TiO2 nanoporous thin-film heterojunctions: Fabrication and electrical characterization

TiO2 nanoporous thin-film heterojunctions: Fabrication and electrical characterization

Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] Contents lists available at ScienceDirect Materials Science in Semiconductor Processi...

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Cu2O/TiO2 nanoporous thin-film heterojunctions: Fabrication and electrical characterization Sajad Hussain a,b,d,n, Chuanbao Cao a,nn, Waheed S. Khan c, Ghulam Nabi a, Zahid Usman a, Abdul Majid d, Thamer Alharbi d, Zulfiqar Ali a, Faheem K Butt a, Muhammad Tahir a, Muhammad Tanveer a, Faryal Idress a a

Research Centre of Materials Science, Beijing Institute of Technology, Beijing 100081, People's Republic of China Department of Physics, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan c National Institute of Biotechnology and Genetic Engineering (NIBGE), P.O. Box no. 577, Jhang Road, Faisalabad, Pakistan d Department of Physics, College of Science, Almajmaah University, P.O. Box no. 1712, Al-Zulfi 11932, Saudi Arabia b

a r t i c l e i n f o

Keywords: p–n junction Cu2O thin film TiO2 nanoporous film Anodization Semiconductor

abstract In this paper, cuprous oxide (Cu2O)/titanium dioxide (TiO2) diodes have been fabricated by a facile and inexpensive method for possible use in solar cells. TiO2 nanoporous films were prepared through anodization of Ti foil and Cu2O films were deposited on it to make the diode through electrodeposition. The structural and morphological characterization was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). In electrical characterization the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the diodes were measured at room temperature. The linear behavior of C–V curve indicated that the carrier concentration was homogeneous in the film region adjacent to the equilibrium space-charge region. The thickness of the depletion region ω ffi 29 nm, carrier concentration N ffi 8  1022 m  3 and built in potential ffi 0.80 V was estimated from C–V graph. The transport mechanism was due to the Poole–Frankel field effect because the experimentally obtained value of β was close to the theoretical value calculated for the Poole–Frenkel in log I against V1/2 graph. The values of several electrical parameters such as ideality factor, barrier height, and series resistances were calculated from I–V, Cheung's and Norde's functions. & 2013 Elsevier Ltd. All rights reserved.

1. Introduction The heterojunction diodes have attracted much attention due to various applications in solar cell, light emitting diode, integrated circuits and so on. Cu2O thin layers deposited on a TiO2 films typically form a p–n heterojunction. This system has been widely used for photocatalysis

n Corresponding author at: Research Centre of Materials Science, Beijing Institute of Technology, Beijing 100081, People's Republic of China. Tel./fax: þ86 10 6891 3792. nn Corresponding author. E-mail addresses: [email protected] (S. Hussain), [email protected] (C. Cao).

and solar cell applications [1,2]. The energies band gap of TiO2 and Cu2O are 3.2 and 2.0 eV [3], respectively. The TiO2 absorb high-energy portion (UV) of the solar light. It has been tried to improve the absorption of visible light of TiO2 by incorporation of transition metals. But it is not suitable for water splitting reactions. Therefore low band gap semiconductor Cu2O has been used with TiO2 for water splitting. [4]. Also the electrons excited to the conduction band of Cu2O would move to TiO2 because the valence band and conduction band lie above those of TiO2; whereas the holes move in opposite direction to Cu2O. Cu2O can absorb the visible light and enhance the efficiency of the solar cell. Due to this reason this heterojunction is very good for photovoltaic applications [4–6]. Hou

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et al. [7] have fabricated Cu2O/TiO2 heterojunction but the rectification ratio was too low. Li et al. [2] also have reported prototype of a scalable core–shell Cu2O/TiO2 solar cell with low efficiency. They have increased the rectification ratio after annealing. Previously, we fabricated the photovoltaic solar cell from the thin film of Cu2O/TiO2 through sputtering techniques but the efficiency was low due to the interface defects and band discontinuity [8]. Interface states decrease the photocurrent due to the carrier's traps or recombination centers. Interface states and band discontinuity formed due to the defects and lattice mismatch of films. The transportation of carriers is affected by these interface states and band discontinuity in diodes. So the conduction mechanism or the nature of barrier height formation at the interface is needed to study. For these purposes different models have been applied to study the different electrical parameters of diodes [9–12]. In our work we have fabricated Cu2O/TiO2 nanoporous films heterojunction with good rectification ratio and successfully decrease the value of ideality factor due to decreasing of interface defects and transportation of charges. First time we measured the electrical parameter of Cu2O/TiO2 nanoporous films heterojunction from current–voltage (I–V) and capacitance–voltage (C–V) curves. The current mechanism has also been discussed in detail. We have used Cheung's and Norde's functions to measure the series resistance, ideality factor and barrier height. This junction is a forward step towards the fabrication of Photovoltaic solar cells after the growth of nanoporous or nanotubes on transparent substrates. 2. Experimental TiO2 nanoporous films were prepared by the anodization process reported by Sadek et al. [13]. In brief, the Ti foil (0.3 mm, 99.5%) was first cleaned by acetone and distilled water to remove the surface grease. Anodization was performed in electrolyte medium of 0.5% (wt) NH4F/ ethylene glycol solution using carbon as a cathode at room temperature for 2 h. TiO2 nanoporous films were then crystallized by an annealing process in air at 500 1C for 4 h. To make p–n heterojunction, p-type Cu2O was electrodeposited on TiO2 nanoporous films in the lactic acid solution at bath temperature 55 1C [14]. The pH 9.5 of lactic acid solution was adjusted with the help of KOH and electrodeposition potential was carried out at 0.5 V. After deposition the sample was washout with distilled water, dried in air and an indium contact was stacked on Cu2O layer. The purity and orientation of heterojunction was examined by X-ray diffraction with a Cu-Kα radiation source. The heterojunction was examined using scanning electron microscopy (SEM). Dark I–V and C–V measurements were performed using a ZAHNER IM6e potentiostat at room temperature. 3. Results and discussion Fig. 1 shows the X-ray spectrum of a Cu2O layer deposited on a TiO2 nanoporous film at  0.5 V and 55 1C for 30 min. All observed peaks can be indexed as peaks

