Study on silicon for photovoltaic application using hot wire chemical vapor deposition techniques
Creators: name: Hossion, M Abul
Publisher: name: Zenodo
Year: 2017
Subjects: HWCVD; thin film; photosensor; silicon; IR laser

description: Large grain polycrystalline silicon thin film on low cost and robust substrate is an <br> interesting area of research for photovoltaic devices. This film can be used to fabricate solar <br> cell which has a potential to achieve 20% efficiency like multi crystalline silicon solar cell with <br> the advantage of thin film fabrication techniques. In this thesis we have investigated the <br> possibility of growing large grain (220) oriented poly crystalline silicon films, 2 to 3 micron <br> thick, from silane‐hydrogen mixture, by hot wire chemical vapour deposition (HWCVD). <br> Several substrates i) Si/SiO2, ii) TiO2 layer on glass, iii) textured nickel‐5% tungsten metal strip, <br> iv) crystalline sapphire and v) alkali free borosilicate glass have been used in this work. <br> The growth of intrinsic poly crystalline silicon film was performed starting with a thin <br> nucleation layer at 400C followed by the thickening stage at 600C with higher silane <br> concentration. We have optimized the growth conditions and one micron of well passivated <br> hydrogenated (220) oriented polycrystalline film were grown successfully. We have used <br> several characterization techniques for the evaluation of the optical, topographical and <br> electrical properties of the film. <br> The growth of boron doped poly crystalline and amorphous silicon film was also <br> performed following by process similar to that used for intrinsic poly silicon film using HWCVD <br> with the addition of diborane to the gas mixture for boron doping the silicon. <br> The HWCVD system used for the synthesis of intrinsic and p‐type polycrystalline <br> silicon films did not have provision for n‐type doping. For synthesis of phosphorous doped n‐<br> type silicon film, we initially explored doping intrinsic polycrystalline silicon film by using two <br> different sources of phosphorous followed by Laser annealing. One approach used i) spin‐on <br> phosphorous dopant film about one micron thick and the other approach used ii) <br> phosphorous ion implantation. <br> For both these approaches, we have investigated the possibility of dopant activation <br> by irradiation with infrared continuous wave laser which can uniformly scan the sample along <br> X‐Y directions. Laser annealing is performed varying the laser power and scan speed to ensure <br> suitable laser annealing condition. Sheet resistivity measurement was carried out to verify the <br> laser annealing process. In this case the several characterization techniques were also <br> followed for the evaluation of the optical, topographical and electrical properties of the film. <br> ‐ 9 ‐ <br> <br> However, the n‐type silicon films synthesised in this manner were not device quality. At this <br> stage, we got access to another HWCVD reactor equipped with synthesis of n‐type silicon <br> using phosphine as the dopant gas. <br> After the above growth experiments, we took initiative to stack all these silicon layers <br> in order to apply them in photovoltaic application. In this case we have fabricated an n‐i‐p <br> structure on glass using two different HWCVD systems, starting with synthesis n‐type silicon <br> on glass in one reactor. This was transferred to the second HWCVD reactor for the synthesis <br> of intrinsic polycrystalline silicon film. After this, a combination of a thin layer of amorphous <br> intrinsic silicon followed by p‐type amorphous silicon film was deposited to form a hetero <br> junction between the intrinsic polycrystalline silicon and p‐type amorphous silicon layer. The <br> structure is well known as heterojunction with intrinsic thin layer (HIT). In our case the intrinsic <br> polycrystalline silicon layer acts as the light absorber layer, the top p‐type amorphous silicon <br> acts as emitter layer, while the bottom n‐type silicon acts as back surface field. The <br> performance of the device was evaluated from dark and illuminated I‐V characterization. It <br> was observed that the light response was very weak. Further work is necessary to improve <br> the photovoltaic action. For this, we have to increase the thickness and material quality of the <br> light absorbing layer, and improve the quality of junctions.