J. Phys. Chem. C 2009, 113, 18515–18522
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Efficient ZnO Nanowire Solid-State Dye-Sensitized Solar Cells Using Organic Dyes and Core-shell Nanostructures Natalie O. V. Plank,*,† Ian Howard,‡ Akshay Rao,‡ Mark W. B. Wilson,‡ Caterina Ducati,§ Rajaram Sakharam Mane,| James S. Bendall,† Rami R. M. Louca,† Neil C. Greenham,‡ Hidetoshi Miura,⊥ Richard H. Friend,‡ Henry J. Snaith,*,| and Mark E. Welland† Nanoscience Centre, The UniVersity of Cambridge, Department of Engineering, 11 JJ Thomson AVenue, Cambridge CB3 0FF, U.K., Optoelectronics Group, The UniVersity of Cambridge, CaVendish Laboratory, Cambridge CB3 0HE, U.K., Department of Materials Science, The UniVersity of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K., Condensed Matter Physics, Department of Physics, The UniVersity of Cambridge, Clarendon Laboratory, Parks Road, Oxford OX13PU, U.K., and Chemicrea Co. Ltd., 2-1-6 Sengen, Tsukuba, Ibaragi 305-0047, Japan ReceiVed: May 26, 2009; ReVised Manuscript ReceiVed: August 28, 2009
We have applied a MgO and a ZrO2 shell deposition method to control the interface between two indolenebased organic dyes in solid-state dye-sensitized solar cells. The shell deposition was carried out at less than 100 °C, and shell thickness was shown to be 2 nm for the ZrO2 and 6-10 nm for the MgO by transmission electron microscopy. X-ray photoelectron spectroscopy has shown the initial ZnO NWs and core-shell structures have little surface water contamination. The use of suitable dyes, D102 and D149, has led to power conversion efficiency for ZnO NW based hybrid solar cells of 0.71%. Transient absorption measurements indicate that enhancements in photoinduced charge generation with core-shell formation are the main factor leading to the improved device efficiency. 1. Introduction Solid-state dye-sensitized solar cells (SDSCs) are a promising technology for energy applications as a result of their low cost fabrication methods and the use of low toxicity materials.1-4 One of the key advantages of solid-state DSCs is that the resulting devices are lightweight and can be “flexible” and transparent in certain wavelength ranges, making solar cells useful for a wide range of applications. Zinc oxide (ZnO) is a wide bandgap semiconducting material with excellent material stability5-7 for which the facile synthesis of ZnO nanowires (NWs) has been demonstrated at low temperatures using a hydrothermal synthesis method.8,9 The use of ZnO NWs as the electron collection material for low temperature fabrication of solar cells has been performed by several groups, notably for the fabrication of ZnO:P3HT hybrid solar cells10-15 and in the development of liquid electrolyte dye-sensitized solar cells (DSCs).16,17 However, in general there are several problems with these devices. Usually polymer hybrid devices exhibit low opencircuit voltages (Voc), of approx 0.2-0.44 V,10-15 whereas for ZnO NW liquid electrolyte DSCs the Voc can reach over 0.6 V.16 For polymer hybrid devices the poor Voc results in power conversion efficiencies (η) from 1%;16 however, this is still lower than the typical laboratory performances of equivalent TiO2 DSCs.1-4 The poor performance of the ZnO NW based hybrid solar cells appears to stem from the non-optimized interface between the light absorber and * To whom correspondence should be addressed. E-mail: (N.O.V.P.)
[email protected]; (H.J.S.)
[email protected]. † Nanoscience Centre. ‡ Optoelectronics Group. § Department of Materials Science. | Condensed Matter Physics. ⊥ Chemicrea Co. Ltd.
