Flexible Sandwich Photodetectors Based on Thick Polythiophene

Apr 6, 2009 - B. Radha , Abhay A. Sagade , and G. U. Kulkarni. ACS Applied Materials & Interfaces 2011 3 (7), 2173-2178. Abstract | Full Text HTML | P...
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J. Phys. Chem. C 2009, 113, 7411–7415

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Flexible Sandwich Photodetectors Based on Thick Polythiophene Films Lei Tong, Chun Li, Feng’en Chen, Hua Bai, Lu Zhao, and Gaoquan Shi* Department of Chemistry and Key Laboratory of Bio-organic Phosphorous Chemistry and Chemical Biology, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: December 29, 2008

We report a flexible photodetector with a sandwich structure of Au/polythiophene (PTh)/Au, which consists of two Au electrodes with a thickness of 30 nm each and a PTh layer with a thickness of 10-40 µm. The devices showed strong and fast photoresponses under excitation through the gold electrodes using white light or a 514 nm laser beam. Upon illumination with a 40 mW/cm2 514 nm laser beam and under a low bias electric field of 0.2 V/µm, the device with a 20 µm thick PTh layer exhibited an external quantum efficiency up to 136%. These devices are flexible and can be bent to large angles and fabricated into large sizes with arbitrary shapes. 1. Introduction Conducting polymers have been widely applied in various electronic devices, such as sensors, light-emitting diodes, and solar cells.1 Photoactive films of conducting polymer sandwiched between two electrodes are usually used as photodetectors.2,3 The photoconductive properties of poly(p-phenylene vinylene) (PPV), polythiophene (PTh), and their derivatives have been studied extensively.4,5 Avalanche multiplication was observed in these strongly disordered organic solids. For example, the electrodecollectionefficiencyofa255nmthickAu/arylamino-PPV/ Al device was measured to be up to 2000% under a high bias field of -20 V/µm and illumination through its Al electrode with 450 nm laser light.4 However, to date, almost all of the active layers of the photodetectors were based on films with thicknesses thinner than 1 µm. Therefore, the films have to be deposited on a metallic sheet or a conductive plastic substrate. The photoconductivities of free-standing thick conducting polymer films have never been reported. On the other hand, conducting polymers are usually brittle and have low mechanical strengths. They cannot be fabricated into desired structures by conventional polymer processing techniques. Therefore, much effort has been devoted to fabricating flexible and low-cost electronic devices using ink jetting and screen printing of soluble conjugated polymers.5,6 In this paper, we report flexible sandwich photodetectors based on thick high-strength PTh films formed by one-step electrochemical polymerization of thiophene in freshly distilled boron trifluoride diethyl etherate (BFEE)7 and sputtering Au layers on both sides of the PTh layer. The Au (30 nm)/PTh (20 µm)/Au (30 nm) device showed an external quantum efficiency (EQE) up to 95% under a low positive bias field of 0.1 V/µm or 136% under 0.2V/ µm upon illumination through the bottom electrode (the side in contact with the working electrode during film growth) with a 40 mW/cm2 514 nm laser beam. 2. Experimental Section 2.1. Electrochemical Deposition of PTh. Thiophene monomer was purchased from Beijing Chemical Plant (Beijing, China), and boron trifluoride diethyl etherate (BFEE) was bought * To whom correspondence should be addressed. Phone: +86-10-62773743. Fax: +86-10-6277-1149. E-mail: [email protected].

