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Correlations for coumarin additive on the electrical and photocatalytic activity of TiO2 modified by thiourea
Correlations for coumarin additive on the electrical and photocatalytic activity of TiO2 modified by thiourea M. Soylu, Ahmed A. Al-Ghamdi, W.A. Farooq, F. Yakuphanoglu PII: DOI: Reference:
S0167-9317(16)30026-0 doi: 10.1016/j.mee.2016.01.026 MEE 10096
To appear in: Received date: Revised date: Accepted date:
3 August 2015 28 December 2015 21 January 2016
Please cite this article as: M. Soylu, Ahmed A. Al-Ghamdi, W.A. Farooq, F. Yakuphanoglu, Correlations for coumarin additive on the electrical and photocatalytic activity of TiO2 modified by thiourea, (2016), doi: 10.1016/j.mee.2016.01.026
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ACCEPTED MANUSCRIPT
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Correlations for coumarin additive on the electrical and photocatalytic activity of TiO2 modified by thiourea
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M. Soylua,*, Ahmed A. Al-Ghamdib, W. A. Farooqc, F. Yakuphanoglub,d *a
Department of Physics, Faculty of Sciences and Arts, Bingol University, Bingol, Turkey Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia c Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi Arabia d Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey
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b
Abstract
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Doping of TiO2 was made with thiourea to introduce the C, N and S into the TiO2. In order to investigate the effect of coumarin additions, coumarin-doped TiO2 samples modified by thiourea
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were synthesized by the same method. X-ray diffraction pattern confirmed the anatase crystalline
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phase in the doped-TiO2 samples. XRD data shows that the addition of coumarin and thiourea does not lead to the rutilization during sample crystallization. In order to investigate the photocatalytic
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performance of coumarin doped-TiO2 samples, the current-voltage and photocapacitance transient measurements of Au/cou-doped TiO2/p-Si heterojunction structures were carried out under
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illumination. The variation in the transient decay after illumination follows the different compositions introduced by the addition of coumarin and thiourea. This suggests that there is a correlation between the decay kinetics and the mechanism such as traps and recombination centers provided by the doping level.
Keywords: TiO2, Sol–gel process, Electrical properties ∗Corresponding
author at: Department of Physics, Faculty of Sciences and Arts, Bingol University, Bingol, Turkey Tel.: +90 426 2160012; Fax: +90 426 2150877. E-mail addresses: [email protected], [email protected] (Murat Soylu)
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ACCEPTED MANUSCRIPT 1. Introduction p–n junction diodes have attracted great interest because of their electronic applications [1-5].
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The interaction of oxygen with amorphous or crystalline Si results in formation of a stable oxide
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layer on the surface of the substrate. The oxide based materials such as TiO2 [6], SiO2 [7], ZnO [8],
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CdO [9], Gd2O3 [10], ZrO2 [11] and Al2O3 [12] with low-k and various high-k dielectric constants have been used as interfacial layer in heterojunction applications. Among the oxide based semiconductors, TiO2 is a wide-band gap (3.2 eV) semiconductor material, and it is an important
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material for photocatalysis. The basic idea of conduction by electrons in the conduction band is
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associated with direct current formation [13-16]. The natural phenomenons due to special circumstances following the electron excitation in TiO2 can be used to start the photocatalytic reaction [17]. Inorganic semiconductor surfaces are not generally flat. It is found that the pinholes
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on top of semiconductor substrate have normally obvious effect on the initial performance of the heterojunction. However, the pinholes on the surface are filled with TiO2 nanoparticles. Recently,
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Xue et a.l [18] synthesized AgI/TiO2 nanocomposites by an ultrasound-assisted preparation method. They reported that coupling with AgI nanoparticles (having narrower band-gap than TiO2
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semiconductors) could increase the photocatalytic activity of visible light. It is assumed that the formation of interface dipole is associated with the thickness of interfacial layer. The thickness of interfacial layer estimated to be larger than 30 0A represents trapped carriers and interface states that interact between each other, but less thickness is required to interact with metal. As the thickness increases, the system is used as metal-oxide-semiconductor device (a MOS capacitor, also known as MOS diode) [19]. While a nitrogen doping to TiO2 decreases the photocatalytic activity [20], producing oxygen vacancies in its lattice, the additive of boron element compared to N-TiO2 can eliminate vacancies and thus improves the activity [21]. Semiconductor nanocrystals of titanium dioxide (TiO2) serve as electrode for photovoltaic cells, as well as relatively little attention in electrochemistry [22]. While zinc oxide (ZnO) exhibits photocatalytic properties under UV and visible light, its photodegradation capacity is less than TiO2. Researchers have tried some strategies 2
ACCEPTED MANUSCRIPT concerning modification of TiO2 for photocatalitic, solar cells and magnetic applications. Among these, doping to TiO2 has been proven to be an effective method for altering the electronic,
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structural, optical and morpholologic properties of TiO2. The influence of silver doping on the UV-
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vis properties of TiO2 was investigated. UV-vis measurements have showed that the increase of the
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silver content significantly reduces the light transmission for wavelength above 400 nm [23]. Properties of undoped and doped TiO2 depend on the synthesis conditions, the source of dopant and precursors used. The highest photocatalytic activity for cholesteryl hemisuccinate (CHOL)
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degradation under the solar light was found for TiO2:Fe powders obtained from FeCl3 in
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comparison with TiO2:Fe obtained from Fe(NO3)3 [24]. Polymer solar cells fabricated with a TiO2:Ag 3% layer achieved open-circuit voltage (Voc) of 0.58 V, compared to that of solar cell with a TiO2:Ag 5% layer, resulting in a Voc value of 0.19 V [25]. Moreover, polymer polyazomethine
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solar cells without TiO2 and TiO2:Ag have the much lower power conversion efficiency (PCE) [26]. In addition, an effort has been made to investigate the influence of the amount of Fe on the
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magnetic properties of TiO2. The magnetic susceptibility of TiO2 powders shows the paramagnetic property with the negative Curie temperature with the increase of Fe, that suggests the ordering of
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antiferromagnetic atoms, while the samples with low Fe content consist of superparamagnetic properties in magnetization [27]. In this study, we reported the effect of coumarin addition on the photocatalytic activity of TiO2 modified by thiourea in Au/p-Si heterojunction structure. We focus on how the distribution of the energy states in the TiO2 band-gap and its bulk or surface modification due to the thiourea and coumarin correlate with the observed photocatalytic activity at Au/p-Si structure. We believe that the results will optimize the preparation of doped TiO2 to be used in nanocrystalline powders, the quantum dot dye-sensitized solar cells based on TiO2 and the peroxidation of organics.
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ACCEPTED MANUSCRIPT 2. Experimental details Titanium tetraisopropoxide and thiourea (Sigma Aldrich) were used as precursor materials.
