Otimização por simulação e desenvolvimento de células solares com emissor posterior formado por pasta de alumínio e difusão em forno de esteira

摘要

Photovoltaic solar energy is the direct convertion of solar energy into electricity and it has low impact to the environment during electric energy production. The main device of this technology is the solar cell and silicon is the substrate most used. The solar cells are electrically connected and encapsulated in order to form the photovoltaic module. The aims of this thesis are to optimize, develop and to analyse n+np+ solar cells processed in n type Si-PV-FZ and with aluminum rear emitter formed in belt furnace. The optimization of solar cells by simulation is an important step before the device development. The software PC-1D and another program developed using Visual Basic language were used. Considering a metal grid formed by evaporation technique in vaccum ambient an efficiency of 16. 8 % may be achieved. With screen printed grid, 15. 8 % efficient solar cells were obtained. From the simulation results it was found that the screen printing metallization may become more viable than evaporation technique because there is low difference in the efficiency and the screen printing is a simpler technique. The experimental optimization of silicon wafers texture process resulted in reflectance of 12 %. This value is tipical for monocrystalline silicon with textured surface. Experimental optimization of phosphorus front surface field shows a sheet resistance of (36 ± 4) `/ı for this region. This region was formed in a thermal step in a conventional furnace with POCl3. It was found that after the phosphorus diffusion occurred gettering to specific temperature and time. It was verified that the minority carrier lifetime in the final of processing is similar to the initial value. The influence of steps sequence of front silver paste firing and rear diffusion/firing aluminium paste, of surface passivation and the influence of dry air flow during the aluminium paste diffusion/firing, of aluminium paste diffusion/firing temperature and of belt speed under the fabricated devices electric characteristics were analysed. The aluminum rear emitter n+np+ silicon solar cells resulted in efficiencie up to 9,5 %. The best solar cells were processed under 900 °C to aluminium paste diffusion/firing and belt speed of 140 cm/min. The efficiency is limited by low values of VOC and FF reached. It was also verified that the local rear emitter formation results in solar cells with higher efficiency than those with homogeneous emitter.