Surface Modification of ZnO Nanorods with Small Organic Molecular

Oct 27, 2011 - (4) They can also provide a direct electron pathway at high carrier mobilities. ..... (44) At slower charge injection, a large number o...
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Surface Modification of ZnO Nanorods with Small Organic Molecular Dyes for Polymer Inorganic Hybrid Solar Cells Pipat Ruankham,† Lea Macaraig,† Takashi Sagawa,*,† Hiroyuki Nakazumi,‡ and Susumu Yoshikawa*,† † ‡

Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan

bS Supporting Information ABSTRACT: ZnO nanorod arrays modified with dye molecules demonstrate specific light-harvesting and charge-collecting properties which are promising for the enhancement of the characteristic performance of hybrid solar cells based on ZnO/poly(3-hexylthiophene). The properties of dyes commonly used for dye-sensitized solar cells were investigated in relation to the performance of polymer inorganic hybrid photovoltaic devices. The use of indoline dye D205, which has dipole moments directing away from the ZnO surface, was found to suppress the reverse saturation dark current density and charge recombination and to consequently lead to higher opencircuit voltage and improved power conversion efficiency (PCE) from 0.22% to 0.71%. Derivatized squaraine molecules were synthesized and were found to improve device performance by extending the lightharvesting range to the near-infrared region, leading to increased short-circuit current density and the highest PCE of 1.02%.

1. INTRODUCTION Hybrid polymer inorganic photovoltaic devices (HPVs) have been broadly studied in recent years because of their fundamental research interest and application potential. HPVs are constructed from a combination of organic materials such as p-type donor polymers and inorganic materials such as n-type acceptor nanostructured metal oxides.1 3 Among inorganic metal oxides, vertically aligned ZnO nanorods have great potential for HPVs and other electronic devices since they can easily be processed at low temperatures.4 They can also provide a direct electron pathway at high carrier mobilities.1,5 Also, the refractive index of ZnO (2.0) shows applicable potential for optical devices.6 However, when ZnO nanorods are matched with the most widely used p-type donor in HPVs and organic photovoltaics (OPVs), poly(3-hexylthiophene) (P3HT),7 9 measured power conversion efficiencies (PCEs) are quite low (less than 0.6%).1,10 13 One effort to improve device performance is the modification of the interface between the polymer and the metal electrode.10,14,15 Hau et al. have reported that the modification of ZnO by the adsorption of a layer of derivatized C60 improved performance by the reduction of charge recombination and an improvement of charge transfer properties, leading to a PCE of 0.72%.15 Improvement with the use of inorganic materials was also established with the use of CdSe-sensitized ZnO nanorods to increase photogenerated charge carriers, corresponding to a PCE of 0.88%.10 Organic sensitizers are also available in the form of dyes. Dyes have been largely used for photovoltaic works in the form of dyesensitized solar cells (DSCs) because of their high absorption r 2011 American Chemical Society

coefficients that benefit the light absorption spectra of the devices. One of the most widely used dyes in DSCs is the Rubased complex N719, which can give PCEs as high as 11% when TiO2 is used as the electrode.16,17 D205 and NKX2677 dyes are also common DSCs giving PCEs of 9.5%18 and 7.7%,19 respectively. Because of the instability of DSCs,20 many researchers have applied dye molecules to HPV work,10,21 23 but the effects on the device mechanism are still unclear. In this work, the effects of dye modification on the device mechanism are investigated to find appropriate properties of dyes for inverted ZnO nanorods/ P3HT devices by the use of the three different types of dyes for DSCs, N719 (Ru-based complex), NKX2677 (coumarin dye), and D205 (indoline dye), as shown in Figure 1. A derivative of squaraine, which is also used for DSCs, was synthesized and studied for HPV work. Squaraines absorb light beyond the 400 600 nm range of the three common DSC dyes and are expected to extend the range of HPV absorption to the nearinfrared region.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Squaraine (Sq) Dye. A mixture of semisquarylic acid24 (1; 0.129 g, 0.497 mmol), 5-carboxy-1-ethyl2,3,3-trimethyl-3H-indorenium iodide25 (2; 0.150 g, 0.418 mmol), and quinoline (200 mg) in butanol/benzene (5:1, v/v; 6 mL) was refluxed at 80 100 °C for 3 h with removal of water using Received: May 9, 2011 Revised: October 18, 2011 Published: October 27, 2011 23809 | J. Phys. Chem. C 2011, 115, 23809–23816