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0.2 mL of ethanolamine (C2H7NO) and 5 mL of 2-methoxyethanol were used as stabilizer and
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solvent, respectively. These materials were mixed in a magnetic mixer at 60 °C for 4 hours. TiO2
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was treated by thiourea of 1%. Thiourea treated-TiO2 includes the C, N and S-elements. TiO2 modified by thiourea was doped with various coumarin contents of 0.005%, 0.01% and 0.03%. TiO2 based samples were prepared on p-type silicon substrates with Al ohmic back contact by spin-
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coating technique at 1000 rpm for 60 seconds. The samples without top contact were annealed at
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500 0C for 2 hours. After depositing of films, 99 percent purity gold was thermally evaporated through molybdenum mask with contact diameter of 2 mm. SCS-4200 Keithley semiconductor characterization system was used in order to examine the electrical characteristics of Au/Cou-doped
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TiO2/p-Si heterojunction structures. The film thicknesses were measured to be about 80±1 nm. The photoresponse measurements of Au/Cou-doped TiO2/p-Si heterojunction structures were performed
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using a 200W halogen lamp. The illumination intensity was monitored by change of the current across the lamp and measured using a solar power meter (TM-206). X-ray diffraction patterns for
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the samples were pointed out using full automated Rigaku Ultima IV advance diffractometer, which scans at a rate of approximately 2 degree/minute.
3. Results and discussion 3.1 The structural characteristics of TiO2 thin films The crystalline forms of three main polymorphs of TiO2 are known as tetragonal anatase, rutile and orthorhombic brookite, affecting its photocatalytic performance [28-33]. Fig. 1 shows XRD patterns of TiO2 samples. The main diffraction peaks of the TiO2 structure are indicated: (101), (110), (112), (200) and (211). The characteristic peaks of anatase phase (JCPDS File No. 211272) appear, indicating a tetragonal structure. The XRD data shows that the addition of coumarin and thiourea does not lead to the rutilization during sample crystallization. Basca et al. [34] 4
ACCEPTED MANUSCRIPT determined that the crystalline phase in the active doped TiO2 sample was anatase. Hassan et al. [35] reported that the photoactivity of anatase was higher than that of rutile. Not only is this
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crystalline form definition, it is also easier in leading us to our goal.
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3.2. The electrical characteristics of Au/Cou-doped TiO2/p-Si heterojunctions The constructed energy-band diagram of Au/n-TiO2/p-Si/Al structure is shown in Fig. 2, considering the Anderson model. s is the surface potential as a function of the applied forward
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bias. δ is the thickness of the interfacial layer. The band gap of TiO2 is 3.2 eV and that of Si is 1.12
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eV. The electron affinities of Si and TiO2 are 4.05 eV and 4.3 eV, respectively [36,37]. The energy barrier ΔEc for an electron is ΔEc=χ(n-TiO2)-χ(p-Si)=4.30–4.05=0.25 eV, and the energy barrier
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ΔEv for hole is ΔEv=Eg(n-TiO2)+ΔEc-Eg(p-Si)=3.2+0.25–1.12=2.33 eV. It is seen that the value of
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ΔEv is higher than ΔEc. It is concluded that the injection of holes from p-type Si to TiO2 is greater than the electron injection from TiO2 to p-Si. The forward-bias current is in the direction from p-
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type semiconductor to metal; it is an exponential function of the forward bias voltage. When the forward bias is applied to Si crystal, electrons can be injected towards p-type Si from n-TiO2 due to
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the small potential barrier, resulting in an increase in the forward current under higher voltage. Fig. 3a-c shows the I–V characteristics of Au/p-Si heterojunction diodes with (a) 0.005%, (b) 0.01% and (c) 0.03% coumarin-doped TiO2. The heterojunctions show rectifying behavior. It is noted that there is an energy barrier between TiO2 and p-Si(100), since TiO2 thin film is n-type semiconductor [38]. The current-voltage measurements were performed at ±5 V. The heterojunction rectifying devices may be destroyed by high voltage. An electric field occurs in the depletion region, when a bias voltage of ±5 V is applied to the TiO2/p-type Si heterojunction. This region contains electrons or holes that tend to flow in the opposite direction to the silicon substrate or TiO2. Thus, current is able to flow through the external circuit. Due to the non-destroy ability of I-V characteristics, all of the samples studied are candidate for developing electronic devices to operate at high voltages. A high-voltage diode has an important role in protection from the damage of high5
ACCEPTED MANUSCRIPT voltage to equipment when a predetermined level of voltage has reached or exceeded. Au/p-Si heterojunction diodes including coumarin-doped TiO2 and the thiourea-modified coumarin-doped
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TiO2 interfacial layers can be configured to provide high voltage protection level. The particularly
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forward bias bowing in heterojunction structures can be explained by specific properties of contact
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materials. This bowing for conventional Si diodes starts at forward bias voltages as low as ~0.4-0.8 V [39-43]. However, the current curve at a forward high-voltage Al/SiO2/p-Si (MIS) Schottky diode having interfacial insulator layer can be related by bulk resistance or contact wires of p-type silicon
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[44]. The physical properties of SiO2 layer are associated with a specific surface area. The presence
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of the oxide layer also explains why the MIS structure's current drops with increasing voltage [45]. The amount of voltage loss that occurs across the oxide layer is attributed to the defined thickness. It is seen that the reverse bias current increases with increasing coumarin (Fig. 3b,c). Also,
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when coumarin-doped TiO2 is modified by thiourea, the current flow and diode parameters are affected in different ways (Fig. 4). It is evaluated that the organosulfur-compound thiourea modifies
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the current transport mechanism. As the amount of thiourea in TiO2 increases, the barrier faced by charge carriers in heterojunction structure is reduced by contribution of the N, S, and C dopants to
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TiO2.
The technical meaning of rectification is as follows:
RR=
I (V fwd )
(1)
I (Vrev )
where, the forward bias (Vfwd) is considered as the greater absolute current |I| while the reverse bias (Vrev) gives the lesser absolute current |I|. The rectification ratios (RRs) through Au/p-Si heterojunction diodes with coumarin-doped TiO2 at ±5 V were determined from Eq. (1). All of the heterojunctions yielded high rectification ratio under dark conditions. The high rectification ratio (RR) of 1.0x105 was found for the sample with 0.005% coumarin-doped TiO2 modified by thiourea 6
ACCEPTED MANUSCRIPT 1.5% (not shown here). The amount and type of contribution must be appropriate to the needs of the application.
