Design and characterization of alkoxy-wrapped push-pull porphyrins for dye-sensitized solar cells

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This article is part of the

Porphyrins &

Phthalocyanines

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Guest editors: Jonathan Sessler, Penny Brothers and

Chang-Hee Lee

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4368 Chem. Commun., 2012,48, 4368–4370 This journal is c The Royal Society of Chemistry 2012

Cite this:

Chem. Commun

., 2012, 48, 4368–4370

Design and characterization of alkoxy-wrapped push–pull porphyrins

for dye-sensitized solar cellswz

Teresa Ripolles-Sanchis,

a

Bo-Cheng Guo,

b

Hui-Ping Wu,

c

Tsung-Yu Pan,

c

Hsuan-Wei Lee,

b

Sonia R. Raga,

a

Francisco Fabregat-Santiago,

a

Juan Bisquert,*

a

Chen-Yu Yeh*

b

and

Eric Wei-Guang Diau*

c

Received 15th February 2012, Accepted 9th March 2012 DOI: 10.1039/c2cc31111a

Three alkoxy-wrapped push–pull porphyrins were designed and synthesized for dye-sensitized solar cell (DSSC) applications. Spectral, electrochemical, photovoltaic and electrochemical impedance spectroscopy properties of these porphyrin sensitizers were well investigated to provide evidence for the molecular design.

Porphyrins are promising candidates as highly eﬃcient sensitizers for dye-sensitized solar cells (DSSC) because of their superior light-harvesting ability in the visible region.1–3Recent advances on the development of a porphyrin sensitizer (YD2-o-C8) with co-sensitization of an organic dye (Y123) using a cobalt-based electrolyte attained a power conversion eﬃciency of 12.3%,4

which is superior to those developed based on Ru complexes5 and becomes a new milestone in this area. The key structural feature on molecular design of a highly efficient porphyrin sensitizer is to bear with long alkoxyl chains in the ortho-positions of the meso-phenyls so as to effectively envelope the porphyrin ring to reduce the degree of dye aggregation for a higher electron injection yield and to form a blocking layer for a better charge collection yield.6In the present study, we further design three porphyrin sensitizers (YD20–YD22, Chart 1) based on the structure of YD2-o-C8 but with extended p-conjugation in order to enhance the light-harvesting ability. Basically all of them have the same ortho-substituted porphyrin core with two phenylethynyl (PE) groups acting as a p-bridge in the meso-position of the ring. YD20 and YD22 dyes have the acceptor group (ethynylbenzoic acid) the same as that of YD2-o-C8 but with different donor groups: YD20 has a triphenylamino group

with two methoxyl substitutes and YD22 has a phenylamino group with two n-butyl chains. On the other hand, YD20 and YD21 dyes have the same donor group but the cyanoacrylic acid was used as an anchoring group in YD21. This approach mimics the molecular design of an organic dye7 _{having the}

acrylonitrile group with strong electron-pulling power to act as an eﬃcient acceptor for the porphyrin dye.

The details for the syntheses, optical and electrochemical characterizations of YD20–YD22 are given in ESI.z These porphyrin dyes were fabricated into DSSC devices for photo-voltaic and electrochemical impedance spectroscopy (EIS) characterizations. Fig. 1a and b show the J–V curves and the corresponding Incident Photon to Current Conversion Eﬃciency (IPCE) action spectra for the YD20–YD22 devices, respectively; the obtained photovoltaic parameters and the amounts of dye-loading are summarized in Table 1. The results indicate that the short-circuit current densities (JSC) exhibit a trend YD20 4

YD22 4 YD21 and the open-circuit voltages (VOC) display a trend

YD20 4 YD22B YD21; the overall power conversion eﬃciencies (Z) show the same order as JSC, which is consistent with the

variations of the IPCE action spectra showing the same order. As a result, YD20 has the highest JSC(17.43 mA cm2) and VOC

(676 mV), which yields the greatest Z (8.1%) among the three porphyrins under investigation. Even though the cyanoacrylic substitute makes YD21 a slight red shift in the absorption spectrum (Fig. S1, ESIz), the floppy feature of the CQC double bond might tilt the molecules adsorbed on TiO2film to significantly decrease its

