In this study, a comprehensive study of 2D ZnO made by sol-gel, atomic layer deposition (ALD) method, 3D ZnO nanorods grown from sol-gel seed layer and ALD seed layer was carried out. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the morphology of the films.
Journal of Science: Advanced Materials and Devices (2018) 428e432 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Physical properties of 2D and 3D ZnO materials fabricated by multi-methods and their photoelectric effect on organic solar cells Sheng Bi a, b, Yu Li a, b, Yun Liu c, Zhongliang Ouyang d, Chengming Jiang a, b, * a Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian, 116024, PR China b Institute of Photoelectric Nanoscience and Nanotechnology, Dalian University of Technology, Dalian, 116024, PR China c School of Physics, Dalian University of Technology, Dalian, 116024, China d Department of Electrical and Computer Engineering, Center for Materials for Information Technology, The University of Alabama, Box# 870209, Tuscaloosa, AL, 35487, USA a r t i c l e i n f o a b s t r a c t Article history: Received October 2018 Received in revised form 12 November 2018 Accepted 12 November 2018 Available online 20 November 2018 ZnO material is a crucial layer for organic solar cell due to its excellent photoelectric properties However, the influence of ZnO fabricated by various methods as well as the effect of 2-dimensional (2D) and 3dimensional (3D) configuration is still under debate In this study, a comprehensive study of 2D ZnO made by sol-gel, atomic layer deposition (ALD) method, 3D ZnO nanorods grown from sol-gel seed layer and ALD seed layer was carried out Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the morphology of the films X-ray photoelectron spectroscopy (XPS) were used to probe bonding nature as well as defects present in different forms of ZnO Band gap and crystal quality were characterized by UV-vis spectra The inverted structure of organic solar cells was fabricated using these forms of ZnO, and the I-V curves as well as the power conversion efficiency (PCE) were measured to evaluate the photoelectric property of the synthesized ZnO nanostructures It is found that 2D ZnO made by the sol-gel method yields the best PCE of 2.53% © 2018 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Introduction Nanoscale control of metal oxide architectures is a crucial technique to enhance the performance of the devices [1e6] Zinc oxide (ZnO) is regarded as one of the most widely used materials due to its extraordinary photoelectric properties, significant physical, chemical, mechanical, and bio-compatible characteristics [7,8] It is a semiconductor with a large bandgap of 3.4 eV and a large excitation binding energy of 60 meV that possesses applications in optoelectronic devices [9], such as solar cells [10e15], light emitting diodes [16], and image sensors [17] Chemical vapor deposition [18], pulsed laser deposition [19], solegel process [20], electrochemical deposition [21] and numerous other methods have been adopted to synthesize different kinds of ZnO nanostructures, such as nanorods, nanotubes, nanobelts and nanofilms etc Among these methods, * Corresponding author Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian, 116024, PR China E-mail address: jiangcm@dlut.edu.cn (C Jiang) Peer review under responsibility of Vietnam National University, Hanoi the sol-gel method with an excellent compositional control, low crystallization temperature and preferential orientation and photoluminescence [20], and atomic layer deposition (ALD) method [22,23] is widely used due to a number of advantages However, for the organic solar cell application, influences of 2-dimensional and 3-dimensional ZnO nanostructures as well as the function of various ZnO fabrication methods are still unknown In this paper, 2-dimensional and 3-dimensional ZnO fabricated by both sol-gel and ALD method were studied and the efficiency of organic solar cells with an inverted structure was evaluated by applying these ZnO layers P3HT and PCBM were used as benchmark for the study Quality of the 2D films and 3D nanorods grown by various methods was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM) Binding energy of the 2D and 3D ZnO films was tested by X-ray photoelectron spectroscopy (XPS) Absorbance properties were measured by ultraviolet-visible (UV-vis) spectrometer Organic solar cell devices fabricated using the above 2D and 3D ZnO further confirm the photoelectric property of the materials made by these methods It is anticipated that our findings will contribute to the development of the field https://doi.org/10.1016/j.jsamd.2018.11.003 2468-2179/© 2018 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) S Bi et al / Journal of Science: Advanced Materials and Devices (2018) 428e432 Experimental P3HT and PCBM were purchased from Solarmer and Nano-C, respectively, and used as received ITO glass was cleaned in detergent, de-ionized water, acetone and isopropyl alcohol in sequence, and treated with oxygen plasma at 30 W for In sol-gel method for 2D ZnO film fabrication, appropriate amount of zinc acetate dihydrate (Zn(CH3COO)2$2H2O) was dissolved in 2-methoxyethanol (CH3OCH2CH2OH) with ethanolamine (NH2CH2CH2OH) as additive and vigorous stirred for 24 h to