Triarylamine (TAA) and related materials have dramatically promoted the development of organic and hybrid photovoltaics during the past decade. The power conversion efficiencies of TAA-based organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs) have exceeded 11%, 14%, and 20%, which are among the best results for these three kinds of devices, respectively. In this review, we summarize the recent advances of the high-performance TAA-based materials in OSCs, DSSCs, and PSCs. We focus our discussion on the structure–property relationship of the TAA-based materials in order to shed light on the solutions to the challenges in the field of organic and hybrid photovoltaics. Some design strategies for improving the materials and device performance and possible research directions in the near future are also proposed.

Triarylamine: Versatile Platform for Organic, Dye-Sensitized, and Perovskite Solar Cells.

Dye-sensitized solar cells (DSSCs) are promising for utilizing solar energy. To achieve high efficiencies, it is vital to synergistically improve the photocurrent (Jsc) and the photovoltage (Voc). In this respect, conjugation framework extension and cosensitization are effective for improving the absorption and the Jsc, which, however, is usually accompanied by undesirably decreased Voc. Herein, based on a rationally optimized porphyrin dye, we develop a targeted coadsorption/cosensitization approach for systematically improving the Voc from 645 to 727, 746, and 760 mV, with synergistical Jsc enhancement from 18.83 to 20.33 mA cm–2. Thus, the efficiency has been dramatically enhanced to 11.5%, which keeps the record for nonruthenium DSSCs using the I2/I3– electrolyte. These results compose an alternative approach for developing highly efficient DSSCs with relatively high Voc using traditional iodine electrolyte.

Porphyrin Cosensitization for a Photovoltaic Efficiency of 11.5%: A Record for Non-Ruthenium Solar Cells Based on Iodine Electrolyte

Increasing energy consumption worldwide and environmental concerns about global warming have attracted great interest in the development of renewable and eco-friendly energy technologies. Dye-sensitized solar cells (DSSCs) have attracted considerable attention over the last 25 years since they offer possible low-cost conversion of photovoltaic energy. The sensitizer is the most important component of a DSSC, as it is largely responsible for light harvesting and charge separation, as well as the dye regeneration process. As a result, there have been tremendous research efforts in developing sensitizers. However, many challenges remain, and a deeper understanding of the design rules of DSSC sensitizers is required to obtain efficient and long-term stable DSSCs. The purpose of this review is to discuss recent progress and the rational design criteria used in the structural design of organic dyes and porphyrin photosensitizers for use in DSSCs. The effects of molecular structural engineering on the photophysical and electrochemical properties, photovoltaic parameters, and efficiency of DSSCs are presented.

Rational design criteria for D–π–A structured organic and porphyrin sensitizers for highly efficient dye-sensitized solar cells

14.2% Efficiency Dye-Sensitized Solar Cells by Co-sensitizing Novel Thieno[3,2-b]indole-Based Organic Dyes with a Promising Porphyrin Sensitizer

Zn(II)–porphyrin sensitizers, coded as SGT-020 and SGT-021, are designed and synthesized through donor structural engineering. The photovoltaic (PV) performances of SGT sensitizer-based dye-sensitized solar cells (DSSCs) are systematically evaluated in a thorough SM315 as a reference sensitizer. The effect of the donor ability and the donor bulkiness on photovoltaic performances is investigated for establishing the structure–performance relationship in the platform of porphyrin-triple bond-benzothiadiazole-acceptor sensitizers. By introducing a more bulky fluorene unit to the amine group in the SM315, the power conversion efficiency (PCE) is enhanced with the increased short-circuit current (Jsc) and open-circuit voltage (Voc), due to the improved light-harvesting ability and the efficient prevention of charge recombination, respectively. As a consequence, a maximum PCE of 12.11% is obtained for SGT-021, whose PCE is much higher than the 11.70% PCE for SM315. To further improve their maximum efficiency, the first parallel tandem DSSCs employing cobalt electrolyte in the top and bottom cells are demonstrated and an extremely high efficiency of 14% is achieved, which is currently the highest reported value for tandem DSSCs. The series tandem DSSCs give a remarkably high Voc value of >1.83 V. From this DSSC tandem configuration, 7.4% applied bias photon-to-current efficiency is achieved for solar water splitting.

Porphyrin Sensitizers with Donor Structural Engineering for Superior Performance Dye‐Sensitized Solar Cells and Tandem Solar Cells for Water Splitting Applications

Over the last decades, porphyrin sensitizers have made a remarkable contribution to performance improvement in dye-sensitized solar cells (DSSCs). In particular, versatile push–pull-type porphyrin sensitizers have achieved power conversion efficiencies (η) over 10% as a result of their improved light-harvesting abilities. Meanwhile, aromatic ring fusion to a porphyrin core is an attractive option for highly efficient DSSCs because of its expanded π-conjugation and resultant red-shifted absorption. Nevertheless, aromatic-fused porphyrin sensitizers have suffered rather low cell performances due to their mismatch of HOMO–LUMO levels, high aggregation tendency, and short lifetime of the excited states. Bearing these in mind, we envisioned that the fusion of substituted methylene-bridged small aromatic ring to a porphyrin core would overcome these drawbacks, boosting the cell performance. Herein, we report a series of substituted methylene-bridged thiophene-fused porphyrins, AfZnP, DfZnP, and DfZnP-iPr. After...

Renaissance of Fused Porphyrins: Substituted Methylene-Bridged Thiophene-Fused Strategy for High-Performance Dye-Sensitized Solar Cells.

A tropolone group has been employed for the first time as an anchoring group for dye-sensitized solar cells (DSSCs). The DSSC based on a porphyrin, YD2-o-C8T, with a tropolone moiety exhibited a power-conversion efficiency of 7.7 %, which is only slightly lower than that observed for a reference porphyrin, YD2-o-C8, with a conventional carboxylic group. More importantly, YD2-o-C8T was found to be superior to YD2-o-C8 with respect to DSSC durability and binding ability to TiO2. These results unambiguously demonstrate that tropolone is a highly promising dye-anchoring group for DSSCs in terms of device durability as well as photovoltaic performance.

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Tropolone as a High-Performance Robust Anchoring Group for Dye-Sensitized Solar Cells.

To evaluate the effects of substituent bulkiness around a porphyrin core on the photovoltaic properties of porphyrin-sensitized solar cells, long alkoxy groups were introduced at the meso-phenyl group (ZnPBAT-o-C8) and the anchoring group (ZnPBAT-o-C8Cn, n = 4, 8) of an asymmetrically substituted push–pull porphyrin with double electron-donating diarylamino groups and a single electron-withdrawing carboxyphenylethynyl anchoring group. The spectroscopic and electrochemical properties of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn were found to be superior to those of a push–pull porphyrin reference (YD2-o-C8), demonstrating their excellent light-harvesting and redox properties for dye-sensitized solar cells. A power conversion efficiency (η) of the ZnPBAT-o-C8-sensitized solar cell (η = 9.1%) is higher than that of the YD2-o-C8-sensitized solar cell (η = 8.6%) using iodine-based electrolyte due to the enhanced light-harvesting ability of ZnPBAT-o-C8. In contrast, the solar cells based on ZnPBAT-o-C8Cn, possessing the ad...

Effects of Bulky Substituents of Push-Pull Porphyrins on Photovoltaic Properties of Dye-Sensitized Solar Cells.

A hydroxamic acid group has been employed for the first time as an anchoring group for cobalt-based dye-sensitized solar cells (DSSCs). The porphyrin dye YD2-o-C8HA including a hydroxamic acid anchoring group exhibited a power conversion efficiency (η) of 6.4 %, which is close to that of YD2-o-C8, a representative porphyrin dye incorporating a conventional carboxylic acid. More importantly, YD2-o-C8HA was found to be superior to YD2-o-C8 in terms of both binding ability to TiO2 and durability of cobalt-based DSSCs. Notably, YD2-o-C8HA photocells revealed a higher η-value (4.1 %) than YD2-o-C8 (2.8 %) after 500 h illumination. These results suggest that the hydroxamic acid can be used for DSSCs with other transition-metal-based redox shuttle to ensure high cell durability as well as excellent photovoltaic performance.

A Hydroxamic Acid Anchoring Group for Durable Dye‐Sensitized Solar Cells Incorporating a Cobalt Redox Shuttle

Anchoring groups for dye-sensitized solar cells (DSSCs) play a decisive role in high-power conversion efficiency (η) and long-term cell durability. To date, a carboxylic acid is the most widely used anchoring group for DSSCs. However, the carboxylic acid tends to dissociate from a TiO2 surface during the cell operation as well as in the presence of water. Considering that the dye dissociation from TiO2 leads to a decrease in the cell performance, stable anchoring groups are highly desirable to achieve long-term durability of DSSCs toward their practical application. In this study, we designed and synthesized a series of porphyrin dyes with the triethoxysilyl anchoring groups, ZnPSi1, ZnPSi2, and ZnPSi3, to evaluate the effects of the silicon-based anchoring group on cell durability and photovoltaic properties. The DSSCs based on ZnPSi1, ZnPSi2, and ZnPSi3 exhibited moderate η-values of 2.2, 4.7, and 2.3%, respectively. It is noteworthy that the η-value of the DSSC based on ZnPSi2 (4.7%) is the highest amo...

Yukihiro Tsuji

Papers

Renaissance of Fused Porphyrins: Substituted Methylene-Bridged Thiophene-Fused Strategy for High-Performance Dye-Sensitized Solar Cells.

Tropolone as a High-Performance Robust Anchoring Group for Dye-Sensitized Solar Cells.

Effects of Bulky Substituents of Push-Pull Porphyrins on Photovoltaic Properties of Dye-Sensitized Solar Cells.

A Hydroxamic Acid Anchoring Group for Durable Dye‐Sensitized Solar Cells Incorporating a Cobalt Redox Shuttle

Photovoltaic Properties and Long-Term Durability of Porphyrin-Sensitized Solar Cells with Silicon-Based Anchoring Groups