IPCE values and the corresponding current density. However, YD20 and YD22 have the same anchoring group and very similar

Chart 1 Molecular structures for YD20–YD22 porphyrin dyes.

a

Photovoltaics and Optoelectronic Devices Group,

Departament de Fı´sica, Universitat Jaume I, 12071 Castello´, Spain. E-mail: [email protected]; Tel: +34-964-38-7540

b_{Department of Chemistry and Center of Nanoscience &}

Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan. E-mail: [email protected];

Fax: +886-4-22862547; Tel: +886-4-22852264

c_{Department of Applied Chemistry and Institute of Molecular}

Science, National Chiao Tung University, Hsinchu 30010, Taiwan. E-mail: [email protected]; Fax: +886-3-5723764;

Tel: +886-3-5131524

w This article is part of the ChemComm ‘‘Porphyrins and Phthalocyanines’’ web themed issue.

z Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cc31111a

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012,48, 4368–4370 4369

absorption spectra (Fig. S1, ESIz), therefore, the diﬀerences in IPCE and photocurrent are related to the eﬀect of the donor groups. Note that the decrease in the IPCE occurs at a nearly constant level for all the wavelengths of the spectra for YD21 compared to YD20. Thus, the loss of electrons is independent of the energy of the absorbed photons. Transport and injection losses may be considered for the decrease in IPCE, which is discussed in the following.

Dye loading measurements yielded 161, 132, and 134 nmol cm2for YD20, YD21 and YD22, respectively. The changes in JSCbetween the dyes with the same anchoring group, YD20

and YD22, may be understood in terms of the diﬀerent amounts of loaded sensitizer. Further explanation is needed for sample YD21 as the decrease in JSCis larger despite the

amount of dye loading in the cell is the same as for YD22. Electrochemical Impedance Spectroscopy was used to complete the analysis of injection and to gain insight into the transport and charge losses characteristics of the DSSC with the different dyes.8 From the fitting of impedance spectra of the DSSC at different applied potentials under 1 sun illumination, we obtained the chemical capacitance (Cm),

transport resistance in the TiO2 (Rtr), recombination resistance

(Rrec), as a function of the Fermi level voltage (VF) shown in

Fig. 2a, b, and c, respectively. Other contributions to the total resistance of the cell such as diﬀusion, counter electrode and FTO resistances were grouped as series resistance (Rs). The eﬀect of Rsin

the applied potential (Vapp) was removed to obtain the VFthat

may be calculated through VF= Vapp jRs. From the plot of

Cmvs.VFshown in Fig. 2a, the position of the conduction band

edge of TiO2 (Ec) may be estimated as reported elsewhere. 9

Through these calculations, we estimated that for YD20 Ec E

0.48 V vs. NHE, while for YD21 Ecwas displaced +4 mV and

YD2210 mV. Data from transport resistance shown in Fig. 2b also provide very small displacements in Ec, corroborating that

all the TiO2conduction bands remain almost unchanged for the

three dyes as obtained from the capacitance data.

To understand the origin of the small diﬀerences in the VOC

found for the three diﬀerent dyes it is needed to analyze the behavior of the recombination resistance in Fig. 2c. In previous studies,8,10 when comparing the recombination resistance of diﬀerent samples it has been found that the higher the value of Rrec, the larger the VOC, while only very large changes in

photo-current produce small variations in VOC. The results here match

very well with this analysis: as it can be seen in Fig. 2c, YD20 has the larger recombination resistance and VOC, whereas YD21 and

YD22 have similar values of Rrecshowing almost the same VOC.

Data from Rrec and Rtr may be used to calculate the

diﬀusion length (Ln) in TiO2ﬁlm shown in Fig. 2d as8

Ln¼ L

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rrec=Rtr p

ð1Þ where L is the ﬁlm thickness (15 mm) represented as a dashed curve in Fig. 2d. The Lnvalues exhibit a systematic trend with

the order YD20 4 YD22 4 YD21 with those of YD20 and YD22 reaching values greater than their film thickness whereas those of YD21 being significantly smaller than the film thickness. This implies that the YD21 device suffers from a poorer collection efficiency of injected electrons what produces the extra decrease in JSCfound for this sample.

