- Email: [email protected]
ZnO heterojunction photovoltaic performance
Superlattices and Microstructures 85 (2015) 908–917
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
The influence of Cu2O crystal structure on the Cu2O/ZnO heterojunction photovoltaic performance Nezar G. Elfadill a,⇑, M.R. Hashim a,b, Khaled M. Chahrour a, M.A. Qaeed a, M. Bououdina c,d a
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, University Sains Malaysia, Penang 11800, Malaysia Institute of Nano-optoelectronics Research & Technology Laboratory (INOR), School of Physics, Universiti Sains Malaysia, USM, 11800 Penang, Malaysia c Nanotechnology Centre, College of Science, University of Bahrain, P.O. Box 32038, Bahrain d Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Bahrain b
a r t i c l e
i n f o
Article history: Received 22 June 2015 Received in revised form 1 July 2015 Accepted 4 July 2015 Available online 6 July 2015 Keywords: Metal oxides Cu2O/ZnO heterojunction Crystal growth Interface properties
a b s t r a c t Cuprous oxide (Cu2O) films were potentiostatically electrodeposited onto platinum (Pt) film coated onto silicon (Si) wafer from lactic solution at pH 9. The influence of applied potential on Cu2O crystal structure was carefully examined. At higher electrochemical applied potential, a polycrystalline structure was observed, and then as the applied potential decreased, a single crystalline structure oriented along (1 1 1) was obtained. Further decrease in the applied potential leads to the formation of a polycrystalline structure and finally at much lower applied potential, a single crystalline structure growing along (2 0 0) orientation (equivalent to (1 0 0) orientation) was revealed. Cu2O/ZnO heterojunction photodiodes based on these three crystal structures were fabricated and studied under dark and illuminated conditions. The best performance of the solar cell efficiency was achieved by the heterojunction based on (1 1 1) oriented Cu2O film (1.45%) compared to other structures (0.34% and 0.25%), which may be attributed to the formation of high quality heterojunction interface due to the heteroepitaxial-like growth of (0 0 2) oriented ZnO. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the fabrication of functional p–n heterojunction metal oxide optoelectronic devices with high interface quality received great interests. The crystal structure and the lattice symmetry of semiconductors play a critical role in determining many physical properties, such as cleavage, electronic band structure, and optical transparency. Compared to polycrystalline and amorphous semiconductors, single crystalline semiconductor possesses enhanced properties such as impeding the process of electron–hole pair recombination at grain boundaries, which in turn results in improving carrier life time and diffusion lengths. Among many metal oxide p–n heterojunctions, p-Cu2O/n-ZnO heterojunction receive great interest due to the non-toxicity and availability of its constituent elements. Cuprous oxide (Cu2O) is one of the most studied metal oxide semiconductor. It is a p-type metal oxide semiconductor with a direct energy bandgap of 2.1 eV and a large free-exciton binding energy of 140 meV [1,2]. On the other hand, the wide-band gap n-type ZnO was considered as a promising material that can be used as window layer in many solar cell structures due to its direct wide band gap (3.37 eV) [3]. The theoretical ⇑ Corresponding author. E-mail address: [email protected] (N.G. Elfadill). http://dx.doi.org/10.1016/j.spmi.2015.07.010 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
909
photovoltaic efficiency of Cu2O based solar cell is as high as 20% [4]. However, the conversion efficiency so far obtained is limited to 3.83% [5], which remains far below than the theoretical value. Growing a layer of single crystalline Cu2O was very much desired in order to improve the electronic properties of p-Cu2O/n-ZnO heterojunction interface. The conventional methods of growing single crystalline semiconductor require either high temperature or high vacuum. In contrast, the growth of crystalline materials using electrochemical deposition (called electrocrystallization) offers a simple and low-cost technique that operates at low temperature. Electrocrystallization is a process that produces conductive crystals from the mass transfer joined with charge transfer. Walsh and Herron [6] define the electrocrystallization as the process (or result) of a direct or indirect electrochemical influence on the crystallization, where direct influence refers to the domination of overpotential in the nucleation and growth of crystals, while indirect refers to the influence of local reaction (e.g. the pH) on the crystallization process. Usually, the electrodeposited Cu2O film is polycrystalline with a preferred orientation along (1 0 0) plane at pH 9 or along (1 1 1) at pH 12 [7– 9]. However, many researchers report the growth of single crystalline Cu2O thin film onto single crystalline metals layers [10,11] and single crystalline semiconductors layers [12,1]. In most heterojunctions, the key factor in building solar cell that can lead to the observed performance difference appears to be the degree of crystallinity and crystal face of the p-type and n-type exposed at the p–n junction interface. Due to the fact that crystal face contains a different atomic arrangement depending on which the plane is exposed at the surface, the termination of atoms and the deviation of surface coordination from ideal bulk coordination are expected to be different. The energy level, density, and distribution of interface states at the p–n junction can be affected by that difference and, therefore, the recombination losses at the interface. Therefore, the purpose of this work is to investigate the effect of the electrodeposition applied potential on both crystal structure and morphology of the deposited Cu2O thin films, as well as the influence of the crystal structure and morphology of the deposited Cu2O on the quality of p-Cu2O/n-ZnO heterojunction interface and photovoltaic properties. It should be noted here that platinum (Pt) thin film as electrode was selected to be pre-deposited onto silicon (Si) wafer because it has similar FCC crystal structure as Cu2O with a relatively low mismatch, i.e. 8.9%.
2. Material and methods As substrate preparation, Pt thin layer was firstly sputtered onto Si wafer. Cu2O films were then potentiostatically electrodeposited on the substrate using a conventional three electrode method. The applied potential was controlled by using eDAQ ER466 Integrated Potentiostat System, while the current during deposition was monitored. Ag/AgCl and Pt mesh (2.5 2.5) cm were used as reference electrode and counter electrode respectively. The electrolyte used to grow Cu2O consisted of 0.45 M cupric sulfate (CuSO4.5H2O, ACS reagent P 98%) and 3 M of lactic acid (C2H4OHCOOH, ACS reagent 85%) at pH = 9 adjusted by the addition of an amount of 5 M sodium hydroxide (NaOH, reagent grade P 98%). All chemicals were purchased from (Sigma Aldrich, USA). The solution was kept at a constant temperature of 70 °C during deposition process using water bath. To determine the electrochemical activities that take place in the reduction of copper-lactate solution, a linear sweep voltammogram was carried out between 0 and 1000 mV vs. Ag/AgCl, at a scan rate of 10 mV/s. The deposition was carried out with an applied potential ranging from 340 to 550 mV vs. Ag/AgCl with an increase step of 30 mV. ZnO nanorods of 1.2 lm were grown hydrothermally on sputtered pre-prepared 50 nm ZnO seed layer on the top of Cu2O/Pt/Si substrates. The solution used to grow ZnO nanostructures was equimolar 1:1 zinc nitrate hexahydrate (ZnNO3.6H2O, reagent grade 98%) and hexamethylenetetramine HMTA ((CH2)6N4, ACS reagent P 99%). All chemicals also were purchased from (Sigma Aldrich, USA). Finally, 200 nm of ITO (Sn-doped In2O3) was sputtered on the top of ZnO nanorods as transparent contact thin layer, and then the structure of the heterojunction was completed by Al electrode which was sputtered onto ITO film. The morphology of the electrodeposited Cu2O films was characterized by a field emission scanning electron microscopy (FESEM) using FEI/Nova NanoSEM 450. The structure of the films was studied by X-ray diffraction using PANalytical X’pert PRO diffractometer equipped with Cu Ka radiation (k = 1.5406 Å) and high-spatial resolution Raman spectroscopy using Jobin Yvon HR800UV spectrometer System. The heterojunction properties of p-Cu2O/n-ZnO were studied by measuring the dark and illuminated I–V characteristics using Keithley model 4200-SCS Semiconductor Characterization System.
