Acid-Treated PEDOT:PSS Polymer and TiO2 Nanorod Schottky

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Acid Treated PEDOT:PSS Polymer and TiO2 Nanorods Schottky Junction Ultraviolet Photodetectors with Ultrahigh External Quantum Efficiency, Detectivity and Responsivity Saurab Dhar, Pinak Chakraborty, Tanmoy Majumder, and Suvra Prakash Mondal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Acid Treated PEDOT:PSS Polymer and TiO2 Nanorods Schottky Junction Ultraviolet Photodetectors with Ultrahigh External Quantum Efficiency, Detectivity and Responsivity Saurab Dhar, Pinak Chakraborty, Tanmoy Majumder and Suvra Prakash Mondal* Department of Physics, National Institute of Technology, Agartala, India -799046. *Corresponding Author’s email: [email protected] and [email protected]

Abstract Vertically aligned TiO2 nanorods (NRs) were synthesized on the fluorine-doped tin oxide (FTO) deposited glass substrate by hydrothermal method. Schottky junction ultraviolet photodetector was fabricated by spin coating of poly 3,4-ethylenedioxythiophene: polystyrene sulfonate (PEDOT:PSS) polymer on TiO2 NRs. For the improvement of device performance, electrical conductivity of the polymer was increased by adding dimethyl sulfoxide (DMSO) and concentrated H2SO4 acid. The diode parameters i.e., work function, series resistance and ideality factor were studied for all devices. Photoresponse behavior of TiO2 nanorods/PEDOT:PSS junction was studied upon illumination of white light of intensity 80 mW/cm2. Our acid treated sample demonstrated highest photocurrent value, which is 10 times larger than DMSO treated and 39 times larger than the untreated sample. Our acid treated device showed superior external quantum efficiency (~12560%), responsivity (~34.43A/W) and detectivity (~1.6×1011Hz1/2/W) at wavelength ~340 nm under -1V bias. Keywords: TiO2 nanorods, PEDOT:PSS, Schottky junction, photodetector, external quantum efficiency, responsivity, detectivity.

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1. Introduction In wide band gap semiconductors family, TiO2 is one of the most popular candidates for its outstanding chemical, physical and optical properties and has been applied in many fields of research areas such as gas sensor, photoelectrochemical cells, quantum dot sensitized solar cell, dye sensitized solar cell etc1-3. Ultraviolet (UV) photodetectors using TiO2 nanostructures has been studied extensively during the past few decades due to its great importance in various technological, civilian and military fields of applications4-6. One-dimensional (1D) TiO2 nanostructures like nanorods, nanowires, nanotubes were studied extensively because of their wide bandgap (~3-3.4 eV), large surface area, high stability, and easy synthesis process. More importantly, vertically aligned TiO2 nanorods facilitate fast electron transport, reduce the recombination rate of photogenerated carriers and increase the photoresponse behavior, which offers great advantage for fabricating highly sensitive UV detectors7-11.Several research works were carried out with TiO2 nanostructured based Schottky junction photodetector and demonstrated superior ultraviolet photodetection properties. Karaagac et al12 has reported highly sensitive Au/TiO2 based Schottky junction photodetector with a maximum responsivity of 134.8 A/W at wavelength (λ) ~350 nm at -3 V bias. Zou et al.9studied TiO2/Ag based Schottky type photodetector and observed high responsivity (~13A/W) with fast response time of 0.5s and recovery time of 0.7 s, respectively. Guller et al13 has also studied TiO2/Ag Schottky junction photodetector and they found high photocurrent value under incident of UV-light (λ~380 nm), with responsivity of 3 A/W at external bias -1V. Double-layered TiO2 nanostructure/Au Schottky contact based UV photodetector was reported by Wang et al14 . In spite of having fast response and good UV sensing properties, TiO2/novel metal based Schottky junction photodetectors suffer


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from high fabrication cost, which arises due to the costly novel metals as well as expensive metal deposition techniques.11-14 Recently, conducting polymer electrodes has been utilized in various optoelectronic devices due their exciting material properties such as high transmittance in UV-Visible region, solution processed deposition technique, low material cost15-17. PEDOT:PSS is a highly conducting conjugated polymer, which has been extensively studied by several researchers in organic solar cells18,19, light emitting diodes20, field effect transistors21,22, and photodetectors23,24. In recent year, this polymer has been used as transparent conducting electrode material because of its high carrier mobility, large work function (5-5.2 eV) and high transmittance (above 90%) over broad spectrum (250 nm to 800 nm)15,19. It has been investigated that, the pristine PEDOT:PSS polymer shows very low electrical conductivity (σ ~ 0.5 to 1 S/cm) in comparison with commercially available transparent conducting oxides (TCO) or metal electrodes (3,300 to 10,000 S/cm ).25,26 The enhancement of conductivity of PEDOT:PSS has been reported by several researchers by adding different acid and organic solvent treatment. Xia et al.25 deposited PEDOT:PSS films of highest conductivity~ 3065 S/cm by treating with concentrated H2SO4. Such high conductive polymer films were used in organic solar cell for transparent conducting electrode25. The conductivity of PEDOT:PSS films was increased up to 4100 S/cm using HNO3 treatment by Yeon el al27. The conductivity of the PEDOT:PSS can also be improved by various organic solvent treatment. Using dimethyl sulfoxide (DMSO), ethylene glycol (EG) and sorbitol, the electrical conductivity of PEDOT:PSS was improved by Ouyang et al.28. The maximum conductivity was obtained after DMSO doping and found to be ~ 1057 S/cm28. Although, there are several reports available on TiO2



