Monograin materials for solar cells

This paper reviews results of studies on different materials and technologies for monograin layer (MGL) solar cells conducted at Tallinn University of Technology. The MGL consists of monograin powder crystals embedded into an organic resin. The MGL combines the superior photoelectrical parameters of single crystals with the advantages of polycrystalline materials, such as the low cost and simple technology of materials and layers preparation and the possibility of making devices of practically unlimited area. A main technological advantage is the separation between absorber and cell formations. The developments in the field of monograin materials of CuInSe₂, Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ and technical parameters of MGL solar cells are summarized.     

The performance of CuInSe2 monograin layer solar cells with variable indium content

The recent results on the characterization of CuInSe₂ monograin layer solar cells are presented. The influence of different Cu/In ratio on solar cell characteristics was studied. It was determined that device-quality CuInSe₂ monograin powder could be grown from CuIn precursor alloys with a composition between 0.9 < Cu/In < 1. For absorber material, these powders were post-treated in sulphur vapour. The cells based on these absorbers showed efficiencies up to 9.5%. However, the quantum efficiency measurements revealed a significant loss in the long-wavelength range of photons λ > 800 nm. The derivative of QE with respect to wavelength showed two peaks at the energy values of ∼ 1.4 eV and ∼ 1.03 eV, which proves that these sulphurised absorber materials consist of two different phases.     

The effect of sodium doping to CuInSe2 monograin powder properties

The effect of sodium doping to the electrical and photoluminescence properties of CuInSe₂ monograin powders was studied. Sodium was added in controlled amounts from 5 × 10¹⁶ cm^− 3 to 1 × 10²⁰ cm^− 3. The photoluminescence spectra of Na-doped stoichiometric CuInSe₂ powders had two bands with peak positions at 0.97 and 0.99 eV. The photoluminescence bands showed the shift of peak positions depending on the Na doping level. Peak positions with maximum energy were observed if added sodium concentration was 1 × 10¹⁹ cm^− 3. This material had the highest carrier concentration 2 × 10¹⁷ cm^− 3. In the case of stoichiometric CuInSe₂ (Cu:In:Se = 25.7:25.3:49.0), Na doping at concentrations of 3 × 10¹⁷ cm^− 3 and higher avoided the precipitation of Cu-Se phase. Solar cells output parameters were dependent on the Na doping level. Sodium concentration 3 × 10¹⁸ cm^− 3 resulted in the best open-circuit voltage.     

Toxicological evaluation of transgenic rice flour with a synthetic cry1Ab gene from Bacillus thuringiensis

Bacillus thuringiensis (Bt) transgenic (KMD1) and non‐transgenic (KMD1′s parental variety Xiushui 11) rice flours were assessed in a 90 day feeding test with rats. KMD1 contained a synthetic cry1Ab gene from Bt, and selection marker genes nptII and hpt linked in tandem. In the≤64 g kg^?1 body weight (BW) dosage range (Bt transgenic rice flour composed 64% of the ingredients of the diet), no adverse effects of Bt rice on rats were observed in terms of animal behaviour, weight gain and feed utilisation rate. Necropsy at the end of the experiment indicated that neither pathological lesions nor histopathological abnormalities were present in organs such as liver, kidneys, intestines and testes of rats in both test and control groups by macroscopic and microscopic pathology. In addition, no significant differences (P > 0.05) were observed in relative organ weights, haemograms and blood indices of rats between test and control groups. Several serum parameters of female rats were found to be significantly different between Bt and non‐Bt diets, but the values of these parameters were still within the normal ranges of values for rats of this age and sex. These results demonstrated that Bt rice flour at a dosage of 64 g kg^?1 BW, Bt toxin and NPTII and HPT proteins have no toxic effects on rats. © 2002 Society of Chemical Industry     

Photoluminescence and Raman study of Cu2ZnSn(SexS1 − x)4 monograins for photovoltaic applications

The quaternary semiconductors Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ have attracted a lot of attention as possible absorber materials for solar cells due to their direct bandgap and high absorption coefficient (> 10⁴ cm⁻¹). In this study we investigate the optical properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograin powders that were synthesized from binary compounds in the liquid phase of potassium iodide (KI) flux materials in evacuated quartz ampoules. Radiative recombination processes in Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using low-temperature photoluminescence (PL) spectroscopy. A continuous shift from 1.3 eV to 0.95 eV of the PL emission peak position with increasing Se concentration was observed indicating the narrowing of the bandgap of the solid solutions. Recombination mechanisms responsible for the PL emission are discussed. Vibrational properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using micro-Raman spectroscopy. The frequencies of the optical modes in the given materials were detected and the bimodal behaviour of the A₁ Raman modes of Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ is established.     

Phase composition of selenized Cu2ZnSnSe4 thin films determined by X-ray diffraction and Raman spectroscopy

Thin layers of Sn onto Cu-Zn alloy with different component ratios were processed at different temperatures. Scrupulous comparative analyses were performed by room temperature Raman spectroscopy and X-ray-diffractometry. An excess of tin on the surface results in isothermal selenization at 450 °C in the hexagonal residuals of unstable SnSe₂ in the well-crystallized Stannite — Cu₂ZnSnSe₄. In similar selenization conditions, copper-rich layers as precursors result in the Stannite phase with micro-immersions of CuSe. Low-temperature photoluminescence spectra of selenized films indicated to two Gaussian shaped bands at 0.81 and 1.16 eV.     

Radiative and nonradiative recombination centers in Cl and Na doped CdTe monograin powders

The wide 1.4 eV photoluminescence band, photoconductivity and dark conductivity of CdTe monograin powders were studied as a function of Na concentration. The donor-acceptor complex Na_CdCl_Te is proposed as the radiative recombination center responsible for the 1.44 eV PL band as well as for the photoconductivity. The high n-conductivity of CdTe monograins with low Na concentration is determined by the ionization of shallow Cl_Te donor defects. The sharp decrease of the conductivity (10⁷ times) in vicinity of equal concentrations of Na and Cl is due to the high level of compensation Cl_Te + e + Na_Cd ^x Na_CdCl_Te .     

CuInSe2 monograin growth in the liquid phase of potassium iodide

The results of monograin CuInSe₂ synthesis from Cu-In alloy and Se in liquid KI are presented. The amounts of CuInSe₂ and KI were nearly equal to fulfil the criterion for the monograin growth (all free volume between the particles has to be filled with liquid). All the grown powder materials with narrow-disperse granularity were chalcopyrite CuInSe₂. The grown crystallites had tetrahedral shapes and homogeneous composition. Particle size distribution was used to describe the growth process. The activation energy of linear growth of crystals was E_d = 0.25 ± 0.05 eV, and the power of time dependence of the crystal growth was l/n = 0.26 ± 0.06. The solubility of CuInSe₂ in KI at 990 K was 0.17 ± 0.05 wt. %. The solubility of potassium and iodine in CuInSe₂ at 990 K was 0.094 wt. %, and 0.0086 wt. %, respectively. As a result, homogeneous p-type CuInSe₂ monograin materials were synthesised in KI solvent.