3. Results and discussion The cathodic reduction of copper-lactate solution was extensively studied, and many reports showed that by varying the electrodeposition applied potential two reduction activities could be observed [13,8,14]. It has been noticed that at lower applied potential, the reduction of Cu2+ to Cu1+ was dominating at the cathode which produces pure phase of Cu2O film. With increasing applied potential, a new reduction activity from Cu2+ to Cu0 takes place and competes with the previous reduction activity (Cu2+ to Cu1+), thereby leading to the formation of a mixture of Cu metal and Cu2O phase. At higher applied potential, the Cu2+ to Cu0 reduction activity was dominating producing only metallic Cu film. The above two reaction activities were clearly observed in the linear sweep voltammogram shown in Fig. 1. However, the specific reduction activity
910
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 1. Linear sweep voltammogram for the reduction of copper-lactate at 70 °C and pH 9.
taking place at these potentials was unknown a priori, but by using XRD characterization one can determine the phase composition of thin films synthesized at these potentials. In Fig. 1, a sharp increase in the cathode current density was started around the applied potential of 340 mV and this was assigned to the reduction of Cu2+ to Cu1+ (only Cu2O phase was produced). This reduction process was extended in a narrow range from (340 to 520) mV. Starting from an applied potential of 550 mV and above, a mixture of Cu and Cu2O was obtained, which indicates that the reduction of Cu2+ to Cu1+ was initiating. Finally, at an applied potential of 780 mV and above the film was pure copper therefore the reduction of Cu2+ to Cu0 was dominating. In order to study the influence of the applied potential on the evolution of both structure and microstructure of electrodeposited Cu2O, the deposition was carried out with different applied potentials in the range of 340 to 520 mV. Fig. 2(a) shows the morphology of sputtered Pt onto Si wafer, showing a packed granular nanostructure. Fig. 2(b–f) shows the top view of electrodeposited Cu2O films deposited under different applied potentials (550, 520, 430, 400 and 340 mV). It can be noticed that the surface morphology of the film deposited at 550 and 520 mV is composed of grains with cubic-like shape, while films with a smooth surface with large grains is observed at 430 mV (Fig. 2(b) and (c). Finally, four sided pyramidal-like grains were revealed at 400 and 340 mV. This difference in the surface morphology was mainly caused by the variation of the films crystal orientation which is governed by the nucleation mechanisms. In general, the nucleation of electrodeposited Cu2O thin film depends on various deposition parameters. Switzer et al. [15], reported that the orientation of Cu2O film at higher over potential follows the orientation of the substrate (known
Fig. 2. FESEM micrographs of the morphology of the electrodeposited Cu2O (a) sputtered Pt film; (b) 550 mV; (c) 520 mV; (d) 430 mV; (e) 400 mV; (f) 340 mV.
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
911
as thermodynamics nucleation and growth mechanism) until it reaches a critical thickness, after which it changes to the kinetically nucleation and growth mechanism (pH dependent). Moreover, at low overpotential, the film follows the orientation of the substrate indefinitely. The thickness of the deposited film can be controlled by measuring the charge passed (Q) during deposition process in a period of time, according to Faraday law expressed by the following equation:
d¼
MQ nAqF
ð1Þ
where M and q are the molar mass and density of the deposit substance, Q the total charge passing during a fixed deposition time, F = 96485 C mol1 is Faraday’s constant, d and A are the thickness and area of Cu2O film, n is the number of electrons transferred by reduction of an ion. The total charge involved in the deposition process can be calculated from the integration of cathode current over a period of time. In our experiment, the total charge deposited was around 2.3 103 C, which expected to deposit a 5 lm of Cu2O layer. Fig. 3 shows the cross section FESEM micrograph of pure phase of Cu2O films prepared with different applied potentials. The cross section micrograph shows that the thickness of Cu2O films are nearly 5 lm except for the film deposited at 520 mV, which shows highest thickness (6 lm). Whereas all films shows a single phase of Cu2O produced by using same amount of transferred charge, this difference can be only attributed to the presence of voids within Cu2O film or in other words the film is not very compact. Fig. 4 shows the time–current transient during the Cu2O film electrodeposition growth process of grown Cu2O at four different applied potentials 520, 430, and 400 and 340 mV. In all experiments, the cathodic current density is high at the beginning of the deposition process then sharply decreases due the nucleation process of Cu2O. After that, the decreases in the cathodic current becomes very slow due to the increase in the resistance of the grown semiconductor film [16]. It is clearly noticed that the applied potential influences the cathodic current density and more importantly the value of current density at the nucleation time (the first 20 s). The current density at this stage determines the driving force for the nucleation process. As the applied potential decreases, the driving force for the nucleation process was reduced. Fig. 5 shows the evolution of XRD patterns of the as-prepared Cu2O films. Fig. 5(a) reveals peaks characteristic of the Pt sputtered layer in agreement with JCPDS cards No. 01-070-2431. Fig. 5(b–f) shows the XRD patterns for the electrodeposited Cu2O films under various potentials. As shown in Fig. 5(b) and (c), the XRD patterns at applied potential 550 and 520 mV reveal the formation of a mixture of Cu metal matching with JCPDS card No. 01-070-3038 and Cu2O phase matching with JCPDS card No. 01-078-0428. Fig. 5(d–f) shows the XRD patterns for the films grown at applied potential from 430 to 340 mV. Interestingly, various phase transformation has been observed: (i) from a single crystalline structure along
Fig. 3. FESEM micrographs of Cu2O cross sections prepared with different applied potentials (a) 520 mV; (b) 430 mV; (c) 400 mV; (d) 340 mV.
912
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 4. Electrodeposition cathodic current during deposition with applied potential (a) 520 mV; (b) 430 mV; (c) 400 mV; (d) 340 mV.
Fig. 5. XRD patterns of the sputtered Pt electrode and the electrodeposited Cu2O films (a) Pt film; (b) 550 mV; (c) 520 mV; (d) 430 mV; (e) 400 mV; (f) 340 mV.
(1 1 1) orientation at applied potential 430 mV; (ii) to a polycrystalline structure with (2 0 0) preferred orientation at 400 mV; (iii) and then to a single crystalline structure along (2 0 0) orientation. Previous studies showed that Cu2O electrodeposited from lactic solution at pH 9 produces polycrystalline structure with different I(2 0 0)/I(1 1 1) intensity ratios depending on the applied potentials [7,8], whereas in our case, only a single (1 1 1) peak of Cu2O was observed and as the growth potential decreases the structure changed to a polycrystalline and then again a single (2 0 0) peak was detected. Fig. 6 shows Raman spectra revealing characteristic peaks of crystalline Cu2O phase [17]. The most intense peak observed at 218 cm1 is assigned as 2EU phonon mode, the peak at 300 cm1 as A2U phonon mode whereas the peaks at 420 and 510 cm1 are assigned as region of multiphonon processes and finally the broad peak is assigned as an overlap of tow peaks (620, 645) cm1. In order to examine the role of different Cu2O crystal structure on the growth and performance of the Cu2O/ZnO solar cell, hydrothermally ZnO nanorods were grown onto Cu2O films using the same growth parameters as described earlier. Fig. 7(a–c) shows the top view of ZnO nanorods grown onto Cu2O substrates with (1 1 1) orientation, polycrystalline and
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
913
Fig. 6. Raman spectra of Cu2O films deposited with applied potential (a) 520 mV; (b) 430 mV; (c) 400 mV; (d) 340 mV.
