- Email: [email protected]
n-Si heterojunction solar cells
Available online at www.sciencedirect.com
Solar Energy 85 (2011) 2589–2594 www.elsevier.com/locate/solener
Fabrication and characterization of amorphous In–Zn–O/SiOx/ n-Si heterojunction solar cells Hau-Wei Fang a, Shiu-Jen Liu b,⇑, Tsung-Eong Hsieh a, Jenh-Yih Juang c, Jang-Hsing Hsieh d b
a Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan Department of Mathematics and Science (Precollege), National Taiwan Normal University, Linkou, Taipei 244, Taiwan c Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan d Department of Materials Engineering, Mingchi University of Technology, Taishan, Taipei 243, Taiwan
Received 2 February 2011; received in revised form 31 May 2011; accepted 30 July 2011 Available online 29 August 2011 Communicated by: Associate Editor Igor Tyukhov
Abstract Amorphous In–Zn–O (a-IZO) films were deposited on SiOx covered n-type Si substrates by using pulsed laser deposition (PLD) technique to form a-IZO/SiOx/n-Si heterojunction solar cells. The a-IZO films grown at 150 °C with various laser power (250–500 mJ/pulse) exhibit low resistivity (2–3 103 X cm) and high transparency (80%) in the visible wavelength range. The highest conversion efficiency of the fabricated a-IZO/SiOx/n-Si solar cells is 2.2% under 100 mW/cm2 illumination (AM1.5 condition). The open-circuit voltage, shortcircuit current density and fill factor of the best device are 0.24 V, 28.4 mA/cm2 and 33.6%, respectively. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Amorphous In–Zn–O film; Semiconductor–insulator–semiconductor solar cell; Pulsed laser deposition; Photovoltaic characteristics
1. Introduction In comparison with Si-base pn junction solar cells, semiconductor-insulator-semiconductor (SIS) solar cells composed of transparent conducting oxide (TCO), i.e. wide bandgap oxide semiconductors, and SiOx covered Si have been proposed to be low-cost technology (Shewchun et al., 1978; Feng et al., 1979). There were several SIS solar cells exhibiting high conversion efficiency (g) reported, including ITO/SiOx/p-Si (g = 12.8%) (Shewchun et al., 1979), ITO/ SiOx/n-Si (g = 12%) (Malik et al., 2008), SnO2/SiOx/n-Si (g = 8.8–14.1%) (Ghosh et al., 1978; Maruska et al., 1983) and ZnO/SiOx/n-Si (g = 6.9%) (Kobayashi et al., 1995). According to the theoretical calculation and experimental results reported by Shewchun et al. (1977, 1978) the ⇑ Corresponding author. Tel.: +886 2 77148400; fax: +886 2 26022617.
E-mail address: [email protected] (S.-J. Liu). 0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.07.016
tunneling process is regarded as the dominant mechanism for the carriers transport from Si to TCO through the interfacial insulator layer. Therefore, the most critical parameters which affect the performance of SIS solar cells are the interfacial layer thickness and interface states which are controlled by the process employed to produce the insulating SiOx layers. The basic requirement for the SiOx layers between TCO and Si substrates is that the thickness must be less than 2 nm (Shewchun et al., 1978). Amorphous transparent conducting oxide films such as In–Zn–O (IZO) films with high transparency and low resistivity can be fabricated at low temperatures (Hosono, 2006). Consequently, the fabrication temperature of SIS solar cells can be lowered by growing amorphous IZO films on SiOx/Si substrates. Moreover, low-temperature process can reduce the possibility of increasing the thickness of the SiOx layers between the TCO layers and Si substrates. On the other hand, a study of epi-n-IZO thin films/h1 0 0i Si
2590
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594
solar cells has been reported previously (Ramamoorthy et al., 2004). However, the conversion efficiency and short-circuit current density (Jsc) of the epi-IZO/n-Si devices fabricated at 450 °C are relatively low (g = 0.0001% and Jsc = 0.24 lA/cm2). High-temperature fabrication process of the crystalline IZO layers would increase the thickness of the interfacial layer between the IZO layer and Si substrate and thus the tunneling transport of carriers is suppressed by the thick insulating interface. The aim of this work is to investigate the feasibility of fabricating solar cells based on SIS structures by depositing amorphous TCO films on SiOx covered n-type Si substrates. Amorphous IZO (a-IZO) films were grown on Si substrates covered by an thin SiOx layer to form a-IZO/ SiOx/n-Si heterojunction devices. The electrical, optical and structural properties of IZO films deposited with various laser power were also investigated. 2. Experiments The P-doped n-type Si (1 0 0) wafer with a resistivity of 1 X cm was firstly cleaned with acetone and deionized water, and then etched with 10% hydrofluoric acid (HF) for 2 min to remove the native oxide layer. A thin SiOx layer was produced by immersing the Si wafer in a hot H2O2 solution for 10 min. The thickness of the SiOx layer was estimated to be 1.7 nm by a high-resolution transmission electron microscopy (HRTEM) measurement. The a-IZO layers were deposited on SiOx covered Si wafer by using pulsed laser deposition (PLD) with ZnO/ In2O3 = 3/1 (at.