Developing renewable energy resources to replace the depleting fossil fuel reserves is the biggest challenge for civilization. Dye-sensitized solar cell (DSSC) has emerged as a key player amongst the green energy sources due to their low cost, easy fabrication and high performance. Although a lot of factors determine the efficiency of DSSCs, the behavior of sensitizer molecules remain utmost important. A sum of research laboratories is devoted to develop new efficient sensitizers. DSSCs sensitized with ruthenium based dyes have achieved efficiencies around 11%. In spite of the high efficiencies, relatively low extinction coefficients, limited resources and high cost of purification make them less attractive for commercialization. Recently Grätzel and co-workers reported a Zn(II) porphyrin with D-π-A push-pull structure showing remarkable power conversion efficiency of 12% when incorporated with Co(II/III)tris(bipyriyl)-based redox electrolyte under illumination with standard AM 1.5G simulated sunlight. Though extension of the porphyrin π-conjugation by modification at β- or meso-positions is an effective strategy to shift the absorption into the NIR region, tedious synthesis is in general unavoidable. It is projected that the accessibility of high efficiency near-infrared (NIR) and panchromatic dyes might further improve the efficiency of DSSCs. Although enormous literature is available on porphyrin based DSSCs, surprisingly an expanded porphyrin has never been used as the sensitizer for DSSCs till date.
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From the spectroscopic point of view, the expanded porphyrins offer a broad range of compounds with absorption energies down to infrared regions. Expanded porphyrins with more than four pyrrole subunits in the porphyrinic macrocycle have been widely used in a variety of applications including anion and metal sensors, NIR sensing dyes, two-photon absorption (TPA) materials, and photodynamic therapy due to their increased cavity sizes and unique properties in association with extended π-conjugation. Even though red-shifted absorption bands of expanded porphyrins match well with the demands for a low energy sensitizer, partly owing to their symmetrical geometries and lacking of proper anchoring groups these expanded porphyrins remain unappealing for DSSC studies.
To uncover the potential of applying expanded porphyrins to DSSC studies, herein we report the syntheses, photophysical and photovoltaic properties of boron chelated oxasmaragdyrins, a class of aromatic core-modified expanded porphyrin with 22 π electrons. The oxasmaragdyrin boron complexes 4a-4e with structures depicted in Scheme 1, demonstrate panchromatic incident photon-to-current efficiencies (IPCEs), high short-circuit photocurrent densities (Jsc), and moderate-to-good overall efficiencies revealing an opportunity to develop expanded porphyrin-based high efficiency sensitizers for DSSCs.
[ï€ª] Sandeep B. Mane, Dr. Chen-Hsuing Hung
Institute of Chemistry, Academia Sinica, Taipei, 115 Taiwan
Fax: (+) 886-2-2783-1237
Dr. Eric Wei-Guang Diau
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu, 300 (Taiwan)
Dr. Liyang Luo,
Department of Chemistry,
Chung Yuan Christian University, Chung Li, 32023 (Taiwan)
Sandeep B. Mane
Department of Chemistry, National Taiwan Normal
University, Taipei, 11677 (Taiwan)
[ï€ªï€ª] This work has been supported by
Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author.
Scheme 1. Synthesis of oxamaragdyrins 4a4e. Reagents and conditions: a) BF3-OEt2, NEt3, CH2Cl2, RT; b) AlCl3, alcohol, dry CH2Cl2, reflux; c) KOH(aq), THF, reflux.
The desired oxasmaragdyrin complexes 4a-4e, were prepared in four steps in decent yields by mild reaction conditions. The oxasmaragdyrin 1 was prepared from a 3+2 condensation of meso-(4-methoxyphenyl)-dipyrromethane and 16-oxatripyrrane in the presence of TFA as the acid catalyst (Scheme 1). The BF2 chelated complex 2 was prepared by treating 1 with triethylamine and BF3-OEt2 in CH2Cl2 at room temperature. The treatment of oxasmaragdyrin-BF2 complex 2 with excess amount of corresponding alcohol in presence of AlCl3 at refluxing temperature for 10 min yielded B(OR)2 complexes 3b-3e. The final oxasmaragdyrins 4a-4e were isolated in moderate yields by hydrolysis of the precursors 2 and 3b-3e with aqueous KOH in THF at reflux conditions. All the final compounds and their corresponding precursors were fully characterized by spectroscopic techniques. In our design, alkoxide with long chains from C2 to C10 are introduced to shield oxasmaragdyrins from aggregation and to increase their solubility.
