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p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO2 Nanoplates with Saturation Photovoltages Exceeding 460 mV Mingzhe Yu, Gayatri Natu, Zhiqiang Ji, and Yiying Wu* Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (Voc). Here we report the first application of delafossite CuGaO2 nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO2 nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of Voc on the illumination intensity to estimate the maximum obtainable Voc from the CuGaO2-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co3+/2+(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a Voc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs. SECTION: Energy Conversion and Storage; Energy and Charge Transport

A

p-type dye-sensitized solar cell (p-DSSC)1 is based on the cathodic sensitization of p-type semiconductors and thus operates in a manner reverse to the conventional n-type DSSC. Recently, the research on p-DSSCs has attracted increasing attention because they can be integrated with n-DSSCs into tandem DSSCs, which hold a great promise for achieving high power conversion efficiencies. For example, Lindquist et al.2 combined a NiO-based photocathode with a TiO2-based photoanode into a tandem DSSC with a Voc equal the sum of the Voc of the separate devices. Since then, much progress has been achieved in the molecular engineering of sensitizers as well as the developments of the redox mediators.3 The recent work by Nattestad et al.4 in particular demonstrated that a tandem DSSC can outperform either a p-DSSC or an n-DSSC. The results clearly show that tandem DSSCs are promising for realizing substantially improved efficiencies. In principle, p-DSSCs should be able to work as efficiently as n-DSSCs. However, in reality, the development of p-DSSCs has been lagging behind that of n-DSSCs. The main reason is because no optimum wide bandgap p-type semiconductor is available that is equivalent to anatase TiO2 as in n-DSSCs. To date, the predominant p-type semiconductor used in p-DSSCs is NiO. However, NiO is not optimal due to the following reasons: (1) NiO absorbs a significant amount of visible light. A 2.3-μm-thick film absorbs 30−40% of the incident photons over most of the visible wavelength range.4 More transparent p-type semiconductors are therefore desired. (2) The valence band (VB) edge of NiO is very close to the redox potential of the commonly used triiodide/iodide (I3−/I−) mediator, resulting in low Voc. The VB edge of NiO is +0.54 V versus normal hydrogen electrode (NHE),1 while the redox potential of © 2012 American Chemical Society

triiodide/iodide is +0.35 V.5 The difference is only 190 mV. Therefore, with just a few exceptions,4,6−9 the reported Voc in most prior works is in the range of 90−125 mV (see Table S1 in the Supporting Information (SI)).1,10−17 (3) NiO has a low hole mobility with an estimated hole diffusion coefficient of only 4 × 10−8 cm2/s,18 which may limit the diffusion length of the hole carriers. Therefore it is crucial to find alternative p-type semiconductors. Some other p-type semiconductors have been investigated, including CuO,13 CuSCN,19 and p-type diamond;20 however, the performances of these solar cells are not particularly better than NiO p-DSSCs. The exploration of complex oxides is a promising approach for identifying new semiconductors for DSSCs due to their widely available compositions and tunable properties.21 The Cu(I) delafossite ternary oxides, CuMO2 (M = B, Al, Ga, In), are a group of ptype semiconductors with wide bandgap energies in the range of 3.4−4.0 eV and lower VB edges than NiO with a valenceband maximum (VBM) determined by the mixing of O 2p orbitals and Cu 3d orbitals.22−25 Therefore, they should be more transparent to visible light than NiO and be able to produce higher photovoltages. The challenge is the difficulty in synthesizing the delafossite CuMO2 nanoparticles. For example, Nattestad et al. reported p-DSSCs based-on CuAlO2.26 However, their synthesis of CuAlO2 was carried out by a high-temperature solid-state reaction followed by ball milling. The resulting CuAlO2 particles were large, and therefore the resultant porous CuAlO2 films exhibited limited surface area. Received: March 23, 2012 Accepted: April 10, 2012 Published: April 10, 2012 1074

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Figure 1. Characterization of our CuGaO2 nanoplates: (a) digital photo of the CuGaO2 (left) and NiO (right) particles dispersed in water; (b) XRD pattern, (c) SEM image, and (d) TEM image for the CuGaO2 nanoplates; (e) HRTEM image and (f) SAED pattern of a nanoplate along the [001] zone axis.