form 0.1 mM sol-gel ZnO solution In ALD method, diethylzinc solution and water were used to synthesize 60 nm-thick ZnO film according to the formula n (C25)2 ỵ ẳ n þ 2C2Н6 In nanorod growth, 10 nm-thick seed layer was deposited onto pre-cleaned ITO glass by either ALD method or sol-gel method followed by hydrothermal method for h at 90 C P3HT-PCBM (1:1 wt, concentration of 25 mg/mL in chlorobenzene) was spin-coated onto the pre-fabricated ZnO film at the spin-speed of 900 rpm for 45 s MoO3 was thermally evaporated at the rate of 0.3 Å/s for 10 nm followed by silver deposition at the rate of Å/s for 60 nm I-V characterization of polymer photovoltaic cells was conducted using a computer-controlled measurement unit from Agilent technologies B1500A semiconductor parameter analyzer under ambient condition with illumination of AM1.5G, 100 mW/cm2 Thin Au was sputtered onto the sample to increase the conductivity of the ITO glass for electron transportation The samples were then transferred into the chamber of SEM for observation Fig illustrates the structure of organic solar cells with 2D and 3D ZnO layers Fig 1(a) is the configuration of the devices directly fabricated by 2D ZnO film from ALD and sol-gel methods, while 3D ZnO nanorods were grown from thin seed layers deposited by ALD and sol-gel methods, as shown in Fig 1(b) Results and discussion Morphology of the 3D ZnO nanorods was taken by SEM as shown in Fig 2(a) and (b) and the roughness of the 2D films were tested by AFM as displayed in Fig 2(c) and (d) The two SEM images share the same scale bar of 100 nm and the scale bars of the inset are both 500 nm In the SEM image, the ZnO nanorods grown from both ALD seed layer and sol-gel seed layer demonstrate a dense 3D structure and it is clearly revealed that the ZnO nanorods were successfully grown from depositional thin seed layer, but the latter exhibits superior vertical orientation Both of the nanorods present a hexagonal crystal growth indicating the presence of the ZnO wurtzite crystal lattice AFM measurements were carried out to test the quality and uniformity of the 2D ZnO film In the 2D film made by sol-gel method, highly ordered surface texture was observed with a roughness of 0.425 nm On the contrary, the morphology of the ALD film indicates a granular property and aggregates nanoparticles The roughness of the ALD 2D film is 3.486 nm 429 Morphology of the 3D ZnO nanorods is crucial for the performance of the organic solar cells There are several reasons that might result in lower device performance with 3D ZnO nanorod Firstly, the density of ZnO nanorod might play an important role The closely packing ZnO nanorods prevent P3HT-PCBM solution from getting well contact with ZnO and ITO layer As a consequence, charge carriers are getting blocked at P3HT-PCBM/ZnO interface, which eventually lead to much lower charge carrier transportation and higher recombination Also, not upright-grown nanorods will also result in the same problem [24] On the other hand, thin but long nanorods will result in a high resistance It is well understood that the resistance depends on the cross section area and the length of the channel The ZnO nanorods act as a channel for charge carrier transportation Higher resistance would result in lower fill factor and S-shape currentedensity curves Furthermore, the length of the ZnO nanorods is also important It is known that longer grow time leads to long but more vertically straight nanorods However, if the nanorods are too long to break through the top electrode of the organic solar cells, there would be a short circuit and a mismatched band diagram, which eventually exhibit no or low efficiency [25] A further study is necessary to explore the structural properties of the 2D ZnO films and 3D ZnO seeded nanorods made by the two methods Zn 2P3=2 were tested using XPS According to Fig 3, preliminary observation indicates the films from sol-gel method have a higher binding energy compared to that from ALD method The binding energy of ZnO nanorods from sol-gel seed layer has the lowest binding energy The Zn 2P3=2 peaks of sol-gel film and ALD film were observed at 1022.1 eV and 1021.9 eV, respectively, which is the binding energy between zinc and oxygen Compared with the films made by ALD, the Zn 2P3=2 peak of nanorods made from ALD seed layers demonstrates a right shift of around 0.4 eV On the contrary, the Zn 2P3=2 peak of nanorods fabricated from sol-gel seed layer nanorods shift to the lower binding energy The positive shift is mainly due to the oxygen vacancies on the utmost surface or the increasing concentration of acceptors such as zinc vacancies, which cause surface energy variation [26,27] Also, the negative shift can be explained by the oxygen vacancies resides within the bulk ZnO [26,27] It is reasonably understood that 2D ZnO films made by solgel and ALD methods may exit internal defects, while 3D ZnO nanorods fabricated from ALD seed layer and sol-gel seed layer may exit surface defects The internal defects may introduce defect level and the surface defects may act as recombination centers for charge carriers The less the surface defects in ZnO film, the less negative effects on and carrier mobility and thus worse power conversion efficiency (PCE) [28] The more surfaces and interfaces