ZnO NW.13 Several solutions have been presented to optimize this interface; surface treatment of the ZnO with alkanethiols,15 treating TiO2 with long chain carbon molecules,18 and the fabrication of core-shell NW structures such as ZnO-TiO213,19 have all led to increases in device performance. However, considerable improvement is still required for ZnO-based hybrid solar cells to become competitive with the most promising future generation photovoltaic concepts. Even in the field of TiO2based liquid electrolyte DSCs, in which commercial products are currently available, the application to the interface of a thin layer of an insulating material (such as Al2O3 or SiO220) or a wide bandgap material (such as ZrO221) has improved photovoltaic device performance and stability in many instances. In a previous study we made use of the facile and scalable process of growing MgO shells hydrothermally directly onto hydrothermally grown ZnO NWs. This overcoating improved the power conversion efficiency solid-state DSCs incorporating ZnO nanowires in conjunction with a ruthenium dye to 0.33% by enhancing the short-circuit current, open-circuit voltage, and fill factor.22 Although the overcoating electronically enhanced the performance in this system, a significant limitation is the inability to create extremely large surface area nanowire arrays, resulting in incomplete solar light harvesting in sensitized films. Organic dyes, with much higher molar extinction coefficients are likely to be more suited to these mesoscopic nanowire arrays.23-28 In this work we have applied an MgO and a ZrO2 shell deposition method to control the interface between two indolenebased organic molecular dyes, termed D102 and D149,25,28 shown in Figure 1a and b, respectively, and ZnO NWs. Solar cells were assembled as solid-state DSCs with spiro-OMeTAD as the molecular hole-transporter. Photovoltaic characterization has shown the maximum solar-to-electric power conversion efficiency to be 0.71% for ZnO-ZrO2 NWs coated with D149.
10.1021/jp904919r CCC: $40.75 2009 American Chemical Society Published on Web 09/30/2009
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Figure 1. Chemical structure of the organic dyes (a) D102 and (b) D149.
The efficiency of all devices has been found to be stable after 2 months. High resolution transmission electron microscopy (HRTEM) analysis has been performed to investigate the nanoscopic core-shell structures. In addition we have also looked at the surface chemistry using XPS to determine the influence of surface hydroxyl groups and possible water contamination. We have investigated the device and photophysics with transient absorption experiments performed directly on the operating solar cells. Surprisingly, these measurements reveal that the deposition of a surface shell of MgO and ZrO2 predominantly enhances the photoinduced charge generation and does not appear to inhibit the electron hole-recombination. 2. Experimental Section 2.1. Nanowire Core/Shell Synthesis. Core-shell nanowire structures were grown by hydrothermal growth methods,8,22,29 in a two-stage process. First the fabrication of a seed layer followed by the growth of the ZnO NWs. Second, the deposition of either an MgO shell or ZrO2 layer onto the ZnO core by further low temperature hydrothermal growth. For all devices ITO on glass substrates was cleaned in acetone and IPA following standard substrate cleaning procedures in a cleanroom environment. A 100 nm (approx) layer of Zn metal was sputtered directly onto the clean ITO substrate, which acts effectively as the seed layer. It should be noted that in contrast to other reports where the ZnO seed layer is also fabricated using a high temperature anneal stage (>300 °C), the sputter deposition process for the zinc layer used here is compatible with many substrate materials, including flexible substrates, in addition to being advantageous for solar cell device characteristics. For the NW synthesis, the growth solution for hydrothermal preparation of ZnO NWs was prepared by mixing 0.025 M zinc nitrate hydrate and 0.025 M hexamethylenetetramine (HMT) in water. The nanowire growth was then carried out by placing the Zn-coated ITO on glass substrates directly into the growth solution. The solution was held at 92 °C for 120 min, before removing the substrates and cleaning them in DI water. Subsequently, the films were dried on a hot plate at 100 °C to remove any excess water. This process resulted in an even and uniform film of NWs that were approximately 600 nm long and 60 nm in diameter.
Plank et al. The ZnO-MgO NW core-shell structures were fabricated by a second hydrothermal growth procedure immediately after removal from the ZnO NW growth system. ZnO NW arrays were submerged in a 10 mM solution of magnesium nitrate mixed with 0.2 M NaOH at 98.5 °C for 40 min using a water bath for even temperature (ZnO-MgO). A secondary method was similarly explored whereby zirconium acetate solution of 10 mM in water was mixed with 0.2 M NaOH at 98.5 °C (ZnOZrO2) for 20 min. The samples were all rinsed and dried on a hot plate at 100 °C. 2.2. Solid-State Dye-Sensitized Solar Cell Fabrication. Solutions of organic based dyes were made using a 0.3 mM solution of both D102 and D149 in 1:1 v/v tert-butanol/ acetonitrile (ACN). To fabricate the solid-state DSCs, the pristine ZnO NWs, ZnO-MgO, and the ZnO-ZrO2, samples were submerged in a solution of D102 or D149 for 15 and 10 min, respectively, in the dark after which the samples were rinsed in anhydrous acetonitrile. The hole transporting material used in the SDSC was 2,2′,7,7′-tetrakis(N,N-dimethoxypheny-amine)9,9′-spirobifluorene (spiro-OMeTAD), which was dissolved in chlorobenzene (CB) (180 mg/mL). tert-Butyl pyridine (tBP) was added straight to the solution (1:57 tBP/CB). Lithium trifluoromethyl sulfonylimide (Li-TFSI) (ionic dopant) was separately predissolved in ACN at 170 mg/mL. These solutions were then added to the hole-transporter solution at 1:27 v/v, respectively, as reported previously.2,21 To complete the devices, 50 nm thick silver top electrodes were deposited by thermal evaporation under high vacuum. 2.3. Microscopy, Photovoltaic Characterization, and Spectroscopy. The nanowire array morphology was confirmed by scanning electron microscopy (SEM) using a LEO 1530 VP microscope. Transmission electron microscopy (TEM) analysis was performed on a FEI Tecnai F20, with 200 kV acceleration voltage, to assess the structure of the MgO shell grown hydrothermally. The spectral responses of the solar cells were characterized using a tungsten lamp in combination with a monochromator and Si reference diode, and the current-voltage response was measured with a Keithley 237 SMU under simulated sun light generated from a 300 W Oriel solar simulator lamp (calibrated using a Si-reference cell bought from and calibrated by the Fraunhofer Institute of Solar Energy, with the solar cell mismatch factor accounted for). The surfaces of the nanowire materials were tested using X-ray electron spectroscopy with an Mg KR (hν ) 1253.6 eV) to determine the oxygen composition on the samples. XPS was performed in an ion pumped VG Microtech CLAM 4 MCD analyzer system; 200 W unmonochromated Mg X-ray excitation was used. The CLAM 4 has variable slits for small area analysis. The largest slit (5 mm) was used in this case with no apertures selected. The analyzer was operated at constant pass energy of 100 eV for wide scans and 20 eV for detailed scans setting the C1s peak at BE 284.8 eV to overcome any sample charging. Data was obtained using SPECTRA version 8 operating system. The peaks were fitted using XPS peak software.30 2.4. Transient Absorption Spectroscopy. To study the photophysics, transient absorption measurements were performed on films of nanorods loaded with dye as well as working devices. The experimental setup is well described elsewhere.31 Briefly, the pump beam was provided by a frequency doubled output of a Q-switched Nd:YVO4 laser (AOT-YVO-25QSPX, Advanced Optical Technology Ltd.). The devices were probed using the 1300 nm signal output of a traveling wave optical parametric amplifier (TOPAS, Light Conversion Ltd.). The TOPAS was pumped by a portion of the 800 nm output of a 1
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Figure 2. (a) Schematic of the solid-state dye-sensitized device. (b) Photograph of a D102 sensitized ZnO NW solid-state dye-sensitized solar cell. (c) Schematic of the energy levels at the photovoltaic heterojunction in the solid-state DSC.
W, 1 kHz regenerative amplifier (Spitfire, Spectra-Physics). Films were probed at 800 nm using a portion of the amplifier output. The delay between pump and probe was introduced using an electronic delay generator (Stanford Research Systems SRSDG535) to vary the trigger of the Q-switched laser relative to the trigger of the amplifier system. Devices were mounted so that pump and probe beams overlapped in the active area of the device, with the probe beam then reflected from the metal electrode to be subsequently guided into the detection apparatus. The devices were biased (and current recorded) using a source measurement unit (Keithley 2400). 3. Results and Discussion 3.1. Solar Cell Performance. Figure 2a shows the schematic device geometry of the solid-state DSC. The Zn Seed layer has been sputtered onto only the central areas of the device. Consequently there is no overlap between the contact edge of the silver electrode forming the device pixel and the ZnO NWs, preventing undesired short-circuits. The photograph of a typical
ZnO nanowire solid-state dye-sensitized solar cell shown in Figure 2b illustrates the uniformity and the semitransparent nature of the device. The ZnO NWs grown hydrothermally for 2 h result in homogeneous well-aligned NWs of approximately 600 nm in length and 60 nm in diameter. The monolayer of dye and the deposited film of spiro-OMeTAD all retain the device transparency, with smooth highly reflective Ag electrodes on the top. The J-V characteristics under the solar simulator AM 1.5 G for the D102 devices 1 day after device preparation are shown in Figure 3a. The important parameters for the solar cell device characterization are the short-circuit current (Jsc), the open-circuit voltage (Voc), and the power conversion efficiency (η). The ZnOZrO2 NW device shows the highest short-circuit current, 2.14 mA/cm2, and the ZnO NW demonstrates the lowest at 0.72 mA/ cm2. All of the devices prepared are shown to have an average Voc of 0.47 V. The η of the devices has been improved for both the ZnO-MgO NW and the ZnO-ZrO2 NW devices to 0.155% and 0.283%, respectively. It should be noted that the data
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Figure 3. Current density-voltage (J-V) characteristics of solid-state dye-sensitized solar cells measured under simulated AM 1.5 G illumination of 100 mW cm-2 incorporating D102 dye with ZnO NWs, ZnO-MgO NWs, and ZnO-ZrO2 NWs after (a) 1 day, (b) 1 month, and (c) 2 months time elapsed post device fabrication and the J-V characteristics of solid-state dye-sensitized solar cell devices at AM 1.5 G with D149 Dye with ZnO NW, ZnO-MgO NW, and ZnO-ZrO2 NW devices after (d) 1 day, (e) 1 month, and (f) 2 months time elapsed post device fabrication.
displayed comes from pixel 7 out of 8 on each device allowing comparison on the same position on each sample. The average errors on Jsc measurements are significantly lower in comparison to alteration between pixels with ranges for D149 devices 1 day after fabrication: ZnO NW 0.64-0.78 mA/cm2 (σ 0.049), ZnO-MgO 1.31-1.45 mA/cm2 (σ 0.057), and ZnO-ZrO2 2.62-3.21 mA/cm2 (σ 0.199). Similarly as the original NWs are produced in the same batch growth the Voc varies little over samples. Referring to Figure 2b, the high uniformity of the sample reflects the likelihood of similarly behaving pixels. To investigate the device performance further several samples were made, using both D102 and D149 dyes. The devices were measured over a period of 2 months. Figure 3 shows the J-V curves of the D102 and D149 devices, respectively, 1 day (a, d), 1 month (b, e), and 2 months (c, f) after fabrication. During this time period the devices were not encapsulated and were stored at room temperature in the dark under ambient atmosphere conditions (in a laboratory drawer). As a result of the enhanced photocurrent for both the ZnO-MgO NW and the
