Synthesis and characterization of diporphyrin sensitizers for dye-sensitized solar cells

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Synthesis and characterization of diporphyrin sensitizers for

dye-sensitized solar cellsw

Chi-Lun Mai,

a

Wei-Kai Huang,

b

Hsueh-Pei Lu,

b

Cheng-Wei Lee,

a

Chien-Lan Chiu,

a

You-Ren Liang,

a

Eric Wei-Guang Diau*

b

and Chen-Yu Yeh*

a

Received (in Cambridge, UK) 21st August 2009, Accepted 4th November 2009 First published as an Advance Article on the web 24th November 2009 DOI: 10.1039/b917316a

Novel porphyrin dimers with broad and strong absorption in the visible and/or near IR regions have been synthesized; the meso–meso-linked porphyrin dimer (YDD1) exhibited the best photovoltaic performance with power conversion eﬃciency 5.2% under AM 1.5G one solar illumination.

The development of clean and renewable energy sources reflects the limited fossil resources and severe environmental problems caused by their combustion. Infinite and inexhaustible solar energy is a key resource to meet a rapidly increasing global demand for energy. Dye-sensitized solar cells (DSSC) are promising devices to generate clean energy as an alter-native to the traditional solar cells based on silicon. A typical DSSC comprises dye-sensitized nanocrystalline films of TiO2

and an iodide/triiodide mediator.1The greatest eﬃciency (Z) for the conversion of solar to electric energy for a DSSC is 410% based on ruthenium polypyridine complexes.1b In view of the cost and environmental concerns about ruthenium dyes, organic dyes have attracted attention because of their diversity, the facile modiﬁcation of their molecular structures, their intense absorption and cheap production.2–11

To generate a large photocurrent response, organic dyes in an eﬃcient DSSC must have broad and intense absorption in the visible and near IR regions.12 Porphyrin sensitizers are dominant candidates for this purpose because of their intense absorption in Soret and Q bands to harvest solar energy eﬃciently in a broad spectral region,13but the existence of a gap between the Soret and Q bands in monomeric porphyrins limits their cell performances. Porphyrin arrays linked with conjugated acetylene bridges exhibit strong electronic coupling between porphyrin rings, resulting in splitting of the Soret band and broadening of the Q bands.14Electronic absorption spectra of meso–meso-linked porphyrin arrays and their corresponding doubly and triply fused porphyrin arrays also show wide absorption covering the visible and near IR region.15 _{By dint of such spectral features, these porphyrin}

arrays are prospectively eﬃcient sensitizers for application in DSSC. Here we report the synthesis and the spectral,

electrochemical and photovoltaic properties of four porphyrin dimers; their molecular structures show diverse connectivity between the two porphyrin macrocyles, as displayed in Scheme 1. Details of their synthetic procedures appear in the ESIw.16–19

Absorption spectra of these porphyrin dyes are shown in Fig. 1; the corresponding spectral properties are listed in Table S1 (ESIw). All porphyrin dimers exhibit a much broader absorption than that of reference compound YD0. Compound YDD0 shows split Soret bands in the range 400–500 nm, and red shifts and broadening of the Q bands due to interporphyrin electronic coupling. Dimer YDD1 also exhibits a split Soret band ascribed to excitonic coupling. The absorption spectra of YDD2 and YDD3 exhibit a typical feature for fused porphyrin dimers with three major bands. Bands I and II of YDD2 and YDD3 feature a range across almost the entire visible region. Bands III appears at 756 and 845 nm for YDD3, whereas those for YDD2 are much wider and range from 900 to 1300 nm. The molar absorption coeﬃcients at the maximum absorption wavelengths for these dimers are all large and fall in the range 0.3–2.9 105dm3mol 1cm 1. Fluorescence spectra of YDD2 and YDD3 were unobtainable because of the sensitivity limit of our detector for this region, but they are expected to fall in the near IR region.20