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The incident light (hv) excites valence electrons and produces electron-hole pairs, generating
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electron-vacancy in valence band. The reaction proposed for TiO2 is as follows [46]:
TiO2 + light (hν) Ti2O3 or TiO (e-+h+)
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Figs. 3,4 show the light dependent I–V characteristics of Au/n-TiO2/p-Si heterojunction diodes. As
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it is seen from figures, the current-voltage measurements consist of characteristics which symbolize the transition from darkness to light reflecting the electron-hole pair formation. While the reverse bias current increases more rapidly with illumination of 10 mW/cm2, the current increase ratio
TE
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decreases with increasing illumination intensity. This rate is much higher for 0.005% coumarindoped TiO2/p-Si structure (e.g. 74.931, 14.942 and 1.580, respectively). Simple solution-processed
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coumarin doped-TiO2 electron transport layer introduces non-stable reverse bias current with high efficiency. It means that the doping of coumarin has remarkable effect on the electrical
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characteristics of Au/TiO2/p-Si heterojunction diodes. In order to determine the diode parameters of coumarin doped and thiourea treated-TiO2/p-Si heterojunctions, the well-known thermionic emission equations [47] were used. The main diode parameters as a function of the current densityvoltage (J–V) characteristics are determined on the basis of the thermionic emission (TE) theory. The current density-voltage relation is given as follows:
q(V JRs A) q(V JRs A) J J 0 exp 1 exp nkT kT
The saturation current density J0 is given by:
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(2)
ACCEPTED MANUSCRIPT qΦ J 0 A*T 2 exp b0 kT
(3)
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where k, n, T, Φb 0 , A, A*, q, J0 and V are Boltzman constant, ideality factor, contact area,
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Richardson constant, electronic charge, saturation current density and applied voltage, respectively. A* has the theoretical value of 32 Acm−2K−2 for the holes in p-type silicon [45]. The term, V-JRsA is specified with respect to voltage drop in diode. Ideality factor and apparent barrier height (BH) of a
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diode can be determined from the slope and intercept of the linear region experimentally using the
kT AA*T 2 ln q I 0
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Φb 0
(4)
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q dV kT d ln(I )
(5)
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n
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plot of In(I) vs. voltage, respectively. The required equations are as follows:
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The values of barrier height ( Φb 0 ) and ideality factor (n) were determined from the y-axis intercept and the slope of ln[J/{1−exp(q(V-JRsA)/kT)}] vs. V. The device parameters, e.g. barrier height and ideality factor varied in the range of 0.72 eV-0.82 eV and 2.1-3.4, respectively. The ideality factor larger than unity describes the non-ideal behavior of TiO2 based silicon heterojunction diodes. The presence of coumarin doped and thiourea treated-TiO2 thin film layers is deemed as one of the several factors influencing the non-ideal regime. The barrier height decreases with increasing doping of coumarin. The trapping and scattering of carriers locate in grain boundaries around Ti-Oorganic molecule, since inhomogeneous Schottky barriers occur in grain boundaries. The barrier height is higher compared to conventional Au/n-Si metal-semiconductor diode (0.66 eV) [48]. The results are in agreement with the barrier heights of similar Au/organic/n-Si heterojunction diodes 8
ACCEPTED MANUSCRIPT [49–51]. It is seen that the barrier height changes with illumination intensity. We conclude that
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there is a relationship between two parameters, as in the following equation:
(6)
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b0 ( P) b0 P
where P and are the illumination intensity and the illumination coefficient, respectively. The coefficient was found to be −5.42×10−4 eV/W for Au/p-Si heterojunction diode with 0.005%
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coumarin-doped TiO2 interfacial layer. The temperature coefficient of Si band gap was reported to
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be −4.73×10−4 eV/K. It's not a coincidence that there is an agreement between these values for silicon. The charge carrier transport is modulated by the temperature coefficient of band gap and the
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illumination coefficient of barrier height, indicating a linear function relationship [52,53]. The
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photoconductivity spectrum contains the generation profile of electron-hole pairs. Phenomenon that combines the long lifetime and the short transit time of charge carriers is evaluated as
Gain
I ph Id
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stated as follows:
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photoconductive gain [54]. The current gain with the help of voltage-dependent measurement is
(7)
The value of gain was found to be 74.93 under illumination intensity of 10 mWcm -2 at 5 V for 0.005% coumarin-doped TiO2/p-Si structure. As it is seen from figures, photocurrent (Iph) increases as the illumination intensity (P) increases (Iph Pγ) [55]. P is the intensity of the incident light and
is an exponent. The exponent values ranged from 0.49 to 1.02 for all of the TiO2/p-Si heterojunction diodes. The localized states within the mobility gap depend on the quantization of [56]. The better values of , determined for heterojunctions based on the organic/inorganic
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ACCEPTED MANUSCRIPT semiconductors are associated with lower density of trapping centers and less defect rate in certain area [57,58].
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The mechanism behind the photoconduction in ZnO has been investigated by many
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researchers [59,60]. In order to quantify the photoconduction, typical photoresponse measurements
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are performed as a function of the wavelength of incident light. By cutting the illumination, the focus has shifted to the characteristics of optical switching. The gain is formulated as (G=Nelectron/Nphoton) that is, it is the ratio between the number of absorbed photons per unit time and
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the number of electron gained per unit time. The relation is expressed as follows [61]:
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I photo I dark hν x q PAabs
(8)
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G
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Transient photocurrent measurements were applied to all samples and given in Figs. 5,6. While the
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photocurrent for sample with 0.01% coumarin-doped TiO2 interfacial layer reaches maximum value of 3.8x10-4 A at the forward bias, it is seen that its value decreases with increasing dopant content. The photoresponse spectrum includes both an increase and a decrease as function of the wavelength
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of the incident light. This is ascribed to photoconduction process that quantifies the relative contribution of the generation and recombination of excitons. The limit of light absorption within the TiO2 photocatalyst film is the cause of current saturation during periodic illumination. The transient photocurrent measurements are suggested as an alternative way to determine the trapped carrier states, which control recombination rate and carrier mobility [62]. The net charge change in the depletion layer of the semiconductor is modulated by carrier states. Also, the gap states tend to lead to opposite signals through transitions. Phototransient characteristic of 0.03% coumarin doped TiO2 sample where a decay is observed, depends on the amount of incident illumination and a higher decay rate is seen at higher illumination intensity. The photoresponse is fitted by an exponential function to evaluate the photocurrent and relaxation time.
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t 1 2
I pc I 0 exp
t
(9)
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Ipc is the photocurrent, τ1 and τ2 are the time constants, α and β depend on the relative contribution of each photo-related process. The values of time constant for 0.03% coumarin doped TiO2 sample
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are 4±0.2 and 6±0.3, respectively. As it is seen, the value of τ1 is lower than τ2. A high value of time
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constant results in slow rise of current.
