Electrolyte Effects on Electron Transport and ... - ACS Publications

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Electrolyte Effects on Electron Transport and Recombination at ZnO Nanorods for Dye-Sensitized Solar Cells Yu Xie,† Prakash Joshi,† Seth B. Darling,‡ Qiliang Chen,† Ting Zhang,§ David Galipeau,† and Qiquan Qiao*,† Center for AdVanced PhotoVoltaics, Department of Electrical Engineering, South Dakota State UniVersity, Brookings, South Dakota 57006, United States, Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States, and Department of Optoelectronics, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China ReceiVed: July 8, 2010; ReVised Manuscript ReceiVed: August 31, 2010

Electrolyte effects on electron transport and recombination at ZnO nanorods (nd-ZnO) were studied by electrochemical impedance spectroscopy (EIS) in dye-sensitized solar cells (DSSCs). Different electrolyte systems were prepared by gradually adding tert-butylpyridine (TBP) and guanidinium thiocyanate (GuSCN) and replacing LiI with 1-butyl-3-methylimidazolium iodide (BMII). The introduction of TBP and GuSCN, and the replacement of LiI with BMII suppressed charge recombination at the nd-ZnO/electrolyte interface, increased electron lifetime (τn), improved electron transport, and reduced collection time (τd) on nd-ZnO. In addition, they improved the electron diffusion coefficient (Dn) and elongated the effective diffusion length (Ln). The electron transfer coefficient (β) at the ZnO/electrolyte interface was increased from 0.34 to 0.44, which was a good sign of improvement in fill factor (FF). In addition, the ohmic series resistance was reduced from 17.35 to 4.99 Ω · cm2 and the charge transfer resistance was decreased from 18.49 to 7.09 Ω · cm2 at the electrolyte/Pt interface. Nd-ZnO DSSCs using different electrolytes were tested and the improvement of open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and incident photon to electron conversion efficiency (IPCE) was in good agreement with the findings from the EIS data. 1. Introduction Dye-sensitized solar cells (DSSCs) have become one of the most promising technologies as a low-cost alternative to conventional solar cells.1-9 A typical DSSC consists of a nanoparticle TiO2 (np-TiO2) photoelectrode and a counterelectrode separated by an iodide-triiodide (I-/I3-) liquid electrolyte.10 The np-TiO2 is sensitized by a dye, which serves as light absorber. After photoexcitation, dye molecules inject electrons into TiO2. The electrons then diffuse along TiO2 to the electrode and reach the counterelectrode through an external circuit. The dye molecules are reduced by the I-/I3- redox couples that convey electrons from the counterelectrode. Although np-TiO2 has a high surface area for dye attachment, structural disorder at the surface contact between adjacent nanoparticles leads to scattering of free electrons and thus reduces carrier transport.11-14 Enhanced electron transport was reported in various nanostructures including nanowires, nanorods, and nanotubes.15-20 High-density aligned ZnO nanorods (nd-ZnO) showed higher efficiency in DSSCs than unaligned nanorods because the high density led to a large surface area while alignment provided improved electron transport.21 Current studies on nd-ZnO DSSCs are mainly focused on effects of nanorod morphology (e.g., degree of alignment, diameter, spacing, and length) and growth parameters (e.g., synthesis temperatures and methods).17,21-25 * To whom correspondence should be addressed: tel 609-688-6965; e-mail [email protected]. † South Dakota State University. ‡ Argonne National Laboratory. § Beijing Institute of Technology.

It is generally agreed that electrolytes containing different components have a strong effect on DSSC performance. The effects of electrolyte composition on np-TiO2 DSSCs were extensively investigated.26-31 For example, the cation Li+ in LiI/ I2 electrolyte adsorbed onto the TiO2 surface and caused a positive shift of the TiO2 conduction band edge (Ec), which decreased open circuit voltage (Voc).26-28 Efficiencies of DSSCs with carbon nanotube (CNT) counterelectrodes were increased by use of 1-butyl-3-methylimidazolium iodide (BMII) instead of LiI, where CNT catalytic stability was improved by removing Li+.32 Guanidinium thiocyanate (GuSCN) and tert-butylpyridine (TBP) were also used as additives to suppress charge recombination at the TiO2/electrolyte interface.33,34 The addition of GuSCN can passivate the recombination sites on np-TiO2 and increase electron lifetime.33 TBP is believed to adsorb to TiO2, preventing Li+ diffusion onto TiO2 and prohibiting the subsequent positive shift in the Ec of TiO2, thus improving Voc.26,35,36 Additionally, the use of low-viscosity solvents enhanced both the long-term stability of the redox cycle37 and ion conductivities in the electrolyte.38 ZnO nanoparticle-based nanostructured electrodes have also been used in DSSCs;9 and the influence of iodide concentration and the effect of Li+ and TBP were studied in a solvent-free electrolyte.39 However, so far there have been few discussions on electrolyte influences on charge transport and lifetime in nd-ZnO DSSCs. It is imperative to understand and reveal electrolyte effects on electron transport and recombination at ZnO nanorods in order to develop an effective charge mediator and ZnO/electrolyte interface for efficient DSSCs. In this work, we studied electrolyte effects on electron transport and recombination in nd-ZnO DSSCs using electrochemical impedance spectroscopy (EIS). The electrolyte compositions were modified by gradually adding TBP and GuSCN

