Ryan Yeh‐Yung Lin

Materials for the Active Layer of Organic Photovoltaics: Ternary Solar Cell Approach

Power conversion efficiencies in excess of 7% have been achieved with bulk heterojunction (BHJ)-type organic solar cells using two components: p- and n-doped materials. The energy level and absorption profile of the active layer can be tuned by introduction of an additional component. Careful design of the additional component is required to achieve optimal panchromatic absorption, suitable energy-level offset, balanced electron and hole mobility, and good light-harvesting efficiency. This article reviews the recent progress on ternary organic photovoltaic systems, including polymer/small molecule/functional fullerene, polymer/polymer/functional fullerene, small molecule/small molecule/functional fullerene, polymer/functional fullerene I/functional fullerene II, and polymer/quantum dot or metal/functional fullerene systems.

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.

Solid-state dye-sensitized solar cells based on spirofluorene (spiro-OMeTAD) and arylamines as hole transporting materials

Dye-sensitized solar cells are a promising solar technology because of their low cost, reliability, and high efficiency, compared with silicon-based solar cells. Efforts over the last two decades have increased solar cell efficiency to 12% based on liquid electrolytes, and more research on solid-state devices is necessary to determine their practical usage and long-term stability. The development of solid-state devices has achieved an overall efficiency over 7% using hole transporting materials. This study reviews current progress on hole transporting materials, sensitizers, and mesoporous TiO(2) in solid-state dye-sensitized solar cells using small organic molecules as the hole transporting material. This study also discusses the key factors, such as molecular structure design and interfacial problems, affecting device performance.

2,6-Conjugated anthracene sensitizers for high-performance dye-sensitized solar cells

Metal-free dyes (Ant1 to Ant4) containing a 2,6-conjugated anthracene unit in the spacer have been synthesized. The conversion efficiency (7.52%) of the dye-sensitized solar cell using Ant3 as the sensitizer reached ∼90% of the standard N719-based cell (8.41%). The co-sensitized DSSC using Ant3 and a near-IR dye (SQ2) exhibited an improved efficiency of 8.08%. With addition of a co-adsorbent, CDCA, the cell efficiency (9.11%) of the Ant3-based DSSC surpassed that of the N719-based standard cell. An even higher efficiency of 10.42% was achieved under weak light irradiation.

Y-shaped metal-free D–π–(A)2 sensitizers for high-performance dye-sensitized solar cells

A series of 2,3,5-substituted thiophene-based metal-free dyes with two anchoring groups, D–π–(A)2 (DA), were synthesized for application in DSSCs. Different arylamines and 2-cyanoacrylic acid were used as the electron donor and the electron acceptor, respectively. The DSSC based on DA5 has the best power conversion efficiency (7.28%) among all devices, which is 2 times higher than that of S1 with only one anchoring group. With addition of a co-adsorbent, CDCA, the device performance of all the DA-based DSSCs are improved by 1.03 to 2-fold, with the best efficiency (7.87%) reaching 95% of that of the N719-based standard cell (8.28%). Compared with the S1 congener with only one anchor, the DA dyes can more effectively suppress charge recombination and increase electron injection efficiency, leading to higher open-circuit voltage and short-circuit current.

Dihydrophenanthrene-based metal-free dyes for highly efficient cosensitized solar cells.

Metal-free dyes (BP-1 to BP-3) containing a 9,10-dihydrophenanthrene unit in the spacer have been synthesized. The dye with the highest cell efficiency, BP-2, was used in combination with SQ2 for cosensitized DSSCs. The cosensitized DSSC in which the ratio of BP-2 and SQ2 is 8:2 (v/v) has a record high efficiency of 8.14% among cosensitized systems using all metal-free sensitizers. Dye distribution along the TiO2 film depth was analyzed by an Auger electron spectroscopy technique.

Benzothiadiazole-containing donor–acceptor–acceptor type organic sensitizers for solar cells with ZnO photoanodes

Dye-sensitized solar cells using nanocrystalline ZnO as the photoanode and a metal-free sensitizer with a benzothiadiazole entity directly connected to the 2-cyanoacrylic acid acceptor exhibited an efficiency (5.18%) higher than those using the TiO(2) photoanode. Use of a hierarchical ZnO back scattering layer further improved the efficiency to 5.82%.

Anthracene/Phenothiazine π-Conjugated Sensitizers for Dye-Sensitized Solar Cells using Redox Mediator in Organic and Water-based Solvents

Metal-free dyes (MD1 to MD5) containing an anthracene/phenothiazine unit in the spacer have been synthesized. The conversion efficiency (7.13 %) of the dye-sensitized solar cell using MD3 as the sensitizer reached approximately 85 % of the N719-based standard cell (8.47 %). The cell efficiency (8.42 %) of MD3-based dye-sensitized solar cells (DSSCs) with addition of chenodeoxycholic acid is comparable with that of N719-based standard cell. The MD3 water-based DSSCs using a dual-TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl)/iodide electrolyte exhibited very promising cell performance of 4.96 % with an excellent Voc of 0.77 V.

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 %.

Ionic Liquid with a Dual‐Redox Couple for Efficient Dye‐Sensitized Solar Cells

A new type of ionic liquid that contains a nitroxide radical (N-O(.)) and iodide as two redox couples, JC-IL, has been successfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I2 in the wavelength region of 350-500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electrolyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electrolyte show promising cell efficiencies if coupled with dyes CR147 (8.12%) or D149 (6.76%). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electrolyte alone.