3.3 Capacitance spectroscopy of Au/Cou-doped TiO2/p-Si heterojunction diodes
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The active layer of metal-semiconductor contact allows us to the creation of e—h+ pairs, effectively absorbing the light. The band gap of TiO2 is 3.15 eV [63]. This value is low compared to
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the excitation energy. Transient photocapacitance relates to defect state distributions in an
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intervalence charge-transfer reaction. The photocapacitance (Cph) measurements are due to the
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density of photo-induced carriers in the depletion region and the radiative ionization of deep levels. The defect levels in CdTe were investigated using photocapacitance measurements by Duke et al. [64]. The transient photocapacitance measurements of Au/cou-doped TiO2/p-Si heterojunction
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diodes under various light intensities are shown in Figs. 7,8. The source of the transient photocapacitance for Au/cou-doped TiO2/p-Si device was positive in sign. The positive photocapacitance transients are due to the reduction in depletion layer and photo-generated carriers inside the intrinsic layer. The photocapacitance increases with increasing light intensity and saturates at different levels, for different light intensities. The increase was about 0.12 pF, when the illumination power was 30 mW/cm2. The value of capacitance returns sharply to its base value and it does not show any decay, when the illumination is cut off. A correlation is observed between the illumination intensity and the amplitude of the photocapacitance. The photocapacitance and characteristic time ( T ) of its transient part are given as follows [65]:
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dV
d / 2
d ph
(10)
dV
(11)
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T
d d2 ph 2V J ph
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T
d ph
where J ph is the photocurrent density, ph is the photo charge density and added to the
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photocapacitance, d is the depletion width and is the mobility. The trapped electron-related photocapacitance is caused by states localized to the conduction band. The probability of photo-
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induced activation of the trapped carriers with respect to the optical band edge increases the depletion capacitance, since the capacitance of photodiode is associated with the junction
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capacitance. The photocapacitance values of Au/p-Si heterojunction diodes with 0.005%, 0.01%
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and 0.03% coumarin-doped TiO2 were determined to be 9.76x10-9 F, 1.34x10-8 F and 9.62x10-9 F,
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respectively. While the photocapacitance increases with increasing coumarin dopant content from 0.005% to 0.01%, it is decreased by 0.03% close to its initial value. A comparison of results obtained from the photocapacitance measurements, with changing coumarin at %, confirmed that
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the electron traps in doped-TiO2 layer were related to coumarin-induced defects. The low frequency component of photocapacitance is in order that interface charges may follow the applied signal. Whereas, it is more difficult to examine the direct influence of the change in the capacitance with illumination under high frequency. This implies that the interface charges will redistribute when an electric field is applied to the device at the lower frequencies. The dependence of junction capacitance on the depletion layer can be written as [66]:
C
0 r A
(12)
Xd
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ACCEPTED MANUSCRIPT where 0 =8.85x10-14 F/cm, A is the diode area, r is the dielectric constant and given as (C/C0), C0
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the capacitance of the free-space capacitor, Xd is the depletion layer width and as follows:
1/ 2
(13)
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2 N N D X d Si A (Vbi Vr ) q NAN D
where NA and ND are the acceptor and donor density, respectively. The values of depletion width
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varied in the range of 2x10-6-4x10-5 m for Au/cou-doped TiO2/p-Si heterojunction diodes. Also, the
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junction capacitance per unit area:
(14)
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1 2 2(Vbi Vr kT / q) C q r 0 A2 N i
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where Vr is the reverse bias voltage, Vbi is the built-in voltage and Ni is the density of ionized acceptors. The variation of depletion width reflects the high tunability of capacitance. The tunability
T (%)
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of depletion layer can be expressed as follows [67]:
rv r 0 x100 r0
(15)
where εr0 is the dielectric constant at zero bias voltage and εrv is the dielectric constant at a certain reverse bias voltage. Figs. 9,10 show the capacitance-voltage characteristics of Au/cou-doped TiO2/p-Si heterojunction diodes at different frequencies. It is seen that the capacitance increases step by step with decreasing frequency at the reverse bias, including a dopant effect, while it remains unchanged at forward bias. While the capacitance increases with increasing coumarin dopant content from 0.005% to 0.01%, it is decreased by 0.03% close to its initial value in dark conditons. This is evidence of the change in the interface states which follow the alternating signal 13
ACCEPTED MANUSCRIPT (a.c.). It has been reported that the capacitance depending on the frequency consists of two modules as the interface trap capacitance (Css) and semiconductor capacitance (Cs) [68]. Also, when 0.005%
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coumarin-doped TiO2 is modified by 1% thiourea, the values of capacitance decrease (Fig. 9a). As
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the amount of thiourea in TiO2 increases, the magnitude of peaks is affected in reverse and forward
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bias region (Fig. 9a-c). We do not observe a sharp decrease of the peaks, particularly 0.01% coumarin-doped TiO2 modified by the increased thiourea content. The capacitance decreases with the increasing amount of thiourea and coumarin. It is concluded that the capacitance can be
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controlled by dopant materials such as thiourea and coumarin.
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The frequency dependent conductance–voltage (G–V) characteristics may be used as an alternative data source to calculate the density of interface states. The conductance–voltage (G–V) characteristics of Au/cou-doped TiO2/p-Si heterojunction diodes appear to have similar nature with
TE
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the capacitance–voltage (C–V) characteristics as function of frequency. But, the conductance decreases with decreasing frequency at the reverse bias, while the capacitance increases step by step
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with decreasing frequency in the range of 10 kHz-1 MHz (not shown here). The measured capacitance and conductance are highly influenced by the series resistance. Their modulus are
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eliminated from the effect of series resistance. The corrected capacitance (CADJ) and corrected conductance (GADJ) are given as follows [69]:
C ADJ
[Gm2 (2fCm )2 ]Cm 2 aR (2fCm )2
GADJ
aR Gm Gm2 (2fCm )2 Rs
Rs
[Gm2 (2fCm ) 2 ]a 2 aR (2fCm ) 2
(16)
(17)
Gm G (2f ) 2 Cm2
(18)
2 m
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ACCEPTED MANUSCRIPT where Cm is the measured capacitance, Gm is the measured conductance, Rs is the series resistance and f is the frequency. The conductance characteristics are governed by layers such as depletion,
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inversion and accumulation. The majority carrier density is much larger in the depleted regions than
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disproportionately large contribution to the conductance [70,71].
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in the weakly inverted regions. Therefore, interface traps in the depleted areas make a
The interface state density (Dit) is estimated from the Hill-Coleman method that is a
2
1 C Cox Cox
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2 qA G 2f
G 2f 2
(19)
TE
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Dit
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variation of conductance technique and as follows [72]:
The results show that the interface state density decreases with increasing frequency. The interface
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state density was found in the range of 2.1x1013–7.9x1013 cm-2eV-1. While the interface state density decreases with increasing coumarin dopant content from 0.005% to 0.01%, it is increased by 0.03%,
interface.
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demonstrating the restructuring, generation and reordering of coumarin-induced states in TiO2
Figs. 11,12 show the variation of series resistance (Rs) with applied bias voltage in the range of 10 kHz-1 MHz frequency for Au/cou-doped TiO2/p-Si heterojunction diodes. The series resistance decreases with increasing frequency. The high value of Rs compared to the high frequency is attributed to the interface states that can follow the a.c. signal, demonstrating the excess capacitance at low frequency [19]. The fixed oxide charges, mobile oxide charges and oxidetrapped charges are the source of correlation between Rs and frequency. The possibility of escape from a local potential energy for the traps is characterized by carrier mobility. The plots of Rs vs. V give peaks with positive and nagative phase, including a dopant effect. The magnitude of peaks can be attributed to the particular distribution of interface states. The series resistance increases with 15
ACCEPTED MANUSCRIPT increasing coumarin dopant content from 0.005% to 0.01%, but, it is decreased by 0.03% (Fig. 11ac). Also, it is seen from Fig. 12 that the addition of thiourea to the TiO2 changes the peak positions
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with frequency in reverse and forward bias region.
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4. Conclusions
Coumarin-doped TiO2 and coumarin-doped TiO2 samples modified by thiourea were prepared by sol-gel method. X-ray diffraction pattern confirmed the anatase crystalline phase in the
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doped-TiO2 samples. It has been made a comparison of coumarin additive on the electrical
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properties and photocatalytic activity of TiO2 modified by thiourea as the sulfur source at Au/TiO2/p-Si heterojunctions. There is evidence that coumarin-doped TiO2 and coumarin-doped TiO2-thiourea can be more effective than using TiO2 alone. The results obtained suggest that
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Au/cou-doped TiO2/p-Si heterojunctions can be used as photodiode in optoelectronic applications. The interface state density extracted from equivalent parallel conductance for Au/cou-doped
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TiO2/p-Si heterojunction diode was found in order of 1013 eV-1cm-2. We believe that both in UV and Vis-regions, the electrical properties and photocatalytic activity of TiO2 can be largely developed by
series.
