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Journal of Nanoelectronics and Optoelectronics Vol. 5, 1–5, 2010

Mist-CVD Growth of High Quality ZnO Thin Films at Low Temperature for Inverted Organic Solar Cells Kwon-Ho Kim1 , Kyung-Sik Shin1 , Brijesh Kumar1 , Kyung-Kook Kim2 , and Sang-Woo Kim1
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∗ 1

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2 Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung 429-793, Republic of Korea 3 SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea We report the deposition of ZnO thin films on indium-doped tin oxide (ITO)-coated glass substrates via Mist chemical vapor deposition (Mist-CVD) at a temperature range of 250, 300 and 350  C, respectively. Surface morphology and crystalline quality were investigated by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD), respectively and optical property by transmission spectra. Further, we have optimized the surface morphology of ZnO films for inverted organic solar cells (IOSCs) by fabricating IOSCs on glass substrate, consisting of Au/MoO3 /PCBM:P3HT/ZnO/ITO. It was found that the photovoltaic power conversion efficiency (PCE) of IOSCs is affected by the transmittance and crystallinity of ZnO thin films.

Keywords: ZnO Thin Film, Mist-CVD, Inverted Organic Solar Cell, Power Conversion Efficiency.

In recent years, various techniques have been extensively used to grow or deposit the zinc oxide (ZnO) nanostructures or thin films; such as pulsed laser deposition (PLD), molecular beam epitaxy (MBE), magnetron sputtering, metal organic chemical vapor deposition (MOCVD), chemical vapor deposition (CVD), as well as solution-based chemistry such as hydrothermal, sol–gel, and electrochemical deposition.1–6 Magnetron sputtering and solution-based chemistry methods produce ZnO thin films of polycrystalline. On the other hand, PLD, MBE and MOCVD can produce high-quality crystalline films. These techniques are complicated and expensive. To grow high quality crystalline thin films at low temperature, the Mist-CVD process is simple and inexpensive. It is a combination of spray pyrolysis and CVD, allowing the deposition to occur from the vapor phase using a spraying solution. The precursor droplets introduced from the bottom upwards by spraying through a well-defined temperature profile result in the formation of a thin film on the substrate. The film’s quality has been found to be better than that deposited by spray pyrolysis.7 ZnO films and nanostructures have attracted substantial attention due to their interesting physical (morphological, structural, optical and electrical) properties and ∗

Author to whom correspondence should be addressed.

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there various potential technological applications in photovoltaic conversion, optoelectronics and piezoelectronics. Inverted organic solar cells (IOSCs) composed of organic semiconductors and inorganic nanostructures are an alternative to organic bi-layer and bulk heterojunction device structures. IOSC is a promising photovoltaic (PV) technology which offers environmental stability, and lowcost manufacturing.8
9 In these devices, a p-type donor organic material such as poly3-hexylthiophene (P3HT) is interfaced with an acceptor organic material, for instance [6, 6]-phenyl C61 butyric acid methyl ester (PCBM). Excitons (electron–hole pairs) are generated when light is absorbed by P3HT. Generated Photocurrent follows dissociation of excitons and charge separation at the heterojunction between the donor material and the organic material. One important key to high performance IOSC is the selection of the electron collection layer. Several semiconducting oxides such as ZnO, TiO2 , CdSe, act as buffer layers for electron transport of the separated charge at the heterojunction. Advantages of ZnO are high carrier mobility, solution process ability, thermal and ambient stability, and a high electron affinity necessary for charge injection from the complementary organic material. In this present work, we have studied the growth of ZnO thin films on ITO/glass substrates at low temperature using the Mist-CVD process as an electron transport layer for IOSCs. IOSCs were fabricated with ZnO thin films, then 1555-130X/2010/5/001/005

doi:10.1166/jno.2010.1103

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1. INTRODUCTION

Mist-CVD Growth of High Quality ZnO Thin Films at Low Temperature for Inverted Organic Solar Cells

photovoltaic power conversion efficiency (PCE) from the devices were measured.