The reduction and oxidation potentials of these porphyrin dyes are summarized in Table S1 (ESIw). The cyclic voltammogram of YDD0 shows two 1-e reversible oxidations at E1/2= +0.89 and +1.08 V (Fig. S1, ESIw). The ﬁrst and

second oxidations are cathodically shifted by 0.15 and 0.40 V relative to those of YD0, reﬂecting elevated energy levels due

Scheme 1 Molecular structures of porphyrin dyes.

a_{Department of Chemistry, National Chung Hsing University,}

Taichung 402, Taiwan. E-mail: [email protected]; Fax: +886 4-2286-2547; Tel: +886 4-2285-2264

b_{Department of Applied Chemistry and Institute of Molecular}

Science, National Chiao Tung University, Hsinchu 300, Taiwan. E-mail: [email protected]; Fax: +886 3-572-3764; Tel: +886 3-513-1524

w Electronic supplementary information (ESI) available: Syntheses, characterizations, devices, measurements, supplementary Table S1 and Fig. S1–S4. See DOI: 10.1039/b917316a

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to strong interporphyrin interaction in YDD0. Compound YDD1 exhibits two overlapping oxidation waves at E1/2= +0.99 and +1.17 V, corresponding to 1-e abstraction

from each porphyrin ring. The cyclic voltammogram of YDD2 displays two oxidations at E1/2= +0.69 and +1.03 V, which

are cathodically shifted by 0.30 and 0.14 V, respectively, relative to those of meso–meso-linked dimer YDD1. Similar electrochemical behavior was observed for fused porphyrin YDD3. The CV results of fused porphyrins show elevated HOMO and decreased LUMO energy levels, caused by extended p-conjugation over the two porphyrin macrocycles, consistent with results obtained from DFT calculations (Fig. S2, ESIw).

Fig. 2 shows energy levels of YDD0–YDD3, with YD0 for comparison. The HOMO levels were derived from the ﬁrst oxidation potentials of the porphyrins and the LUMO levels from the diﬀerence of the HOMO level and the absorption threshold of each porphyrin. The HOMO levels of these porphyrin dimers are all more positive than the oxidation potential for I /I3 , indicating that dye regeneration might be

feasible for all sensitizers; the LUMO energy levels of only YDD0 and YDD1 are more negative than the conduction-band (CB) edge of TiO2, whereas those of YDD2 and YDD3

are not negative enough for eﬀective injection of the excited-state electrons into the CB of TiO2.

Fig. 3a and b show the current–voltage characteristics and eﬃciencies of conversion of incident photons to current

(IPCE) of porphyrin-based DSSC; the corresponding photo-voltaic parameters are summarized in Table S1 (ESIw). Both porphyrin dimers YDD0 and YDD1 perform similarly to the reference compound (YD0), but the fused porphyrins (YDD2 and YDD3) exhibit poor cell performance. In particular, the photocurrents of YDD2 and YDD3 are small, consistent with the potential feature shown in Fig. 2. For YDD2, essentially no injected electrons were observed (Fig. 3b), because the energy level of LUMO is substantially lower than the CB edge of TiO2. For YDD3, a small response in the IPCE action

spectrum corresponds to the contribution of broad Bands I and II of the fused porphyrin, but no injected electrons were observed for the broad Band III in region 700–900 nm. We infer that electron injection from the excited states of YDD3 to TiO2 competed with energy relaxation from higher excited

states (Bands I/II) to the lowest excited state (Band III), and there was insuﬃcient kinetic energy for the electrons to inject from Band III of YDD3 to the CB of TiO2.

