Study Of Oxvaen-related Defects In Czochralski Silicon Used For Photovoltaics

Oxygen is one of the most important and inevitable impurities in CZ silicon.Oxygen is typically at the interstitial position with a concentration in the range of(5-10)× 1017 cm-3.The interstitial oxygen atoms can combine with impurities atoms or vacancies to form oxygen related complexes,which could have detrimental effects on the silicon.The oxygen atoms trend to form thermal donors during low temperature heat treatment,and oxygen precipitates during high temperature heat treatment,which could degrade the performance of materials and devices.Therefore,manipulation of oxygen-related defects in silicon is of great significance in terms of the impacts on the materials and devices.In this dissertation,effects of oxygen-related thermal donors on the performance of silicon heterojunction(SHJ)solar cells were explored experimentally.Also,the effects of iron contamination and hydrogen passivation on the properties of oxygen precipitates were investigated.The primary achievements have been listed below.(1)The influence of oxygen-related thermal donors(TDs)on the performance of silicon heterojunction(SHJ)solar cells was explored experimentally.It is found that a certain number of thermal donors could do much harm to the performance of SHJ solar cells.The efficiency of SHJ solar cells is usually reduced by a value of?1%absolutely in the case of thermal donors with a concentration of nearly 1015 cm-3.The microwave photoconductance decay and Hall Effect studies have proved that the TDs can significantly reduce the carrier lifetime of n-type silicon substrate,but have no influence on the carrier mobility.Deep level transient spectroscopy(DLTS)measurements have further demonstrated that the TDs can cause an energy level at Ec-0.13 eV with carrier capture cross-section of 10-15 cm2,and therefore reduce the.carrier lifetime and the solar cell efficiency.(2)The effects of iron contamination and hydrogen passivation on the properties of oxygen precipitates were investigated in this paper.We used the low-high two-step method to intentionally produce oxygen precipitation,which is monitored by means of etch pit observation and FTIR.It is found that oxygen precipitates in iron contaminated samples are smaller and denser than those without contamination.And then the electrical property is characterized by DLTS and EBIC.It is well-known that oxygen precipitates without metallic contamination generate relatively shallower energy levels,as detected by DLTS,while oxygen precipitates with metallic contamination have much deeper energy levels.The temperature dependence of EBIC contrast shows that iron contamination has the decisive influence on oxygen precipitates.The EBIC contrast of oxygen precipitates with contamination increases with the increased temperature,while those without contamination shows the opposite trend,as previously reported by literature.Finally,Plasma-Enhanced Chemical Vapor Deposition(PECVD)is used to develop a film of SiNx to passivate the defects.It is proved that hydrogen can effectively passivate the oxygen precipitates,no matter iron contamination is performed or not.The evolution of energy levels and recombination activity are then demonstrated.