Study On The Silicon-based Violet-enhanced Photoelectric Cell Of SINP And Device With SIS Hetero-junction Configuration

In order to reduce the technical complexity of the fabrication and simultaneously to improve the quantum efficiency in the range of 400-600 nm, a new type of ITO/SiO2/np-Silicon frame (named as SINP) hetero-interface solar cell has been designed and developed by combining the p-n junction with ITO transparent conductive oxide thin film as top layer of the device, according to the energy band evolution. A series of advanced techniques have been applied to the fabrication process, such as shallow junction, back surface field of aluminum, ultra-thin SiO2 passivation layer, and ITO coating. On the other hand, the novel semiconductor-insulator-semiconductor (SIS) hetero-junction structure has been fabricated by thermally growing an ultrathin silicon dioxide at low temperature and subsequently magnetron sputtering deposition of Al-doped n-ZnO (named as AZO) layer on p-Si (100) wafer. The microstructure, optics and electrical properties of the ITO and AZO films were characterized by XRD, SEM, UV-VIS spectrophotometer and Hall effect measurement, respectively. The influence of the substrate temperatures on the properties of ITO and AZO films were deeply studied. The current-voltage (I-V),capacitance -voltage (C-V) and spectral response characteristics of SINP hetero-interface solar cell and SIS heterojunctions were analyzed in detail.1. ITO film possesses high quality in terms of antireflection and electrode functions. The influence of the substrate temperatures on the property of ITO film was deeply studied. The results indicated that ITO film possesses high quality , when the substrate temperature is at 480℃. The resistivity is as low as 9.42×10-5?·cm, the carrier concentration and mobility are as high as 3.46×1021 atom/cm3 and 19.1 cm2/V·s, respectively. The average transmittance of the film is about 95% in the visible region. The film has a bixbyite structure and a preferred growth along the (222) orientation, perpendicular to the substrate surface. The configuration of the ITO layer on the textured Si is suitable for the blue and ultraviolet cells owing to the less loss of the number of photons with a short wavelength, which is more favorable for high efficiency cells.2. The ultra-thin SiO2 layer and SINP structure samples were analyzed by x-ray photoelectron spectroscopy (XPS). The results showed that the non-stoichiometric matter were present on the surface layers and which resulted in the electronic defects to the minority carrier transportation. 3. The I-V curves of SINP violet photoelectric cell showed the fairly good rectifying behaviors. The values of IF/IR (IF and IR stand for forward and reverse currents, respectively) at 1.0 V for the violet SINP device is as high as 324.7,and the ideality factor of the violet SINP photoelectric cell is 1.84. The conversion efficiencies of the violet SINP cell under illumination of continue light source (AM 1.5 class A) is12.26%. Because the high quality crystallite and the good conductivity of ITO film magnetron-sputtered on SiO2 lead to a great decrease of the lateral resistance, it is possible to decrease the power loss induced by the lateral resistance of emitter region in the new type solar cell with SINP structure. The short-circuit current density of the SINP violet solar cell is as high as JSC = 38.7 mA/cm2. The external quantum efficiencies (EQE) and responsivity of the violet SINP device are 70% and 285mA/W at 500nm, respectively. The spectral responsivity peak of the violet SINP photoelectric cell is 487mA/W at about 800nm.The latest result indicates that the violet SINP photoelectric cell with 60?/□shallow junction can greatly improve the violet response. The calculated results indicates that the high quantum efficiency and responsivity of violet and blue enhanced photovoltaic cell is attributed to the shallow junction and the good conducting, violet and blue antireflection coating of ITO optical film. We found that the main feature of our PV cell is the enhanced violet response and optoelectronic conversion. The improved short-circuit current, open-circuit voltage, and filled factor indicate that the device is promising to be developed into an ultraviolet and blue enhanced photovoltaic device in the future.4. The AZO films were prepared by direct-current (DC) magnetron sputtering. The target was sintered ceramic disks of ZnO doped with 2 wt% Al2O3 (purity 99.99%). The AZO film possesses high quality in terms of electrode functions, when substrate temperature is at 480℃. The resistivity is as low as 3.483×10-3?·cm. The average transmittance of the film is about 90% in the visible region. The AZO film has a hexagonal wurtzite structure with its dominant orientation along the c-axis perpendicular to the substrate surface.5. Polycrystalline AZO films were prepared by direct-current (DC) reactive sputtering technique with Zn/Al alloy target. The AZO film possesses high quality in terms of electrode functions, when substrate temperature is 480℃. The resistivity is as low as 4.973×10-4?·cm, the carrier concentration and mobility are as high as 1.103×1021 atom/cm3 and 11.4 cm2/V·s, respectively. The average transmittance of the film is about 90% in the visible region. The AZO film has a hexagonal wurtzite structure with its dominant orientation along the c-axis perpendicular to the substrate surface.6. The properties of the electronic junctions were investigated by I-V measurement, which reveals that the SIS heterojunction showed a rectifying behavior under a dark condition. It shows fairly good rectifying behavior indicating the formation of a diode between AZO and p-Si. By the comparison of AZO films and SIS heterojunction prepared by different sputtering technique, the conductivity of AZO film prepared by DC reactive sputtering is much higher than that of the AZO film prepared by DC sputtering. Therefore, more photon induced current can easily flow through AZO layer prepared by DC reactive sputtering and enter into the Cu front contact. Thus, higher photocurrent is obtained under a reverse bias. Because the resistivity of ITO film is much lower than that of AZO films, the magnetron- sputtering of the ITO film with the high quality crystallite and the good conductivity on AZO/SiO2/p-Si lead to a great decrease of the lateral resistance. The ITO layer was as the large area surface electrode. Thus, the photon induced current can easily flow through ITO layer entering the Cu front contact, since there is a parallel connection and a high mobility of the top layer of the ITO/AZO/SiO2/p-Si SIS heterojunction. Most high photocurrent can be obtained under a reverse bias. The results indicated that the novel SIS heterojunctions can be not only used for low cost solar cell, but also used for high quantum efficiency of UV and visible lights enhanced photodiode for various applications.