Chun-Ting Li

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.

Platinum-Free Counter Electrode Comprised of Metal-Organic-Framework (MOF)-Derived Cobalt Sulfide Nanoparticles for Efficient Dye-Sensitized Solar Cells (DSSCs)

We fabricated a highly efficient (with a solar-to-electricity conversion efficiency (η) of 8.1%) Pt-free dye-sensitized solar cell (DSSC). The counter electrode was made of cobalt sulfide (CoS) nanoparticles synthesized via surfactant-assisted preparation of a metal organic framework, ZIF-67, with controllable particle sizes (50 to 320 nm) and subsequent oxidation and sulfide conversion. In contrast to conventional Pt counter electrodes, the synthesized CoS nanoparticles exhibited higher external surface areas and roughness factors, as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM) element mapping, and electrochemical analysis. Incident photon-to-current conversion efficiency (IPCE) results showed an increase in the open circuit voltage (VOC) and a decrease in the short-circuit photocurrent density (Jsc) for CoS-based DSSCs compared to Pt-based DSSCs, resulting in a similar power conversion efficiency. The CoS-based DSSC fabricated in the study show great potential for economically friendly production of Pt-free DSSCs.

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.

Copper zinc tin sulfide as a catalytic material for counter electrodes in dye-sensitized solar cells

Quaternary Cu2ZnSnS4 (CZTS) semiconductor thin films, deposited onto an indium-doped tin oxide (ITO) conducting substrate by sulfurizing direct-current (DC) magnetron sputtered Cu–Zn–Sn metal alloys, were used as the counter electrode (CE) of dye-sensitized solar cells (DSSCs). X-ray diffraction (XRD) and Raman spectroscopy reveal that all the samples show the kesterite Cu2ZnSnS4 phase. The morphology becomes smoother for the Cu-rich or Zn-rich sample, as confirmed by atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). The influence of the [Cu]/[Zn] + [Sn] molar ratio in the CZTS samples on the catalytic performance of DSSCs was also investigated. The electrocatalytic ability and electrochemical properties of the CEs were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel plots. The performance of DSSCs with various CEs was verified by J–V curves and incident photon-to-current conversion efficiency (IPCE) curves. The maximum solar-to-electrical power efficiency of DSSCs using CZTS as the CE approached 7.94%. These results showed the attractive potential of CZTS to replace the Pt CE (8.55%). The influence of different molar ratios of CZTS CEs on the DSSC performance is discussed in detail.

Fluorene-Based Sensitizers with a Phenothiazine Donor: Effect of Mode of Donor Tethering on the Performance of Dye-Sensitized Solar Cells

Two types of fluorene-based organic dyes featuring T-shape/rod-shape molecular configuration with phenothiazine donor and cyanoacrylic acid acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. Phenothiazine is functionalized at either nitrogen (N10) or carbon (C3) to obtain T-shape and rod-like organic dyes, respectively. The effect of structural alternation on the optical, electrochemical, and the photovoltaic properties is investigated. The crystal structure determination of the dye containing phenyl linker revealed cofacial slip-stack columnar packing of the molecules. The trends in the optical properties of the dyes are interpreted using time-dependent density functional theory (TDDFT) computations. The rod-shaped dyes exhibited longer wavelength absorption and low oxidation potentials when compared to the corresponding T-shaped dyes attributable to the favorable electronic overlap between the phenothiazine unit and the rest of the molecule in the former dyes. However, the T-shaped dyes showed better photovoltaic properties due to the lowest unoccupied molecular orbital (LUMO) energy level favorable for electron injection into the conduction band of TiO2 and appropriate orientation of the phenothiazine unit rendering effective surface blocking to suppress the recombination of electrons between the electrolyte I3(-) and TiO2. The electrochemical impedance spectroscopy investigations provide further support for the variations in the electron injection and transfer kinetics due to the structural modifications.

Organic dyes containing fluoren-9-ylidene chromophores for efficient dye-sensitized solar cells

Novel organic sensitizers comprising one or two fluorenylidene moieties in the donor part of triarylamine, cyanoacrylic acid as the acceptor/anchoring group and a fluorene and oligothiophene spacer in a D–π–A architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO2-based dye-sensitized solar cells. Their optical, electrochemical and photovoltaic properties are compared with the electron accepting dicyanovinyl unit containing the organic sensitizer. Incorporation of the fluorenylidene moiety dominates the optical properties of the dyes in terms of relatively broad and high molar extinction coefficient absorption when compared to the dicyanovinyl derivatives. Theoretical investigations using TDDFT simulations indicate that the trends in the excitation energies are consistent with the solution spectral data for higher wavelength absorption and the lower wavelength absorptions attributed to the amine to auxiliary acceptor charge transfer. The electrochemical properties are influenced by the number of fluorenylidene chromophores and the electron richness of the linking segment. The dye-sensitized solar cells fabricated using fluorenylidene-based sensitizers showed higher power conversion efficiency than the dicyanovinyl derivatives attributed to their higher photocurrent density. A fluorenylidene-based dye exhibited a high power conversion efficiency of 6.13% under full sunlight (AM 1.5G, 100 mW cm−2).

Functional tuning of phenothiazine-based dyes by a benzimidazole auxiliary chromophore: an account of optical and photovoltaic studies

We have designed and synthesized a series of organic dyes featuring phenothiazine donors containing an N-phenylbenzimidazole substituent and a cyanoacrylic acid acceptor. Benzimidazole incorporation red-shifted the charge transfer transition and increased the molar extinction coefficient of the peak. It also helped to fine-tune the HOMO and LUMO energies of the dyes due to the electron-withdrawing nature of the benzimidazole. The dye-sensitized solar cells fabricated using the dyes possessing N-phenylbenzimidazole showed efficiency better than the analogous dye without benzimidazole. The hike in the efficiency resulted from the increase in short-circuit current and open circuit voltage. The electrochemical impedance spectroscopy of the devices revealed that the integration of benzimidazole in the dye enhances the life time of the injected electrons in the conduction band of TiO2 and resists electron recombination at the TiO2/dye/electrolyte interface.

Structure–Performance Correlations of Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety for Dye‐Sensitized Solar Cells

The high performances of dye-sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of 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 DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon-to-electron conversion efficiencies extends to the onset at the near-infrared region due to strong internal charge-transfer transition as well as the effect of electron-deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO2 to the electrolyte and boosts the photovoltage, comparable to their Ru(II) counterparts. Detailed spectroscopic studies have revealed the dye structure-cell performance correlations, to allow future design of efficient light-harvesting organic dyes.

High‐Performance Aqueous/Organic Dye‐Sensitized Solar Cells Based on Sensitizers Containing Triethylene Oxide Methyl Ether

Metal-free dyes (EO1 to EO4) containing the hydrophilic triethylene oxide methyl ether (TEOME) unit in the spacer have been synthesized and used in dye-sensitized solar cells (DSSCs). Efficient lithium-ion trapping by TEOME results in improved open-circuit voltage (VOC ), leading to excellent conversion efficiency of the cells, ranging from 9.02 to 9.98 % with I(-) /I3 (-) electrolyte in acetonitrile under AM 1.5 illumination. The TEOME unit also enhances the wettability of the dye molecules for application in aqueous-based DSSCs. Aqueous-based DSSCs with a dual TEMPO/iodide electrolyte exhibit high VOC values (0.80-0.88 V) and very promising cell performances of up to 5.97 %.