nanostructure/metal Schottky junction UV photodetector, however, there is no study available on TiO2/PEDOT:PSS polymer junction based devices. Here, we have investigated the growth and characteristics of TiO2nanorod/PEDOT:PSS Schottky junction based UV photodetector. The junction properties has been studied after modification of electrical conductivity of PEDOT:PSS by using concentrated acid treatment. The external quantum efficiency (EQE), detectivity (Dλ), responsivity (Rλ), and response time of the photodetector have been investigated in this report. 2. Experimental Methods 2.1 Growth of TiO2 nanorods TiO2 nanorods were grown on fluorine-doped tin oxide (FTO) deposited glass substrates (procured from Sigma Aldrich) by hydrothermal process. Prior to synthesis, the FTO substrates were cleaned with de-ionized (DI) water followed by 2-propanol and acetone. For the growth of nanorods, at first, 10 mL of concentrated hydrochloric acid (Merck 37% by weight) was mixed with DI water to make a total volume of 30 mL. Afterwards, 400 µL of titanium (IV) butoxide (Sigma Aldrich, 97%) was added to the above solution and stirred for 15 minutes. The mixture solution was poured in a Teflon-lined stainless steel autoclave. All cleaned FTO substrates were dipped in the precursor solution for hydrothermal process and the temperature was maintained at 150°C for 3 hours. The nanorods were washed thoroughly with DI water and annealed at 500°C for 1 hour. 2.2 Device Fabrication The photodetector samples were fabricated by spin coating of PEDOT:PSS polymer (SigmaAldrich, conductive grade,1.3 wt % dispersion in H2O) on TiO2 nanorods at a spin speed of 2000 4 ACS Paragon Plus Environment

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rpm. All samples were dried in air at 120°C for few minutes to remove the solvents. For organic solvent treatment, DMSO (Merck 98%) solution was mixed in PEDOT:PSS solution of 3:4 volume ratio and spin coated on TiO2 NR samples. The detailed experimental procedure of DMSO modification was discussed elsewhere24. To enhance the electrical conductivity by acid treatment, PEDOT:PSS coated samples were immersed into concentrated sulfuric acid (Merck 98%) for 10 minutes. Afterwards, the H2SO4 treated samples were washed thoroughly with DI water and dried at 120°C for 15 minutes. To measure the photodetector characteristics, all electrical connections were taken from FTO electrode and top PEDOT:PSS polymer by applying a little amount of conducting Ag paste (Ted Pella, USA). 2.3 Sample Characterizations Surface morphology of TiO2 nanorods were investigated using a Nova Nano (FEI, USA) scanning electron microscope. Scanning tunnelling microscopy study (STM) (Bruker, MultiMode-8) of H2SO4 modified PEDOT:PSS films were carried out in air environment using a tungsten tip. Ultraviolet-visible (UV-Vis) absorption spectra of TiO2 NRs were recorded by Shimadzu UV-3600 Plus spectrophotometer. Current-voltage (I-V) characteristics were measured using an Agilent, USA source measurement unit (Model No: B2912A). Photosensing measurements were carried out under incident of light from a monochromator connected with a broadband light source (Science Tech, Canada). 3. Result and Discussion 3.1 Characterization of TiO2 Nanorods Figure 1(a) illustrates the top view SEM micrograph of TiO2 nanorod arrays grown on FTO substrate. TiO2 nanorods are vertically aligned on the substrate with average diameter 80 nm and 5 ACS Paragon Plus Environment


nanorod density per unit area is 62×108cm-2. The typical XRD pattern of TiO2 nanorods is represented in Figure 1(b). TiO2 nanorods are highly crystalline and the major diffraction peaks at (101), (200), (210), (310) and (221) planes confirmed the formation of rutile phase29. However, the strong peak at (004) is assigned to anatase phase of TiO229. Such type of mixed phase generally observed in hydrothermally grown TiO2 nanorods. The typical UV-visible absorption spectrum of TiO2 NRs is shown in Figure 1(c). The sharp absorption edge at 400 nm wavelength is attributed to the formation of rutile TiO2 nanorods30. The band gap (Eg) of TiO2nanorods was calculated using Tauc’s formula31, 1