Fig. 7. FESEM top view images of hydrothermally grown ZnO nanorods on (a) (1 1 1) oriented Cu2O; (b) polycrystalline Cu2O; (c) (2 0 0) oriented Cu2O.
Cu2O with (2 0 0) orientation, respectively. It is clear from Fig. 7 that all ZnO nanorods were hexagonal and aligned normal to the substrate surface. The ZnO nanorods grown onto Cu2O substrates with (1 1 1) orientation have the higher density and almost covered the entire substrate and become like a thin film (see also Fig. 8). The compactness of ZnO nanorods grown onto Cu2O with (1 1 1) orientation may be attributed to the higher nucleating density with lesser interface defects due to the small lattice mismatch 7.1% [18,19] between (1 1 1) plane of cubic Cu2O and (0 0 1) plane of hexagonal ZnO. For the other samples, the nanorods compactness and density was dramatically reduced. The Cu2O/ZnO heterojunctions crystal structure was confirmed by studying X-ray diffraction and Raman spectroscopy. Fig. 9(a) shows XRD pattern of the heterojunction prepared based on (1 1 1) oriented Cu2O film. Only a single (2 0 0) peak of ZnO was detected (matching with JCPDS card No. 36-1451), showing a heteroepitaxial-like growth of ZnO along (0 0 1) direction compared to the polycrystalline ZnO for the other samples as shown in Fig. 9(b) (sample based on (2 0 0) oriented Cu2O as representative). In addition, the Raman spectrum shown in Fig. 10 reveals characteristic peaks of ZnO and Cu2O phases in all samples; the peak located at 437 cm1 was assigned for the E2 (high) phonon mode [20] and the peak located at 331 cm1corresponds to the second order Raman spectrum arising from zone-boundary phonons E2 [21], respectively. In order to investigate the functionality of the grown nanostructures as solar cell, a typical structure of Pt/p-Cu2O/n-ZnO heterojunction diode was fabricated and shown in Fig. 8(b). The dark I–V characteristics of the three selected p-Cu2O/n-ZnO heterojunctions were shown in Fig. 11. The heterojunctions show a rectifying behavior. The external diode’s electrical properties such as rectifying ratio (R), saturation current density (Jsc) and ideality factor (n) are listed in Table 1. These parameters were extracted from semi-logarithmic plot of I–V curve using the following diode equation:
914
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 8. The microstructure of the Cu2O/ZnO heterojunction photodiode (a) FESEM cross section image (based on (1 1 1) oriented Cu2O); (b) schematic diagram.
Fig. 9. XRD patterns of the fabricated Cu2O-film/ZnO-nanorods prepared based on (a) (1 1 1) oriented Cu2O film; (b) (2 0 0) oriented Cu2O film.
Fig. 10. Raman spectrum of the fabricated Cu2O/ZnO (based on (1 1 1) oriented Cu2O).
qV 1 J ¼ J s exp nkT
ð2Þ
where Js is the saturation current density, n is the ideality factor, T is the absolute temperature and k the Boltzmann constant. The low extracted saturation current density (3.55 109Am/cm2) and the moderate value of ideality factor (2.5) reported in Table 1, for the heterojunction based on (1 1 1) Cu2O film compared to other samples indicate that the Cu2O/ZnO interface having lower interface defect density [22–24] and it is sufficiently good. This suggest that there was lower number of tunneling traps and consequently lower carrier leakage in the depletion region which in turn enhance the photovoltaic output of the heterojunction solar cell. The calculated rectifying ratio of the fabricated devices was very low in general, but the rectifying ratio for the device fabricated onto (1 1 1) oriented Cu2O (three orders of magnitude) is
915
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 11. Dark I–V characteristic of Cu2O/ZnO heterojunctions fabricated based on (a) (1 1 1) oriented Cu2O film; (b) polycrystalline Cu2O; (c) (2 0 0) oriented Cu2O film.
Table 1 Characteristic parameters of current–voltage measurements for Cu2O/ZnO heterojunctions. Cu2O preparation applied potential
Rectifying ratio (R)
Ideality factor (n)
Saturation current density Jo (A/cm2)
(1 1 1) Oriented Cu2O Polycrystalline (2 0 0) Oriented Cu2O
1900 32 16
2.5 3.06 3.22
3.55 109 3.16 108 4.0 108
Table 2 The photovoltaic parameters for the Cu2O/ZnO heterojunctions. Cu2O preparation applied potential
ISC (mA)
VOC (V)
Fill factor FF
Efficiency (%)
(1 1 1) Oriented Cu2O Polycrystalline (2 0 0) Oriented Cu2O
9.07 3.8 3.03
0.39 0.27 0.32
0.41 0.33 0.26
1.45 0.34 0.25
Fig. 12. Solar cell I–V curves of Cu2O/ZnO heterojunctions fabricated based on (a) (1 1 1) oriented Cu2O film; (b) polycrystalline Cu2O; (c) (2 0 0) oriented Cu2O film.
much better than the other devices. It seems to be the heteroepitaxial-like growth of (0 0 2) ZnO nanorods on the (1 1 1) Cu2O single crystal film that generates fewer surface states in the inter-band region, and hence better performance in the heterojunction. Also, it is found that all devices exhibit turn-on voltage around (1.2 V), which is comparable to turn-on voltage of Cu2O/ZnO heterojunction prepared by Wei et al. [25,26].
916
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Finally, the photovoltaic effects in Pt/p-Cu2O/n-ZnO/Al heterojunctions were investigated under AM 1.5 at 100 mW/cm2 illumination. Fig. 12 shows the J–V curve of the three selected heterojunctions solar cells. A typical solar cell behavior was observed for the three selected heterojunctions. The heterojunction solar cell prepared based on (1 1 1) Cu2O film shows the highest efficiency. The short circuit current density, open circuit voltage, fill factor and efficiency were measured; i.e. 9.07 mA cm2, 0.39 V, 0.41% and 1.45%, respectively. The PV parameters for the other heterojunctions were measured and listed in Table 2. The enhancement in the efficiency for the solar cell prepared based on (1 1 1) Cu2O film was notable. However, compared to the electrodeposited Cu2O based solar cells, the efficiency in this study is higher than some previous research works reported in the literature [27–30]. On the other hand, the obtained results remain smaller to some extent compared to Cu2O/ZnO solar cells [5] (3.83%), which was fabricated by oxidation process. We attributed this difference to the high Cu2O resistivity of heterojunction fabricated in this work. Generally, the photovoltaic parameters are still very low for practical applications. 