10.1021/jp106302m  2010 American Chemical Society Published on Web 09/28/2010

Electrolyte Effects on ZnO Nanorods for DSSCs

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17881

and replacing LiI with BMII. The EIS results showed that the use of TBP and GuSCN in concert with the replacement of LiI with BMII significantly improved charge transport in nd-ZnO and suppressed charge recombination from nd-ZnO to the electrolyte. The improved nd-ZnO DSSC performance in terms of open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and incident photon to electron conversion efficiency (IPCE) was consistent with these observations from the EIS data. 2. Experimental Section 2.1. ZnO Nanorod Preparation and Characterizations. All reagents and chemicals were purchased from Fisher Scientific and Sigma Aldrich. The substrates were cleaned with detergent, deionized (DI) water, acetone, and 2-propanol in an ultrasonic bath for 10 min each, followed by oxygen plasma for 10 min. A two-step hydrothermal method was used to prepare ZnO nanorods. First, ZnO nanocrystal seeds were coated onto cleaned fluorine-doped tin oxide (FTO) substrates as follows. Solutions of both 0.75 M ethanolamine (NH2CH2CH2O) and zinc acetate dehydrate [Zn(CH3COO)2 · 2H2O] were made with 2-methoxyethanol as solvent. Then the solution was coated onto a FTO substrate and baked at 360 °C. Second, a mixed solution of 0.005 M poly(ethylenimine), 0.025 M hexamethylenetetramine, and 0.025 M zinc nitrate hydrate was made in DI water. The solution was then heated to ∼92 °C and the samples coated with seeds were immersed. Afterward the substrates were rinsed via DI water and then ethanol or 2-propanol. They were then dried by N2 flow and sintered at 400 °C. Scanning electron microscopy (SEM) images were taken via a Hitachi S-4300SE/N SEM with an accelerating voltage of 15 kV under high-vacuum mode at room temperature. X-ray diffraction data were obtained on a Rigaku Ultima plus X-ray diffractometer (40 kW, 40 mA) with Cu KR radiation (wavelength λ ) 1.54 Å). 2.2. Photovoltaic Device Fabrication and Testing. The monopotassium salt of Z907 dye (Z907K, Z907 ) cisRu(H2dcpby)(dnpby)(NCS)2; H2dcbpy ) 4,4′-dicarboxylic acid2,2′-bipyridine; dnpby ) 4,4′-dinonyl-2,2′-bipyridine) was obtained from Konarka Technologies and used as the sensitizer. ZnO nanorod samples were immersed into dye solutions to attach Z907K molecules. After soaking, samples were rinsed by methanol and then dried by a compressed N2 flow. Pt counterelectrodes were made by depositing H2PtCl6 · 6H2O onto FTO glass and then heating at 380 °C.40 Parafilm was used as a spacer to assemble the devices. Three different electrolytes were prepared and injected into the cells. The electrolytes are (1) LiI/I2, 0.50 M LiI and 0.05 M I2 in propylene carbonate; (2) BMII/LiI/I2/TBP (LiI partially replaced by BMII; TBP and GuSCN partially added with a lower concentration), 0.30 M BMII, 0.25 M LiI, 0.04 M I2, 0.05 M GuSCN, and 0.25 M TBP in propylene carbonate/acetonitrile/valeronitrile 50:42.5:7.5 volume ratio; and (3) BMII/I2/TBP (LiI entirely replaced by BMII; TBP and GuSCN fully added with a higher concentration), 0.60 M BMII, 0.10 M GuSCN, 0.03 M I2, and 0.50 M TBP in acetonitrile/valeronitrile 85:15 volume ratio. Hot glue was used to seal the cells. To precisely compare electrolyte effects on the electron transport and recombination, all other parameters were kept constant. DSSCs were tested under a xenon lamp AM 1.5 solar simulator. Lamp power was calibrated by an Orion TH power meter with an Ophir 3A sensor. The incident photon to electron conversion efficiency (IPCE) spectral response was tested via a M-QE Kit system from Newport. Provided by the system, a

Figure 1. (a) Top view and (b) cross section SEM image of nd-ZnO.