from Beijing Chang Yang Chemical Plant (Beijing, China). PTh films were synthesized following a modified procedure reported previously.7 Briefly, PTh films were deposited onto a stainless steel sheet (AISI 304) by electrolysis of freshly distilled BFEE solution containing 30 mM thiophene at a constant applied potential of 1.3 V, using a CHI440 potentiostat (CH Instruments Inc. USA). The counter and reference electrodes were a stainless steel (AISI 304) sheet and an Ag/AgCl wire, respectively. To provide a more general reference, a correction of 0.069 V was needed to bring the measured potentials in BFEE originally versus Ag/AgCl to potentials versus the standard hydrogen electrode.7 The electrolyte was deoxygenated by bubbling nitrogen gas, and a slight overpressure was maintained during electrosynthesis. The thickness of the PTh film was controlled by the total charge passed through the electrochemical cell. 2.2. Fabrication of the Sandwich Photodetectors. The configuration of the sandwich photodetector is illustrated in Figure 1a. An as-grown PTh film was peeled off from the stainless steel working electrode and washed repeatedly with diethyl ether and deionized water. Then, it was dedoped with an aqueous solution of 1.0 M NaOH or 80% (by volume) hydrazine for 24 h. Successively, the film was washed thoroughly with deionized water and dried under vacuum. Finally, both sides of the PTh film were coated with 30 nm Au layers, each by sputtering. The resulting photodetector is flexible and can be bent into large angles, as shown in Figure 1b. 2.3. Characterizations. UV-vis spectra were recorded by using a U-3010 spectrophotometer (Hitachi, Japan). Electron spin resonance (ESR) spectra were recorded on a 200D SRC X-band spectrometer (Bruker, Germany). Microwave frequencies were measured with an SP3382A microwave counter. The magnetic fields were calibrated by using an ER 035 M NMR gaussmeter. The morphology of the films was examined by the use of an FEI Sirion 200 scanning electron microscope (JEOL, Japan). J-V curves of the photodetector were taken out by using a 2400 source meter (Keithley, USA) under AM1.5 simulated sunlight irradiation or a 514 nm laser beam. 3. Results and Discussion 3.1. UV-vis Spectra. Figure 2 shows the absorption spectra of a 200 nm PTh film dedoped with an aqueous solution of hydrazine (80%, by volume) and a glass sheet coated with a 30

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Figure 3. SEM images of the bottom (a, c) and top (b, d) surfaces of a 20 µm PTh film before (a, b) and after (c, d) sputtering 30 nm gold layers. The inset of (b) shows a cross-sectional view of the film.

Figure 1. (a) Configuration of the sandwich photodetector. (b) A photograph of the flexible photodetector.

Figure 4. ESR spectra of 10 µm (O), 20 µm (4), and 40 µm (0) PTh films dedoped in 1.0 M aqueous NaOH solution; 20 µm PTh film in the doped state (top) and in hydrazine (bottom).

Figure 2. Absorption spectra of a 200 nm PTh film dedoped by hydrazine and 30 nm Au layer coated on a glass sheet by sputtering.

nm Au layer. It is clear that the PTh film has a strong and broad absorption band with a maximum around 500 nm. (This spectrum is the same as that of the PTh film dedoped with 1.0 M NaOH.) The 30 nm gold layer shows strong absorptions at wavelengths lower than 450 nm and higher than 550 nm and exhibits a semitransparent optical window centered around 500 nm. The transmittance of a 514 nm laser light through a 30 nm gold coating was determined to be about 25%. Accordingly, the white light and 514 nm laser light can partly penetrate through the 30 nm gold electrode and be absorbed by the PTh layer to generate excitons. 3.2. Morphology. The scanning electron micrographs (SEMs) of the top (in contact with the electrolyte) and bottom (in contact with the electrode) surfaces of a 20 µm PTh film are shown in Figure 3. It is clear that the surface of the bottom side in Figure 3a is more compact and smoother than that of the top side in Figure 3b. After a 30 nm gold film was deposited on each surface by sputtering, this morphology difference was kept, as shown in Figure 3c,d, and gold nanoparticles on the PTh surfaces formed continuous conductive layers. This morphology implied that the top surface can scatter more light than the bottom surface because of its higher roughness. Therefore, we studied the photoresponses of the devices by illumination through their bottom surfaces.

3.3. ESR Studies and Conductivity Measurements. The doping states of the thick PTh films were studied by ESR spectroscopy. It is widely accepted that the charge carriers of conducting polymers are polarons and bipolarons, and both of them can propagate the electrical current along the polymer chains.8-10 In highly doped PTh, spinless bipolarons dominate, and they cannot be quantitatively evaluated by ESR.10 Polarons have magnetic moments and can be determined by using ESR spectroscopy due to their paramagnetic character. Polarons coexist with bipolarons in conducting polymers with high or medium doping levels, and their ESR signals are sensitive to the chemical environment. The ESR spectra shown in Figure 4 indicate that the polaron content of PTh films dedoped with NaOH is much higher than that of as-grown and hydrazinededoped PTh films. (All samples have the same weights.) The ESR g-factors of as-grown, 1.0 M aqueous NaOH solution or hydrazine-dedoped PTh films were calculated to be 2.0088, 2.0075 (40 µm in NaOH for 24 h), 2.0074 (20 µm in NaOH for 24 h), 2.0071 (10 µm in NaOH for 24 h), and 2.0063 using the following equation:

g ) hν/[µBBr] where h is Planck’s constant, ν is the microwave frequency, µB is the Bohr magneton, and Br is the resonance magnetic field. A complex ESR line consists of Gauss and Lorentz components related to two different groups of paramagnetic centers present in polymers.11,12 The g-factors of the PTh films described above are remarkably high (>2.0023, the free electron value) due to

