Changing the thickness of two layers: i-ZnO nanorods, p-Cu2O and its influence on the carriers transport mechanism of the p-Cu2O/i-ZnO nanorods/n-IGZO heterojunction
© The Author(s) 2016
Received: 3 November 2015
Accepted: 29 May 2016
Published: 13 June 2016
In this study, two layers: i-ZnO nanorods and p-Cu2O were fabricated by electrochemical deposition. The fabricating process was the initial formation of ZnO nanorods layer on the n-IGZO thin film which was prepared by sputtering method, then a p-Cu2O layer was deposited on top of rods to form the p-Cu2O/i-ZnO nanorods/n-ZnO heterojunction. The XRD, SEM, UV–VIS, I–V characteristics methods were used to define structure, optical and electrical properties of these heterojunction layers. The fabricating conditions and thickness of the Cu2O layers significantly affected to the formation, microstructure, electrical and optical properties of the junction. The length of i-ZnO nanorods layer in the structure of the heterojunction has strongly affected to the carriers transport mechanism and performance of this heterojunction.
KeywordsElectrochemical method ZnO nanorods Heterojunction Cu2O layer Solar cells
Renewable energy is expected to replace depleting fossil energy sources in order to ensure energy security and overcome the problem of global climate change. Currently, when the demand for energy is increasing, the manufacture of cheap and durable solar cells is an essential requirement. As opposed to the high cost of single crystal silicon, the metal oxide semiconductors are suitable options for solar cells fabrication because of the diversity and simplicity in manufacturing of them (Abdu and Musa 2009). The metal oxide semiconductors prepared by thin film technology can save material and production costs. In addition, the structure of these semiconductors can be easily adjusted. Therefore, the suitable electrical and optical properties are easily obtained for forming the optoelectronic devices based on heterojunctions (Chen 2013).
Among the oxide semiconductors, zinc oxide (ZnO) and cuprous oxide (Cu2O) are attractive of many scientists because they have favorable photoelectric properties and economic values, such as suitable bandgap, good thermal stability, low-cost and environment-friendly material (Wang et al. 2011). ZnO is an n-type semiconductor with a direct bandgap. The bandgap energy of ZnO is about 3.37 eV corresponding to exciton bounding energy of 60 meV. The improving in optical and electronic properties of ZnO by doping metal atoms such as Ga, In, Al etc. made it specially suitable for n-type electrode materials of solar cells because of the high transmittance in the visible wavelength region and high electron concentration (Kidowaki et al. 2012). Especially, the 1D ZnO nanostructures that have the larger surface area and high electron mobility are promising in enhancing the ability to the separation and transmission of carriers (Baek et al. 2013). However, there are many difficult problems in preparing of the p-type ZnO semiconductor that lead to unstable electrical capacity. Therefore, it is difficult to get homojunction based on n-type and p-type of ZnO (Gershon et al. 2013). As noted above, the p-type ZnO layer need to be replaced by another semiconductor. Among metal oxide semiconductors, Cu2O shows up as a bright candidate. Naturally, Cu2O is a p-type semiconductor due to the present of Cu+ vacancy in crystalline structure. Its potential for solar cells was revived during the mid-seventies as a possible low-cost material (De Jongh et al. 1999). The bandgap of Cu2O semiconductor is about 2.17 eV and this kind of semiconductor has absorption edge in visible range. The absorption coefficient of Cu2O is higher than single crystalline Si therefore it has been considered as a potential material for the light absorbing layer in solar cells (Zoolfakar et al. 2012).
Base on Shockley–Queisser theory, the power conversion efficiency is about 20 % could be obtained from the thin film solar cell made of n-type ZnO and p-type Cu2O heterojunction (Cheng et al. 2013). In such heterojunction cells, the Cu2O layers are generally prepared by many physical and chemical techniques such as thermal oxidation of metallic Cu sheet, DC and RF sputtering, pulse laser deposition, photochemical deposition, chemical vapor deposition, and electrochemical deposition. Among them, the electrochemical deposition method has several advantages such as low cost, saving material, simple fabrication, and easy application (Jeong et al. 2008). However, results from many reports have showed that the conversion efficiency of ZnO/Cu2O heterojunction prepared by electrochemical method was still low in range 0.007–0.2 % because of two main reasons: the quality of crystal structure affected the electrical conductivity and absorption capacity of the Cu2O layer, and the defects at interface between two layers trapped carriers and produced a tunnel recombination process (Lv et al. 2013).
In present work, a ZnO nanorods layer was deposited in the middle of two layers: n-IGZO and p-Cu2O to investigate the carriers transport mechanisms and performance of the junction. The ZnO nanorods layer was also prepared by electrochemical deposition. This layer worked as intrinsic layer and had effective contributions in separate, transport mechanisms of carriers.