monochromatic beam penetrated a set of lenses for focusing on samples. Spectral signals were tested and recorded with the pace of 1 point per 5 nm. 2.3. Electrochemical Impedance Spectroscopy. EIS measurements on the DSSC devices with different electrolytes were performed in the dark environment on an Ametek Versastat3200 potentiostat with frequency analysis module (FDA). The AC signal was used with amplitude of 10 mV and frequency range from 0.05 to 105 Hz. The EIS measurement was run at different forward biases, in which the nd-ZnO electrode was negatively biased and the counterelectrode was positively biased.13 EIS Spectrum Analyzer was used to fit the impedance data via transmission line modeling. 3. Results and Discussion 3.1. ZnO Nanorod Characterizations. SEM images of the nd-ZnO array are shown in Figure 1. ZnO nanorods were grown on the FTO substrate as an electron acceptor, providing a continuous electron transport pathway. The nanorod density is about 8 × 109 cm-2. The average diameter and length of the individual nanorods are ∼40 nm and 1 µm, respectively. Figure 2 shows an X-ray diffraction (XRD) pattern of the nd-ZnO arrays coated on a FTO glass substrate. Sharp and distinct peaks imply a high crystallinity of the nd-ZnO arrays. There was a strong (002) peak of ZnO in the XRD; other major peaks were all from the FTO substrate. Several lower intensity features corresponded to (100), (101), (102), (110), and (103) peaks of ZnO, respectively. The results indicated that some ZnO nanorods grew along the [001] axis while the others grew in other directions.21 This result was consistent with the SEM images discussed above. 3.2. Nd-ZnO DSSC Efficiencies and IPCE Spectra. Three electrolytes were used to fabricate nd-ZnO DSSCs. The

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Figure 2. XRD pattern of ZnO nanorod film on FTO glass. Red peaks are from nd-ZnO, while blue peaks are from FTO substrate.

Figure 4. (a) Current density-voltage (J-V) curves and (b) incident photon-to-current conversion efficiency (IPCE) spectra of the ZnO nanorod DSSCs with various electrolytes: LiI/I2 (red), BMII/LiI/I2/TBP (blue), and BMII/I2/TBP (green).

TABLE 1: Comparison of Photovoltaic Parameters with Three Different Electrolytes Figure 3. Schematic of DSSC fabricated with nd-ZnO.

schematic of the DSSC fabricated from nd-ZnO is shown in Figure 3. Figure 4a shows cell performance for three electrolytes tested under a simulated AM 1.5 illumination at an intensity of 100 mW cm-2. The comparison of cell parameters is summarized in Table 1. The LiI/I2-based electrolyte device exhibited a Jsc of 5.0 mA · cm-2, Voc of 0.63 V, and η of 1.3%. The partial addition of TBP and GuSCN and partial replacement of LiI with BMII in the electrolyte increased Voc from 0.63 to 0.72 V, leading to an efficiency increase from 1.3% to 1.7%. When LiI was completely replaced by BMII, the cell performance was improved even higher with values for Jsc and η of 6.4 mA · cm-2 and 2.3%, respectively. In np-TiO2 DSSCs, the cation Li+ in LiI/I2 electrolyte was reported to adsorb onto the TiO2 surface and lead to a positive shift of the conduction band edge (Ec), which decreased Voc but improved charge injection into TiO2.26-28 Voc was also increased here when Li+ cations were partly or entirely replaced by BMI+, and this is possibly caused by the same mechanism in the nd-ZnO here. In the LiI/I2 electrolyte, Li+ cations adsorbed onto the ZnO surface may also lead to a positive shift of Ec and thus reduction of Voc. When TBP is introduced into the electrolyte, it may adsorb onto nd-ZnO, thereby preventing Li+ diffusion onto the nd-ZnO surface and subsequent positive shift of Ec in ZnO. The addition of GuSCN may passivate the recombination sites on nd-ZnO and increase electron lifetime.33 In addition, when the nd-ZnO is electroncharged, the cation BMI+ may adsorb onto nd-ZnO and form a thicker electrical double layer, leading to a lower amount of I3- on the nd-ZnO surface. A similar interpretation was reported for np-TiO2 DSSCs.26 Therefore, the increase of Voc from 0.63 to 0.72 V can be interpreted with the mechanisms described above.

liquid electrolyte

Jsc (mA/cm2)

Voc (V)

FF (%)

η (%)

LiI/I2 BMII/LiI/I2/TBP BMII/I2/TBP

5.0 5.1 6.4

0.63 0.72 0.72

39.8 46.3 49.4

1.3 1.7 2.3

The IPCE spectral responses for three electrolyte-based DSSCs were also measured and are shown in Figure 4b. The BMII/I2/TBP-based device achieved an IPCE of ∼56%, corresponding to 6.4 mA · cm-2, while the LiI/I2- and BMII/LiI/I2/ TBP-based devices gave comparable IPCEs at ∼43% and ∼44%, at the level of a Jsc of ∼5.0 and ∼5.1 mA · cm-2, respectively. When IPCE is known, Jsc can be estimated by

Jsc )

