Improving efficiency of organic solar cells by preparing aluminum-doped zinc oxide films by ion beam-assisted sputtering

Organic Electronics 14 (2013) 182–186

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Letter

Improving efficiency of organic solar cells by preparing aluminum-doped zinc oxide films by ion beam-assisted sputtering Kuang-Teng Hung, Hsuan-Ta Wu, Sheng-Wen Fu, Hui-Ju Chen, Chu-Yun Hsiao, Chuan-Feng Shih ⇑ Department of Electrical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan

a r t i c l e

i n f o

Article history: Received 20 August 2012 Received in revised form 12 October 2012 Accepted 15 October 2012 Available online 29 October 2012 Keywords: Organic solar cells Alumina-doped zinc oxide Ion beam assisted sputtering Work function Contact resistance

a b s t r a c t This paper reports on the improvement of the efficiency of organic solar cells (OSCs) by using ion beam-assisted sputtering (IBAS) to synthesize the alumina-doped zinc oxide (AZO) anodes that are used therein. The anode voltage of the ion source and the substrate temperature during IBAS were varied. When the substrate temperature was 460 °C, increasing the anode voltage from 0 to 40 V increased the efficiency of the OSCs by 50%. The optical, electrical, morphological, and interfacial properties of the AZO and organic layers were investigated. The improvement of OSCs was associated with an increase in the conductivity, FOM, surface work function of the AZO films, and a decrease in the contact resistance in AZO/CuPc interface. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Owing to rapid advances in photovoltaics, touch panels, and flat-panel displays, a transparent conductive oxide (TCO) with high transmittance and conductivity is urgently required. Aluminum-doped zinc oxide (AZO) has been used to replace indium-contained oxide and zinc oxide owing to its low cost and high conductivity. Several techniques have been used to fabricate AZO films, including chemical vapor deposition [1], magnetron sputtering [2], spray pyrolysis [3], evaporation [4], sol–gel [5], and pulse laser deposition [6,7]. Among those methods, magnetron sputtering provides such advantages as high c-axis orientation, large area deposition, good adhesion of the films, and a low processing temperature. However, sputtering unavoidably produces more defects and a greater stress in the films than other methods. Recently, organic semiconductors with high efficiencies have been demonstrated following successes advances in interfacial engineering [8], the control of nanostructures

⇑ Corresponding author. Tel.: +886 6 2757575x62398; fax: +886 6 2080687. E-mail address: [email protected] (C.-F. Shih). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.10.010

[9], and modification of electrodes [10] and device structures [11]. AZO films have been used as transparent anodes or cathodes of organic solar cells (OSCs), and especially in inverted OSCs. Post-treatments such as ozone treatment and an ZnO buffer layer [12] have been used to improve the performance of AZO electrodes in OSCs. In this work, ion beam-assisted sputtering (IBAS) was used to improve the AZO films. The morphological, electrical and optical properties of the AZO films were investigated. A change in the surface work function, an increase in figure of merit of AZO, and a decrease in contact resistance in AZO/organic interface were the main factors that dominated the improvement of OSCs. 2. Experimental Three inch ZnO/Al2O3 (98/2 wt.%) target was used to sputter AZO thin films on a 4 cm  4 cm glass substrate by IBAS. The background pressure was 4.7  106 torr and the substrate temperatures (Ts) were room temperature and 460 °C. The anode voltage (Va) of ion source determined the kinetic energy of the species in the ion beam, and was varied from 0 to 40 V. Ar (20 sccm) gas was introduced to the chamber for sputtering and into

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3. Results and discussions Fig. 1a–d characterizes solar cells that were prepared using the IBAS-prepared AZO films. As Va was increased

VOC (V)

0.39

(c)

