Design and characterization of highly efficient porphyrin sensitizers for green see-through dye-sensitized solar cells

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www.rsc.org/books

Foreword by Roald Hoff mann

Introduction by Ahmed H Zewail Edited by Kenneth D M Harris and Peter P Edwards

Turning Points in Solid-State, Materials and Surface Science

A Book in Celebration of the Life and Work of Sir John Meurig Thomas

Edited by: Kenneth D M Harris and

Peter P Edwards

Foreword by: Roald Hoffmann

Introduction by: Ahmed H Zewail

Publication date: November 2007

Publisher: RSC Publishing

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Price: £99.95

Turning Points in Solid-State, Materials and Surface Science

provides a state-of-the-art survey of some of the most

important recent developments across the spectrum of

solid-state, materials and surface sciences, while at the same

time reflecting on key turning points in the evolution of this

scientific discipline and projecting into the directions for

future research progress.

The book serves as a timely tribute to the life and work

of Professor Sir John Meurig Thomas FRS, who has

made monumental contributions to this field of science

throughout his distinguished 50-year career in research,

during which he has initiated, developed and exploited

many important branches of this field. Indeed, the depth

and breadth of his contributions towards the evolution and

advancement of this scientific discipline, and his critical

role in elevating this field to the important position that it

now occupies within modern science, are demonstrated

recurrently throughout the chapters of this book.

Turning Points in Solid-State,

Materials and Surface Science

A Book in Celebration of the Life and Work of

Sir John Meurig Thomas

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www.rsc.org/pccp Volume 11 | Number 44 | 28 November 2009 | Pages 10229–10528

ISSN 1463-9076

Physical Chemistry Chemical Physics

PERSPECTIVE

Devlin et al.

Clathrate hydrates with

COVER ARTICLE

Diau et al.

Design and characterization of highly efficient porphyrin sensitizers for green

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Design and characterization of highly eﬃcient porphyrin sensitizers

for green see-through dye-sensitized solar cellsw

Hsueh-Pei Lu,

a

Chi-Lun Mai,

b

Chen-Yuan Tsia,

a

Shun-Ju Hsu,

a

Chou-Pou Hsieh,

b

Chien-Lan Chiu,

b

Chen-Yu Yeh*

b

and Eric Wei-Guang Diau*

a

Received 22nd August 2009, Accepted 23rd September 2009 First published as an Advance Article on the web 5th October 2009 DOI: 10.1039/b917271h

YD12 (g = 6.7%) is a green sensitizer remarkable for its outstanding cell performance beyond that of N719 (g = 6.1%) with no added scattering layer; the additional scattering layer assists N719 in promoting the eﬃciency in the red shoulder of the spectrum, but has only a small eﬀect on the improvement of the cell performance for porphyrins.

As a cost-effective energy-conversion device, dye-sensitized solar cells (DSSC) have received much attention after the pioneering work of Gra¨tzel and co-workers.1 These devices with Ru complexes as photosensitizers have attained the greatest efficiency (Z B11%) of conversion of photovoltaic power,2 but the limited availability of Ru dyes and their environmental concerns have stimulated much effort to find cheaper and safer alternative organic-based dyes.3 Among those non-Ru-based dyes, porphyrin chromophores are promising candidates because of their efficient capture of solar energy in the visible region. Numerous reports on porphyrin-based DSSC have appeared.4 _{In principle, the molecular}

design of a porphyrin sensitizer is based on a P–B–A structure, in which B represents a p-conjugation bridge serving as a spacer between the porphyrin light-harvesting center P and the carboxyl anchoring group A. A DSSC device using porphyrin sensitizers with the B–A unit functionalized at the b-position is reported to have attained a cell performance as great as Z = 7.1%;4dthe meso-substituted porphyrins gave smaller Z values.4f,g,i,jThe eﬀects of porphyrin aggregation on the TiO2

surface,4e,5 the bridge length of the sensitizers,5c,6 and the position and the number of the B–A units4e,j,6ahave all been investigated to rationalize the performance of the cell. We have reported a novel zinc porphyrin dye (YD1) with a molecular design based on a concept of a D–P–B–A structure (Scheme 1), simply adding an electron-donating diarylamino group (D) attached at the meso-position of P opposite the meso-substituted phenylethynylcarboxyl anchoring group.7 A device made from this porphyrin dye has a cell performance similar to that of a Ru-based DSSC, making the push-pull porphyrin the most eﬃcient green dye for DSSC applications.7