The small diﬀerences found for the position of the conduction band edge (Ec) may also help to ﬁne tune the roles of the linker in

these Zn–porphyrin dyes. If the Fermi level potential is shifted the amounts found for the displacement of Ec, it is possible to compare

the recombination resistance of the DSSC at the potential level with the same number of injected electrons. To do this we deﬁne the potential at the equivalent conduction band position8

Vecb= VF DEc/e (2)

where e is the electron charge and DEc= Ec Ec,ref, for which

Ec,refis the position of the conduction band of YD20. Based on Fig. 1 (a) Current vs. voltage characteristics of DSSC devices prepared

with YD20 (black), YD21 (red), and YD22 (green) under illumination of simulated AM 1.5 full sunlight (100 W cm2) with an active layer of 0.16 cm2and (b) the corresponding action spectra for the eﬃciency of incident photon-to-current conversion (IPCE).

Table 1 Photovoltaic parameters of porphyrin-based dye-sensitized solar cells (active layer 0.16 cm2) under 100 mW cm2light illumination (AM 1.5 G) for YD20–YD22

Dye Dye loading/ nmol cm2 _{mA cm}JSC/2 V_mVOC/ _FF _(%)Z YD20 161 17.43 676 0.686 8.1 YD21 132 12.05 631 0.721 5.5 YD22 134 14.87 634 0.700 6.6

Fig. 2 (a) Capacitance, (b) transport resistance, (c) recombination resistance, and (d) diﬀusion length of YD20–YD22 dyes in DSSC plotted with respect to the Fermi level voltage (VF) with removing

the eﬀect of series resistance.

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4370 Chem. Commun., 2012,48, 4368–4370 This journal is c The Royal Society of Chemistry 2012 these conditions, we transfer Fig. 2a–c into Fig. 3a–c, which

show Cm(a), Rtr(b), and Rrec(c) as a function ofVecb. While

the chemical capacitance (Fig. 3a) and the transport resistance (Fig. 3b) of the three dyes match quite well, the recombination resistance (Fig. 3c) of the YD21 device is much smaller compared to that of the YD20 and YD22 devices. In other words, charge recombination is a major problem for the poor performance of the YD21 device. These results allow us to make a conclusion: compared to the YD20 device, the smaller VOCof YD22 was due to a small shift in conduction band but

the smaller VOC of YD21 was due to a signiﬁcant charge

recombination. From the structural viewpoint, the use of cyanoacrylic acid as an acceptor and an anchoring group in YD21 might provide more free space (less amount of dye-loading) for the charge recombination than the use of the rigid ethynylbenzoic acid in YD20 and YD22. Moreover, YD21 might be tilted on the surface of TiO2for the charge

recombination to occur more easily.

In conclusion, although the concept for molecular design with the cyanoacrylic acid acceptor has been widely applied in highly eﬃcient organic dyes,7_{such an approach does not work}

well for the porphyrin sensitizers as demonstrated herein. The greater performance in the YD20 device than the other two devices is attributed to its rigid structural feature for a larger amount of dye-loading, which combined with the higher recombination resistance and diﬀusion length yields to larger JSC and VOC. Modiﬁcation of the porphyrin structure with

extended p-conjugation for better light harvesting is feasible to boost up the device performance in the near future.

This work was partially supported by National Science Council of Taiwan and Ministry of Education of Taiwan, under

the ATU program. JB acknowledges support by projects from Ministerio de Ciencia e Innovacio´n (MICINN) of Spain (Consolider HOPE CSD2007-00007, MAT2010-19827), and Generalitat Valenciana (PROMETEO/2009/058). SRR thanks ﬁnancial support from Bancaixa foundation under project Innova 11I272. CYY and EWGD acknowledge support by projects from National Science Council of Taiwan and Ministry of Education of Taiwan, under the ATU program.

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Fig. 3 (a) Capacitance, (b) transport resistance, and (c) recombina-tion resistance of YD20–YD22 dyes in DSSC plotted with respect to the equivalent common conduction band voltage (Vecb).