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tailoring with coumarin additive and modification by thiourea as the sulfur source at Au and silicon
Acknowlegments The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project No. RG 1435-059.
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Graphical abstract
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We have fabricated nanostructure TiO2 by spin coating on the p-Si The photocurrent in the reverse bias voltage is increased by increasing photo-illumination
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S0167-9317(16)30026-0 doi: 10.1016/j.mee.2016.01.026 MEE 10096
To appear in: Received date: Revised date: Accepted date:
3 August 2015 28 December 2015 21 January 2016
Please cite this article as: M. Soylu, Ahmed A. Al-Ghamdi, W.A. Farooq, F. Yakuphanoglu, Correlations for coumarin additive on the electrical and photocatalytic activity of TiO2 modified by thiourea, (2016), doi: 10.1016/j.mee.2016.01.026
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Correlations for coumarin additive on the electrical and photocatalytic activity of TiO2 modified by thiourea
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M. Soylua,*, Ahmed A. Al-Ghamdib, W. A. Farooqc, F. Yakuphanoglub,d *a
Department of Physics, Faculty of Sciences and Arts, Bingol University, Bingol, Turkey Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia c Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi Arabia d Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey
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b
Abstract
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Doping of TiO2 was made with thiourea to introduce the C, N and S into the TiO2. In order to investigate the effect of coumarin additions, coumarin-doped TiO2 samples modified by thiourea
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were synthesized by the same method. X-ray diffraction pattern confirmed the anatase crystalline
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phase in the doped-TiO2 samples. XRD data shows that the addition of coumarin and thiourea does not lead to the rutilization during sample crystallization. In order to investigate the photocatalytic
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performance of coumarin doped-TiO2 samples, the current-voltage and photocapacitance transient measurements of Au/cou-doped TiO2/p-Si heterojunction structures were carried out under
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illumination. The variation in the transient decay after illumination follows the different compositions introduced by the addition of coumarin and thiourea. This suggests that there is a correlation between the decay kinetics and the mechanism such as traps and recombination centers provided by the doping level.
Keywords: TiO2, Sol–gel process, Electrical properties ∗Corresponding
author at: Department of Physics, Faculty of Sciences and Arts, Bingol University, Bingol, Turkey Tel.: +90 426 2160012; Fax: +90 426 2150877. E-mail addresses: [email protected], [email protected] (Murat Soylu)
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ACCEPTED MANUSCRIPT 1. Introduction p–n junction diodes have attracted great interest because of their electronic applications [1-5].
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The interaction of oxygen with amorphous or crystalline Si results in formation of a stable oxide
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layer on the surface of the substrate. The oxide based materials such as TiO2 [6], SiO2 [7], ZnO [8],
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CdO [9], Gd2O3 [10], ZrO2 [11] and Al2O3 [12] with low-k and various high-k dielectric constants have been used as interfacial layer in heterojunction applications. Among the oxide based semiconductors, TiO2 is a wide-band gap (3.2 eV) semiconductor material, and it is an important
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material for photocatalysis. The basic idea of conduction by electrons in the conduction band is
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associated with direct current formation [13-16]. The natural phenomenons due to special circumstances following the electron excitation in TiO2 can be used to start the photocatalytic reaction [17]. Inorganic semiconductor surfaces are not generally flat. It is found that the pinholes
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on top of semiconductor substrate have normally obvious effect on the initial performance of the heterojunction. However, the pinholes on the surface are filled with TiO2 nanoparticles. Recently,
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Xue et a.l [18] synthesized AgI/TiO2 nanocomposites by an ultrasound-assisted preparation method. They reported that coupling with AgI nanoparticles (having narrower band-gap than TiO2
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semiconductors) could increase the photocatalytic activity of visible light. It is assumed that the formation of interface dipole is associated with the thickness of interfacial layer. The thickness of interfacial layer estimated to be larger than 30 0A represents trapped carriers and interface states that interact between each other, but less thickness is required to interact with metal. As the thickness increases, the system is used as metal-oxide-semiconductor device (a MOS capacitor, also known as MOS diode) [19]. While a nitrogen doping to TiO2 decreases the photocatalytic activity [20], producing oxygen vacancies in its lattice, the additive of boron element compared to N-TiO2 can eliminate vacancies and thus improves the activity [21]. Semiconductor nanocrystals of titanium dioxide (TiO2) serve as electrode for photovoltaic cells, as well as relatively little attention in electrochemistry [22]. While zinc oxide (ZnO) exhibits photocatalytic properties under UV and visible light, its photodegradation capacity is less than TiO2. Researchers have tried some strategies 2
ACCEPTED MANUSCRIPT concerning modification of TiO2 for photocatalitic, solar cells and magnetic applications. Among these, doping to TiO2 has been proven to be an effective method for altering the electronic,
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structural, optical and morpholologic properties of TiO2. The influence of silver doping on the UV-
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vis properties of TiO2 was investigated. UV-vis measurements have showed that the increase of the
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silver content significantly reduces the light transmission for wavelength above 400 nm [23]. Properties of undoped and doped TiO2 depend on the synthesis conditions, the source of dopant and precursors used. The highest photocatalytic activity for cholesteryl hemisuccinate (CHOL)
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degradation under the solar light was found for TiO2:Fe powders obtained from FeCl3 in
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comparison with TiO2:Fe obtained from Fe(NO3)3 [24]. Polymer solar cells fabricated with a TiO2:Ag 3% layer achieved open-circuit voltage (Voc) of 0.58 V, compared to that of solar cell with a TiO2:Ag 5% layer, resulting in a Voc value of 0.19 V [25]. Moreover, polymer polyazomethine
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solar cells without TiO2 and TiO2:Ag have the much lower power conversion efficiency (PCE) [26]. In addition, an effort has been made to investigate the influence of the amount of Fe on the
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magnetic properties of TiO2. The magnetic susceptibility of TiO2 powders shows the paramagnetic property with the negative Curie temperature with the increase of Fe, that suggests the ordering of
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antiferromagnetic atoms, while the samples with low Fe content consist of superparamagnetic properties in magnetization [27]. In this study, we reported the effect of coumarin addition on the photocatalytic activity of TiO2 modified by thiourea in Au/p-Si heterojunction structure. We focus on how the distribution of the energy states in the TiO2 band-gap and its bulk or surface modification due to the thiourea and coumarin correlate with the observed photocatalytic activity at Au/p-Si structure. We believe that the results will optimize the preparation of doped TiO2 to be used in nanocrystalline powders, the quantum dot dye-sensitized solar cells based on TiO2 and the peroxidation of organics.
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ACCEPTED MANUSCRIPT 2. Experimental details Titanium tetraisopropoxide and thiourea (Sigma Aldrich) were used as precursor materials.
