光伏多晶硅晶界杂质吸收与钝化的层次结构

This thesis examines grain boundaries, dislocations and metallic impurities in  photovoltaic multicrystalline silicon (PV mc-Si). The interaction dynamics between  hierarchy of grain boundaries (GBs), nickel, iron, and intragrain defects during thermal  and hydrogenation processes have been studied. Localized Deep Level Transient  Spectroscopy (DLTS) analysis was applied to quantify the changes induced by rapid  thermal annealing (RTA) in trap states and accompanying decrease of carrier  recombination in respect to the hierarchy of GBs and the dislocation density. A unique  in-situ microscopic masking process for Schottky diode preparation which practically  enables the localized probing of specific electrical active defect structure has been  developed to investigate the local properties of mc-Si wafers. The reduction of  recombination activity at GBs after RTA was found to be dominated by impurity  decoration and less influenced by the relative structural mismatch for a specific type of  GB. A new type of spectra was observed in PV thin mc-Si and its origins are attributed to  the additional barrier height caused by GB hetro-junction.

The gettering capabilities of GBs were found to follow the sigma order in high sigma GB  group but no consistent gettering relationship was found between gettering strength and  low sigma group (Σ3 and twins). Electron bean induced current (EBIC) result shows that  most of dislocation sites and twins (intragrain defects) present a weak or no impurity  gettering tendency if they co-exist with GBs. GBs present as a more favored gettering  sites to attract impurities under annealing condition. GB gettering strength presents a higher dependence on the hierarchy of crystal defect and lower dependence on the  impurity type. GB plane also plays an influential role in the gettering behavior of GBs.  On the other hand, the concentration of electrically “inactive” metals due to the presence  of structural defect was quantified by subtracting the difference between the bulk metallic  impurity concentration and electrical trap concentration. The electrical inactive level of  nickel was found a least one order of magnitude higher than iron.

Photovoltaic solar cells are an important contributor to the exploitation of renewable  energy. More than 90% of current production uses silicon as the active material in these  devices and at the present time about half of this is multi-crystalline material. This type of  silicon is considerably cheaper that higher quality single crystal, but its efficiency as a  photovoltaic material is lower. The present situation is that continuing improvements in  multi-crystalline silicon (mc-Si) solar cell efficiencies has reached a stage where, for  further progress, it is essential to understand the underlying materials physics that  controls the electrically activity of structural defects, impurities and their mutual  interactions.

The performance of crystalline solar cells is generally limited by lattice defects,  which reduce the minority carrier lifetime. Dislocations, grain boundaries and intragrain defects, such as certain impurities, small clusters of atoms or precipitates, are mainly  responsible for the recombination processes. Particularly localized regions of high  dislocation densities are known to be rather detrimental. 2 Dislocations and grain  boundaries play an important dynamic role in the determination of electrical properties  because of their gettering and storage capacity for certain mobile metallic impurities. If  dissolved in the lattice, most of the metal impurities are very recombination active.  Because of the high mobility down to lower temperatures, they can accumulate at the  core of extended defects, which is energetically more favorable. In the bound state, the  impurities are, in general, less recombination active.

Fig1

These effort discussed in the previous paragraph verified that the oxidation- induced  dislocations are better correlated with shallow trap E with E_V + 0.10 eV and misfit  dislocations can be attributed to trap B. This further provides the explanation on the  higher amplitude of trap B in HOB-2 than it is in HOB. The second oxidation at elevated  temperatures caused further dislocation reactions including glide and climb processes so that the concentration of dislocations in twice-oxidation sample HOB-2 is higher than one  time oxidation sample HOB.