Polycrystalline CuGaSe2 Thin Films Research Articles

Deep level emission in polycrystalline CuGaSe2 thin-films observed by micro-photoluminescence

Polycrystalline CuGaSe2 thin-films with various Cu-contents were deposited by three-stage co-evaporation method. Radiative transitions in the CuGaSe2 were studied by micro-photoluminescence technique in relation to the Cu/Ga ratio in the films. In addition to a peak DAP1 around 1.60 eV, a peak DAP2 at relatively lower energy of 1.35 eV and a broadened deep-level emission DL roughly centered at 1.05 eV were observed in the photoluminescence (PL) spectra of the CuGaSe2 films. Excitation-power dependence along with temperature dependence of the PL suggests that dominant recombination mechanism due to donor–acceptor pair recombination at lower temperature becomes impurity-to-band transition at higher temperature for all the three peaks. A deep donor-level, roughly 630 meV below the conduction band, which corresponds to DL-emission has been confirmed, and was investigated further in relation to the compositional variation in the CuGaSe2 films.

Effect of Ga/Cu Ratio on Polycrystalline Thin Film Solar Cell

Structural and electrical properties of polycrystalline CuGaSe2 thin films have been studied by changing the Ga/Cu ratio in the films. CuGaSe2 thin films with various Ga/Cu ratio were grown over Mo-coated soda-lime glass substrates. With the increase of Ga content in CuGaSe2, morphology of the films was found to deteriorate which is associated with the smaller grain size and the appearance of impurity phases presumably due to the phase transition from the chalcopyrite structure to the defect-related phase on the surface of the films. Properties of the Ga poor films were affected by the Cu rich secondary phases. Electrical properties of the films were strongly influenced by the structural properties and degraded with increasing the Ga/Cu ratio in the film. Device performances, fabricated with the corresponding CuGaSe2 films, were found to be correlated with the Ga/Cu ratio in the films and consistent with the observed structural and electrical properties.

Cu-dependent phase transition in polycrystalline CuGaSe2 thin films grown by three-stage process

The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.

Impact of Cu/III ratio on the near-surface defects in polycrystalline CuGaSe2 thin films

Polycrystalline CuGaSe2 thin films grown with various Cu/III(=Cu/Ga) ratios were investigated by positron annihilation spectroscopy (PAS). The line-shape parameter S of the spectra was used to characterize defects in CuGaSe2 films. The S-parameter in positron annihilation spectra increased with decreasing bulk Cu/III ratio in the CuGaSe2 film. Experimental results combined with theoretical calculation show the formation of multiple vacancy-type defect complexes in the near-surface region of the CuGaSe2 film when Cu-content in the film is decreased. These point defects appear to cause the higher S-parameter in PAS measurement.

The effect of composition on structural and electronic properties in polycrystalline CuGaSe2 thin film

Polycrystalline CuGaSe2 thin films on Mo-coated soda-lime glass substrates have been synthesized by co evaporation process from Cu, Ga and Se sources. Structural and electrical properties of the as-grown CuGaSe2 films strongly depend on the film composition. Stoichiometric CuGaSe2 is fabricated, as indicated by x-ray dirraction spectroscope (XRD) and x-ray fluorescence (XRF). A two-phase region is composed of CuGaSe2 and Cu2-xSe phases for Cu-rich films, and CuGaSe2 and CuGa3Se5 phases for Ga-rich films respectively. Morphological properties are detected by scanning electron microscope (SEM) for various compositional films, the grain sizes of the CuGaSe2 films decrease with the extent of deviation from stoichiometric composition. Raman spectroscopy of Cu-rich samples shows that there exist large Cu-Se particles on the film surface. The results from Hall effect measurements for typical samples indicate that CuGaSe2 films are always of p-type semiconductor from Cu-rich to Ga-rich. Stoichiometric CuGaSe2 films exhibit relatively large mobility than any other compositional films. Finally, polycrystalline CuGaSe2 thin film solar cell with a best conversion effciency of 6.02% has been achieved under the standard air mass (AM)1.5 spectrum for 100 mW/cm2 at room temperature (aperture area, 0.24 cm2). The open circuit voltage of the CuGaSe2 solar cells is close to 770 mV.