Here we report the cell performance for three new porphyrin sensitizers (YD11–YD13, Scheme 2): YD11 was modified from YD1 for which the two tert-butyl groups in the diarylamino substituent were replaced by two hydrocarbon long chains to improve its thermal and photochemical stability in a device. This design mimics the strategy applied in an amphiphilic ruthenium polypyridyl sensitizer (Z907) that has shown excellent stability toward water-induced desorption under both thermal stress and light-soaking conditions.8 YD12 and YD13 have the same diarylamino substituent as in YD11 but with the phenyl group in B being replaced by naphthalene and anthracene, respectively. Fig. 1 shows the absorption spectra of YD11–YD13 in ethanol solution; the Soret and Q bands shift toward longer wavelengths as the p system in B is expanded, because of the spectral coupling between the aromatic substituent and the porphyrin ring. This coupling effect was exceptionally pronounced in YD13, for which both Soret and Q bands become significantly broad and red-shifted. We investigated the electrochemical properties of these porphyrins using cyclic voltammetry. The measured oxidation and reduction potentials of YD11–YD13 are summarized in Table S1;w the corresponding energy-level diagram showing the HOMO and the LUMO of each porphyrin is depicted in Fig. S1.w These results indicate that electron injection from the LUMO of the excited porphyrin to the conduction band of TiO2 and dye regeneration from redox

couple I/I3to the HOMO of the porphyrin cation are feasible.

Because of the large absorption coefficients of these porphyrins, we expect that thinner TiO2 films might suffice

to produce a reasonable cell performance. We accordingly prepared three TiO2/FTO working electrodes with TiO2ﬁlm

thicknesses (L)B5, B10 and B(10 + 4) mm for photovoltaic

Scheme 1 Molecular design of YD1 based on a D–P–B–A structure.

a

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan. E-mail: [email protected]

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

Taichung 402, Taiwan. E-mail: [email protected]

w Electronic supplementary information (ESI) available: The details of synthetic procedures, electrode preparation and device fabrication, dye-loading examination, photovoltaic characterization, femtosecond ﬂuorescence spectroscopy, and supplementary tables (Table S1–S5) and ﬁgures (Fig. S1–S5). See DOI: 10.1039/b917271h

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measurements (the corresponding SEM images are shown in Fig. S2w). As the ﬁrst two TiO2 ﬁlms (A and B) contain

only the active layer (particle size B20 nm), the ﬁlms were essentially transparent before dye loading and they can be see-through after dye loading. The third TiO2ﬁlm (C) has the

same active layer as the second one (B10 mm) but with an additional scattering layer (B4 mm, particle size 200–600 nm) to improve further the light-harvesting eﬃciency. The corres-ponding DSSC devices were fabricated according to a standard procedure reported elsewhere.6b,7,9

The photovoltaic measurements were performed with three to four identical working electrodes for each porphyrin (YD11–YD13) adsorbed on each TiO2 ﬁlm (A–C) under the

same experimental conditions. The raw data from each J–V measurement are summarized in Tables S2–S4w for ﬁlms A–C, respectively; the corresponding averaged photovoltaic para-meters are summarized in Table 1. Fig. 2a–c show one set of typical J–V curves (working electrode ‘‘a’’ in Tables S2–S4w) of the porphyrin-based DSSC devices for TiO2 ﬁlms A–C,

respectively; for comparison, the cell performances of the devices made of N719 dye with the same TiO2ﬁlms as for YD11–YD13

are shown as dashed curves in each plot. Our results indicate that both YD11 and YD12 exhibit exceptionally superior perfor-mance relative to N719 dye; the poor perforperfor-mance of YD13 is remarkable and is discussed below. The short-circuit photo-current densities (JSC) of the two promising porphyrin-based

devices are significantly greater than those of the N719 devices, in particular for those of film B (Fig. 2b). Even though the open-circuit photovoltages (VOC) and fill factors (FF) for the

former are smaller than for the latter, the net effects of these variations make the overall efficiencies of power conversion of the YD11 and YD12 devices outperform those of the N719 devices at L B 10 mm without a scattering layer (Fig. 2b). YD11 (Z = 6.6%) and YD12 (Z = 6.7%) are thus two remark-able green sensitizers for their outstanding cell performances relative to that of N719 (Z = 6.1%) without an added scattering layer (film B) for light-penetrable DSSC applications. When the B10-mm TiO2films were covered with a scattering layer (film C),

we found that the cell performance of N719 was signiﬁcantly improved to Z = 7.3% whereas the performances of the porphyrin dyes increased only slightly (Z = 6.8% and 7.0% for YD11 and YD12, respectively, Fig. 2c). Our results indicate that a substantial increase in JSCfor the N719 device is a key