%) ceramic oxide formed into a pellet as the target prepared by conventional solid state reactions from stoichiometric amounts of ZnO and In2O3 powders of 99.99% purity. After being sintered at 1200 °C in air for 2 h, the chemical composition of the target was examined by energy-dispersive X-ray spectroscopy (EDX) and the Zn content ratio Zn/(In + Zn) of the target is 69 at.%. A KrF excimer laser was used to ablate the target with power of 250–500 mJ/pulse and repetition rate of 5 Hz. The PLD chamber was evacuated to a base pressure lower than 1 106 torr before PLD. In order to enhance the conductivity of IZO films, following argon was used as the working gas and the chamber pressure was kept at 4 103 torr. Owing to the absence of oxygen during PLD, large amount of oxygen vacancies are expected to be produced in the amorphous films and the carrier density would be increased. The distance between the target and substrates was 4–5 cm. The temperature of substrates was kept at 150 °C during PLD. The thickness of all IZO films is in the range of 25–35 nm. The rear Si surface is covered by a Al metal layer as ohmic contact electrode by dc magnetron sputtering after removing the native SiOx layer. And Al metal grids were also formed on the IZO surface for current collection. The IZO films were simultaneously deposited on Corning 1737 glasses for electrical and optical properties measurements. The crystal structures of the IZO target
and films were studied by X-ray diffraction (XRD) scans with Cu Ka radiation. The Zn/(Zn + In) atomic ratios of films were determined by EDX. The electrical properties including resistivity, carrier concentration and carrier mobility were carried out using the four-probe van der Pauw method. Atomic-force microscopy (AFM) measurements were employed to examine the surface roughness of IZO films. The optical measurements of the films, transmission and reflection, were recorded using a UV–VIS double beam spectrometer in the wavelength of 200–900 nm.
3. Results and discussion Fig. 1 presents the schematic energy-band diagram of the IZO/SiOx/n-Si hetero-junction structure. /IZO and /Si are the IZO-to-SiOx and Si-to-SiOx barrier heights related to the work functions of IZO and Si, respectively. While the heterojunction is under illumination, the light-generated current is primarily due to the photoexcited electron-hole pairs in the Si. /B is the difference between /IZO and /Si and is the barrier height which dominates the open-circuit voltage (Voc) of the photovoltaic devices. However, there are interface states exist at IZO/SiOx and SiOx/Si interfaces, which directly affect the performance of SIS solar cells. The XRD measurement indicates that the IZO target used in this study is a mixture of In2O3 and Zn4In2O7 phases. On the other hand, no peak was observed in the XRD patterns of the IZO films deposited on glasses with various laser power, indicating the amorphous characteristics of IZO films grown at 150 °C. The XRD patterns are not shown here. Fig. 2 shows the Zn content ratio Zn/(Zn + In) of IZO films as a function of the laser power used for the film deposition. The horizontal dash line in the plot represents the Zn/(Zn + In) value of the target, i.e. 69 at.%. The Zn content ratios of films are higher than that of the target and decrease approximately with increasing the laser
φ IZO
φ Si φB
EF CB
Eg-Si
CB EF VB
Eg-IZO
VB
Interface States d IZO
SiOx
n-Si
Fig. 1. Schematic energy band diagram of the IZO/SiOx/n-Si heterojunction structure.
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594 85
Zn / (Zn+In) (at.%)
80
75
70
target
65
60 250
300
350
400
450
500
Laser Power (mJ/pulse) Fig. 2. The dependence of the Zn content ratios Zn/(In + Zn) of IZO films on the laser power used for film deposition.
2591
power. The higher Zn content ratios of films are possibly resulted from the different ablation rates of Zn4In2O7 and In2O3 phases of the target owing to low laser power. The Zn content ratio of films grown with a laser power of 500 mJ/pulse is 70 at.% which is almost the same with that of the target. The AFM images of IZO films are shown in Fig. 3. The scan area is 1 lm 1 lm and the z-scale is 20 nm per division. It is clearly observed that the surface roughness of IZO films increases monotonically with increasing the laser power. Since the crystallinity of the a-IZO films was not enhanced by increasing the laser power, which is deduced from the XRD patterns, the increase of the surface roughness is attributed to the bombardment damage by particles with high kinetic energy ablated from the target by high laser power. The mean roughness of the a-IZO films ranges between 0.3 nm (250 mJ/pulse) and 15.3 nm (500 mJ/ pulse). Fig. 4 shows the resistivity (q), carrier density (N) and mobility (l) of IZO films as a function of laser power. As
(a) 250 mJ
(b) 300 mJ
(c) 350 mJ
(d) 400 mJ
(e) 450 mJ
(f) 500 mJ
Fig. 3. AFM images of IZO films deposited with various laser power.
