K. Timmo

Cu2ZnSnSe4 Monograin Powders for Solar Cell Application

Cu₂ZnSnSe₄ monograin powders with different Zn/Sn concentration ratios were synthesized from binaries and elemental selenium, p-type CCu₂ZnSnSe₄ monograins of stannite structure had tetragonal shape with rounded edges. PL spectra showed one symmetrical band with peak position at 0.81 eV. Monograin layer solar cell structures graphite/Cu₂ZnSnSe₄/CdS/ZnO had open circuit voltage over 400 mV, short circuit current 15.5 mA/cm² and FF41 %

Chemical etching of Cu 2 ZnSn(S,Se) 4 monograin powder

Cu2ZnSn(S,Se)4 (CZTS,Se) monograin powders were synthesized in the liquid phase of molten KI as flux material from binary compounds in evacuated quartz ampoules. Monograin powders were subjected to various chemical treatments with several etchants (HCl, KCN, NH4OH and Br in methanol (Br2-MeOH)) to modify the crystal surface. Polarographic analyses of leaching solutions showed that Sn and Se were removed preferably by HCl etching. Treatment with 10% KCN dissolved mainly Cu, Sn and chalcogen, and ammonia solution removed selectively Cu and chalcogen in an approximate ratio of 1∶2. From XPS measurements we found that after etching with 1% Br2-MeOH the material surfaces were Sn-rich. The prepared monograin powders were used as absorber materials in monograin layer solar cells: ZnO/CdS/CZT(S,Se)/graphite. A combination of chemical treatments before the deposition of CdS led to the best parameters of Cu2ZnSn(S,Se)4 monograin layer solar cells. The here achieved efficiencies of solar cells were above 4%.

CZTS Monograin Powders and Thin Films

This paper reviews results of studies on different materials and technologies for polycrystalline solar cells conducted at Tallinn University of Technology. Structural properties and defect structure of kesterite CZTS compounds (Cu2ZnSnSe4, Cu2ZnSn(SSe)4) were studied. Influence of selenization parameters of a Zn-Cu-Sn stacked layer on the CZTS layer growth and on the morphology, distribution of elements was analyzed. All the results obtained have been used to optimize the technology of producing solar cell structures in different designs. Cu2ZnSnSe4 and, Cu2ZnSn(SSe)4 based monograin layer solar cells were developed.

Reaction pathway to Cu2ZnSnSe4 formation in CdI2

We investigated various possible chemical interactions between individual precursor compounds (ZnSe, SnSe, and CuSe) and CdI2 as a flux material used in the CZTSe monograin powder synthesis–growth process in closed vacuum ampoules. The processes occurring in these mixtures were detected by the differential thermal analysis method. The phase changes in these processes were determined using X-ray diffraction and Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and mass spectrometry. The analyses showed that molten CdI2 was chemically active, forming Zn1−xCdxSe in the CdI2–ZnSe mixture by a chemical dissolution reaction; CdSe, SnI2 and SnI4 formed in the CdI2–SnSe mixtures and CdSe; and CuI formed in the CdI2–CuSe mixtures by exchange reactions.

Reaction pathway to Cu 2 ZnSnSe 4 formation in CdI 2

We investigated various possible chemical interactions between individual precursor compounds (ZnSe, SnSe, and CuSe) and CdI2 as a flux material used in the CZTSe monograin powder synthesis–growth process in closed vacuum ampoules. The processes occurring in these mixtures were detected by the differential thermal analysis method. The phase changes in these processes were determined using X-ray diffraction and Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and mass spectrometry. The analyses showed that molten CdI2 was chemically active, forming Zn1−xCdxSe in the CdI2–ZnSe mixture by a chemical dissolution reaction; CdSe, SnI2 and SnI4 formed in the CdI2–SnSe mixtures and CdSe; and CuI formed in the CdI2–CuSe mixtures by exchange reactions.

Growth of CuInSe 2 monograin powders with different compositions

CuInSe 2 monograin powders (MGP) were synthesized from Cu-In alloys of different Cu/In concentration ratios and elemental Se in liquid phase of flux material in evacuated quartz ampoules. The surface morphology, phase structure, and composition of the powder crystals were analyzed by scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray analysis respectively. Bulk composition was analyzed polarographically. Photoluminescence spectra were measured at 9 K. It was found that the composition of MGP material (Cu/In concentration ratio) can be controlled by the concentration ratio of precursor Cu-In alloys. Single phase CuInSe 2 growth is realisable between 0.7 2 . Samples with high In content exhibited two broad bands with peak positions at 0.86 and 0.93 eV.

Tailoring the composition and properties of CuInSe2 materials for solar cell application

Ternary semiconductor compound CuInSe2 is one of the most promising absorber material used in solar cells. In this study, we used CuInSe2 powder materials synthesized in molten salts. Modifying the preparation conditions, we studied the relationships between the initial and final composition, and determined the preparation conditions for the single- phase growth of CuInSe2. The as-grown samples were annealed in selenium or sulfur vapor at various temperatures for a different time period to change the CuInSe2 material properties. Se vapor treatment has been found to influence the bulk composition. Sulfur vapor treatment has been found to improve the open-circuit voltage of the completed cells by about 100 mV. It could be attributed to an increase of the band gap at the surface of the absorber due to the formation of a wider band gap CuIn(S,Se)2 material. The incorporation of sulfur into CuInSe2 reduces the carrier recombination in the interface region, which was indicated as an improvement of the fill factor of the cells.