factor for the improvement of the cell performance with the addition of a scattering layer. To understand why the scattering layer was insensitive to the cell performances of the porphyrin-based devices, we performed measurements of the incident photon-to-current conversion eﬃciency (IPCE) for each device.10 Fig. 3a–c show the eﬃciency spectra of the same DSSC devices of which the corresponding J–V characteristics are shown in Fig. 2a–c. Integrating the IPCE over the AM 1.5G solar spectrum gives a calculated JSCsimilar to the collected

value for all devices under investigation (Fig. S3w). There are three important points deduced from our IPCE results. First, the eﬃciency spectra of both YD11 and YD12 sensitizers are similar for all three TiO2ﬁlms, but the spectra of YD12 have a

Q-band shoulder slightly extended to longer wavelengths, which increases slightly JSCfor YD12 relative to YD11. This

Scheme 2 Structures of YD11–YD13.

Fig. 1 Calibrated absorption spectra of YD11–YD13 in ethanol.

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eﬀect is consistent with the absorption spectral feature shown in Fig. 1. Second, the eﬃciency spectra of YD13 show smaller values than those of YD11 and YD12, which explains its poor

performance. This effect is inconsistent with the absorption feature shown in Fig. 1 and is discussed in the next section. Third, the efficiency spectra of YD11 and YD12 involve a large gap between the Soret and the Q bands in film A (Fig. 3a), but this gap became smaller when thicker TiO2 films (B) were

applied (Fig. 3b). On addition of another scattering layer (ﬁlm C), the gaps in YD11 and YD12 became even smaller

Fig. 2 Current–voltage characteristics of DSSC devices (working

electrode ‘‘a’’ in ESIw) with sensitizers of YD11–YD13 as indicated (open squares: YD11; ﬁlled circles: YD12; open triangles: YD13) under

illumination of simulated AM1.5 full sunlight (100 mW cm2) with an

active area 0.16 cm2of three ﬁlm thicknesses: (a) B5 mm (ﬁlm A);

(b)_{B10 mm (ﬁlm B); (c) B(10 + 4) mm (ﬁlm C). The dashed curves}

show the results of the N719 devices for comparison.

Table 1 Photovoltaic parameters of DSSC with photosensitizers YD11–YD13 and N719 as a function of TiO2ﬁlm thickness (L) under simulated

AM-1.5 illumination (power 100 mW cm2) and active area 0.16 cm2a

L/mm Dye Dye-loading/nmol cm2 _J SC/mA cm2 VOC/V FF Z/% B5 YD11 75 10.54 0.33 0.723 0.013 0.73 0.01 5.54 0.11 (Film A) YD12 82 10.75 0.34 0.724 0.009 0.72 0.02 5.60 0.09 YD13 62 3.30 0.16 0.633 0.006 0.71 0.01 1.49 0.08 N719 90 9.27 0.13 0.794 0.006 0.74 0.01 5.47 0.03 B10 YD11 154 12.99 0.83 0.715 0.006 0.71 0.03 6.56 0.09 (Film B) YD12 160 13.77 0.40 0.714 0.005 0.68 0.02 6.69 0.09 YD13 129 3.97 0.10 0.618 0.002 0.72 0.01 1.76 0.04 N719 178 10.97 0.42 0.769 0.002 0.73 0.02 6.16 0.14 B(10 + 4) YD11 b 14.01 0.14 0.716 0.003 0.68 0.01 6.79 0.12 (Film C) YD12 b _14.23_0.82 _0.717_0.008 _0.68_0.03 _6.91_0.15 YD13 b 4.12 0.08 0.630 0.002 0.72 0.01 1.86 0.04 N719 b 13.08 0.31 0.786 0.007 0.71 0.01 7.27 0.14 a

The photovoltaic parameters are the averaged values obtained from analysis of the J–V curves of three-four identical working electrodes for each device fabricated and characterized under the same experimental conditions; the raw data of each J–V measurement are summarized in

Tables S2–S4,w for ﬁlms A–C, respectively; the uncertainties represent two standard deviations of the measurements.bThe dye-loading amounts

are similar to those of the corresponding ﬁlms of the same thickness without a scattering layer (ﬁlm B).

Fig. 3 Corresponding IPCE action spectra of the same DSSC devices

as those shown in Fig. 2. The dashed curves show the results of the N719 devices for comparison.

