M. Grossberg

The role of structural properties on deep defect states in Cu2ZnSnS4 studied by photoluminescence spectroscopy

In this study, we investigated the photoluminescence (PL) properties of Cu2ZnSnS4 polycrystals. Two PL bands at 1.27 eV and 1.35 eV at T = 10 K were detected. Similar behaviour with temperature and excitation power was found for both PL bands and attributed to the band-to-impurity recombination. Interestingly, the thermal activation energies determined from the temperature dependence of the PL bands coincide. With the support of the Raman results, we propose that the observed PL bands arise from the band-to-impurity-recombination process involving the same deep acceptor defect with ionization energy of around 280 meV but different Cu2ZnSnS4 phase with different bandgap energy.

Optical properties of high quality Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

Photoluminescence studies of heavily doped CuInTe2 crystals

The photoluminescence spectra of heavily doped CuInTe2 and their dependence on the temperature and excitation power were measured. At 10 K an asymmetric broad peak at 0.98 eV was observed. The PL peak position did not depend on the excitation power, but had a characteristic dependence on the sample temperature. Our computer simulations proved that this behaviour is in good compliance with the Shklovskij/Efros model of heavily doped semiconductors with spatially varying potential fluctuations. Therefore, the PL band was attributed to the band-toimpurity type recombination and the corresponding level to the single acceptor at 70 meV, which is most probably caused by copper vacancy.

Microphotoluminescence study of Cu2ZnSnS4 polycrystals

Abstract. Microphotoluminescence studies of Cu2ZnSnS4 polycrystals were performed. At room temperature, two photoluminescence (PL) bands were detected at 1.39 and 1.53 eV and attributed to band-to-tail (BT) and band-to-band (BB) recombination, respectively. At lower temperatures, band-to-impurity recombination always dominates. The results show that the model of heavily doped semiconductors applies to Cu2ZnSnS4 and that, in contrast to the ternary chalcopyrites, the BT recombination in Cu2ZnSnS4 has very low intensity. The laser power dependency of the PL intensity shows that the recombination mechanism of BT and BB bands exhibits an exciton-like behavior.

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%.

Temperature dependent photoreflectance study of Cu2SnS3 thin films produced by pulsed laser deposition

The energy band structure of Cu2SnS3 (CTS) thin films fabricated by pulsed laser deposition was studied by photoreflectance spectroscopy (PR). The temperature-dependent PR spectra were measured in the range of T = 10–150 K. According to the Raman scattering analysis, the monoclinic crystal structure (C1c1) prevails in the studied CTS thin film; however, a weak contribution from cubic CTS (F-43m) was also detected. The PR spectra revealed the valence band splitting of CTS. Optical transitions at EA = 0.92 eV, EB = 1.04 eV, and EC = 1.08 eV were found for monoclinic CTS at low-temperature (T = 10 K). Additional optical transition was detected at EAC = 0.94 eV, and it was attributed to the low-temperature band gap of cubic CTS. All the identified optical transition energies showed a blueshift with increasing temperature, and the temperature coefficient dE/dT was about 0.1 meV/K.

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.

SEM analysis and selenization of Cu-Zn-Sn sequential films produced by evaporation of metals

The formation of Cu2SnZnSe4 thin films in the selenization of different sequential metallic and alloy films is investigated. It is shown that the main process of low temperature selenization (up to 300degC) is the formation of different binary copper selenides on the layer surface. High temperature selenization (over 400degC) leads to the formation of Cu2ZnSnSe4 phase with some excess of a separate ZnSe phase. The content of ZnSe diminishes with the rise of the selenization temperature, but the selenized films stayed always multiphased. The size of the formed Cu2ZnSnSe4 crystals is controlled by the composition of the precursor.

High Temperature Properties of CdTe Crystals, doped by Sb

Electrical properties of CdTe single crystals doped by Sb were studied in situ at high temperature point defect equilibrium under well defined Cd and Te vapour pressure. Up to samples revealed p-type conductivity both under Cd and Te saturation. The position of the deep acceptor level was determined using hole density temperature dependency at 350-650 K. The point defects responsible for hole density are supposed to be acceptors. The n-type conductivity above is determined by intrinsic point defects: electrons under Te saturation due to native disorder and Cd interstitials under Cd saturation.