NANO LETTERS
High Open-Circuit Voltage Solid-State Dye-Sensitized Solar Cells with Organic Dye
2009 Vol. 9, No. 6 2487-2492
Peter Chen,† Jun Ho Yum,† Filippo De Angelis,‡ Edoardo Mosconi,‡ Simona Fantacci,‡ Soo-Jin Moon,† Robin Humphry Baker,† Jaejung Ko,†,§ Md. K. Nazeeruddin,*,† and Michael Gra¨tzel* Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH - 1015 Lausanne, Switzerland, Istituto CNR di Scienze e Tecnologie Molecolari and Dipartimento di Chimica, UniVersita` di Perugia, I-06123, Perugia, Italy, and Department of New Material Chemistry, Korea UniVersity, Jochiwon, Chungnam 339-700, Korea Received April 20, 2009; Revised Manuscript Received May 4, 2009
ABSTRACT Solid-state dye-sensitized solar cells were fabricated using an organic dye, 2-cyanoacrylic acid-4-(bis-dimethylfluoreneaniline)dithiophene (JK2), which exhibits more than 1 V open-circuit potential (Voc). To scrutinize the origin of high voltage in these cells, transient Voc decay measurements and density functional theroy calculations of the interacting dye/semiconductor surface were performed. A negative conduction band shift was observed due to the favorable dipolar field exerted by the JK2 sensitizer to the TiO2 surface, at variance with heteroleptic Ru(II)-dyes for which an opposite dipole effect was found, providing an increased Voc.
Dye-sensitized solar cell (DSC) technology is one of the most promising alternatives to compete with the traditional inorganic semiconductor-based solar cells.1,2 In particular, the solid-state dye-sensitized solar cell (SSDSC) has drawn intense research activity since its first publication.3-9 The operating mechanism of SSDSC is very similar to the liquidbased electrolyte system where upon photoexcitation the dye injects an electron and a hole into the n- and p-type materials, respectively, generating free charge carriers.6 The advantages of SSDSC are layer-by-layer fabrication processes and less demanding sealing materials. Nevertheless, the efficiency is lower than its liquid-DSCs counterpart, albeit significant progress in SSDSC has been made in the past decade. SSDSCs have reached highest efficiencies of about 4% utilizing organic dyes,10 ion coordinating dyes,11 and hydrophobic dyes.12 Recently, 5% SSDSC efficiency obtained by using a strong optical reflector has also been reported.13 One major challenge for SSDSCs is that the optical harvesting capability is low due to the limitation in the mesoporous TiO2 film thickness. Organic dye molecules are attractive materials in this respect, because of their high optical * To whom correspondence should
[email protected]. † Swiss Federal Institute of Technology. ‡ CNR-ISTM. § Korea University. 10.1021/nl901246g CCC: $40.75 Published on Web 05/13/2009
be
addressed.
E-mail:
2009 American Chemical Society
absorption extinction coefficient, adjustable spectral response, and environmental benignity. These properties make them promising materials as the light-absorbing layer in SSDSC. A critical issue in the DSC seems to be the low open-circuit voltage (0.7∼0.8 V) compared to the band gap (∼1.5 eV) of the light absorber, due to the necessity of concomitantly ensuring a high driving force for both electron injection into TiO2 and rapid regeneration of the oxidized dye. Organic hole-conductor materials, where the redox potential is closer to the dye HOMO than the redox potential of the I-/I3couple used in liquid-DSCs, have the potential to achieve higher voltage devices if the interfacial charge recombination is effectively controlled. In this article, we report a SSDSC device using 2-cyanoacrylic acid-4-(bis-dimethylfluoreneaniline)dithiophene (JK2) organic dye,14 giving the highest voltage reported in solid-DSC devices. Device Fabrication. The solid-state dye-sensitized solar cells were prepared using the following procedure. FTO glass was cleaned with acetone, Helmax (soap), ethanol, and dried with air then cleaned by UV light for 20 min just before deposition of a dense TiO2 film (100 nm) by spray pyrolysis.15 The precursor solution for spray pyrolysis is bis(acetylacetonate)diisopropoxide titanium in ethanol with 1:9 ratio by volume. The 30 nm powder was received from Showa
Titanium Company, and the TiO2 paste was prepared using a previously reported procedure.16 The TiO2 paste was doctor bladed with an automatic moving bar to form a colloidal thin film. After few minutes drying in air, the film was transferred to a programmable hot plate for sintering. The temperature heats up from room temperature to 500 °C in 60 min and stays there for another 30 min. After cooling down to room temperature, the substrate was then immersed into a 0.02 M TiCl4 solution at room temperature for 12 h. The substrate was rinsed again with water and sintered again at 450 C for 30 min then dipped into dye solution for sensitizing process. The dye-coated porous films were spin coated with 2,2′,7,7′-tetrakis(N,N-di-p-methoxypheny-amine)9,9′-spirobi-fluorene (spiro-OMeTAD) solution (0.17 M in chlorobenzene) containing two additives, Li(CF3SO2)2N (19.5 mM) and tertiary-butylpyridine (0.12 M). Seventy-two microliters of the above solution was applied to a 2.5 × 2.5 cm substrate and left stationary for 1 min in order to penetrate the electrolyte solution into TiO2 pores. Then, the substrate was spun up to 2000 rpm for 30 s with an acceleration speed of 500 rpm/second. A 30 nm thin film of gold was thermal evaporated on top of the HTM as counter electrode. Density Functional Theory (DFT) Calculations. Geometry optimization of the JK2 dye adsorbed onto a TiO2 nanoparticle, represented by a (TiO2)38 cluster,17,18 have been performed by means of the Car-Parrinello method.19 In this stage, we use a plane-wave basis set, ultrasoft pseudopotentials20,21 and the PBE exchange-correlation functional.22 For comparison, a model Ru(II)-heteroleptic complex Ru(4,4′dicarboxylic acid-2,2′-bipyridine)(4,4′-ditridecyl-2,2′-bipyridine)(NCS)2 (N621) in which the long aliphatic chains of the real complex are replaced by methyl groups is also considered.23 Calculations of the sensitizers’ dipole moment are performed at the geometry optimized for the dye adsorbed on the surface, saturating the negatively charged carboxylic groups by a proton (JK2) or by Na+ counterions (N621). These calculations are performed using the B3LYP hybrid exchange-correlation functional24 with the similar 6-31G* and DVZP basis sets for JK2 and N621, respectively, including solvation effects by means of the conductor-like polarizable continuum model (C-PCM),25,26 as implemented in the Gaussian 03 program package.27 Figure 1 shows current-voltage and photocurrent action spectra of JK2 dye sensitized TiO2 films, the corresponding values are gathered in Table 1. The SSDSC device using the JK2 dye with 30 nm TiO2 particles gives a record high voltage of 1087 mV, which is 190 mV higher than that measured for the same cell using the NaRu(4-carboxylic acid-4′-carboxylate)(4,4′-bis[(triethyleneglycolmethylether)heptylether]-2,2′-bipyridine)(NCS)2 (K68) complex, which is the best-performing dye among Ru(II)-heteroleptic complexes.13 The K68 complex includes the “ion-coordinating” ethylene oxide chains, which enhances both the cell opencircuit voltage and fill factor, with approximately 20% increase in device efficiency when compared to standard Ru(4,4′-dicarboxylic acid-2,2′-bipyridine)(4,4′-dinonyl-2,2′bipyridine)(NCS)2 (Z907) complex.28 The photocurrent action spectrum (incident monochromatic photon-to-current ef2488
Figure 1. (a) The I-V characteristics of SSDSCs based on JK2 (gray line) and K68 (black line) dyes. (b) The IPCE of the JK2 SSDSC.
Table 1. I-V Characteristics of SSD Devices Made of JK2 and K68 dye
Voc (mV)
Jsc (mA/cm2)
FF (%)
efficiency (%)
JK2 K68
1087.5 896.7
3.85 5.7
67.7 76.1
3.17 3.88
ficiency, IPCE), Figure 1b, of the JK2-sensitized cell is characteristic of DSC using a thin TiO2 film and shows tow peaks around 370 and 460 nm, which correspond well with the absorption spectrum of JK2 in solution. These data are different from the liquid cell IPCE response where high values (∼70-90%) extended across the entire 400-600 nm range, due to use of a thick TiO2 film that saturates the light absorption. We also notice that the open circuit voltage of these devices are higher than the values reported with the similar ion-coordinating dyes K68.11,29 The SEM image of the doctor-bladed film made of the 30 nm TiO2 particles after sintering on FTO is shown in Figure 2. The film is homogeneous without big agglomerates, having surface area and porosity of 57.6 m2/g and 63.5%, respectively. This decrease of surface area measured with the 30 nm TiO2 particles results in reduced photocurrent and higher voltage comparing to those cells made of the same dyes with 20 nm TiO2.. We consider this decrease of surface area to reduce the contact area between the TiO2 and hole transporter, thus reducing the recombination pathway between injected elecNano Lett., Vol. 9, No. 6, 2009
Figure 2. The SEM image of the 30 nm TiO2 mesoporous film on FTO. The scale bar is 200 nm.
trons and the oxidized hole transporter. From the transient Voc decay measurement, cells made of the 30 nm would possess slightly longer electron lifetime at the same Voc than those cells made with 20 nm particles.30 The maximum value of Voc is determined by the difference between the quasi Fermi level in TiO2 under illumination and the oxidation potential of the hole transporting material. The theoretical value is limited to about 1.2-1.3 V if we take the value of -0.5/-0.4 V (vs normal hydrogen electrode (NHE)) for the TiO2 conduction band edge and 0.82 V for the one-electron oxidation (E° HTM/HTM+ vs NHE) for Spiro-OMeTAD.31 Our data, 1.09 V, is very close to the theoretical maximum value of Voc assuming the position of the conduction band (CB) edge is not shifted. What is the origin of such high voltage in SSDSCs based on JK2 compared to the liquid electrolyte based DSC? Primarily, the origin of large Voc variation among a solid-state and a liquid electrolyte based DSC comes from a 400 mV difference between the redox potential of I-/I3- (0.4 V vs NHE) and Spiro-OMeTAD (0.8 vs NHE). Added to this is the dipole influence, which in a liquid cell will be affected by the mobile ionic species in electrolyte (such as Li+) and ion absorption at the TiO2/liquid interface. In a comparison of the transient photovoltage measurements, Voc of the solidstate DSC based on the two different dyes JK2 and K68 at the same level of photon-induced charge density in the film (Figure 3) shows that the Voc of the device based on the JK2 dye is almost 200 mV higher than that of the device employing the K68 dye at fixed charge density. This method has been applied to estimate the effect of adding coadsorbent into the electrolyte or dye solution, and the change in the CB band edge energy or in the recombination rate can be distinguished by this technique.32-35 From Figure 3 and 4, we do see the difference in the Voc at the same charge density level or same capacitance of the film, implying that the high Voc is related to an upward shift of the TiO2 CB edge toward more negative potentials.32-35 Another important factor that has an influence on the voltage of DSCs is the electron lifetime which is the Nano Lett., Vol. 9, No. 6, 2009
Figure 3. Open circuit voltage at different charge density level for cells made using JK2 and K68 dyes.
Figure 4. Phototransient induce capacitance at different Voc level.
Figure 5. Electron lifetime at different charge density level.
reciprocal recombination rate of injected electron leading to a change of electron density in TiO2. The electron lifetime at different charge densities are shown in Figure 5; the values are similar for the two cells using different dyes and are in the typical regime for SSDSC. Since the recombination rate 2489
Figure 6. Optimized geometrical structure of the JK2 (left) and N621 (right) dyes adsorbed onto the (TiO2)38 cluster. The reference axes are also reported.
of the JK2 cell is not showing any dramatic change compared to similar SSDSC based on different dyes, the high Voc is not the result of reduced interfacial charge recombination. Our results therefore suggest that the high photovoltage approaching the theoretical maximum value is due to a shift in the CB edge toward negative potentials. The dipole effects on the chemical potential shift at organic-inorganic interface were first measured in OLEDs devices.36-38 The position of the band edge level can be shifted by adsorption of dipolar molecules onto the TiO2 surfaces. This surface modification strategy has shown improvement for the Voc of liquid dye sensitized solar cells.33,35,39 For the SSDSC, it has been demonstrated that the dipole pointing outward from the surface would suppress the dark current in a flat TiO2/dipolar molecular/HTM/gold junction and change the band alignment at the organicinorganic interface.40 Therefore, it is clear that the dipole moment of the adsorbed molecules has strong effects on the rectifying behavior. This effect is likely to induce a shift in the position of the semiconductor conduction band, which yields the high open-circuit voltage in JK2 dyes via offsetting the energy level between the n-type TiO2 and the p-type HTM. To verify whether the JK2 dye could induce a favorable CB shift in TiO2 due to its dipole moment, we performed DFT calculations on JK2 adsorbed onto TiO2. For comparison, we also considered a typical heteroleptic Ru(II)-dye such as N621. As previously described, it is the sensitizer dipole component normal to the surface that can induce a shift in the TiO2 CB energy.32 It is therefore mandatory to have detailed information concerning the dye adsorption geometry 2490
