Device Performance Related to Amphiphilic Modification at Charge

Feb 16, 2011 - Hind Fadhil Oleiwi , Azmi Zakaria , Chi Chin Yap , Sin Tee Tan , Hock Beng Lee , Chun Hui Tan , Riski Titian Ginting , Abdelelah Alshan...
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ARTICLE pubs.acs.org/JPCC

Device Performance Related to Amphiphilic Modification at Charge Separation Interface in Hybrid Solar Cells with Vertically Aligned ZnO Nanorod Arrays Dongqin Bi, Fan Wu, Qiyun Qu, Wenjin Yue, Qi Cui, Wei Shen, Ruiqiang Chen, Changwen Liu, Zeliang Qiu, and Mingtai Wang* Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China Key Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, People's Republic of China

bS Supporting Information ABSTRACT: This paper reports the chemical modification effects at charge separation interface on the performance of the hybrid solar cells consisting of poly(2-methoxy-5(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as an electron donor (D) and vertically aligned ZnO nanorod arrays as an electron acceptor (A). Results show that, with increasing the modification time Ts for grafting dye Z907 onto the ZnO surface from 0 to 8 h, the charge transfer efficiency at the MEH-PPV/ZnO interface keeps increasing, the short circuit current (Jsc) increases and reaches a peak value at Ts = 8 h, but the open circuit voltage (Voc) increases within Ts = 1-3 h and reduces with further increasing Ts up to g6 h. By controlling the Ts, a peak power conversion efficiency of η = 0.61% at AM 1.5 illumination (100 mW/cm2) is obtained for Ts = 6 h. It is revealed that the Z907 modification mainly contributes to the enhanced Jsc by increasing the charge separation efficiency as a result of the improved electronic coupling property at the D/A interface rather than the light harvesting; on the other hand, the Z907 modification reduces (Ts = 1-3 h) or increases (Ts g 6 h) the surface defect concentration of the ZnO nanorods, resulting in the increased or reduced Voc (or electron lifetime τe). It is demonstrated that trapping electrons by the surface defects may facilitate the charge separation at the D/A interface in the MEH-PPV/ZnO devices, and both Voc and τe correlate to the occupation of injected electrons in conduction band and surface defects. Further analysis provides the relation between Voc and τe in those devices.

1. INTRODUCTION Polymer-based solar cells (PSCs) that use conjugated polymers as an electron donor (D) and nanocrystals as an electron acceptor (A) in a bulk-heterojunction structure are promising for low-cost application of solar energy.1,2 Recently, an encouraging power conversion efficiency (η) up to 7% has been realized by using fullerenes as the organic electron acceptors for such devices.3 Using inorganic semiconductor nanoparticles as alternatives to the fullerenes produces hybrid PSCs that combine advantages of both organic polymers (e.g., easy processability, lightweight, and good flexibility) and inorganics (e.g., high electron mobility, high electron affinity, and good stability).2 However, the state-of-the-art efficiency of the hybrid PSCs (η = 2-3%)4 is not high yet. Among the limiting factors in the hybrid devices, the poor compatibility between inorganic A and organic D components and the serious charge recombination at the D/A interface are the major interfacial difficulties for efficient devices. The incompatibility between hydrophilic nanoparticles and hydrophobic r 2011 American Chemical Society

polymers frequently causes a macroscopic phase separation and a bad interfacial contact between A and D components, resulting in a low efficiency of the charge transfer from D to A materials; moreover, the normally low carrier mobility (ca. 10-1-10-9 cm2 V-1 s-1)1a,2d in conjugated polymers often leads to a slow hole transport away from the D/A interface and consequently a poor spatial separation of photogenerated charge carriers (electron and hole), yielding an easy interfacial charge recombination. Therefore, optimization of the D/A interface to enhance the charge separation efficiency and reduce the charge recombination is an important issue for efficient PSC devices.5 Surface modification of nanoparticles with small organic photoactive molecules4b,6-8 or conducting polymers9 has been an efficient strategy to improve the interfacial properties and the device performance in their PSCs. Received: November 22, 2010 Revised: January 23, 2011 Published: February 16, 2011 3745