(𝛼ℎ𝜈)𝑛 = 𝐴(ℎ𝜈 ― 𝐸𝑔)………………………………………………(1) The symbols α is the absorption coefficient, ℎ𝜈 is the incident photon energy, A is a constant, and the value of n depends on the nature of transition. In case of the direct allowed transition, n = ½ and n=2 corresponds to indirect allowed transition. Figure 1(d) and 1(e) shows (αhν)1/n vs hν plots for n=1/2 and n=2, respectively. The optical band gaps were calculated by extrapolating the linear region of the (αhν)1/n vs hν plots to the energy axis (hν) and found to be 3.1eV and 2.8eV, respectively. 3.2 Electrical properties of TiO2 nanorods-PEDOT:PSS junctions Figures 2(a) and 2(b) represent the cross-sectional SEM micrograph and schematic diagram of TiO2 nanorods-PEDOT-PSS Schottky junction photodetector, respectively. To improve the junction property of such metal oxide semiconductor-polymer junction, the conductivity of PEDOT-PSS has been enhanced by using concentrated sulfuric acid and organic solvent like DMSO. The conductivity study of PEDOT:PSS films was investigated by STM study and four 6 ACS Paragon Plus Environment

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probe measurements. Figures 3a, 3b and 3c represent STM current image of pristine PEDOT:PSS, DMSO treated and H2SO4 treated films, respectively. The tunneling current of the films were recorded at 500mV bias with current set point 2.5nA. In the topography, darker spots indicate low conductive region, whereas the brighter spots signify high conductive region. It is obvious from the figures, that the conductivity of the polymer has been improved significantly after treatment. As for example, the average current (STM current) for pristine, DMSO modified and H2SO4 treated PEDOT:PSS are found to be 0.93nA, 2.50 nA and 10.10nA, respectively. The STM IV characteristics of pristine, DMSO treated and H2SO4 treated films are shown in Figure 3(d). Interestingly, the tunneling current was increased significantly after acid treatment of the film. More importantly, H2SO4 treated samples showed highest electrical conductivity compared to other samples. The conductivity study of all samples was also carried by four probe method. The conductivity of pristine, DMSO and H2SO4 treated films were found to be 0.16, 1.65 and 160 S/cm, respectively. In PEDOT:PSS polymer, the conducting PEDOT+ core are encapsulated by the insulating PSSmatrix. As PSS- matrix has better water solubility, PEDOT:PSS polymer is highly water soluble. When organic solvents (e.g. DMSO) are added in PEDOT:PSS, the solvents interact with the sulfonic groups of PSS and separates it from the conducting PEDOT grains by weakening the Coulombic attraction. The removal of excess PSS by addition of organic solvent results the enhancement of conductivity of the film24. On the other hand, when PEDOT:PSS are treated with highly concentrated acids, H+ ions from the acid solutions neutralizes the PSS- to PSSH, which can be removed by washing with DI water25. This process exposes conducting PEDOT cores, which enhances the conductivity of the films.



To study the photodetector properties of TiO2 NRs-PEDOT:PSS Schottky junctions, we have prepared three different types of samples, (i) FTO/TiO2NRs-PEDOT:PSS (S1) (ii) FTO/ TiO2 NRs-DMSO modified PEDOT:PSS (S2) (iii) FTO/TiO2 NRs- H2SO4 treated PEDOT:PSS (S3). Figure 4(a) shows current density vs. voltage (J-V) plots of S1, S2 and S3 samples. The typical semi-logarithmic J-V characteristic of S3 sample is plotted at the inset of Figure 4(a). The J-V plots of S1 and S2 samples are shown in figure 4(b) for better visibility. All the devices showed Schottky nature having rectification ratio more than 150 at ±1V. The diode parameters such as diode ideality factor (η), barrier height (𝜙𝐵 ), and series resistance (Rs) of S1, S2 and S3 samples were calculated from Cheung-Cheung method using the following formula.32 𝐾 𝐵𝑇 𝑑𝑉 = 𝐼𝑅𝑠 + 𝜂 𝑑(𝑙𝑛𝐼) 𝑞

𝐻(𝐼) = 𝑉 ― 𝜂

𝐾 𝐵𝑇

……………………(2)

( )( 𝑞

𝑙𝑛

𝐼

)

𝐴𝐴 ∗ 𝑇2

𝐻(𝐼) = 𝐼𝑅𝑠 + 𝜂𝜙𝐵

…………………(3)

………………….(4)

The symbols V, η, q, KB, T, A and A* in the above expressions are mentioned as applied bias voltage, ideality factor, electronic charge, Boltzmann's constant, absolute temperature in Kelvin, effective area of the device, and the Richardson’s constant (~1200 Acm-2K-2 for TiO2)33 , respectively. Figure 4(c) and 4(d) represent the dV/d(lnI) vs. I and H(I) vs. I plots of S3 sample. The similar plots of S1 and S2 samples are given in the supporting information (Figure S1). In the Figure 4(c) and 4(d), the open circles show the experimental results and the solid lines indicate the fitted equation of 2 and 4, respectively. The calculated values of Schottky diode parameters (Rs, ΦB 8 ACS Paragon Plus Environment

