Physical routes for the synthesis of kesterite

This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4 thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. Afinal section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading1. R Caballero and M León acknowledge financial support via the Spanish Ministry of Science, Innovation and Universities project (WINCOST, ENE2016-80788-C5-2-R) and thank H2020 EU Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968). 2. S Canulescu and J Schou acknowledge the support from Innovation Fund Denmark. 3. D-H Kim acknowledges financial support via the DGIST R&D Program of the Ministry of Science and ICT, KOREA (18-BD-05). 4.C. Malerba acknowledges the support from the Italian Ministry of Economic development in the framework of the Operating Agreement with ENEA for the Research on the Electric System. 5.A Redinger acknowledges financial support via the FNR Attract program, Project : SUNSPOT, Nr.11244141. 6. E Saucedo thanks H2020 EU Programme under the projects STARCELL (H2020-NMBP-03-2016-720907) and INFINITE-CELL (H2020-MSCA-RISE-2017-777968), the Spanish Ministry of Science, Innovation and Universities for the IGNITE project (ENE2017-87671-C3-1-R), and the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the ‘Generalitat de Catalunya’ (Ref. 2017 SGR 862). 7. Taltech acknowledges financial support via the Estonian Ministry of Education and Research funding project IUT19-28 and the European Union Regional Development Fund, Project TK141. 8. B Vermang has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (Grant Agreement No 715027

Characterization of the locally fluctuating absorbance and quasi Fermi level splitting of Cu In,Ga S2 thin films by spatially resolved photoluminescence measurements

3.3 CIS and Other II VI Ternary Thin Film Solar Cells Characterization of the locally fluctuating absorbance and quasi Fermi level splitting of Cu In,Ga S2 thin amp; 64257;lms by spatially resolved photoluminescence measurements F. Heidemann1, L. Gütay1, R. Brüggemann1, S. Merdes2, A. Meeder3 and G.H. Bauer2 1 Institut für Physik, Carl von Ossietzky Universität Oldenburg, D 26111Oldenburg, Germany, florian.heidemann uni oldenburg.de 2 Helmholtz Zentrum Berlin, Solarenergieforschung SE2, Abteilung Heterogene Materialsysteme, Glienicker Str. 100, D 14109 Berlin, Germany 3 SULFURCELL Solartechnik GmbH, Barbara McClintock Str. 11, D 12489 Berlin, Germany Chalcopyrite absorbers gain more and more importance in thin film photovoltaics. Besides Cu In,Ga Se2, which reaches efficiencies of up to 19,9 , the sulfurised counterpart CuInS2 or Cu In,Ga S2 is presently being considered as an alternative providing a larger band gap and thus open circuit voltage. Analogously with to Cu In,Ga Se2, Cu In,Ga S2 shows a high degree of spatial inhomogeneities in structural, optical and optoelectronic properties in the length scale of grain sizes and above which is caused by the grainy structure and the inhomogeneous growth of absorber layers. To analyze these locally fluctuating magnitudes spectrally resolved photoluminescence measurements with a high lateral resolution amp; 8804; 1 m in a confocal microscope setup have been performed. Based on these data sets and on Planck s generalized law the determination of the spatial variation in the splitting of the quasi Fermi levels amp; 916; Efn Efp and a calculation of the local absorbance of the material is possible. A comparison of these properties, which are crucial for the solar light conversion efficiency of a final cell, is made for CuInS2 and Cu In,Ga S2 absorber layers for data obtained from statistically representative scan areas. The results show that an increase in the band gap and the mean amp; 916; Efn Efp due to an incorporation of gallium does not come along with a decrease in the variation of amp; 916; Efn Efp over the absorber layer. A further analysis by a detailed cross correlation between the splitting of the quasi Fermi levels and the local absorbance of an absorber leads to the conclusion that the local amp; 916; Efn Efp is reduced by the excess carrier recombination via deep defects

Discrimination and detection limits of secondary phases in Cu2ZnSnS4 using X ray diffraction and Raman spectroscopy

The formation of single phase Cu2ZnSnS4 thin films is known to be challenging, mainly due to the difficulties to detect secondary phases in the Cu2ZnSnS4 system. Here, the ability to quantitatively discriminate the most likely secondary phases ZnS and Cu2SnS3 from Cu2ZnSnS4 using common approaches but also using more complex and time consuming Rietveld refinement analysis techniques to analyse X ray diffractograms is investigated in a comparative study to the peak analysis of Raman spectra measured with standard conditions. In studying not only individual samples of the respective phases but also a phase gradient sample containing various amounts of Cu2SnS3 and ZnS alongside Cu2ZnSnS4, we found that refinement analyses can only discriminate more than 10 ZnS and 50 Cu2SnS3 from Cu2ZnSnS4, respectively. In comparison, Raman measurements performed with green wavelength excitation can discern more than 30 Cu2SnS3 from Cu2ZnSnS4 while ZnS is indiscernible. The results show that the identification of secondary phases in the Cu2ZnSnS4 system is more difficult than currently assumed in literature. Furthermore, the potential of multiple wavelength Raman spectroscopy as a tool to identify ZnS secondary phases is shown. Characterization of a Sn rich sample composition nearly Cu2ZnSn3S8 shows no sign of a Sn rich quaternary phase, questioning its existence under typical annealing condition

Reaction Pathway for Efficient Cu2ZnSnSe4 Solar Cells from Alloyed Cu Sn Precursor via a Cu Rich Selenization Stage

The selenization of stacked elemental metallic layers Cu Sn Zn is a commonly reported approach in kesterite Cu2ZnSnSe4 CZTSe processing. CZTSe formation via this approach usually involves a reaction route containing binary selenides, such as SnSe2 amp; 8722;x. The high volatility of these phases at the necessary annealing temperatures 500 550 amp; 8201; C makes this reaction pathway prone to Sn loss, which makes it challenging to control the composition and quality of the grown material. Herein, an approach based on stacked elemental and alloyed precursors is reported, and the benefits of using a Zn Cu Sn Zn configuration are discussed. The absence of nonalloyed elemental Sn helps in suppressing the formation and subsequent evaporation of SnSe2 amp; 8722;x phases, preventing Sn loss from the film during selenization. This reaction pathway involves a process scheme which 1 starts with the growth of CZTSe in a Cu amp; 8208;rich environment, 2 includes a shift of the composition by supply of SnSe2 amp; 8722;x vapor, and 3 terminates in the Cu amp; 8208;poor regime, leading to device efficiencies above 10 . This composition shift in the presented process appears similar to the final stage of the commonly known CIGSe three amp; 8208;stage coevaporatio

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells