Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent

Recent advances in the approaches for improving the photovoltaic performance of porphyrin-based DSSCs

Among other photovoltaic techniques including perovskite solar cells and organic solar cells, dye-sensitized solar cells (DSSCs) are considered to be a potential alternative to conventional silicon solar cells. Porphyrins are promising dyes with the properties of easy modification and superior light-harvesting capability. However, porphyrin dyes still suffer from a number of unfavorable aspects, which need to be addressed in order to improve the photovoltaic performance. This feature article briefly summarizes the recent progress in improving the Voc and Jsc of porphyrin-based DSSCs in terms of molecular engineering by modifying the porphyrin macrocycle, donor and acceptor moieties of the porphyrin dyes, coadsorption of the porphrin dyes with bulky coadsorbents like chenodeoxycholic acid (CDCA), and cosensitization of the porphyrin dyes with metal-free organic dyes. Notably, concerted companion (CC) dyes are described in detail, which have been constructed by linking a porphyrin dye subunit and a metal-free organic dye subunit with flexible alkoxy chains to achieve panchromatic absorption and concerted enhancement of Voc and Jsc. In one sentence, this article is expected to provide further insights into the development of high performance DSSCs through the design and syntheses of efficient porphyrin dyes and CC dyes in combination with device optimization to achieve simultaneously elevated Voc and Jsc, which may inspire and promote further progress in the commercialization of the DSSCs.

Efficient Solar Cells Based on Porphyrin and Concerted Companion Dyes Featuring Benzo 12-Crown-4 for Suppressing Charge Recombination and Enhancing Dye Loading

In recent years, various porphyrin dyes have been designed to develop efficient dye-sensitized solar cells (DSSCs). Based on our previously reported porphyrin dye XW43, which contains a phenothiazine donor with two diethylene glycol (DEG)-derived substituents, we herein report a porphyrin dye XW89 by introducing a benzo 12-crown-4 (BCE) unit onto the N atom of the phenothiazine donor. On this basis, XW90 and XW91 have been synthesized by replacing a DEG chain in XW89 with two DEG chains and a 12-crown-4 unit, respectively. For iodine electrolyte-based DSSCs, dyes XW89-XW91 exhibit VOC values of 765-779 mV, higher than that of XW43 (755 mV), which may be related to the strong capability of the BCE group in binding Li+ and thus suppressing the downward shift of the TiO2 conduction band and interfacial charge recombination. Moreover, the smaller size of 12-crown-4 than the DEG unit enables higher adsorption amounts of the dyes than XW43, contributing to an enhanced JSC value. Due to the presence of two BCE units, dye XW91 exhibits the highest dye loading amount and JSC of 1.86 × 10-7 mol cm-2 and 19.79 mA cm-2, respectively, affording a high PCE of 11.1%. To further enhance the light-harvesting ability, a concerted companion (CC) dye XW92 has been constructed by linking the two subdye units corresponding to the porphyrin dye XW91 and an organic dye. As a result, XW92 affords an enhanced JSC and efficiency. Further coadsorption of XW92 with chenodeoxycholic acid achieved the highest efficiency of 12.1%. This work provides an effective approach for fabricating efficient DSSCs sensitized by porphyrin and CC dyes based on the introduction of crown ether units with smaller sizes and stronger Li+ affinities.

D-π-A Structured Porphyrin and Organic Dyes with Easily Synthesizable Donor Units for Low-Cost and Efficient Dye-Sensitized Solar Cells

This study aimed to develop low-cost D-π-A structured porphyrin and organic dyes with easily synthesizable donor units instead of the conventional complex multistep synthetic donor unit of Hexyloxy-BPFA [bis(7-(2,4-bis(hexyloxy)phenyl)-9,9-dimethyl-9H-fluoren-2-yl)amine] used in SGT-021 and SGT-149 as well-known record cosensitizers with an extremely high power conversion efficiency (PCE). The design strategy concerned the easier synthesis of low-cost donor units with inversion structures in donor groups via donor structural engineering, particularly by changing the position of the fluorene and phenylene units in the donor moiety while keeping the π-bridge and acceptor unit unchanged, leading to the synthesis of two D-π-A structured porphyrins [SGT-021(D0) and SGT-021(D)] and one D-π-A structured organic sensitizer [SGT-149(D)] for dye-sensitized solar cells (DSSCs). Specifically, porphyrin SGT-021(D0) incorporated two hexyl chains into the 9-position of each fluorene, while SGT-021(D) and SGT-149(D) substituted two hexyloxy chain units to the terminal position of each fluorene in the donor groups of porphyrin dyes. The effect of the position of the fluorene and phenylene units in the donor moiety on the photochemical and electrochemical properties, as well as the photovoltaic performance, was compared with the reference dyes of SGT-021 and SGT-149, previously reported by the research group. After optimizing the DSSC devices, SGT-021(D) and SGT-021(D0) achieved a high PCE of 11.6 and 10.5%, respectively, while SGT-149(D) exhibited a little lower PCE of 10.3% under the standard AM 1.5G light intensity. The cell performance of DSSC devices based on SGT-021(D) and SGT-149(D) was inferior to the corresponding reference dyes of SGT-021 and SGT-149 due to their lower donating ability of Hexyloxy-BPFA than Hexyloxy-BFPA.