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and η) are listed in Table-1. For control device (sample S1), the values of ΦB and Rs were found to be 0.86 eV and 897.75 Ω, respectively. Whereas, in case of DMSO modified sample (S2 sample), ΦB and Rs were decreased to 0.82 eV and 304.89 Ω, respectively. Interestingly, the lowest barrier height and series resistance have been observed for acid treated PEDOT:PSS based Schottky junction (S3 sample) and found to be 0.71eV and 36.39 Ω, respectively. The decrease of barrier height and series resistance of S2, S3 samples is ascribed to the lowering of work function as well as improvement of electrical conductivity of PEDOT:PSS polymer34. However, for all samples, the diode ideality factor has been deviated from the ideal Schottky diode nature (η~1), which can be attributed to the presence of surface trap states in TiO2 nanorods 23,35,36. 3.3 Photodetector Properties Figure 5 shows the transient photocurrent (J-t) plots for sample S1, S2 and S3 under illumination of white light of intensity 80mW/cm2. All the devices exhibit excellent ON/OFF switching behavior and maximum photocurrent (Jlight - Jdark) was observed for S3 sample. Interestingly, the photocurrent changes under ON/OFF condition for sample S1, S2 and S3 are obtained as 0.59, 2.31 and 23.49 mA/cm2, respectively. Such improvement of photocurrent for S3 device (~39 times larger than S1) is accredited to superior electrical conductivity of the PEDOT:PSS polymer resulting better charge collection process. To study the wavelength dependency of our devices, EQE was determind from the following equation,23,24

𝐸𝑄𝐸(%) =

1240 𝐽𝜆 𝜆𝑃𝜆

× 100…………………………………….(5)



The symbols Jλ and Pλ

are photocurrent density (mA/cm2) and intensity of incident light

(mW/cm2) at wavelength λ (nm), respectively. The plot of EQE vs. λ at -1V bias voltage of sample S3 is shown in figure 6. The similar plots for sample S1 and S2 is depicted at the inset of figure 6. For all samples, the maximum EQE value was obtained at wavelength λ ~340nm. The highest EQE values for S1, S2 and S3 samples were found to be 178%, 916% and 12560%, respectively. Interestingly, the EQE values for all samples are more than 100% and such phenomena can be explained by trap assisted carrier injection process, which usually observed in metal oxide semiconductors.37 The schematic representation of the process has been described in figure 7. Under the reverse bias condition, ambient oxygen molecules are adsorbed on the surface of nanorods by capturing free electrons present in the nanorods. Therefore, a low conductive depletion region is formed at the surface of the nanorods and the energy bands (conduction and valance band) moves upward. Upon illumination of UV light, electron-hole pairs are generated and holes are migrated to the surface of the nanorods. The tapped oxygen molecules are released when these photogenerated holes recombine with the electrons and the energy bands move downward. Under reverse bias condition, extra charge carriers are injected along with the photogenerated carriers due to the decrease of barrier height and shrinking of depletion region, which promotes tunneling of electrons. It has been observed that, after DMSO/acid modification, the EQE of the device was enhanced several times due to the improvement of conductivity of polymer and decrease in series resistance of the device. The enhancement of conductivity of PEDOT:PSS results better charge transport at the junction and decrease of series resistance reduces the

power loss in the photodetector device. The

responsivity (Rλ) of a photodetector describes the amount of current generated at the external


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circuit per incident optical power. The responsivity has been calculated by the following relation23,24

𝑅𝜆 =

𝐽𝐿𝑖𝑔ℎ𝑡 ― 𝐽𝐷𝑎𝑟𝑘 ………………………………………(6) 𝑃𝑜𝑝𝑡

Where Jlight and Jdark are current density under light and dark condition, respectively. Popt represents intensity of incident light at a wavelength λ. Figure 8(a) shows

𝑅𝜆 vs. λ plot of

sample S3. The similar plots of samples S1 and S2 are presented at the inset. Similar to EQE spectra, the maximum responsivity value of all samples has been observed at 340nm. The peak value of 𝑅𝜆 of samples S1, S2 and S3 are obtained as 0.48, 2.51, 34.43 A/W, respectively. The parameter detectivity (Dλ) describes the smallest detectable signal of a photodetector and was determined by the following formula23,24

𝐷𝜆 =

𝑅𝜆 2𝑞𝐽𝑑𝑎𝑟𝑘

…………………………….(7)

The symbol q is the electronic charge in the above equation. Figure 8(b) represents the detectivity vs. wavelength plot of sample S3. The similar plots for S1 and S2 samples are shown at the inset. The peak values of Dλ at λ ~ 340 nm, for S1, S2 and S3 samples are obtained as 5.4×1010, 1.4×1011 and 1.6×1011 Hz1/2/W, respectively. Figure 9(a) represents transient photoresponse plot of sample S3 under incident of UV light of λ~340 nm at intensity ~0.38 mW/cm2. The device demonstrated excellent switching behavior with on/off current ratio ~22.



