Evolution of phases in two-stage vacuum processed thin film Cu2ZnSnSe4 absorber layers

Abstract

This paper reports the formation of CZTSe absorber layers at temperatures between 380–550°C using a two-stage process. Energy dispersive X-ray spectroscopy (EDS) analysis identified the presence of Cu rich and Zn rich regions that are consistent with the presence of secondary phases. These phases could evolve at lower temperatures during the formation of the CZTSe phase. The absorber layers exhibit an evolution in the morphology with conversion temperature, with the highest temperature yielding the largest and most compact grains. In the past, Cu–Zn alloys have been identified in the sputtered metallic precursor produced with our baseline process. In this study, we investigate the selenisation of Cu–Zn precursors under the same conditions as used for the CZTSe in order to understand and identify the formation of potential secondary phases that might form in our CZTSe absorber layers. The results showed that Cu_1·8Se and ZnSe were formed as separate phases. There is evidence to suggest that the Cu_1·8Se evolves into the Cu₂Se phase as the temperature increases above 520°C. This provides an insight into the phases formed during processing and their possible influence on kesterite formation.     

Processing effects on the structure of CdTe, CdS and SnO2 thin films

Thin films of CdTe and CdTe/CdS and SnO₂ used for heterojunction solar cells were deposited on glass substrates. The effects resulting from the processing with thermal heat and CdCl₂ treatments are investigated. The optical properties are determined by photoluminescence (PL) and transmission spectra. The compositional changes within the CdTe film structures are studied by 2 MeV ⁴He⁺ beam using the Rutherford backscattering (RBS) technique. The optical and the RBS data are then correlated to the evolution of high-efficiency solar cells.     

A note on the Hall mobility and carrier concentration in pyrite thin films

This note presents results on Hall effect characteristics (mobility and density of majority charge carriers) of undoped p-type and n-doped pyrite thin films. Carrier mobilities between 200 and 0.07 cm²/Vs have been measured in undoped p-type films. Doped n-type films present values which vary from 200 to 0.1 cm²/Vs. The corresponding carrier densities change from 2×10¹⁸ to 10²² cm⁻³ in p-type films and from 6×10¹⁷ to 10²¹ cm⁻³ in n-type films. These results reinforce the physical basis and conclusions of the modelling of pyrite solar cells accomplished by Altermatt et al. (Sol. Energy Mater. Sol. Cells 71 (2002) 181).     

Hydrogen annealing induced physical properties of ZnTe thin films

The ZnTe material has an unprecedented role in the fabrication of high efficiency CdTe thin film solar cells and optimization of hydrogen annealing induced physical properties of ZnTe films is next required step. Consequently, in the present work, the impact of Hydrogen annealing temperature on the structural, optical, electrical, topographical, morphological, and compositional properties of ZnTe films is explored. The ZnTe thin films (having 300 nm thickness) are grown via electron-beam evaporation technique on glass and ITO substrates followed by annealing at different temperatures under a Hydrogen atmosphere. The ZnTe films are found to crystallize in cubic phase with (111) predominant peak having crystallite size in the range of 19–28 nm, whereas annealed films demonstrated lower optical transmittance vis-à-vis to pristine films. The PL spectra exhibit two luminescence peaks with a stronger band at ～351 nm and a weaker band at ～450 nm. Ohmic behavior of ZnTe films is assured through I–V characteristics, while the AFM images revealed hill-like surface topographies. The FESEM image of pristine films demonstrated a homogeneous surface comprising spherical grains whereas annealed films have spherical, stone, and blisters like morphologies. The EDS patterns assured the Te element richness as well as successful ZnTe films deposition. The observed findings signify that the Hydrogen annealing at different temperatures notably modified the physical properties of ZnTe films.     

