**9.1 Introduction**

132 Solar Cells – Thin-Film Technologies

deep junction SINP photovolatic device violet and blue enhanced SINP photovolatic device

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deep junction SINP photovolatic device violet and blue enhanced SINP photovolatic device

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wavelength(nm) Fig. 12. Comparison of the responsivity for the violet and blue SINP photovoltaic device and

The novel ITO/SiO2/np Silicon SINP violet and blue enhanced photovoltaic device has been fabricated by thermal diffusion of phosphorus for shallow junction to enhance the spectral responsivity within the wavelength range of 400-600nm, the low temperature thermally grown a very thin silicon dioxide and RF sputtering ITO antireflection coating to reduce the reflected light and enhance the sensitivity. The ITO film was evinced to a high quality by UV-VIS spectrophotometer, four point probe and Hall-effect measurement. Fairly good

wavelength(nm)

Fig. 11. Comparison of EQE for violet and the blue SINP photovoltaic device and the deep

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the deep junction SINP photovoltaic device.

**8.3.4 Conclusions** 

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Responsivity(mA/W)

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External quantum efficiency(%)

junction SINP photovoltaic device.

As shown in the previous work, semiconductor-insulator-semiconductor (SIS) diodes have certain features, which make them more attractive for the solar energy conversion than conventional Shottky, MIS, or other heterojunction structures (Mridha et al., 2007). For example, efficient SIS solar cells such as indium tin oxide (ITO) on silicon have been reported, where the crystal structures and the lattice parameters of Si (diamond, a = 0.5431 nm), SnO2 (tetragonal, a = 0.4737 nm, c = 0.3185 nm), In2O3 (cubic, a = 1.0118 nm) show that they are not particularly compatible and thus not likely to form good devices. However, the SIS structure is potentially more stable and theoretically more efficient than either a Schottky or a MIS structure. The origins of this potential superiority are the suppression of majority-carrier tunneling in the high potential barrier region of SIS structure, and the existence of thin interface layer which minimizes the amount and the impact of the interface states. This results in an extensive choice of the p-n junction partner with a matching band gap in the front layer. In addition, the top semiconductor film can serve as an antireflection coating (Dengyuan et al., 2002), a low-resistance window, and the collector of the p-n junction as well.

Furthermore, the semiconductor with a wide band gap as the top layer of SIS structure can eliminate the surface dead layer which often occurs within the homojunction devices, such as the normal bulk silicon based solar cells. On the other side, this absence of the light absorption of visible region in a surface layer can improve the ultraviolet response of the internal quantum efficiency. Among many transparent conductive oxides (TCO) of the transition metals, ZnO:Al is one the best n-type semiconductor layer. It has high conductivity, high transmittance, optimized surface texture for light trapping, and large band gap of Eg≈ 3.3 eV. Thus, in this description, we show a photovoltaic device with AZO/SiO2/p-Si frame, as an attempt to study its opto-electronic conversion property and the I-V features as well. The schematic and the bandgap structure of the novel AZO/SiO2/p-Si SIS heterojunction device was show here (Fig.13).

#### **9.2 Experimental in details**

For the purpose of fabricating SIS structure, p-type Si (100) wafers were used as the substrates of the heterojunction device. The wafers were firstly prepared by a stand cleaning procedure, then, they were dipped in 10% HF solution for one minute to remove native oxide layer. Finally, the wafers were dried in a flow gas of nitrogen.

By thermal evaporation, 1 μm-thick Al electrode was deposited on the back side. Then the samples were annealed at 500°C for 20 min in N2:O2=4:1 condition to form good ohmic contact and a very thin oxide layer (about 15~20Å) was grown on the p-Si surface.