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0.2 mL of ethanolamine (C2H7NO) and 5 mL of 2-methoxyethanol were used as stabilizer and
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solvent, respectively. These materials were mixed in a magnetic mixer at 60 °C for 4 hours. TiO2
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was treated by thiourea of 1%. Thiourea treated-TiO2 includes the C, N and S-elements. TiO2 modified by thiourea was doped with various coumarin contents of 0.005%, 0.01% and 0.03%. TiO2 based samples were prepared on p-type silicon substrates with Al ohmic back contact by spin-
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coating technique at 1000 rpm for 60 seconds. The samples without top contact were annealed at
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500 0C for 2 hours. After depositing of films, 99 percent purity gold was thermally evaporated through molybdenum mask with contact diameter of 2 mm. SCS-4200 Keithley semiconductor characterization system was used in order to examine the electrical characteristics of Au/Cou-doped
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TiO2/p-Si heterojunction structures. The film thicknesses were measured to be about 80±1 nm. The photoresponse measurements of Au/Cou-doped TiO2/p-Si heterojunction structures were performed
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using a 200W halogen lamp. The illumination intensity was monitored by change of the current across the lamp and measured using a solar power meter (TM-206). X-ray diffraction patterns for
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the samples were pointed out using full automated Rigaku Ultima IV advance diffractometer, which scans at a rate of approximately 2 degree/minute.
3. Results and discussion 3.1 The structural characteristics of TiO2 thin films The crystalline forms of three main polymorphs of TiO2 are known as tetragonal anatase, rutile and orthorhombic brookite, affecting its photocatalytic performance [28-33]. Fig. 1 shows XRD patterns of TiO2 samples. The main diffraction peaks of the TiO2 structure are indicated: (101), (110), (112), (200) and (211). The characteristic peaks of anatase phase (JCPDS File No. 211272) appear, indicating a tetragonal structure. The XRD data shows that the addition of coumarin and thiourea does not lead to the rutilization during sample crystallization. Basca et al. [34] 4
ACCEPTED MANUSCRIPT determined that the crystalline phase in the active doped TiO2 sample was anatase. Hassan et al. [35] reported that the photoactivity of anatase was higher than that of rutile. Not only is this
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crystalline form definition, it is also easier in leading us to our goal.
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3.2. The electrical characteristics of Au/Cou-doped TiO2/p-Si heterojunctions The constructed energy-band diagram of Au/n-TiO2/p-Si/Al structure is shown in Fig. 2, considering the Anderson model. s is the surface potential as a function of the applied forward
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bias. δ is the thickness of the interfacial layer. The band gap of TiO2 is 3.2 eV and that of Si is 1.12
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eV. The electron affinities of Si and TiO2 are 4.05 eV and 4.3 eV, respectively [36,37]. The energy barrier ΔEc for an electron is ΔEc=χ(n-TiO2)-χ(p-Si)=4.30–4.05=0.25 eV, and the energy barrier
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ΔEv for hole is ΔEv=Eg(n-TiO2)+ΔEc-Eg(p-Si)=3.2+0.25–1.12=2.33 eV. It is seen that the value of
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ΔEv is higher than ΔEc. It is concluded that the injection of holes from p-type Si to TiO2 is greater than the electron injection from TiO2 to p-Si. The forward-bias current is in the direction from p-
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type semiconductor to metal; it is an exponential function of the forward bias voltage. When the forward bias is applied to Si crystal, electrons can be injected towards p-type Si from n-TiO2 due to
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the small potential barrier, resulting in an increase in the forward current under higher voltage. Fig. 3a-c shows the I–V characteristics of Au/p-Si heterojunction diodes with (a) 0.005%, (b) 0.01% and (c) 0.03% coumarin-doped TiO2. The heterojunctions show rectifying behavior. It is noted that there is an energy barrier between TiO2 and p-Si(100), since TiO2 thin film is n-type semiconductor [38]. The current-voltage measurements were performed at ±5 V. The heterojunction rectifying devices may be destroyed by high voltage. An electric field occurs in the depletion region, when a bias voltage of ±5 V is applied to the TiO2/p-type Si heterojunction. This region contains electrons or holes that tend to flow in the opposite direction to the silicon substrate or TiO2. Thus, current is able to flow through the external circuit. Due to the non-destroy ability of I-V characteristics, all of the samples studied are candidate for developing electronic devices to operate at high voltages. A high-voltage diode has an important role in protection from the damage of high5
ACCEPTED MANUSCRIPT voltage to equipment when a predetermined level of voltage has reached or exceeded. Au/p-Si heterojunction diodes including coumarin-doped TiO2 and the thiourea-modified coumarin-doped
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TiO2 interfacial layers can be configured to provide high voltage protection level. The particularly
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forward bias bowing in heterojunction structures can be explained by specific properties of contact
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materials. This bowing for conventional Si diodes starts at forward bias voltages as low as ~0.4-0.8 V [39-43]. However, the current curve at a forward high-voltage Al/SiO2/p-Si (MIS) Schottky diode having interfacial insulator layer can be related by bulk resistance or contact wires of p-type silicon
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[44]. The physical properties of SiO2 layer are associated with a specific surface area. The presence
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of the oxide layer also explains why the MIS structure's current drops with increasing voltage [45]. The amount of voltage loss that occurs across the oxide layer is attributed to the defined thickness. It is seen that the reverse bias current increases with increasing coumarin (Fig. 3b,c). Also,
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when coumarin-doped TiO2 is modified by thiourea, the current flow and diode parameters are affected in different ways (Fig. 4). It is evaluated that the organosulfur-compound thiourea modifies
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the current transport mechanism. As the amount of thiourea in TiO2 increases, the barrier faced by charge carriers in heterojunction structure is reduced by contribution of the N, S, and C dopants to
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TiO2.
The technical meaning of rectification is as follows:
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I (Vrev )
where, the forward bias (Vfwd) is considered as the greater absolute current |I| while the reverse bias (Vrev) gives the lesser absolute current |I|. The rectification ratios (RRs) through Au/p-Si heterojunction diodes with coumarin-doped TiO2 at ±5 V were determined from Eq. (1). All of the heterojunctions yielded high rectification ratio under dark conditions. The high rectification ratio (RR) of 1.0x105 was found for the sample with 0.005% coumarin-doped TiO2 modified by thiourea 6
ACCEPTED MANUSCRIPT 1.5% (not shown here). The amount and type of contribution must be appropriate to the needs of the application.