In this work, we report delafossite CuGaO2 nanoplates and their first application for p-DSSCs that produce remarkably high photovoltages. We chose CuGaO2 for the following reasons: (1) its hydrothermal synthesis has been reported by Poeppelmeier27 and Jobic.28 Although these prior studies did not produce nanoparticles, the hydrothermal synthesis is promising to achieve smaller particle size than the CuAlO2 material synthesized from the solid-state synthesis. (2) CuGaO2’s band-structures have been calculated and experimentally investigated.29−32 It shows a direct forbidden bandgap of about 3.6−3.8 eV. An interesting phenomenon for the delafossite CuMO2 is that the optically measured direct bandgap increases from M = Al, Ga to In.31 Therefore, CuGaO2 should exhibit better transparency than CuAlO2. (3) The VB edge of CuGaO2 is about +0.6 V v.s. NHE, (i.e., +5.1 eV below vacuum level),29 which is lower than that of NiO.33 CuGaO2-based p-DSSCs should thus be promising for producing high photovoltages. Our detailed procedure of the hydrothermal synthesis is shown in the SI, which was modified from a prior report.28 Consistent with its wide bandgap, our CuGaO2 product is white with a pale yellow tinge (Figure 1a). Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for light absorption. This is an attractive advantage over the dark-colored NiO films commonly used in p-DSSCs. The powder X-ray diffraction (XRD) showed that all observed peak positions matched the diffraction pattern of CuGaO2 with a delafossite structure from the International Centre for Diffraction Data powder diffraction file (ICDD PDF#41-0255) (Figure 1b). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the obtained particles had the nanoplate morphology with an average size of about 200 nm and a

thickness of about 45 nm (Figure 1c,d). Each nanoplate is single-crystalline as shown in the high-resolution TEM (HRTEM) (Figure 1e) and selected-area electron diffraction (SAED) (Figure 1f) images. CuGaO2 is known to be thermodynamically unstable below 600 °C, and the oxidation of Cu(I) in air would cause the formation of CuO and CuGa2O4.34,35 This raised the concern of whether our CuGaO2 nanoplates could survive the thermal annealing process typically at 350−450 °C in the fabrication process of DSSCs, which is used to remove the organic residues in the films and to enhance the connection between the nanoparticles. Therefore, XRD tests were performed to check any possible phase changes after the samples were heated at different temperatures in air. As shown in Figure 2, our

Figure 2. XRD pattern for CuGaO2 treated at different temperatures. 1075

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Figure 3. Dependence of Voc on light intensity: (a) CuGaO2-DSSC and NiO-DSSC with simple I3−/I− electrolyte; (b) CuGaO2-DSSC and NiODSSC with Co3+/2+(dtb-bpy) electrolyte.

CuGaO2-based p-DSSCs produce a saturation Voc of 243 mV. By contrast, the saturation photovoltage of NiO cells is only 132 mV. More promisingly, with the Co3+/2+(dtb-bpy) electrolyte, the saturation Voc of CuGaO2-DSSCs increases further to 464 mV, which is an increase of more than 100% compared to the NiO-DSSCs under the same conditions. The saturation photovoltage is determined by the difference between the VBM of CuGaO2 and the redox potential of the redox couple. The redox potential is +0.35 V vs NHE for I3−/ I−, and +0.39 V vs NHE for Co3+/2+(dtb-bpy)/MeCN.7 The large increase in the saturation photovoltage cannot be explained by the small difference in the redox potentials of the two redox couples. Therefore, the most likely reason is the change of the CuGaO2/electrolyte interface. We speculate that different preferential ion adsorption occurs in different electrolytes. The different surface potential can induce the shift of the VBM relative to the electrolyte and thus the change in photovoltage. Zeta potential measurements are ongoing to confirm the surface charge. Figure 4 shows the photocurrent−voltage curves of the CuGaO2 cells under 1 Sun AM 1.5 illumination. At this condition, the Voc is 180 mV, which is significantly higher than the common NiO-based p-DSSCs in the similar LiI/I 2 electrolyte solutions. More remarkably, when the 0.1 M Co3+/2+(dtb-bpy)/MeCN electrolyte was used, a Voc of 357 mV was obtained. This is among the highest values reported to date (see Table S1 in the SI).4,6−9 The current density of the cell using I3−/I− is higher than that of cobalt electrolyte. A recent study from Hamann group shows that the recombination in cobalt(III/II) electrolyte is faster than I3−/I− due to a lower outers-sphere reorganization energy compared to the inner-sphere reorganization energy for the I3−/I− redox couple.38 We think our results are probably due to the same reason. The major limitation of our current CuGaO2 cells is their low current densities. Control experiments have been carried out to confirm the photocurrent is from the dye-sensitized CGO (see SI Figure S2). The photocurrent density of DSSCs is determined by the product of the light harvesting efficiency, the hole injection efficiency from the P1 dye to CuGaO2, and the collection efficiency of the hole carriers. The above saturation Voc measurements show that the VB edge of CuGaO2 is 243 mV more positive than the redox potential of

CuGaO2 nanoplates are stable when the temperature is at or below 350 °C. This agrees with Kumekawa et al.’s conclusion that the CuGaO2 is thermo-kinetically stable below 360 °C.35 However, when the temperature is higher than 400 °C, CuO and CuGa2O4 diffraction peaks also appear, indicating that the following decomposition reaction takes place: 4CuGaO2 + ≥ 400 ° C