2592
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594 100
-3
ρ (10 Ω-cm)
6
Transmission / Reflection (%)
7
(a)
5 4 3 2 1
3
4.0
20
N (10 /cm )
4.5
3.5
(b)
60
40
Reflection 20
300
400
500
600
700
800
Wavelength (nm)
3.0
3000
(b) Laser power: 250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
0.5
(c)
(αhν) (cm eV)
-1
10 8
2000
0.5
-1 -1
Laser power: 250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
0
2.0 12
2
Transmission
80
2.5
μ (cm V s )
(a)
6 4 250
300
350
400
450
1000
500
Laser power (mJ/pulse)
0 1.5
Fig. 4. Electrical properties including resistivity (q), carrier density (N) and carrier mobility (l) of IZO films deposited with various laser power.
seen in Fig. 4a, except the film grown with a laser power of 250 mJ/pulse, the resistivity of films is about 2– 3 103 X cm. Moreover, the carrier density of these films are in the range of 2.3–4.5 1020/cm3, similar to the previously reported values (Hosono, 2006). However, except the film grown with a laser power of 250 mJ/pulse, the values of carrier mobility of the IZO films are in the range of 8– 11 cm2 V1 s1 which are much smaller than the reported values of 30–40 cm2 V1 s1 (Hosono, 2006). The most possible reason is the existence of a large number of oxygen vacancies produced during the deposition process owing to the absence of oxygen in the PLD chamber and the transport of carriers is restricted by these oxygen vacancies. Optical transmission and reflection spectra of a-IZO films are illustrated in Fig. 5a. The average transmission in the visible range of all films is about 80% and almost the same. The similarity between spectra of these samples indicates that the transmission was not affected by the laser power used to ablate the target during PLD. The reflection of all films in the wavelength longer than 300 nm is lower than 30%, as shown in Fig. 5a. The transparent IZO films can also be used as the anti-reflection coating in the devices. The optical band gaps (Eg) of these films can be estimated by the relationship between absorption coefficient (a) and photon energy (hm) of the form (ahm) (hm Eg)r with r = 2 suggested by Tauc for amorphous
2.0
2.5
3.0
3.5
4.0
4.5
Photon energy (eV) Fig. 5. (a) Transmission and reflection spectra of IZO films grown on glasses. (b) The optical bandgap of IZO films is estimated to be 3.1 eV.
semiconductors (Tauc, 1979, Normura et al., 2008). The optical bandgap of IZO films is estimated to be 3.2 eV by linear extrapolation of (ahm)0.5 to the hm-axis, as depicted in Fig. 5b. Fig. 6 shows the cross-sectional HRTEM image of a SIS structure of the a-IZO/SiOx/n-Si which exhibits the highest conversion efficiency. The Si substrate is confirmed to be covered by a SiOx layer with a thickness of 1.7 nm.
1.7 nm
IZO SiOx Si
5 nm Fig. 6. The cross-sectional TEM image of a SIS structure of the a-IZO/ SiOx/n-Si device fabricated with a laser power of 400 mJ/pulse.
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594 30
Table 1 Photovoltaic characteristics including open-circuit voltage (Voc), shortcircuit current density (Jsc), and fill factor (FF) of a-IZO/SiOx/n-Si devices fabricated with various laser power.
(a)
25
250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
2
J (mA/cm )
20 15 10 5
2593
P (mJ/pulse)
g (%)
Voc (V)
Jsc (mA/cm2)
FF (%)
250 300 350 400 450 500
1.2 1.1 0.9 2.2 0.7 1.3
0.15 0.20 0.21 0.24 0.21 0.21
24.1 19.6 17.2 28.4 15.6 23.2
31.8 29.8 26.3 33.6 23.3 25.2
0 -5 -1.0
-0.5
0.0
0.5
1.0
V (V) 30
250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
20
2
J (mA/cm )
25
15 10 5
(b)
0 0.00
0.05
0.10
0.15
0.20
0.25
V (V) Fig. 7. Current density (J) vs. voltage (V) curves of a-IZO/SiOx/n-Si devices measured in dark (a) and under AM1.5 100 mW/cm2 illumination (b). P is the laser power used to deposite IZO films.