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The incident light (hv) excites valence electrons and produces electron-hole pairs, generating
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electron-vacancy in valence band. The reaction proposed for TiO2 is as follows [46]:
TiO2 + light (hν) Ti2O3 or TiO (e-+h+)
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Figs. 3,4 show the light dependent I–V characteristics of Au/n-TiO2/p-Si heterojunction diodes. As
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it is seen from figures, the current-voltage measurements consist of characteristics which symbolize the transition from darkness to light reflecting the electron-hole pair formation. While the reverse bias current increases more rapidly with illumination of 10 mW/cm2, the current increase ratio
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decreases with increasing illumination intensity. This rate is much higher for 0.005% coumarindoped TiO2/p-Si structure (e.g. 74.931, 14.942 and 1.580, respectively). Simple solution-processed
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coumarin doped-TiO2 electron transport layer introduces non-stable reverse bias current with high efficiency. It means that the doping of coumarin has remarkable effect on the electrical
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characteristics of Au/TiO2/p-Si heterojunction diodes. In order to determine the diode parameters of coumarin doped and thiourea treated-TiO2/p-Si heterojunctions, the well-known thermionic emission equations [47] were used. The main diode parameters as a function of the current densityvoltage (J–V) characteristics are determined on the basis of the thermionic emission (TE) theory. The current density-voltage relation is given as follows:
q(V JRs A) q(V JRs A) J J 0 exp 1 exp nkT kT
The saturation current density J0 is given by:
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(2)
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(3)
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where k, n, T, Φb 0 , A, A*, q, J0 and V are Boltzman constant, ideality factor, contact area,
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Richardson constant, electronic charge, saturation current density and applied voltage, respectively. A* has the theoretical value of 32 Acm−2K−2 for the holes in p-type silicon [45]. The term, V-JRsA is specified with respect to voltage drop in diode. Ideality factor and apparent barrier height (BH) of a
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diode can be determined from the slope and intercept of the linear region experimentally using the
kT AA*T 2 ln q I 0
TE
Φb 0
(4)
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q dV kT d ln(I )
(5)
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n
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plot of In(I) vs. voltage, respectively. The required equations are as follows:
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The values of barrier height ( Φb 0 ) and ideality factor (n) were determined from the y-axis intercept and the slope of ln[J/{1−exp(q(V-JRsA)/kT)}] vs. V. The device parameters, e.g. barrier height and ideality factor varied in the range of 0.72 eV-0.82 eV and 2.1-3.4, respectively. The ideality factor larger than unity describes the non-ideal behavior of TiO2 based silicon heterojunction diodes. The presence of coumarin doped and thiourea treated-TiO2 thin film layers is deemed as one of the several factors influencing the non-ideal regime. The barrier height decreases with increasing doping of coumarin. The trapping and scattering of carriers locate in grain boundaries around Ti-Oorganic molecule, since inhomogeneous Schottky barriers occur in grain boundaries. The barrier height is higher compared to conventional Au/n-Si metal-semiconductor diode (0.66 eV) [48]. The results are in agreement with the barrier heights of similar Au/organic/n-Si heterojunction diodes 8
ACCEPTED MANUSCRIPT [49–51]. It is seen that the barrier height changes with illumination intensity. We conclude that
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there is a relationship between two parameters, as in the following equation:
(6)
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b0 ( P) b0 P
where P and are the illumination intensity and the illumination coefficient, respectively. The coefficient was found to be −5.42×10−4 eV/W for Au/p-Si heterojunction diode with 0.005%
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coumarin-doped TiO2 interfacial layer. The temperature coefficient of Si band gap was reported to
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be −4.73×10−4 eV/K. It's not a coincidence that there is an agreement between these values for silicon. The charge carrier transport is modulated by the temperature coefficient of band gap and the
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illumination coefficient of barrier height, indicating a linear function relationship [52,53]. The
TE
photoconductivity spectrum contains the generation profile of electron-hole pairs. Phenomenon that combines the long lifetime and the short transit time of charge carriers is evaluated as
Gain
I ph Id
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stated as follows:
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photoconductive gain [54]. The current gain with the help of voltage-dependent measurement is
(7)
The value of gain was found to be 74.93 under illumination intensity of 10 mWcm -2 at 5 V for 0.005% coumarin-doped TiO2/p-Si structure. As it is seen from figures, photocurrent (Iph) increases as the illumination intensity (P) increases (Iph Pγ) [55]. P is the intensity of the incident light and
is an exponent. The exponent values ranged from 0.49 to 1.02 for all of the TiO2/p-Si heterojunction diodes. The localized states within the mobility gap depend on the quantization of [56]. The better values of , determined for heterojunctions based on the organic/inorganic
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ACCEPTED MANUSCRIPT semiconductors are associated with lower density of trapping centers and less defect rate in certain area [57,58].
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The mechanism behind the photoconduction in ZnO has been investigated by many
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researchers [59,60]. In order to quantify the photoconduction, typical photoresponse measurements
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are performed as a function of the wavelength of incident light. By cutting the illumination, the focus has shifted to the characteristics of optical switching. The gain is formulated as (G=Nelectron/Nphoton) that is, it is the ratio between the number of absorbed photons per unit time and
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the number of electron gained per unit time. The relation is expressed as follows [61]:
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I photo I dark hν x q PAabs
(8)
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G
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Transient photocurrent measurements were applied to all samples and given in Figs. 5,6. While the
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photocurrent for sample with 0.01% coumarin-doped TiO2 interfacial layer reaches maximum value of 3.8x10-4 A at the forward bias, it is seen that its value decreases with increasing dopant content. The photoresponse spectrum includes both an increase and a decrease as function of the wavelength
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of the incident light. This is ascribed to photoconduction process that quantifies the relative contribution of the generation and recombination of excitons. The limit of light absorption within the TiO2 photocatalyst film is the cause of current saturation during periodic illumination. The transient photocurrent measurements are suggested as an alternative way to determine the trapped carrier states, which control recombination rate and carrier mobility [62]. The net charge change in the depletion layer of the semiconductor is modulated by carrier states. Also, the gap states tend to lead to opposite signals through transitions. Phototransient characteristic of 0.03% coumarin doped TiO2 sample where a decay is observed, depends on the amount of incident illumination and a higher decay rate is seen at higher illumination intensity. The photoresponse is fitted by an exponential function to evaluate the photocurrent and relaxation time.
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t 1 2
I pc I 0 exp
t
(9)
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Ipc is the photocurrent, τ1 and τ2 are the time constants, α and β depend on the relative contribution of each photo-related process. The values of time constant for 0.03% coumarin doped TiO2 sample
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are 4±0.2 and 6±0.3, respectively. As it is seen, the value of τ1 is lower than τ2. A high value of time
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constant results in slow rise of current.