∫λλ eηIPCE(λ)Nph(λ) dλ 1

0

(1)

where ηIPCE(λ) and Nph(λ) are the IPCE and photon flux density at wavelength λ, respectively. By use of eq 1, the integration of the measured IPCE spectral response with the AM 1.5 solar spectrum (100 mW · cm-2) results in an estimated Jsc of 4.8 and 4.9 mA · cm-2 for LiI/I2and BMII/LiI/I2/TBP-based cells, respectively, and 5.8 mA · cm-2 for BMII/I2/TBP-based devices. These calculated Jscvalues were found to be close to the measured Jsc values from the J-V curves, showing that the device testing is appropriate. The IPCE for BMII/I2/TBP-based cells was >26% higher than that of LiI/I2- and BMII/LiI/I2/TBP-based cells. This demonstrated that using BMII/I2/TBP electrolyte converted photons into electrons and regenerated the dye more efficiently than the cells using LiI/I2 and BMII/LiI/I2/TBP electrolytes due to improved electron transport and I3- reduction. It was reported that the addition of TBP could suppress electron-triiodide recombination41,42 and led to the negative shift of the band edge



Figure 5. General transmission line model of nd-ZnO DSSCs. rt is electron transport resistance in the nd-ZnO film; rct is the charge transfer resistance at the nd-ZnO/electrolyte interface; cµ is the chemical capacitance in nd-ZnO film; Zd is the Nernst diffusion impedance; Rct,Pt and CPt are the charge transfer resistance and capacitance at the Pt/electrolyte interface, respectively; RFTO/ZnO and CFTO/ZnO are the resistance and capacitance at the contact between nd-ZnO and FTO, respectively; RFTO/el and CFTO/el are the charge transfer resistance and capacitance at the FTO/electrolyte interface on the photoanode side, respectively; Rs is the sheet resistance.

with increased electron lifetime.35 However, it was also reported that the TBP caused only the negative shift of the band edge without any significant influence on electron-triiodide recombination and electron lifetime in the presence of Li+ or tetran-butylammonium (TBA+).26 Later on, Boschloo et al.43 studied the precise behavior of TBP in DSSCs and suggested the mechanism of TBP in increasing Voc as follows: (1) TBP caused a negative shift of the band edge and contributed to 60% of the Voc increase. This effect was the same at both a high and low concentrations of TBP. (2) TBP suppressed the electron-triiodide recombination by keeping triiodide away from the TiO2 surface. This influence was significant (contributing 40% of the Voc increase) at a high concentration of TBP but much less pronounced at a lower concentration. Because the surface adsorption of Li+ can cause a positive shift of the semiconductor conduction band edge (Ec) and lead to a larger energy offset between the Ec and the excited state of the dye, the charge injection efficiency may then be improved, but the Voc is reduced.26,27,38,44 In our work here, the addition of TBP in the electrolyte of BMII/LiI/I2/TBP in the presence of Li+ might cause a negative shift of the band edge back to compensate the positive shift induced by the Li+. Therefore, the DSSCs with Li+ containing TBP would not necessarily have higher charge injection efficiency than those with BMI+. The improved IPCE and Jsc in the BMII/I2/TBP-based cells might be caused by several factors: (1) decrease in charge recombination at the ndZnO/electrolyte interface, (2) increase of electron lifetime (τn), (3) reduction of electron transport/collection time (τd), (3) improvement in electron diffusion coefficient (Dn), (4) elongation in effective diffusion length (Ln), and (5) improved I3reduction and electrocatalytic activity at the electrolyte/Pt interface. In the discussion below, electrochemical impedance spectroscopy was used to study the reasons for the higher performance in BMII/I2/TBP-based DSSCs. 3.3. Electrochemical Impedance Spectroscopy. The role of the electrolyte in DSSCs is to facilitate the reduction of the photo-oxidized dyes, and it is an important factor to achieve high efficiency. The electrolyte also affects electron transport and recombination because charge transfer can occur from semiconductor to electrolyte at their interface. An effective electrolyte would be capable of suppressing charge transfer or recombination at the semiconductor/electrolyte interface while supporting efficient charge transport on the semiconductor side. EIS is an effective tool to study electrolyte effects on electron transport and recombination at the photoanode (TiO2, ZnO, etc).15,45 In the nd-ZnO DSSCs at forward bias, electrons are injected from FTO into nd-ZnO, and then the nd-ZnO is charged by electrons. Simultaneously, a portion of the injected electrons are lost by charge transfer from nd-ZnO to I3- in the electrolyte.

Figure 6. (a) Examples of electrochemical impedance spectra of DSSC device at 0.6, 0.65, 0.7, and 0.75 V forward bias in the dark with BMII/ I2/TBP electrolyte. (b) Zoomed spectra in the high-frequency region.