RT o 460 C

(a)

from 0 to 40 V, the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) of samples that were prepared at roomtemperature increased from 0.349 to 0.353 V, 0.01 to 0.566 mA/cm2, 0.226 to 0.254, and 0.008% to 0.051%, respectively. As Ts was raised to 460 °C, Voc, Jsc, FF, and PCE increased further to 0.355 V, 3.33 mA/cm2, 0.276%, and 0.33%, respectively. These results demonstrate that the OSCs performed well when the AZO films were prepared at high temperature. The high-temperature preparation of AZO is known to yield high crystallinity [14]. As Va was increased from 0 to 40 V at 460 °C, Voc, Jsc, FF, and PCE increased monotonically from 0.355 to 0.369 V, 3.33 to 4.07 mA/cm2, 0.276 to 0.364, and 0.33% to 0.55%, respectively. Use of the IBAS-prepared AZO films increased the PCE of the OSCs by 50%. The effects of electric, optical, and morphological properties of the AZO films on the performance of the OSCs were investigated to clarify the mechanism that dominated the improvement of the OSCs. Fig. 2a–c plots the resistivity (qs), transmittance, and figure of merit (FOM) as functions of Va. The average transmittance (Tavg) was derived from the transmittance spectrum in the range of 300–800 nm, and the FOM was defined as T 10 avg =qs [15], which is commonly used to evaluate the properties of a transparent and conducting film. Obviously, substrate temperature markedly influenced the properties of the AZO films. All of the qs, Tavg, and FOM increased with substrate temperature. The resistivity decreased from 3.56  101 to 7.83  104 X cm as the substrate temperature was increased from RT to 460 °C. Increasing Va reduced qs and increased the transmittance and FOM of AZO films. The resistivity of IBAS-AZO decreased from 7.83  104 to 6.29  104 X- cm as Va was increased from 0 to 30 V and then increased to 7.62  104 X- cm as Va was further increased to 40 V. Notably, the resistivity of AZO could not be further reduced in our system because the reduction of

0.4

0.3

0.36

FF

the ion gun and the plasma-bridge neutralizer. The overall working pressure was maintained at 1  103 torr. The rfpower of magnetron sputtering was 280 W. The target was cleaned by pre-sputtering for 30 min. The cathode current of the ion source, the solenoid current of the ion source, the bias voltage of the plasma-bridge neutralizer, and the anode voltage of the plasma-bridge neutralizer were 5 A, 300 mA, 10 V, and 35 V, respectively. The thickness of all of the AZO thin films was measured using an ellipsometer. The surface roughness and grain size of the AZO thin films were determined by atomic-force microscopy (AFM, Digital Instruments Inc., NanoScope E) and high-resolution transmission-electron microscopy (HR-AEM, Jeol JEM2100F CS STEM), respectively. Hall measurements (Ecopia HMS-3000) were made to measure the resistivity. The transmittance spectrum was obtained using a UV/VIS spectrophotometer. The work function was measured using a photoelectron spectrometer (Riken-Keiki Co., AC-2). The certified repeatable accuracy of the work function measurement was ±0.02 eV [13]. The atomic contents were calculated by quantitative analysis based on X-ray photoelectron spectroscopy (XPS). Copper phthalocyanine (CuPc, 98%, Aldrich), buckminsterfullerene (C60, 99.95%, Aldrich), bathocuproine (BCP, 98%, Alfa Aesar), and Al were used as the electron transport layer, the hole transport layer, the exciton blocking layer (EBL), and the cathode of the organic solar cells, respectively. The device structure was glass/AZO (200 nm)/CuPc (20 nm)/C60 (40 nm)/BCP (7 nm)/Al (130 nm). The size of the solar cell device was 6.45 mm2. The characteristic parameters of all devices were measured using Agilent E5270B under 100 mW/cm2 AM 1.5G simulated solar light.

0.2 0.33

(b)

(d)

0.75 0.50

3 0.25

2 1

0.00

0 0

10

20

30

Anode Voltage e (V)

40

0

10

20

30

Anode Voltage e (V)

Fig. 1. Cell parameters of OSCs. (a) Voc, (b) Jsc, (c) FF, (d) PCE.

