Low-Temperature Solution Processed Utraviolet Photodetector Based

ARTICLE pubs.acs.org/JPCC

Low-Temperature Solution Processed Utraviolet Photodetector Based on an Ordered TiO2 Nanorod Array Polymer Hybrid Yangang Han,†,‡ Congcheng Fan,† Gang Wu,*,† Hong-Zheng Chen,† and Mang Wang† †

MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, State Key Lab of Silicon Materials, Hangzhou 310027, People's Republic of China ‡ Zhejiang Test Academy of Quality and Technical Supervision, Hangzhou 310013, People's Republic of China

bS Supporting Information ABSTRACT: A highly ordered, vertically oriented TiO2 nanorod array is successfully synthesized on a transparent conductive substrate with a facile, free-template, and low-temperature solution method and applied in the fabrication of a hybrid ultraviolet photodetector with polyfluorene. With TiCl4 treatment, the device exhibits high UV-sensitive photoductivity and fast response time. A maximum response of 33.2 mA/W at 395 nm is reached under zero bias, which provides a novel route to fabricate a low-cost photodetector with large area.

’ INTRODUCTION A hybrid ultraviolet (UV) photodetector, which combines the unique properties of inorganic semiconductors with the filmforming properties of the conjugated polymers, has attracted more and more attention.1,2 Like a hybrid solar cell, an effective strategy for a hybrid UV photodetector fabrication is to use blends of nanocrystals with semiconductive polymers as bulk heterojunction.3 Excitons generated upon illumination can be separated into free charge carriers very efficiently at the large interface between the inorganic and organic components in the hybrid film. However, the transportation process is not as efficient as expected, which leads to recombination of the carriers and lowers the photoresponse.4 Furthermore, high surface tension of small inorganic nanocrystals makes them tend to aggregate and grow to larger particles by Ostwald ripening.5 Ordered bulk heterojunction was investigated to offer controllable phase size and speedways for charge transfer. The donor and acceptor within the bulk heterojunction are interspaced with an average length of 10 20 nm, which is the exciton diffusion length. The two phases are interdigitated in percolated highways to ensure high mobility with reduced recombination. One effective strategy to form such a well-organized nanostructure is to fill an inorganic template with organic semiconductors.6 For example, Lin fabricated a UV photodetector based on hybrid polymer/zinc oxide nanorods.7 The device exhibited a nearly 3 order difference while illuminated under UV and visible light with a responsivity of 0.18 A/W at 300 nm. Up to now, two shortcomings of the solution based UV-sensitive materials were still not overcome. One is the r 2011 American Chemical Society

slow response speed, and the other is the existence of persistent photoconduction. Due to high electron injection efficiency from the excited organic molecules along with good chemical stability, TiO2 is considered to be a good alternative to ZnO in the hybrid solar cell field.8,9 However, solution based synthesis of a TiO2 nanorod template is much harder, and a single crystalline TiO2 nanorod array is usually preferred over polycrystalline ones. To date, most of the growth methods used are based on physical synthesis, and there are only a few reports that describe heterogeneous growth of oriented single crystalline TiO2 nanorods or nanowires through an inexpensive solution method.10 Aydil11 fabricated dye-sensitized solar cells (DSSCs) from hydrothermal synthesized oriented, single-crystalline rutile TiO2 nanorods which had a length of several micrometers and a light-to-electricity conversion of 3% was achieved. Wang prepared an aligned single crystalline TiO2 nanorod array on a pretreated glass substrate by a hydrothermal approach and found that it exhibited relatively higher photocatalytic activity.10 Up to now, none was prepared for fabrication of UV photodetectors. In this study, we synthesized a well-organized, single crystalline rutile TiO2 nanorods array with a length of several hundred nanometers on untreated transparent conductive substrate in large scale through a facile, low-temperature solution method and monitored the process of the array growth by capturing the Received: February 13, 2011 Revised: May 20, 2011 Published: June 09, 2011 13438

dx.doi.org/10.1021/jp201413m | J. Phys. Chem. C 2011, 115, 13438–13445

The Journal of Physical Chemistry C

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Figure 1. Typical FESEM images of TiO2 nanorod arrays synthesized with 0.75 mL of TBOT at 180 °C for 2 h: (a) top view at low-magnification; (b) top view at high-magnification; and (c) cross-sectional view. (d) EDX spectrum of the corresponding TiO2 nanorod array.