These processes can be illustrated by the transmission line model shown in Figure 5, which has been developed by others previously.15,45,46 If L is the nd-ZnO length, the overall electron transport resistance, interfacial charge transfer resistance, and chemical capacitance in the nd-ZnO film can be described as Rt () rtL), Rct () rct/L), and Cµ () cµL), respectively. Figure 6 shows representative impedance plots of a nd-ZnO DSSC device at forward biases of 0.6, 0.65, 0.7, and 0.75 V, respectively. The low frequency large arcs (the semicircles shown in Figure 6a) are attributed to the nd-ZnO/electrolyte interfacial charge transfer (recombination) resistance in parallel with the total capacitance in the bulk ZnO nanorods and the ZnO/electrolyte interface. The small quarter-arcs (Figure 6b at high bias potential) in the high-frequency region are assigned to the charge transfer process at the electrolyte/counterelectrode interface with the resistance (Rct,Pt) and capacitance (CPt) in parallel. As the forward bias potential decreased from 0.75 to 0.6 V, the impedance spectra showed a Warburg-like behavior in the nd-ZnO film. Also the semicircles from the charge recombination at the nd-ZnO/electrolyte interface became larger as the forward bias decreased. This phenomenon is also observed in np-TiO2 DSSC devices.45 As discussed above, the electrolyte

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Figure 7. (a) Charge recombination resistance (Rct) of DSSC devices with three different electrolytes: LiI/I2 (black squares), BMII/LiI/I2/ TBP (red circles), and BMII/I2/TBP (blue triangles) at various forward biases in the dark. (b) Dependence of charge transfer coefficient (β) and fill factor (FF) on electrolyte composition.

directly affects the charge transfer at both nd-ZnO/electrolyte and electrolyte/Pt interfaces. Three different electrolyte systems (LiI/I2, BMII/LiI/I2/TBP, and BMII/I2/TBP) were used to study the effects on electron transport in nd-ZnO and recombination at the interface of nd-ZnO/electrolyte, as well as their influences on the charge transfer process on the counterelectrode side. The charge transfer (recombination) resistance (Rct) at the interface of nd-ZnO/electrolyte was studied in nd-ZnO with three different electrolytes at various forward biases in the dark. The fitted charge transfer resistances (Rct) at various biases are shown in Figure 7. The Rct values for the three electrolytes are in the order LiI/I2 < BMII/LiI/I2/TBP < BMII/I2/TBP. The smaller Rct means that the ZnO nanorod/electrolyte interface has a larger dark saturation current density (J0). J0 can be used as a measure of recombination in DSSCs. By fitting the current density-voltage curves in Figure 4a, the dark saturation current density (J0) for LiI/I2-, BMII/LiI/I2/TBP-, and BMII/I2/TBP-based cells were obtained at 5.0 × 10-9, 4.7 × 10-10, and 3.9 × 10-10 A · cm-2, respectively. The decreased J0 caused by the suppressed electron-electrolyte recombination indicated that the Voc could be increased for the BMII/I2/TBP electrolyte, in which the TBP and GuSCN may passivate the recombination sites on nd-ZnO. This is consistent with other reports in np-TiO2 DSSCs.41,42 It agrees well with our results for a higher Voc in BMII/I2/TBP and BMII/LiI/I2/TBP cells than those in LiI/I2. In addition to the suppressed recombination, another reason for increase in Voc is the negative shift of the semiconductor band edge (Ec) caused by the addition of TBP.26,35,43 The dependence of Rct on the applied forward bias can be expressed by46

(

Rct ) R0 exp -β

qV kBT

)

(2)

Figure 8. Forward bias dependence of (a) electron transport resistance (Rt) and (b) conductivity (σn) of the DSSCs with different electrolytes: LiI/I2 (black squares), BMII/LiI/I2/TBP (red circles), and BMII/I2/TBP (blue triangles).

where β is the transfer coefficient, R0 is a constant, q is the elementary charge, kB is the Boltzmann constant, and T is the temperature. By use of eq 2, the β for each electrolyte was obtained and shown in Figure 7b. This implies that the forward bias dependence of Rct is much stronger in BMII/I2/TBP than in LiI/ I2 and BMII/LiI/I2/TBP. It was also reported by others that a larger transfer coefficient (β) would lead to a higher fill factor (FF) in nanotube ZnO (nt-ZnO) and np-TiO2 based cells.15,47 When the electrolyte was changed from LiI/I2 f BMII/LiI/I2/ TBP f BMII/I2/TBP, the charge transfer coefficient (β) was increased, leading to an increase of FF in DSSCs. This is consistent with results found in the J-V measurement where the FF was improved from 39.8% for LiI/I2 to 49.4% for BMII/ I2/TBP, which is also depicted in Figure 7b. The value of β in np-TiO2 DSSCs was ∼0.6,45 which is higher than that observed in our nd-ZnO DSSCs. This could be the reason why FFs in our nd-ZnO DSSCs are lower than those in np-TiO2 cells. The transport resistance (Rt) and electron conductivity (σn) from the fit of impedance spectroscopy are shown in Figure 8, panels a and b, respectively. Rt was observed to be much smaller than the charge transfer (recombination) resistance (Rct) discussed above in nd-ZnO, which is typically required for a highefficiency DSSC device.15 The Rt in nd-ZnO was found to be independent of the electrolyte at low forward biases (V < 0.55 V); however, for higher bias (V > 0.55 V), Rt was reduced when the electrolyte was changed from LiI/I2 f BMII/LiI/I2/TBP f BMII/I2/TBP. Fabregat-Santiago et al.48 reported that Rt in a np-TiO2 DSSC could be described as