The current density (J)–voltage (V) curves of all a-IZO/ SiOx/n-Si cells measured in the dark and under 100 mW/ cm2 illumination (AM1.5 condition) are shown in Fig. 7a and b, respectively. As seen in Fig. 7a, all devices exhibit rectifying behaviors. However, the reverse current mainly contributed from leakage and nearly linear behavior at high forward bias indicate low barrier height between two semiconductors and possible additional transport via intermediate states at IZO/SiOx and SiOx/Si interfaces. The photovoltaic characteristics including conversion efficiency (g), Voc, Jsc and fill factor (FF) of the a-IZO/ SiOx/n-Si devices derived from the J V curves plotted in Fig. 7b are listed in Table 1. The device exhibiting the highest conversion efficiency g = 2.2% is the one fabricated with a laser power of 400 mJ/pulse. And the Voc, Jsc and FF of this device are 0.24 V, 28.4 mA/cm2 and 33.6%, respectively. The Voc of SIS solar cells is mainly determined by the barrier height (denoted as /B in Fig. 1) related to the difference between the work functions of the IZO layer and Si substrate (Shewchun et al., 1978). Since the work function of IZO (4.9 eV) is larger than that of ITO (4.6 eV) (Minami et al., 1998), the Voc of IZO/SiOx/Si cells is expected to be larger than that of ITO/SiOx/Si cells. However, the largest
Voc obtained in this study is 0.24 V which is only one half of the reported value (0.48–0.5 V) of ITO/SiOx/Si structures (Shewchun et al., 1979, Feng et al., 1979). Moreover, the photovoltaic characteristics of the IZO/SiOx/Si devices fabricated with various laser power do not exhibit dependence on the electrical and optical properties of IZO films. It is reported by Spitzer et al. that the Voc decreases with increasing the density of interface states at the SiOx/Si interfaces (Spitzer et al., 1980), owing to that the interface states play three roles as, firstly, charge traps which can change the barrier height /B, secondly, recombination centers which supply a shunt to light-generated current, and thirdly, intermediate local states which provide additional tunneling paths through the insulating layer. The low Voc of fabricated devices in this work is attributed to the existence of interface states at SiOx/Si interfaces, since the Voc of fabricated devices show no correlation with the properties of IZO films. It can be deduced from the results obtained in this work that the transport of carriers in the IZO/SiOx/Si heterojunction is dominated by the insulating SiOx layer. Therefore, a process to produce thin SiOx layers with few SiOx/Si interface states on Si wafers is necessary for fabricating TCO/SiOx/Si solar cells with high conversion efficiency. In comparison with the epitaxial films, amorphous TCO films grown at low temperatures are more suitable for fabricating SIS solar cells. 4. Conclusions This work demonstrates the feasibility of using amorphous In–Zn–O (a-IZO) films to form SIS solar cells consist of ultra thin SiOx layers covered n-type Si substrates and a-IZO films deposited by using pulsed laser deposition technique. The resistivity and average transimission in the visible range of a-IZO films deposited at 150 °C are 2– 3 103 X cm and 80%, respectively. The best a-IZO/ SiOx/n-Si device exhibits a conversion efficiency of 2.2%, and the open-circuit voltage, short-circuit current density and fill factor of this device are 0.24 V, 28.4 mA/cm2 and 33.6%, respectively. Acknowledgment This work was supported by the National Science Council of Taiwan, under Grant No. NSC 98-2112-M-003-005MY3.
2594
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594
References Feng, T., Ghosh, A.K., Fishman, C., 1979. Appl. Phys. Lett. 35, 266. Feng, T., Ghosh, A.K., Fishman, C., 1979. J. Appl. Phys. 50, 4972. Ghosh, A., Fishman, C., Feng, T., 1978. J. Appl. Phys. 49, 3490. Hosono, H., 2006. J. Non-Cryst. Solids 352, 851. Kobayashi, H., Mori, H., Ishida, T., Nakato, Y., 1995. J. Appl. Phys. 77, 1301. Malik, O., De la Hidalga-W, F.J., Zu´n˜iga-I, C., Ruiz-T, G., 2008. J. NonCryst. Solids 354, 2472. Maruska, H.P., Ghosh, A.K., Eustace, D.J., Feng, T., 1983. J. Appl. Phys. 54, 2489. Minami, T., Miyata, T., Yamamoto, T., 1998. Surf. Coat. Technol. 108, 583.
Normura, K., Kamiya, T., Jayaraj, M.K., Saji, K.J., Hosono, H., 2008. J. Vac. Sci. Technol. B 26, 495. Ramamoorthy, K., Jayachandran, M., Sankaranarayanan, K., Misra, Pankaj, Kukreja, L.M., Sanjeeviraja, C., 2004. Sol. Energy 77, 193. Shewchun, J., Singh, R., Green, A., 1977. J. Appl. Phys. 48, 765. Shewchun, J., Dubow, J., Myszkowski, A., Singh, R., 1978. J. Appl. Phys. 49, 855. Shewchun, J., Dubow, J., Wilsen, C., Singh, R., Burk, D., Wager, J., 1979. J. Appl. Phys. 50, 2832. Spitzer, M., Shewchun, J., Burk, D., 1980. J. Appl. Phys. 51, 6399. Tauc, J., 1979. Amorphous and Liquid Semiconductors. Plenum, New York.