images of the array at different reaction time. To enhance the response speed of the solution-processed UV-sensitive materials and eliminate the persistent photoconduction, the array was applied to fabricate hybrid UV photodetector with poly(9,9dihexylfluorene) (PFH) through spin-coating method. The UV photodetector gives a high phototo-dark current ratio of 3 orders of magnitude and a fast response speed less than 200 ms. Furthermore, the UV photodetecting ability and the influences of treatments are also investigated.

’ EXPERIMENTAL SECTION Hydrothermal Synthesis of TiO2 Nanorod Arrays. FTO glasses were cleaned ultrasonically with isopropanol, acetone, and ethanol in sequence and dried. Twenty-four milliliters of deionized water and 24 mL of HCl (37%) were mixed and stirred for 5 min. Then 0.5 1 mL of Ti(OC4H9)4 (TBOT) was added to the mixture dropwise and stirred for another 5 min. The precursor solution was transferred into a Teflon-lined autoclave (100 mL), and then two pieces of FTO glass were immersed into the solution. Hydrothermal growth of the TiO2 nanorod array was carried out at 160 200 °C for 1 3 h. After being cooled to room temperature, the FTO glasses were taken out, thoroughly rinsed with deionized water, and dried in ambient air. The crystal structure of TiO2 nanorod arrays was identified by X-ray diffraction (XRD) (D/MAX 2550/PC, RIGAKU) using Cu KR radiation. The morphology of the samples was investigated by FESEM (S-4800, Hitachi) and HRTEM (JEM-2100F, JEOL Co. Ltd.). The TiO2 nanorods were scratched from an FTO substrate and dispersed in ethanol, and one drop of the suspension was dropped onto a carbon-coated copper grid for HRTEM and SAED measurements. The elemental analysis was studied with an energy dispersive X-ray (EDX) microanalysis system (EX-64175JMH, JEOL Co. Ltd.).

Formation of an Ordered Polymer TiO2 Nanorod Array Hybrid. In the synthesis of ordered polymer TiO2 nanorod

array hybrids, PFH (Mw = 24000, PDI = 2.4) was dissolved in CHCl3 to reach a concentration of 12 50 mg/mL. PFH solution was spin-coated onto the FTO-supported TiO2 nanorod array to form the ordered hybrid. The nanostructure was investigated by a field emission scanning electron microscope. Device Fabrication. A hybrid UV photodetector was assembled using a TiO2 nanorod array on a FTO substrate. Before spin-coating of PFH solution, the as-prepared TiO2 nanorods were placed in 0.1 M TiCl4 solution. After being rinsed with ethanol, the sample was annealed at 450 °C for 0.5 h in air to remove the defects and traps. Then PFH solution was deposited onto the TiO2 nanorods. A 50 nm thick layer of Au was evaporated under vacuum as the top electrode and the effective cell area was adjusted to 0.3 cm2. Two types of UV light sources were applied. One named as UV irradiation is the light directly from the high-pressure mercury lamp (CHG-200) and the other named as 365 nm irradiation is the UV irradiation filtered by a 365 nm bandpass filter from the same lamp. Photoresponse of the device was recorded with a Keithley 236 source measure unit. A 500 W xeon lamp and a grating monochromator were used to obtain the spectral response. All the tests were performed in the air.