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The Journal of Physical Chemistry C As they are nontoxic and stable in the environment and transparent to visible light, the nanoparticles of metal oxides (e.g., TiO2 and ZnO) are often used as A materials for PSCs. As compared with the nanostructures unmodified or modified with different modifiers, the surface modification of TiO24b,6a-6f and ZnO7 with carboxylated organic molecules/dyes normally leads to the increase in both short circuit current (Jsc) and open circuit voltage (Voc), for which the increased Jsc is mainly attributed to the enhanced charge separation efficiency as a result of the improved interfacial contact by the surface modification, and the increased Voc is ascribed to the reduced charge recombination at the D/A interface. Different modification effects on the device performance have also been suggested.8,9a For example, Said and co-workers8 found that, with increasing the concentration of tetra(4-carboxyphenyl)porphyrin (TCPP) chemically bound on ZnO nanorods, the devices consisting of poly(3-hexylthiophene) (P3HT) and ZnO-TCPP nanorods exhibit a reduced Jsc, along with an increased Voc and recombination rate for a low TCPP concentration, where the increased phase separation in the P3HT/ZnO blends by the interfacial modification, as well as the enhanced recombination rate, reduces the overall photocurrent by overshadowing the beneficial effect of TCPP on light harvesting. Note, the origins for the increased Voc and charge recombination rate have not been clarified for the TCPP modification.8 Even though the present understanding of the interfacial modification can explain the observed device performance to a certain extent,4b,6,7 there is still insufficient knowledge concerning the intrinsic correlation between the surface property of nanostructures and the device performance due to interfacial modification. On the other hand, application of vertically aligned ZnO nanorod arrays to PSCs,7,10 instead of the disordered A-phase pathways in the simple blends of polymer and nanoparticles, can provide an ideal architecture for efficient devices,1,2 because the preformed nanorod arrays offer direct pathways for electron transport and stably maintain the spatial distribution of the D/A interface at exposure to evaluated temperatures. Particularly, using the preformed ZnO nanorod arrays to study the interfacial modification effects on device performance can avoid the possible change in the spatial distribution of D/A interface subjected to the modification8 and provide exclusively the information on the modification effects at the D/A interface. In this paper, vertically aligned ZnO nanorod arrays are chemically modified with amphiphilic and carboxylated dye molecules (i.e., Z907)11 at different soaking times (Ts) for grafting Z907 onto ZnO surfaces, and the modification effects on the performance of the polymer/ ZnO solar cells with poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV) as the polymer are systematically investigated. Our data show that, with increasing the time Ts, the device Jsc is greatly enhanced, but the device Voc, as well as the electron lifetime τe, gets increased or reduced depending on the Ts ranges. It is shown that the presence of Z907 enhances the charge separation efficiency at the D/A interface for the increased Jsc but reduces or creates more defects at the D/A interface for the increased or decreased Voc. Note that dye Z907 has been previously applied to modify ZnO nanorod arrays for P3HT/ZnO solar cells,7 where the Z907 modification normally increases both Jsc and Voc of the devices as compared with unmodified samples. The authors suggested that the Z907 modification increases the device Jsc by improving the wetting property of ZnO surface and further increasing the D/A interfacial area for charge separation, and enhances the device Voc

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Scheme 1

by reducing the charge recombination at the D/A interface due to the blocking effect of Z907. It should be identified that the present study is significantly different from the previous reports. In previous reports, the modification effect was studied in comparison with unmodified samples. In contrast, the present study is carried out by using a series of ZnO nanorod arrays modified with different loads of Z907 that are controlled by the soaking time Ts and focuses on the device performance correlating with the interfacial property altered by the surface modification.

2. EXPERIMENTAL SECTION Device Fabrication. MEH-PPV (average Mn = 4000070000, Aldrich) was commercially obtained. Dye Z907 (cisRuLL0 (SCN)2, L = 4,40 -dicarboxylic acid-2,20 -bipyridine, L0 = 4,40 -dinonyl-2,20 -bipyridine) was synthesized as described elsewhere (Scheme 1).11 Chlorobenzene (AR, Sinopharm Chemical Reagent Co., Ltd.) was distilled under reduced pressure before use. The vertically aligned ZnO nanorod array (170 nm in length and 30-40 nm in diameter) was grown in 10 min on the ZnO dense layer coated on indium tin oxide (ITO) conducting glass (e15 Ω/0, Shenzhen Laibao Hi-Tech Co., Ltd., China) by electrochemical deposition, as described previously.10 The aligned ZnO nanorod array was annealed in air at 100 and 220 °C for 20 min, respectively, providing the pristine ZnO nanorod array. After cooling to 80 °C, the ZnO nanorod array was immediately immersed in a solution (3  10-5 M, room temperature) of Z907 in a mixture of acetonitrile/tert-butyl alcohol (=1/1 in volume) to adsorb Z907 for a defined soaking time Ts (1-8 h), followed by a rinse with anhydrous alcohol (3 times) to remove the excess dye, providing the sample ZnOZ907. Solar cells were fabricated by the procedure reported previously,10 where the polymer layer was deposited on the top of ZnO nanorods by spin-coating (2500 rpm, 20 s) the MEHPPV solution in chlorobenzene (10 mg/mL) under ambient conditions, followed by annealing at 150 °C under N2 atmosphere for 20 min to ensure the polymer infiltration into the interspaces between nanorods. A gold electrode (100 nm thick) of 1  4 mm2 defined the active area of each device. Note, the pristine ZnO nanorod array in this experiment is a bit different from that used in our previous report10 in that the previous one has not been thermally annealed after growth. Characterization. To prepare the samples for transmission electron microscopy (TEM) and FT-IR studies, the ZnO and ZnO-Z907 nanorod arrays were scraped off the ITO substrates. The TEM samples were prepared by drying the MEH-PPV/ZnO and MEH-PPV/ZnO-Z907 composites formed in chlorobenzene on TEM copper grids, and the TEM studies were performed on a JEOL-2010 microscope under an acceleration voltage of 200 kV. 3746