Sandwich Photodetectors Based on Polythiophene Films a strong spin-orbit coupling,13,14 and the Gauss component is the principal factor. The g-factors of Lorentz lines are peculiarly smaller than 2.0023 and less sensitive to doping levels than those of the Gauss line.10 Upon the dedoping process, the g-factor decreases gradually. On the basis of the results of ESR, one can conclude that PTh dedoped by NaOH has a medium doping level, and 1.0 M aqueous NaOH solution cannot dedope micrometer thick PTh film extensively as hydrazine does. Furthermore, the 20 µm PTh film has more polarons compared with the 10 and 40 µm films. This is possibly due to the fact that the 40 µm film still contains bipolarons, whereas the polarons in the 10 µm film have been partially reduced, and its reduced level is higher than that of the 20 µm film. The dc conductivity measurements also confirmed the ESR results described above. The conductivity of the as-grown PTh films was measured to be about 10 S/cm by the conventional four-probe technique. After the samples were dedoped with 1.0 M aqueous NaOH solution for 24 h, the conductivities of the 40 and 20 µm PTh films were decreased to around 10-2 S/cm, whereas that of the 10 µm was about 10-3 S/cm. However, these values are still much higher than that of the neutral PTh films reported previously (10-5-10-10 S/cm).15 This is mainly due to the fact that the PTh films dedoped with NaOH contain a high content of conductive polarons, as observed from their ESR spectra (Figure 4). Furthermore, the PTh film electrosynthesized in the electrolyte of BFEE has a long chain length (about 80 units) with few chain defects.16,17 The high conductivities of these PTh films are propitious to the charge generation and transportation of the photodetectors. In comparison, the dc conductivities of the PTh films dedoped with hydrazine solution for 24 h were measured to be in the range of 10-7-10-8 S/cm. This is mainly due to the fact that hydrazine has a much stronger reduction activity than aqueous NaOH solution and reduces PTh polarons more efficiently. Furthermore, because of diffusion limitations, the doping level and conductivity of these three films increased with their thickness. 3.4. Photovoltaic Tests. Photovoltaic properties were measured in a quartz cell filled with high-purity N2 for protection. Figure 5 illustrates the current density-voltage (or field) J-V curves of the Au (30 nm)/PTh (20 µm)/Au (30 nm) photodetector in the dark and illuminated through the bottom Au electrode with 100 mW/cm2 AM1.5 white light. The bias field was defined as positive when the potential of the illuminated side is higher than that of the opposite side of the film. In Figure 5a, the J-V curve of the device under dark conditions is asymmetrical because of the heterogeneity of the film in the vertical direction formed by electrochemical deposition.18,19 Under illumination, the current density of the device increased dramatically even under a small bias field because of the contribution of photocurrent (Jph). As can be seen from Figure 5b, under a given positive bias field, the Jph recorded by illumination through the bottom electrode is higher than that under a negative bias field. These results can be explained as follows. First, for the electrosynthesized conducting polymer films, the polymer chains in contact with the electrolyte (the top side) have more defects and a shorter conjugation length than the chains in contact with the electrode (bottom side), where more perfectly conjugated chain segments are probably found in the inner layers close to the substrate. These could also be found in another conducting polymer electropolymerization process.19 Second, excitons are generated at the interface of the gold electrode and the PTh layer under illumination. With the help of an outside bias field, the excitons separated into charge carriers: electrons and holes. Under a positive bias field,

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Figure 5. J-V characteristics of a Au (30 nm)/PTh (20 µm)/Au (30 nm) device under dark and illumination conditions: (a) Illumination from the bottom side using 100 mW/cm2 white light with an AM1.5 filter. (b) The photocurrent density at various bias fields derived from (a). (c) The photoresponses of the device illuminated with a 40 mW/ cm2 514 nm laser under 0.1 V/µm (black curve) and 0.2 V/µm (gray curve). The time interval is 60 s.

electrons can easily move to the gold electrode; however, under a negative bias field, electrons have to move into the polymer matrix and are partly quenched by PTh polarons. The process of electron transfer shown with a band diagram of the Au/PTh/ Au device is given in Figure 6. The HOMO energy level of PTh was measured to be -4.9 eV by cyclic voltammetry. The optical band gap (Eg) of PTh was calculated to be 1.9 eV, according to the onset wavelengths (λonset) of the absorption spectra of PTh (Eg ) 1240/λonset). Thus, the LUMO level was calculated to be around -3 eV. The energy level of the gold electrode is -5.1 eV. Upon illumination with a 40 mW/cm2 laser through the bottom electrode of the Au (30 nm)/PTh (20 µm)/Au (30 nm) photodetector, the device exhibited fast and stable photoresponses, as shown in Figure 5c. The apparent external quantum efficiency of the device was calculated to be 23.8 (black curve) and 34% (gray curve) under 0.1 and 0.2 V/µm, respectively, using the equation: EQE (%) ) (100 × 1240)Jph/[λPi]; where

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Figure 6. Band diagram of the Au/PTh/Au device.