[ (

Rt ) Rt0 exp -

EF,redox - Ecb e Va + kT e

)]

(3)

where Rt0 is a constant, Va is the applied bias, Ecb is the lower edge of the conduction band, and EF is the Fermi level position of the semiconductor. From eq 3, the theoretical value of the



slope of Rt is -e/(2.3kT).45 At room temperature, this value is 16.7 dec/V and its inverse is about 60 mV/dec. In the np-TiO2 DSSC devices, the dependence of Rt on the forward bias potential was found to follow an exponential behavior with a slope of 66 mV/dec, which was close to the ideal slope of about 60 mV/dec.48 In the nd-ZnO DSSCs here, we observed that Rt decreased monotonically as the forward bias increased. However, the slope showed a nonideal behavior at a bias potential larger than 0.6 V, which was different from the typical findings in np-TiO2 DSSC devices. In the ZnO nanorods, the electrons transport only via the neutral core region, which is separated from the surface by a depletion layer.46,49,50 The diameter of the neutral core region increases with the forward bias potential, thereby leading to a decrease in Rt upon increasing the bias voltage. When the depletion region becomes very thin or disappears at high bias potential, the electron transport along the nanorods is affected by the electrolyte, and this might be the reason for the nonideal behavior of Rt at high bias potentials with a very thin or no depletion layer. Efforts to study this nonideal slope in detail are in progress. The conductivity (σn) of nd-ZnO can be calculated as

σn )

L1 A Rt

(4)

where L is the length of the nd-ZnO, A is the cross sectional area of the conductive region in the nanorods, and Rt is the electron transport resistance. The nanorod density is about 8 × 109 cm-2. The average size of the individual nanorods is 1 µm in length and 40 nm in diameter (Figure 1). As discussed above, the potential across the depletion region decreases when the forward bias increases, and the radius of the neutral region increases so that the entirety of the nanorods and their surfaces can be involved in the charge transport. It has also been reported that the traps are surface-related.51 Once the nd-ZnO surface region transports electrons, the surface traps will enhance Rt and reduce σn. When TBP and GuSCN are added to the electrolyte to passivate the nd-ZnO surface, the trap-related recombination may be suppressed, which can decrease Rt and increase σn. The reduced Rt was consistent with the observation of increased charge transfer resistance (Rct) at the nanorod/ electrolyte interface. However, at low biases, the surface region of nd-ZnO was depleted and not involved in electron transport. Therefore, under such conditions the reduced recombination at the nanorod surface did not affect Rt even when TBP and GuSCN were introduced. Figure 8b shows the dependence of σn on the biases, which was calculated via eq 4 by assuming a conducting diameter of 20 nm in the neutral core region.46 Even though the σn (6.32 × 10-4 Ω-1 · cm-1) obtained in our ndZnO is much lower than that (∼10-2 Ω-1 · cm-1) in nanotubeZnO grown via atomic layer deposition (ALD),15 it is close to the value (6 × 10-4 Ω-1 · cm-1) in other nd-ZnO systems reported by He et al.46 As shown in Figure 9, the capacitance was found to increase exponentially with the potential at low forward bias (V < 0.6 V) in all three electrolyte-based DSSC devices, indicating a high charge accumulation and effective electrical communication between the quasi-Fermi level in ZnO and FTO.15 The exponential increase upon potential less than 0.6 V was possibly caused by the chemical capacitance following exponential distribution of states in the gap.15,46 In the exponential dependence region, the capacitance can be estimated as

Figure 9. Capacitance of ZnO nanorod DSSCs with three different electrolytes: LiI/I2 (black squares), BMII/LiI/I2/TBP (red circles), and BMII/I2/TBP (blue triangles).

[

Cµ ) C0,µ exp -R

qV kBT

]

(5)

where R ) T/T0, T0 is a characteristic temperature indicating the depth of the distribution, and C0,µ is a constant. At higher potential (V > 0.6 V), Figure 9 shows that the slopes of Cµ for all three electrolytes become smaller. The phenomenon of the shift in Cµ dependence on potential at high bias was also observed by He et al.46 When the chemical capacitance reflects a distribution of trap states in the gap, it is reasonable to describe the electron transport/collection time (τd) and lifetime (τn) by15,52

τd ) RtCµ ) L2 /Dn

(6)