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Figure 2. TEM images of MEH-PPV/ZnO (a) and MEH-PPV/ZnOZ907 with Ts = 6 h (b). The arrows on (b) indicate the presence of the polymer.

3. RESULTS AND DISCUSSION

Figure 1. (a) Absorption and (b) room temperature PL (excited at 500 nm) spectra of MEH-PPV/ZnO (Ts = 0 h) and MEH-PPV/ZnO-Z907 (Ts = 1-8 h) composite films on ITO substrates. The inset to (a) shows the corresponding absorption spectra of ZnO-Z907 layers, while that to (b) is the room temperature PL spectrum of pure MEH-PPV film on an ITO substrate.

FT-IR spectra of the samples in KBr pellets were recorded on a Nicolet Magna-IR 750 spectrometer with a maximum resolution of 0.1 cm-1. The absorption and room temperature photoluminescence (PL) properties were measured in ambient conditions. The absorption spectra were recorded with a Shimadzu UV-2550 spectrophotometer, while PL measurements were performed on a Hitachi F-7000 spectrofluorophotometer. The MEH-PPV solution (10 mg/mL) was used to prepare the MEH-PPV, MEH-PPV/ZnO, and MEH-PPV/ZnO-Z907 samples for optical measurements by spin coating (2500 rpm, 20 s), and the samples were annealed at 150 °C for 20 min under N2 atmosphere before the optical measurements. The Z907 samples on SiO2 and ZnO surfaces for PL measurements were prepared by drop-coating the Z907 solution in the acetonitrile/tert-butyl alcohol mixture on quartz substrate and the dense ZnO layer on ITO substrate, respectively, followed by drying in air. Incident photon to current efficiency (IPCE) spectra of the solar cells were measured on a QE/IPCE Measurement Kit (Newport, USA) in the spectral range of 300-900 nm, as described elsewhere.12 The steady-state J-V curves of solar cells under AM 1.5 illumination (100 mW/cm2) were measured in ambient conditions on a 94043A Oriel Sol3A solar simulator (Newport Stratford, Inc.) with 450 W xenon lamp as light source. The electron lifetime (τe) in the devices was obtained by intensity modulated photovoltage spectroscopy (IMVS). Dynamic IMVS responses and the J-V curves in the dark of the devices were measured by controlled intensity modulated photo spectroscopy (CIMPS) (Zahner Co., Germany) in ambient conditions, with a blue light emitting diode (LED) as light source (I0 = 15.85 mW/cm2, modulation depth δ ≈ 0.1) for IMVS, as shown previously.10 The experiments for each sample have been repeated at least 3 times, and remarkably reproducible data were obtained.