Role of precursor solution in controlling the opto-electronic properties of spray pyrolysed Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄, a potential candidate for application as absorber layer in thin film solar cells, were successfully deposited on soda lime glass substrates using spray pyrolysis and the effect of variation of precursor on the structural and opto-electronic properties was investigated. We used stannous as well as stannic chloride as precursors of tin in the spray solution. All the films exhibited kesterite structure with preferential orientation along the (1 1 2) direction. But crystallinity and grain size were better for stannic chloride based films. Also they possessed a direct band gap of 1.5 eV and the absorption coefficient was >10⁴ cm⁻¹. Carrier concentration and mobility could be enhanced and the resistivity reduced by two orders by using stannic chloride in spray solution. Junction trials were performed with CZTS films prepared using stannic chloride precursor as the absorber layer and indium sulfide as the buffer layer. XPS depth profiling of the junction was done. Formation of CZTS could be confirmed and also information about the junction interface could be obtained from the XPS results. We obtained an open-circuit voltage of 380 mV and short-circuit current density of 2.4 mA/cm².     

MS2 (M = W, Mo) photosensitive thin films for solar cells

Photosensitive textured WS₂ and MoS₂ films can be obtained by the techniques of reactive sputtering and solid state reaction, as long as the substrates used are each coated with a 10–20 nm Ni layer. When MS₂ (M = W, Mo) layers are deposited onto these substrates and then annealed for half an hour at 1073k in an argon atmosphere, textured films crystallized in the 2H-MS₂ structure are obtained, with their c crystallite axes perpendicular to the plane of the substrate. The films are nearly stoichiometric. The crystallinity enhancement of the films can be attributed to an improvement in the crystallization process related to liquid NiS phases present at the grain boundaries during annealing. Residual phases (Ni_xS_y; Ni;…) are distributed at the grain boundaries and do not strongly disturb the properties of the WS₂ and MoS₂ films. The optical absorption spectra are similar to those of single crystals, and the high photosensitivity of the films is attributed to a grain size enhancement by the NiS phase.     

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σ_ph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p–i–n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm²) at room temperature.     

Influence of the growth parameters of p-CdTe thin films on the performance of Au-Cu/p-CdTe/n-CdO type solar cells

R.F. sputter deposition of Sb doped CdTe thin films was carried out with targets containing different amounts of antimony (C_T: 0, 2.5, 10 and 20 at.%). The substrates were kept at different temperatures (T_s) of 200, 275, 350 and 450 °C. Three different argon pressure values: 2.5, 5 and 15 mTorr were used. The lowest dark resistivity (ρ) at room temperature (RT) was 9.0 × 10⁵ Ω cm, which is one of the lowest values reported in the literature for Sb doped CdTe. Highly transparent (∼90%) and conductive (ρ = 3.7 × 10⁻⁴ Ω cm) F doped CdO (n-type) thin films, prepared at room temperature by the sol-gel method, were employed as window and top-contact. The configuration of the fabricated solar cell was (Au-Cu)/p-CdTe/n-CdO/glass. Open-circuit voltage (V_oc) and short-circuit current density (J_sc) at room temperature have the highest values for high T_s, low P_g and C_T = 10 at.%. Despite the fact that V_oc and J_sc are lower than those reported in the literature, we think this work is useful as a basis for the search of more competitive CdTe/CdO based PV devices.     

Development of thermoelectric oxides for renewable energy conversion technologies

Geothermal and solar heat can be directly converted into electricity by using thermoelectric generators. Perovskite-type metal oxides are potential materials to improve the efficiency of these devices. Cobaltates with p-type conductivity and n-type manganates are considered for the development of a ceramic thermoelectric converter.Sintered pellets and thin PLD films with the composition La_1−xCa_xMO_3−δ (x=0, 0.3, 0.4) (M = Co, Mn) were synthesised and characterised concerning their thermoelectric properties in a broad temperature range. It was found that similar to polycrystalline samples the electrical conductivity of LaCoO₃ increases significantly with 40% Ca-substitution due to the formation of Co⁴⁺ ions while the thermopower decreases. The thermopower values of the La_0.8Ca_0.2MnO_3−δ films have a negative sign, but become large and positive at temperatures of 1000 K.     

