Study on semiconductor material silicon for photovoltaic application using hot wire chemical vapor deposition techniques

Abstract

Large grain polycrystalline silicon thin film on low cost and robust substrate is an interesting area of research for photovoltaic devices. This film can be used to fabricate solar cell which has a potential to achieve 20% efficiency like multi crystalline silicon solar cell with the advantage of thin film fabrication techniques. In this thesis we have investigated the possibility of growing large grain (220) oriented poly crystalline silicon films, 2 to 3 micron thick, from silane-hydrogen mixture, by hot wire chemical vapour deposition (HWCVD). Several substrates i) Si/SiO2, ii) TiO2 layer on glass, iii) textured nickel-5% tungsten metal strip, iv) crystalline sapphire and v) alkali free borosilicate glass have been used in this work. The growth of intrinsic poly crystalline silicon film was performed starting with a thin nucleation layer at 400C followed by the thickening stage at 600C with higher silane concentration. We have optimized the growth conditions and one micron of well passivated hydrogenated (220) oriented polycrystalline film were grown successfully. We have used several characterization techniques for the evaluation of the optical, topographical and electrical properties of the film. The growth of boron doped poly crystalline and amorphous silicon film was also performed following by process similar to that used for intrinsic poly silicon film using HWCVD with the addition of diborane to the gas mixture for boron doping the silicon. The HWCVD system used for the synthesis of intrinsic and p-type polycrystalline silicon films did not have provision for n-type doping. For synthesis of phosphorous doped n-type silicon film, we initially explored doping intrinsic polycrystalline silicon film by using two different sources of phosphorous followed by Laser annealing. One approach used i) spin-on phosphorous dopant film about one micron thick and the other approach used ii) phosphorous ion implantation. For both these approaches, we have investigated the possibility of dopant activation by irradiation with infrared continuous wave laser which can uniformly scan the sample along X-Y directions. Laser annealing is performed varying the laser power and scan speed to ensure suitable laser annealing condition. Sheet resistivity measurement was carried out to verify the laser annealing process. In this case the several characterization techniques were also followed for the evaluation of the optical, topographical and electrical properties of the film. However, the n-type silicon films synthesised in this manner were not device quality. At this stage, we got access to another HWCVD reactor equipped with synthesis of n-type silicon using phosphine as the dopant gas. After the above growth experiments, we took initiative to stack all these silicon layers in order to apply them in photovoltaic application. In this case we have fabricated an n-i-p structure on glass using two different HWCVD systems, starting with synthesis n-type silicon on glass in one reactor. This was transferred to the second HWCVD reactor for the synthesis of intrinsic polycrystalline silicon film. After this, a combination of a thin layer of amorphous intrinsic silicon followed by p-type amorphous silicon film was deposited to form a hetero junction between the intrinsic polycrystalline silicon and p-type amorphous silicon layer. The structure is well known as heterojunction with intrinsic thin layer (HIT). In our case the intrinsic polycrystalline silicon layer acts as the light absorber layer, the top p-type amorphous silicon acts as emitter layer, while the bottom n-type silicon acts as back surface field. The performance of the device was evaluated from dark and illuminated I-V characterization. It was observed that the light response was very weak. Further work is necessary to improve the photovoltaic action. For this, we have to increase the thickness and material quality of the light absorbing layer, and improve the quality of junctions.