3.1. ZnO Modification. The absorption and PL spectra of MEH-PPV/ZnO-Z907 composites with different soaking times, Ts, are shown in Figure 1. The absorption spectra of Z907 in solution and solid state are shown in Figure S1 in Supporting Information. The absorption around 500 nm (inset to Figure 1a) of ZnO-Z907 samples is attributed to the characteristic metal-toligand charge-transfer (MLCT) transition of Z907.13 The absorption intensity of Z907 on the ZnO surfaces is clearly in the order 8 h > 6 h > 3 h > 1 h, even though the samples with Ts = 1 and 3 h, as well as 6 and 8 h, do not exhibit a great difference in the absorption intensity; moreover, a significant increase in the Z907 absorption intensity appears when increasing Ts from 3 to 6 h. These results indicate that the Z907 content on ZnO surface increases for a longer Ts, the adsorption of Z907 onto the ZnO surface is not very rapid, the adsorption of major Z907 molecules onto ZnO surface occurs in Ts = 3-6 h, and the loading of Z907 onto the ZnO surface becomes nearly saturated in ∼6 h. As shown in Figure 1a, all of the composite samples exhibit the absorption onset at 386 nm (3.21 eV) corresponding to the band gap absorption of ZnO; moreover, the absorption from 400 to 600 nm corresponding to the π-π* band of MEH-PPV in the MEH-PPV/ZnO-Z907 samples is a bit stronger than that in the MEH-PPV/ZnO film, similar to the absorption enhancement in P3HT/ZnO-N719 composite film due to the adsorption of dye N719 on ZnO nanorods.14 As compared with the MEH-PPV and MEH-PPV/ZnO films, the MEH-PPV/ZnO-Z907 samples exhibit additionally a weak absorption in 600-700 nm, in agreement with the absorption of Z907 (Figure S1, Supporting Information). Obviously, the absorption spectra of the MEHPPV/ZnO-Z907 samples are simply the sum of the absorption spectra of MEH-PPV, ZnO, and Z907 components, where no significant change in the absorption spectra indicates a very faint contribution of the adsorbed Z907 to light harvesting.6d The PL peak of the MEH-PPV film at 595 nm is the emission of excited π-electrons relaxing to the ground state (inset to Figure 1b). Analogous to our previous observation,10 the emission profiles of the MEH-PPV/ZnO and MEH-PPV/ZnO-Z907 composite films are dominated by that of the MEH-PPV in the measured range from 500 to 800 nm, indicating that the remaining luminescence in the composite films originates from the radiative decay of the excitons from the polymer. The PL intensity of the composite films is significantly lower than that of the pristine MEH-PPV film, indicating the effective charge transfer from MEH-PPV to ZnO as a result of the exciton dissociation at the MEH-PPV/ZnO interface.15 Here, the PL quenching efficiency (PL-QE) is calculated by comparing the maximum emission 3747

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Figure 3. Typical J-V curves of MEH-PPV/ZnO (Ts = 0 h) and MEHPPV/ZnO-Z907 (Ts = 1-8 h) devices under AM1.5 illumination (100 mW/cm2). The inset is the J-V properties of the devices in the dark.

Table 1. Device Performance of MEH-PPV/ZnO (Ts = 0 h) and MEH-PPV/ZnO-Z907 (Ts = 1-8 h) Solar Cells under AM 1.5 Illumination (100 mW/cm2)a Ts (h) Jsc (mA/cm2) 0

3.55 ( 0.21

Voc (V)

FF

η (%)

τe (ms)

0.30 ( 0.01 0.29 ( 0.01 0.31 ( 0.01 0.81 ( 0.03

1

4.01 ( 0.14

0.33 ( 0.02 0.32 ( 0.02 0.42 ( 0.02 0.96 ( 0.03

3

5.04 ( 0.24

0.35 ( 0.02 0.30 ( 0.01 0.52 ( 0.02 1.02 ( 0.02

6

6.53 ( 0.26

0.29 ( 0.01 0.32 ( 0.01 0.61 ( 0.02 0.77 ( 0.03

8

7.14 ( 0.29

0.26 ( 0.01 0.29 ( 0.03 0.55 ( 0.02 0.62 ( 0.03

a

Each of the data with standard deviations represents the average of three devices.

intensity of the composite sample to that of the MEH-PPV film. The PL-QE of ZnO nanorods was increased from 74% to 85% as Ts increased from 0 to 8 h (refer to Figure 4b). Clearly, the presence of Z907 enhances the charge transfer efficiency at the MEH-PPV/ZnO interface, leading to a higher charge separation efficiency for a longer Ts in the tested Ts range. Figure 2 compares the TEM images of the pristine ZnO and ZnO-Z907 nanorod arrays blended with MEH-PPV. For the MEH-PPV/ZnO sample, bare ZnO nanorod surfaces and the empty interspaces between the nanorods were clearly identified under TEM; however, the modification with Z907 produced the uneven morphology around ZnO nanorods and the interspaces between the nanorods filled with the polymer.9c The TEM results indicate that MEH-PPV is more easily adhered to the ZnO nanorod surface after Z907 modification. Therefore, modification of the ZnO nanorod surface with amphiphilic Z907 improves the compatibility between ZnO and MEH-PPV. The hydrophobic alkyl groups of Z907 (Scheme 1) have a good compatibility with the hydrophobic polymer matrix and are able to enhance the interfacial contact at the MEH-PPV/ZnO interface.6 FT-IR spectrum (Figure S2, Supporting Information) shows that the ZnO-Z907 sample exhibits the asymmetric (νas, at 1547 cm-1) and symmetric (νs, at 1380 cm-1) stretching bands of carboxylate anion (COO-) complexed with surface Zn centers,8,16 as well as the band at 2101 cm-1 due to the thiocyanato group (NCS);16a however, the typical stretching band of carboxylic acid group (νCdO, at ∼1720 cm-1)13a was absent from the ZnO-Z907 sample. The FT-IR results suggest that each Z907 molecule is chemically grafted onto the ZnO surface with two carboxylic acid groups. Furthermore, the separation Δνa-s (=167 cm-1) between νas and νs is much higher than that (∼94 cm-1) of the ionic salt zinc acetate, indicating that the Z907 molecules are chemically grafted onto the ZnO surface with the formation of carboxylate species in a unidentate