’ RESULTS AND DISCUSSION Structural and Morphological Characterization of TiO2 Nanorod Arrays. Panels a c of Figure 1 give the typical field

emission scanning electron microscope (FESEM) images of a TiO2 nanorod array synthesized at 180 °C for 2 h. The images at different locations and magnifications reveal that the whole SnO2:F (FTO) substrate is covered with TiO2 nanorods, most of which are uniform in shape and perpendicular to the substrate surface. The average length and diameter are 540 and 70 nm, 13439

dx.doi.org/10.1021/jp201413m |J. Phys. Chem. C 2011, 115, 13438–13445

The Journal of Physical Chemistry C respectively. The chemical stoichiometry of the nanorods is examined with an energy dispersive X-ray (EDX) microanalysis system (Figure 1d), which indicates the presence of Ti and O with an atomic ratio of 1:1.95, in agreement with the stoichiometric composition of TiO2. The observed peaks of C and Cu originate from the carbon-coated copper grid for high-resolution transmission electron microscopy (HRTEM) measurement. The X-ray diffraction (XRD) pattern of the free-standing TiO2 nanorod array on FTO substrate is presented in Figure 2. All the diffraction peaks that appear after nanorod growth agree well with the tetragonal rutile phase (JCPDS No. 88-1175). Absence of diffraction peaks which are normally present in polycrystalline sample indicates that the nanorods are single crystalline. The XRD pattern of a bare FTO substrate is also given for comparison. The structure of the TiO2 nanorods was further characterized by HRTEM. Lattice fringes with interplanar spacings of 3.247 and 2.959 Å observed correspond to the (110) and (001) lattice planes, respectively. The HRTEM image (Figure 3a) and the selected-area electron diffraction (SAED) pattern (Figure 3b) reveal that the rutile TiO2 nanorod is single crystalline in nature and grows along the [001] direction, which are consistent with the XRD results.

Figure 2. XRD spectra of a TiO2 nanorod array on FTO substrate synthesized with 0.75 mL of TBOT at 180 °C for 2 h and bare FTO substrate.

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The growth process of the TiO2 nanorods on the FTO substrate was monitored by tracing the morphology of the nanorods with different growth time (Figure 4). When the time is less than 1.5 h, the FTO substrate remains transparent and no nanorods are found. However, a thin layer of nanoparticles is observed on the substrate, which is caused by the homogeneous nucleation of TiO2.12,13 When the reaction time prolongs to 1 h and 35 min, for the reason that the TiO2 nanoparticles can serve as the seeds for further growth of the nanorods as well as the heterogeneous nucleation of TiO2, the nanorods growing with the axis perpendicular to the substrate can be detected. Figure 4c presents the morphology evolution from nanoparticles to nanorods. The average nanorod length is 540 nm after 2 h of growth and increases to 1.45 μm after 3 h. The influence of the growth time on the morphology of the array is summarized in Table S1 in the Supporting Information. If the reaction time exceeds 6 h, a white film composed of TiO2 nanorods longer than 5 μm begins to peel off from the substrate, which could be attributed to the competition between crystal growth and crystal dissolution. In the case of a short reaction time, the rate of growth exceeds that of dissolution and the nanorods grow longer and denser. With the reaction time prolaonged, the rate of crystal growth starts to decrease and dissolution begins to be dominant at the highenergy interfaces such as the FTO nanorod interface, which leads to peeling off the TiO2 nanorod film.11 Figure 5 shows the FESEM images of TiO2 nanorod arrays synthesized at different temperatures with other conditions identical. TiO2 nanorods could be observed only when the reaction temperature is higher than 160 °C. Increasing the temperature from 170 to 180 °C increases the growth rate effectively so that denser and longer TiO2 nanorods can be detected. At 200 °C, the thickness of the free-standing film could reach several micrometers. The influence of the growth temperature on the morphology of the array is summarized in Table S2 (Supporting Information). However, the film begins to peel off from the substrate if we further increase the temperature to 220 °C, which is also the result of dissolution at the FTO nanorod interface. It is found that the density and size of the TiO2 nanorods could be adjusted by varying the precursor content in the initial reaction solution. From Figure 6 we can see that when the precursor content is low (0.5 mL), the nanorods grow sparsely on the substrate and do not align well. Furthermore, their length