Solar Energy 85 (2011) 2589–2594 www.elsevier.com/locate/solener
Fabrication and characterization of amorphous In–Zn–O/SiOx/ n-Si heterojunction solar cells Hau-Wei Fang a, Shiu-Jen Liu b,⇑, Tsung-Eong Hsieh a, Jenh-Yih Juang c, Jang-Hsing Hsieh d b
a Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan Department of Mathematics and Science (Precollege), National Taiwan Normal University, Linkou, Taipei 244, Taiwan c Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan d Department of Materials Engineering, Mingchi University of Technology, Taishan, Taipei 243, Taiwan
Received 2 February 2011; received in revised form 31 May 2011; accepted 30 July 2011 Available online 29 August 2011 Communicated by: Associate Editor Igor Tyukhov
Abstract Amorphous In–Zn–O (a-IZO) films were deposited on SiOx covered n-type Si substrates by using pulsed laser deposition (PLD) technique to form a-IZO/SiOx/n-Si heterojunction solar cells. The a-IZO films grown at 150 °C with various laser power (250–500 mJ/pulse) exhibit low resistivity (2–3 103 X cm) and high transparency (80%) in the visible wavelength range. The highest conversion efficiency of the fabricated a-IZO/SiOx/n-Si solar cells is 2.2% under 100 mW/cm2 illumination (AM1.5 condition). The open-circuit voltage, shortcircuit current density and fill factor of the best device are 0.24 V, 28.4 mA/cm2 and 33.6%, respectively. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Amorphous In–Zn–O film; Semiconductor–insulator–semiconductor solar cell; Pulsed laser deposition; Photovoltaic characteristics
1. Introduction In comparison with Si-base pn junction solar cells, semiconductor-insulator-semiconductor (SIS) solar cells composed of transparent conducting oxide (TCO), i.e. wide bandgap oxide semiconductors, and SiOx covered Si have been proposed to be low-cost technology (Shewchun et al., 1978; Feng et al., 1979). There were several SIS solar cells exhibiting high conversion efficiency (g) reported, including ITO/SiOx/p-Si (g = 12.8%) (Shewchun et al., 1979), ITO/ SiOx/n-Si (g = 12%) (Malik et al., 2008), SnO2/SiOx/n-Si (g = 8.8–14.1%) (Ghosh et al., 1978; Maruska et al., 1983) and ZnO/SiOx/n-Si (g = 6.9%) (Kobayashi et al., 1995). According to the theoretical calculation and experimental results reported by Shewchun et al. (1977, 1978) the ⇑ Corresponding author. Tel.: +886 2 77148400; fax: +886 2 26022617.
E-mail address: [email protected] (S.-J. Liu). 0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.07.016
tunneling process is regarded as the dominant mechanism for the carriers transport from Si to TCO through the interfacial insulator layer. Therefore, the most critical parameters which affect the performance of SIS solar cells are the interfacial layer thickness and interface states which are controlled by the process employed to produce the insulating SiOx layers. The basic requirement for the SiOx layers between TCO and Si substrates is that the thickness must be less than 2 nm (Shewchun et al., 1978). Amorphous transparent conducting oxide films such as In–Zn–O (IZO) films with high transparency and low resistivity can be fabricated at low temperatures (Hosono, 2006). Consequently, the fabrication temperature of SIS solar cells can be lowered by growing amorphous IZO films on SiOx/Si substrates. Moreover, low-temperature process can reduce the possibility of increasing the thickness of the SiOx layers between the TCO layers and Si substrates. On the other hand, a study of epi-n-IZO thin films/h1 0 0i Si
2590
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594
solar cells has been reported previously (Ramamoorthy et al., 2004). However, the conversion efficiency and short-circuit current density (Jsc) of the epi-IZO/n-Si devices fabricated at 450 °C are relatively low (g = 0.0001% and Jsc = 0.24 lA/cm2). High-temperature fabrication process of the crystalline IZO layers would increase the thickness of the interfacial layer between the IZO layer and Si substrate and thus the tunneling transport of carriers is suppressed by the thick insulating interface. The aim of this work is to investigate the feasibility of fabricating solar cells based on SIS structures by depositing amorphous TCO films on SiOx covered n-type Si substrates. Amorphous IZO (a-IZO) films were grown on Si substrates covered by an thin SiOx layer to form a-IZO/ SiOx/n-Si heterojunction devices. The electrical, optical and structural properties of IZO films deposited with various laser power were also investigated. 2. Experiments The P-doped n-type Si (1 0 0) wafer with a resistivity of 1 X cm was firstly cleaned with acetone and deionized water, and then etched with 10% hydrofluoric acid (HF) for 2 min to remove the native oxide layer. A thin SiOx layer was produced by immersing the Si wafer in a hot H2O2 solution for 10 min. The thickness of the SiOx layer was estimated to be 1.7 nm by a high-resolution transmission electron microscopy (HRTEM) measurement. The a-IZO layers were deposited on SiOx covered Si wafer by using pulsed laser deposition (PLD) with ZnO/ In2O3 = 3/1 (at.