Figure 4. (a) IPCE spectra of MEH-PPV/ZnO (Ts = 0 h) and MEHPPV/ZnO-Z907 (Ts = 1-8 h) devices. (b) Dependence of PL quenching efficiency and averaged Jsc on Ts. Notes to (b), the scatters are the experimental data and the lines were calculated by linear regression for those scatters; the numeral marked above each line is the line slope.

binding mode.16b As the ZnO nanorod array has been stably preformed on the ITO substrate before Z907 modification, the change in the spatial distribution of D/A interface due to the modification is not the case in this experiment (see Introduction). Therefore, the observed increase in the charge separation efficiency upon the Z907 modification (Figure 1b) is reasonably due to the better electronic coupling between ZnO and MEH-PPV at the MEH-PPV/ZnO interface,4b,5c,17 where the enhanced compatibility between MEH-PPV and ZnO brings the polymer to the ZnO surface and increases, to a certain extent though not predominant (refer to later discussion), the D/A interfacial area for charge separation,7,18 resulting in the improved electronic coupling property at the D/A interface for charge transfer. 3.2. Solar Cells. The J-V characteristics of MEH-PPV/ZnOZ907 solar cells under AM 1.5 illumination and in the dark are typically shown in Figure 3. The J-V curves in the dark of all the devices pass through the origin where no potential results in no current (inset to Figure 3), consistent with those of heterojunction solar cells.19 The device Jsc increases remarkably with increasing Ts; however, Voc first increases when Ts increases from 0 to 3 h, and then decreases as Ts g 6 h. The data of each sample are remarkably reproducible. Table 1 shows the averaged overall photovoltaic performance of three individual devices for each sample. Except for the similar fill factor (FF) of ∼30%, the device Jsc and Voc are significantly dependent on the Z907 modification, resulting in a peak power conversion efficiency of η = 0.61% at AM1.5 illumination (100 mW/cm2) for Ts = 6 h. The Jsc and η values for Ts g 6 h are higher than 2-3 times the values reported for the similarly structured P3HT/ZnO-Z907 device (Jsc ≈ 2.0 mA/cm2, Voc ≈ 0.25 V, η = 0.2%) where Z907 was adsorbed at 100 °C.7b The much lower Jsc may be due to the poor interfacial contact of ZnO/P3HT as a result of the high crystallinity in P3HT.20 3.2.1. Modification Effect on Jsc. The exciton generation and dissociation are key processes for photocurrent generation. Others have shown that interfacial modification may contribute 3748