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The response and recovery time of S3 sample was estimated from the photoresponse curves by using the following equations38,

(

𝐽𝑅 = 𝐴 1 ― 𝑒

𝐽𝐷 = 𝐴𝑒

―

―

𝑡

𝜏1

𝑡 𝜏1

𝑡

) + 𝐵(1 ― 𝑒 )……………..(8) ―

+ 𝐵𝑒

―

𝜏2

𝑡 𝜏2

……………………(9)

In the above equations, the symbols 𝐽𝑅and 𝐽𝐷 are the photocurrent density during rise and decay process, A and B are constant, t is the on and off time of UV light, τ1 and τ2 are time constants. In Figure 9(b), the open blue circles correspond to experimental data and the solid red lines represent the fitted curves. The estimated time constants for photocurrent rise are found to be τ1 ~ 0.7 sc, τ2~4.15 sec. During the photocurrent decay, the time constants are found to be τ1~ 1.5 sec and τ2~13.9 sec. The existence of fast and slow time constants can be described by the following mechanism. Upon illumination, the band to band transition of photogenerated carriers occurs very fast and shows fast growth of photocurrent. In the meanwhile, photogenerated holes wander to the surface of nanorods and discharge the adsorbed oxygen molecules. Due to the shrinkage of depletion layer, the photocurrent also increases. However, this process is slower compared to band to band transition. Under light off condition, the rapid decrease of photocurrent is responsible for band-to-band recombination of electrons and holes. On the other hand, the slower part of photocurrent decay is due to the increase of depletion layer by re-adsorption of ambient oxygen at nanorod surface. To demonstrate the linear detection range of the device, the photocurrent was measured under illumination of various intensity of UV light (λ~340nm).

Fig. 10 shows the variation of

photocurrent (J) vs. light intensity at wavelength 340 nm under -1V bias. All experimental 12 ACS Paragon Plus Environment

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results (solid circle) have been fitted with linear relation (solid line). Interestingly, at λ~340nm the photocurrent of all devices (S1, S2 and S3) demonstrated linear relationship with incident power. More importantly, the acid treated sample showed higher slope (34.91 mA/mW) compared to S1 (3.63 mA/mW) and S2 (1.9 mA/mW) samples. We have listed the photodetector performance of TiO2 nanorod based devices in Table-2. Our TiO2 NRs/ polymer Schottky junction demonstrated better device performance in terms of operating voltage, photocurrent and responsivity. 4. Conclusions In conclusion, we have successfully developed TiO2 nanorod /PEDOT:PSS conducting polymer Schottky junction UV photodetector. The performance of the device was improved after increasing the electrical conductivity of PEDOT:PSS polymer by DMSO and sulphuric acid treatment. The enhancement of conductivity of the polymer was investigated by four probe method and STM analysis. The Schottky diode characteristics of control (S1) as well as DMSO and H2SO4 treated devices (S2 and S3) were studied in details. Sulphuric acid treated device demonstrated highest sensitivity to the UV light with external quantum efficiency (EQE)(~12560%), responsivity (~34.43A/W) and detectivity (~1.6×1011Hz1/2/W) at wavelength 340 nm under 1V reverse bias. The large EQE value (greater than 100%) was attributed to the decrease of barrier height due to oxygen adsorption and deadsorption of oxygen molecules at the TiO2 surface. The response and recovery time of acid treated device was faster compared to DMSO modified device. Acid treated sample showed superior photocurrent change even under exposure of very low intensity of UV light. The photocurrent of the device also demonstrated linear relationship with incident light intensity.



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Acknowledgement We acknowledged our central research facility (CRF) at NIT Agartala for, XRD and AFM characterizations. We also acknowledged DST FIST project at the department of Physics for UV-VIS-NIR spectroscopy measurements. Supporting Information The plots of dV/d(lnI) vs. I and H(I) vs. I for S1 and S2 sample are shown in the supporting results. References 1. Bai, J.; Zhou, B. Titanium Dioxide Nanomaterials for Sensor Applications. Chemical Reviews2014, 114 (19), 10131–10176. 2. Bang, J. H.; Kamat, P. V. Solar Cells by Design: Photoelectrochemistry of TiO2Nanorod Arrays Decorated with CdSe. Advanced Functional Materials2010, 20 (12), 1970–1976. 3. Sobuś, J.; Burdziński, G.; Karolczak, J.; Idígoras, J.; Anta, J. A.; Ziółek, M. Comparison of TiO2 and ZnO Solar Cells Sensitized with an Indoline Dye: Time-Resolved Laser Spectroscopy Studies of Partial Charge Separation Processes. Langmuir2014, 30 (9), 2505–2512. 4. Selman, A. M.; Hassan, Z.; Husham, M.; Ahmed, N. M. A High-Sensitivity, FastResponse,

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UV‐Photodetectors and Third‐Generation Solar Cells. Phys. Status Solidi A 2018, 215, 1700404 14. Wang, H.; Qin, P.; Yi, G.; Zu, X.; Zhang, L.; Hong, W.; Chen, X. A High-Sensitive Ultraviolet Photodetector Composed of Double-Layered TiO2 Nanostructure and Au Nanoparticles Film Based on Schottky Junction. Materials Chemistry and Physics2017, 194, 42–48.



