us Just

Determination of secondary phases in kesterite Cu2ZnSnS4 thin films by x-ray absorption near edge structure analysis

Secondary phases in Cu2ZnSnS4 (CZTS) are investigated by x-ray absorption spectroscopy. Evaluating the x-ray absorption near edge structure at the sulfur K-edge, we show that secondary phases exhibit sufficiently distinct features to allow their quantitative determination with high accuracy. We are able to quantify the ZnS fraction with an absolute accuracy of ±3%, by applying linear combination analysis using reference spectra. We find that even in CZTS thin films with [Sn]/[Zn] ≈ 1, a significant amount of ZnS can be present. A strong correlation of the ZnS-content with the degradation of the electrical performance of solar cells is observed.

The role of interparticle heterogeneities in the selenization pathway of Cu–Zn–Sn–S nanoparticle thin films: a real-time study

Real-time energy dispersive X-ray diffraction (EDXRD) analysis has been utilized to observe the selenization of Cu–Zn–Sn–S nanoparticle films coated from three nanoparticle populations: Cu- and Sn-rich particles roughly 5 nm in size, Zn-rich nanoparticles ranging from 10 to 20 nm in diameter, and a mixture of both types of nanoparticles (roughly 1 : 1 by mass), which corresponds to a synthesis recipe yielding CZTSSe solar cells with reported total-area efficiencies as high as 7.9%. The EDXRD studies presented herein show that the formation of copper selenide intermediates during the selenization of mixed-particle films can be primarily attributed to the small, Cu- and Sn-rich particles. Moreover, the formation of these copper selenide phases represents the first stage of the CZTSSe grain growth mechanism. The large, Zn-rich particles subsequently contribute their composition to form micrometer-sized CZTSSe grains. These findings enable further development of a previously proposed selenization pathway to account for the roles of interparticle heterogeneities, which in turn provides a valuable guide for future optimization of processes to synthesize high quality CZTSSe absorber layers.

Compositional dependence of charge carrier transport in kesterite Cu2ZnSnS4 solar cells

Cu2ZnSnS4 solar cells deposited by thermal co-evaporation have been characterized structurally and electronically to determine the dependence of the electronic properties on the elemental composition of the kesterite phase, which can significantly deviate from the total sample composition. To this end, the kesterite phase content and composition were determined by a combination of X-ray fluorescence and X-ray absorption measurements. The electronic properties, such as carrier density and minority carrier diffusion length, were determined by electron beam induced current measurements and capacitance-voltage profiling. The charge-carrier transport properties are found to strongly depend on the Cu/(Sn+Zn) ratio of the kesterite phase. For the Cu-poor sample, a minority carrier diffusion length of 270 nm and a total collection length of approx. 500 nm are deduced, indicating that current collection should not be an issue in thin devices.

Correlation between composition and photovoltaic properties of Cu 2 ZnSnS 4 thin film solar cells

The influence of composition on the quaternary Cu2ZnSnS4 (CZTS) absorber material on the secondary phase content, recombination characteristics and solar cell performance is investigated. Best solar efficiencies are found in a very narrow Cu-poor, Zn-rich composition range. Photoluminescence measurements performed at 12 K indicate strong compensation for Cu-poor composition, similar to what is observed in chalcopyrite-type compounds. We find that strongly copper-deficient CZTS contains significant amounts of ZnS, which degrades the solar cell performance.

Raman spectroscopy study on in-situ monitoring of Cu2ZnSnS4 synthesis

This study investigates the possibility of Raman spectroscopy as an in-situ monitoring tool for the synthesis of the solar cell material Cu₂ZnSnS₄ (CZTS) by annealing a stacked precursor ZnS/Cu₂SnS₃ on a Mo-coated glass substrate. Temperature dependent behaviour of Raman scattering for ZnS and Cu₂SnS₃ is studied. Both phases can still be detected at respectively 450 °C and 550 °C. Annealing of Mo/CTS/ZnS precursor stacks resulted in in-situ observation of kesterite CZTS formation with Raman spectroscopy. This is a step towards in-situ optical process control desired in kesterite fabrication.

In Situ Monitoring of Cu2ZnSnS4 Absorber Formation With Raman Spectroscopy During Mo/Cu2SnS3/ZnS Thin-Film Stack Annealing

In recent years, Cu₂ZnSn(S,Se)₄ (kesterite) has become increasingly popular as a sustainable alternative absorber material. Many processes for kesterite synthesis involve a high temperature annealing step (>450 °C). This study investigates the possibility of Raman spectroscopy as an <italic>in situ</italic> monitoring technique during high temperature annealing up to 550 °C. Temperature-dependent behavior of Cu₂SnS₃ (CTS) and Cu₂ZnSnS₄ (CZTS) was studied for reference purposes. The synthesis of CZTS was performed by annealing a stacked Mo/CTS/ZnS precursor on a glass substrate. Annealing of the precursor stack resulted in formation of kesterite and could be monitored <italic>in situ</italic> by its main A-mode at 338 cm^–1. At higher temperatures, this mode shifts to lower wavenumbers, is broadened and reduced in intensity. This can be attributed to combined effects of thermal expansion and anharmonic phonon coupling. The shift of the peak position is linearly proportional to the temperature. Thus, given proper calibration, fitting the peak position of the 338 cm^–1 mode during the process yields the sample temperature. Implementation of <italic>in situ</italic> monitoring with Raman spectroscopy would be a step forward toward desired process control and monitoring during this crucial high temperature annealing step in kesterite synthesis.

Radiative recombination from localized states in CZT(S, Se) investigated by combined PL and TRPL at low temperatures

We investigate the dominant recombination process in CZTS thin film absorber layers by time-resolved and steady state photoluminescence spectroscopy at low temperatures. We show that the emission peak at low temperatures shifts monotonically over many orders of magnitude of excitation power, which is inconsistent with electrostatic potential fluctuations causing this behavior. Time-resolved photoluminescence at 7 K shows very long emission times, which are consistent with donor-acceptor pair recombination involving acceptors densities in the mid 1017 cm-3. Spectrally resolved photoluminescence transients show a shift of the luminescence maximum to lower energies at short times after excitation, which we attribute to trapping of photoexcited carriers in defect band states.

Defects in Cu 2 ZnSn(S,Se) 4 solar cells studied by photoluminescence, admittance and IVT

Cu2ZnSn(S,Se)4 thin film devices fabricated with a new, fast and environmental friendly preparation method are investigated by defect spectroscopy and transport measurements. Defect levels in sulfide-based as well as sulfur-selenide based kesterite devices are studied using admittance, temperature-dependent current-voltage analysis and photoluminescence. We find that the series resistance of the devices is activated with an energy of 80-100meV while the defect activation energies from admittance analysis shows activation energies of 200meV and 260meV for sulfur-based and Cu2ZnSn(S0.6Se0.4)4 devices, respectively. These admittance derived energies are consistent with the photoluminescence transition at a room temperature. The photoluminescence quenching activation energies of 130-140meV were determined for both devices and attributed to donor level of the quasi donor acceptor pair transition observed at low temperature.