%) ceramic oxide formed into a pellet as the target prepared by conventional solid state reactions from stoichiometric amounts of ZnO and In2O3 powders of 99.99% purity. After being sintered at 1200 °C in air for 2 h, the chemical composition of the target was examined by energy-dispersive X-ray spectroscopy (EDX) and the Zn content ratio Zn/(In + Zn) of the target is 69 at.%. A KrF excimer laser was used to ablate the target with power of 250–500 mJ/pulse and repetition rate of 5 Hz. The PLD chamber was evacuated to a base pressure lower than 1 106 torr before PLD. In order to enhance the conductivity of IZO films, following argon was used as the working gas and the chamber pressure was kept at 4 103 torr. Owing to the absence of oxygen during PLD, large amount of oxygen vacancies are expected to be produced in the amorphous films and the carrier density would be increased. The distance between the target and substrates was 4–5 cm. The temperature of substrates was kept at 150 °C during PLD. The thickness of all IZO films is in the range of 25–35 nm. The rear Si surface is covered by a Al metal layer as ohmic contact electrode by dc magnetron sputtering after removing the native SiOx layer. And Al metal grids were also formed on the IZO surface for current collection. The IZO films were simultaneously deposited on Corning 1737 glasses for electrical and optical properties measurements. The crystal structures of the IZO target
and films were studied by X-ray diffraction (XRD) scans with Cu Ka radiation. The Zn/(Zn + In) atomic ratios of films were determined by EDX. The electrical properties including resistivity, carrier concentration and carrier mobility were carried out using the four-probe van der Pauw method. Atomic-force microscopy (AFM) measurements were employed to examine the surface roughness of IZO films. The optical measurements of the films, transmission and reflection, were recorded using a UV–VIS double beam spectrometer in the wavelength of 200–900 nm.
3. Results and discussion Fig. 1 presents the schematic energy-band diagram of the IZO/SiOx/n-Si hetero-junction structure. /IZO and /Si are the IZO-to-SiOx and Si-to-SiOx barrier heights related to the work functions of IZO and Si, respectively. While the heterojunction is under illumination, the light-generated current is primarily due to the photoexcited electron-hole pairs in the Si. /B is the difference between /IZO and /Si and is the barrier height which dominates the open-circuit voltage (Voc) of the photovoltaic devices. However, there are interface states exist at IZO/SiOx and SiOx/Si interfaces, which directly affect the performance of SIS solar cells. The XRD measurement indicates that the IZO target used in this study is a mixture of In2O3 and Zn4In2O7 phases. On the other hand, no peak was observed in the XRD patterns of the IZO films deposited on glasses with various laser power, indicating the amorphous characteristics of IZO films grown at 150 °C. The XRD patterns are not shown here. Fig. 2 shows the Zn content ratio Zn/(Zn + In) of IZO films as a function of the laser power used for the film deposition. The horizontal dash line in the plot represents the Zn/(Zn + In) value of the target, i.e. 69 at.%. The Zn content ratios of films are higher than that of the target and decrease approximately with increasing the laser
φ IZO
φ Si φB
EF CB
Eg-Si
CB EF VB
Eg-IZO
VB
Interface States d IZO
SiOx
n-Si
Fig. 1. Schematic energy band diagram of the IZO/SiOx/n-Si heterojunction structure.
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594 85
Zn / (Zn+In) (at.%)
80
75
70
target
65
60 250
300
350
400
450
500
Laser Power (mJ/pulse) Fig. 2. The dependence of the Zn content ratios Zn/(In + Zn) of IZO films on the laser power used for film deposition.
2591
power. The higher Zn content ratios of films are possibly resulted from the different ablation rates of Zn4In2O7 and In2O3 phases of the target owing to low laser power. The Zn content ratio of films grown with a laser power of 500 mJ/pulse is 70 at.% which is almost the same with that of the target. The AFM images of IZO films are shown in Fig. 3. The scan area is 1 lm 1 lm and the z-scale is 20 nm per division. It is clearly observed that the surface roughness of IZO films increases monotonically with increasing the laser power. Since the crystallinity of the a-IZO films was not enhanced by increasing the laser power, which is deduced from the XRD patterns, the increase of the surface roughness is attributed to the bombardment damage by particles with high kinetic energy ablated from the target by high laser power. The mean roughness of the a-IZO films ranges between 0.3 nm (250 mJ/pulse) and 15.3 nm (500 mJ/ pulse). Fig. 4 shows the resistivity (q), carrier density (N) and mobility (l) of IZO films as a function of laser power. As
(a) 250 mJ
(b) 300 mJ
(c) 350 mJ
(d) 400 mJ
(e) 450 mJ
(f) 500 mJ
Fig. 3. AFM images of IZO films deposited with various laser power.