TCO-Si Based Heterojunction Photovoltaic Devices 135

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Fig. 15. I-V characteristic of the AZO/SiO2/p-Si/Al heterojunction device in dark and light

A linear I-V behavior between the two electrodes on the surface of ZnO:Al film indicates a good ohmic contact. The current-voltage characteristic of the AZO/SiO2/p-Si/Al heterojunction device was measured at room temperature in the dark (Fig.14). Typical rectifying is observed for this heterojunction with polar to covalent semiconductors structure. The weak photon irradiation I-V characteristics were measured under two kinds of illumination by low power white light (6.3mW/cm2) lamp and 20W halogen lamp (in Fig.15). The good rectifying with the increase of photoelectric current was observed for the typical interface mismatching device. Under reverse bias conditions the photocurrent caused by the ZnO surfaces exposing in the low power white light lamp and 20W halogen lamp was obviously much larger than the dark current. For example, when the reverse bias is -5V, the dark current is only 3.05×10-3A.While the photocurrent reach to 4.06×10-3A and 6.99×10-3A under low power white light and halogen lamp illumination, respectively.

The novel AZO/SiO2/p-Si/ heterojunction has been fabricated by magnetron sputtering deposition AZO film on p-Si substrate. Fairly good rectifying and photoelectric behaviors are observed and analyzed by I-V measurements in detail. The ideality factor and the saturation current of this diode is 20.1 and 1.19×10-4A, respectively. The results indicated that the novel AZO/SiO2/p-Si/ heterojunction device could be not only used for low cost solar cell, but also could be used for the high quantum efficiency enhanced photodiode in

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**I SC=0.128mA**

**VOC=130mV**

(light-1: 6.3mW/cm2 white light; Light-2: 20W halogen lamp)

UV and visible lights, and also for other applications.

**Voltage(V)**

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**9.3 I-V characteristics** 

**9.4 Conclusions** 

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The Al doped ZnO films were deposited on the oxidized silicon substrates in a RF magnetron sputtering system. The target was a sintered ceramic disk of ZnO doped with 2 wt% Al2O3 (purity 99.99%). The base pressure inside the chamber was pumped down to less than 5×10-4 Pa. Sputtering was carried out at a working gas (pure Ar) pressure of 1Pa. The Ar flow ratio was 30 sccm. The RF power and the temperature on substrates were kept at 100W and 300°C, respectively. The sputtering was proceeded for 2.5 hours. The area is 2×2 cm2.

The thickness of AZO film was measured by step profiler. The optical transmission of the films was measured by UV-VIS spectrophotometer. The electrical properties of Al doped ZnO films were characterized by four point probe. The current-voltage characteristics of the device was measured by Agilent 4155C semiconductor parameter analyzer (with probe station, the point diameter of a probe is 5 μm).

Fig. 13. The structure of AZO/SiO2/p-Si heterojunction PV device.

Fig. 14. I-V curve of the Al/AZO/SiO2/p-Si/Al heterojunction device in dark.

The Al doped ZnO films were deposited on the oxidized silicon substrates in a RF magnetron sputtering system. The target was a sintered ceramic disk of ZnO doped with 2 wt% Al2O3 (purity 99.99%). The base pressure inside the chamber was pumped down to less than 5×10-4 Pa. Sputtering was carried out at a working gas (pure Ar) pressure of 1Pa. The Ar flow ratio was 30 sccm. The RF power and the temperature on substrates were kept at 100W and 300°C, respectively. The sputtering was proceeded for 2.5 hours. The area is

The thickness of AZO film was measured by step profiler. The optical transmission of the films was measured by UV-VIS spectrophotometer. The electrical properties of Al doped ZnO films were characterized by four point probe. The current-voltage characteristics of the device was measured by Agilent 4155C semiconductor parameter analyzer (with probe

**-5 -4 -3 -2 -1 0 1 2 3 4 5**

**Voltage(V)**

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Fig. 14. I-V curve of the Al/AZO/SiO2/p-Si/Al heterojunction device in dark.

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2×2 cm2.

station, the point diameter of a probe is 5 μm).

Fig. 13. The structure of AZO/SiO2/p-Si heterojunction PV device.

**-5 -4 -3 -2 -1 0 1 2 3 4 5**

**-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020 Current(A)**

Fig. 15. I-V characteristic of the AZO/SiO2/p-Si/Al heterojunction device in dark and light (light-1: 6.3mW/cm2 white light; Light-2: 20W halogen lamp)