O2⎯⎯⎯⎯⎯⎯⎯⎯→2CuGa2O4 + 2CuO. The above result confirms that our CuGaO2 nanoplates are compatible with the fabrication procedure as long as the annealing temperature is below 350 °C. Films made of our CuGaO2 nanoplates with thicknesses of 3 μm were sensitized by the organic P1 dye (Figure 4, inset), which contains triphenylamine as the donor and was first reported by Qin et al.12 Two electrolytes were used: 0.1 M I2/ 1.0 M LiI/methoxypropionitrile (MPN) or 0.1 M Co3+/2+(dtbbpy)/MeCN electrolyte. We have measured the dependence of Voc on the illumination intensity in order to estimate the maximum Voc that can potentially be obtained from the CuGaO2-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed in our CuGaO2-based p-DSSCs with the cobalt (III/II) electrolyte. It has been shown that the DSSC’s current−voltage characteristics follow the expression of a constant current source connected in parallel with a diode:36 Voc =

nkT ⎛ Jphoto ⎞ ⎟⎟ ln⎜⎜ e ⎝ J0 ⎠

Here, n is the ideality factor of the solar cell, k is the Boltzmann constant, T is the absolute temperature, e is the electric charge, Jphoto is the photocurrent density, and J0 is the saturation current density. Since the Jphoto is proportional to the incident light intensity I0,37 the Voc should be proportional to the logarithm of the incident illumination intensity I0. This linear relationship would hold until the Voc reaches its saturation value, which is determined by the VB edge of the semiconductor and the redox potential of the electrolyte. Once the Voc gets saturated, the increasing I0 would not further increase the Voc, and that saturated value is the maximum Voc we can obtain from the DSSCs. Voc as a function of the illumination intensity is shown in Figure 3 for the CuGaO2- and NiO-based p-DSSCs in the 0.1 M I2/ 1.0 M LiI/MPN electrolyte or 0.1 M Co3+/2+(dtb-bpy)/ MeCN electrolyte. With the I3−/I− redox electrolyte, the 1076

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Letter

ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis and characterization. A summary table of the major prior works on NiO-based p-DSSCs. The current− voltage curves of the CuGaO2 p-DSSCs with different CuGaO2 film thicknesses. The current−voltage curves of the dyesensitized FTO cells as “blank tests”. The dye loading isotherm measurement information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+1) 614-247-7810; Fax: (+1) 614-292-1685; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 4. Photocurrent−voltage curves for our CuGaO2-based pDSSCs in an I3−/I− electrolyte (blue) and in a Co(III/II) electrolyte (red). A “blank cell” without any dye-adsorption was also made (black) to confirm that the photocurrent was generated from the dyesensitization.

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ACKNOWLEDGMENTS The authors acknowledge the funding support from the U.S. Department of Energy (Award No. DE-FG02-07ER46427).

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I3−/I−. Considering the redox potential of I3−/ I− is 0.35 V (versus the normal hydrogen electrode, NHE; same below),39 the VB edge of CuGaO2 is 0.59 V. The highest occupied molecular orbital (HOMO) position of P1 dye is known to be at 1.35 eV.12 Therefore, the hole injection from the HOMO of the dye into the VB of CuGaO2 is thermodynamically favorable with a driving force of 0.76 eV. With this consideration, together with the desired donor−acceptor geometry of the P1 dye, we think the hole injection efficiency should not be the factor limiting the photocurrent density. In order to evaluate the light harvesting efficiency, we have measured the dye adsorption isotherms. As shown in Figure S3 in the SI, the maximum dye loading for a 1-μm thick film is 66.77 nanomol/cm2 for NiO and 23.35 nanomol/cm2 for CuGaO2. This result shows that the low dye loading is clearly a factor that limits the current density of CuGaO2 cells. We attribute the low dye loading to the relatively large particle size of the CuGaO2 nanoplates, which results in a small surface area for the adsorption of dye molecules. Another possible limitation is the recombination between CuGaO 2 and the dye, considering recombination has been identified to be a major problem in NiO-based p-DSSCs.7,33,40 The low dye loading and the recombination issue will also limit the Voc of the CuGaO2DSSCs. Further work on reducing the CuGaO2-particle size and understanding the device physics is ongoing. In conclusion, we report the first application of CuGaO2 nanoplates in p-DSSCs that produce high photovoltages. These nanoplates are thermally stable up to 350 °C, and therefore, are compatible with the DSSC fabrication process. In contrast to the brown color of NiO, these CuGaO2 nanoplates are offwhite. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. A Voc of 357 mV has been achieved when a Co3+/2+(dtb-bpy) electrolyte was used as the redox shuttle under 1 Sun AM 1.5 illumination. More remarkably, a saturation photovoltage of 464 mV has been achieved with the increasing illumination intensity. Our current efforts are to further decrease the sizes of the nanoplates and therefore to increase the dye loading.

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