86%) patients had fever at presentation, 22 (27%) developed shock during hospitalization, and eight (10%) died of S. Choleraesuis bacteraemia. Patients with immunocompromised conditions had a higher risk of developing primary bacteraemia (OR 18.442, P < 0.001). Hypertension (OR 15.434, P = 0.002) and male gender (OR 7.422, P = 0.039) were associated with mycotic aneurysm. Patients with mycotic aneurysm were more frequently infected with ciprofloxacin-susceptible isolates (P = 0.028) and ciprofloxacin-susceptible isolates were also more frequently associated with recurrent infection than ciprofloxacin-resistant isolates (P = 0.038). The incidence of S. Choleraesuis bacteraemia has increased in the past 8 years, and this increase is associated with the upsurge of ciprofloxacin-resistant isolates.

Self-assembled all-conjugated block copolymer as an effective hole conductor for solid-state dye-sensitized solar cells.

An all-conjugated diblock copolymer, poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-hexylthiophene) (PPP-b-P3HT), was synthesized and applied as a hole transport material (HTM) for the fabrication of solid-state dye-sensitized solar cells (ss-DSCs). This copolymer is characterized by an enhanced crystallinity, enabling its P3HT component to self-organize into interpenetrated and long-range-ordered crystalline fibrils upon spin-drying and ultimately endowing itself to have a faster hole mobility than that of the parent P3HT homopolymer. Transient photovoltage measurements indicate that the photovoltaic cell based on PPP-b-P3HT as the HTM has a longer electron lifetime than that of the reference device based on P3HT homopolymer. Moreover, comparing the two ss-DSCs in terms of the electrochemical impedance spectra reveals that the electron density in the TiO2 conduction band is substantially higher in the PPP-b-P3HT device than in the P3HT cell. Above observations suggest that the PPP block facilitates an intimate contact between the copolymer and dye molecules absorbed on the nanoporous TiO2 layer, which significantly enhances the performance of the resulting device. Consequently, the PPP-b-P3HT ss-DSC exhibits a promising power conversion efficiency of 4.65%. This study demonstrates that conjugated block copolymers can function as superior HTMs of highly efficient ss-DSCs.