Hung-Yu Wei

Highly conductive PEDOT:PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells

We proposed a simple yet robust film treatment method with methanol having only one hydroxyl group to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by four orders of magnitude. Different methods of film treatment: immersing PEDOT:PSS film in the methanol solution; dropping methanol on the film; and a combination of these are employed and the results are compared. The conductivity of PEDOT:PSS films was enhanced from 0.3 S cm−1 to 1362 S cm−1 after film treatment with methanol. Other alcohols like ethanol and propanol were also used to treat the PEDOT:PSS film and showed inferior conductivity enhancement compared to methanol. The conductivity enhancement was greatly affected by the hydrophilicity and dielectric constant of the alcohols used. The mechanism of conductivity enhancement was investigated through various characterization techniques including FTIR, XPS and AFM. Removal of the insulator PSS from the film, and morphology and conformational changes are the mechanisms for the conductivity enhancement. The treated films also showed high transmittance and low sheet resistance desirable for a standalone electrode. ITO-free polymer solar cells were fabricated using PEDOT:PSS electrodes treated with methanol and showed almost equal performance to ITO electrodes.

Synthesis and applications of novel low bandgap star-burst molecules containing a triphenylamine core and dialkylated diketopyrrolopyrrole arms for organic photovoltaics

In this study, we used facile synthetic routes to construct two well-defined starburst donor/acceptor conjugated small molecules with broad absorption features; in TPAKP-2 and TPAKP-3, triphenylamine (TPA) moieties served as electron donor core units and dialkylated diketopyrrolopyrrole (DKP) moieties with symmetrical thiophene units served as electron acceptors, in 1:2 and 1:3 ratios, respectively. Our investigation of the photophysical properties indicated that the absorption bands of TPAKP-2, and TPAKP-3 extended up to 793 nm, with low optical band gaps of 1.56 and 1.65 eV respectively. Under illumination with AM 1.5 white light (100 mW cm−2), we investigated the performance of bulk heterojunction (BHJ) photovoltaic devices incorporating an active layer of an electron-donor small molecule (TPAKP-2 or TPAKP-3) blended with an electron acceptor: [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) at various weight ratios. The photovoltaic device containing the donor TPAKP-3 and the acceptor PC71BM at a 1:3 weight ratio exhibited the best power conversion efficiency (1.81%), with an open circuit voltage of 0.66 V, a short circuit current density of 7.93 mA cm−2, and a fill factor of 34.7%.

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.

rGO/SWCNT composites as novel electrode materials for electrochemical biosensing

In this study we performed electrochemical sensing using conductive carbon composite films containing reduced graphene oxide (rGO) and single-walled carbon nanotubes (SWCNTs) as electrode modifiers on glassy carbon electrodes (GCEs). Raman spectroscopy, transmission electron microscopy, atomic force microscopy, and scanning electron microscopy all suggested that the rGO acted as a surfactant, covering and smoothing out the surface, and that the SWCNTs acted as a conducting bridge to connect the isolated rGO sheets, thereby (i) minimizing the barrier for charge transfer between the rGO sheets and (ii) increasing the conductivity of the film. We used the rGO/SWCNT-modified GCE as a sensor to analyze hydrogen peroxide (H2O2) and β-nicotinamide adenine dinucleotide (NADH), obtaining substantial improvements in electrochemical reactivity and detection limits relative to those obtained from rGO- and SWCNT-modified electrodes, presumably because of the higher conductivity and greater coverage on the GCE, due to π-π interactions originating from the graphitic structures of the rGO and SWCNTs. The electrocatalysis response was measured by cyclic voltammetry and amperometric current-time (i-t) curve techniques. The linear concentration range of H2O2 and NADH detection at rGO/SWCNT-modified electrode is 0.5-5M and 20-400μM. The sensitivity for H2O2 and NADH detection is 2732.4 and 204μAmM(-1)cm(-2), and the limit of detection is 1.3 and 0.078μM respectively. Furthermore, interference tests indicated that the carbon composite film exhibited high selectivity toward H2O2 and NADH. Using GO as a solubilizing agent for SWCNTs establishes a new class of carbon electrodes for electrochemical sensors.