Figure 3. (a) HRTEM image of the single TiO2 nanorod synthesized with 0.75 mL of TBOT at 180 °C for 2 h and (b) its SAED pattern. 13440



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Figure 4. Cross-sectional FESEM images of a TiO2 nanorod array synthesized with 0.75 mL of TBOT at 180 °C for different time durations: (a) 0 h, (b) 1 h and 30 min, (c) 1 h and 35 min, (d) 1 h and 45 min, (e) 2 h, and (f) 3 h.

Figure 5. Cross-sectional FESEM images of TiO2 nanorod arrays synthesized with 0.75 mL of TBOT for 2 h at different temperatures: (a) 160 °C, (b) 170 °C, (c) 180 °C, and (d) 200 °C.

and diameter are also much smaller than those grown with high precursor contents. When the precursor content increases to 1 mL, the nanorods could reach several micrometers in length with a higher growing density. The influence of the precursor content on the morphology of the array is summarized in Table S3 (Supporting Information). That phenomenon results from the increasing of the nucleation density. When the nucleation density is low, fewer seeds could be deposited on the substrate for future nanorod growth. Because FTO has a rather rough surface, the nanorods could grow at an angle to the substrate surface normal without hindrance. Higher nucleation density leads to

more positions on the substrate for TiO2 nanorods to nucleate and grow. Meanwhile, the nanorods grown at an angle to the substrate surface normal will run into the nanorods nearby and stop growing. That is to say, only the nanorods perpendicular to the substrate could grow longer. Morphology of the PFH TiO2 Nanorods Array Hybrids. As the length of the nanorods increases, the interface between the TiO2 nanorods and the polymers also increases. Thus more excitons can reach the interface for separation conveniently. However, the polymer could not fill the interspaces between the TiO2 nanorods effectively when the nanorods are too long. 13441



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Figure 6. Top view and cross-sectional view FESEM images of TiO2 nanorods array synthesized at 180 °C for 2 h with different content of TBOT: (a, d) 0.5 mL, (b, e) 0.75 mL, and (c, f) 1 mL.

Figure 7. Top view and cross-sectional view of FESEM images of the PFH-TiO2 nanorod array hybrids made by spin-coating PFH solution with different concentrations: (a, d) 12 mg/mL, (b, e) 30 mg/mL, and (c, f) 50 mg/mL.

Considering the uniformity and integrity of the film, the TiO2 nanorods with a length of 540 nm are selected for preparing hybrid film. The morphology of the ordered PFH TiO2 nanorod array hybrids made by spin-coating from PFH solutions with different concentrations is shown in Figure 7. As seen in panels a and d of Figure 7, when the PFH solution with a low concentration is spin-coated onto the TiO2 nanorods, no dramatic change could be observed compared with pure TiO2 nanorods. With the increasing of the PFH concentration, the interspaces between the TiO2 nanorods are gradually filled with PFH. When the PFH concentration increases up to 50 mg/mL, the top surface of the TiO2 nanorods is covered with smooth PFH

film completely and few TiO2 nanorods could be identified from the cross-sectional view image as shown in panels c and f of Figure 7. The polymer interpenetrates inside the TiO2 nanorods array and occupies nearly the entire space surrounding the nanorods. The solid film above the TiO2 nanorods could potentially prevent electron back transfer and reduce the dark current.14 However, in polymer layers of improper thickness, parts of the photogenerated carriers could recombine before reaching the electrodes.15 UV Photodetecting Properties. The PFH solution with the concentration of 50 mg/mL was spin-coated onto the TiCl4treated TiO2 nanorod arrays to form the active layer, and the typical structure of the device is shown in Figure 8a. The highest 13442



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Figure 8. (a) Schematic representation of the device assembled from the PFH TiO2 nanorod array hybrid made by spin-coating 50 mg/mL PFH solution. (b) Schematic energy band diagram of the device. (c) Current voltage characteristics of the device under 3.2 mW/cm2 UV irradiation and 0.7 mW/cm2 irradiation at 365 nm on a semilog scale.

