Direct Evidence for a Reduced Density of Deep Level Defects at Grain Boundaries of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ Thin Films

The unusual optoelectronic properties of chalcopyrite grain boundaries (GBs) have become the subject of an intense debate in recent years. In this work we investigate the defect density at GBs of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ by scanning tunneling spectroscopy. Contrary to our expectation, our results give evidence for a reduced density of deep level defects and point to an increased density of defect levels in resonance with the lower conduction band at GBs. Our findings imply low recombination activity at GBs, and thus can explain the low impact of GBs on the efficiency of chalcopyrite based solar cells.

Figure 1

$(1\times 1)\mu {\mathrm{m}}^2$ $dI/dU$-CITS images at different bias voltages, extracted from $100\times 100$ $I(U)$ spectra [see also the $dI/dU$ movie 12 ]. The blue areas indicate sites, where the current limit of the $I(U)$ measurement is exceeded.

Figure 2

(a) $dI/dU$ spectra recorded in the vicinity of a GB. The three bold lines are averaged spectra, where the locations of all single spectra used for the averaging are marked by symbols at the locations $A$, $B$, and $C$ in the inset CITS image [see gray box in Fig. 1 at $U=1.0\mathrm{V}$]. The single spectra $d$, $e$, and $f$ visualize a band bending effect towards the GB. (b) Schematic energy level diagram at a GB. Simulated $dI/dU$ spectra clarifying the influence of (c) band bending and (d) additional defect levels.