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so that the efficiency spectra display a nearly flat nature in the entire visible region, 400–700 nm (Fig. 3c). The shoulders of the efficiency spectra on the red side extended no further beyond the edge of the Q band in the presence of a scattering layer; for this reason only a slight improvement in JSCwas found for the

porphyrin-based DSSC with a scattering layer. In contrast, a significant improvement in cell performance was found for a N719-based DSSC with a scattering layer, because of the effective scattering effect in the red shoulder of the efficiency spectrum. Based on the above observations, we conclude that the involvement of the partially allowed triplet MLCT states of ruthenium complexes2a _{is responsible for the enhanced}

efficiency in the red shoulder of the IPCE spectrum of N719, whereas the effect of spin–orbit coupling in zinc porphyrins was insufficient for S0 - T1 transitions to occur; the additional

scattering layer thus provides no improvement of the eﬃciency spectra of YD11–YD13 beyond the Q-band absorptions.

We applied femtosecond ﬂuorescence decays to investigate the dynamics of interfacial electron injection in the porphyrin/TiO2

films (film A), with porphyrin/Al2O3films serving as references

(L B 5 mm). The corresponding absorption spectra of the thin-ﬁlm samples appear in Fig. S4.w An additional band at B800 nm occurred for porphyrin-sensitized TiO2 ﬁlms

(B820 nm for YD13), but this spectral feature was absent from all porphyrin-sensitized Al2O3 ﬁlms. In our previous work on

the spectroelectrochemistry of YD1, a characteristic band at B800 nm was observed for the oxidized species of YD1.7

The presence of such a band atB800 nm for the YD1/TiO2ﬁlm thus

indicates the formation of species YD1+; the same is true for all other porphyrin sensitizers (YD11–YD13) sensitized on TiO2

ﬁlms found here. For the sensitized Al2O3 ﬁlms, the cationic

spectral feature was absent because the excited electrons of the porphyrin sensitizers cannot inject into the conduction band of Al2O3to form the cationic species on the Al2O3surface.11

Femtosecond excitation of the thin-ﬁlm samples was performed at 430 nm using a ﬂuorescence up-conversion system described elsewhere.5b The emissions at the intensity maximum were optically gated with the fundamental pulse (860 nm) to yield the emission decays of YD11–YD13/TiO2

ﬁlms shown in Fig. 4a–c; those of YD11–YD13/Al2O3ﬁlms are

shown for comparison. The temporal profiles of all samples show multi-exponential decay, and the corresponding time coefficients were obtained on analyzing the data with a parallel kinetic model.5b,12_{With the time coefficients weighted by their}

relative amplitudes (shown in parentheses), the average time coeﬃcients of the TiO2ﬁlms are determined to be all similar

(tTiO2 1:8 ps) for YD11–YD13; those of the Al2O3ﬁlms are

determined to be tAl2O3¼ 10:8, 10.6, and 4.8 ps, respectively.

The emission decays of the Al2O3 ﬁlms reﬂect only the

intermolecular energy transfer due to aggregation of the dye on the Al2O3surface, but the emission decays of the TiO2ﬁlms

contain not only the aggregate-induced energy transfer but also rapid electron injection from the excited state of a porphyrin into the conduction band of TiO2. If we assume

that the extent of dye aggregation on both TiO2and Al2O3

ﬁlms is similar (Fig. S4w), based on the same amount of dye molecules adsorbed on the ﬁlms, the quantum yields of YD11, YD12 and YD13 for electron injection on a TiO2surface become

evaluated to be Finj= 0.83, 0.83 and 0.62, respectively. 13

In Fig. 4, the fluorescence decays are similar for all three porphyrins sensitized on TiO2films, but the fluorescence decay

of YD13 was much more rapid than that of YD11 or YD12 sensitized on Al2O3ﬁlms. Hence the presence of the anthracene

group in the bridge from YD13 to TiO2did not hamper the rate of

interfacial electron transfer for the observed small injection yield. We consider two other possibilities responsible for the small injection yield of YD13: one is the anthracene-induced rapid intramolecular relaxation due to effective vibronic coupling, and the other is the anthracene-induced rapid relaxation of inter-molecular energy due to aggregation. To examine the first possibility, we measured time-correlated single-photon counting (TCSPC) to determine the lifetimes of excited state of YD11–YD13 in dilute solutions. All three porphyrins in ethanol (2 105 M) have similar lifetimes in a nanosecond range (1.3–1.4 ns). Ultrarapid non-radiative relaxation through intra-molecular channel is thus excluded. For the other possibility, we performed photovoltaic measurements for YD11–YD13 co-adsorbed with chenodeoxycholic acid (CDCA) on TiO2(film B)

in a ratio [porphyrin] : [CDCA] = 1 : 2; the results appear in Fig. S5,w and the obtained photovoltaic parameters are summarized in Table S5.w The presence of CDCA slightly decreases JSCfor YD11 and YD12 through a slight reduction of

the amount of dye loading, but CDCA plays a role to improve dye aggregation to some extent so as to signiﬁcantly enhance the