Fig. 1. XRD pattern of the Cu2O/TiO2 nanoporous films heterojunction.

generated by Cu2O, TiO2 nanoporous films. There is no any other peak of Cu or related impurities present in the XRD spectrum. Fig. 2(a) shows the top SEM image of TiO2 nanoporous film. Fig. 2(b) shows the Cu2O deposited on TiO2 nanoporous film, it is clear from the SEM images that Cu2O particles has filled the nanoporous film of TiO2. The Cu2O solution penetrated into the nanoporus film of TiO2 and got deposited into the TiO2 thin films as explained by [15]. The forward and reverse bias I–V characteristics for Cu2O/TiO2 nanoporous films heterojunction is shown in Fig. 3, where the curve exhibits a good pn junction behavior. The ohmic behavior was measured for In/Cu2O/ Ti foil as shown in Fig. 3 (inset) after depositing the Cu2O on Ti foil. The indium contact was stuck on top of the Cu2O thin film. The I–V characteristics show that the barrier formed at interface because from  1 to 0.5 V no charge carriers flow across the junction, where the built-in potential was developed. The rectification ratio (RR) at 1 V of the applied voltage (RR¼ IF/IR) was approximately 44 which is ten times more than the reported by Hou et al. [7]. This average RR ratio reveals that there is no trace amount of Cu2 þ and no carriers from Ti foil which is also confirmed from the XRD analysis [16]. The turn on voltage of the device was 0.520 V and which is less than 0.9 V reported by Tsai et al. [17]. Luo et al. used this pn junction for photovoltaic applications by using of FTO substrate [18]. The reverse-bias I–V characteristics at room temperature plotted in the form of log I against V1/2 is shown in Fig. 4. It is clear from Fig. 4, that there are two distinct linear regions for each characteristic which may be explained in the form of either the Schottky effect or the Poole–Frenkel effect. The I–V expressions for these processes are [19] I ¼ AAn T 2 expð

Φb β V 1=2 Þexpð RS 1=2 Þ kT kTω

ð1Þ

for the Schottky effect and β V 1=2 I ¼ I o expð PF 1=2 Þ kTω

ð2Þ

for the Poole–Frenkel effect, where ω is the thickness of

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Fig. 2. Top SEM image of TiO2 nanoporous film (a), and top SEM image of Cu2O/TiO2 nanoporous films heterojunction (b).

Fig. 3. I–V characteristic of Cu2O/TiO2 nanoporous films heterojunction. Inset the ohmic behavior between Cu2O and Ti foil.

Fig. 5. C  2–V characteristic of Cu2O/TiO2 nanoporous films heterojunction.

where A is the area of the diode, ε is the relative permittivity and Co is the capacitance of the cell at zero bias as shown in Fig. 5. Theoretical values of βRS and βPF coefficients are given by [21]  2βRS ¼ βPF ¼

Fig. 4. ln(I)–V1/2 plot of Cu2O/TiO2 nanoporous films heterojunction.

the depletion region, βRS and βPF are the Schottky and Poole–Frenkel field lowering coefficients respectively. The value of ωffi 29 nm was obtained from the following relation [20] ω¼