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The absorption spectra of oxasmaragdyrins 4a-4e display split Soret band in 400-500 nm and Q-bands in 550-750 nm region as shown in Figure 1a. Markedly, the split Soret band covers a broader range of absorption wavelengths than regular porphyrins. Additionally, the Q-bands, which are more intense than typical zinc or free base porphyrins, have a reverse ethio-type pattern to give the highest ε value at lowest energy band-I at 710 nm, shifting almost 100 nm more into the NIR region than Zn(TPP). The broadened Soret bands and high extinction coefficient Q-bands result in panchromatic absorption after adsorbed on TiO2 (Figure 1b) to increase the light harvesting efficiency and photocurrent density in the NIR photo-excitation. Noticeably, slight bathochromic shift in the absorption of complexes 4b-4e compared to complex 4a indicates the negligible influence of the electron donating alkoxide chains to the electronic structure of macrocycle.
Figure 1. Absorption spectra of oxasmaragdyrins 4a-4e a) in THF, (in inset: expansion of Q-bands) and b) adsorbed on TiO2.
The structural optimization on the compounds 4a-4e using the density functional theory (DFT) under B3LYP functional and the 6-31G basis set gave planar macrocycles for all oxasmaragdyrins (see supporting information for more details). Two alkoxide long chains lay on opposite sides of the oxasmaragdyrin plane and extended over the furan ring. Importantly, for all of the molecules, the calculated molecular orbitals (MO) confirm that in the HOMO (highest occupied molecular orbital) and HOMO-1 orbitals, majority of the electron density localizes on the macrocyclic π-system of oxasmaragdyrin ring whereas in the lowest unoccupied molecular orbital (LUMO) and LUMO+2 orbitals, in addition to the oxasmaragdyrin π-system, significant amount of electron density populates on the meso-carboxyphenyl anchoring group. The effective electron density redistribution from oxasmaragdyrin core moving toward anchoring carboxyphenyl ring upon the photo-excitation from ground state to excited state should facilitate effective electron injection from the excited state of oxasmaragdyrin to conduction band of TiO2.
Table 1. Photophysical and Electrochemical data for oxasmaragdy- rin sensitizers.
λabs [nm] (ε [103M−1cm−1]) [a]
446 (278), 474 (113),
450 (303), 479 (121),
450 (291), 479 (117),
450 (242), 480 (98),
450 (188), 480 (79),
[a] Absorption maximum of oxasmaragdyrins in THF [b] First oxidation potentials vs. NHE determined by CV in THF calibrated by Fc/Fc+ redox couple. [c] E(0,0) values were estimated from the intersection of the absorption and emission spectra. [d] Excited state oxidation potentials approximated from Eox and E(0,0).
Another criterion for an effective electron injection into TiO2 and fast regeneration of oxidized dye is proper matching of LUMO and HOMO energy levels with TiO2 conducting band and energy level of I−/I3− couple, respectively. The cyclic voltammetry (CV) measurements of all oxasmaragdyrins 4a-4e were performed to obtain the redox potentials of the sensitizers and found that all voltammograms observed two reversible oxidations and one reversible reduction couples (Figure S-15, supporting information). The steady potentials and currents under multiple scans during the CV measurements for oxasmaragdyrins suggest high stability and reversibility of these expanded porphyrins toward redox processes suitable for application as sensitizer in DSSCs. The data in Table 1 shows that the oxidation potentials of oxasmaragdyrins 4b-4e, shifted towards less positive by 140 to 170 mV resulting in elevated HOMO levels compared to oxasmaragdyrin-BF2 4a. However, the reduction potentials of 4b-4e shifted toward more negative by approximately 120 mV than oxasmaragdyrin-BF2 complex. Based on E(0,0) obtained from absorption and emission spectra and the first oxidation potential from CV, the ground state oxidation potentials (Eox) and excited state oxidation potentials (Eox*) were calculated as listed in Table 1. For all of the oxasmaragdyrin boron complexes, Eox* are more negative than the conduction band edge of the TiO2 electrode (-0.5 V vs. normal hydrogen electrode (NHE)) while the Eox are more positive than the 0.4 V vs. NHE for the redox potential of I−/I3−. Although the red-shifting in absorption band of oxasmaragdyrins stands for smaller band gap between HOMO and LUMO orbitals, the electrochemical data confirms the high driving force for both electron injection and dye regeneration.
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The BF2 and B(OR)2 chelated oxasmaragdyrins 4a-4e were fabricated into the solar cell devices according to the procedures illustrated in the experimental section. The photovoltaic performance of the devices were measured under standard AM 1.5 G simulated solar conditions using THF as the immersion solvent and are summarized in Table 2. The DSSCs sensitized with oxasmaragdyrin BF2 complex 4a obtained a short circuit photocurrent (Jsc) of 12.19 mA cm-2, an open circuit voltage (Voc) of 0.51 V, and a fill factor (FF) of 0.68, which corresponds to overall photon to current conversion efficiency of 4.26%. It is obvious from the data in Table 2 that the efficiencies of the oxasmaragdyrins 4b-4e decrease as the alkoxy chain length increase. Our current data suggest that the lower performance of B(OR)2 complexes can be attributed to lower dye loadings. The dye loading experiments delivered highest loading of 2.39 - 10−5 mol−1 cm−2 for 4a and 1.54, 1.32, 1.19 and 0.68 - 10−5 mol−1 cm−2 for the B(OR)2 complexes 4b-4e, respectively. Apparently, a longer alkyl chain will dramatically increase the intermolecular steric repulsion resulting in lower dye loading and consequently decreasing the photocurrent density. The presence of chenodeoxycholic acid (CDCA) as an additive to the oxasmaragdyrin dyes failed to further improve the photon to current conversion efficiencies. For all the measurements, tetrahydrofuran was picked as the solvent for dye adsorption to TiO2 for the sake of a higher solubility. Strikingly, using ethanol as the immersion solvent for 4a, calculated a maximum efficiency of 4.95% with a Jsc of 11.19 mA cm-2, a Voc of 0.61 V, and a fill factor of 0.72.
Table 2. Photovoltaic properties of devices with 4a-4e as the sensitizers.[a]
Jsc [mA cm-2]
[a] Immersion in THF at 25 oC for 24 h; measured at AM 1.5G one sun; [b] Immersion in EtOH at room temperature for 6 h.
The current-voltage characteristics and incident-photon-to-electron efficiency (IPCE) action spectra of the corresponding devices are given in Figure 2. IPCE spectra of the oxasmaragdyrins were similar with the absorbance of the dyes adsorbed on TiO2. Consistent with the efficiency measurements; the oxasmaragdyrin 4a gave prominent conversion efficiency in the region of Soret band covering absorption wavelengths from 380 to 520 nm with a maximum of 75% efficiency at 410 nm. The tailing off of the Soret absorption has overlapped with the increasing IPCE of Q band to cover the entire visible region with a maximum efficiency of 52% at 702 nm. The onset of the IPCE spectrum was at 800 nm which showed a bathochromic shift of approximately 100 nm compared with the D-π-A zinc porphyrin dye YD-2 and the IPCE in Q band region was well comparable with phthalocyanines.[5d,14] Usually the B(OR)2 chelated oxasmaragdyrins give lower IPCEs in both Soret band and Q-band region except that the efficiency reaches near 80% at 440 nm for B(OEt)2 chelated oxasmaragdyrin 4b. The abnormality can also be visualized from the J-V curve with 4b exhibiting the highest Voc among devices prepared using THF as the immersion solvent.
Figure 2. a) J-V characteristics and b) IPCE action spectra of DSSCs fabricated with oxasmaragdyrins 4a-4e measured under 100% sun (AM 1.5 G).
