The characterisation and fabrication of a-SiOx:H/c-Si heterojunction solar cells

Amongst the silisium based photovoltaic technologies a-Si:H/c-Si heterojunctions are getting more important because of their potentials to obtain high efficiency and low production costs. Low production costs because of ability to grow thin film layers on extensive surfaces and low production heat (roughly 200C ), better heat coefficients and thinner silisium substrates used for better surface passivation Heterojunction solar cells are more advantageous to the homojunction technologies used widely because of the low silisium consumption. While this thesis was being worked on (in 2013) the Japanese company Panasonic has shown that the high efficiency by heterojunctions with extensive surfaces having a surface area of 101.8 cm2 and 24.7% efficiency is possible within the industrial production conditions by producing these solar cells. Nevertheless the full potential of these structures have not yet been identified completely and some wide range researches have been carried on by some groups. This thesis is focused on understanding the effects of the different cleaning processes of the c-Si substrate surfaces to the interface defect passivation of the doped and intrinsic SiOX:H compounds by analyzing the electrical and electrooptical features. The passivation of the crystal silisium (c-Si) substrate and reducing the interface defects are the basic needs of the growth of the highly efficient a-Si:H/c-Si heterojunction solar cells widely known as silisium heterojunctions. Surface passivation is provided mainly by developing silisium substrate cleaning processes in detail and determining the PECVD parameters necessary to grow the hydrogenated amorph silisium layers. Nevertheless, producing the (i) layer with high temperatures (>130C) results in an epitaxial layer on c-Sİ that seriously reduces the efficiency of the solar cells This epitaxial growth (i) restraints the growing process of a-Si:H layer and make the optimization of the solar cell difficult. The biggest advantage of doped and intrinsic a-SiOx:H layer is that they suppress the epi Si growth on c-Si substrate by the a-SiOx:H (i) layer Specially a-SiOx:H i layer enables us to freely change the PECVD conditions before the epi Si layer is formed. The application of the a-SiOx:H (i) layer improves the repeatability of the solar cells since the a-Si:H (i) layers' optimum deposition conditions are at the a-Si:H/epi-Si phase limit for the maximum performance, photovoltaic structures having emitter qualification with wider band width is suggested in the past. For the a-Si:H/c-Si solar cells, a-Si:H based compound having a wider band width than a-Si:H is preferred to improve the Jsc short circuit current. The aim of this thesis is to produce solar cells which have a structure of metal /(n) (p) a-Si:H/(i) a-Si:H/(p) (n) c-Si/(i) qa-Si:H/n+ a-Si:H HIT type and at the layers of (n) (p) a-SiOx:H/(i) a-SiOx:H/(p) c-Si. Within this aim different film growing conditions have been tried, solar cell parameters and the optimum parameters of the growing conditions have been determined and the solar cells have been produced using these conditions. When examined from this angle film conditions of which a-Si:H films have been used as buffer layer were observed. After the structural features like spectroscopic ellipsometer (SE), FTIR have been analyzed for the films that were grown over the Corning glass and quartz substrate, the undoped (intrinsic) a-Si:H films were grown on the p- type doped c-Si substrate. Also the plasma and chemical cleaning methods of c-Sİ substrate have been comparatively analysed. After then the substrate heat, PECVD parameters (RF power, pressure, SiH4 gas flow), photo conductivity, dark conductivity and dielectric constant have been followed and analyzed for the undoped a-Si films that are proper for making device. The homogenity and the smoothness of the surface of a-Si:H films have been analyzed by SEM and AFM techniques. Afterwards the films have been grown on c-Si with these conditions and a-Si:H/c-Si heterojunctions have been obtained. By following the open circuit tension the optimum doping ratio is identified as 5x103 ppm and 5x104 ppm [60] for both undoped a-Si:H films' passivation success and for doped (n-type) a-Si:H films that were grown in order to reduce the resistance The results of the I-V characteristics of light condition for the measurements of the solar cell which was grown 5x103 ppm and has a structure of Al/(n) a-Si:H/i a-Si:H/p c-Si/Al have been determined as 4.6% effectiveness and over the active area of 72 cm2 This value is the highest efficiency value of the highest area but the effectiveness value is fairly low. It has been understood from the electroluminescence measurements that the surface of the undoped layer has not been effectively passivated and grown partially epitaxial. That's why the surface of the c-Si has been poorly passivated. Instead of using 200C as the most appropriate substrate temperature, another series of solar cell with lower temperature and dilution of hydrogen have been produced. The first group has a temperature 190C and higher substrates whereas the other group is made up of solar cells which has a temperature lower than190C doped layer was grown with the constant temperature between 190C and 225C. According to the results when 190C is chosen for the doped layer, the ideal temperature of undoped layer is between 190C-210C, whereas if it is chosen as 225C then the ideal temperature is found out as 150C. It has been understood that the expected efficiency of the solar cells cannot be met by using c-Si surface passivation, undoped a-Si:H films, instead of a-Si:H films silicon suboxide(a-SiOX:H) is decided to be used to meet this target. To reach this target the literature review is carried out and at the existing system conditions of [SiH4]:[CO2]:[H2] sccm the amount of CO2 has been changed in the range of 1-10 sccm. The produced a-SiOX:H have been analyzed by SE method and by considering dielectric constant. Also the changes of a series of solar cells produced by using a-SiOX:H buffer layer and dilution of H2 have been followed up. The results of the first trials showed that worse solar cells have been produced. The highest short circuit current and open circuit tension is observed in the series of the films that were grown by the dilution ratios of R=7 and 10 and tempered for 2 minutes. Additionally the correlation between the dilution of hydrogen and the series resistance have been followed up and it has been observed that when the dilution amount is increased the series resistance is decreased so it is passivated relatively better. Additionally the effect of the thickness of a-SiOX:H to the solar cell parameters has been analyzed by following up both by finding out the specifications of the cell and the quantum efficiency. After determining that the highest response to blue light is achieved within a-SiOX:H films it is understood that like the i a-Si:H layer it has to be grown by 60 s. Afterwards, how the efficiency of the solar cell have been affected using n type doped a-SiOX:H window layer films instead of n type a-Si:H films additional to i a-SiOX:H layers have been analyzed by following up the solar cell parameters and quantum efficiencies. For this purpose the literature review is carried out and at the existing system conditions of [PH3]:[SiH4]:[CO2] sccm the amount of PH3 has been changed in the range of 20-60 sccm. In these series of samples for the solar cells which have maximum efficiency with a doping ratio of 40:40:20 sccm, the optimum thickness of n-type doped a-SiOX:H films is tried to be determined by using solar cell parameters and quantum efficiency It has been observed that the tempering heat of these solar cells are 300C which is higher than their growing temperature of 190C and tempering period of 80-90 minutes. The most appropriate growing period is observed as 80s.