Integrating the IPCE over the AM 1.5G solar spectrum yields a calculated JSCsimilar to the collected value for devices

of YD0, YDD0 and YDD1 (Fig. S3, ESIw), conﬁrming the accuracy of the current–voltage results shown in Fig. 3a. As shown there, the short-circuit photocurrent density (JSC) of

YDD1 is greater than that of YD0, whereas the open-circuit voltage (VOC) of the former is smaller than the latter; the net

effect produces a slightly greater power-conversion efficiency of the former than that of the latter (Z = 5.23 vs. 5.14%). The fact that the JSCvalue of YDD1 was significantly greater than

that of YD0 is understood to be due to the effective excitonic coupling of the two porphyrin macrocycles in the dimer, whereas such a character was absent in the monomer. As a result, the IPCE spectrum of YDD1 exhibits a flat response over the entire visible region whereas that of YD0 shows a large gap between the Soret and Q bands (Fig. 3b). In contrast, the overall efficiency of YDD0 was significantly smaller than that of YDD1 (4.07 vs. 5.23%) because of the smaller JSCand

VOCvalues of the former. Even though the IPCE spectrum of

YDD0 shown in Fig. 3b displays a broad feature covering spectral range 400–800 nm, the IPCE values of YDD0 were much smaller than those of YDD1 in the region of spectral response.

Fig. 1 Calibrated absorption spectra of YD0 and YDD0–YDD3 in CH2Cl2/pyridine (100/1).

Fig. 2 Schematic energy levels of porphyrins YD0 and YDD0–YDD3 based on absorption and electrochemical data in ESIw.

Fig. 3 (a) Current–voltage characteristics of DSSC devices with sensitizers YD0-YDD3 under illumination of simulated AM1.5 full sunlight (100 mW cm 2) with active area 0.16 cm2; (b) Corresponding action spectra for the eﬃciency of incident photon-to-current conversion.

810 | Chem. Commun., 2010, 46, 809–811 This journal is c _{The Royal Society of Chemistry 2010}

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We notice that the absorption coeﬃcients of YDD0 are, in general, greater than those of YD0 and YDD1 (Fig. 1), but the values of IPCE of the former are substantially lower than those of the latter (Fig. 3b). As the results of dye-loading experiments (Fig. S4, ESIw) show similar amounts of dye molecules (B75 nmol cm 2) being sensitized on TiO2 ﬁlms

for YD0-YDD1, the smaller external quantum eﬃciencies of YDD0 relative to those of YD0 or YDD1 are thus inferred to arise from the eﬃciency of electron injection into TiO2, which

was smaller for the former than for the latter. There are two reasons that explain these eﬀects; one is that the LUMO energy level of YDD0 is much lower than that of YD0 or YDD1, which might impede electron injection for the former. The other is that the nearly planar structure of YDD0 facilitates p-conjugation between two porphyrin macrocycles and provides a decreased driving force to push electrons toward TiO2; this explanation is consistent with the frontier

orbital pictures shown in the supplementary Fig. S2 (ESIw). The planar geometry of YDD0 might also facilitate the formation of dye aggregates that signiﬁcantly decrease the eﬃciency of electron injection.

In conclusion, we have synthesized porphyrin dimers with varied connectivity (YDD0–YDD3) between the two porphyrin moieties; their nature significantly influences their spectral, electrochemical and photovoltaic properties. Among these porphyrin dimers, YDD1 exhibited the greatest photocurrent density because of its flat IPCE spectrum with external quantum efficiencies B70% covering the entire visible spectral region. Although YDD0 displayed a further broad IPCE spectrum extending to the near-IR region, the cell performance was not improved because of the smaller quantum efficiencies (producing a smaller short-circuit current density) and open-circuit voltage. The best photovoltaic performance of YDD1 attained 5.2%, which is slightly greater than, but comparable with, that of the reference monoporphyrin YD0. Introduction of an electron-donating group such as diarylamine into the meso-position opposite the anchoring group significantly increases the efficiency of conversion of solar energy to electricity.21 _{Preparation of diporphyrin dyes incorporating}

electron-donating groups to improve the cell performance for DSSC applications is in progress.

National Science Council of Taiwan and Ministry of Education of Taiwan, under the ATU program, provided support for this project.

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