The Journal of Physical Chemistry C a Dean Stark distillation apparatus. The reaction of the synthesis is shown in Figure 2. After cooling, the solvent was evaporated in vacuo, and the residue was purified by silica gel column chromatography (CHCl3/MeOH, 10:1, as eluent). Recrystallization from CHCl3/MeOH/hexane afforded a crystal of Sq 3 H2O (3; 0.062 g, 0.126 mmol, yield 30%). The structure was confirmed through 1H NMR analysis (section 2 in the Supporting Information). 2.2. Preparation of ZnO Nanorods. The synthesis of ZnO nanorods was patterned after the method described in previously reported procedures.26,27 Dense ZnO thin films serving as the seed layers were deposited on indium tin oxide (ITO) substrates by spin-coating using zinc acetate solution (0.3 M) with a mixture of monoethanolamine (0.3 M) in 2-methoxyethanol. Then the films were annealed on a hot plate at 300 °C for 10 min. This process was repeated twice, yielding films approximately 40 nm thick. The hydrothermal growth of ZnO nanorod arrays was performed by suspending the as-prepared ZnO substrate in a polypropylene container filled with an aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine (50 mM). The container was next aged in an oil bath at 90 °C for 0 60 min. The substrate was carefully rinsed with distilled water several times and finally annealed at 150 °C for 10 min to remove organic residuals from inorganic materials. 2.3. Surface Modification by Dye Molecules. The as-prepared ZnO nanorod substrates were immersed in solutions of various dyes (N719, NKX2677, D205, squaraine) in a concentration of 0.3 mM in a mixture of acetonitrile and tert-butyl alcohol (1:1 by volume) at 60 °C for 1 h (see section 7 in the Supporting Information). The chemical structures of the dyes are shown in Figure 1. Then the substrates were rinsed by acetonitrile before fabrication of the solar cells.

Figure 1. Molecular structures of dyes used in this work.


2.4. Fabrication of Hybrid ZnO/P3HT Solar Cells. A solution of P3HT in chlorobenzene (30 mg/mL) was spin-coated on top of dye-modified ZnO nanorods. Subsequently, the films were annealed at 150 °C in a N2-filled glovebox for 3 min. Finally, the Ag top electrode (100 nm) was thermally evaporated in the vacuum evaporation system. 2.5. Characterization. Field emission scanning electron microscopy (FE-SEM) was performed to measure the length of ZnO nanorods (section 3 in the Supporting Information). Transmission and absorption measurements were carried out using a UV vis spectrophotometer (UV-2450, Shimadazu). Fourier transform infrared (FT-IR) spectroscopy was carried out (Iraffinity-1, Shimadzu). The photocurrent voltage characteristics were measured under an ambient atmosphere and simulated solar light, AM 1.5, 100 mW/cm2 (CEP-2000, Bunkoh-Keiki). The light intensity of the illumination source was calibrated by using a standard silicon photodiode (BS520, Bunkoh-Keiki). A photomask of 0.0503 cm2 was used to define active areas of the device irradiated by the light.

3. RESULTS AND DISCUSSION 3.1. Properties of the Dyes. 3.1.1. Dye Adsorption Behavior on the ZnO Surface. Dye molecules attach to metal oxide surfaces

by their carboxylic or carboxylate group.28,29 FT-IR techniques were used to investigate the types of these carboxylate coordinations. The carboxylate differences in the IR spectra have been used as a standard to reveal the type of bonding between the metal oxide surface and adsorbates.28 31 The difference is defined by Δν = νasym(COO ) νsym(COO ), where νasym(COO ) and νsym(COO ) are wavenumbers for asymmetric and symmetric vibrational modes of carboxylate, respectively. The type of anchoring mode is indicated by the following criteria: if Δνads > Δνsalt, the anchoring mode is monodentate, if Δνads < Δνsalt, the anchoring mode is a chelate or bridge, and if Δνads , Δνsalt, the anchoring mode is a chetate,28,32 where Δνads and Δνsalt are the carboxylate differences for the adsorbed state and salt state of dyes. In Figure 3a, the peak at about 1690 1750 cm 1 for salt dyes is characteristic of the CdO stretching band of the carboxyl group. Upon attachment onto the ZnO surface, the peak at ν(CdO) disappears due to deprotonation,11 indicating a shift to lower energy (lower ν) as the vibrational mode becomes coupled to another oxygen. This gives rise to an asymmetric COO (νasym) vibration (between 1540 and 1650 cm 1). Similarly, the C OH vibrational mode at 1200 1300 cm 1 shifts to higher energy on deprotonation, resulting in a symmetric COO (νsym) mode between 1300 and 1420 cm 1. The νasym(COO ) and νsym(COO ) values were carefully determined from IR spectra, and the values are summarized in Table 1.