τn ) RctCµ

(7)

where Dn is the electron diffusion coefficient and L is the length of the ZnO nanorods. Figure 10 shows the comparison of τd and τn in the cells with different electrolytes. In all three electrolytes, τn is at least 1 order of magnitude higher than τd, indicating that electron transport/collection is very efficient since it is within the charge lifetime. For comparison in specific electrolytes, we found that τn has the order LiI/I2 < BMII/LiI/I2/TBP < BMII/I2/TBP, while τd exhibits an opposite order LiI/I2 > BMII/LiI/I2/TBP > BMII/ I2/TBP. This shows BMII/I2/TBP has the longest lifetime but shortest collection time; therefore it is the most efficient electrolyte for charge collection. This is in good agreement with IPCE spectral response results (Figure 4b) and also indicates that BMII/I2/TBP-based devices have the highest IPCE. Other parameters that are important for charge collection efficiency include the electron diffusion coefficient (Dn) and effective diffusion length (Ln). Dn and Ln can be calculated by15,53

Dn )

L2 L2 ) τd RtCµ

Ln ) √Dnτn ) L

(8)

τn )L τd

Rct Rt

(9)

As shown in Figure 11, both Dn and Ln increase as the electrolyte changes from LiI/I2 f BMII/LiI/I2/TBP f BMII/ I2/TBP. This shows that the electrolyte influences Dn and Ln. It has been reported that the surface adsorption of cations (e.g.,

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Figure 10. Comparison of the electron lifetime (solid symbols, τn) and electron transport/collection time (open symbols, τd) for nd-ZnO DSSCs with electrolytes LiI/I2 (black squares), BMII/LiI/I2/TBP (red circles), and BMII/I2/TBP (blue triangles).

Figure 12. (a) Impedance spectra and (b) high-frequency regions of DSSC devices with three different electrolytes: LiI/I2 (black squares), BMII/LiI/I2/TBP (red circles), and BMII/I2/TBP (blue triangles) at forward bias of 0.7 V in the dark.

TABLE 2: Fitted Parameters on the Counterelectrode Side from Impedance Spectra with Three Different Electrolytes

Figure 11. Comparison of (a) electron diffusion coefficient (Dn) and (b) effective diffusion length (Ln) of DSSCs with electrolytes LiI/I2 (black squares), BMII/LiI/I2/TBP (red circles), and BMII/I2/TBP (blue triangles).

Li+ and DMPIm+) can influence electron lifetime (τn) and diffusion coefficient (Dn).26,54 Figure 11b shows that Ln for BMII/ I2/TBP-based DSSCs is much larger than the nanorod length itself at about 1 µm compared to the other two electrolytes. By use of eq 9, the increased Ln upon the use of this electrolyte can be interpreted by two possible reasons: (1) the introduction of TBP and GuSCN may suppress electron-electrolyte recombination on the nd-ZnO surface, thus reducing the charge loss and increasing Rct;33,41-43 or (2) the use of BMI+ instead of Li+ elongated the electron lifetime (τn) because BMI+ possibly forms a thicker double layer resulting in a smaller amount of I3- on the nd-ZnO surface. The observed Ln substantially exceeds the length (1 µm) of the ZnO nanorod with τd < τn and Rtr < Rct, suggesting a high charge collection efficiency.15 Interestingly, we found that Ln decreased as the forward bias increased, different from the results observed in typical np-TiO2 and nt-ZnO, in which Ln became larger as potential increased.15,45 Recently, Wang el al.55 have also reported that the electron diffusion length decreased as the forward bias potential increased under a 23.8% sunlight intensity illumination. The decrease of Ln on potential we found here is consistent with the observations from other reported ZnO nanorods when eq 9 is used to calculate Ln.46 This is possibly caused by the depletion region in the ZnO

electrolyte

Rd,Pt (Ω · cm2)

CPt (F)

Rct,Pt (Ω · cm2)

LiI/I2 BMII/LiI/I2/TBP BMII/I2/TBP

17.35 10.12 4.99

7.15 × 10-5 3.08 × 10-4 3.32 × 10-4

18.49 9.89 7.09

nanorods. It was reported that electron transport occurred through the neutral region in the core of the nanorods.49 As the forward bias increases, the potential across the depletion region decreases and the radius of the neutral region increases so that the entirety of the nanorods and their surfaces can be involved in the charge transport. Once the nd-ZnO surface region disappears or is thin enough to be involved in electron transport, the surface-related traps will affect and enhance the charge transfer from nd-ZnO to electrolyte. In comparison of Figures 7a and 8a, it can be seen that decrease of Rct upon potential takes place more rapidly than that of Rt upon forward bias. Then, from eq 9, it is reasonable to find that Ln decreases as potential increases. Figure 12 shows the impedance spectra of DSSC devices with three different electrolytes (LiI/I2, BMII/LiI/I2/TBP, and BMII/ I2/TBP) at forward bias of 0.7 V. All three impedance spectra displayed large semicircles at low frequencies (Figure 12a) and small quarter-circles at high frequencies (Figure 12b). The large semicircles are attributed to electron transfer from the ZnO nanorods to the electrolytes (charge losses), while the small quarter-circles are from the charge transfer process of the electrolyte/counterelectrode interface. Fitting results of the charge transfer process at the electrolyte/counterelectrode interface are summarized in Table 2. When the electrolyte was optimized from LiI/I2 f BMII/LiI/I2/TBP f BMII/I2/TBP, the charge transfer resistance (Rct,Pt) at the electrolyte/counterelectrode interface was reduced from 18.49 f 9.89 f 7.09 Ω · cm2, and the capacitance (CPt) increased from 7.15 × 10-5 f 3.08 × 10-4 f 3.32 × 10-4 F, indicating that BMII/I2/TBP is the most efficient electrolyte for the reduction of triiodide (I3-). In addition, the series resistance (Rd,Pt) was decreased from 17.35

