Levent Gütay

Raman analysis of monoclinic Cu2SnS3 thin films

Secondary phases like Cu2SnS3 are major obstacles for kesterite thin film solar cell applications. We prepare Cu2SnS3 using identical annealing conditions as used for the kesterite films. By x-ray diffraction, the crystal structure of Cu2SnS3 was identified as monoclinic. Polarization-dependent Raman investigations allowed the identification of the dominant peaks at 290 cm−1 and 352 cm−1 with the main A′ symmetry vibrational modes from the monoclinic Cu2SnS3 phase. Furthermore, micro-resolved Raman investigations revealed local variations in the spectra that are attributed to a secondary phase (possibly Cu2Sn3S7). This exemplifies the abilities of micro-resolved Raman measurements in the detection of secondary phases.

Detecting ZnSe secondary phase in Cu2ZnSnSe4 by room temperature photoluminescence

Secondary phases, such as ZnSe, occur in Cu2ZnSnSe4 and can be detrimental to the resulting solar cell performance. Therefore, it is important to have simple tools to detect them. We introduce subband gap defect excitation room temperature photoluminescence of ZnSe as a practical and non-destructive method to discern the ZnSe secondary phase in the solar cell absorber. The PL is excited by the green emission of an Ar ion laser and is detected in the energy range of 1.2–1.3 eV. A clear spatial correlation with the ZnSe Raman signal confirms this attribution.

Degradation and passivation of CuInSe2

The degradation of CuInSe2 absorbers in ambient air is observed by the decay of the quasi-Fermi level splitting under well defined illumination with time. The decay is faster and stronger in absorb ...

Subgrain size inhomogeneities in the luminescence spectra of thin film chalcopyrites

We report near-field photoluminescence (PL) spectra of Cu(InGa)Se2 thin films recorded with a lateral optical resolution of ≈200 nm and simultaneous detection of the sample topography. Our results reveal significant local variations in the PL spectra, specifically the PL yield, on length scales of 0.2–1.5 μm. Local variations in both the splitting of quasi-Fermi levels μ and the band gap energy are quantitatively extracted from the PL spectra by applying Planck’s generalized law. We show pronounced fluctuations of μ and the band gap on length scales below the grain size. These fluctuations are only weakly correlated with the topographic film structure.

The three A symmetry Raman modes of kesterite in Cu 2 ZnSnSe 4

We investigate CZTSe films by polarization dependent Raman spectroscopy. The main peaks at 170 cm(-1), and 195 cm(-1) are found to have A symmetry. The Raman signal at 170 cm(-1) is found to be composed of two modes at 168 cm(-1) and 172 cm(-1). We attribute these three Raman peaks to the three A symmetry modes predicted for kesterite ordered Cu(2)ZnSnSe(4). The main Raman peak is asymmetrically broadened towards lower energies. Possible sources of the broadening are tested through temperature and depth dependent measurements. The broadening is attributed to phonon confinement effects related to the presence of lattice defects.

Influence of copper excess on the absorber quality of CuInSe2

The compositional dependence of the optoelectronic quality of CuInSe2 thin film absorbers is investigated on single- and poly-crystalline films with varying [Cu]/[In]-ratios. We quantify the quality of the absorbers by the splitting of quasi-Fermi levels, determined by spectral photoluminescence. This quantity determines the maximum achievable open circuit voltage by an absorber. Our results indicate a significant increase of this value for growth under Cu-excess, indicating a decrease of recombination losses. By comparison of the predicted achievable open circuit voltage and the actually measured ones of finished solar cells, we find a huge “un-utilized potential” for the Cu-rich devices.The compositional dependence of the optoelectronic quality of CuInSe2 thin film absorbers is investigated on single- and poly-crystalline films with varying [Cu]/[In]-ratios. We quantify the quality of the absorbers by the splitting of quasi-Fermi levels, determined by spectral photoluminescence. This quantity determines the maximum achievable open circuit voltage by an absorber. Our results indicate a significant increase of this value for growth under Cu-excess, indicating a decrease of recombination losses. By comparison of the predicted achievable open circuit voltage and the actually measured ones of finished solar cells, we find a huge “un-utilized potential” for the Cu-rich devices.

Lone conduction band in Cu2ZnSnSe4

We present experimental proof for a narrow first conduction band in Cu2ZnSnSe4 semiconductor films as it has been predicted by theoretical calculations. The optical absorption characteristics of Cu2ZnSnSe4 thin films are analyzed by optical transmission spectroscopy. The experimental data show strong evidence for three absorption edges, as expected from theory, involving the valence band and two conduction bands, which are separated by a second band gap.

Defect levels in the epitaxial and polycrystalline CuGaSe2 by photocurrent and capacitance methods

The defect levels in epitaxial and polycrystalline wide bandgap chalcopyrite CuGaSe2 with various stoichiometry deviations were investigated using modulated photocurrent spectroscopy. The results were analyzed as a function of light intensity and Fermi-level position. Comparison of the results from epitaxial and polycrystalline material distinguished levels belonging to intrinsic defects and their correlation with the material stoichiometry. We also compared the fingerprints of defect levels by MPC to the results derived from capacitance spectroscopy performed on Schottky diodes fabricated on both epitaxial and polycrystalline layers. This allowed us to attribute unambiguously levels observed in the capacitance response to bulk point defects. In the final conclusions we provide information on the electronic parameters of nine defect levels observed in CuGaSe2 and their correlation with the material stoichiometry. These results should help to identify intrinsic defects that are important for the photovolta...

Influence of secondary phase CuxSe on the optoelectronic quality of chalcopyrite thin films

The effect of secondary phase CuxSe on the absorber quality of epitaxial CuGaSe2 films is studied by spatially resolved photoluminescence combined with scanning electron microscopy. We show that the presence of CuxSe crystals on the absorber during growth under Cu excess results in a locally increased crystal quality in the vicinity of the CuxSe crystallites, affecting the defect structure of the absorber. This effect remains after removal of CuxSe and induces a local reduction in the recombination centers, resulting in locally higher excess carrier concentrations and larger splitting of quasi-Fermi levels.

Multiple phases of Cu2ZnSnSe4 detected by room temperature photoluminescence

Cu2ZnSnSe4 based solar cells are promising but suffer from low open circuit voltage relative to their band gap. Additionally, the bandgap as extrapolated from quantum efficiency (QE) measurements varies without clear correlation to the growth conditions. Using room temperature photoluminescence, we show that different materials with different bandgaps coexist within micrometer sized areas of the absorbers. Simulations of the effect of multiple bandgaps on both the absorption and the Shockley-Queisser radiative recombination limit, explain the variations of the bandgap extrapolated from QE and the deficiencies of the solar cell parameters.