Figure 2. Synthesis of squaraine dye. 23810 |J. Phys. Chem. C 2011, 115, 23809–23816

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Figure 3. FT-IR spectra of dyes for the salt state (top) and adsorbed state on the ZnO nanrods (bottom) at (a) low wavenumber and (b) high wavenumber.

Table 1. νasym, νsym, Δνads, and Δνsalt for Dyes in the Salt State and Adsorbed Statea salt state dye N719


adsorbed state

νasym νsym



Δνads Δνsalt anchoring mode

1609 1362






NKX2677 1607 1301






D205 Sq

1539 1595

1361 1350

199 233

178 245

bidentate monodentate

1562 1363 1585 1352

Details for νasym, νsym, Δνads, and Δνsalt are described in the text.

The carboxylate difference for the salt N719 is 247 cm 1, which is lower than that observed in the adsorbed state (254 cm 1). This reveals a monodentate mode for N719 dye adsorbed on the ZnO surface, while a bidentate mode for N719 dye adsorbed on the TiO2 surface has been investigated.28,33 Similarly, anchoring modes of NKX2677, D205, and Sq to the ZnO surface were determined to be bidentate, bidentate, and monodentate, respectively. Not all dye molecules however covalently bind through monodentate and bidentate modes with ZnO. Attachment through H-bonding between the hydroxyl groups of the ZnO surface and carboxylic acid group of the dye molecule can occur. The presence of hydroxyl groups on the ZnO surface was confirmed through photoluminescence for ZnO nanorods prepared by hydrothermal growth.34 Free hydroxyl groups are indicated by sharp peaks at around 3600 cm 1, which are observed in the IR spectrum of pure ZnO nanorods (Figure 3b) at 3628 3734 cm 1. Interestingly, these peaks cannot be seen in the spectrum of squaraine-modified ZnO nanorods, indicating that the carboxylic acid group of the dye has been deprotonated as it is adsorbed onto the ZnO surface and such COO groups interact with free hydroxyl groups of the ZnO surface through hydrogen bonding. However, for NKX2677 and D205, sharp peaks at around 3600 cm 1 can still be

Figure 4. Semiempirical calculation of the stabilized molecular structures and dipole moments (pink arrows) of the dyes.

moderately observed. This indicates that a small concentration of free hydroxyl groups is still present. This may largely come from the native hydroxyl groups of the ZnO surface that did not interact with carboxylic groups of the dyes. NKX2677 and D205 both have bulky groups that may strictly hinder the hydrogen bonding. For N719 the sharp peaks at this region are more pronounced. These peaks can be attributed to both the free carboxyl groups of the dye and the free hydroxyl groups of ZnO that cannot H-bond with the carboxylic groups of the dyes since they are strictly hindered by the bulky part of the dye. The coordination of carboxylic groups of all dyes to the ZnO surface is confirmed by the broad O H stretch peak at 2600 4000 cm 1, which indicates a H-bonded hydroxyl group. The spectra of salt dyes exhibit the usual ν(C H) stretches from hydrocarbon chains in the molecules, the absorption appearing between 2850 and 2970 cm 1. These peaks seem to decrease in intensity after absorption due to the domination of broad H-bonded O H stretching. 3.1.2. Dipole Moment of Dye Molecules. To understand the effects of interface modification on the photovoltaic performance, the dipole moments of the dyes were calculated using semiempirical models from the HyperChem software. The evaluated magnitudes and directions are shown in Figure 4. For N719 dye, De Angelis et al. have reported the dipole moments of the three dye isomers.35 The possible isomers for ZnO modification have dipole moment values of 28.0 and 25.0 D along the z-axis when the tetrabutylammonium groups are located on the carboxylic groups cis and trans, respectively, to the NCS ligands.35 For calculation simplicity and comparison, the orientations of the dye molecules used for calculating dipole moments are in such a way that the C2 axis of the carboxylate is parallel to the zaxis.29 The ZnO surface plane is parallel to the x- and y-axes. After the adsorption of dye molecules on the ZnO surface, the dipole moment may change in magnitude since the carboxylic group lessens in its electron-withdrawing property. However, this change is not significant as explained by Goh et al.22 Since the orientation of the molecules for dipole moment calculation was done on the basis of the orientation of the adsorbed dye molecule 23811 |J. Phys. Chem. C 2011, 115, 23809–23816