4. Conclusion The effect of electrochemical applied potential on the evolution of morphology and crystal structure of the electrodeposited Cu2O films was studied. At higher potential (520 to 460 mV), a polycrystalline Cu2O film was revealed. At lower applied potential (430 to 340 mV), interesting changes in the crystal structure of Cu2O films were observed: at 430 mV a smooth surface single crystalline structure oriented along (1 1 1), then at 340 mV a polycrystalline structure and finally at 340 mV a single crystalline structure growing along (2 0 0) orientation (equivalent to (1 0 0) orientation). This was clearly confirmed by XRD analysis. Additionally, Cu2O film/ZnO nanorods heterojunction solar cell based on different Cu2O crystal structures were fabricated. The best solar cell efficiency was achieved by the heterojunction based on (1 1 1) oriented Cu2O film (1.45%) compared to other structures (0.34% and 0.25%), which may be attributed to the formation of high quality heterojunction interface due to the heteroepitaxial-like growth of (0 0 2) oriented ZnO. Acknowledgments The provision of financial support from the Institute of Postgraduate Studies (IPS) Universiti Sains Malaysia (USM) Malaysia Fellowship and RU Grant No. 1001/PFIZIK/811175 are gratefully acknowledged. References [1] R. Liu, E.W. Bohannan, J.A. Switzer, F. Oba, F. Ernst, Epitaxial electrodeposition of Cu2O films onto InP (0 0 1), Appl. Phys. Lett. 83 (2003) 1944–1946. [2] N.G. Elfadill, M. Hashim, K. Saron, K.M. Chahrour, M. Qaeed, M. Bououdina, Ultraviolet–visible photo-response of p-Cu2O/n-ZnO heterojunction prepared on flexible (PET) substrate, Mater. Chem. Phys. 156 (2015) 54–60. [3] M. Zamfirescu, A. Kavokin, B. Gil, G. Malpuech, M. Kaliteevski, ZnO as a material mostly adapted for the realization of room-temperature polariton lasers, Phys. Rev. B 65 (2002) 161205–161209. [4] K. Cheng, Q.Q. Li, J. Meng, X. Han, Y.Q. Wu, S.J. Wang, L. Qian, Z.L. Du, Interface engineering for efficient charge collection in Cu2O/ZnO heterojunction solar cells with ordered ZnO cavity-like nanopatterns, Sol. Energy Mater. Sol. Cells 116 (2013) 120–125. [5] T. Minami, Y. Nishi, T. Miyata, J. Nomoto, High-efficiency oxide solar cells with ZnO/Cu2O heterojunction fabricated on thermally oxidized Cu2O sheets, Appl. Phys. Exp. 4 (2011) 23011–23013. [6] F.C. Walsh, M.E. Herron, Electrocrystallization and electrochemical control of crystal growth: fundamental considerations and electrodeposition of metals, J. Phys. D Appl. Phys. 24 (1991) 217–225. [7] S. Bijani, L. Martinez, M. Gabas, E.A. Dalchiele, J.R. Ramos-Barrado, Low-temperature electrodeposition of Cu2O thin films: modulation of micronanostructure by modifying the applied potential and electrolytic bath pH, J. Phys. Chem. C 113 (2009) 19482–19487. [8] L. Wang, N. De Tacconi, C. Chenthamarakshan, K. Rajeshwar, M. Tao, Electrodeposited copper oxide films: effect of bath pH on grain orientation and orientation-dependent interfacial behavior, Thin Solid Films 515 (2007) 3090–3095. [9] T.D. Golden, M.G. Shumsky, Y.C. Zhou, R.A. VanderWerf, R.A. VanLeeuwen, J.A. Switzer, Electrochemical deposition of copper(I) oxide films, Chem. Mater. 8 (1996) 2499–2504. [10] E.W. Bohannan, M.G. Shumsky, J.A. Switzer, Epitaxial electrodeposition of copper (I) oxide on single-crystal gold (1 0 0), Chem. Mater. 11 (1999) 2289– 2291. [11] J.K. Barton, A.A. Vertegel, E.W. Bohannan, J.A. Switzer, Epitaxial electrodeposition of copper (I) oxide on single-crystal copper, Chem. Mater. 13 (2001) 952–959. [12] J.A. Switzer, R. Liu, E.W. Bohannan, F. Ernst, Epitaxial electrodeposition of a crystalline metal oxide onto single-crystalline silicon, J. Phys. Chem. B 106 (2002) 12369–12372. [13] A.-L. Daltin, A. Addad, J.-P. Chopart, Potentiostatic deposition and characterization of cuprous oxide films and nanowires, J. Cryst. Growth 282 (2005) 414–420. [14] W. Septina, S. Ikeda, M.A. Khan, T. Hirai, T. Harada, M. Matsumura, L.M. Peter, Potentiostatic electrodeposition of cuprous oxide thin films for photovoltaic applications, Electrochim. Acta 56 (2011) 4882–4888. [15] J.A. Switzer, H.M. Kothari, E.W. Bohannan, Thermodynamic to kinetic transition in epitaxial electrodeposition, J. Phys. Chem. B 106 (2002) 4027–4031. [16] R. Inguanta, S. Piazza, C. Sunseri, Template electrosynthesis of aligned Cu2O nanowires: Part I. Fabrication and characterization, Electrochim. Acta 53 (2008) 6504–6512. [17] P. Dawson, M. Hargreave, G. Wilkinson, The dielectric and lattice vibrational spectrum of cuprous oxide, J. Phys. Chem. Solids 34 (1973) 2201–2208. [18] S.-M. Chou, M.-H. Hon, C. Leu, Y.-H. Lee, Al-doped ZnO/Cu2O heterojunction fabricated on (2 0 0) and (1 1 1)-orientated Cu2O substrates, J. Electrochem. Soc. 155 (2008) H923–H928. [19] B. Mohamad Fariza, J. Sasano, T. Shinagawa, H. Nakano, S. Watase, M. Izaki, Electrochemical growth of (0 0 0 1)-n-ZnO film on (1 1 1)-p-Cu2O film and the characterization of the heterojunction diode, J. Electrochem. Soc. 158 (2011) D621. [20] K. Saron, M. Hashim, M. Farrukh, Stress control in ZnO films on GaN/Al2O3 via wet oxidation of Zn under various temperatures, Appl. Surf. Sci. 258 (2012) 5200–5205. [21] K.C. Chin, C.K. Poh, G.L. Chong, J. Lin, C.H. Sow, A.T.S. Wee, Large area, rapid growth of two-dimensional ZnO nanosheets and their field emission performances, Appl. Phys. A 90 (2007) 623–627.
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
917
[22] N.G. Elfadill, M.R. Hashim, K.A.T. Thabit, The role of using seed-layer assisted electrodeposition method on the growth and the photovoltaic properties of p-Cu2O/n-Si heterojunctions, J. Mater. Sci.: Mater. Electron. 26 (2014) 985–991. [23] P. Lv, W. Zheng, L. Lin, F. Peng, Z. Huang, F. Lai, I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction, Physica B 406 (2011) 1253–1257. [24] C.-L. Kuo, R.-C. Wang, J.-L. Huang, C.-P. Liu, C.-K. Wang, S.-P. Chang, W.-H. Chu, C.-H. Wang, C.-H. Tu, The synthesis and electrical characterization of Cu2O/Al: ZnO radial p–n junction nanowire arrays, Nanotechnology 20 (2009) 365603–365607. [25] P. Lv, L. Lin, W. Zheng, M. Zheng, F. Lai, Photosensitivity of ZnO/Cu2O thin film heterojunction, Optik 124 (2013) 2654–2657. [26] H.M. Wei, H.B. Gong, Y.Z. Wang, X.L. Hu, L. Chen, H.Y. Xu, P. Liu, B.Q. Cao, Three kinds of Cu2O/ZnO heterostructure solar cells fabricated with electrochemical deposition and their structure-related photovoltaic properties, CrystEngComm 13 (2011) 6065–6070. [27] J.B. Cui, U.J. Gibson, A simple two-step electrodeposition of Cu2O/ZnO nanopillar solar cells, J. Phys. Chem. C 114 (2010) 6408–6412. [28] J. Katayama, K. Ito, M. Matsuoka, J. Tamaki, Performance of Cu2O/ZnO solar cell prepared by two-step electrodeposition, J. Appl. Electrochem. 34 (2004) 687–692. [29] Y.-K. Hsu, H.-H. Lin, J.-R. Wu, M.-H. Chen, Y.-C. Chen, Y.-G. Lin, Electrochemical growth and characterization of a p-Cu2O thin film on n-ZnO nanorods for solar cell application, RSC Adv. 4 (2014) 7655–7659. [30] K. Cheng, Q. Li, J. Meng, X. Han, Y. Wu, S. Wang, L. Qian, Z. Du, Interface engineering for efficient charge collection in Cu2O/ZnO heterojunction solar cells with ordered ZnO cavity-like nanopatterns, Sol. Energy Mater. Sol. Cells 116 (2013) 120–125.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
The influence of Cu2O crystal structure on the Cu2O/ZnO heterojunction photovoltaic performance Nezar G. Elfadill a,⇑, M.R. Hashim a,b, Khaled M. Chahrour a, M.A. Qaeed a, M. Bououdina c,d a
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, University Sains Malaysia, Penang 11800, Malaysia Institute of Nano-optoelectronics Research & Technology Laboratory (INOR), School of Physics, Universiti Sains Malaysia, USM, 11800 Penang, Malaysia c Nanotechnology Centre, College of Science, University of Bahrain, P.O. Box 32038, Bahrain d Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Bahrain b
a r t i c l e
i n f o
Article history: Received 22 June 2015 Received in revised form 1 July 2015 Accepted 4 July 2015 Available online 6 July 2015 Keywords: Metal oxides Cu2O/ZnO heterojunction Crystal growth Interface properties
