Long Lifetime Hole Traps at Grain Boundaries in CdTe Thin-Film Photovoltaics

A novel time-resolved cathodoluminescence method, where a pulsed electron beam is generated via the photoelectric effect, is used to probe individual CdTe grain boundaries. Excitons have a short lifetime ($\le 100\mathrm{ps}$) within the grains and are rapidly quenched at the grain boundary. However, a $\sim 47\mathrm{meV}$ shallow acceptor, believed to be due to oxygen, can act as a long lifetime hole trap, even at the grain boundaries where their concentration is higher. This provides direct evidence supporting recent observations of hopping conduction across grain boundaries in highly doped CdTe at low temperature.

Figure 1

Continuous wave CL results from polycrystalline CdTe at 12 K. (a) Panchromatic “image” extracted from a hyperspectral map. CL spectra extracted from the grain “G1” and grain boundary “GB” in (a) are shown in (b); the total intensity has been normalized. Normalized intensity distributions for the $A^{0}X$, $e{A}^0/{\mathrm{DAP}}_1$, and ${\mathrm{DAP}}_2$ spectral features are shown in (c)–(e), respectively. The total intensity at each pixel has been normalized to remove effects of enhanced nonradiative recombination at grain boundaries. Panel (f) shows the ${\mathrm{DAP}}_2$ central wavelength in nanometers as a function of position.

Figure 2

Pulsed CL data from polycrystalline CdTe at 12 K showing (a) time-integrated spectra from G1 and GB positions in Fig. 1 (total intensity has been normalized). The peak labeled “LO” is the longitudinal optical phonon replica of the $e{A}^0/{\mathrm{DAP}}_1$ transition. Panels (b) and (c) show decay curves for the $A^{0}X$ and $e{A}^0/{\mathrm{DAP}}_1$ peaks, respectively (note the difference in time scale).

Figure 3

Room temperature, continuous wave CL from polycrystalline CdTe showing (a) spectra from a grain interior G1 and grain boundary GB; the total intensity has been normalized. Panels (b) and (c) show the normalized (with respect to total signal) intensity distribution of the 818 nm [arrowed in (a)] and above band gap (i.e., 750–790 nm) emission, respectively. The spatial positions G1 and GB are indicated in (b).