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2. EXPERIMENTAL DETAILS High quality ZnO thin films for IOSCs were prepared using the Mist-CVD process. Glass and ITO/glass substrates were cleaned by ultrasonic wave in acetone, methanol, and DI-water, respectively. The source solution was prepared in a solution of 0.01 M zinc acetate [Zn(CH3 COO)2 ] dissolved in acetone. The precursor in the solution is ultrasonically atomized into the aerosol particles. N2 flow (5000 sccm) was introduced as a carrier gas. The substrate temperatures were maintained at 250  C, 300  C, 350  C in order to control crystallinity and surface morphology of ZnO thin films. To produce IOSCs based on ZnO thin films, a polymer blend of poly (3-hexylthiophene) (P3HT):(6, 6)-phenyl C61 butyric acid methyl ester (PCBM) (1:1 vol.% in chlorobenzene) was spin-coated onto ZnO thin films at 3000 rpm for 120 seconds and then annealed at 150  C for 10 minutes. Molybdenum oxide (MoO3  as an electronblocking layer and a gold(Au) anode were deposited by thermal evaporation. The surface morphologies of the films were examined by employing a field-emission scanning electron microscopy (FE-SEM), and crystalline property of the films were observed from X-ray diffraction (XRD) measurement. The optical transmissions of the ZnO films were measured in the wavelength range of 300–800 nm by using a UV-VIS spectrophotometer (Shimadzu UV-3600). Electrical parameters of the solar cells were measured under simulated illumination at AM 1.5 G, 100 mW/cm2 .

3. RESULTS AND DISCUSSION Figure 2 shows the surface morphologies of ZnO thin films deposited on glass and ITO/glass substrates, while varying the growth temperature. ZnO thin films on glass substrates have plate-like morphology with an average of 200 nm of grain size at temperature 250  C and 300  C, as shown Figures 2(a and b). However, ZnO thin films comprising dense nanoparticles with an average of 50 nm of grain size were deposited at 350  C, as shown in Figure 2(c). Figures 2(d–f) are images of the surface morphology from ZnO thin films deposited on ITO/glass, which have similar surface morphology as of ZnO thin films deposited on glass. However, the grain size in Figures 2(d and e) is decreased approximately 60–70 nm, and the grain size in Figure 2(f) become nearly 30 nm. As a result, total roughness and grain size of ZnO thin film deposited on ITO/glass are decreased. We concluded from the SEM images that morphology of ZnO thin films varies with growth temperature and substrate, keeping all growth conditions the same. 2

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Figure 3 shows the XRD diffraction pattern of ZnO thin films deposited on glass substrate. In the case of ZnO thin films deposited at a growth temperature of 250  C on glass, it is this along (101) crystalline orientation. ZnO is generally grown along the (002) crystalline orientation due to the low surface energy of (002) plane. The reason of prevailing (101) crystalline orientation growth is lack of sufficient energy at low temperatures for atoms to move to low energy sites, which induces strain in the films. No XRD peak is visible in the case of ZnO thin films deposited at 300  C. It could be due to the detection limits for XRD and polycrystalline features (101) and (002) of the films. The energy gain at this temperature can be enough for a few atoms to move to lower energy sites, yet not enough for all atoms to move to lower energy sites. As the growth temperature increases up to 350  C, ZnO thin films are grown in the (002) crystalline orientation. Increasing growth temperature, crystalline property transformed from (101) orientation to (002) orientation. We consider that growth temperature at 350  C supports enough energy for the diffusion of atoms adsorbed on the substrate and accelerates the migration of atoms to energetically favorable positions. The growth temperature plays an important role in determining the structure of ZnO films.10
11 Therefore, we can control the crystallinity of ZnO thin films by varying the growth temperature. Figure 4 shows the optical transmittance properties of the ZnO films deposited at the different temperatures of 250  C, 300  C and 350  C, respectively. Transmittance of ZnO thin films on ITO/glass is improved near 400 nm wave-length. The spectra show high transparency up to 85%–89% in the visible range above 550 nm. High transparency of the ZnO thin films is mainly attributed to the formation of a rough surface on ITO/glass by Mist-CVD. A rough surface on ZnO thin films affects external light reflection and scatters light outward.12
13 In other words, ZnO thin films deposited via Mist-CVD lead to a dispersed angular distribution. Therefore, ZnO thin films with rough surface morphology have high optical transmittance. We fabricated IOSCs to confirm the solar power performances based on various temperature-dependent deposited

Fig. 1. The schematic diagram for the Mist-CVD growth system.

