Large Neutral Barrier at Grain Boundaries in Chalcopyrite Thin Films

The electronic structure of grain boundaries in polycrystalline $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ thin films and their role on solar cell device efficiency is currently under intense investigation. A neutral barrier of about 0.5 eV has been suggested as the reason for the benign behavior of grain boundaries in chalcopyrites. Previous experimental investigations have in fact shown a neutral barrier but only a few 10 meV high, which cannot be expected to have a significant influence on the solar cell efficiency. Here we show that a full investigation of the electrical behavior of charged and neutral grain boundaries shows the existence of an additional narrow neutral barrier, several 100 meV high, which is tunneled through by the majority carriers but is sufficiently high to explain the benign behavior of the grain boundaries.

Figure 1

KPFM measurement on the ${\mathrm{CuGaSe}}_2$ bicrystal containing a $\Sigma 9$ grain boundary. (a) Topography and (b) CPD showing a dip along the GB. Averaged line profiles across the GB were extracted in the gray box indicated in the images for (c) topography and (d) CPD.

Figure 2

(a) Hall mobility of the $\Sigma 9$-GB showing a thermally activated barrier of $\sim 100\mathrm{meV}$. (b) Temperature dependence of the resistance for transport across the $\Sigma 9$-GB (solid squares) with fits considering only TE (open triangles) and TE and tunneling (open circles).

Figure 3

(a) Electronic structure model of the $\Sigma 9$-GB, consisting of a rectangular barrier and a SCR, used for simulating the electrical transport by Eq. (3). (b) Detailed view of the region close to the GB plane, including an atomistic sphere model (Cu—medium green, Ga—small gray, Se—large red).

Figure 4

(a) Temperature dependence of the resistance of the $\Sigma 3$-GB (solid squares) and fits using only TE (open triangles) and a combination of TE and tunneling (open circles), considering the (b) electronic structure model comprising a high and thin barrier (${\varphi }_b$) surrounded by lower rectangular barriers (${\varphi }_0$) and an atomistic sphere model (same colors as in Fig. 3).