15. Nakano, M.; Makino, T.; Tsukazaki, A.; Ueno, K.; Ohtomo, A.; Fukumura, T.; Yuji, H.; Akasaka, S.; Tamura, K.; Nakahara, K.; Tanabe, T.; Kamisawa, A.; Kawasaki, M. Transparent Polymer Schottky Contact for a High Performance Visible-Blind Ultraviolet Photodiode Based on ZnO. Applied Physics Letters2008, 93 (12), 123309. 16. Vempati, S.; Chirakkara, S.; Mitra, J.; Dawson, P.; Nanda, K. K.; Krupanidhi, S. B. Unusual Photoresponse of Indium Doped ZnO/Organic Thin Film Heterojunction. Applied Physics Letters2012, 100 (16), 162104. 17. Ranjith, K. S.; Kumar, R. T. R. Facile Construction of Vertically Aligned ZnONanorod/PEDOT:PSS Hybrid Heterojunction-Based Ultraviolet Light Sensors: Efficient Performance and Mechanism. Nanotechnology2016, 27 (9), 095304. 18. Hau, S. K.; Yip, H.-L.; Zou, J.; Jen, A. K.-Y. Indium Tin Oxide-Free Semi-Transparent Inverted Polymer Solar Cells Using Conducting Polymer as Both Bottom and Top Electrodes. Organic Electronics2009, 10 (7), 1401–1407. 19. Vosgueritchian, M.; Lipomi, D. J.; Bao, Z. Highly Conductive and Transparent PEDOT:PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes. Advanced Functional Materials2011, 22 (2), 421–428. 20. Sakamoto, S.; Okumura, M.; Zhao, Z.; Furukawa, Y. Raman Spectral Changes of PEDOT–PSS in Polymer Light-Emitting Diodes upon Operation. Chemical Physics Letters2005, 412 (4-6), 395–398. 21. Stutzmann N.; Friend, RH.; Sirringhaus, H.; Self-Aligned, Vertical-Channel, Polymer Field-Effect Transistors. Science299, (2003), 1881–1884. 22. Rost, H.; Ficker, J.; Alonso, J.S.; Leenders, L.; Mcculloch, I.; Air-stable all-polymer field-effect transistors with organic electrodes, Synthetic Metals. 145 (2004) 83–85. 23. Dhar, S.; Majumder, T.; Mondal, S. P. Graphene Quantum Dot-Sensitized ZnONanorod/Polymer Schottky Junction UV Detector with Superior External Quantum Efficiency, Detectivity, and Responsivity. ACS Applied Materials & Interfaces2016, 8 (46), 31822–31831. 16 ACS Paragon Plus Environment

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24. Dhar, S.; Majumder, T.; Chakraborty, P.; Mondal, S. P. DMSO Modified PEDOT:PSS Polymer/ZnONanorodsSchottky Junction Ultraviolet Photodetector: Photoresponse, External Quantum Efficiency, Detectivity, and Responsivity Augmentation Using N Doped Graphene Quantum Dots. Organic Electronics2018, 53, 101–110. 25. Xia, Y.; Sun, K.; Ouyang, J. Solution-Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices. Advanced Materials2012, 24 (18), 2436–2440. 26. Fortunato, E.; Ginley, D.; Hosono, H.; Paine, D. C. Transparent Conducting Oxides for Photovoltaics. MRS Bulletin2007, 32 (03), 242–247 27. Yeon, C.; Yun, S. J.; Kim, J.; Lim, J. W. PEDOT:PSS Films with Greatly Enhanced Conductivity via Nitric Acid Treatment at Room Temperature and Their Application as Pt/TCO-Free Counter Electrodes in Dye-Sensitized Solar Cells. Advanced Electronic Materials2015, 1 (10), 1500121. 28. Ouyang, L.; Musumeci, C.; Jafari, M. J.; Ederth, T.; Inganäs, O. Imaging the Phase Separation Between PEDOT and Polyelectrolytes During Processing of Highly Conductive PEDOT:PSS Films. ACS Applied Materials & Interfaces2015, 7 (35), 19764–19773. 29. Wang, Y.; Li, L.; Huang, X.; Li, Q.; Li, G. New Insights into Fluorinated TiO2 (Brookite, Anatase and Rutile) Nanoparticles as Efficient Photocatalytic Redox Catalysts. RSC Advances2015, 5 (43), 34302–34313. 30. Wang, H.; Bai, Y.; Zhang, H.; Zhang, Z.; Li, J.; Guo, L. CdS Quantum Dots-Sensitized TiO2 Nanorod Array on Transparent Conductive Glass Photoelectrodes. The Journal of Physical Chemistry C2010, 114 (39), 16451–16455. 31. Majumder, T.; Dhar, S.; Debnath, K.; Mondal, S. P. Role of S, N Co-Doped Graphene Quantum Dots as a Green Photosensitizer with Ag-Doped ZnONanorods for Improved Electrochemical Solar Energy Conversion. Materials Research Bulletin2017, 93, 214– 222.



