Susanne Siebentritt

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.

In-depth resolved Raman scattering analysis for the identification of secondary phases: Characterization of Cu2ZnSnS4 layers for solar cell applications

This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.

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.

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.

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.

Charge and doping distributions by capacitance profiling in Cu(In,Ga)Se2 solar cells

Doping distributions in the Cu(In,Ga)Se2 solar cells with various gallium contents are analyzed by the use of capacitance-voltage and drive-level capacitance profiling. The influence of deep traps on the evaluation of the spatial-doping distribution in the bulk of Cu(In,Ga)Se2 absorbers is discussed. An analysis is presented, which shows that traps labeled N1, commonly observed in these devices, are interface states or compensating donors and their input to the capacitance is related only to the width of the depleted n-type insulating layer. We attribute the apparent increase of doping density toward the back electrode to the accumulation of the electrostatic charge in deep bulk acceptors with a concentration at an order of magnitude higher than net shallow doping. The metastable changes of doping distributions induced by light or reverse bias are also investigated and interpreted in terms of the Lany–Zunger model of VSe-VCu divacancies with negative-U property. All conclusions have been tested by numeric...

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.

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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.