Kuan-Lin Wu

Ruthenium(II) Sensitizers with Heteroleptic Tridentate Chelates for Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are a promising technology with the potential to harvest sunlight at low cost. Specifically, DSSCs based on either organometallic dyes or organic push-and-pull dyes adsorbed on nanocrystalline TiO2 photoanodes have attracted intensive research interest in the past two decades. The use of ruthenium(II)-based sensitizers is still the most attractive approach, because their photophysical and electrochemical properties can be systematically fine-tuned to achieve optimal material characteristics. Key examples of Ru sensitizers for DSSCs include the wellestablished N3 and N719 dyes and their functionalized derivatives. 6] These dyes are commonly assembled by incorporation of at least one 4,4’-dicarboxy-2,2’-bipyridine chelate (Scheme 1), on which the carboxy anchors allow efficient electron injection into the TiO2 nanoparticles. The accompanying thiocyanate ancillary ligands are pointed away from the TiO2 surface, thus providing intimate contact to the I /I3 redox couple in the electrolyte and subsequently triggering rapid regeneration of the oxidized sensitizer. The lower-lying absorption bands are attributed to metal-toligand charge transfer (MLCT) and display absorption thresholds close to 800 nm, unmatched by the majority of organic dyes known to date. For optimizing Ru sensitizers, it has been reported that, upon introduction of 4,4’,4’’-tricarboxy-2,2’:6,2’’-terpyridine (H3tctpy), the lowest energy transition could be extended toward the near-IR region, thus affording the panchromatic sensitizer known as N749, or black dye [nBu4N]3[Ru(Htctpy)(NCS)3]. [7] Although this design seems to show satisfactory sensitization up to 900 nm, it still possesses two major deficiencies. One is the inferior absorptivity in the visible region, attributed to the lack of effective auxochrome. The second is the presence of three thiocyanate ligands, which not only reduce the synthetic yield owing to coordination isomerism but also deteriorate the lifespan of the as-fabricated cell units, because the weak Ru NCS dative bonding causes notable dye decomposition during operation. In conjunction with current endeavors, we described one panchromatic dye PRT4, which possesses a styrene-substituted pyridyl pyrazolate (pypz) auxochrome. Its recorded solar-cell performance supersedes our best reference cells using N749, thus providing the first success for our research initiative. Herein we present a breakthrough design employing both the H3tctpy anchor and a newly designed tridentate ancillary ligand; the latter is also targeted for replacing the last remaining thiocyanate in our PRT4 dye as well as all three thiocyanates in N749 sensitizer. Thiophene derivatives or other auxochrome units were tethered to the central pyridyl group in an attempt to increase the light-harvesting capability. In this synthetic protocol (Scheme 2), the functionalized 2,6-bis(5-pyrazolyl)pyridine chelate was first prepared using a multistep process starting from chlorination of chelidamic acid, except for the synthesis of parent 2,6-bis(5-pyrazolyl)pyridine, which employed the commercially available 2,6diacetylpyridine. Bis-tridentate Ru complexes were then obtained from addition of the relevant 2,6-dipyrazolylpyridine derivative with source complex [Ru(tectpy)Cl3] in ethanol solution (tectpy = 4,4’,4’’-triethoxycarbonyl2,2’:6’,2’’-terpyridine). The crude ethoxycarbonyl Ru products were purified on a silica gel column and eluted with a mixture of hexane and ethyl acetate. To confirm their molecular structure, single-crystal X-ray analysis of one derivative (TF-2OEt; TF = thiocyanate-free) was carried Scheme 1. Ru sensitizers N3, N719, N749, and PRT4.

Donor–acceptor dyes with fluorine substituted phenylene spacer for dye-sensitized solar cells

A new series of organic sensitizers LJB-H, LJB-Fo and LJB-Fm with triphenylamine, 3,4-ethylenedioxythiophene plus various functionalized phenylenes, and cyanoacrylic acid as the donor, π-spacer, and anchoring group, respectively. These organic sensitizers exhibit much higher molar extinction coefficients than the parent LJ1 dye as well as reveal a remarkable fluorine substituent effect. LJB-Fm with fluoro substitution at the meta position relative to cyanoacrylic acid possesses a lower S0–S1 gap and weaker coupling with the TiO2 electrode (cf.LJB-H and LJB-Fo). Conversely, due to the resonance effect with respect to cyanoacrylic acid, the ortho-F-substituted LJB-Fo has a larger energy gap but stronger TiO2 affinity. As a result, LJB-Fo shows better DSSC performance with Jsc = 15.58 mA cm−2, Voc = 787 mV, FF = 0.67 and η = 8.22%.