32. Cheung, S. K.; Cheung, N. W. Extraction of Schottky Diode Parameters from Forward Current‐Voltage Characteristics. Applied Physics Letters1986, 49 (2), 85–87. 33. Rawat, G.; Kumar, H.; Kumar, Y.; Kumar, C.; Somvanshi, D.; Jit, S. Effective Richardson Constant of Sol-Gel Derived TiO2Films in n-TiO2/p-Si Heterojunctions. IEEE Electron Device Letters2017, 38 (5), 633–636. 34. Lee, I.; Kim, G. W.; Yang, M.; Kim, T.-S. Simultaneously Enhancing the Cohesion and Electrical Conductivity of PEDOT:PSS Conductive Polymer Films Using DMSO Additives. ACS Applied Materials & Interfaces2015, 8 (1), 302–310. 35. Sze, S.M.; Ng, K.K. Physics of Semiconductor Devices, third ed, Wiley, India, 2014. 36. Ruan, C-H.; Lin, Y-J. High Schottky barrier height of Au contact on Si-nanowire arrays with sulfide treatment. Journal of Applied Physics 2013, 114, 143710. 37. Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chemical Reviews1995, 95 (3), 735–758. 38. Sun, Z.: Liu, Z.; Li, J.; Tai, G.-A.; Lau, S.-P.; Yan, F.; Infrared Photodetectors Based on CVD-Grown Graphene and PbS Quantum Dots with Ultrahigh Responsivity, Advanced Materials. 24 (2012) 5878–5883. 39. Han, Y.; Wu, G.; Wang, M.; Chen, H. Hybrid Ultraviolet Photodetectors with High Photosensitivity Based on TiO2Nanorods Array and Polyfluorene. Applied Surface Science2009, 256 (5), 1530–1533. 40. Selman, A. M.; Hassan, Z. Highly Sensitive Fast-Response UV Photodiode Fabricated from Rutile TiO2 Nanorod Array on Silicon Substrate. Sensors and Actuators A: Physical2015, 221, 15–21. 41. Chinnamuthu, P.; Dhar, J. C.; Mondal, A.; Bhattacharyya, A.; Singh, N. K. Ultraviolet Detection Using TiO2 nanowire Array with Ag Schottky Contact. Journal of Physics D: Applied Physics2012, 45 (13), 135102.


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42. Patel, D. B.; Chauhan, K. R.; Park, W.-H.; Kim, H.-S.; Kim, J.; Yun, J.-H. Tunable TiO2 Films for High-Performing Transparent SchottkyPhotodetector. Materials Science in Semiconductor Processing2017, 61, 45–49. 43. Abbas, S.; Kumar, M.; Kim, H.-S.; Kim, J.; Lee, J.-H. Silver-Nanowire-Embedded Transparent Metal-Oxide HeterojunctionSchottkyPhotodetector. ACS Applied Materials & Interfaces2018, 10 (17), 14292–14298



30

(200)R

(101)R

*

*

40

50

-1 1/2 (nm /eV)

2

2.0

30

1.5

1/2

20

600

Wavelength(nm)

700

1.0

h

2 500

70

2.5

-1

h (nm /eV)

400

60

2

40

300

*

(221)R

(310)R

*

TiO2 Nanorods FTO

(004)A

Intensity(a.u.)

(210)R

Figures

Absoption(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

10

0.5

0 2.5

3.0

heV

3.5

0.0 2.50 2.75 3.00 3.25 3.50

heV

Figure 1: (a) Top view SEM micrograph of TiO2 nanorods grown on FTO coated glass substrate. (b) XRD pattern of TiO2 nanorods grown on FTO substrate. (c) Typical UV-visible absorption spectrum of TiO2 nanorods, (d) (αhν)1/2 vs. hν plot and (e) (αhν)2 vs. hν plot.


Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: (a) Cross-sectional SEM micrograph and (b) schematic device architecture of the TiO2 NRs/ PEDOT:PSS Schottky junction photodetector.



100 80 60

Current (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

40 20 0 Acid treated

-20

DMSO treated

-40 -60 -500

Pristine -250

0

250

500

Voltage(mV)

Figure 3: STM current imaging of (a) pristine (b) DMSO treated and (c) sulphuric acid (H2SO4) treated PEDOT:PSS, all STM current topography were recorded under bias voltage 0.5V and current set point 2.5nA. (d) STM current- voltage (I-V) plot of pristine, sulphuric acid treated and DMSO treated PEDOT:PSS films.


Page 23 of 32

40 30 20

10 1 0.1 0.01

S3 dark

1E-3 -1.0

-0.5

0.0

0.5

1.0

Voltage(V)

10

S1

S2

S3

-0.5

0.0

0.5

4 3 2 1 0 -1

1.0

Voltage(V)

0.30

0.26

2.86

0.24

2.84

H(I)

2.88

0.22 0.20

2.78 2.76

dV/dln(I) Linear Fit of dV/dln(I) 1.0x10-3

2.0x10-3

3.0x10-3

0.0

Voltage(V)

0.5

1.0

2.80

0.16

0.0

-0.5

2.82

0.18

0.12

-1.0

2.90

0.28

0.14

S2

5

0 -1.0

S1

6

Current Density(mA/cm2)

50

7

100



60

dV/dln(I)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.0x10-3

2.74

H(I) Linear Fit of H(I)

2.72 0.0

5.0x10-3

1.0x10-3

Current (A)

2.0x10-3

3.0x10-3

4.0x10-3

5.0x10-3

Current (A)

Figure 4: (a) J-V characteristics of S1, S2 and S3 devices at dark. Semi logarithmic J-V plot of S3 sample is plotted at inset. (b) J-V plots of sample S1and S2.(c) dV/d(lnI) vs I and (d) H(I) vs I plots for sample S3.