From a better matching of the peak shapes between absorption spectra in solution states and in solid states for oxasmaragdyrins-B(OR)2 (Figure S-14), it appears that the alkoxy substituents can effectively prevent the dye aggregation. However, the observed lower net potentials for cells with B(OR)2 (R = C4H9, C7H15 and C10H21) chelated oxasmaragdyrins as sensitizer suggested that the decreased dye loadings and therefore lower surface coverage will encourage the back electron transfer from TiO2 surface to I3−. The highest Voc for 4b is presumably due to a delicate balance between the positive effect of a lesser degree of dye aggregation and the negative effects of decreased dye loading from steric constrain.
In conclusion, without extensive fabrications on the substituents of dye and before exhausted device optimizations, we were able to demonstrate for the first time that boron complexes of oxasmaragdyrin, a core-modified expanded porphyrin, can be effective sensitizers for DSSC. In contrast with the relatively low efficiencies for the devices using BODIPY as the sensitizer, boron chelated oxasmaragdyrins are aromatic compounds with high extinction coefficients, high stability, and good power conversion efficiency. The photophysical characterizations suggest that the boron chelation to oxasmaragdyrins can reduce intermolecular aggregation and also these dyes exhibit the desired redox potential required for an effective sensitizer in DSSCs. More importantly, broad absorption spreading the entire visible region and its lower energy Q band covering the NIR region make this class of compounds an optimistic candidate for being one of the future selections of porphyrin-based sensitizing dyes. Given that the expanded porphyrins have already demonstrated unique photophysical properties with absorptions reaching IR region, more studies to apply the expanded porphyrins as the sensitizer for DSSC are ongoing in our lab.
Photovoltaic characterization: The current-voltage characteristics of the devices were measured with a digital source meter (Keithley 2400) under one sun AM 1.5G irradiation from a solar simulator (SAN-EI, XES-502S) calibrated with a silicon-based reference cell (S1133, Hamamatsu). The incident photons to current efficiency (IPCE) spectra of the corresponding devices were recorded with a system comprising a Xe lamp (PTi A-1010, 150 W), a monochromator (PTi, 1200 g mm-1 blazed at 500 nm), long passed filter and a source meter.
Solar cell fabrication: TiO2 nanoparticles were prepared with a sol-gel method as reported. A paste composed of TiO2 nano particles (20 nm) was coated with screen printing on a TiCl4-pretreated FTO glass (TEC7, Hartford, USA). A scattering layer (200-600 nm) was screen-printed additionally on the active layer to improve the performance of the solar cell. The TiO2 films were annealed according to a programmed procedure: heating at 80 oC for 15 min; heating at 135 oC for 10 min; heating at 325 oC for 30 min; heating at 375 oC for 5 min; heating at 450 oC for 15 min; and heating at 500 oC for 15 min. The thickness of the transparent active layer was 10 µm and that of the scattering layer was 4 µm. The TiO2 films (active size 0.4-0.4 cm2) were sensitized by dipping it in the solution of oxasmaragdyrins (0.2 mM) in THF for 24 h at ambient temperature. After being washed with ethanol, the sensitized working electrode was assembled with a Pt counter electrode and sealed with a hotmelt film. An electrolyte solution (0.05 M I2, 0.1 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and 0.6 M tert-butyl pyridine (t-BP) in a mixture of acetonitrile and valeronitrile 18/15 (v/v) was used in all devices.
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Keywords: dye sensitized solar cells · porphyrinoids · energy conversion · smaragdyrins
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Dye Sensitized Solar cell
Expand it Further: Liquid DSSCs containing a 22-π-electron expanded core-modified oxasmaragdyrin as the sensitizer with panchromatic absorption extended to the near-IR region are prepared to achieve a promising energy conversion efficiency up to 4.95%, with a short-circuit current density (Jsc) of 11.19 mA cm-2, an open-circuit voltage (Voc) of 610 mV and a fill factor (FF) of 0.72.
S. B. Mane, L. Luo, Eric W.-G. Diau, C. H. Hung* __________ Page - Page
Oxasmaragdyrin Boron Complexes: First High-Efficiency Panchromatic Expanded Core-Modified Porphyrin for DSSCs.