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Figure 5. Semilog plot and double log plot (inset) of dark current voltage characteristics (open symbols) for dye-modified ZnO nanorod/ P3HT hybrid devices under forward bias, together with the exponential fitted curve (gray line) and SCLC (black line). A down arrow indicates a position of built-in voltage.

Table 2. Voc and Vbi of Dye-Modified Devices with ZnO Nanorods of 180 nm Length dipole moment of dye μx

































modifying dye no N719b


a Direction with respect to the ZnO. b Dipole moment reported by De Angelis et al.34

onto ZnO, the total direction and the trend of the dipole moment of the dyes will not vary from the computed values, although the magnitude may differ as the bulk reorients due to steric strain35 and when the molecule bends for dyes connected in monodentate mode. 3.2. Electrical Properties. 3.2.1. Built-in Voltage Analysis. To investigate the effects of the dipole moments of dye molecules on the built-in voltage of the devices, the current density voltage (J V) behavior in the dark condition was collected. The double logarithmic plot of dark J V curves (inset of Figure 5) can be observed clearly in three distinct regimes:36,37 leakage-dominated current (linear), diffusion-dominated current (exponential), and space-charge-limited current (SCLC). Consequently, the built-in voltage (Vbi) can be read at the point where the current starts to follow a quadratic contribution.36 The values at this point are listed in Table 2. The difference in built-in voltage can be largely explained by the dipole moment of the dye molecules adsorbed onto the ZnO nanorods. Generally, for unmodified devices, O2 adsorption induces a positive space-charge width in the range of less than 10 nm at the surface of the ZnO nanorods.38 The internal electric field along the c-axis of the ZnO nanorods,39 which is formed by the polar space-charge layer, is significant since the dimensions of the ZnO nanorods are larger than those of the P3HT materials. This internal electric field of the nanorods can be enhanced or weakened by the adsorption of dye molecules on the surface in place of O2 molecules. In the case of molecules with their dipole moment pointing away from the ZnO surface (negative charge at

the ZnO surface), the thickness of the positive space-charge layer at the interface is wider and more homogeneous in comparison to that of the unmodified nanorod. The wider space-charge layer induces a stronger internal electric field of the nanorods and supports the electric field generated from the difference in the work functions of the metals (points from ITO to Ag), resulting in an enhancement of Vbi. However, the effect on the internal electric field and Vbi is reversed in the case of the dye molecules with their dipole moments pointing toward the ZnO surface. These mechanisms are shown briefly in Figure 10. It is clear from Figure 5 and Table 2 that the devices modified by NKX2677 (μz = 1.09 D) and D205 (μz = 2.63 D) dyes show an enhancement in the built-in voltage of unmodified devices, while the N719- and Sq-modified devices have lower Vbi as compared with pristine ZnO. Vbi is lowest for the devices modified by N719. This is due to the significantly large dipole moment (μz = 25.0 D). 3.2.2. Photovoltaic Characteristics. The dye-modified and unmodified ZnO nanorods were used for hybrid solar cells. The performance of the devices with different ZnO nanorod lengths is plotted in Figure 6. It is seen that surface modification with dye molecules improves the PCE of conventional devices from 0.22% to 0.26%, 0.71%, and 0.82% for N719, D205, and Sq modification, respectively. Unfortunately, the PCE decreases to 0.14% for NKX2677 modification. These PCE values are at the optimum nanorod length for each dye. It can be seen clearly from Figure 6c that the short-circuit current density (Jsc) increases with the length of the ZnO nanorods. This is largely due to the increase in the surface area, where the charge transfer occurs rather than light absorption of the ZnO nanorods as discussed in section 4 in the Supporting Information. The dye modification of the ZnO nanorod surface improved Jsc for all dyes (N719, D205, and Sq dye) except for NKX2677 dye. Interestingly, the Sq-modified devices show a significant increase of Jsc. More in-depth discussion on Jsc improvement by dye modification is detailed in section 3.3. Figure 6d shows the open-circuit voltage (Voc) of the dyemodified ZnO devices with various lengths of nanorods. The maximum Voc of 0.65 V was obtained by using D205-modified ZnO (180 nm). The significant increase from the Voc of the pure ZnO device (0.44 V) is largely due to the dipole moment contribution of D205 to the built-in voltage as described above. However, enhancement of Voc at this length is not observable for all dyes. For nanorods in the range of 180 290 nm, N719- and Sq-modified devices have lower Voc than unmodified devices. This can be attributed to the opposite direction of their dipoles adsorbed on the ZnO surface. However, devices with shorter (290 nm) lengths modified with these two dyes have improved Voc. To understand the contribution of the nanorod length and dye modification to the change of Voc, a single diode model was fitted to the J V curves. 3.2.3. Single Diode Model Fitting. The single diode model fitting is an effective method to investigate the effect of specific aspects in solar cells since the parameters of the model can be related to different mechanisms in the photocurrent conversion.40,41 The model can be fitted well to experimental data (section 5 in the Supporting Information), and the extracted parameters from the fits can be seen in Figure 7. For the pure ZnO devices, J0 decreased by a factor of 5 with lengthening of the ZnO nanorods to ∼290 nm and increased by 1 order of magnitude as the length of the nanorods was further increased (∼570 nm). On the other hand, the ideality factor (n), referring 23812 |J. Phys. Chem. C 2011, 115, 23809–23816

