Alex Redinger

The Consequences of Kesterite Equilibria for Efficient Solar Cells

Copper-zinc-tin-chalcogenide kesterites, Cu(2)ZnSnS(4) and Cu(2)ZnSnSe(4) (CZTS(e)) are ideal candidates for the production of thin film solar cells on large scales due to the high natural abundance of all constituents, a tunable direct band gap ranging from 1.0 to 1.5 eV, a large absorption coefficient, and demonstrated power conversion efficiencies close to 10%. However, Sn losses through desorption of SnS(e) from CZTS(e) at elevated temperatures (above 400 °C) impede the thorough control of film composition and film homogeneity. No robust and feasible fabrication process is currently available. Here we show that understanding the formation reaction of the kesterite absorber is the key to control the growth process and to drastically improve the solar cell efficiency. Furthermore, we demonstrate that this knowledge can be used to simplify the four-dimensional parameter space (spanned by the four different elements) to an easy and robust two-dimensional process. Sufficiently high partial pressures of SnS(e) and S(e) (a) prevent the decomposition reaction of the CZTS(e) at elevated temperatures and (b) introduce any missing Sn into a Sn-deficient film. This finding enables us to simplify the precursor to a film containing only Cu and Zn, whereas Sn and S(e) are introduced from the gas phase by a self-regulating process.

Detection of a ZnSe secondary phase in coevaporated Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 (CZTSe) thin films are grown by coevaporation. Composition depth profiles reveal that a Zn rich phase is present at the CZTSe/Mo interface. Raman measurements on the as grown films are used to study the near surface region and the CZTSe/Mo interface, after mechanically removing the thin film from the Mo coated glass. These measurements provide direct experimental evidence of the formation of a ZnSe phase at the CZTSe/Mo interface. While the Raman spectra at the surface region are dominated by CZTSe modes, those measured at the CZTSe/Mo interface are dominated by ZnSe and MoSe2 modes.

Coevaporation of Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films grown by coevaporation are investigated in a wide temperature range and for different Se partial pressures during growth. At temperatures higher than 350 °C Sn is re-evaporating as SnSe from the surface whereas Zn is lost at temperatures higher than 430 °C. Moreover the Se partial pressure dramatically changes the Zn and Sn concentrations in the resulting film. Interrupted processes at 380 °C show that single-stage coevaporation intrinsically induces a secondary phase at the substrate/film interface.

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.

Atom probe study of Cu2ZnSnSe4 thin-films prepared by co-evaporation and post-deposition annealing

We use atom probe tomography (APT) for resolving nanometer scale compositional fluctuations in Cu2ZnSnSe4 (CZTSe) thin-films prepared by co-evaporation and post-deposition annealing. We detect a complex, nanometer–sized network of CZTSe and ZnSe domains in these films. Some of the ZnSe domains contain precipitates having a Cu- and Sn-rich composition, where the composition cannot be assigned to any of the known equilibrium phases. Furthermore, Na impurities are found to be segregated at the CZTSe/ZnSe interface. The insights given by APT are essential for understanding the growth of CZTSe absorber layers for thin-film solar cells and for optimizing their optoelectronic properties.

Route Toward High-Efficiency Single-Phase Cu

Thin-film chalcogenide kesterites Cu2ZnSnS4 and Cu2 ZnSnSe4 (CZTSSe) are promising candidates for the next-generation thin-film solar cells. They exhibit a high natural abundance of Cu, Zn, Sn and S2, a high absorption coefficient, and a tunable direct bandgap between 1.0 and 1.5 eV. A prerequisite for the use of CZTSSe as absorber layers in photovoltaic applications on large scales is a detailed knowledge of the formation reaction. Recently, we have shown that a decomposition/formation equilibrium governs the formation reaction. The presence of Sn(S,Se) during the high-temperature preparation steps is essential to prevent decomposition. This improves the solar cell efficiency from 0.02% to 6.1%. In this paper, we show that the decomposition is universal. Absorbers produced by high-temperature coevaporation and samples produced by low-temperature precursor fabrication followed by annealing in a tube furnace in S or Se atmosphere are compared in order to elucidate that in all cases, the loss of Sn(S,Se) forms a degraded surface region. We demonstrate that the degraded surface of CZTSe absorbers contains grains of ZnSe. These new insights can be used to explain why some of the synthesis routines described in the literature yield much better efficiencies than others.

_{\bf 2}

The molecular structure of the wetting layer of ice on Pt(111) is resolved using scanning tunneling microscopy (STM). Two structures observed previously by diffraction techniques are imaged for coverages at or close to completion of the wetting layer. At 140K only a sqrt(37) x sqrt(37) R25.3{\deg} superstructure can be established, while at 130K also a sqrt(39) x sqrt(39) R16.1{\deg} superstructure with slightly higher molecular density is formed. In the temperature range under concern the superstructures reversibly transform into each other by slight changes in coverage through adsorption or desorption. The superstructures exhibit a complex pattern of molecules in different geometries.

ZnSn(S,Se)

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.

_{\bf 4}

Unlike Cu(In,Ga)Se2 based solar cells, Cu2ZnSn(S,Se)4 solar cells show a strong increase in series resistance with decreasing temperature. In this study we deduce the series resistance from temperature dependent current-voltage measurements on a 5.5% efficient Cu2ZnSnSe4 solar cell. By applying a simple circuit model an increasing series resistance with decreasing temperature alone results in a capacitance step within the C-f profile. We show that this step needs to be distinguished from a step caused by a defect state or a carrier freeze-out. Consequently, the deduced activation energy is strongly distorted by the circuit response.

Thin-Film Solar Cells: Model Experiments and Literature Review

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.