J. Raudoja

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.

Monograin layer solar cells

The paper presents results of studies directed towards the production of monograin powders of CuInSe for possible use in 2 solar cells preparation.The results concern the tailoring of chemical and defect composition of materials, development of the technology of manufacturing monograin layers (MGL) on the base of developed materials and the cleaning of open surfaces of the grains in the MGL by different mechanical, chemical and electrochemical methods.It is shown that up to now the low efficiency of MGL solar cells is associated with the chemical and defect composition of the monograin powder materials and with difficulties in cleaning the surfaces of the crystals in the MGL before depositing active contacts. 2003 Elsevier Science B.V. All rights reserved.

Nature of the native deep localized defect recombination centers in the chalcopyrite and orthorhombic AgInS2

We studied the deep photoluminescence (PL) emission in polycrystalline chalcopyrite and orthorhombic AgInS2. In both phases several PL bands were detected at 8 K. On the energy scale these deep PL bands are positioned according to a regular pattern. This is explained as being due to electron-hole recombination within very close deep donor-deep acceptor pairs, with different distances between donor and acceptor defects. The deep donor defect is an interstitial silver Agi and the native deep acceptor defect appears to be situated at the Ag or In place. The two different crystal modifications also cause slightly different distances between donor and acceptor defects in the AgInS2 lattice and, as a result of this, different spectral positions of the deep PL bands. It is shown that these deep localized donor–acceptor pairs can be reasonably efficient radiative recombination centers up to distances of 5.3 A between the deep donor and the deep acceptor and, thus, up to six distinct deep PL bands are visible in A...

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.

On the Shape of the Close-to-Band-Edge Photoluminescent Emission Spectrum in Compensated CuGaSe2

Photoluminescence (PL) properties of compensated as-grown and air-annealed CuGaSe2 single crystals, grown by the vertical Bridgman technique, in the edge emission spectral region were studied. The intensity maximum of the broad asymmetrical PL band at T= 8 K was found to be at hν max = 1.586 eV. After air annealing at 673 K for 15 min the PL band shifts towards higher energies, and its intensity slightly decreases but the shape remains the same. It is shown that this typical asymmetric PL band is not associated with a certain acceptor level but originates from the bandtail recombination. The valence band tail is formed by the potential fluctuations of charged defects. The average depth of these fluctuations is determined by the Debye-Huckel correlation in the distribution of donors and acceptors. The low-temperature air-annealing reduces the concentration of charged defects, but the sample remains highly compensated.

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.

Annealing effect for SnS thin films prepared by high-vacuum evaporation

Thin films of SnS are deposited onto molybdenum-coated soda lime glass substrates using the high-vacuum evaporation technique at a substrate temperature of 300 °C. The as-deposited SnS layers are then annealed in three different media: (1) H2S, (2) argon, and (3) vacuum, for different periods and temperatures to study the changes in the microstructural properties of the layers and to prepare single-phase SnS photoabsorber films. It is found that annealing the layers in H2S at 400 °C changes the stoichiometry of the as-deposited SnS films and leads to the formation of a dominant SnS2 phase. Annealing in an argon atmosphere for 1 h, however, causes no deviations in the composition of the SnS films, though the surface morphology of the annealed SnS layers changes significantly as a result of a 2 h annealing process. The crystalline structure, surface morphology, and photosensitivity of the as-deposited SnS films improves significantly as the result of annealing in vacuum, and the vacuum-annealed films are fo...

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%.

Photoluminescence and the tetragonal distortion in CuInS2

Abstract Low temperature photoluminescence (PL) measurement of a deep–donor–deep–acceptor (DD–DA) pair recombination emission of close DA pairs in CuInS 2 at h ν=0.62 eV reveals two well resolved lattice vibrational modes, of energy ℏ ω 1 =40.5 meV and ℏ ω 2 =8 meV. Furthermore, the recombination emission has an additional fine structure of Δ E =2 meV. It appears that the 2 meV fine structure can be interpreted as being due to a small difference in the DA separation between two otherwise equivalent interstitial donor sites. This is due to the tetragonal lattice distortion in the CuInS 2 structure. The measured full width at half maximum, inclusive of instrumental resolution, of these two zero-phonon-emission lines was approximately 0.6 meV at T =8 K.