2592
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594 100
-3
ρ (10 Ω-cm)
6
Transmission / Reflection (%)
7
(a)
5 4 3 2 1
3
4.0
20
N (10 /cm )
4.5
3.5
(b)
60
40
Reflection 20
300
400
500
600
700
800
Wavelength (nm)
3.0
3000
(b) Laser power: 250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
0.5
(c)
(αhν) (cm eV)
-1
10 8
2000
0.5
-1 -1
Laser power: 250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
0
2.0 12
2
Transmission
80
2.5
μ (cm V s )
(a)
6 4 250
300
350
400
450
1000
500
Laser power (mJ/pulse)
0 1.5
Fig. 4. Electrical properties including resistivity (q), carrier density (N) and carrier mobility (l) of IZO films deposited with various laser power.
seen in Fig. 4a, except the film grown with a laser power of 250 mJ/pulse, the resistivity of films is about 2– 3 103 X cm. Moreover, the carrier density of these films are in the range of 2.3–4.5 1020/cm3, similar to the previously reported values (Hosono, 2006). However, except the film grown with a laser power of 250 mJ/pulse, the values of carrier mobility of the IZO films are in the range of 8– 11 cm2 V1 s1 which are much smaller than the reported values of 30–40 cm2 V1 s1 (Hosono, 2006). The most possible reason is the existence of a large number of oxygen vacancies produced during the deposition process owing to the absence of oxygen in the PLD chamber and the transport of carriers is restricted by these oxygen vacancies. Optical transmission and reflection spectra of a-IZO films are illustrated in Fig. 5a. The average transmission in the visible range of all films is about 80% and almost the same. The similarity between spectra of these samples indicates that the transmission was not affected by the laser power used to ablate the target during PLD. The reflection of all films in the wavelength longer than 300 nm is lower than 30%, as shown in Fig. 5a. The transparent IZO films can also be used as the anti-reflection coating in the devices. The optical band gaps (Eg) of these films can be estimated by the relationship between absorption coefficient (a) and photon energy (hm) of the form (ahm) (hm Eg)r with r = 2 suggested by Tauc for amorphous
2.0
2.5
3.0
3.5
4.0
4.5
Photon energy (eV) Fig. 5. (a) Transmission and reflection spectra of IZO films grown on glasses. (b) The optical bandgap of IZO films is estimated to be 3.1 eV.
semiconductors (Tauc, 1979, Normura et al., 2008). The optical bandgap of IZO films is estimated to be 3.2 eV by linear extrapolation of (ahm)0.5 to the hm-axis, as depicted in Fig. 5b. Fig. 6 shows the cross-sectional HRTEM image of a SIS structure of the a-IZO/SiOx/n-Si which exhibits the highest conversion efficiency. The Si substrate is confirmed to be covered by a SiOx layer with a thickness of 1.7 nm.
1.7 nm
IZO SiOx Si
5 nm Fig. 6. The cross-sectional TEM image of a SIS structure of the a-IZO/ SiOx/n-Si device fabricated with a laser power of 400 mJ/pulse.
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594 30
Table 1 Photovoltaic characteristics including open-circuit voltage (Voc), shortcircuit current density (Jsc), and fill factor (FF) of a-IZO/SiOx/n-Si devices fabricated with various laser power.
(a)
25
250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
2
J (mA/cm )
20 15 10 5
2593
P (mJ/pulse)
g (%)
Voc (V)
Jsc (mA/cm2)
FF (%)
250 300 350 400 450 500
1.2 1.1 0.9 2.2 0.7 1.3
0.15 0.20 0.21 0.24 0.21 0.21
24.1 19.6 17.2 28.4 15.6 23.2
31.8 29.8 26.3 33.6 23.3 25.2
0 -5 -1.0
-0.5
0.0
0.5
1.0
V (V) 30
250 mJ/pulse 300 mJ/pulse 350 mJ/pulse 400 mJ/pulse 450 mJ/pulse 500 mJ/pulse
20
2
J (mA/cm )
25
15 10 5
(b)
0 0.00
0.05
0.10
0.15
0.20
0.25
V (V) Fig. 7. Current density (J) vs. voltage (V) curves of a-IZO/SiOx/n-Si devices measured in dark (a) and under AM1.5 100 mW/cm2 illumination (b). P is the laser power used to deposite IZO films.