3.3 Capacitance spectroscopy of Au/Cou-doped TiO2/p-Si heterojunction diodes
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The active layer of metal-semiconductor contact allows us to the creation of e—h+ pairs, effectively absorbing the light. The band gap of TiO2 is 3.15 eV [63]. This value is low compared to
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the excitation energy. Transient photocapacitance relates to defect state distributions in an
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intervalence charge-transfer reaction. The photocapacitance (Cph) measurements are due to the
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density of photo-induced carriers in the depletion region and the radiative ionization of deep levels. The defect levels in CdTe were investigated using photocapacitance measurements by Duke et al. [64]. The transient photocapacitance measurements of Au/cou-doped TiO2/p-Si heterojunction
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diodes under various light intensities are shown in Figs. 7,8. The source of the transient photocapacitance for Au/cou-doped TiO2/p-Si device was positive in sign. The positive photocapacitance transients are due to the reduction in depletion layer and photo-generated carriers inside the intrinsic layer. The photocapacitance increases with increasing light intensity and saturates at different levels, for different light intensities. The increase was about 0.12 pF, when the illumination power was 30 mW/cm2. The value of capacitance returns sharply to its base value and it does not show any decay, when the illumination is cut off. A correlation is observed between the illumination intensity and the amplitude of the photocapacitance. The photocapacitance and characteristic time ( T ) of its transient part are given as follows [65]:
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dV
d / 2
d ph
(10)
dV
(11)
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d d2 ph 2V J ph
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d ph
where J ph is the photocurrent density, ph is the photo charge density and added to the
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photocapacitance, d is the depletion width and is the mobility. The trapped electron-related photocapacitance is caused by states localized to the conduction band. The probability of photo-
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induced activation of the trapped carriers with respect to the optical band edge increases the depletion capacitance, since the capacitance of photodiode is associated with the junction
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capacitance. The photocapacitance values of Au/p-Si heterojunction diodes with 0.005%, 0.01%
TE
and 0.03% coumarin-doped TiO2 were determined to be 9.76x10-9 F, 1.34x10-8 F and 9.62x10-9 F,
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respectively. While the photocapacitance increases with increasing coumarin dopant content from 0.005% to 0.01%, it is decreased by 0.03% close to its initial value. A comparison of results obtained from the photocapacitance measurements, with changing coumarin at %, confirmed that
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the electron traps in doped-TiO2 layer were related to coumarin-induced defects. The low frequency component of photocapacitance is in order that interface charges may follow the applied signal. Whereas, it is more difficult to examine the direct influence of the change in the capacitance with illumination under high frequency. This implies that the interface charges will redistribute when an electric field is applied to the device at the lower frequencies. The dependence of junction capacitance on the depletion layer can be written as [66]:
C
0 r A
(12)
Xd
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the capacitance of the free-space capacitor, Xd is the depletion layer width and as follows:
1/ 2
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2 N N D X d Si A (Vbi Vr ) q NAN D
where NA and ND are the acceptor and donor density, respectively. The values of depletion width
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varied in the range of 2x10-6-4x10-5 m for Au/cou-doped TiO2/p-Si heterojunction diodes. Also, the
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junction capacitance per unit area:
(14)
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1 2 2(Vbi Vr kT / q) C q r 0 A2 N i
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where Vr is the reverse bias voltage, Vbi is the built-in voltage and Ni is the density of ionized acceptors. The variation of depletion width reflects the high tunability of capacitance. The tunability
T (%)
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of depletion layer can be expressed as follows [67]:
rv r 0 x100 r0
(15)
where εr0 is the dielectric constant at zero bias voltage and εrv is the dielectric constant at a certain reverse bias voltage. Figs. 9,10 show the capacitance-voltage characteristics of Au/cou-doped TiO2/p-Si heterojunction diodes at different frequencies. It is seen that the capacitance increases step by step with decreasing frequency at the reverse bias, including a dopant effect, while it remains unchanged at forward bias. While the capacitance increases with increasing coumarin dopant content from 0.005% to 0.01%, it is decreased by 0.03% close to its initial value in dark conditons. This is evidence of the change in the interface states which follow the alternating signal 13
ACCEPTED MANUSCRIPT (a.c.). It has been reported that the capacitance depending on the frequency consists of two modules as the interface trap capacitance (Css) and semiconductor capacitance (Cs) [68]. Also, when 0.005%
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coumarin-doped TiO2 is modified by 1% thiourea, the values of capacitance decrease (Fig. 9a). As
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the amount of thiourea in TiO2 increases, the magnitude of peaks is affected in reverse and forward
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bias region (Fig. 9a-c). We do not observe a sharp decrease of the peaks, particularly 0.01% coumarin-doped TiO2 modified by the increased thiourea content. The capacitance decreases with the increasing amount of thiourea and coumarin. It is concluded that the capacitance can be
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controlled by dopant materials such as thiourea and coumarin.
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The frequency dependent conductance–voltage (G–V) characteristics may be used as an alternative data source to calculate the density of interface states. The conductance–voltage (G–V) characteristics of Au/cou-doped TiO2/p-Si heterojunction diodes appear to have similar nature with
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the capacitance–voltage (C–V) characteristics as function of frequency. But, the conductance decreases with decreasing frequency at the reverse bias, while the capacitance increases step by step
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with decreasing frequency in the range of 10 kHz-1 MHz (not shown here). The measured capacitance and conductance are highly influenced by the series resistance. Their modulus are
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eliminated from the effect of series resistance. The corrected capacitance (CADJ) and corrected conductance (GADJ) are given as follows [69]:
C ADJ
[Gm2 (2fCm )2 ]Cm 2 aR (2fCm )2
GADJ
aR Gm Gm2 (2fCm )2 Rs
Rs
[Gm2 (2fCm ) 2 ]a 2 aR (2fCm ) 2
(16)
(17)
Gm G (2f ) 2 Cm2
(18)
2 m
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inversion and accumulation. The majority carrier density is much larger in the depleted regions than
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disproportionately large contribution to the conductance [70,71].
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in the weakly inverted regions. Therefore, interface traps in the depleted areas make a
The interface state density (Dit) is estimated from the Hill-Coleman method that is a
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1 C Cox Cox
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2 qA G 2f
G 2f 2
(19)
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Dit
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variation of conductance technique and as follows [72]:
The results show that the interface state density decreases with increasing frequency. The interface
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state density was found in the range of 2.1x1013–7.9x1013 cm-2eV-1. While the interface state density decreases with increasing coumarin dopant content from 0.005% to 0.01%, it is increased by 0.03%,
interface.
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demonstrating the restructuring, generation and reordering of coumarin-induced states in TiO2
Figs. 11,12 show the variation of series resistance (Rs) with applied bias voltage in the range of 10 kHz-1 MHz frequency for Au/cou-doped TiO2/p-Si heterojunction diodes. The series resistance decreases with increasing frequency. The high value of Rs compared to the high frequency is attributed to the interface states that can follow the a.c. signal, demonstrating the excess capacitance at low frequency [19]. The fixed oxide charges, mobile oxide charges and oxidetrapped charges are the source of correlation between Rs and frequency. The possibility of escape from a local potential energy for the traps is characterized by carrier mobility. The plots of Rs vs. V give peaks with positive and nagative phase, including a dopant effect. The magnitude of peaks can be attributed to the particular distribution of interface states. The series resistance increases with 15
ACCEPTED MANUSCRIPT increasing coumarin dopant content from 0.005% to 0.01%, but, it is decreased by 0.03% (Fig. 11ac). Also, it is seen from Fig. 12 that the addition of thiourea to the TiO2 changes the peak positions
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with frequency in reverse and forward bias region.
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4. Conclusions
Coumarin-doped TiO2 and coumarin-doped TiO2 samples modified by thiourea were prepared by sol-gel method. X-ray diffraction pattern confirmed the anatase crystalline phase in the
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doped-TiO2 samples. It has been made a comparison of coumarin additive on the electrical
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properties and photocatalytic activity of TiO2 modified by thiourea as the sulfur source at Au/TiO2/p-Si heterojunctions. There is evidence that coumarin-doped TiO2 and coumarin-doped TiO2-thiourea can be more effective than using TiO2 alone. The results obtained suggest that
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Au/cou-doped TiO2/p-Si heterojunctions can be used as photodiode in optoelectronic applications. The interface state density extracted from equivalent parallel conductance for Au/cou-doped
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TiO2/p-Si heterojunction diode was found in order of 1013 eV-1cm-2. We believe that both in UV and Vis-regions, the electrical properties and photocatalytic activity of TiO2 can be largely developed by
series.
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tailoring with coumarin additive and modification by thiourea as the sulfur source at Au and silicon
Acknowlegments The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project No. RG 1435-059.
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Graphical abstract
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►
We have fabricated nanostructure TiO2 by spin coating on the p-Si The photocurrent in the reverse bias voltage is increased by increasing photo-illumination
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intensity The structure behaves as a photodiode
►
The Al/TiO2/p-Si diode can be used for optical sensor applications.
AC
CE P
TE
D
MA
NU
SC R
►
34