Physical properties of 2D and 3D ZnO materials fabricated by multi-methods and their photoelectric effect on organic solar cells

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.

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Original Article

Physical properties of 2D and 3D ZnO materials fabricated by

multi-methods and their photoelectric effect on organic solar cells

Sheng Bia,b, Yu Lia,b, Yun Liuc, Zhongliang Ouyangd, Chengming Jianga,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

Article history:

Received 9 October 2018

Received in revised form

12 November 2018

Accepted 12 November 2018

Available online 20 November 2018

a b s t r a c t 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 3-dimensional (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 thefilms 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%

open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 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

phys-ical, chemphys-ical, mechanphys-ical, 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,

the sol-gel method with an excellent compositional control, low crystallization temperature and preferential orientation and pho-toluminescence[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 fabri-cated 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 2Dfilms 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 ZnOfilms 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 theﬁeld

* Corresponding author Key Laboratory for Precision and Non-traditional

Machining Technology of the Ministry of Education, Dalian University of

Technol-ogy, Dalian, 116024, PR China.

E-mail address: jiangcm@dlut.edu.cn (C Jiang).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.11.003

Journal of Science: Advanced Materials and Devices 3 (2018) 428e432

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2 Experimental

P3HT and PCBM were purchased from Solarmer and Nano-C,

respectively, and used as received ITO glass was cleaned in

deter-gent, de-ionized water, acetone and isopropyl alcohol in sequence,

and treated with oxygen plasma at 30 W for 5 min In sol-gel method

for 2D ZnOﬁlm 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 Ζn

(C2Н5)2þ Н2Ο ¼ ΖnΟ 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

2 h at 90C P3HT-PCBM (1:1 wt, concentration of 25 mg/mL in

chlorobenzene) was spin-coated onto the pre-fabricated ZnOﬁlm at

the spin-speed of 900 rpm for 45 s MoO3was thermally evaporated

at the rate of 0.3Å/s for 10 nm followed by silver deposition at the

rate of 1Å/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 1

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 inFig 1(b)

3 Results and discussion

Morphology of the 3D ZnO nanorods was taken by SEM as

shown inFig 2(a) and (b) and the roughness of the 2Dﬁlms were

tested by AFM as displayed inFig 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 ZnOﬁlm 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 ALDﬁlm indicates a granular property and aggregates

nano-particles The roughness of the ALD 2Dﬁlm is 3.486 nm

Morphology of the 3D ZnO nanorods is crucial for the perfor-mance 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 lowerﬁll 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 mis-matched band diagram, which eventually exhibit no or low

efﬁciency[25]

A further study is necessary to explore the structural properties

of the 2D ZnOfilms 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=2peaks of sol-gelfilm 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 =2peak of nanorods made from ALD

seed layers demonstrates a right shift of around 0.4 eV On the contrary, the Zn 2P3=2peak 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 ZnOﬁlms made by sol-gel 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 ZnOﬁlm, 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

ﬁlm and (b) 3-dimensional seeded ZnO nanorods.

S Bi et al / Journal of Science: Advanced Materials and Devices 3 (2018) 428e432 429

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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）1n= ¼ A hv Eg

(1) wherea is absorption coefﬁcient, h is Planck constant, V is

fre-quency, Eg is semiconductor forbidden bandwidth, A is constant

which depends on the type of the semiconductor: 0.5 for direct

band and 2 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

concen-tration[29] It is well known that defect is one of the reasons that

causes band gap variation UV-vis spectra of both 2Dﬁlms and 3D

nanorods were illustrated inFig 4to 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

indicating the enhanced oxidability On the contrary, the sol-gel 2Dfilms 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 de-vice, 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 elec-tricity It is observed from thefigure 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 quan-tum efficiency (EQE) to the device

Performance of the organic solar cells fabricated with different kinds of ZnOﬁlms was measured and I-V curve were plotted in

Fig 2 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.

Fig 3 XPS curves of (a) the sol-gel film, (b) ALD film, (c) ALD seeded nanorods, and (d) Fig 4 UV-vis spectra of the ALD film, sol-gel film, ALD seeded nanorods and sol-gel

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Fig 5 The ZnOﬁlm from sol-gel methods gives a beautiful curve

with highest open circuit voltage (VOC) and short circuit current

(ISC), while the ALDﬁlm 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 1 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

4 Conclusion

The 2D and 3D ZnO materials prepared by ALD, sol-gel,

ALD-seed and sol-gel ALD-seed methods were adopted to fabricate

P3HT-PCBM based solar cells The SEM and AFM characterizations

indi-cate that the morphology of ZnO made by these methods has a

distinct variation in the aspect ofﬁlm 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 intrinsic ZnO, while the band gaps of

ZnO fabricated by other methods are lower than that of intrinsic

ZnO Furthermore, UV-vis spectra exhibit not only the transmission

property of ZnO, but also the quality of crystal structures, which is

reﬂected by the red-shift and blue-shift in the spectra It is found

that 2D ZnO from the sol-gel method has the best crystal structure

while 3D ZnO from the sol-gel seed layer gives the worst property

By fabricating the inverted P3HT-PCBM solar cell with these ZnO

layers, it is found that the one with the 2D sol-gel method yields the

highest PCE of 2.53% We hope our ﬁnding will promote the development of theﬁeld

Acknowledgments This project wasﬁnancially supported by National Natural Sci-ence Foundation of China (NSFC, 51702035 and 51602056), and Dalian University of Technology, China, DUT16RC(3)051

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Fig 5 I-V curves of ZnO made from ALD, sol-gel method, ALD seeded nanorods and

sol-gel seeded nanorods.

Table 1

Physical parameters of solar cell with different kinds of ZnO deposition methods:

open circuit voltage (V OC ), short circuit current (I SC ), ﬁll factor (FF) and power

con-version efﬁciency (PCE).

V OC (V) I SC (mA/cm 2 ) FF (%) PCE (%)

sol-gel 0.57 8.221 53.99 2.53

ALD seeded 0.27 5.388 41.72 0.61

sol-gel seeded 0.17 6.690 37.28 0.42

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