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Figure 6. Photovoltaic performance of hybrid solar cells with different ZnO nanorod lengths before and after modification with dye: (a) PCE, (b) fill factor, (c) short-circuit current density (Jsc), and (d) open-circuit voltage (Voc).

Figure 7. (a) Reverse current density (J0) and (b) ideality factor (n) parameters extracted from the single diode model fitting for devices before and after adsorption of dyes.

to the charge recombination mechanism, decreased from 3.31 for the devices with only a ZnO seed layer to 1.90 for devices with 570 nm nanorods. However, the large ideality factor (n > 2) is still under discussion. One explanation of this high value is a trapassisted tunneling via the defect levels or thermionic emission.42 This tunneling electron flows in a reverse direction to the photogenerated electron (from P3HT to ZnO), yielding the reverse current density. Moreover, taking into account that the ZnO nanorod solar cell has a 3D interface, it is possible that electrons from the ITO electrode leak to the P3HT layer via the thin and dense layer of the ZnO nanorods or from the defect level in the nanorods, resulting in charge recombination. The tunneling effect can be seen clearly for devices with only a dense ZnO layer (∼30 nm), which have an ideality factor of 3.31 and a J0 of 7 μA/cm2. However, as the thickness is increased by the lengthening of the ZnO nanorods, the thermionic effect is suppressed and J0 decreases, but when the length of the ZnO nanorods is further increased beyond an optimum point (180 290 nm), the ZnO material becomes too close to the Ag electrode, allowing for a photogenerated electron to transport to the Ag electrode, which is charged positively under forward bias. This leads to a significant increase of J0 and a reduction of Voc (Figure 6d). The decreasing trend of the n value with the length of the nanorods is still followed at high values (>290 nm) since the thermionic effect is eliminated such that the values approach the ideal value of 2.43 J0 of devices modified with N719 and Sq is greater than that of unmodified devices. It is possible that the charged groups in N719 and Sq molecules generate excess electrons, which can

Figure 8. Contour plot showing the open-circuit voltage (Voc) versus the ratio of Jsc to J0 and ideality factor (n) of devices modified with dyes.

serve as the leakage current. Moreover, their dipole moments, directed toward ZnO, support electron tunneling at the ITO/ ZnO surface, increasing J0. With increased reverse current, the possibility of charge recombination at the interface increases, giving higher n values. Also, both dyes are anchored to ZnO nanorods in monodentate mode, wherein charge injection is slower than that in the bidentate mode by an order of magnitude.44 At slower charge injection, a large number of carriers are available for recombination near the ZnO surface. This is especially noticeable for devices with longer nanorods. At longer lengths, n values of devices modified by these two dyes are significantly high due to the increased concentration of recombination centers at the dye molecules. On the other hand, the devices modified by D205 and NKX2677 dyes show a reduction in both J0 and n. Even for the long nanorod devices, wherein a short circuit (leakage) can easily occur, J0 is smaller by ∼1 order of magnitude than in unmodified devices. For devices at optimum length (around 290 nm), J0 decreases by 2 3 orders of magnitude, indicating that the dyes suppress electron injection from ZnO to P3HT (reverse current). This is attributed to the electric field generated by the dipole moment of the dye, which induces effective leakage-blocking behavior. Subsequently, charge recombination, corresponding to the n value (Figure 7b), decreases due to the presence of less leakage carriers as compared to that of other devices. An n value of ∼2 for NKX2677-modified devices at all lengths tested and an n value of 2 3 for D205-modified devices were obtained, indicating the domination of a single defect level recombination or band to band emission. The low n value also confirms the leakage-blocking behavior. 23813 |J. Phys. Chem. C 2011, 115, 23809–23816