The current density (J)–voltage (V) curves of all a-IZO/ SiOx/n-Si cells measured in the dark and under 100 mW/ cm2 illumination (AM1.5 condition) are shown in Fig. 7a and b, respectively. As seen in Fig. 7a, all devices exhibit rectifying behaviors. However, the reverse current mainly contributed from leakage and nearly linear behavior at high forward bias indicate low barrier height between two semiconductors and possible additional transport via intermediate states at IZO/SiOx and SiOx/Si interfaces. The photovoltaic characteristics including conversion efficiency (g), Voc, Jsc and fill factor (FF) of the a-IZO/ SiOx/n-Si devices derived from the J V curves plotted in Fig. 7b are listed in Table 1. The device exhibiting the highest conversion efficiency g = 2.2% is the one fabricated with a laser power of 400 mJ/pulse. And the Voc, Jsc and FF of this device are 0.24 V, 28.4 mA/cm2 and 33.6%, respectively. The Voc of SIS solar cells is mainly determined by the barrier height (denoted as /B in Fig. 1) related to the difference between the work functions of the IZO layer and Si substrate (Shewchun et al., 1978). Since the work function of IZO (4.9 eV) is larger than that of ITO (4.6 eV) (Minami et al., 1998), the Voc of IZO/SiOx/Si cells is expected to be larger than that of ITO/SiOx/Si cells. However, the largest
Voc obtained in this study is 0.24 V which is only one half of the reported value (0.48–0.5 V) of ITO/SiOx/Si structures (Shewchun et al., 1979, Feng et al., 1979). Moreover, the photovoltaic characteristics of the IZO/SiOx/Si devices fabricated with various laser power do not exhibit dependence on the electrical and optical properties of IZO films. It is reported by Spitzer et al. that the Voc decreases with increasing the density of interface states at the SiOx/Si interfaces (Spitzer et al., 1980), owing to that the interface states play three roles as, firstly, charge traps which can change the barrier height /B, secondly, recombination centers which supply a shunt to light-generated current, and thirdly, intermediate local states which provide additional tunneling paths through the insulating layer. The low Voc of fabricated devices in this work is attributed to the existence of interface states at SiOx/Si interfaces, since the Voc of fabricated devices show no correlation with the properties of IZO films. It can be deduced from the results obtained in this work that the transport of carriers in the IZO/SiOx/Si heterojunction is dominated by the insulating SiOx layer. Therefore, a process to produce thin SiOx layers with few SiOx/Si interface states on Si wafers is necessary for fabricating TCO/SiOx/Si solar cells with high conversion efficiency. In comparison with the epitaxial films, amorphous TCO films grown at low temperatures are more suitable for fabricating SIS solar cells. 4. Conclusions This work demonstrates the feasibility of using amorphous In–Zn–O (a-IZO) films to form SIS solar cells consist of ultra thin SiOx layers covered n-type Si substrates and a-IZO films deposited by using pulsed laser deposition technique. The resistivity and average transimission in the visible range of a-IZO films deposited at 150 °C are 2– 3 103 X cm and 80%, respectively. The best a-IZO/ SiOx/n-Si device exhibits a conversion efficiency of 2.2%, and the open-circuit voltage, short-circuit current density and fill factor of this device are 0.24 V, 28.4 mA/cm2 and 33.6%, respectively. Acknowledgment This work was supported by the National Science Council of Taiwan, under Grant No. NSC 98-2112-M-003-005MY3.
2594
H.-W. Fang et al. / Solar Energy 85 (2011) 2589–2594
References Feng, T., Ghosh, A.K., Fishman, C., 1979. Appl. Phys. Lett. 35, 266. Feng, T., Ghosh, A.K., Fishman, C., 1979. J. Appl. Phys. 50, 4972. Ghosh, A., Fishman, C., Feng, T., 1978. J. Appl. Phys. 49, 3490. Hosono, H., 2006. J. Non-Cryst. Solids 352, 851. Kobayashi, H., Mori, H., Ishida, T., Nakato, Y., 1995. J. Appl. Phys. 77, 1301. Malik, O., De la Hidalga-W, F.J., Zu´n˜iga-I, C., Ruiz-T, G., 2008. J. NonCryst. Solids 354, 2472. Maruska, H.P., Ghosh, A.K., Eustace, D.J., Feng, T., 1983. J. Appl. Phys. 54, 2489. Minami, T., Miyata, T., Yamamoto, T., 1998. Surf. Coat. Technol. 108, 583.
Normura, K., Kamiya, T., Jayaraj, M.K., Saji, K.J., Hosono, H., 2008. J. Vac. Sci. Technol. B 26, 495. Ramamoorthy, K., Jayachandran, M., Sankaranarayanan, K., Misra, Pankaj, Kukreja, L.M., Sanjeeviraja, C., 2004. Sol. Energy 77, 193. Shewchun, J., Singh, R., Green, A., 1977. J. Appl. Phys. 48, 765. Shewchun, J., Dubow, J., Myszkowski, A., Singh, R., 1978. J. Appl. Phys. 49, 855. Shewchun, J., Dubow, J., Wilsen, C., Singh, R., Burk, D., Wager, J., 1979. J. Appl. Phys. 50, 2832. Spitzer, M., Shewchun, J., Burk, D., 1980. J. Appl. Phys. 51, 6399. Tauc, J., 1979. Amorphous and Liquid Semiconductors. Plenum, New York.