Advances and prospects of porphyrin derivatives in the energy field

At present, porphyrin is developing rapidly in the fields of medicine, energy, catalysts, etc. More and more reports on its application are being published. This paper mainly takes the ingenious utilization of porphyrin derivatives in perovskite solar cells, dye-sensitized solar cells, and lithium batteries as the background to review the design idea of functional materials based on the porphyrin structural unit in the energy sector. In addition, the modification and improvement strategies of porphyrin are presented by visually showing the molecular structures or the design synthesis routes of its functional materials. Finally, we provide some insights into the development of novel energy storage materials based on porphyrin frameworks.

Efficient Solar Cells Sensitized by Organic Concerted Companion Dyes Suitable for Indoor Lamps

In this work, organic concerted companion (CC) dyes CCOD-1 and CCOD-2 were constructed by covalently linking two organic dye units with complementary absorption spectra. Both CC dyes exhibited intense absorption from 300 to 650 nm with the band edges extended to 700 nm. These CC dyes were used to fabricate dye-sensitized solar cells (DSSCs), and the photovoltaic performance was investigated using different light sources. CCOD-2 possessed bulkier outer shelter than CCOD-1 owing to the longer carbon chains (C₁₂ ) at the donor moiety, and thus it had stronger anti-aggregation and anti-charge-recombination ability. Under simulated sunlight (AM1.5G), CCOD-2 exhibited enhanced photovoltaic behavior with an open-circuit voltage (V_OC ) of 759 mV, short-circuit current density (J_SC ) of 19.23 mA ⋅ cm^-2 , and power conversion efficiency (PCE) of 10.4 %, respectively. Notably, under the illumination of the indoor T5 fluorescent lamp (2500 lux), CCOD-2 afforded an enhanced PCE of 28.0 % with remarkable V_OC and J_SC of 692 mV and 0.424 mA cm^-2 , respectively. Notably, the PCE achieved for CCOD-2 outperformed those of the reference sensitizer N719 and our previously reported CC dyes XW61 and XW70-C8 under the same indoor lamp conditions. In summary, the novel organic CC dyes developed in this work were demonstrated to be promising for fabricating DSSCs to efficiently harvest the energy of indoor lamps.

Efficient Dye-Sensitized Solar Cells Based on a New Class of Doubly Concerted Companion Dyes

To develop efficient dye-sensitized solar cells (DSSCs), concerted companion (CC) dyes XW60-XW63 constructed from the covalent linkage of a strapped porphyrin dye unit and an organic dye unit have been reported to exhibit panchromatic absorption and excellent photovoltaic performance. However, these CC dyes only afforded moderate VOC values of ca. 763 mV, demonstrating relatively weak antiaggregation ability, which remains an obstacle for further enhancing the photovoltaic behavior. To address this problem, we herein develop porphyrin dyes XW77-XW80 with the macrocycles wrapped with alkoxy chains of various lengths (OC6H13-OC22H45) and the corresponding CC dyes XW81-XW84 containing these porphyrin dye units. Interestingly, the new CC dyes XW81-XW83 exhibit increasing VOC from 745 to 784 mV with the chain lengths extended from C6 to C18, and a lowered VOC of 762 mV was obtained for XW84 when the chain length was further extended to C22. As a result, XW83 afforded the highest PCE of 12.2%, which is, to the best of our knowledge, the record efficiency for the iodine electrolyte-based solar cells sensitized with a single dye. These results can be rationalized by the so-called doubly concerted companion (DCC) effects, that is, the two subdye units exhibit not only complementary absorption but also concerted antiaggregation with the long wrapping chains on the porphyrins unit simultaneously protecting the porphyrin macrocycle and the neighboring organic subdye unit, thus affording panchromatic absorption and strong antiaggregation and anticharge-recombination ability. These results provide a new approach for constructing a class of DCC dyes to achieve high-performance DSSCs without using any antiaggregating coadsorbent or absorption-enhancing cosensitizer.

Polyimide-Based Covalent Organic Framework as a Photocurrent Enhancer for Efficient Dye-Sensitized Solar Cells

Covalent organic frameworks (COFs) are of great interest in the energy and optoelectronic fields due to their high porosity, superior thermal stability, and highly ordered conjugated architecture, which are beneficial for charge migration, charge separation, and light harvesting. In this study, polyimide COFs (PI-COFs) are synthesized through the condensation reaction of pyromellitic dianhydride (PMDA) with tris(4-aminophenyl) amine (TAPA) and then doped in the TiO₂ photoelectrode of a dye-sensitized solar cell (DSSC) to co-work with N719 dye to explore their functionality. As a benchmark, the pristine DSSC without the doping of PI-COFs exhibits a power conversion efficiency of 9.05% under simulated one sun illumination. The doping of 0.04 wt % PI-COFs contributes an enhanced short-circuit current density (J_SC) from 17.43 to 19.03 mA/cm², and therefore, the cell efficiency is enhanced to 9.93%. The enhancement of J_SC is attributed to the bifunctionality of PI-COFs, which enhances the charge transfer/injection and suppresses the charge recombination through the host (PI-COF)-guest (N719 dye) interaction. In addition, the PI-COFs also function as a cosensitizer and contribute a small quantity of photoinduced electrons upon sunlight illumination. Surface modification of oxygen plasma improves the hydrophilicity of PI-COF particles and reinforces the heterogeneous linkage between PI-COF and TiO₂ nanoparticles, giving rise to more efficient charge injection. As a result, the champion cell exhibits a high power conversion efficiency of 10.46% with an enhanced J_SC of 19.43 mA/cm². This methodology of increasing solar efficiency by modification of the photoelectrode with the doping of PI-COFs in the TiO₂ nanoparticles is promising in the development of DSSCs.