a b s t r a c t Cuprous oxide (Cu2O) films were potentiostatically electrodeposited onto platinum (Pt) film coated onto silicon (Si) wafer from lactic solution at pH 9. The influence of applied potential on Cu2O crystal structure was carefully examined. At higher electrochemical applied potential, a polycrystalline structure was observed, and then as the applied potential decreased, a single crystalline structure oriented along (1 1 1) was obtained. Further decrease in the applied potential leads to the formation of a polycrystalline structure and finally at much lower applied potential, a single crystalline structure growing along (2 0 0) orientation (equivalent to (1 0 0) orientation) was revealed. Cu2O/ZnO heterojunction photodiodes based on these three crystal structures were fabricated and studied under dark and illuminated conditions. The best performance of the solar cell efficiency was achieved by the heterojunction based on (1 1 1) oriented Cu2O film (1.45%) compared to other structures (0.34% and 0.25%), which may be attributed to the formation of high quality heterojunction interface due to the heteroepitaxial-like growth of (0 0 2) oriented ZnO. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the fabrication of functional p–n heterojunction metal oxide optoelectronic devices with high interface quality received great interests. The crystal structure and the lattice symmetry of semiconductors play a critical role in determining many physical properties, such as cleavage, electronic band structure, and optical transparency. Compared to polycrystalline and amorphous semiconductors, single crystalline semiconductor possesses enhanced properties such as impeding the process of electron–hole pair recombination at grain boundaries, which in turn results in improving carrier life time and diffusion lengths. Among many metal oxide p–n heterojunctions, p-Cu2O/n-ZnO heterojunction receive great interest due to the non-toxicity and availability of its constituent elements. Cuprous oxide (Cu2O) is one of the most studied metal oxide semiconductor. It is a p-type metal oxide semiconductor with a direct energy bandgap of 2.1 eV and a large free-exciton binding energy of 140 meV [1,2]. On the other hand, the wide-band gap n-type ZnO was considered as a promising material that can be used as window layer in many solar cell structures due to its direct wide band gap (3.37 eV) [3]. The theoretical ⇑ Corresponding author. E-mail address: [email protected] (N.G. Elfadill). http://dx.doi.org/10.1016/j.spmi.2015.07.010 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
909
photovoltaic efficiency of Cu2O based solar cell is as high as 20% [4]. However, the conversion efficiency so far obtained is limited to 3.83% [5], which remains far below than the theoretical value. Growing a layer of single crystalline Cu2O was very much desired in order to improve the electronic properties of p-Cu2O/n-ZnO heterojunction interface. The conventional methods of growing single crystalline semiconductor require either high temperature or high vacuum. In contrast, the growth of crystalline materials using electrochemical deposition (called electrocrystallization) offers a simple and low-cost technique that operates at low temperature. Electrocrystallization is a process that produces conductive crystals from the mass transfer joined with charge transfer. Walsh and Herron [6] define the electrocrystallization as the process (or result) of a direct or indirect electrochemical influence on the crystallization, where direct influence refers to the domination of overpotential in the nucleation and growth of crystals, while indirect refers to the influence of local reaction (e.g. the pH) on the crystallization process. Usually, the electrodeposited Cu2O film is polycrystalline with a preferred orientation along (1 0 0) plane at pH 9 or along (1 1 1) at pH 12 [7– 9]. However, many researchers report the growth of single crystalline Cu2O thin film onto single crystalline metals layers [10,11] and single crystalline semiconductors layers [12,1]. In most heterojunctions, the key factor in building solar cell that can lead to the observed performance difference appears to be the degree of crystallinity and crystal face of the p-type and n-type exposed at the p–n junction interface. Due to the fact that crystal face contains a different atomic arrangement depending on which the plane is exposed at the surface, the termination of atoms and the deviation of surface coordination from ideal bulk coordination are expected to be different. The energy level, density, and distribution of interface states at the p–n junction can be affected by that difference and, therefore, the recombination losses at the interface. Therefore, the purpose of this work is to investigate the effect of the electrodeposition applied potential on both crystal structure and morphology of the deposited Cu2O thin films, as well as the influence of the crystal structure and morphology of the deposited Cu2O on the quality of p-Cu2O/n-ZnO heterojunction interface and photovoltaic properties. It should be noted here that platinum (Pt) thin film as electrode was selected to be pre-deposited onto silicon (Si) wafer because it has similar FCC crystal structure as Cu2O with a relatively low mismatch, i.e. 8.9%.
2. Material and methods As substrate preparation, Pt thin layer was firstly sputtered onto Si wafer. Cu2O films were then potentiostatically electrodeposited on the substrate using a conventional three electrode method. The applied potential was controlled by using eDAQ ER466 Integrated Potentiostat System, while the current during deposition was monitored. Ag/AgCl and Pt mesh (2.5 2.5) cm were used as reference electrode and counter electrode respectively. The electrolyte used to grow Cu2O consisted of 0.45 M cupric sulfate (CuSO4.5H2O, ACS reagent P 98%) and 3 M of lactic acid (C2H4OHCOOH, ACS reagent 85%) at pH = 9 adjusted by the addition of an amount of 5 M sodium hydroxide (NaOH, reagent grade P 98%). All chemicals were purchased from (Sigma Aldrich, USA). The solution was kept at a constant temperature of 70 °C during deposition process using water bath. To determine the electrochemical activities that take place in the reduction of copper-lactate solution, a linear sweep voltammogram was carried out between 0 and 1000 mV vs. Ag/AgCl, at a scan rate of 10 mV/s. The deposition was carried out with an applied potential ranging from 340 to 550 mV vs. Ag/AgCl with an increase step of 30 mV. ZnO nanorods of 1.2 lm were grown hydrothermally on sputtered pre-prepared 50 nm ZnO seed layer on the top of Cu2O/Pt/Si substrates. The solution used to grow ZnO nanostructures was equimolar 1:1 zinc nitrate hexahydrate (ZnNO3.6H2O, reagent grade 98%) and hexamethylenetetramine HMTA ((CH2)6N4, ACS reagent P 99%). All chemicals also were purchased from (Sigma Aldrich, USA). Finally, 200 nm of ITO (Sn-doped In2O3) was sputtered on the top of ZnO nanorods as transparent contact thin layer, and then the structure of the heterojunction was completed by Al electrode which was sputtered onto ITO film. The morphology of the electrodeposited Cu2O films was characterized by a field emission scanning electron microscopy (FESEM) using FEI/Nova NanoSEM 450. The structure of the films was studied by X-ray diffraction using PANalytical X’pert PRO diffractometer equipped with Cu Ka radiation (k = 1.5406 Å) and high-spatial resolution Raman spectroscopy using Jobin Yvon HR800UV spectrometer System. The heterojunction properties of p-Cu2O/n-ZnO were studied by measuring the dark and illuminated I–V characteristics using Keithley model 4200-SCS Semiconductor Characterization System.
