Modeling the effects of Na incorporation on CIGS solar cells

Abstract

In this paper, we quantitatively evaluate the relative importance of three mechanisms that are proffered by various groups to interpret the effects of sodium (Na) incorporation in copper indium gallium diselenide solar cells. The suggested mechanisms are 1) increase in the carrier density due to defect passivation; 2) spatial redistribution of gallium (Ga); and 3) change in the crystal orientation. The simulation framework which is developed for this purpose indicates that, among these three coexisting effects, the increase in the carrier density with Na incorporation is likely to be most important. If the grain boundaries (GBs) initially contain donor-like traps that are subsequently passivated by Na, the increase in carrier density can improve the cell efficiency significantly. On the other hand, we find that the effects of Ga redistribution and change in crystal orientation are limited.

Homogeneous Na incorporation for industrial‐scale application of Cu(In,Ga)(Se,S)2 solar cells

“…Na diffusion supplied from glass significantly enhances both carrier density and lifetime. This is consistent with previous studies as well as earlier discussion, where Na plays important roles in the defect passivation and thereby increases V OC and J SC . However, variations in the carrier density and lifetime are less pronounced, in the case of further Na incorporation using the Na layers, apparently to be saturated.…”

“…Following 20‐nm Na layer incorporation, the creation of nanometer‐/micrometer‐scale voids is observed at the CIGSS/Mo interface. Furthermore, we found that the CIGSS crystal size is gradually reduced in proximity to the Mo layer after Na incorporation, which is similar with previous reports . For quantitative analysis on the voids, we intentionally decapped the CIGSS layer from the Mo layer and measured the surface FE‐SEM images on the CIGSS layer of the decapped side as described in Figure C,D.…”

“…Since the change in R S is consistent with that in FF, R S appears to be a more determinant factor in the dramatic variation in the FF. In general, the presence of Na at CIGSS layers leads to the increase in the carrier density, resulting from the passivation of defects such as grain boundaries and vacancies and/or the Na doping effects . In terms of the defect passivation, especially in the case of grain boundaries, the decrease in the trap density can enhance the lifetime as well as mobility of itinerant charge carriers .…”

“…In general, the presence of Na at CIGSS layers leads to the increase in the carrier density, resulting from the passivation of defects such as grain boundaries and vacancies and/or the Na doping effects . In terms of the defect passivation, especially in the case of grain boundaries, the decrease in the trap density can enhance the lifetime as well as mobility of itinerant charge carriers . These changes following Na incorporation can be the reasons for the increases in J SC and V OC in the Na‐doped CIGSS samples (SLG).…”

“…This scaling‐up issue is primarily due to large‐area inhomogeneity in the crystal quality of the CIGSS absorber layers, complicatedly depending on defects, grain boundaries, and surface/interface quality; so it is necessary to develop uniform process technologies of the absorber formation for reliable industrial‐scale production. In particular, it is important to uniformly incorporate alkali elements like sodium (Na) into the absorber layer, because they are deeply concerned with the passivation of the crystal defects and the consequential improvement in device performance …”