Figure 8. Photocurrent densities of the Au (30 nm)/PTh (20 µm)/Au (30 nm) devices dedoped with 1.0 M NaOH for 48 h (a) and 80% (by volume) aqueous hydrazine solution for 24 h (b) under illumination through the bottom Au electrode with AM1.5 100 mW/cm2 white light and at various bias fields.

Figure 7. Photoresponse of the 40 µm (a) and 10 µm (b) devices whose bottom sides were illuminated with a 40 mW/cm2 514 nm laser under 0.1 V/µm. The time interval is 60 s.

λ and Pi are the wavelength (nm) and intensity (mW/cm2) of incident light, respectively, and Jph is the photocurrent density (mA/cm2). The diameter of the laser beam is 3 mm. Taking into account the transmittance of the electrode at 514 nm (25%), we corrected the actual quantum efficiency of the device to be 95 (black curve) and 136% (gray curve). The high charge efficiency can be explained by photocurrent multiplication at the interface of the gold electrode and PTh layer.4 The performance of the photodetector depends strongly on the conductivity or doping level of its PTh layer. As shown in Figure 7a, under a 0.1 V/µm bias field, the photoresponse of the device with a 40 µm PTh layer is much lower than that of the device with a 20 µm PTh layer (Figure 5c). The apparent and actual EQEs of the thicker device were measured to be about 8.5 and 34%, respectively, upon illumination with a 40 mW/ cm2 514 nm laser light (Figure 7a). As the thickness of the PTh layer was reduced to 10 µm, the apparent and actual EQEs of the device were measured to be only about 1.7 and 6.8%,

respectively, under the same excitation condition. These results indicated that the device with a 20 µm film can exhibit the strongest photoresponse because of its medium conductivity and doping level. If the as-grown 20 µm PTh films were dedoped with 1.0 M aqueous NaOH solution for a longer time (e.g., 48 h) or with hydrazine solution for 24 h, the resulting devices showed much weaker photoresponses (Figure 8) than those shown in Figure 5d. This is mainly due to the fact that these films have much lower conductivities than those of the films dedoped by 1.0 M NaOH for 24 h, as described above. Interfacial excitions can be generated only on neutral species of PTh; however, the neutral PTh is an insulator. Therefore, a device with a highly doped PTh cannot be photoexcited to generate a large amount of excitons, whereas the device with a low doped PTh has a large resistance, which limits charge separation and transportation. Thus, a PTh film with a medium doping level can provide the device with a balance between excition generation and resistance and exhibits a maximum photoresponse. This conclusion was partly supported by a similar result observed in the solar cells based on polyaniline films.20 4. Conclusions In summary, PTh films synthesized by direct oxidation of thiophene in BFEE and dedoped by 1.0 M aqueous NaOH solution for 24 h contain a high content of polarons and have much higher conductivities than those of neutral PTh films. The Au/PTh/Au sandwich photodetectors based on these films with thicknesses of 10, 20, and 40 µm are flexible and exhibit strong

Sandwich Photodetectors Based on Polythiophene Films photoresponses. The avalanche multiplication of the photodetector with a 20 µm PTh layer under a low bias field of 0.2 V/µm was achieved, and its EQE was measured to be up to 136% upon illumination with 40 mW/cm2 514 nm laser light. The conductivity or doping level of the PTh layer plays an important role in controlling the performance of the device. This work developed a simple route to fabricate flexible photodetectors based on conducting polymers without using supporting substrates. The easy fabrication of the devices into large sizes or desired shapes provides various potential applications. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50533030, 20774056, and 20604013) and 863 project (2006AA03Z105). References and Notes (1) Law, K. Y. Chem. ReV. 1993, 93, 449. (2) Yu, G.; Zhang, C.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 1540. (3) Yu, G.; Pakbaz, K.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 3422. (4) Daubler, T. K.; Neher, D.; Rost, H.; Horhold, H. H. Phys. ReV. B 1999, 59, 1964. (5) Basavaraj, V. K.; Manoj, A. G.; Narayan, K. S. IEE Proc., Part G 2003, 150, 552.

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