Chuan-Pei Lee

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.

Organic dyes containing carbazole as donor and π-linker: optical, electrochemical, and photovoltaic properties.

A series of new metal free organic dyes containing carbazole as donor and π-linker have been synthesized and characterized as effective sensitizers for dye sensitized solar cells (DSSCs). The carbazole functionalized at C-2 and C-7 served as electron-rich bridge. The donor property of the carbazole is substantially enhanced on introduction of tert-butyl groups at C-3 and C-6 positions and the oxidation propensity of the dyes increased on insertion of thiophene unit in the conjugation pathway. These structural modifications fine-tuned the optical and electrochemical properties of the dyes. Additionally, the presence of tert-butyl groups on the carbazole nucleus minimized the intermolecular interactions which benefited the performance of DSSCs. The dyes served as efficient sensitizers in DSSCs owing to their promising optical and electrochemical properties. The efficiency of DSSCs utilizing these dyes as sensitizers ranged from 4.22 to 6.04%. The tert-butyl groups were found to suppress the recombination of injected electrons which contributed to the increment in the photocurrent generation (JSC) and open circuit voltage (VOC). A dye with carbazole donor functionalized with tert-butyl groups and the conjugation bridge composed of 2,7-disubstituted carbazole and thiophene fragments exhibited higher VOC value. However, the best device efficiency was observed for a dye with unsubstituted carbazole donor and the π-linker featuring carbazole and bithiophene units due to the high photocurrent generation arising from the facile injection of photogenerated electrons into the conduction band of titanium dioxide (TiO2) facilitated by the low-lying LUMO.

Iodine-free high efficient quasi solid-state dye-sensitized solar cell containing ionic liquid and polyaniline-loaded carbon black

An incombustible and non-volatile paste with carbon black (CB), a conducting polymer (CP), and an ionic liquid (1-buty-3-methylimidazolium iodide, BMII or 1-methyl-3-propyl imidazolium iodide, PMII) was placed between the dye-sensitized porous TiO2 and the Pt counter electrode to fabricate a quasi solid-state DSSC, without the addition of iodine. While the solar-to-electricity efficiencies (η) were measured to be 4.38% and 3.68% for the cells with PMII/CB and BMII/CB, respectively, the corresponding values without CB were 0.6% and 0.3%; indicating the remarkable role played by the carbon material in the electrolyte. When the CB was replaced with polyaniline-loaded carbon black (PACB), an efficiency of 5.81% was obtained, at 100 mW cm−2 AM1.5 illumination using PMII, the highest ever reported for a quasi solid-state DSSC made without iodine. High thermal stability up to 250 °C for each component in the composite electrolytes was confirmed by thermogravimetric analyses (TGA). At-rest durability of the DSSC with PACB was studied both at room temperature and at 70 °C and was found to be far superior to that of a cell with an organic solvent electrolyte. Electrochemical impedance spectroscopy (EIS) and dark current measurements were used to substantiate the results.

Unsymmetrical squaraines incorporating the thiophene unit for panchromatic dye-sensitized solar cells.

Two unsymmetrical squaraines, where the electron-rich 3,4-ethylenedioxythiophene or bithiophene conjugated fragment was used to link unconventionally the squaraine core and the hexyloxyphenyl amino group, were applied for DSCs. The corresponding photovoltaic devices exhibit an attractively panchromatic response and also convert a portion of the near-infrared photons into electricity.

2,7-Diaminofluorene-Based Organic Dyes for Dye-Sensitized Solar Cells: Effect of Auxiliary Donor on Optical and Electrochemical Properties

New organic dyes containing a diarylaminofluorene unit as an electron donor and cyanoacrylic acid as acceptor and anchoring group in a donor-π-donor-π-acceptor architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO(2)-based dye-sensitized solar cells. They have shown three major electronic absorptions originating from the π-π* and charge-transfer transitions covering the broad visible range (250-550 nm) in solution. The charge-transfer transition of the dyes exhibited negative solvatochromism, suggesting a polarized ground state. They have also displayed acidochromism in solution owing to the presence of a protonation-deprotonation equilibrium. On comparison with the triphenylamine and carbazole-based parent dyes (E)-2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid and (E)-2-cyano-3-(9-ethyl-9H-carbazol-3-yl)acrylic acid they exhibited longer wavelength absorptions and facile oxidation, indicating the stronger electron-donating ability of the auxiliary chromophores. In addition, they exhibited nearly two times larger light-to-electron conversion efficiency under simulated AM 1.5 G irradiation (100 mW cm(-2)) with an aperture mask when compared to the parent dyes. Among the new dyes, the one containing the naphthylphenylamine segment showed better device characteristics attributable to the higher HOMO energy level which probably facilitates the regeneration of the dye and effective suppression of the back reaction of the injected electrons with the I(3)(-) in the electrolyte. The optical properties of the dyes were modeled using TDDFT simulations employing different theoretical models (B3LYP, CAM-B3LYP, and MPW1K), and the best correlations with the observed parameters have been found for CAM-B3LYP and MPW1K calculations. The electron lifetimes extracted from the electrochemical impedance measurements of the dye-sensitized solar cells were used to interpret the solar cell efficiency alternations.

Novel Pyrenoimidazole-Based Organic Dyes for Dye-Sensitized Solar Cells

A novel class of organic dyes containing pyrenoimidazole donors, cyanoacrylic acid acceptors, and oligothiophene π-linkers has been synthesized and characterized. The electro-optical properties of these dyes can be tuned by changing the conjugation length of the π-linkers. A dye containing terthiophene in the conjugation pathway exhibited a solar energy-to-electricity conversion efficiency of 5.65%.

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.

High‐Performance Dipolar Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety in the π‐Conjugation Framework for Dye‐Sensitized Solar Cells

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye-sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron-deficient diphenylquinoxaline moiety integrated in the π-conjugated spacer between the electron donor and acceptor moieties. The incorporation of the electron-deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35 %, which translates to approximately 79 % of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO(2) surface, rapid dye regeneration, and effective retardation of charge recombination.

A coral-like film of Ni@NiS with core–shell particles for the counter electrode of an efficient dye-sensitized solar cell

A coral-like film of nickel@nickel sulfide (Ni@NiS) was obtained on a conducting glass through an electrochemical method, in which the Ni functioned as a template. Three types of Ni thin films were electrodeposited on fluorine-doped tin oxide (FTO) substrates by a pulse current technique at the passed charge densities of 100, 200, and 300 mC cm−2, which rendered custard apple-like, coral-like, and cracked nanostructures, respectively. Subsequently, nickel sulfide films were coated on these Ni films by using a pulse potential technique. Due to the template effect of the Ni films, the composite films of Ni@NiS also assumed the same structures as those of their nickel templates. In each case of the films the particle of the film assumed a core–shell structure. The Ni@NiS coated FTO glasses were used as the counter electrodes for dye-sensitized solar cells (DSSCs). The DSSC with the coral-like Ni@NiS film on its counter electrode exhibits the highest power conversion efficiency (η) of 7.84%, while the DSSC with platinum film on its counter electrode shows an η of 8.11%. The coral-like Ni@NiS film exhibits multiple functions, i.e., large surface area, high conductivity, and great electrocatalytic ability for iodine/triiodine (I−/I3−) reduction. X-ray photoelectron spectroscopy (XPS), X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), and four-point probe technique were used to characterize the films. The photovoltaic parameters are substantiated using incident photon-to-current conversion efficiency (IPCE) curves, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization plots. The IPCE curves were further used to calculate theoretical short-current densities of the cells.

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.