40

PCE (%)

4

2

Jsc (mA/cm )

5

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100

Resistivity (Ω cm)

10

(a)

RT o

460 C

-1

-2

10

-3

10

-4

10

94

(b)

Tavg (%)

93

92

91

1

10

-3

-1

Figure of merit (10 Ω )

(c)

0

10

-1

10

-2

10

0

10

20

30

40

50

Anode Voltage (V) Fig. 2. (a) Resistivity, (b) transmittance, (c) FOM of IBAS-prepared AZO films with different Va and temperature (RT: room temperature).

residual oxygen in chamber was limited by the lowest background pressure (3  106 torr). (Fig. S1 a (Supplementary material)) plots the transmittance as functions of ion-beam voltage. The optical band gap was

substantially increased by increasing substrate temperature and slightly increased with the anode voltage. Similarly, the full-width at half maximum (FWHM) of the X-ray diffraction pattern of the AZO films was greatly decreased by increasing the substrate temperature and slightly reduced by increasing the anode voltage. The reduction of the X-ray FWHM was directly associated with the improvement of crystallinity, but the shift of the optical band gap was a more complicated effect that was associated with changes in stoichiometry, crystallinity or electronic structure of the surface. Notably, Fig. 2b and c shows that the optimal Tavg and FOM were 93.0% and 15.34 103 X1, respectively, when Ts was 460 °C and Va was 30 V. However, the electrical and optical properties of AZO were optimized when Va was 30 V, but the cell properties increased monotonically with Va, indicating that other properties of the IBAS-AZO films should be responsible for the enhanced performance of the OSCs when Va was larger than 30 V. The surface work function is known to associate with Voc, based on the metal–insulator–metal model [16]. Therefore, the work function of AZO films versus the anode voltage was measured, as shown in Fig. 3a. The work function increased with Va and was unchanged when Va exceeded 40 V. Additionally, the work functions of the AZO films that were prepared at 460 °C were higher than those prepared at RT. This change of work function was found to be affected by the oxygen content in the AZO films, as revealed by Fig. 3b. Fig. 3b plots the work functions that varied with either temperature or the anode voltage of the ion beam as functions of the oxygen content in the AZO films. The atomic ratio of the AZO films was measured by XPS and used to determine the relative oxygen content. When the atomic ratio of Al + Zn:O was 1:1, the relative oxygen content was defined to be 0%. Therefore, the zero of the horizontal axis corresponded to 50% oxygen in the AZO films. The linear dependence of the work function on the oxygen content was consistent with the literature [17]. Clearly, the work functions of AZO increased with the anode voltage and the temperature because of a reduction of oxygen content, which was caused by the different sputtering yields of oxygen and zinc. Fig. S2 (Supplementary material) shows the sputtering yields (Y(E)) of oxygen and zinc, which were calculated according to the following equation, YðEÞ ¼ 0:042 aSn ðEÞ, Ub where E is the energy of the initially incident particle; a

Fig. 3. (a) Work function of AZO thin films with different anode voltages. (b) Work function versus relative oxygen content in AZO films. (c) Band diagram of AZO/CuPc based OSC.

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coefficient of the 200-nm thick AZO were measured by ellipsometer. Monochromatic light with a wavelength of 600 nm was chosen to simulate the optical field, because it fitted the absorption of CuPc. The results of the simulation show that most of the change of electric field fell in the AZO region. No change was found within the active layer. Therefore, the possibility that Jsc was improved by changing the optical properties of AZO was excluded. Table 1 presents in detail the properties of IBAS AZO thin films. The variation in the surface morphology of AZO that is caused by the assisted ion beam was another possible factor that altered the Jsc Fig. 4a–c presents the AFM images of AZO films with different Va at Ts = 460 °C. As Va was increased from 0 V to 40 V, the rms roughness increased slightly from 1.71 to 2.00 nm (Fig. 4d). A larger-scale scan of the surface morphology by scanning-electron microscopy confirmed the AFM results, as shown in supplementary Fig. S3 (Supplementary material). The grain size of AZO thin films was calculated from transmission-electron microscopic (Supplementary material, Fig. S4) images and was shown in Fig. 4d. Although the roughness considerably increased with Va, its influence on optical absorption was neglected because the roughness was much smaller than the wavelength of light. The grain size also increased with Va, suggesting that a reduction of area of grain boundaries may reduce the leakage current and improve cell performance. The variation of surface conditions that were caused by changing the anode voltage during IBAS, including roughness, stoichiometry, and work function of the AZO films, should have affected the electrical properties of the CuPc/ AZO interface. Therefore, current–voltage (I–V) relationship of CuPc/AZO interface was examined by the following

