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We investigated the near infrared enhancement in Cu(In,Ga)Se2 (CIGS)- based solar cells utilizing a hydrogen-doping ZnO (ZnO:H) window layer. The results show that the carrier concentration of ZnO:H film is lower than that of AZO film which can increase the transmittance in the NIR. The advantage of ZnO:H film is higher Hall mobility than AZO film. Thus ZnO:H film has similar resistivity to AZO film. It was found that the cell efficiency was 12.4 and 13% for the AZO device and the ZnO:H device, respectively. The cell efficiency is enhanced by 4.8%. Furthermore, the results indicate that, the ZnO:H film is superior to the AZO film as the window layer for CIGS-based solar cells.
©2012 Optical Society of America
1. Introduction
Zinc Oxide (ZnO) film is a transparent semiconductor with a high bandgap that can be used for transparent electrodes in thin-film solar cells and light-emitting diodes (LEDs) [1–3]. Although undoped ZnO film exhibits high resistivity, the conductivity of the ZnO film can be improved by impurity-doping such as B, Al, or Ga [4,5]. Al-doping ZnO (AZO) films are the material commonly used as the window layer in the Cu(In,Ga)Se2 (CIGS)–based thin film solar cells. The requirements for good performance large-area CIGS modules are high transmission in order to transmit the sunlight to the absorber layer and low sheet resistance as an electrode to reduce the series resistance of the device. The optimum thickness of the AZO film needs to be considered. However, one drawback is that AZO films exhibit free carrier absorption loss, which leads to a decrease in quantum efficiency in the near infrared region (NIR). The low sheet resistance is incompatible with high optical transmission when the thickness of the AZO film increases [6, 7].
Recently, it has been reported by Van de Wall that hydrogen impurities act as shallow donors in ZnO [8, 9]. Based on the density function theory, H+ is the stable and lowest-energy state for all Fermi-level positions and can be incorporated into ZnO as a promising source of n-type conductivity. Experimental results have shown the high transparency and conductivity of hydrogen-doping ZnO (ZnO:H) film fabricated by various hydrogenation processes [10,11]. In this study, we used the highly transparent ZnO:H film to replace AZO film as the window layer in the CIGS-based solar cells to improve the quantum efficiency in the NIR.
2. Experiments
The CIGS thin films were fabricated using a two-step selenization process. In the first step, 650-nm thick films of CuInGa precursors were deposited by the DC magnetron sputtering of a Cu0.9In0.75Ga0.25 ternary target onto Mo-coated soda-lime glass substrates. In the second step, the above mentioned metallic precursors were selenized using H2Se vapor in a quartz tube furnace. The selenization process was accomplished by ramping up from room temperature to the final reaction temperature of 550°C for 20 min. 50-nm thick Cadmium Sulfur (CdS) buffer layers were deposited on top of the CIGS thin films using the Chemical Bath Deposition (CBD) method. The 50-nm intrinsic ZnO (i-ZnO) and TCO window layers were deposited by a radio frequency (RF) magnetron sputtering using an intrinsic ZnO target and AZO target, respectively. In this study, Al-doping ZnO (AZO) and hydrogen-doping ZnO (ZnO:H) window layers are discussed. The ZnO:H film was deposited with hydrogen and argon gases. Finally, Ni (50 nm) / Al (3 μm) grids were fabricated by DC magnetron sputtering to complete the solar cell fabrication. The electrical resistivity, Hall mobility, and carrier concentration were measured by using a Hall measurement system (BIO-RAD HL5500). The optical transmittance of TCO films with glass were measured by a UV-visible spectrometer (Hitachi U4100). The illuminated J–V characteristics of the devices were measured under the standard measurement conditions. (100 mW cm−2, A.M. 1.5 spectrum at 25 °C)
3. Results and discussion
The electrical properties of the ZnO:H films under the different hydrogen flow ratios are shown in Fig. 1 . The carrier concentration of the ZnO:H films can be significantly increased by increasing the hydrogen flow ratio. This is because hydrogen atoms behave as shallow donors in the ZnO film [12]. Even the concentration of hydrogen atoms cannot be analyzed easily [13], the result shows that the effective hydrogen concentration should be depended on the hydrogen flow ratio. The resistivity of the ZnO:H films deposited at room temperature decreases with increasing hydrogen flow ratio, reaching a minimum at a hydrogen flow ratio of 10%. The lowest resistivity of the ZnO:H film is 1.29 × 10−3 Ω-cm. The decrease in resistivity is attributed to an increase in both the Hall mobility and carrier concentration. The corresponding Hall mobility and carrier concentration reach 32.2 cm2/V s and 1.5 × 1020 cm−3, respectively. However, when the hydrogen flow ratio increases further to 10%, the resistivity of the ZnO:H films is increased due to a rapid decrease of Hall mobility. This is caused by the beneficial influence of hydrogen atoms incorporated into the ZnO film thereby resulting in an increase in the density of electron scattering centers. As shown in Fig. 2 , all of the ZnO:H films have a diffraction peak around 34.3° in the XRD spectrum, corresponding to the (0002) diffraction of the wurzite-type ZnO. The (0002) peaks in the XRD spectra shift to lower 2θ values with the increase in the hydrogen flow ratio, indicating an expansion of the crystal lattice. The phenomenon occurs is because the hydrogen atoms can be significantly incorporated into the ZnO films with an increase in the hydrogen flow ratio during deposition.
