Mario Lang

Reversible order-disorder related band gap changes in Cu2ZnSn(S,Se)4 via post-annealing of solar cells measured by electroreflectance

We report on order–disorder related band gap changes in Cu2ZnSn(S,Se)4 solar cells which are induced by post-annealing. The band gap changes of the absorber are detected utilizing electroreflectance and analyzed by comparison with predictions of the stochastic Vineyard model. This yields a critical temperature of TC=195 °C above which the Cu2ZnSn(S,Se)4 absorber layer is entirely disordered within the Cu–Zn layers of the kesterite unit cell. The temporal evolution of the band gap during annealing shows that the equilibrium value is reached on a timescale in the order of hours, depending on the annealing temperature. In contrast to other experimental techniques, electroreflectance precisely measures the band gap and is not influenced by defect-mediated radiative recombination.

Order-disorder related band gap changes in Cu2ZnSn(S,Se)4: Impact on solar cell performance

We investigate the impact of order-disorder related band gap changes on finished Cu2ZnSn(S,Se)4 solar cells. Solution-processed solar cells are post-annealed in a tube furnace in order to modify the Cu-Zn disorder within the kesterite unit cell. A decrease in this disorder leads to an increased band gap of the absorber. The latter is detected using electroreflectance spectroscopy. In our experiments the change in the open-circuit voltage follows the change in the absorber band gap. However, the open-circuit voltage deficit of the solar cell remains unaltered. Still, power conversion efficiencies could be increased by 1%, mainly due to an increased open-circuit voltage.

Influence of the Cu Content in Cu2ZnSn(S,Se)4 solar cell absorbers on order-disorder related band gap changes

We investigate the electronic structure and the radiative recombination in wet-chemically fabricated Cu2ZnSn(S,Se)4 solar cell absorbers utilizing photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy, focusing especially on the effects of varying Cu content. This includes the impact of the latter on the band gap energy and the change in band gap energy related to the order-disorder transition. Characteristic PL and PLE parameters like the energetic position of the PL maximum and the PL yield as a function of the excitation power as well as the PLE tailing parameter do not depend on composition indicating that the nature of the radiative transition is not altered by the Cu content. However, the band gap energy Eg significantly increases as a function of decreasing Cu content. This increase is more pronounced in the disordered than in the ordered atomic arrangement of Cu and Zn atoms in the Cu–Zn planes of the kesterite crystal structure.

Fabrication of polycrystalline Cu2ZnSnSe4 layers with strongly preferential grain orientation via selenization of Sn/Cu/ZnSe(001)/GaAs(001) structures

We report on a simple approach for the fabrication of polycrystalline Cu2ZnSnSe4 films on GaAs with strongly preferential grain orientation. Such layers are important for both fundamental spectroscopic investigations as well as studies of the impact of grain boundaries on the performance of solar cells. Cu2ZnSnSe4 (CZTSe) layers were fabricated by growing a Sn/Cu/ZnSe(001) stack on a GaAs(001) substrate in a molecular-beam epitaxy system followed by selenization in a tube furnace. Raman spectroscopy as well as X-ray diffraction measurements combined with a microstructural investigation indicate the presence of the kesterite phase. Further, X-ray analysis such as rocking curves and ϕ-scans proves a preferential grain orientation of the obtained CZTSe films in all three dimensions.

The influence of the degree of Cu-Zn disorder on the radiative recombination transitions in Cu2ZnSn(S,Se)4 solar cells

Cu2ZnSn(S,Se)4 absorbers show disorder in the Cu-Zn lattice planes, which affects the band gap energy. The degree of disorder can be changed via post-annealing treatments. We report on the influence of this Cu-Zn disorder on the radiative recombination transitions in Cu2ZnSn(S,Se)4 solar cells. Temperature- and power-dependent photoluminescence measurements were performed to evaluate this influence. With increasing excitation power the samples show a large blue-shift of the photoluminescence peak energy while with increasing temperature at constant excitation power they show a red-shift. From an analysis of the shifting behavior as a function of the degree of Cu-Zn disorder we conclude that the degree of disorder does not alter the dominant recombination mechanism significantly.

Impact of the degree of Cu–Zn order in Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails

In order to identify the impact of the degree of Cu–Zn order in kesterite Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails, we perform photoluminescence (PL), photoluminescence excitation, and time-resolved photoluminescence (TRPL) spectroscopy. The PL lineshape and further PL characteristics such as state filling are analysed as a function of Cu–Zn order. Furthermore, TRPL decays and band tails are quantified. No significant modification in the defect states is caused by changes in Cu–Zn order, meaning that the formation of the defect states is not mainly determined by disorder in the Cu–Zn plane. In regard to band tailing, a small tendency to a decrease in the tailing parameter for the states with a high degree of Cu–Zn order compared to states with a low degree of Cu–Zn order is obvious. However, this reduction is small compared to the reduction of the defect density accompanied by the increase in the degree of Cu–Zn order. Hence, band tails are not mainly formed due to disorder in the Cu–Zn planes.In order to identify the impact of the degree of Cu–Zn order in kesterite Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails, we perform photoluminescence (PL), photoluminescence excitation, and time-resolved photoluminescence (TRPL) spectroscopy. The PL lineshape and further PL characteristics such as state filling are analysed as a function of Cu–Zn order. Furthermore, TRPL decays and band tails are quantified. No significant modification in the defect states is caused by changes in Cu–Zn order, meaning that the formation of the defect states is not mainly determined by disorder in the Cu–Zn plane. In regard to band tailing, a small tendency to a decrease in the tailing parameter for the states with a high degree of Cu–Zn order compared to states with a low degree of Cu–Zn order is obvious. However, this reduction is small compared to the reduction of the defect density accompanied by the increase in the degree of Cu–Zn order. Hence, band tails are not mainly formed due to disorder in t...

Analysis of the radiative transitions in CZTSSe solar cells with varying degree of Cu-Zn order

Cu2ZnSn(S, Se)4 (CZTSSe) shows broad and asymmetric photoluminescence spectra situated far below the absorption edge at low temperatures. The physical recombination paths for the observed transitions could not be assigned unambiguously yet in literature. Nevertheless we show in this contribution that the peak position of the photoluminescence at low temperatures can be used as an indirect measure of the degree of order in the Cu-Zn planes of the kesterite crystal structure. The degree of order can be changed easily by thermal annealing procedures. The photoluminescence for different degrees of order comprises an additional contribution which seems not to change its energetic position with the degree of order which is in contrast to the main radiative contribution and band parameters, i.e., the band gap. We attribute this transition to a secondary phase or some deep defect level which does not follow the CZTSSe band edge as determined by electroreflectance.

Raman investigation of epitaxial Cu 2 ZnSnSe 4 layers from annealed Sn/Cu/ZnSe(001) precursors on GaAs(001)

We investigate the crystal unit cell orientation of epitaxial Cu2ZnSnSe4 films on GaAs(001) using polarized Raman spectroscopy. An epitaxial ZnSe(001) layer is grown in a molecular-beam epitaxy system followed by the deposition of Cu and Sn and annealing in Se atmosphere. This method results in an epitaxially ordered Cu2ZnSnSe4 film on GaAs as proven by Electron Backscatter Diffraction (EBSD) measurements and X-ray diffraction. Polarization-dependent Raman spectra of the layers not only exhibit the typical kesterite main modes but also intensity oscillations when the sample is rotated around the optical axis. These oscillations yield information about the orientation of the kesterite crystal unit cell.