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The Journal of Physical Chemistry C to light harvesting.6c,8 To gain insight into the increased Jsc upon the Z907 modification, the IPCE spectra of the devices were measured (Figure 4a). All of the devices exhibit the highest IPCE at the wavelength close to the maximum absorption of MEHPPV. Normally, the MEH-PPV/ZnO-Z907 device has a higher IPCE than the MEH-PPV/ZnO one, and the Ts influence on the IPCE agrees with that on Jsc (Table 1). The MEH-PPV/ZnO device has a spectral response from 300 to 600 nm, while the IPCE spectra of the MEH-PPV/ZnO-Z907 ones have a weak response extending to 600-700 nm, indicating the very limited contribution in the range of 600-700 nm where there is no absorption of the MEH-PPV. Therefore, the Z907 modification increases the device IPCE and photocurrent, to which the contribution from the absorption of the grafted Z907 is negligible though Z907 absorbs weakly in 600-700 nm (Figure 1a and Figure S1, Supporting Information). Others have also observed no contribution of the Z907 as a modifier to the photocurrent generation in the similarly structured P3HT/ZnO-Z907 device,7b as well as in the bilayer P3HT/TiO2 devices.6b This phenomenon is believed to be due to the absorption of Z907 in the devices being much lower with respect to that of MEH-PPV and the device photocurrent is still dominantly generated from the optical absorption of the polymer. Known from Figure 4b, moreover, the dependence of PL quenching efficiency (i.e., PL-QE) and Jsc on the soaking time Ts follows a similar trend in the tested Ts range, indicating the photocurrent correlates tightly with the charge transfer efficiency. Therefore, the increased photocurrent (i.e., Jsc) upon the Z907 modification originates from the enhanced charge separation efficiency at the MEHPPV/ZnO interface with the improved electronic coupling property for charge transfer rather than the contribution of Z907 to light harvesting. This is further supported by the absorption and PL results (Figures 1 and 4b) in that the absorption intensities of MEH-PPV/ZnO-Z907 composite films with Ts = 1 and 3 h are almost the same, as well as those with Ts = 6 and 8 h, but their PL quenching efficiencies differ remarkably. Note that the above conclusion is in agreement with the findings in P3HT/ ZnO devices.7 3.2.2. Modification Effect on Voc. To fully understand the device performance as a result of the Z907 modification, the electron lifetime τe in the devices was measured with IMVS (Figure S3, Supporting Information).10 IMVS measures the periodic photovoltage response to a small sinusoidal light perturbation superimposed on a large steady background level I0, and the time constant τ of the IMVS response provides a good estimation of the electron lifetime τe related to the interfacial charge recombination dynamics under open circuit condition. The τe values of the devices are presented in Table 1. The τe = 0.81 ms in the MEH-PPV/ZnO device is a bit higher than the value (0.48 ms) reported previously for the same grown but not thermally annealed ZnO nanorod array,10 which is very likely due to the different thermal history of the ZnO nanorod array in this experiment (see Experimental Section), because the thermal annealing will inevitably change the surface defects21 that may impose great influences on the charge recombination dynamics.10,22 Upon the Z907 modification, τe initially increases from 0.81 ms (Ts = 0 h) to 1.02 ms (Ts = 3 h); however, as Ts g 6 h, τe gets shorter than that in the pristine ZnO nanorod array (Table 1). Obviously, the Z907 modification remarkably suppresses the interfacial charge recombination as Ts e 3 h but promotes the charge recombination for Ts g 6 h. Generally, the reduced charge recombination rate upon the interfacial modification

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Figure 5. (a) Room temperature PL spectra of ZnO (Ts = 0 h) and ZnO-Z907 (Ts = 1-8 h) under excitation at 325 nm, where the inset is the room temperature PL spectra of Z907 films drop-coated on quartz and ZnO film surfaces. (b) Dependences of IDLE/INBE, averaged τe, and averaged Voc on Ts. Note, IDLE/INBE values are determined by the ratio between the emission intensities at 382 nm (INBE) and 550 nm (IDLE) in panel a.

in comparison with the unmodified PSC samples has been attributed to the energy barrier formed at the D/A interface by Z9076b,7 or other small modifiers,4b,6c,6d,6f which increases the spatial separation of photogenerated electrons and holes. In this experiment, the different effects of Z907 modification on τe were observed for Ts e 3 h and Ts g 6 h, which obviously cannot be simply accounted for in the energy barrier point of view; otherwise, an increased trend for the τe within Ts = 0-8 h should be expected with increasing Ts. In order to reveal the intrinsic effects of Z907 modification on Voc and τe, the PL spectra of the ZnO-Z907 nanorod arrays with different Ts values were measured, as shown in Figure 5a. The pristine ZnO nanorod array (Ts = 0 h) exhibits a sharp emission at 382 nm (3.25 eV) and a weak and broad emission at 500-600 nm (2.48-2.07 eV), which are the same as our previous observation.10 The emission at 382 nm is the near band edge (NBE) emission, while the one at 500-600 nm is the deep level emission (DLE) due to surface defects.23 The PL profile of the ZnO-Z907 samples is the same as that of the pristine ZnO nanorod array. Known from Figure 5a, the NBE emission intensity is hardly influenced but the DLE intensity is remarkably reduced by the Z907 modification with Ts = 1-3 h; however, further increasing Ts makes the DLE of the ZnO-Z907 samples similar to (Ts = 6 h) or higher than (Ts = 8 h) that of the pristine ZnO nanorod array, accompanied by a remarkable decrease in the NBE emission intensity. To clarify whether the change in the DLE intensity is related to the emission of Z907, the room temperature PL spectra of Z907 films drop-coated on SiO2 and ZnO surfaces, in which the SiO2 surface is a nonquenching one,24 were measured with an excitation at 325 nm (inset to Figure 5a). Clearly, almost no PL of Z907 was detected in 500-600 nm on both nonquenching and quenching surfaces under the excitation at 325 nm. Note, the excitation of Z907 in solution at 325 nm also produces no PL peak in the 500-650 nm range (Figure S1, Supporting Information). Therefore, the observed change in the 3749

