Phosphorous Diffusion Gettering of Trapping Centers in Upgraded Metallurgical‐Grade Solar Silicon

Abstract

in mc-Si produced by metallic impurities acting as traps and recombination centers.

Reduction of trapping and recombination in upgraded metallurgical grade silicon: Impact of phosphorous diffusion gettering

“…Figure 3b shows, in a box-type plot, the t2 values obtained for all A-type and B-type wafers before and after the gettering. As it can be observed, similar values of t2, ranging around 15 ms, are found after PDG in both types of samples.This indicates that the PDG process has been effective in removing the slow traps originally present in B-type wafers, being probably related with a getterable impurity [27]. This is however not the case for the fast trap present in A-type wafers, which is PDGinsensitive.…”

“…To characterize the trapping processes, the PC raw measurements were directly analyzed, thereby avoiding the assumptions required to convert these data into lifetime values. PC values were satisfactorily fitted to a two-exponential function of the form y=A1•exp(-x/t1)+A2•exp(-x/t2), as shown in Figure 1, following the method thoroughly explained in [27]. Note that the photovoltage signal is proportional to the PC, and that when the time constant of the first exponential t1 follows the light decay, the time constant of the second exponential t2 can be assigned to the dominant trap.…”

“…Since it is well known that these trapping centers may increase the recombination of the bulk [25], some works have tried to eliminate trapping centers through procedures such as PDG and laser treatments [21,26,27]. Attempts to unveil the microscopic origin of traps in Si have only been partly successful: e.g., hints have been found about a linear relationship between oxygen concentration and trap concentration, as concluded from quantitative analyses [11,28].…”

Boosting Charge Carrier Mobilities in Upgraded Metallurgical Grade Silicon by Phosphorous Diffusion Gettering

“…The characteristic apparent lifetime increase observed at low injection levels in both samples is a well-known effect associated with carrier trapping. [22,23] Carrier trapping effectively delays the consumption of excess carriers upon recombination, which is reflected in the aforementioned equation as an apparent increase in τ. Bulk carrier lifetimes are therefore better estimated at sufficient injection levels so as to avoid entering the exponential-like, trap-related curvature.…”

External Gettering of Metallic Impurities by Black Silicon Layer

“…Gettering is especially widely used in Si-based solar cell production, for which low-cost solar-grade Si is increasingly used. [3][4][5][6][7][8][9][10] Such material contains various undesirable metal impurities (Au, Cu, Fe, Ni, etc.) and structural defects, such as grain boundaries, dislocations, second-phase precipitates, oxidation-induced stacking faults (OISF), etc.…”