3. Results and discussion The cathodic reduction of copper-lactate solution was extensively studied, and many reports showed that by varying the electrodeposition applied potential two reduction activities could be observed [13,8,14]. It has been noticed that at lower applied potential, the reduction of Cu2+ to Cu1+ was dominating at the cathode which produces pure phase of Cu2O film. With increasing applied potential, a new reduction activity from Cu2+ to Cu0 takes place and competes with the previous reduction activity (Cu2+ to Cu1+), thereby leading to the formation of a mixture of Cu metal and Cu2O phase. At higher applied potential, the Cu2+ to Cu0 reduction activity was dominating producing only metallic Cu film. The above two reaction activities were clearly observed in the linear sweep voltammogram shown in Fig. 1. However, the specific reduction activity
910
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 1. Linear sweep voltammogram for the reduction of copper-lactate at 70 °C and pH 9.
taking place at these potentials was unknown a priori, but by using XRD characterization one can determine the phase composition of thin films synthesized at these potentials. In Fig. 1, a sharp increase in the cathode current density was started around the applied potential of 340 mV and this was assigned to the reduction of Cu2+ to Cu1+ (only Cu2O phase was produced). This reduction process was extended in a narrow range from (340 to 520) mV. Starting from an applied potential of 550 mV and above, a mixture of Cu and Cu2O was obtained, which indicates that the reduction of Cu2+ to Cu1+ was initiating. Finally, at an applied potential of 780 mV and above the film was pure copper therefore the reduction of Cu2+ to Cu0 was dominating. In order to study the influence of the applied potential on the evolution of both structure and microstructure of electrodeposited Cu2O, the deposition was carried out with different applied potentials in the range of 340 to 520 mV. Fig. 2(a) shows the morphology of sputtered Pt onto Si wafer, showing a packed granular nanostructure. Fig. 2(b–f) shows the top view of electrodeposited Cu2O films deposited under different applied potentials (550, 520, 430, 400 and 340 mV). It can be noticed that the surface morphology of the film deposited at 550 and 520 mV is composed of grains with cubic-like shape, while films with a smooth surface with large grains is observed at 430 mV (Fig. 2(b) and (c). Finally, four sided pyramidal-like grains were revealed at 400 and 340 mV. This difference in the surface morphology was mainly caused by the variation of the films crystal orientation which is governed by the nucleation mechanisms. In general, the nucleation of electrodeposited Cu2O thin film depends on various deposition parameters. Switzer et al. [15], reported that the orientation of Cu2O film at higher over potential follows the orientation of the substrate (known
Fig. 2. FESEM micrographs of the morphology of the electrodeposited Cu2O (a) sputtered Pt film; (b) 550 mV; (c) 520 mV; (d) 430 mV; (e) 400 mV; (f) 340 mV.
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
911
as thermodynamics nucleation and growth mechanism) until it reaches a critical thickness, after which it changes to the kinetically nucleation and growth mechanism (pH dependent). Moreover, at low overpotential, the film follows the orientation of the substrate indefinitely. The thickness of the deposited film can be controlled by measuring the charge passed (Q) during deposition process in a period of time, according to Faraday law expressed by the following equation:
d¼
MQ nAqF
ð1Þ
where M and q are the molar mass and density of the deposit substance, Q the total charge passing during a fixed deposition time, F = 96485 C mol1 is Faraday’s constant, d and A are the thickness and area of Cu2O film, n is the number of electrons transferred by reduction of an ion. The total charge involved in the deposition process can be calculated from the integration of cathode current over a period of time. In our experiment, the total charge deposited was around 2.3 103 C, which expected to deposit a 5 lm of Cu2O layer. Fig. 3 shows the cross section FESEM micrograph of pure phase of Cu2O films prepared with different applied potentials. The cross section micrograph shows that the thickness of Cu2O films are nearly 5 lm except for the film deposited at 520 mV, which shows highest thickness (6 lm). Whereas all films shows a single phase of Cu2O produced by using same amount of transferred charge, this difference can be only attributed to the presence of voids within Cu2O film or in other words the film is not very compact. Fig. 4 shows the time–current transient during the Cu2O film electrodeposition growth process of grown Cu2O at four different applied potentials 520, 430, and 400 and 340 mV. In all experiments, the cathodic current density is high at the beginning of the deposition process then sharply decreases due the nucleation process of Cu2O. After that, the decreases in the cathodic current becomes very slow due to the increase in the resistance of the grown semiconductor film [16]. It is clearly noticed that the applied potential influences the cathodic current density and more importantly the value of current density at the nucleation time (the first 20 s). The current density at this stage determines the driving force for the nucleation process. As the applied potential decreases, the driving force for the nucleation process was reduced. Fig. 5 shows the evolution of XRD patterns of the as-prepared Cu2O films. Fig. 5(a) reveals peaks characteristic of the Pt sputtered layer in agreement with JCPDS cards No. 01-070-2431. Fig. 5(b–f) shows the XRD patterns for the electrodeposited Cu2O films under various potentials. As shown in Fig. 5(b) and (c), the XRD patterns at applied potential 550 and 520 mV reveal the formation of a mixture of Cu metal matching with JCPDS card No. 01-070-3038 and Cu2O phase matching with JCPDS card No. 01-078-0428. Fig. 5(d–f) shows the XRD patterns for the films grown at applied potential from 430 to 340 mV. Interestingly, various phase transformation has been observed: (i) from a single crystalline structure along
Fig. 3. FESEM micrographs of Cu2O cross sections prepared with different applied potentials (a) 520 mV; (b) 430 mV; (c) 400 mV; (d) 340 mV.
912
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 4. Electrodeposition cathodic current during deposition with applied potential (a) 520 mV; (b) 430 mV; (c) 400 mV; (d) 340 mV.
Fig. 5. XRD patterns of the sputtered Pt electrode and the electrodeposited Cu2O films (a) Pt film; (b) 550 mV; (c) 520 mV; (d) 430 mV; (e) 400 mV; (f) 340 mV.
(1 1 1) orientation at applied potential 430 mV; (ii) to a polycrystalline structure with (2 0 0) preferred orientation at 400 mV; (iii) and then to a single crystalline structure along (2 0 0) orientation. Previous studies showed that Cu2O electrodeposited from lactic solution at pH 9 produces polycrystalline structure with different I(2 0 0)/I(1 1 1) intensity ratios depending on the applied potentials [7,8], whereas in our case, only a single (1 1 1) peak of Cu2O was observed and as the growth potential decreases the structure changed to a polycrystalline and then again a single (2 0 0) peak was detected. Fig. 6 shows Raman spectra revealing characteristic peaks of crystalline Cu2O phase [17]. The most intense peak observed at 218 cm1 is assigned as 2EU phonon mode, the peak at 300 cm1 as A2U phonon mode whereas the peaks at 420 and 510 cm1 are assigned as region of multiphonon processes and finally the broad peak is assigned as an overlap of tow peaks (620, 645) cm1. In order to examine the role of different Cu2O crystal structure on the growth and performance of the Cu2O/ZnO solar cell, hydrothermally ZnO nanorods were grown onto Cu2O films using the same growth parameters as described earlier. Fig. 7(a–c) shows the top view of ZnO nanorods grown onto Cu2O substrates with (1 1 1) orientation, polycrystalline and
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
913
Fig. 6. Raman spectra of Cu2O films deposited with applied potential (a) 520 mV; (b) 430 mV; (c) 400 mV; (d) 340 mV.
Fig. 7. FESEM top view images of hydrothermally grown ZnO nanorods on (a) (1 1 1) oriented Cu2O; (b) polycrystalline Cu2O; (c) (2 0 0) oriented Cu2O.