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Mist-CVD Growth of High Quality ZnO Thin Films at Low Temperature for Inverted Organic Solar Cells (a)

(b)

(c)

(d)

(e)

(f)

Fig. 2. FE-SEM images of (a) 250  C (b) 300  C (c) 350  C ZnO thin films deposited on glass substrates and, (d) 250  C (e) 300  C (f) 350  C ZnO thin films deposited on ITO/glass via the Mist-CVD.

and FF being decreased to 8.35 mA/cm2 , 0.536 V and 42.5%, respectively. The PCE of IOSC fabricated with ZnO thin films at a growth temperature of 250  C is higher. At first, different transmittance has an effect on generating photocurrent and Jsc properties. Improved transmittance of ZnO thin films (Fig. 4) will enhance the absorption in the polymer the active layer. In other words, increasing absorption in the active layer generates more photocurrent and affects current density. As a result, transmittance is high at a growth temperature of 250  C and 300  C and which will affect Jsc , PCE in the IOSCs. However, IOSC fabricated with ZnO thin films at a growth temperature of 350  C show lower Jsc and PCE. This is due to relatively low transmittance and an increase of ITO resistivity as deposited at a relatively higher growth temperature. For the films deposited at higher growth temperatures, the resistivity was found to increase due to the contamination of the

Fig. 3. XRD pattern of 250  C, 350  C deposited ZnO thin films on glass via the Mist-CVD.

Fig. 4. Transmittance spectra of ZnO thin films deposited on ITO/glass at different temperature.

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ZnO thin films. Current density–voltage J –V  measurements were carried out using a solar simulator under standard AM 1.5 G solar illumination. A schematic diagram of IOSCs device is shown in Figure 5, and performance is summarized in Table I. Figure 6 shows J –V curves for IOSCs with ZnO thin films as the electron transport. This device with ZnO thin film deposited at a growth temperature 250  C, reveals a short circuit current (Jsc  of 11.2 mA/cm2 , an open circuit voltage (Voc  of 0.545 V and a fill factor (FF) of 52.5%, resulting in PCE 3.21%. With ZnO films in the device deposited at a growth temperature of 300  C, PCE was decreased to 0.13% under the same conditions, as a result of the decreased values to 10.8 mA/cm2 , 0.542 V and 52% for Jsc , Voc and FF, respectively. When ZnO thin film deposited at a growth temperature of 350  C, was used in the device, PCE was greatly far decreased, to 1.90%. This is due to the Jsc , Voc

Mist-CVD Growth of High Quality ZnO Thin Films at Low Temperature for Inverted Organic Solar Cells

Fig. 5. Schematic diagram of the IOSC structure consisting of Au (100 nm)/MoO3 (20 nm)/PCBM:P3HT (250 nm)/ZnO thin films (50 nm)/ITO (150 nm) /glass.

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As demonstrated in previous studies,16–18 the Jsc is also strongly dependent on the crystallinity of functional material layers. As shown in Figure 3, we confirmed deposited ZnO thin film crystalline orientation from (101) to (002) with increased growth temperature from 250  C to 350  C. Better crystallinity of ZnO thin films have an effect on carrier mobility, and Jsc of IOSCs can improve. In the case of ZnO film deposited at a growth temperature 350  C, with (002) crystalline orientation affecting Jsc , we proposed that Jsc decrease by other factors of low transmittance and ITO damage. Consequently, high transmittance and crystallinity of ZnO thin films have an effect on Jsc improvement in IOSCs by controlled growth temperature condition. By controlling the growth temperature one can enhance the efficiency of IOSCs.

Table I. The summary of device performance. Deposition temperature ( C) 250 300 350

Jsc (mA/cm2 )

Voc (V)

FF (%)

PCE (%)

112 108 835

0.545 0.542 0.536

52.5 52.5 42.5

3.21 3.08 1.90

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films by alkali ions from the glass substrates.14
15 Series resistance of IOSCs is strongly affected by the increasing resistivity of ITO electrodes, higher series resistance generally leading to a lower Jsc  (a)

4. CONCLUSION Growth temperature-dependent surface morphologies, crystalline properties and optical quality of ZnO thin films prepared using the Mist-CVD method are optimized for inverted inorganic–organic solar cell structures. ZnO thin films have high transmittance up to 85%–89% in the visible range. It has been suggested that the PCE of the IOSCs is affected by carrier mobility related to the transmittance and crystallinity. Acknowledgment: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100015035 and 2009-0077682). by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean Government Ministry of Knowledge Economy (2009T100100614).

References and Notes

(b)

Fig. 6. (a) J –V characteristics for IOSCs with ZnO thin films under AM 1.5 G irradiation, (b) J –V characteristics for IOSCs with ZnO thin films under dark.

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Mist-CVD Growth of High Quality ZnO Thin Films at Low Temperature for Inverted Organic Solar Cells

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Received: 30 April 2010. Accepted: 16 June 2010.

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