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The Journal of Physical Chemistry C DLE intensity with Ts mainly correlates with the surface defect concentration of ZnO nanorods, and the DLE can serve as a helpful probe to monitor the change in surface defects subjected to the Z907 modification.4b,25 The intensity ratio between the DLE and NBE emission (IDLE/INBE) is an indication of crystal quality, and a larger ratio of IDLE/INBE means a higher concentration of surface defects.23 Accordingly, the concentration of surface defects on ZnO nanorods is significantly reduced by the Z907 modification with Ts = 1-3 h, which is similar to the findings in modifying TiO2 nanoparticles with dye N719.6f Since the unidentate carboxylate complex on a ZnO surface can efficiently trap the photogenerated holes during ultraviolet radiation to intensify the visible luminescence through the electron-rich carbonyl group,16b,26 on the other hand, the increased IDLE/INBE ratios for Ts = 6-8 h suggest that more surface defects are created on the ZnO nanorods as compared to the modification with Ts = 1-3 h. The Ts dependence of the IDLE/INBE ratio, the electron lifetime τe, and the device Voc are depicted in Figure 5b. It is clear that both τe and Voc have a similar dependence on Ts with a longer τe for a larger Voc, and the reduced concentration of surface defects leads to a larger Voc and longer τe. Obviously, the observed Ts dependence of Voc and τe in the MEH-PPV/ZnO devices correlates with the changed concentration of surface defects on ZnO nanorods by the Z907 modification. The Voc of the polymer/nanostructure solar cells is normally determined by the difference between the quasi-Fermi levels of the electrons in the nanostructure and the holes in the polymer.4b,6b,27 As the defect concentration is reduced by the Z907 modification with Ts = 1-3 h, more electrons will accommodate in conduction band, with an increase in the difference between the quasi-Fermi levels of electrons and holes for a higher Voc.28 In contrast, for Ts = 6-8 h, more surface defects are created at the MEH-PPV/ ZnO interface by the carboxylate groups to trap the injected electrons to the energy level below the conduction band edge, resulting in a reduced Voc.4b,27 At the same time, as more electrons are trapped in surface defects, they will not easily escape from the MEH-PPV/ZnO interface where a substantial number of holes exist, leading to a faster recombination or smaller τe.22,28b Therefore, the Voc and τe in the MEH-PPV/ZnO devices are tightly related to the occupation of injected electrons in conduction band and surface defects, and the Z907 modification alters the surface defect concentration to change such occupation, which is responsible for the observed increased or reduced Voc and τe. Additionally, it is observed that the Z907 modification increases the charge transfer efficiency at the MEH-PPV/ZnO interface with increasing Ts from 0 to 8 h (Figures 1b and 4b), and more defects exist in the samples with Ts g 6 h than in the unmodified one (Ts = 0 h) (Figure 5); these indicate that trapping electrons by surface defects may facilitate the charge transfer at the D/A interface for efficient charge separation.5c,28a 3.2.3. Remarks on Relation between Voc and τe. A higher Voc is accompanied by a longer τe in this experiment (Table 1 and Figure 5b). This agrees with the reports on PSCs4b,6a-6f,7 or dye-sensitized solar cells29 by others. One question may arise, concerning whether there is a direct dependence of Voc on τe in the PSCs. In the MEH-PPV/ZnO nanorod array solar cells,10 the devices, which have different ZnO nanorod lengths (Ln) (i.e., Ln = 110 and 170 nm) and almost the same surface defect concentration of ZnO nanorods, exhibit a slightly different Voc (0.39 and 0.32 V, respectively) together with a significantly different τe (0.60 and 0.48 ms, respectively), suggesting that the Voc is slightly

