Phonon coupling and shallow defects in $\mathrm{CuIn}{\mathrm{S}}_2$

$\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right){\mathrm{S}}_2$ is an emerging material for solar cells, reaching already efficiencies above 15%. For any solar cell material, it is essential to understand the recombination processes and doping behavior of its defects. Therefore, in this work, we present a detailed photoluminescence and admittance investigation of polycrystalline $\mathrm{CuIn}{\mathrm{S}}_2$ thin films to study the shallow intrinsic defects. We find evidence for two acceptors, 105 and 145 meV from the valence band, their predominance depending on the Cu excess, plus one donor, 30 meV from the conduction band. The high crystalline and electronic quality of our samples allows the detection of low intensity luminescence peaks. Based on these emissions we can analyze the phonon coupling of the observed defects and show that an emission previously attributed to a second donor is in fact a phonon replica of the first donor-acceptor transition.

Figure 1

Photoluminescence spectra of $\mathrm{CuIn}{\mathrm{S}}_2$ with different compositions measured at 10 K. The excitonic luminescence for the two samples with the lowest Cu/In ratio is magnified for a better visibility.

Figure 2

Intensity dependence of the band edge photoluminescence spectra measured at 10 K on the sample with a Cu/In 1.8. The spectra have been normalized and shifted in intensity for a better visualization. The vertical lines mark the energies of the different emissions.

Figure 3

(a) Excitation intensity dependence of the integrated PL flux of the four band edge emissions taken from the sample with a Cu/In ratio of 1.8. (b) Trend of the 1.520 eV emission (line C) together with 2 power laws with a $k$ value of 3/2 and 2/2, respectively. ${\mathrm{\Phi }}_0$ indicates the excitation threshold.

Figure 4

Measured photoluminescence spectra at 10 K (black) of $\mathrm{CuIn}{\mathrm{S}}_2$ thin film with Cu/In (a) 1.00 and (b) 1.83. Both spectra are fitted reasonably well with a Huang-Rhys spectrum of the DA1 and the DA2 plus a contribution of an additional deep emission (purple dotted line).

Figure 5

Excitation intensity dependent energy position of the two DA transitions in $\mathrm{CuIn}{\mathrm{S}}_2$ taken from the spectra of the samples with Cu/In ratio of (a) 1.00 and (b) 1.82.

Figure 6

Excitation intensity dependent intensity of the two DA transitions in $\mathrm{CuIn}{\mathrm{S}}_2$ taken from the spectra of the samples with Cu/In ratio of (a) 1.00 and (b) 1.82. The dash-dotted lines represent the single power law, while the line of multiple power laws, with the corresponding $k$ values, are reported as well. ${\mathrm{\Phi }}_1$ and ${\mathrm{\Phi }}_2$ indicate the excitation thresholds.

Figure 7

Excitation intensity dependent spectra of the two FB transitions in $\mathrm{CuIn}{\mathrm{S}}_2$ taken from the spectra with Cu/In ratio of (a) 1.00 for FB1 and (b) 1.82 for FB2.

Figure 8

Arrhenius plot for Mo/$\mathrm{CuIn}{\mathrm{S}}_2$/Al stack indicating the activation energy of the main capacitance step for the CIS samples grown in Cu-rich excess.

Figure 9

Recombination model for $\mathrm{CuIn}{\mathrm{S}}_2$ based on PL measurements. The defect energies reported have to be assumed with an error of about ± 5 meV.