Table 1 Resistivity, average transmittance, figure of merit, work function, and refractive index (at 600 nm) of AZO films. Device Ts Va qs (X cm) (°C) (V) 1 2 3 4 5 6

25 25 25 600 600 600

0 30 40 0 30 40

Tavg (%)

3.56  101 6.11  102 3.40  102 7.83  104 6.29  104 7.62  104

Refractive FOM Work (103 X1) function index (k = 600 nm) (eV)

91.64 0.02 91.86 0.14 92.13 0.26 92.64 11.89 93.00 15.34 92.70 12.29

4.57 4.64 4.71 4.79 4.82 4.90

1.85 1.99 1.94 2.07 2.16 2.14

is associated with the mass of the incident particle and the target particle, and Sn(E) is the energy-dependent nuclear stopping cross section. Obviously, the sputtering yield of oxygen was larger than that of zinc under Ar bombardment. Fig. 3c displays the band diagram of CuPc/C60-based OSCs, which shows that an increase in the work function of AZO reduced the potential difference between AZO and the highest occupied molecular orbital (HOMO) of CuPc. This variation reduced the barrier height of the holes and thus increased Jsc. Other than the effect of the work function of AZO, many factors can improve Jsc. For example, the IBAS changed the transmittance of the AZO films, potentially changing the intensity and distribution of the optical wave inside the OSC. The position of maximum strength of the electromagnetic field has been known to influence the cell efficiency [18]. To clarify this point, the intensity of the optical wave inside the layered OSCs was simulated (Supplementary material, Fig. S1 b). The refractive index and extinction

Fig. 4. Atomic force microscopic images of IBAS-prepared AZO with various Va (a) 0, (b) 30, (c) 40 V. Variation of roughness and grain size were summarized in (d).

(a) 460

o

C

300

(b) 460

o

C

Rsh

100

Rs

0.0 -0.5 -1.0

75 200 50

2

Rsh ( Ω cm

2

)

0.5

IBAS (0 V) IBAS (30 V) IBAS (40 V)

Rs (Ω cm )

Current (mA)

1.0

100 -2

-1

0

Voltage (V)

1

2

0

10

20

30

40

Anode Volta age e (V)

Fig. 5. (a) I–V curves of the CuPc/AZO interface. Inset shows top view of specimen. (b) Shunt and series resistances of OSCs with different Va.

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procedure. CuPc was deposited on the stripped AZO films, and the I–V relationship was measured by probing two of the AZO strips. Fig. 5a plots the I–V curves and the inset shows the top view of the specimen. The slope of the I–V curve is the reciprocal of the sum of contact resistance and the resistances of both AZO and CuPc. Notably, the conventional transmission line method (TLM) was not applicable in this material system because the top-probing damaged the organic layer, the poor CuPc/AZO contact caused a considerable deviation of I–V characteristic, and the highly resistive organic layer restricted the distance between the TLM patterns. The I–V measurements were reproducible, although the actual contact resistance cannot be obtained. The variation of the slope was ascribed to the reduction of the contact resistance of the CuPc/AZO interface. Fig. 5b shows the shunt resistances (Rsh) and series resistances (Rs) that were calculated from the I–V characteristics of the AZO/CuPc/C60/BCP/Al OPV, as the Va of AZO was varied. Rsh increased but Rs decreased as Va increased. The results agreed well with the I–V measurements in Fig. 5a. Accordingly, the monotonic increase in the cell properties with Va cannot be explained by only the electrical and optical properties of the AZO films; the surface and contact properties must also be considered. 4. Conclusions IBAS was used to deposit and improve AZO films as anodes of OSCs. The IBAS- prepared AZO thin films has a tunable work function (4.57–4.90 eV), high transmittance (93%), low resistivity (6.29  104 X- cm), and high FOM when Ts was 460 °C and Va was 30 V. As Va was increased from 0 to 40 V for Ts = 460 °C, the PCE of the OSCs increased by 50%. The work function of AZO was found to be related to oxygen content, which affected the band alignment and the transport efficiency. Finally, the contact resistance of the AZO/CuPc interface was found to be reduced by the IBAS technique, reducing the series resistance of the OSCs.

Acknowledgment This work was supported by the National Science Council under Contract No. 100-2221-E-006-130-MY2. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2012.10.010. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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