A counter electrode based on hollow spherical particles of polyaniline for a dye-sensitized solar cell

Hollow spherical polyaniline (hsPANI) particles are synthesized and deposited on an ITO/glass substrate to prepare a counter electrode (designated as hsPANI-CE) for a dye-sensitized solar cell (DSSC). The structure and crystallization of the hsPANI particles are characterized by using high resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectra. A power-conversion efficiency (η) of 6.84% is obtained for the DSSC with the hsPANI-CE, while it is 6.02% in the case of the DSSC with a CE based on pristine PANI (designated as PANI-CE). Such enhancement is attributed to the hsPANI film having a larger active surface area (A) of 0.191 cm2, compared to that of the PANI film (A = 0.126 cm2), both values being estimated by a rotating disk electrode (RDE). Cyclic voltammetric (CV) curves have evidenced that the electro-catalytic ability of the hsPANI-CE for the reduction of tri-iodide (I3−) ions is higher than that of the PANI-CE. As a reference, the DSSC with a Pt-sputtered CE gives an η of 7.17%. Electrochemical impedance spectroscopic (EIS) spectra are used to substantiate the photovoltaic behaviors. The results suggest that the film consisting of hsPANI particles can be a potential catalytic layer for the replacement of Pt in the CE of a DSSC.

Using a low temperature crystallization process to prepare anatase TiO2 buffer layers for air-stable inverted polymer solar cells

In this study, we fabricated inverted polymer solar cells featuring titanium dioxide (TiO2) as the electron collection layer and vanadium (V) oxide (V2O5) as the hole collection layer. TiO2 films (anatase phase) were prepared by combining electrochemical deposition with high-pressure crystallization. The low temperature process used to obtain the TiO2 films minimized interdiffusion of Ti and In species between the TiO2 and ITO films and maintained the conductivity of the indium tin oxide substrate. The inverted device reached a power conversion efficiency of 3.22% and exhibited much better stability under ambient conditions relative to that of the corresponding conventional device.

A high performance electrochemical sensor for acetaminophen based on a rGO–PEDOT nanotube composite modified electrode

In this study, we perform an electrochemical sensing using a conductive composite film containing reduced graphene oxide (rGO) and poly(3,4-ethylenedioxythiophene) nanotubes (PEDOT NTs) as an electrode modifier on a glassy carbon electrode (GCE). Scanning electron microscopy suggests that the rGO covers the surface of GCE uniformly and the PEDOT NTs act as a conducting bridge to connect the isolated rGO sheets. By combining these two materials, the conductivity and the surface coverage of the film can be enhanced, which is beneficial for electrochemical sensing. The rGO–PEDOT NT composite modified electrode is applied for an effective sensor to analyze acetaminophen. The obtained electrochemical activity is much higher than those obtained by the rGO- and PEDOT NT-modified electrodes; the higher electrochemical activity may be attributed to the higher conductivity and greater coverage of the rGO–PEDOT NT composite film on the GCE. Furthermore, interference tests indicate that the rGO–PEDOT NT composite modified electrode exhibits high selectivity toward the analyte. This novel method for combining the rGO and PEDOT NTs establishes a new class of carbon material-based electrodes for electrochemical sensors.

Wet-milled transition metal oxide nanoparticles as buffer layers for bulk heterojunction solar cells

In this study, we used high-energy grinding to prepare solutions of well-dispersed transition metal oxides (TMOs), allowing the fabrication of crystalline TMO films with high surface coverage through solution processing without post-annealing. Moreover, because this solution-based method did not require surfactants, it preserved the intrinsic electronic and optical properties of the TMOs. The resulting smooth, continuous, and highly transparent TMO films were readily integrated into organic solar cells. After incorporating MoO3 thin films into bulk heterojunction solar cells, we obtained devices delivering power conversion efficiencies of up to 3.68%, rivalling those of devices fabricated with commercial poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as the hole collection layer.

Dual-color electrochromic films incorporating a periodic polymer nanostructure

In this study, we used electrochemical deposition with a polystyrene (PS) bead template to prepare thin layers of poly(3,4-ethylenedioxythiophene) (PEDOT) that has a nanometer-scale honeycomb periodicity. To achieve dual-color electrochromic films, we then used pulsed electrochemical deposition to form polyaniline (PANI) layers on the rims of the PEDOT honeycomb film. Modifying the depositing charge density of PANI allowed tuning of the color of the composite films. We estimated the coloration efficiencies with respect to the values expected for corresponding amounts of pristine single layers of PEDOT and PANI. As a result of improved contact between the polymer film and the electrolyte, the coloration efficiencies of the structured composite films were much closer to their theoretical values than those of the corresponding bilayer composite films. Moreover, the switching times of the structured composite films were much faster than those of the bilayer composite films prepared without the nanostructures.