Figure 9. Time dependence of the photocurrent rise and decay of the device assembled from the PFH TiO2 nanorod array hybrid under periodic illumination of 3.2 mW/cm2 UV irradiation and 0.7 mW/cm2 365 nm light. The bias is 0 V.

occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level of PFH are 5.6 and 2.7 eV, respectively.16 The HOMO level and the LUMO level of rutile TiO2 are 7.4 and 4.4 eV, respectively.17 Considering that the vacuum work function of FTO is 4.5 eV and Au is 5.3 eV,18,19 the energy band diagram of the device, FTO/TiO2/ PFH/Au, is drawn in Figure 8b. The LUMO level of PFH is above the bottom of the conduction band of TiO2, which favors the electron injection from PFH to TiO2 energetically.20 The corresponding current voltage (I V) characteristics of the device under 3.2 mW/cm2 UV irradiation and 0.7 mW/cm2 irradiation at 365 nm are shown in Figure 8c. Though the junction is not formed well at the interface, photogenerated excitons might be separated into electrons and holes by the interaction between the excitons and the trap centers on the

surface of the nanorods.21 Thus, a dramatic UV-sensitive photoconductivity can be detected as the device is exposed to either UV irradiation or 365 nm irradiation. The photoenhanced current increases by more than 1 order of magnitude over the dark current. Especially upon application of a reverse bias, the current density increases due to the enhanced separation efficiency of geminate charge pairs and the improved extraction efficiency of the separated carriers from electrodes at higher electric fields.22 The response speed of the device upon switching light on and off was also investigated through the time-dependence measurements as shown in Figure 9. As one of the key factors for 1Dstructure-based photodetector performance, a slow response time that ranges from seconds to several hours usually limits the real application.23 25 However, both rise time and decay time of the present device are shorter than the limit of our measurement limit (200 ms). The fast rise and decay could be attributed to the solid-state process in which excitons are generated instantaneously by UV light and recombination of photogenerated excitons.26 The present device also shows a good reproducibility and stability. The results are reproducible under either UV irradiation or 365 nm monochromatic light. Under 3.2 mW/ cm2 UV illumination, the measured current density increases significantly (by near 3 orders of magnitude) as the dark current density of the present device is in the range of 10 11 A. When the device is illuminated with 0.7 mW/cm2 365 nm monochromatic light, the device demonstrates 2 orders of magnitude increase with a fixed dark current level. The high photoconductivity, the short response time both on rise and decay, and the stable photocurrent are undoubtedly determined by the single crystalline TiO2 nanostructure, the enhanced charge transport route to the electrodes by using 1D TiO2 nanorods array, the good energy levels matching between TiO2 and PFH, and the improved charge separation efficiency at the large interface.27 13443



Figure 10. Absorption spectra and photoresponse as a function of wavelength of incident light for the device assembled from the PFH TiO2 nanorod array hybrid.

Figure 11. Time dependence of the photocurrent rise and decay of the device assembled from the PFH TiO2 nanorod array hybrid and pretreated with/without TiCl4 under periodic illumination of 0.7 mW/cm2 365 nm light. The bias is 0 V.