Second, the i-ZnO nanorods layer was grown on IGZO substrates by using the electrochemical deposition. The electrolyte was prepared by adding of 0.05 M Zn(NO3)2·6H2O and 0.05 M C6H12N4. This solution in electrolytic tank was heated to a temperature of 80 °C. In the solution, there were many happened processes, first Zn(NO3)2 was dissociated to form two Zn2+ and NO3 − ions then NO3 − ions combined with water in solution and two electrons to form two ions: NO2 − and OH−. Besides, C6H12N4 decomposed into NH3 and HCHO. The NH3 reacted with water to produce NH4+ and OH−. Through two process, OH− ions were continuously provided for the formation of Zn(OH)2 which subsequently formed ZnO. The ZnO nanorods layer was electrodeposited at 80 °C for 60 min. After that, the sample was rinsed with distilled water and transferred in the Cu2O electrodeposition bath. The Cu2O layer was prepared in solution of Copper (II) sulfate (CuSO4, 0.02 mol L−1) and Lactic acid (4 mol L−1). The pH of the electrochemical solution was adjusted to 11 by adding NaOH. The electrolyte temperature was kept at 70 °C during electrochemical process. The current density at 0.1 and 0.15 mA cm−2 for two steps were set up to growth of Cu2O crystals. In step 1, the seed layer was prepared according to current density of 0.1 mA cm−2 in order to make the bonding ability between Cu2O seeds and ZnO nanorods. After that, the thickness of Cu2O was grown by step 2 and the sample thickness could be adjusted by the change of deposition time (Jeong et al. 2013). The silver paste was used as a back contact of the p-Cu2O/i-ZnO nanorods/n-IGZO heterojunction.
The morphology and size of the product were analyzed by using scanning electron microscopy (SEM). X-ray diffraction (XRD) patterns to determine the crystalline structure of the samples were obtained by using a D8 ADVANCE-BRUKER system with Cu Kα primary X-rays. The optical spectra were recorded by using UV–Vis Jasco V-530 in the wavelength range of 200 to 1100 nm. The Keithley K2612A source and Agilent 4294 Precision Impedance Analyser were used to measure the electrical properties of the heterojunction. The photoelectric properties were performed by using a solar simulator (XES-40S1, San-Ei) equipped with AM 1.5 G filters used at 100 mW/cm2. The solar cells were illuminated through the side of the IGZO substrate, and the illuminated area was 0.25 cm2.
Results and discussion
Morphology and crystal structure
The top-view of Cu2O surface revealed the fact that the cubic crystalline structure of Cu2O was obtained from electrochemical method. The size of Cu2O crystal is about 1–2 μm and no ZnO nanorod is observed on the surface have proved the well-contact between two layers: ZnO nanorods and Cu2O. In confirmation, uniformly distinctive hexagonal morphologies with clear boundaries observed in the SEM images will lead to decreasing of defects at interface and improving charge collection efficiency of heterojunction (Baek et al. 2013).
In the case of the ZnO nanorods on IGZO substrate, the high intensity peak at 2θ = 34.3o revealed the fact that ZnO nanorods have wurtzite structure and orientation of (002) plane. When a Cu2O layer was deposited on top of ZnO nanorods, the intensity of ZnO (002) peak was also decreased clearly due to the Cu2O layer on the ZnO nanorods. The effect of IGZO substrate and ZnO nanorods layer was so strong that no peak of Cu2O structure was observed clearly. However, the XRD pattern of Cu2O/ZnO nanorods/IGZO heterojunction in range of 2θ from 35° to 60° has revealed cubic structure of Cu2O at (111) and (200) planes according to 2θ = 36.45° and 42.35° (Jeong et al. 2013). The weak peak at Cu2O (111) orientation compared with the strong ZnO (002) peak has indicated that the seeds layer of Cu2O was formed on the surface of ZnO nanorods and small crystals of Cu2O were randomly distributed on interface of heterojunction (Perng et al. 2013). This lead to some advances in carriers transport capability of solar cell based on the p-Cu2O/i-ZnO nanorods/n-IGZO heterojunction.
In this work, the p-Cu2O/i-ZnO nanorods/n-IGZO heterojunction was fabricated by electrochemical, sputtering method. The ZnO nanorods layer was deposited between two layers: n-IGZO and p-Cu2O to investigate the carrier transport mechanisms and performance of the junction. The clear boundaries were observed between two layers. The absorption range of the IGZO/ZnO nanorods/Cu2O heterojunction was expanded from 400 to 800 nm because of the high absorption coefficient in the visible range of the Cu2O layer. It is found that the length ZnO nanorods layer has contributed to carriers-collection and carriers-separation capability of heterojunction.
LVTH leads the research group. NHK analyzed the results and wrote this manuscript, LTTT, PKP, PTKL did experiments and calculations. DAT, NHT, CVT discussed and supplied IGZO thin films. All authors read and approved the final manuscript.
The authors would like to thank Lam Nguyen for his constructive suggestions.
The authors declare that they have no competing interests.
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