Ruthenium and Osmium Complexes That Bear Functional Azolate Chelates for Dye‐Sensitized Solar Cells

The preparation of sensitizers for dye-sensitized solar cells (DSSCs) represents an active area of research for both sustainability and renewable energy. Both Ru(II) and Os(II) metal sensitizers offer unique photophysical and electrochemical properties that arise from the intrinsic electronic properties, that is, the higher propensity to form the lower-energy metal-to-ligand charge-transfer (MLCT) transition, and their capability to support chelates with multiple carboxy groups, which serve as a bridge to the metal oxide and enable efficient injection of the photoelectron. Here we present an overview of the synthesis and testing of these metal sensitizers that bear functional azolate chelates (both pyrazolate and triazolate), which are capable of modifying the metal sensitizers in a systematic and beneficial manner. Basic principles of the molecular designs, the structural relationship to the photophysical and electrochemical properties, and performances of the as-fabricated DSSCs are highlighted. The success in the breakthrough of the synthetic protocols and potential applications might provide strong stimulus for the future development of technologies such as DSSCs, organic light-emitting diodes, solar water splitting, and so forth.

Engineering of thiocyanate-free Ru(II) sensitizers for high efficiency dye-sensitized solar cells

We synthesized a series of Ru(II) metal complexes TFRS-1, -2, -4, -21, -22 and -24 with a single 4,4′-dicarboxylic acid-2,2′-bipyridine together with two functionalized pyridyl azolate ancillary ligands consisting of pyrazolate or triazolate groups. Both photophysical measurements and DFT/TDDFT calculations were conducted to gain insight into their electronic and optical properties. The triazolate series of sensitizers TFRS-21, -22 and -24 showed an enlarged optical band gap with respect to their pyrazolate counterparts TFRS-1, -2 and -4. When employed in dye sensitized solar cells (DSCs), the triazolate sensitizers show slightly inferior JSC values due to the poor incident photon-to-current conversion efficiencies recorded compared to the pyrazolate series. Moreover, the endowed 5-(hexylthio)thiophen-2-yl substituents exert a notable hyperchromic effect and bathochromic shift in the absorption spectra, which then improves the short circuit current JSC to 18.7 and 15.5 mA cm−2 and the overall conversion efficiency to η = 10.2% and 8.25% for TFRS-4 and TFRS-24, respectively. For the evaluation of VOC, transient photocurrent and photovoltage decay measurements were carried out to compare the rates of interfacial recombination of electrons from the TiO2 conduction band to electrolyte.

Harnessing the open-circuit voltage via a new series of Ru(II) sensitizers bearing (iso-)quinolinyl pyrazolate ancillaries

A novel class of Ru(II) sensitizers (TFRS-51–TFRS-54) with a 4,4′-dicarboxy-2,2′-bipyridine anchoring ligand and two trans-oriented isoquinolinyl (or quinolinyl) pyrazolate ancillaries were designed, characterized and used to fabricate dye sensitized solar cell (DSC) devices. In sharp contrast to Ru(II) sensitizers that employ functionalized thiophene appendages in their ancillary bipyridal ligands, the extended π-conjugation introduced by the isoquinolinyl or quinolinyl groups improves the optical absorptivity, particularly for the absorption located at ∼500 nm when compared with the parent sensitizer TFRS-1 possessing less conjugated 5-pyrid-2-yl pyrazolate ancillaries. As a result, DSCs incorporated with these dyes show much improved JSC compared with the reference device. Moreover, the use of bulky t-butyl substituents on the ancillary ligands improves the cell performance with excellent VOC of up to 830 mV recorded. Also, the addition of tetra-n-butyl ammonium deoxycholate [TBA][DOC] as co-adsorbent to the dye solution further improves the power conversion efficiency (η). The best solar cell parameters recorded were JSC = 16.3 mA cm−2, VOC = 860 mV, FF = 0.72, and η = 10.1% for a device sensitized with TFRS-52. The markedly high open-circuit voltage is confirmed by the longer electron lifetime revealed in transient photovoltage (TPV) measurement versus the TFRS-1 sensitizer, and is probably derived from a combination of the higher conduction band edge of TiO2 induced by the in situ metathesis of carboxylate anchors and the reduced recombination contributed by the bulky sensitizer.