Cu2O with (2 0 0) orientation, respectively. It is clear from Fig. 7 that all ZnO nanorods were hexagonal and aligned normal to the substrate surface. The ZnO nanorods grown onto Cu2O substrates with (1 1 1) orientation have the higher density and almost covered the entire substrate and become like a thin film (see also Fig. 8). The compactness of ZnO nanorods grown onto Cu2O with (1 1 1) orientation may be attributed to the higher nucleating density with lesser interface defects due to the small lattice mismatch 7.1% [18,19] between (1 1 1) plane of cubic Cu2O and (0 0 1) plane of hexagonal ZnO. For the other samples, the nanorods compactness and density was dramatically reduced. The Cu2O/ZnO heterojunctions crystal structure was confirmed by studying X-ray diffraction and Raman spectroscopy. Fig. 9(a) shows XRD pattern of the heterojunction prepared based on (1 1 1) oriented Cu2O film. Only a single (2 0 0) peak of ZnO was detected (matching with JCPDS card No. 36-1451), showing a heteroepitaxial-like growth of ZnO along (0 0 1) direction compared to the polycrystalline ZnO for the other samples as shown in Fig. 9(b) (sample based on (2 0 0) oriented Cu2O as representative). In addition, the Raman spectrum shown in Fig. 10 reveals characteristic peaks of ZnO and Cu2O phases in all samples; the peak located at 437 cm1 was assigned for the E2 (high) phonon mode [20] and the peak located at 331 cm1corresponds to the second order Raman spectrum arising from zone-boundary phonons E2 [21], respectively. In order to investigate the functionality of the grown nanostructures as solar cell, a typical structure of Pt/p-Cu2O/n-ZnO heterojunction diode was fabricated and shown in Fig. 8(b). The dark I–V characteristics of the three selected p-Cu2O/n-ZnO heterojunctions were shown in Fig. 11. The heterojunctions show a rectifying behavior. The external diode’s electrical properties such as rectifying ratio (R), saturation current density (Jsc) and ideality factor (n) are listed in Table 1. These parameters were extracted from semi-logarithmic plot of I–V curve using the following diode equation:
914
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 8. The microstructure of the Cu2O/ZnO heterojunction photodiode (a) FESEM cross section image (based on (1 1 1) oriented Cu2O); (b) schematic diagram.
Fig. 9. XRD patterns of the fabricated Cu2O-film/ZnO-nanorods prepared based on (a) (1 1 1) oriented Cu2O film; (b) (2 0 0) oriented Cu2O film.
Fig. 10. Raman spectrum of the fabricated Cu2O/ZnO (based on (1 1 1) oriented Cu2O).
qV 1 J ¼ J s exp nkT
ð2Þ
where Js is the saturation current density, n is the ideality factor, T is the absolute temperature and k the Boltzmann constant. The low extracted saturation current density (3.55 109Am/cm2) and the moderate value of ideality factor (2.5) reported in Table 1, for the heterojunction based on (1 1 1) Cu2O film compared to other samples indicate that the Cu2O/ZnO interface having lower interface defect density [22–24] and it is sufficiently good. This suggest that there was lower number of tunneling traps and consequently lower carrier leakage in the depletion region which in turn enhance the photovoltaic output of the heterojunction solar cell. The calculated rectifying ratio of the fabricated devices was very low in general, but the rectifying ratio for the device fabricated onto (1 1 1) oriented Cu2O (three orders of magnitude) is
915
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Fig. 11. Dark I–V characteristic of Cu2O/ZnO heterojunctions fabricated based on (a) (1 1 1) oriented Cu2O film; (b) polycrystalline Cu2O; (c) (2 0 0) oriented Cu2O film.
Table 1 Characteristic parameters of current–voltage measurements for Cu2O/ZnO heterojunctions. Cu2O preparation applied potential
Rectifying ratio (R)
Ideality factor (n)
Saturation current density Jo (A/cm2)
(1 1 1) Oriented Cu2O Polycrystalline (2 0 0) Oriented Cu2O
1900 32 16
2.5 3.06 3.22
3.55 109 3.16 108 4.0 108
Table 2 The photovoltaic parameters for the Cu2O/ZnO heterojunctions. Cu2O preparation applied potential
ISC (mA)
VOC (V)
Fill factor FF
Efficiency (%)
(1 1 1) Oriented Cu2O Polycrystalline (2 0 0) Oriented Cu2O
9.07 3.8 3.03
0.39 0.27 0.32
0.41 0.33 0.26
1.45 0.34 0.25
Fig. 12. Solar cell I–V curves of Cu2O/ZnO heterojunctions fabricated based on (a) (1 1 1) oriented Cu2O film; (b) polycrystalline Cu2O; (c) (2 0 0) oriented Cu2O film.
much better than the other devices. It seems to be the heteroepitaxial-like growth of (0 0 2) ZnO nanorods on the (1 1 1) Cu2O single crystal film that generates fewer surface states in the inter-band region, and hence better performance in the heterojunction. Also, it is found that all devices exhibit turn-on voltage around (1.2 V), which is comparable to turn-on voltage of Cu2O/ZnO heterojunction prepared by Wei et al. [25,26].
916
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
Finally, the photovoltaic effects in Pt/p-Cu2O/n-ZnO/Al heterojunctions were investigated under AM 1.5 at 100 mW/cm2 illumination. Fig. 12 shows the J–V curve of the three selected heterojunctions solar cells. A typical solar cell behavior was observed for the three selected heterojunctions. The heterojunction solar cell prepared based on (1 1 1) Cu2O film shows the highest efficiency. The short circuit current density, open circuit voltage, fill factor and efficiency were measured; i.e. 9.07 mA cm2, 0.39 V, 0.41% and 1.45%, respectively. The PV parameters for the other heterojunctions were measured and listed in Table 2. The enhancement in the efficiency for the solar cell prepared based on (1 1 1) Cu2O film was notable. However, compared to the electrodeposited Cu2O based solar cells, the efficiency in this study is higher than some previous research works reported in the literature [27–30]. On the other hand, the obtained results remain smaller to some extent compared to Cu2O/ZnO solar cells [5] (3.83%), which was fabricated by oxidation process. We attributed this difference to the high Cu2O resistivity of heterojunction fabricated in this work. Generally, the photovoltaic parameters are still very low for practical applications. 