in nanorods also increase the possibility of recombination, which lowers the PCE of organic solar cells Band gap as well as absorbance of ZnO are other key factors that possess great influence on the performance of the solar cells The band gap calculated by Tacu plot (in Equation (1)) method for ZnO Fig Solar cell structure of ITO\ZnO\P3HT-PCBM\MoO3\Ag with (a) 2-dimensional ZnO film and (b) 3-dimensional seeded ZnO nanorods 430 S Bi et al / Journal of Science: Advanced Materials and Devices (2018) 428e432 Fig Top view of SEM image of 3D ZnO film from (a) sol-gel method seed layer and (b) ALD seed layer ZnO nanorods with side-view image in the inset AFM image of the 2D ZnO film fabricated by (c) sol-gel method and (d) ALD method fabricated by sol-gel, ALD, ALD seed, sol-gel seed are 3.4 eV, 3.25 eV, 3.21 eV and 3.1 eV, respectively = （ahv） n À Á ¼ A hv À Eg (1) where a is absorption coefficient, h is Planck constant, V is frequency, Eg is semiconductor forbidden bandwidth, A is constant which depends on the type of the semiconductor: 0.5 for direct band and for indirect band Compared with the band gap of intrinsic ZnO, which is 3.302 eV, the band gap of ZnO made by sol-gel method is higher than that of intrinsic ZnO According to the Burstein-Moss band gap compensation effect, the Fermi energy of ZnO made by sol-gel method is closer to the bottom of conduction band which leads to a wider band gap and higher carrier concentration [29] It is well known that defect is one of the reasons that causes band gap variation UV-vis spectra of both 2D films and 3D nanorods were illustrated in Fig to explore the type of defect as well as crystal quality Compared with 3D nanorods made from ALD seed layer, the 2D ALD films show a red-shift in absorbance Fig XPS curves of (a) the sol-gel film, (b) ALD film, (c) ALD seeded nanorods, and (d) sol-gel seeded nanorods indicating the enhanced oxidability On the contrary, the sol-gel 2D films display a blue-shift in absorbance compared to the 3D nanorods fabricated from sol-gel seed layer demonstrating improved reductibility Moreover, when light shines onto the device, absorption, reflection and transmission will act on films The more transmission, the better chance the light will reach the active layer which will generate charge carriers and thus generate electricity It is observed from the figure that ZnO made by ALD has the highest absorbance which might result in less light reaches to P3HT and PCBM layer The absorbance of the ZnO fabricated by sol-gel method is the lowest, which might give the best external quantum efficiency (EQE) to the device Performance of the organic solar cells fabricated with different kinds of ZnO films was measured and I-V curve were plotted in Fig UV-vis spectra of the ALD film, sol-gel film, ALD seeded nanorods and sol-gel seeded nanorods S Bi et al / Journal of Science: Advanced Materials and Devices (2018) 428e432 431 highest PCE of 2.53% We hope our finding will promote the development of the field Acknowledgments This project was financially supported by National Natural Science Foundation of China (NSFC, 51702035 and 51602056), and Dalian University of Technology, China, DUT16RC(3)051 References Fig I-V curves of ZnO made from ALD, sol-gel method, ALD seeded nanorods and sol-gel seeded nanorods Table Physical parameters of solar cell with different kinds of ZnO deposition methods: open circuit voltage (VOC), short circuit current (ISC), fill factor (FF) and power conversion efficiency (PCE) ALD sol-gel ALD seeded sol-gel seeded VOC (V) ISC (mA/cm2) FF (%) PCE (%) 0.49 0.57 0.27 0.17 6.255 8.221 5.388 6.690 31.05 53.99 41.72 37.28 0.95 2.53 0.61 0.42 Fig The ZnO film from sol-gel methods gives a beautiful curve with highest open circuit voltage (VOC) and short circuit current (ISC), while the ALD film shows an S-shape curve The ones from the ZnO nanorods have comparably low VOC The physical parameters of the devices made by various ZnO nanostructures are summarized in Table The performance of the solar cells with 2D ZnO fabricated by the sol-gel method is better than the 3D ZnO from the sol-gel seed layer The performance of the solar cells with 2D ZnO fabricated by the ALD method is also better than the 3D ZnO from the ALD seed layer The 2D ZnO made by the sol-gel method is better than that from the ALD method, while the 3D ZnO fabricated from the sol-gel seed layer is worse than that from the ALD seed layer Conclusion The 2D and 3D ZnO materials prepared by ALD, sol-gel, ALDseed and sol-gel seed methods were adopted to fabricate P3HTPCBM based solar cells The SEM and AFM characterizations indicate that the morphology of ZnO made by these methods has a distinct variation in the aspect of film roughness and the dimension as well as the direction of nanorods Also, the existence of zinc and oxygen vacancies in ZnO from different fabrication methods leads to band gap alternation ZnO fabricated by 2D sol-gel method has a higher band gap than that of 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than the 3D ZnO from the ALD seed layer The 2D ZnO made by the... gap compensation effect, the Fermi energy of ZnO made by sol-gel method is closer to the bottom of conduction band which leads to a wider band gap and higher carrier concentration [29] It is