Ru(II) sensitizers with a tridentate heterocyclic cyclometalate for dye-sensitized solar cells

A new series of bis-tridentate, thiocyanate-free Ru(II) complexes (TF-21–TF-24), containing the 2-azolyl-6-phenylpyridine cyclometalate, were synthesized. Upon switching from a pyrazolyl to a more electron deficient triazolyl moiety, the oxidation potentials were fine-tuned to readily accept electrons from the I−/I3− redox couple, so that the dye regeneration proceeds efficiently amid the device operation. This, together with high concentrations of 1,3-dimethylimidazolium iodide in the electrolyte which accelerates dye regeneration, attains a short-circuit photocurrent density as high as 18.09 mA cm−2 and an overall efficiency of 9.04%. The trifling decrease (3–6%) in efficiency after 1000 h of illumination at 60 °C manifests the high device stability, due to the much suppressed solvent volatility in combination with the robust bis-tridentate structure involved in cyclometalating the 2-azolyl-6-phenylpyridine ancillary.

Panchromatic Ru(II) sensitizers bearing single thiocyanate for high efficiency dye sensitized solar cells

We report on a new series of Ru(II) sensitizers PRT-21–PRT-24 suitable for high performance dye sensitized solar cells (DSCs). Their molecular design consists of a tridentate anchor, a bidentate pyrazolate ancillary and a single thiocyanate. On this architecture, we examined two types of anchor, incorporating the traditional 4,4′4′′-tricarboxy-2,2′:6′,2′′-terpyridine (i.e. tctpy) and newly evaluated 4,4′-dicarboxy-6-quinolin-8-yl-2,2′-bipyridine (i.e. Qbpy). This modification, along with the synergistic incorporation of either 5-hexylthien-2-yl or 5-(hexylthio)thien-2-yl substituent at the 4-position of the pyridyl pyrazolate chelate for enhancing the optical response, leads to the achievement of DSC with a prominent JSC of 20.4 mA cm−2 and VOC = 740 mV, and thus a high photon conversion efficiency of 11.16% using PRT-22. Comprehensive charge extraction, transient photovoltage and transient absorption measurements have been carried out to gain an insight into the fundamental mechanism of these new dyes and the associated device properties. These panchromatic Ru(II) sensitizers offer better product yields, higher stability and lower synthetic costs compared to that of black dye (N749), adding another dimension for better sensitizers en route to high performance DSCs.

Dye sensitized solar cells with cobalt and iodine-based electrolyte: the role of thiocyanate-free ruthenium sensitizers

Three isomeric Ru(II) metal complexes with distinctively oriented tpiq ancillary chelates, TFRS-80a, 80b and 80c, were prepared from the condensation of Ru(4,4′-diethoxycarbonyl-2,2′-bipyridine) (p-cymene)Cl with tpiqH, i.e. 6-(5-(2,6-bis(hexyloxy)phenyl)thiophen-2-yl)-1-(3-(trifluoromethyl)-1H-pyrazol-5-yl)isoquinoline. Photophysical and electrochemical investigations, together with DFT and TD-DFT calculations, allowed a comprehensive understanding of their basic properties in both solution state and on TiO2 surface. DSC cells with both an ultra-thin layer of transparent TiO2 (3.6 μm) and I−/I3− electrolyte were fabricated, for which the symmetric sensitizers TFRS-80a and 80c showed better performances (η = 8.37 and 8.26%) over that of the asymmetric counterpart TFRS-80b (η = 5.55%), the latter suffered from poor dye loading and consequently lowered JSC and VOC. In sharp contrast, all DSC cells with [Co(phen)3]2+/3+ electrolyte gave superior efficiencies (η = 8.36–9.06%), for which the thiocyanate-free architecture, the improved light harvesting capability, and the possession of conjugated and bulky 5-(2,6-bis(hexyloxy)phenyl)thiophen-2-yl functional moieties are three primary factors governing the observed results.

Molecularly Engineered Ru(II) Sensitizers Compatible with Cobalt(II/III) Redox Mediators for Dye-Sensitized Solar Cells

Thiocyanate-free isoquinazolylpyrazolate Ru(II) complexes were synthesized and applied as sensitizers in dye-sensitized solar cells (DSCs). Unlike most other successful Ru sensitizers, Co-based electrolytes were used, and resulting record efficiency of 9.53% was obtained under simulated sunlight with an intensity of 100 mW cm(-2). Specifically, dye 51-57dht.1 and an electrolyte based on Co(phen)3 led to measurement of a JSC of 13.89 mA cm(-2), VOC of 900 mV, and FF of 0.762 to yield 9.53% efficiency. The improved device performances were achieved by the inclusion of 2-hexylthiophene units onto the isoquinoline subunits, in addition to lengthening the perfluoroalkyl chain on the pyrazolate chelating group, which worked to increase light absorption and decrease recombination effects when using the Co-based electrolyte. As this study shows, Ru(II) sensitizers bearing sterically demanding ligands can allow successful utilization of important Co electrolytes and high performance.