ZnO-ZrO2 NW in comparison to the ZnO NW devices, there is a higher overall power conversion efficiency for all of the coated NWs. Table 1 lists the main device performance parameters. It is clearly observed from Figure 3 that the ZnO NW devices show the poorest fill factors and least rectification, whereas the ZnO-ZrO2 NW devices have the best overall performance. During the period of 2 months the devices have remained stable. The highest performance of 0.7% was achieved for devices incorporating the ZnO-ZrO2 NW arrays in conjunction with D149 after 1 month. To the best of our knowledge this represents the highest efficiency reported for a ZnO nanowire hybrid solar cell incorporating an organic hole-transporter. In the context of previous literature in the field, these results show significant improvement over solid-state DSCs that employ a ruthenium-based sensitizer.21 Although it was previously shown that Ru-based dyes may not be advantageous for ZnObased solar cell applications due to the tendency for the dyes to agglomerate and hence limit charge transfer,32 work by other groups had shown the ZnO Ru system to be highly effective in
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TABLE 1: Solar Cell Performance Characteristics as prepared cells
after 1 month 2
after 2 months
device
η (%)
Voc (V)
Jsc (mA/cm )
η (%)
Voc
Jsc (mA/cm )
η (%)
Voc
Jsc (mA/cm2)
D102_ZnO NWs D102_ZnO-MgO NWs D102_ZnO-ZrO2 NWs D149_ZnO NWs D149_ZnO-MgO NWs D149_ZnO-ZrO2 NWs
0.093 0.156 0.283 0.088 0.278 0.596
0.47 0.49 0.47 0.47 0.58 0.57
0.73 1.12 2.14 0.71 1.53 3.02
0.173 0.358 0.611 0.249 0.301 0.717
0.56 0.52 0.52 0.57 0.52 0.6
1.43 1.75 3.51 1.05 2.03 2.79
0.095 0.143 0.426 0.132 0.319 0.537
0.43 0.37 0.47 0.5 0.44 0.52
0.83 1.32 2.76 0.96 2.18 2.82
nanocrystalline liquid electrolyte DSSCs with η comparable to that of TiO2 device performances.33,34 Here the application of small-molecule organic based dyes, with high molar extinction coefficient,23-27 used at dilute concentrations with short dye loading times, has produced an improvement in the pristine ZnO NW devices and the ZnO-MgO NW and ZnO-ZrO2 NW devices. These organic dyes have performed excellently in standard mesoporous TiO2 electrolyte-based DSCs,27 with η exceeding 9%,35 and on mesoporous ZnO layers short-circuit photocurrents of over 10 mA/cm2 have been reached.23 Work by others has also shown a direct improvement in solar cell performance using D102 in comparison to the Ru-based N719 dye in TiO2 solid-state DSCs,25 whereby the D102 causes an increase in the fraction of absorbed light in the cell. Overall our results showing that D149 dye outperforms the D102 dye agrees with previous studies in the literature.26,27 The critical improvement point appears to be the second rhodanine unit on the D149 dye (see Figure 1), which extends the π conjugation, thus red-shifting the absorption of the dye. Although the ZnO NW devices presented here still suffer from a relatively low η of under 1%, one clear issue is that the significantly smaller surface area of the ZnO nanowires due to the 2 h growth time leads to inferior dye loading and solar light harvesting in comparison to nanocrystalline films with total thickness of several micrometers. ZnO and SnO2-ZnO coated nanocrystalline film DSCs have clearly shown the suitability of ZnO as a photovoltaic material with η greater than 2% and even as high as 8%.33,34,36 The single crystalline nature of our ZnO NWs and their direct contact to the collection electrode should lead to faster electron transport in comparison to nanocrystalline films. The development of longer ZnO NWs and optimization of the fabrication procedure for these longer arrays could improve the power conversion efficiency.3,16 3.2. Transient Absorption. It is clear that the “insulating” and wide band gap shells of MgO and ZrO2 greatly enhance the photovolatic process. The broad intention of using these core-shell materials, in many different systems, is to “passivate” the semiconductor surface, primarily to inhibit electron-hole recombination.19,20,37-39 To investigate if this is occurring in this system, we have performed transient absorption spectroscopy on complete devices and dye-sensitized NW arrays. We first investigate the transient response of the optical transmission at 800 nm for films of the ZnO NWs, ZnO-MgO NWs, and ZnOZrO2 NWs that have been sensitized with D102 dye (there is no hole-transporter or top electrode in this instance). Studying the dye-sensitized electrode enables direct probing of the photoinduced electron transfer from the excited dye to the oxide, the initial step in the photovolatic process. This is probed through the optical absorption associated with dye cation at 800 nm,19 following electron transfer to the NW. Figure 4a shows the transient absorption signal of the dye cation on a nanosecond time scale. All samples were pumped with the same excitation fluence and exhibited similar optical absorption. Contrary to what we expected, the charge generation is increased on the