Electrolyte Effects on ZnO Nanorods for DSSCs f 10.12 f 4.99 Ω · cm2. This can be interpreted as an indication that BMII, which stays in liquid state at room temperature and possesses lower viscosity, shows a higher diffusion coefficient.56,57 In addition, the electrolyte solvents including acetonitrile and valeronitrile have relatively low viscosities and can enhance ion conductivity for rapid diffusion of mediators (I-/I3-).38,58,59 The reduced charge transfer resistance (Rct,Pt) and series resistance (Rd,Pt) at the Pt/electrolyte interface imply that the IPCE and Jsc can be increased by use of BMII/I2/TBP electrolyte, which is in good agreement with DSSC performance (Figure 4). 4. Conclusions EIS results at forward biases showed that electrolyte composition affects electron transport and recombination on ndZnO in DSSCs. The introduction of TBP and GuSCN suppressed charge recombination at the ZnO/electrolyte interface. At high potentials, they also reduced the charge transport resistance (Rt) and increased electron conductivity (σn) of ndZnO. In addition, the use of TBP, GuSCN, and BMII increased electron lifetime (τn), reduced transport/collection time (τd), improved electron diffusion coefficient (Dn), and elongated effective diffusion length (Ln) on the nd-ZnO. The series resistance (Rd,Pt) was reduced from 17.35 to 4.99 Ω · cm2, and the charge transfer resistance (Rct,Pt) was decreased from 18.49 to 7.09 Ω · cm2 at the electrolyte/Pt interface. The improved ndZnO DSSC performance in terms of Voc, Jsc, FF, and IPCE could be interpreted from the findings from the EIS data. Acknowledgment. We appreciate the partial financial support from the NSF CAREER (ECCS-0950731), NASA EPSCoR (NNX09AP67A), ACS PRF DNI (48733DNI10), and South Dakota NSF EPSCoR/PANS program. We also thank Dr. Mahdi F. Baroughi, Department of Electrical Engineering & Computer Science, South Dakota State University, for his great help in setting up the J-V and IPCE measurement systems. Use of the Center for Nanoscale Materials at the Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. References and Notes (1) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (2) Alibabaei, L.; Wang, M. K.; Giovannetti, R.; Teuscher, J.; Di Censo, D.; Moser, J. E.; Comte, P.; Pucciarelli, F.; Zakeeruddin, S. M.; Gra¨tzel, M. Energy EnViron. Sci. 2010, 3, 956. (3) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. AdV. Mater. 2000, 12, 1214. (4) Abbotto, A.; Manfredi, N.; Marinzi, C.; De Angelis, F.; Mosconi, E.; Yum, J. H.; Zhang, X. X.; Nazeeruddin, M. K.; Gra¨tzel, M. Energy EnViron. Sci. 2009, 2, 1094. (5) Yoshida, T.; Iwaya, M.; Ando, H.; Oekermann, T.; Nonomura, K.; Schlettwein, D.; Wohrle, D.; Minoura, H. Chem. Commun. 2004, 400. (6) Gajjela, S. R.; Ananthanarayanan, K.; Yap, C.; Gra¨tzel, M.; Balaya, P. Energy EnViron. Sci. 2010, 3, 838. (7) Yum, J. H.; Baranoff, E.; Hardin, B. E.; Hoke, E. T.; McGehee, M. D.; Nuesch, F.; Gra¨tzel, M.; Nazeeruddin, M. K. Energy EnViron. Sci. 2010, 3, 434. (8) Zhang, G. L.; Bai, Y.; Li, R. Z.; Shi, D.; Wenger, S.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. Energy EnViron. Sci. 2009, 2, 92. (9) Keis, K.; Magnusson, E.; Lindstro¨m, H.; Lindquist, S.-E.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2002, 73, 51. (10) Joshi, P.; Xie, Y.; Ropp, M.; Galipeau, D.; Bailey, S.; Qiao, Q. Energy EnViron. Sci. 2009, 2, 426. (11) Martinson, A. B. F.; Elam, J. W.; Liu, J.; Pellin, M. J.; Marks, T. J.; Hupp, J. T. Nano Lett. 2008, 8, 2862. (12) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (13) Yang, Z.; Xu, T.; Ito, Y.; Welp, U.; Kwok, W. K. J. Phys. Chem. C 2009, 113, 20521.

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