Fig. 4 Femtosecond emission decay curves of thin-ﬁlm samples

excited at the Soret band (lex= 430 nm) and probed at the wavelength

of maximum emission intensity. Panels (a)–(c) represent the transients

of YD11–YD13 on TiO2(circles) and on Al2O3(squares) ﬁlms. Solid

curves represent theoretical ﬁts with the corresponding time coeﬃ-cients and relative amplitudes (in parentheses) as indicated.

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JSCfor YD13. As a result, the eﬃciency of YD13 greatly increased

from 1.8% to 2.9% in the presence of CDCA. Anthracene thus induces much more rapid intermolecular energy transfer due to dye aggregation, leading to a cell performance for YD13 poorer than for YD11 and YD12.

The electron injection yield is an important factor to be considered to improve further the cell performance of the devices for organic dyes with a tendency to aggregate. For a Ru-based DSSC, in contrast, Finj was expected to be

B1.0,1b,3b,14

but a smaller value (B0.9) was reported.15For porphyrin sensitizers such as YD11 and YD12, even though several bulky tert-butyl groups and hydrophobic long alkyl chains were incorporated around the porphyrin macrocycle, the eﬀect of aggregation cannot be completely eliminated: there is still room for improvement of porphyrin-based solar cells from the approach of the molecular design.

In summary, three push–pull zinc porphyrin sensitizers (YD11–YD13) were designed, synthesized and characterized for highly efficient green see-through photovoltaic applications. Both YD11- and YD12-sensitized solar cells exhibit excellent cell performances that outperform those of N719-based DSSC because of their exceptionally large photocurrents generated in the 400–700 nm region with no added scattering layer. The additional scattering layer assists N719 in promoting the efficiency in the red shoulder of the spectrum, but has only a small effect on the improvement of cell performance for porphyrins. Moreover, the efficiency spectra of YD12 have a Q-band shoulder slightly extended to longer wavelengths, which increases JSCslightly and

thus also increases Z for YD12 relative to YD11. With the addition of a scattering layer, the cell performance of YD12 was comparable to that of N719 (Z = 6.91 0.15% vs. 7.27 0.14%). These porphyrins adsorbed on TiO2ﬁlms have a long-lived cationic state

occurring at B800 nm for its protracted charge recombination. Measurements of femtosecond ﬂuorescence decay for all three porphyrins sensitized on TiO2 and Al2O3 ﬁlms determined the

electron injection yields for YD11, YD12, and YD13 to be 0.83, 0.83 and 0.62, respectively. The poor performance of YD13 is understood to be due to the rapid aggregate-induced energy transfer in the presence of an anthracene group in the bridge. Work is in progress to diminish the eﬀect of dye aggregation in the device, to improve its stability and to apply this series of porphyrin dyes for prospective applications in solid-state DSSC.4d,16

Acknowledgements

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

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11 R. J. Ellingson, J. B. Asbury, S. Ferrere, H. N. Ghosh, J. R. Sprague, T. Lian and A. J. Nozik, J. Phys. Chem. B, 1998, 102, 6455. 12 The emission decays shown in Fig. 4 were well ﬁtted on

convolu-tion of the multi-exponential decay funcconvolu-tions (P

i

AiexpðttiÞ) with

the laser pulse (a Gaussian function with FWHMB220 fs), which

yielded time coeﬃcients ti(relative amplitudes Ai) of YD11–YD13

sensitized on both ﬁlms indicated in each plot.

13 Electron injection from the electronically excited state of porphyrin into

the conduction band of TiO2competes with other radiative or

non-radiative relaxation channels. The electron-injection quantum yield (Finj) deﬁnes the fraction of photons absorbed by the porphyrin that

are converted into electrons in the conduction band of TiO2, which is

formulated as Finj¼ kinj kinjþkaggþknrþkrﬃ ðt 1 TiO2 t 1 Al2O3Þ tTiO2¼ 1 tTiO2

t_Al2O3, in which kinjand kaggrepresent the non-radiative rate

coeﬃcients of electron injection and aggregate-induced energy

transfer, respectively; krand knrdenote radiative and other

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