As shown in Fig. 3 , the average optical transmittance of the ZnO:H films is above 80% in the visible range (400 nm- 800 nm). It is obvious that the optical transmittance of ZnO:H films shows a decrease in the NIR when increasing the hydrogen flow ratio. This is caused by the free carrier absorption due to an increase of hydrogen atoms being incorporated into the ZnO film [14]. Thus the increase of carrier concentration leads to increasing the absorption in the longer wavelength region. To apply the ZnO:H film as the window layer of a CIGS solar cell, the hydrogen flow ratio of 10% is the better process for the film with the less resistivity and higher transmittance in the NIR.
CIGS solar cells with the Al/ Ni/ TCO (window layer)/ i-ZnO/ CdS/ CIGS/ Mo structure shown in Fig. 4 were fabricated utilizing the process shown in Section 2. We use the ZnO:H film to replace the AZO film as the window layer in order to investigate the difference in cell performance. The AZO and ZnO:H window layers were both deposited at room temperature in order to decrease the diffusion processes in the interface between CdS and CIGS [15]. Figure 5 shows the optical transmission for different thickness of the AZO and ZnO:H films. When we increase the thicknesses of films from 400 nm to 600 nm, the average transmission (400 nm - 1300 nm) of the AZO films decreases from 80.9% to 78% while that of the ZnO:H films remains near 83.5%. A drastic change in the NIR can be seen for the AZO film which can be attributed to the higher carrier concentration than the ZnO:H film, resulting in an increase in free carrier absorption, as shown in Table 1 . However, the ZnO:H film has similar resistivity to the AZO film. This is because the Hall mobility of the ZnO:H film is higher than that of AZO film. The results suggest that when increasing the thickness of the film, we can obtain a lower sheet resistance and higher transmittance of ZnO:H films at the same time.
Table 1. Electrical Properties of the AZO and ZnO:H Films
The ZnO:H film is superior to the AZO film for using as a window layer in CIGS-based solar cells. Figure 6 shows the J-V characteristics of the CIGS solar cell fabricated using AZO and ZnO:H window layers. All the window layers are 400 nm. The active area of the CIGS solar cell is 0.4 cm2. Comparison with the AZO and ZnO:H devices show that the fill factor (FF) and open circuit voltage (Voc) are very similar. It can be seen that the cell efficiencies (Eff.) are 12.4% and 13.0% for the AZO device and ZnO:H device, respectively. The cell efficiency is enhanced by 4.8% for the ZnO:H device. This is attributed to the improvement in the short-circuit current (Jsc) from 34.5 to 35.6 mA/cm2 with 3.2% of improvement. As shown in Fig. 7 , the external quantum efficiency (EQE) of the CIGS solar cell fabricated using a AZO film is slightly higher than that of the ZnO:H film in the short wavelength. This is because the carrier concentration of AZO film is higher than ZnO:H film resulting in the transmittance edge shifts to a lower wavelength. This phenomenon is due to the filling of the conduction band by electrons, which is known as Burstein-Moss effect. However, the EQE of the CIGS solar cell fabricated using a ZnO:H film is higher than that of the AZO film in the NIR. To integrate the total amount of EQE in Fig. 7, the improvement is enhanced by 3.2% for the ZnO:H device. The results are consistent with Jsc. We can say that the increase in Jsc is caused by the higher optical transmission of the ZnO:H film in the NIR due to the lower free carrier absorption loss.
Fig. 7 External quantum efficiency curves of the CIGS solar cell fabricated using AZO and ZnO:H window layers.
4. Conclusion
Near infrared enhancement in CIGS-based solar cells utilizing a ZnO:H window layer have been investigated. The resistivity improvement of ZnO:H film is attributed to the hydrogen atoms behaving as shallow donors within the ZnO film. The lowest resistivity of the ZnO:H film is 1.29 × 10−3 Ω cm under process parameter of the hydrogen flow ratio 10%. The corresponding Hall mobility and carrier concentration reach 32.2 cm2/V s and 1.5 × 1020 cm−3, respectively. The carrier concentration of ZnO:H film is lower than that of AZO film which can increase the transmittance in the NIR. The advantage of ZnO:H film is higher Hall mobility than AZO film. Thus ZnO:H film has similar resistivity to AZO film. It is found that the cell efficiency is enhanced by 4.8% for the ZnO:H device. This is attributed to the fact that the ZnO:H film has higher transmittance than the AZO film in the NIR which results in the improvement of short-circuit current (Jsc) from 34.5 to 35.6 mA/cm2. In conclusion, the ZnO:H film is superior to the AZO film as the window layer for CIGS-based solar cells.
Acknowledgments
The authors would like to thank the National Science Council of Taiwan for financial support of this research.
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