εεo A Co

ð3Þ

q3 πεεo

1=2 ð4Þ

The calculated values were βRS ¼1.26  10  5 eV (m V  1)1/2 and βPF ¼2.52  10  5 eV (m V  1)1/2. The experimental calculated values of β from the slopes of Fig. 4 were found to be 8.86  10  6 eV (m V  1)1/2 and 2.34  10  5 eV (m V  1)1/2 for the higher and lower-voltage regions, respectively. The experimentally calculated value of β for the low field region is about 1.86 times the theoretical value of βRS and 0.93 times the theoretical value of βPF. The calculated value of β for the high voltage region is 0.70 times the theoretical value of βRS and 0.35 times the theoretical value of βPF. The experimentally obtained value of β for the high voltage region was close to the theoretical value calculated for the Poole–Frenkel field effect. The capacitance–voltage (C  2–V) characteristics Cu2O/ TiO2 nanoporous films heterojunction have been measured at room temperature as shown in Fig. 5. It has been

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observed that the junction has specific value of capacitance due to the space charge in the depletion region. It varies with the width of the depletion region and therefore, depends on junction voltage [22]. The plots of 1/C1/2 vs. forward bias voltage are linear which indicates, the carrier concentration is homogeneous in the film region adjacent to the equilibrium space-charge region. The effective carrier concentration N ffi 8  1022 m  3 was calculated from the slope of C  2–V curve, and is given by [23] dðC  2 Þ 2 ¼ dV εεo qNS2

ð5Þ

The built in potential (Vbi) ffi0.80 V was calculated from extrapolated intercept on voltage axis. The maximum barrier field attainable in the depletion layer was calculated from following relation 2V bi Emax ¼ d

ð6Þ 1

and it was 55 V mm . The high carrier photogeneration efficiency depends upon this high field for the photovoltaic devices [20]. Cheung's [24] method has been used to calculate the ideality factor, barrier height and series resistance. Series resistance RS plays a vital role in the electrical characterization of a diode, so in view of the series resistance RS, the current in a diode is expressed as   qðV  IRs Þ ð7Þ I ¼ I o exp nkT From above relation, Cheung's functions are defined as dV nkT ¼ þ IRs d lnðIÞ q 

HðIÞ ¼ V 

nkT Io ln q AAn T 2

HðIÞ ¼ IRs þnΦb

Fig. 7. H(I)–I plot of Cu2O/TiO2 nanoporous films heterojunction.

ð8Þ  ð9Þ ð10Þ

The RS and n values were calculated from the slope and y-axis intercept of the dV/d ln I–I plot and were equal to be 2.5 kΩ and 6.3, respectively as shown in Fig. 6. The higher values of n attribute to oxide layer on the surface of metal electrodes, series resistance, interfacial states and the voltage drop across the interfacial layer [24]. The value of

ideality factor is almost equal to p-Si/n-ZnO nanorods [25] diode and 3.5 times less than Mutabar et al. 2010 [26] reported ideality factor. The ideality factor is also less than doped TiO2 diode [27] at room temperature. The barrier height Φb and RS values were measured from the y-axis intercept and slope of the H(I)–I plot of Eq. (13) as shown in Fig. 7 and were found to be 0.77 eV and 6.450 kΩ, respectively. The barrier height and series resistance could also be measured by Norde's method [28]. The Norde function is expressed as   V kT I FðVÞ ¼  ln ð11Þ n 2 γ q AA T The barrier height is given by Φb ¼ FðV o Þ þ

RS ¼

V o kT  q γ

kTðγ  nÞ qIo

ð12Þ

ð13Þ

where F(Vo) is the minimum value of F(V) and γ is a dimensionless integer greater than the ideality factor. Fig. 8 shows the plot of F(V) vs. V of the diode. From the graph RS and Φb values were measured to be 2.532 kΩ and 0.76 eV respectively. For the transport of charges in pn junction, there were interface defects and band discontinuity which make the conduction process little away from ideal. Cheung's and Norde's method gave different values of series resistance. This difference is slightly less than the difference reported by the Shah et al. [26]. This difference may be attributing to the fact that Cheung's model is valid in the high-voltage region of the forward bias ln I–V characteristics, while Norde's model is applied to the full-voltage range of forward bias ln I–V characteristics of the junctions. 4. Conclusion

Fig. 6. dV/d ln(I)  I plot of Cu2O/TiO2 nanoporous films heterojunction.

The Cu2O/TiO2 nanoporous films heterojunction were prepared through anodization of Ti foil and electrodeposition of Cu2O. The diode was characterized by XRD, SEM

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Fig. 8. F(V)–I plot of Cu2O/TiO2 nanopores films heterojunction.

and electrical techniques. The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the diode were measured. The turn on voltage of the device was 0.520 V. The thickness of the depletion region 29 nm, carrier concentration N ffi8  1022 m  3 and built in potential (Vbi) ffi 0.80 V was estimated from C  2–V graph. The experimentally obtained value of β for the high voltage region was close to the theoretical value calculated for the Poole–Frenkel field effect. The values of several electrical parameters such as ideality factor, barrier height, and series resistances were calculated through Cheung's and Norde's functions. Acknowledgments This work was supported by the National Natural Science Foundation of China (20471007 and 50972017) and the Research Fund for the Doctoral Program of Higher Education of China (20101101110026). References

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