Effect of grain boundaries and grains on electrical properties of FeS2 films

Abstract

Because of its band gap and excellent optical absorption polycrystalline pyrite is a candidate material for absorber layers of solar cells. Pyrite films with different thicknesses were obtained by sol–gel method and following sulphurisation. The structural characteristics of the films were determined by X-ray diffraction, scanning electron microscopy and atomic force microscopy, while the electrical properties were investigated by Hall instrument and impedance spectroscopy. The grain and grain boundary contributions to the electrical transport mechanism were studied using the brick layer model. The equivalent circuit parameters were calculated by simulating experimental data. According to the results, the pyrite films were observed to crystallise more effectively, and the root mean square roughness tended to increase corresponding with increased film thickness. The grains govern the conductivity mechanism of FeS₂ film with a thickness of 109 nm. Both the grains and grain boundaries contribute to the electrical transport, and the grain contribution to the electrical transport mechanism has been further enhanced as the film thickness increases from 230 to 400 nm.     

CuInSe2 thin films deposited by UV laser ablation

Application of pulsed laser ablation method for deposition of CuInSe₂ films was studied. The special time–temperature regime was developed. The homogeneous amorphous and polycrystalline CuInSe₂ films were prepared and investigated with XRD, SEM-EDS and optical spectroscopy.     

Electrodeposition and characterization of CdTe thin films on Mo foils using a two voltage technique

Polycrystalline thin film CdTe-based solar cells are one of the most promising candidates for low-cost terrestrial conversion of solar energy because of the optimum energy band gap (E_g=1.44eV) and high absorption coefficient of CdTe. In this work, a two-step electrodeposition technique has been used to prepare CdTe thin films from acidic solutions. In the first step, a thin Te rich CdTe layer was deposited at –300 mV (SCE) on a Mo foil substrate. On top of this film, a Cd rich CdTe layer was deposited at more negative voltages. The resulting films showed good adherence to the substrate and very low contact resistance between the substrate and the p+ Te rich CdTe layer. The composite film was of nearly stoichiometric composition. From X-ray diffraction results, the as-deposited films show very small grain sizes but after annealing the grain size increases considerably showing very well-defined peaks. The morphological, structural and composition results of CdTe thin films obtained by Scanning Electron microscopy, X-ray diffraction, and X-ray fluorescence will be presented. Electrical properties such as conductivity type and contact resistance values for the Mo/CdTe structures will also be presented.     

Diffusion processes for doping of C60 (fullerene) thin films

As part of our ongoing research program to produce a high-efficiency, low-cost, photovoltaic cell based on the fullerene C₆₀, we report here on our first attempts at the intercalative doping of C₆₀ thin films by the electrodiffusion of metals. Semiconductor behavior with decreased values of conductivity activation energy has been demonstrated for the doped samples. The results are explained by electrodiffusion of Au from an electrode, dominated by grain boundary diffusion.     

Photoelectrical properties of ZnS thin films deposited from aqueous solution using pulsed electrochemical deposition

ZnS is an n-type semiconductor with a wide direct band gap (3.7 eV at room temperature), and it is very suitable as a window layer in heterojunction photovoltaic solar cells. We deposited ZnS thin films on Sn-doped In₂O₃-coated glass substrate using pulsed electrochemical deposition (ECD) from aqueous solutions containing Na₂S₂O₃ and ZnSO₄ with two different compositions, the first group grown from ZnSO₄-rich solution, and the second grown from Na₂S₂O₃-rich solution. We investigated electrical properties of the ZnS thin films and properties of contacts with different metals evaporated on the surfaces. We found that Au and In contacts have Ohmic-like characteristics to ZnS. Furthermore, we observed photoconductivity of the ZnS thin films by means of photoelectrochemical (PEC) measurements. We found that for both the groups of ZnS thin films, the as-deposited film shows weak photosensitivity and after annealing at 300 °C the photosensitivity improved.     