4. Conclusion The effect of electrochemical applied potential on the evolution of morphology and crystal structure of the electrodeposited Cu2O films was studied. At higher potential (520 to 460 mV), a polycrystalline Cu2O film was revealed. At lower applied potential (430 to 340 mV), interesting changes in the crystal structure of Cu2O films were observed: at 430 mV a smooth surface single crystalline structure oriented along (1 1 1), then at 340 mV a polycrystalline structure and finally at 340 mV a single crystalline structure growing along (2 0 0) orientation (equivalent to (1 0 0) orientation). This was clearly confirmed by XRD analysis. Additionally, Cu2O film/ZnO nanorods heterojunction solar cell based on different Cu2O crystal structures were fabricated. The best solar cell efficiency was achieved by the heterojunction based on (1 1 1) oriented Cu2O film (1.45%) compared to other structures (0.34% and 0.25%), which may be attributed to the formation of high quality heterojunction interface due to the heteroepitaxial-like growth of (0 0 2) oriented ZnO. Acknowledgments The provision of financial support from the Institute of Postgraduate Studies (IPS) Universiti Sains Malaysia (USM) Malaysia Fellowship and RU Grant No. 1001/PFIZIK/811175 are gratefully acknowledged. References [1] R. Liu, E.W. Bohannan, J.A. Switzer, F. Oba, F. Ernst, Epitaxial electrodeposition of Cu2O films onto InP (0 0 1), Appl. Phys. Lett. 83 (2003) 1944–1946. [2] N.G. Elfadill, M. Hashim, K. Saron, K.M. Chahrour, M. Qaeed, M. Bououdina, Ultraviolet–visible photo-response of p-Cu2O/n-ZnO heterojunction prepared on flexible (PET) substrate, Mater. Chem. Phys. 156 (2015) 54–60. [3] M. Zamfirescu, A. Kavokin, B. Gil, G. Malpuech, M. Kaliteevski, ZnO as a material mostly adapted for the realization of room-temperature polariton lasers, Phys. Rev. B 65 (2002) 161205–161209. [4] K. Cheng, Q.Q. Li, J. Meng, X. Han, Y.Q. Wu, S.J. Wang, L. Qian, Z.L. Du, Interface engineering for efficient charge collection in Cu2O/ZnO heterojunction solar cells with ordered ZnO cavity-like nanopatterns, Sol. Energy Mater. Sol. Cells 116 (2013) 120–125. [5] T. Minami, Y. Nishi, T. Miyata, J. Nomoto, High-efficiency oxide solar cells with ZnO/Cu2O heterojunction fabricated on thermally oxidized Cu2O sheets, Appl. Phys. Exp. 4 (2011) 23011–23013. [6] F.C. Walsh, M.E. Herron, Electrocrystallization and electrochemical control of crystal growth: fundamental considerations and electrodeposition of metals, J. Phys. D Appl. Phys. 24 (1991) 217–225. [7] S. Bijani, L. Martinez, M. Gabas, E.A. Dalchiele, J.R. Ramos-Barrado, Low-temperature electrodeposition of Cu2O thin films: modulation of micronanostructure by modifying the applied potential and electrolytic bath pH, J. Phys. Chem. C 113 (2009) 19482–19487. [8] L. Wang, N. De Tacconi, C. Chenthamarakshan, K. Rajeshwar, M. Tao, Electrodeposited copper oxide films: effect of bath pH on grain orientation and orientation-dependent interfacial behavior, Thin Solid Films 515 (2007) 3090–3095. [9] T.D. Golden, M.G. Shumsky, Y.C. Zhou, R.A. VanderWerf, R.A. VanLeeuwen, J.A. Switzer, Electrochemical deposition of copper(I) oxide films, Chem. Mater. 8 (1996) 2499–2504. [10] E.W. Bohannan, M.G. Shumsky, J.A. Switzer, Epitaxial electrodeposition of copper (I) oxide on single-crystal gold (1 0 0), Chem. Mater. 11 (1999) 2289– 2291. [11] J.K. Barton, A.A. Vertegel, E.W. Bohannan, J.A. Switzer, Epitaxial electrodeposition of copper (I) oxide on single-crystal copper, Chem. Mater. 13 (2001) 952–959. [12] J.A. Switzer, R. Liu, E.W. Bohannan, F. Ernst, Epitaxial electrodeposition of a crystalline metal oxide onto single-crystalline silicon, J. Phys. Chem. B 106 (2002) 12369–12372. [13] A.-L. Daltin, A. Addad, J.-P. Chopart, Potentiostatic deposition and characterization of cuprous oxide films and nanowires, J. Cryst. Growth 282 (2005) 414–420. [14] W. Septina, S. Ikeda, M.A. Khan, T. Hirai, T. Harada, M. Matsumura, L.M. Peter, Potentiostatic electrodeposition of cuprous oxide thin films for photovoltaic applications, Electrochim. Acta 56 (2011) 4882–4888. [15] J.A. Switzer, H.M. Kothari, E.W. Bohannan, Thermodynamic to kinetic transition in epitaxial electrodeposition, J. Phys. Chem. B 106 (2002) 4027–4031. [16] R. Inguanta, S. Piazza, C. Sunseri, Template electrosynthesis of aligned Cu2O nanowires: Part I. Fabrication and characterization, Electrochim. Acta 53 (2008) 6504–6512. [17] P. Dawson, M. Hargreave, G. Wilkinson, The dielectric and lattice vibrational spectrum of cuprous oxide, J. Phys. Chem. Solids 34 (1973) 2201–2208. [18] S.-M. Chou, M.-H. Hon, C. Leu, Y.-H. Lee, Al-doped ZnO/Cu2O heterojunction fabricated on (2 0 0) and (1 1 1)-orientated Cu2O substrates, J. Electrochem. Soc. 155 (2008) H923–H928. [19] B. Mohamad Fariza, J. Sasano, T. Shinagawa, H. Nakano, S. Watase, M. Izaki, Electrochemical growth of (0 0 0 1)-n-ZnO film on (1 1 1)-p-Cu2O film and the characterization of the heterojunction diode, J. Electrochem. Soc. 158 (2011) D621. [20] K. Saron, M. Hashim, M. Farrukh, Stress control in ZnO films on GaN/Al2O3 via wet oxidation of Zn under various temperatures, Appl. Surf. Sci. 258 (2012) 5200–5205. [21] K.C. Chin, C.K. Poh, G.L. Chong, J. Lin, C.H. Sow, A.T.S. Wee, Large area, rapid growth of two-dimensional ZnO nanosheets and their field emission performances, Appl. Phys. A 90 (2007) 623–627.
N.G. Elfadill et al. / Superlattices and Microstructures 85 (2015) 908–917
917
[22] N.G. Elfadill, M.R. Hashim, K.A.T. Thabit, The role of using seed-layer assisted electrodeposition method on the growth and the photovoltaic properties of p-Cu2O/n-Si heterojunctions, J. Mater. Sci.: Mater. Electron. 26 (2014) 985–991. [23] P. Lv, W. Zheng, L. Lin, F. Peng, Z. Huang, F. Lai, I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction, Physica B 406 (2011) 1253–1257. [24] C.-L. Kuo, R.-C. Wang, J.-L. Huang, C.-P. Liu, C.-K. Wang, S.-P. Chang, W.-H. Chu, C.-H. Wang, C.-H. Tu, The synthesis and electrical characterization of Cu2O/Al: ZnO radial p–n junction nanowire arrays, Nanotechnology 20 (2009) 365603–365607. [25] P. Lv, L. Lin, W. Zheng, M. Zheng, F. Lai, Photosensitivity of ZnO/Cu2O thin film heterojunction, Optik 124 (2013) 2654–2657. [26] H.M. Wei, H.B. Gong, Y.Z. Wang, X.L. Hu, L. Chen, H.Y. Xu, P. Liu, B.Q. Cao, Three kinds of Cu2O/ZnO heterostructure solar cells fabricated with electrochemical deposition and their structure-related photovoltaic properties, CrystEngComm 13 (2011) 6065–6070. [27] J.B. Cui, U.J. Gibson, A simple two-step electrodeposition of Cu2O/ZnO nanopillar solar cells, J. Phys. Chem. C 114 (2010) 6408–6412. [28] J. Katayama, K. Ito, M. Matsuoka, J. Tamaki, Performance of Cu2O/ZnO solar cell prepared by two-step electrodeposition, J. Appl. Electrochem. 34 (2004) 687–692. [29] Y.-K. Hsu, H.-H. Lin, J.-R. Wu, M.-H. Chen, Y.-C. Chen, Y.-G. Lin, Electrochemical growth and characterization of a p-Cu2O thin film on n-ZnO nanorods for solar cell application, RSC Adv. 4 (2014) 7655–7659. [30] K. Cheng, Q. Li, J. Meng, X. Han, Y. Wu, S. Wang, L. Qian, Z. Du, Interface engineering for efficient charge collection in Cu2O/ZnO heterojunction solar cells with ordered ZnO cavity-like nanopatterns, Sol. Energy Mater. Sol. Cells 116 (2013) 120–125.