Hsien-Hsin Chou

Recent developments in molecule-based organic materials for dye-sensitized solar cells

Photoanodes such as TiO2, sensitizers, electrolytes, and photocathodes play important roles in dye-sensitized solar cells (DSSCs). Organic materials are often used for these components. This feature article reviews the recent development of organic sensitizers for both n- and p-type DSSCs, with an emphasis on molecular design strategies. This paper also discusses dyes for co-sensitized and tandem DSSCs, and metal-containing and metal-free redox organic mediators. Finally this paper briefly discusses organic materials for counter electrodes. The article provides readers with an overview of new trends in organic materials for DSSCs.

Organic Dyes Containing Furan Moiety for High-Performance Dye-Sensitized Solar Cells

New metal-free dyes with a furan moiety in the conjugated spacer between the arylamine donor and the 2-cyanoacrylic acid acceptor have been synthesized, and high efficiency dye-sensitized solar cells were fabricated using these molecules as light-harvesting sensitizers.

Dipolar compounds containing fluorene and a heteroaromatic ring as the conjugating bridge for high-performance dye-sensitized solar cells.

A novel series of dipolar organic dyes containing diarylamine as the electron donor, 2-cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conjugating bridge have been developed and characterized. These metal-free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fabricated as efficient nanocrystalline TiO(2) dye-sensitized solar cells (DSSCs). The solar-energy-to-electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88%, which reached 68-96% of a standard device of N719 fabricated and measured under the same conditions. With a TiO(2) film thickness of 6 microm, DSSCs based on these dyes had photocurrents surpassing that of the N719-based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.

Arylamine-based dyes for p-type dye-sensitized solar cells.

New arylamine-based sensitizers for p-type dye-sensitized solar cells (DSSCs) have been synthesized and used for p-type DSSCs. The best conversion efficiency reaches ∼0.1%. Sensitizers with two anchoring carboxylic acids lead to higher open-circuit voltages, short-circuit currents, and energy conversion efficiencies.

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.

High-performance dye-sensitized solar cells based on 5,6-bis-hexyloxy-benzo[2,1,3]thiadiazole

New dipolar compounds containing a 5,6-bis-hexyloxy-benzo[2,1,3]thiadiazole entity in the conjugated spacer between the electron donating arylamine and the electron accepting 2-cyanoacrylic acid have been synthesized. These compounds were used as the sensitizers for efficient dye-sensitized solar cells. Compared to the congeners without the hexyloxy chains, the new dyes effectively suppress the dark currents and significantly improve the cell performance. The best conversion efficiency reached ∼92% of the N719-based standard DSSC fabricated and measured under similar conditions (AM 1.5 irradiation).

Benzotriazole-Containing D–π–A Conjugated Organic Dyes for Dye-Sensitized Solar Cells

A series of metal-free benzotriazole-based dipolar dyes have been developed as sensitizers for dye-sensitized solar cells (DSSCs). Different heteroaromatic rings such as furan, thiophene, and selenophene, were used in combination with benzotriazole as the conjugated spacer group. Light harvesting, charge recombination, and electron injection of the cells fabricated are affected by the heteroaromatic ring used in the spacer. The DSSC with the thiophene-containing dye has the highest conversion efficiency of 6.20%, which reaches 85% of the standard cell based on N719.

Dibenzo[f,h]thieno[3,4-b] quinoxaline-based small molecules for efficient bulk-heterojunction solar cells.

Two isomeric compounds (1 and 2) containing a dibenzo[f,h]thieno[3,4-b]quinoxaline core and two peripheral arylamines were synthesized. Solution-processed bulk heterojunction (BHJ) solar cells based on these sensitizers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are reported. The cell fabricated from 1/67 wt % of PCBM exhibited a high power conversion efficiency of 1.70% and an external quantum yield of 55%. The film of the cell was found to have balanced electron and hole mobility and good film morphology.

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.

Incorporating a New 2H-[1,2,3]Triazolo[4,5-c]pyridine Moiety To Construct D–A−π–A Organic Sensitizers for High Performance Solar Cells

Two new organic dyes (PTN1 and NPT1) of the configuration D-A-π-A, based on 2H-[1,2,3]triazolo[4,5-c]pyridine (PT) as a central linker, have been synthesized and used as the sensitizers for dye-sensitized solar cells. Compared with pyridal[2,1,3]thiadiazole-containing congeners, the new dyes have conversion efficiencies nearly 1 order higher due to alleviation of charge trapping. The best conversion efficiency of the cell reaches 6.05% (under AM 1.5G irradiation). Upon addition of the coadsorbent CDCA, the efficiency is boosted to 6.76%, which reaches ∼90% of the standard based on N719.