Influence of dispersion forces on phase equilibria between thin liquid films and their vapour

Heat transfer and nucleation processes in nucleate boiling strongly depend on the phase equilibrium at the liquid-vapour interface. In a certain region between heated wall and a vapour bubble where a thin liquid film is adsorbed, phase equilibria are considerably influenced by dispersion forces acting on the liquid film. As shown in the paper in such systems the chemical potential, decisive for phase equilibria between liquid films and their vapour, contains an additional term for the action of dispersion forces, and differs from the chemical potential of dispersion-free systems, though their chemical potential is usually taken in the literature for systems with dispersion forces. With the aid of the chemical potential the Kelvin equations for the pressures at the liquid-vapour interface were derived. It turned out that the Gibbs assumption of a geometrical interface between extremely thin liquid films in equilibrium with its vapour does not hold. Instead, following the ideas of van der Waals junior, the small but finite transition interlayer between both phases had to be introduced.As numerical examples illustrate, the dispersion forces considerably influence the pressures at the liquid-vapour interface. In nucleate boiling processes the driving pressure difference for evaporation undergoes a maximum within a tiny area underneath vapour bubbles. As could be shown the maximum driving pressure difference between gas-side interface and gas-core is a considerable fraction of the vapour pressure itself and contributes significantly to the high heat fluxes in nucleate boiling.     

CuInTe2 and In-rich telluride chalcopyrites thin films obtained by electrodeposition techniques

In order to obtain single-phase thin films of the system Cu–In–Te with optoelectronic properties adequate for solar cells, electrodeposition techniques were used on substrates of molybdenum supported by glass. Different annealings in Te atmosphere have been done that affect the Te concentration and In/Cu atomic ratio. Single chalcopyrite phase appears in two ranges of composition where the In/Cu atomic ratio varies between 0.21–0.76 and 0.90–3.46, respectively. Morphology, cell lattice parameters and electrical resistance for single-phase samples depend strongly on the composition in the annealed samples. The cell parameters ranges are a=6.141–6.183 Å and c=12.201–12.375 Å.     

Deposition of thin conducting films of CuI on glass

A method is described for coating thin optically transparent conducting films of CuI (p-type semiconductor of band gap 3.1 eV) on glass. The dependence of the sheet resistance of the film on the level of iodine doping and other characteristics of the film are described. A minimum sheet resistance 25 Ω/□ (for a film of thickness 10 μm) was obtained through an optimization of iodine doping, sintering time and temperature.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

Preparation and properties of sprayed CuGa0.5In0.5Se2 thin films

CuGa_0.5In_0.5Se₂ thin films were prepared by spray pyrolysis technique at substrate temperatures (T_s) in the range 100–400°C. The films prepared at T_s = 300–350°C were nearly stoichiometric, polycrystalline with a strong preferred (112) orientation. The resistivity of the films varied in the range, 50–1000 Ω cm and the evaluated optical band gap was 1.35 eV.     

Physical basis for the design of CdS/CdTe thin film solar cells

Solar cells based on polycrystalline semiconductor thin films have great potential for decreasing the cost of photovoltaic energy. However, this kind of solar cells has characteristics very different from those fabricated on crystalline silicon for which the carrier-transport and behavior is clearly known. Instead, for hetero-junction solar cells made on less known polycrystalline materials the design is almost empirical. In this work, several physical aspects related to the behavior of polycrystalline thin film solar cells will be discussed, and some considerations for an adequate design of this kind of solar cells will be made. For example, the recombination at the grain boundaries and its influence on the short circuit current as a function of the crystallite sizes on the active material is considered. Based on this, the appropriate thickness of each layer and their resistivity will be discussed. As an example, these considerations will be applied to CdS/CdTe heterojunction solar cells, taking into account typical properties of CdTe thin films used for solar cells.