The spectral photoresponse of the hybrid device at zero bias is shown in Figure 10. For comparison, the UV vis absorption spectra are also given. The pristine PFH film exhibits a broad absorption ranging from 300 to 420 nm, and the TiO2 nanorod array has an absorption peak around 338 nm. The absorption of the hybrid is the superposition of the absorption spectra of the constituents. However, the spectral photoresponse is quite different. The shape of the spectrum is quite similar to that of the absorption spectrum of PFH, revealing that light absorbed by the polymer contributes mostly to the photosensitivity. A maximum photoresponse of 33.2 mA/W is obtained at 395 nm, which means that 395 nm monochromatic light contributes most to the photocurrent. The beneficial effects of TiCl4 treatment are well-known in TiO2 nanoparticle cells, nanotube cells, and nanowire cells due to higher photocurrents.28 31 We also applied this technique to the photodetectors field. As seen in Figure 11, the photocurrent of the TiCl4-pretreated device (1.78 10 5 mA/cm2) increases by one time compared with the untreated sample (0.89 10 5 mA/cm2) under 0.7 mW/cm2 365 nm light and the dark current remains the same. Furthermore, the TiCl4 treatment does not influence the response speed. There are two main reasons for the doubled

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Figure 12. Time dependence of the photocurrent rise and decay of the device assembled from the PFH TiO2 nanorod array hybrid with/ without annealing under periodic illumination of 0.7 mW/cm2 365 nm light. The bias is 0 V.

photo-to-dark current ratio. One role of the TiCl4 treatment is to roughen the surface of the TiO2 nanorods and increase PFH adsorption.11 In the untreated sample, some excited PFH molecules fail to inject electrons into the TiO2 conduction band because of a conduction band potential that is near or above the oxidation potential of the excited molecules. In that case, the electron injection rate could be slow that other reactions can compete with injection to some extent, for example, recombination. Under that explanation, a downward shift in the band edge caused by TiCl4 treatment would be expected to give a higher efficiency of injection and thus a higher photocurrent,32 as observed. Thermal annealing is another way to improve cell efficiency.33 However, the influence of annealing on the polyfluorene based devices is still not clear. In this work, the device was annealed at 150 °C for 30 min under vacuum before the Au electrodes deposition and its photoresponses before and after annealing treatment are presented in Figure 12. It is found that response time of the annealed device was also shorter than 200 ms. However, the photocurrent of the annealed device (0.87 10 5 mA/cm2) decreases by 50% from the as-prepared sample (1.78 10 5 mA/cm2) under 0.7 mW/cm2 365 nm light, while the dark current remains the same. Because the treatment was carried out under vacuum, there was no chemical structure change to PFH related to oxygen and moisture. And the annealing temperature was much lower than the decomposition temperature of PFH (higher than 400 °C). Therefore, the reason for the decreased photo-to-dark current ratio might lie in the physical aggregation of polymer molecules induced by annealing.34

’ CONCLUSIONS In summary, we have successfully synthesized single crystalline rutile TiO2 nanorod arrays on transparent conductive substrate without a template. An ordered hybrid UV photodetector composed of PFH and TiCl4-pretreated TiO2 nanorods 540 nm in length was fabricated. A high photo-to-dark current ratio of 3 orders of magnitude and a fast response time less than 200 ms were achieved. According to the spectral response, the device shows a peak response of 33.2 mA/W at 395 nm while PFH dominates the response of the device. Furthermore, the 13444


The Journal of Physical Chemistry C thermal treatment is found to have no positive effect on the device performance. With the high performance of the hybrid material and low cost fabrication for large-area, fast response, persistent photoconduction free, it is believed that this system could be promising in the area of UV detection.

’ ASSOCIATED CONTENT

bS

Supporting Information. The detail of the influence of the growth time, growth temperature, and precursor content on the morphology of TiO2 nanorods array. This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The work is financially supported by the National Natural Science Foundation of China (Grant Nos. 50773067, 50520150165, 51011130028, and 50990063)) and of the Research Fund for the Doctoral Program of Higher Education in China (No. 20060335078). The authors also thank the developing program of Changjiang Scholar and Innovation Team from the Ministry of Education of China (Grant No. IRT0651).

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Low-Temperature Solution Processed Utraviolet Photodetector Based

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