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influenced by Ln-dependent charge recombination rate (i.e., τe); however, the devices with a great difference in both ZnO nanorod length (i.e., Ln = 170 and 320 nm) and surface defect concentration have a greatly different Voc (0.32 and 0.15 V, respectively) and a similar τe (0.48 and 0.44 ms, respectively) due to the exponential attenuation of incident light intensity in the device, indicating that the Voc is very sensitive to the surface defect concentration. The location for charge recombination is mainly at the D/A interface where the photogenerated electrons and holes are present.30 In this experiment, while the improved interfacial compatibility increases the D/A interfacial area7 for charge generation (Figures 1b and 2), the D/A interface for recombination will inevitably get increased. If the τe in the devices were mainly determined by the increased D/A interface due to the Z907 modification, the τe value should be expected to continuously get smaller with increasing Ts,10,30 as is obviously in contradiction with the experimental observation (Figure 5b). The similar dependences of Voc and τe on the Ts or IDLE/INBE strongly suggest that the Voc and τe in the MEH-PPV/ZnO devices are mainly determined by the surface defect concentration (Figure 5b) rather than the somewhat increased D/A interfacial area due to the improved interfacial contact by the Z907 modification. This suggestion is reasonable, because the major factors determining the D/A interfacial area, which are mainly in the ZnO nanorod arrays (length, diameter and spatial distribution), remain actually unchanged during adsorption of Z907, resulting in the nonpredominant increase in the D/A interfacial area by the Z907 modification. Known from the above results, the Voc in the MEH-PPV/ZnO devices can be influenced by both τe (i.e., charge recombination rate) and intraband surface defects but is dominantly determined by the defect concentration at the D/A interface; however, the τe mainly correlates to the D/A interfacial area for charge recombination10,30 and the interfacial property (e.g., defect concentration,10 and energy barrier for recombination4b,6b-6d,7). Even though there exists no direct dependence of Voc on τe as they are influenced by different factors in the MEH-PPV/ZnO devices,10 they will change in a similar trend with the factor that simultaneously imposes influences on them (Figure 5b). It should be noted that, since interfacial modification may cause the morphological changes in the spatial distribution and area of the D/A interface to alter the charge recombination dynamics,8 the morphological characteristics in the D/A blend layer should be taken into account when correlating τe with Voc in the hybrid PSCs. In this experiment, with the unchanged spatial distribution of D/A interface (see Introduction), the increase in the D/A interfacial area by the Z907 modification is not a predominant factor to influence τe, resulting in both τe and Voc determined by the defect concentration.

4. CONCLUSION Amphiphilic modification with dye Z907 at charge separation interface is applied to the MEH-PPV/ZnO solar cells based on ZnO nanorod arrays. The presence of Z907 improves the compatibility between ZnO and MEH-PPV and increases the charge separation efficiency at the MEH-PPV/ZnO interface as a result of the improved electronic coupling property at the interface for charge transfer. The Z907 modification mainly contributes to the enhanced Jsc by increasing the charge separation efficiency at the D/A interface rather than the light harvesting. 3750

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The Journal of Physical Chemistry C However, the modification can reduce the surface defect concentration of ZnO nanorods to increase Voc and τe or create more defects at the MEH-PPV/ZnO interface due to the electron-rich carbonyl groups to reduce Voc and τe, depending on the Z907 content on ZnO surface determined by the modification time Ts. Our results clearly demonstrate that trapping electrons by surface defects may facilitate the charge transfer at the D/A interface for efficient charge separation in MEH-PPV/ZnO devices, and both Voc and τe in them correlate to the occupation of injected electrons in conduction band and surface defects. Combinational analysis of the present and previous10 data shows that the Voc in the MEH-PPV/ZnO devices can be affected by both τe (i.e., charge recombination rate) and intraband surface defects but is dominantly determined by the defect concentration at the D/A interface; however, the τe mainly correlates to the D/A interfacial area for charge recombination and the interfacial property. While there exists no direct dependence of Voc on τe as they are influenced by different factors in the MEH-PPV/ZnO devices, they will change similarly with the factor that simultaneously imposes influences on them. The present study provides new insights into the interfacial modification effects on the hybrid PSCs for optimization of the interfacial properties in highly efficient devices.

’ ASSOCIATED CONTENT

bS

Supporting Information. Absorption and PL spectra of Z907, FT-IR spectra of ZnO and ZnO-Z907, and IMVS responses of solar cells. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the “100-Talent Program” of the Chinese Academy of Sciences, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, the National 973 project (2007CB936602), and the President Foundation of Hefei Institute of Physical Sciences. We also acknowledge the referees involved for their generous advice on revision. ’ REFERENCES (1) (a) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533–4542. (b) G€unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (c) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77. (2) (a) Boucle, J.; Ravirajan, P.; Nelson, J. J. Mater. Chem. 2007, 17, 3141–3153. (b) Saunders, B. R.; Turner, M. L. Adv. Colloid Interface Sci. 2008, 138, 1–23. (c) Hillhouse, H. W.; Beard, M. C. Curr. Opin. Colloid Interface Sci. 2009, 14, 245–259. (d) Skompska, M. Synth. Met. 2010, 160, 1–15. (3) (a) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li., G. Nat. Photonics 2009, 3, 649–653. (b) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135–E138. (c) Po, R.; Maggini, M.; Camaioni, N. J. Phys. Chem. C 2010, 114, 695–706. (4) (a) Takanezawa, K.; Hirota, K.; Wei, Q.-S.; Tajima, K.; Hashimoto, K. J. Phys. Chem. C 2007, 111, 7218–7223. (b) Lin, Y.-Y.; Chu, T.-H.; Li, S.-S.; Chuang, C.-H.; Chang, C.-H.; Su, W.-F.; Chang, C.-P.; Chu,

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