5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

S1

S2

S3

0 -5 -10 -15 OFF

-20

ON

OFF

OFF

ON

ON

OFF

-25 -30

0

20

40

60

80

100

Time(s) Figure 5: The transient photoresponce (J-t curve) for S1, S2 and S3 samples under ON/OFF of white light of intensity 80 mW/cm2 at -1V bias.


Page 25 of 32

1200

14000

S1 S2

1000

12000

800

EQE%

10000

EQE%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8000

600

400

200

6000 0

4000

300

2000

350

400

450

500

Wavelength(nm)

550

600

S3

0 300

350

400

450

500

550

600

Wavelength(nm)

Figure 6: EQE vs wavelength plot of S3 sample at –1 V bias. EQE vs. λ plots of S1 and S2 samples are presented at the inset.



Figure 7: Schematic diagram of carrier transport mechanism at TiO2 NRs/polymer interface.


Page 26 of 32

20

2.5 2.0 1.5 1.0 0.5 0.0 300

10

11

1.8x10 1/2

30

S1 S2

350

400

450

500

Wavelength(nm)

550

600

S3

1.8x10

11

1.6x10

11

1.4x10

11

1.2x10

11

1.0x10

S1 S2

11

1.5x10

11

1.2x10

10

9.0x10

10

6.0x10

10

3.0x10

10

8.0x10

0.0 300

10

6.0x10

350

400

450

500

550

600

Time(s)

10

4.0x10

10

2.0x10

0

11

(b)

1/2

3.0

Detectivity (Hz /W)

(a)

Responsivity(A/W)

40

Responsivity(A/W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Detectivity(Hz /W)

Page 27 of 32

S3

0.0 300

350

400

450

500

550

600

300

Wavelength(nm)

350

400

450

500

550

600

Wavelength(nm)

Figure 8: (a) Rλ vs. λ plot of S3 sample at –1 V bias. Rλ vs. λ plot for S1 and S2 are shown at the inset. (b) Dλ vs. λ plot of S3 sample at –1 V bias. Dλ vs. λ for S1 and S2 are presented at the inset.


-2 -4 -6 -8

-10 -12 -14

OFF

OFF

OFF

OFF

-16 -18

0

20

40

60

Time(s)

80

100

14 12 10 8 6 4 2

2

16

Page 28 of 32

Photocurrent Density(mA/cm )

ON

ON

ON

2

0

Photocurrent Density(mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



16 14 12 10 8

Experimental Fitted curve

6 4 2 0 -2

0

5

10

15

Time(s)

20

25

30

0 20 25 30 35 40 45 50 55 60 65 70 75 80

Time(s)

Figure 9: (a)Transient photoresponse of S3 device under ON/OFF of UV light of λ~340nm, intensity 0.38 mW/cm−2 at -1V bias, (b) Experimental results (blue open circle) and fitted curves (solid red line) of response and recovery process of photocurrent for S3 sample.


Page 29 of 32

16 Photocurrent Density(mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S3 S2 S1 Fitted curve

14 12 10 8 6 4 2 0

0.10

0.15

0.20

0.25

0.30

0.35 2

0.40

Power Density (mW/cm ) Figure 10: Photocurrent density vs incident light intensity plots at λ~340 nm of S1, S2 and S3 samples.



Page 30 of 32

Table 1: Calculated Schottky diode parameters of S1, S2 and S3 device at dark condition Samples

Series Resistance RS (Ω)

Barrier Height ΦB (eV)

Diode Ideality Factor (η)

S1

897.75

0.86

2.89

S2

304.89

0.84

3.10

S3

36.39

0.71

4.14


Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Photosensing performance of some reported TiO2 nanostructure based photodetectors Device structure

Applied bias

Photocurrent

Wavelength

Responsivity

Reference

TiO2 nanorod (Electrochemical photosensor) PFH/TiO2 Schottky junction

0

12.87µA/cm2

365nm

………

7

5V

~0.5 µA/cm2

UV light of intensity 3.6mW/cm2

………

39

Au/TiO2 nanorod Schottky junction TiO2 nanowire/Ag

3V

………

134.8 A/W

12

5V

………

350nm (58 μW/cm2) 350nm

3.1 A/W

11

TiO2 NRs/p-Si(111)

5V

6.09×10-4 A

325

0.46 A/W

40

TiO2 nanotube /Ag

2.5V

………

312 nm (1.06 mW/cm2)

13A/W

9

TiO2 nano-branched arrays (UV photodetectors using electrochemical cell) TiO2 nanorod/Ag

0V

373µA cm-2

………

0.22A/W

6

1V

………

3A/W

13

Ag/TiO2 nanowires Cu/TiO2/FTO

6.5V -1V

0.75 µA ………

0.18 A/W 0.897A/W

41 42

AgNWs/Cu4O3/TiO2/FT O

0V

………

380nm (58µW/cm2) 414nm 365(1.06 mw/cm2) 365nm (6mW/cm2)

0.187 A/W

43

TiO2/PEDOT:PSS Schottky junction

-1V

15.69 mA/cm2

340 (0.38mW/cm2)

34.43 A/W

This Paper



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Page 32 of 32

Acid-Treated PEDOT:PSS Polymer and TiO2 Nanorod Schottky

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