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Figure 9. Comparison of IPCE spectra of devices with ZnO nanorods before and after modification with (a) N719, (b) NKX2677, (c) D205, and (d) Sq dye. Comparison of normalized absorbance of P3HT, ZnO nanorods (5 magnification), and the dyes adsorbed on the ZnO nanorods (5 magnification) (e h).

Figure 10. Device mechanisms along the ZnO nanorods (a) for unmodified devices, (b) for D205-modified devices, (c) for NKX2677-modified devices, and (d) for N719- and Sq-modified devices. Ein is the internal electric field of the ZnO nanorods. Red and blue dotted lines are the electron and hole transport pathways, and the gray dotted line is the charge leakage.

The changes in J0 and n contribute to the change in Voc by the equation Voc = (nkT/q) ln(Jsc/J0 + 1), where k and T are the Boltzmann constant and temperature, respectively. From the equation, Voc has a direct relationship with the n value and the logarithm of the ratio of Jsc to J0. From Figure 8, the trend of Voc can be predicted, with high Voc values expected for devices with n at 2 2.5 and Jsc/J0 of more than 10 000. The calculated value of Voc obtained from the equation is a very good approximation of the measured Voc as tabulated in the Supporting Information (Table S1). 3.3. Expansion of the Absorption Range. To investigate the light-harvesting improvement due to dye modification, the absorption of dye-modified ZnO nanorods in the UV vis region was measured (Figure 9e h) and an incident photon to current efficiency (IPCE) measurement was performed for dye-modified and unmodified devices. The IPCEs of dye-modified and unmodified devices with ZnO nanorods of 180 nm length are shown in Figure 9. In devices without surface modification, the IPCE peaks at 370 and 520 nm are attributed to the photogenerated charge carrier of ZnO and P3HT, respectively.12,45

The device modified by N719 demonstrates the enhancement in IPCE spectra (Figure 9a) in both the UV region and the visible region. The additional peak at ∼400 nm in the IPCE spectra for this device corresponds to the absorption spectra of N719 adsorbed on ZnO (Figure 9e). The same could be said for devices modified with D205, wherein improvements of IPCE at ∼380 and ∼520 nm (Figure 9c) are found. It is clear that the peak at 380 nm is attributed to the absorption of D205 (Figure 9g). At 520 nm, the IPCE improvement is attributed to both the absorbance of D205 and the absorbance of P3HT, which overlap at 450 650 nm. This improvement due to dye absorption at the visible range is most interesting for Sq-modified devices. The IPCEs of Sqmodified devices show significant enhancement (Figure 9d) in the region of 600 700 nm. This improvement is largely attributed to the blue Sq dye. At this region, the energy of the photons is very low such that very few electrons are excited in the P3HT material. This extension of the photoconversion range of the devices into the orange-red energy level is very important since a 23814 |J. Phys. Chem. C 2011, 115, 23809–23816

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P3HT and VOx layers, PCEs were equal to or slightly improved from that of the previous structure used. Among all dyes used, it is the PCE of devices with squaraine dye that are the highest and showed good improvement. Sq was used to modify the ZnO surface since among the four dyes, it gave the best performance. The performance of the device modified with Sq dye increases to 1.02% from 0.82% (Figure 11). This number is an improvement to previously reported PCEs of inverted hybrid ZnO devices.10,15,21,47,48

Figure 11. PCEs of devices structured as (A) ITO/ZnO nanorods/ P3HT (regular)/Ag and (B) ITO/ZnO nanorods/P3HT (electronic)/ VOx (5 nm)/Ag.