on the TiO2 surface to properly evaluate the sensitizer dipole components relative to the surface plane. To provide such an information for the JK2 dye, we optimized the geometrical structure of the dye adsorbed onto TiO2. Our results, reported in Figure 6, show that adsorption takes place via a bidentate coordination mode through the dye carboxylic group with transfer of the acidic proton to the surface. This dissociative bidentate coordination, already found for the N719 dye,41 is at variance with what reported for formic acid on TiO242 and is probably related to the increased acidity of the JK2 cyanoacrylic group compared to that of formic acid. The calculated Ti-O distances between the carboxylic group and the TiO2 surface are 2.02 and 2.14 Å, reflecting quite a strong interaction between the deprotonated dye and the surface. The optimized geometry of the N621 dye, a as model of the heteroleptic K68 dye employed in the current investigation, is also reported in Figure 6.43 To further simplify the calculations, in this case we limited our attention to the deprotonated dye, thus avoiding the investigation of the protons and counterions location on the surface. While these effects might substantially alter the dye/semiconductor electronic coupling,41 here we are simply interested in the relative orientation of the dye with respect to the surface, which is to a first approximation, largely independent from the precise locations of the protons and counterions. As found previously,43 heteroleptic dyes, such as N621 and K68, adsorb onto TiO2 by using two carboxylic groups residing on the same bipyridine ligand. This is opposed to the N3 or N719 homoleptic complexes, which are found to adsorb onto TiO2 exploiting two or three anchoring groups residing on two different bipyridine ligands.41,43 The different Nano Lett., Vol. 9, No. 6, 2009
Table 2. Calculated Dipole Components for the JK2 and N621 Dyesa dipole components sensitizer
µx
µy
JK2 6.8 -1.3 N621 -16.8 1.8 a The z axis corresponds to the TiO2 surface normal.
µz 7.7 -30.1
adsorption geometries between homoleptic and heteroleptic dyes have been found to induce different dipolar fields at the TiO2 surface, which in turn lead to reduced open circuit potentials in DSCs employing the heteroleptic dyes.43 Similar dipole arguments were recently invoked to explain the Voc behavior of SSDSC employing different organic and organometallic dyes.44,45 The dipole moments calculated for JK2 and N621 at their adsorption geometry are reported in Table 2. Because the dipole moment is origin-dependent for charged species, we considered here the JK2 dye at its adsorption geometry with one proton added on the oxygens in carboxylic group. This gives rise to two possibilities, protonation of the oxygen cis or trans to the cyano group, which are found to provide consistent dipole values. For N621, we considered the system saturated with two Na+ counterions.43 Here we adopt the definition for the dipole sign in which the dipole points from the negative to the positive charge. As can be immediately noticed from Table 2, the two dyes have extremely different dipoles, both in sign and magnitude. Considering the relevant z component, that is, the dipole component normal to the TiO2 surface plane, we notice that while JK2 has a positive dipole along this direction, N621 shows a negative sign of this dipole component. In terms of charge distributions, this means that in JK2 the negative pole is localized close to the TiO2 surface, while in N621 the opposite holds, due to the prevailing negative contribution of the thiocyanate ligands, pointing in the opposite direction with respect to the surface. Even though the N621 case explored here is just a model for the much more complex K68 dye subject of the present investigation, altogether our data indicate that TiO2 CB shifts of the opposite sign can be envisioned for JK2 and Ru(II)-heteroleptic dyes. The positive dipole exerted by JK2 onto the TiO2 surface leads, as previously ascertained by calculations including artificial dipoles of different sign on the same TiO2 nanocluster,43 to a CB energy upshift, which in turn leads to the experimentally observed increase in the open circuit potential of DSCs employing this dye. Conclusion. In brief, we found an extra high voltage exceeding 1 V in SSDSCs using an organic sensitizer resulting in an overall efficiency of 3.17%. Such a high voltage results mainly from the shift of TiO2 conduction band edge rather than slow recombination, as clearly demonstrated by transient photovoltage decay and DFT calculations. Acknowledgment. P.C. thanks the Taiwan Merit Scholarships Program (TMS-094-2A-026) for supporting his study. F.D.A. and E.M. thank MIUR-FIRB 2003 and INSTMPRISMA 2007 for financial support. The 30 nm TiO2 particles were received as a gift from Showa Titanium Company. Nano Lett., Vol. 9, No. 6, 2009
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NL901246G
Nano Lett., Vol. 9, No. 6, 2009