2
nanosecond time scale for the MgO coated NWs and even more so for the ZrO2 coated NWs. Indeed similar improvements have been observed in SnO2 DSCs coated with MgO.33,38 This could be due to favorable electron transfer through or to the core-shell materials as compared to bare ZnO. However, we note that this may also due to the presence of the ZrO2 layer reducing early time recombination. Figure 4b shows the transient absorption of complete NW devices sensitized with D102, measured 1 month after the cell fabrication. The hole species in spiro-OMeTAD absorbs strongly in the IR region;37 thus a 1.3 µm probe was used to monitor the charge dynamics on the hole transporter. Once more, all cells were excited with close to identical fluences, and optical absorption was similar. A clear difference between the photoinduced absorption of the devices indicates that the photogenerated charge density after 1 µs is considerably enhanced by the overcoating. Notably, the decay lifetimes for all materials are similar, indicating that the “insulating” and wide band gap shells have little influence on electron-hole recombination. These observations, in conjunction with the device measurements, strongly suggest that the overcoating layers significantly enhance the photoinduced charge generation and separation but have little influence upon charge recombination. Though this may at first appear surprising, it is not entirely implausible: binding to ZnO through carboxylic acid groups is known to be problematic because of the possibility of Zn complex formation. Hence, coating the ZnO with a material more stable to molecular
Figure 4. (a) Transient absorption of films of nanorods sensitized with D102 having identical optical densities. The absorption of the dye cation was probed at 800 nm after an excitation pulse at 533 nm with a fluence of 5 × 1014 photons/cm2. (b) Transient absorption of charged species in the various D102 solar cells 1 month after fabrication. The solar cells are held at open-circuit voltage. Transient absorption was measured at 1.3 µm after an excitation pulse at 533 nm with a fluence of 1 × 1014 photons/cm2. Greater transient absorption due to a higher concentration of charged species is observed in the more efficient devices.
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Figure 5. (a) TEM image of a ZnO-ZrO2 NW. (b) Zr 3d peak of a ZnO-ZrO2 NW sample both with and without dye.
adsorption could feasibly enhance the dye coupling to the oxide. The recombination in solid-state DSCs is known to be strongly suppressed by the presence of the lithium salts in the holetransporter matrix. This may mask any small difference that should be observed by the introduction of an insulating oxide surface layer. 3.3. Surface Characterization by Micropscopy and Spectroscopy. TEM analysis has been used to indicate the structural integrity of the NWs post hydrothermal shell fabrication. The MgO shell has been previously characterized22 and has been shown to be highly reproducible over many samples with a ∼6 nm amorphous MgO shell. Work by others on TiO2-ZrO2 core-shell nanoparticles for liquid electrolyte dye-sensitized solar cells has shown a ∼0.5 nm thick ZrO2 shell21 as confirmed by TEM and XPS, where a percent concentration of Zr similar to that seen on our ZnO-ZrO2 NWs was present over the bulk film. The addition of the thin ZrO2 shell improved η for those devices21 as has been the case for our ZnO-ZrO2 NW devices. From high resolution TEM images of ZnO-ZrO2 NW, Figure 5a, a 2 nm amorphous ZrO2 coating is observed, which over small areas has been seen to increase up to 5 nm. In Figure 5b the Zr 3d XPS spectra is shown for the ZnO-ZrO2 NW sample, both pristine and post dye loading. The dye-loaded sample shows a lower intensity of the ZrO2 peak due to the monolayer of carbon-rich dye covering the NW surface. The spectra has the characteristics ZrO2 3d peak with 3d5/2 at 182.8 eV and 3d3/2 185.4 eV21,40,41 confirming the presence of ZrO2 on these films. At present we are comparing various solid-state devices using the 2 h ZnO NW hydrothermal growth process. The ZnO-ZrO2 NW devices reported here show the highest η to date at 0.71% 1 month post fabrication. The improvement of ZnO NW solar cell devices over time has been observed by other groups, where after 1 month for ZnO-TiO2 core-shell NW devices13 η increased from