large number of photons from sunlight are at this energy level. This is seen in the dramatic Jsc improvement of Sq-modified devices, which dominates over that of the other dye-modified and unmodified devices (Figure 6c). 3.4. Compatibility between the Dyes and P3HT. For the devices modified with NKX2677, it can be seen that there is a decrease in the measured IPCE in the UV region and the visible region (Figure 9b) from unmodified devices. The shape of the IPCE spectrum is similar to that of the absorbance of NKX2677modified ZnO nanorods (Figure 9f). This indicates that the measured charge carriers are only from those generated from NKX2677, indicating that the excitons of P3HT do not separate at the NKX2677/P3HT interface. This may be due to the fact that the aromatic group of NKX2677 is blocked by the bulky terminal group that cannot accept electrons from P3HT (Figure 1). This problem is not seen when NKX2677 is used for DSCs19 because in DSCs, the electrolyte molecules are minute so they can interact with the aromatic portion of the NKX2677 even if they are hindered sterically. However, for hybrid devices, the structure and orientation of the functional groups in the dye molecule are important points to consider since charge transfer from the donor occurs at the interface of the dye molecule and P3HT. For N719-, D205-, and Sq-modified devices, the increase of IPCE of unmodified devices in the regions wherein the absorption of the dye molecules and absorption of P3HT overlap indicates that charges from P3HT can be separated at the dye/P3HT interface and the dye molecule can transport these electron charges and the electrons it generates to ZnO. The device mechanisms are summarized in Figure 10. For unmodified devices and D205- and N719-modified devices, the excitons of P3HT can separate and the free electrons can transport through the interface (red line in Figure 10), while this does not occur for NKX2677-modified devices. The induced space-charge layer for D205- and NKX2677-modified devices can block a flow of charge leakage22 (gray dotted line in Figure 10), while this does not occur for unmodified devices and N719- and Sq-modified devices. 3.5. Improvement of Device Performance. To improve the PCE of dye-modified ZnO devices, an electronic grade of P3HT (4002-E, Rieke Metals) was used because of its higher purity compared to regular grade (4002). Moreover, a VOx layer was inserted at the P3HT/Ag interface by thermal evaporation. The buffer VOx layer can prevent the recombination of the charge carriers at the P3HT/Ag electrode interface, leading to better device performance.46 For all the devices with electronic grade

4. CONCLUSIONS In conclusion, a workable approach for the enhancement of photovoltaic performance of HPVs based on ZnO nanorods and P3HT by dye modification was presented. Sq-modified devices gave the best PCE comparatively (0.82%), with PCEs of devices with improved structure reaching as high as 1.02%. The relatively high performance of Sq-modified devices was mainly attributed to the extension of the absorption range of the device to the nearinfrared range. The additional charges generated by Sq at the near-infrared region and its ability to accept electrons from P3HT (unlike NKX2677) led to high Jsc. Although the PCE was highest for Sq, D205-modified devices had better Voc. This is because the dipole orientation of Sq weakens the internal electric field of the ZnO nanorods (opposite that of D205), which increases J0, and the monodentate (bidentate for D205) anchoring mode of Sq, which leads to a comparatively higher n. Higher HPV performance can be obtained for dyes that can address these issues of high reverse current and recombination factor but still have the extended absorption range of Sq in the nearinfrared region. ’ ASSOCIATED CONTENT


Supporting Information. Characterization of Sq dye, morphology of ZnO nanorods, absorbance and IPCE spectra of devices for various nanorod lengths, single diode model fitting, comparison of calculated Voc and measured Voc, and dye adsorption conditions. This material is available free of charge via the Internet at

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (T.S.); [email protected] (S.Y.). Phone: +81-774-38-4580. Fax: +81-774-38-3508.

’ ACKNOWLEDGMENT We gratefully acknowledge the New Energy and Industrial Technology Development Organization (NEDO) of the Ministry of Economy, Trade, and Industry (METI) and the Core Research of Evolutional Science & Technology Agency (CREST) of the Japan Science Technology Agency (JST). We also thank Dr. Hirokuni Jintoku and Prof. Hirotaka Ihara of Kumamoto University for fruitful scientific discussions on the calculation of the dipole moment. ’ REFERENCES (1) Briseno, A. L.; Holcombe, T. W.; Boukai, A. I.; Garnett, E. C.; Shelton, S. W.; Frechet, J. J. M.; Yang, P. D. Nano Lett. 2010, 10, 334. (2) Leventis, H. C.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Nano Lett. 2010, 10, 1253. 23815 |J. Phys. Chem. C 2011, 115, 23809–23816

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