Jan Sendler

The band gap of Cu2ZnSnSe4: Effect of order-disorder

The order-disorder transition in kesterite Cu2ZnSnSe4 (CZTSe), an interesting material for solar cell, has been investigated by spectrophotometry, photoluminescence (PL), and Raman spectroscopy. Like Cu2ZnSnS4, CZTSe is prone to disorder by Cu-Zn exchanges depending on temperature. Absorption measurements have been used to monitor the changes in band gap energy (Eg) of solar cell grade thin films as a function of the annealing temperature. We show that ordering can increase Eg by 110 meV as compared to fully disordered material. Kinetics simulations show that Eg can be used as an order parameter and the critical temperature for the CZTSe order-disorder transition is 200 ± 20 °C. On the one hand, ordering was found to increase the correlation length of the crystal. But on the other hand, except the change in Eg, ordering did not influence the PL signal of the CZTSe.

Different Bandgaps in Cu

We present a high-temperature Cu

_2

_2

ZnSnSe

ZnSnSe

_4

_4

: A High Temperature Coevaporation Study

coevaporation study, where solar cells with a power conversion efficiency of 7.1% have been achieved. The process is monitored with laser light scattering in order to follow the incorporation of the Sn into the film. We observe the segregation of ZnSe at the Mo/CZTSe interface. Optical analysis has been carried out with photoluminescence and spectrophotometry. We observe strong band tailing and a bandgap, which is significantly lower than in other reported efficient CZTSe absorbers. The photoluminescence at room temperature is lower than the bandgap due to the existence of a large quantity of tail states. Finally, we present effects of low-temperature postannealing of the absorbers on ordering of the Cu/Zn atoms in CZTSe and solar cell parameters. We observe strong changes in all solar cell parameters upon annealing. The efficiency of the annealed devices is significantly reduced, although ordering is improved compared with ones made from nonannealed absorbers.

Molecular beam epitaxy of Cu 2 ZnSnSe 4 thin films grown on GaAs(001)

Cu2ZnSnSe4 (CZTSe) thin films are grown on GaAs(001) by molecular beam epitaxy in order to produce epitaxial reference material for the characterization of the fundamental physical properties of the semiconductor. The epitaxy is made possible by a new high temperature co-evaporation process which takes into account that CZTSe easily decomposes at the growth temperatures used. The co-evaporation process can easily be transferred to polycrystalline films on molybdenum coated glass substrates for absorbers in solar cells. By using a considerable pressure of SnSe, the decomposition can be avoided. This manuscript describes the growth process and how a specific composition can be grown with high precision. X-Ray diffraction measurements confirm that CZTSe grows epitaxially on GaAs. Raman spectroscopy and Photoluminescence measurements are used to study the occurrence of secondary phases and the quality of the films.

Photoluminescence studies in epitaxial CZTSe thin films

Epitaxial Cu2ZnSnSe4 (CZTSe) thin films were grown by molecular beam epitaxy on GaAs(001) using two different growth processes, one containing an in-situ annealing stage as used for solar cell absorbers and one for which this step was omitted. Photoluminescences (PL) measurements carried out on these samples show no dependence of the emission shape on the excitation intensity at different temperatures ranging from 4 K to 300 K. To describe the PL measurements, we employ a model with fluctuating band edges in which the density of states of the resulting tail states does not seem to depend on the excited charge carrier density. In this interpretation, the PL measurements show that the annealing stage removes a defect level, which is present in the samples without this annealing.