Material properties of CuGaSe2 absorber films prepared by rapid thermal annealing of stacked elemental layers

In this study, the reaction kinetics and material properties of polycrystalline CuGaSe2 thin films are reported. The films were prepared by the rapid thermal processing of thermally evaporated glass/Ga/Se/Cu//Ga/Se stacked precursors. In order to gain insight into the chalcopyrite formation process, stacked elemental layers with different Cu/Ga atomic ratios were annealed in an Ar/Se atmosphere under different thermal conditions. For typical annealing temperatures below 600°C, x-ray fluorescence studies indicated that a homogeneous element distribution could only be obtained for stoichiometric or near-stoichiometric samples. Systematic studies indicated that temperatures above 600°C are required in order to enhance the crystallinity and in-depth compositional uniformity of Ga-rich CuGaSe2 thin films. These results also highlighted the structural limitation of high gallium containing chalcopyrite thin films.

Solar Cells Based on PVD Grown CuGaSe2 – Absorber and Device Properties

ABSTRACTFor solar cells based on CuGaSe2(CGS) absorber layers physical vapour deposition (PVD) was employed to deposit polycrystalline CGS thin films. Efficiencies up to 7.9% were achieved by applying a two-stage deposition process for absorber preparation. The high structural quality of the absorbers is shown. For highest cell efficiencies our standard recipe for the CdS buffer deposition had to be re-adapted. Voltage dependent spectral response measurements of the device in conjunction with absorption measurements of the absorber layers allow calculation of the minority carrier diffusion length, space charge region (SCR) width, absorber carrier concentration and the built-in voltage. The defect state density at the buffer/absorber interface is estimated. It can be shown that the benefical effect of the modified buffer deposition is mainly due to a reduced acceptor concentration at the absorber/buffer interface.

Study of polycrystalline CuGaSe2 thin films deposited by MOCVD onto ZnO substrates

Metalorganic chemical vapour deposition (MOCVD) from three organometallic precursors has been applied to the growth of CuGaSe2. Polycrystalline thin films were obtained by co-deposition onto ZnO-coated glass substrates. Structural, morphological, optical and photoluminescence characterizations reveal that the properties of the material strongly depend on the composition. The various characteristics will be detailed. Stoichiometric CuGaSe2 was obtained, as shown by XRD, with a gap of about 1.68 eV. A two-phase region composed of CuGaSe2 and CuGa3Se5 was evidenced for Ga-rich compounds. The absorption edge shifts toward higher energies when the Ga-content increases and for CuGa3Se5 a direct optical gap was found at about 1.85 eV. Photoluminescence measurements show only a large and intense peak from Cu-rich to Ga-rich compositions which is attributed to a donor-acceptor transition.

A New Approach to Grow Polycrystalline CuGaSe2 Thin Films: Chemical Vapor Deposition with I2 as Transport Agent

Polycrystalline CuGaSe2 thin films have been prepared onto glass by a chemical vapor deposition (CVD) method which uses iodine as the transport agent. The source material was a polycrystalline CuGaSe2 powder which was pressed into pellets. Single phase CuGaSe2 films were prepared in a temperature range from 460°C to 560°C and had grain sizes between 0.5 and 5 µm. Thermochemical calculations were performed for the system under equilibrium conditions to model the deposition process. The necessary values for the heat of formation H298 and the standard entropy S298 for CuGaSe2 were estimated to be H298=-251 kJ/mol and S298=155 J/K mol. The calculations are in good agreement with the experimental observations.

Ambipolar Diffusion Length in CuGaSe2 Thin Films for Solar Cell Applications Measured by Steady-State Photocarrier Grating Technique

The steady-state photocarrier grating technique is used to measure the transport properties of polycrystalline CuGaSe2 thin films. The influence of composition and growth temperature is analysed in bilayers grown exactly the same way as absorbers for solar cells. The photocurrent losses in solar cells in `substrate' and `superstrate' configuration are related to the measured diffusion lengths.

Composition-structure relationships for multisource evaporated CuGaSe2 thin films

X-ray diffraction (XRD) analysis of doublet formation and peak shifts were used to observe the compositional dependence of the order–disorder transition in polycrystalline CuGaSe2 thin films deposited by multisource evaporation. Cu-poor material had a strong tendency to disorder as evidenced by the simultaneous presence of both the chalcopyrite and sphalerite phases. Stoichiometric and Cu-rich material contained only the tetragonal phase as observed by XRD. Comparison of Cu-poor XRD patterns with theoretical calculations reflecting probable defect chemistries (VCu, GaCu, VSe) suggests an interesting microstructure. The absence of higher index group (iii) reflections, notably the (103) and (211) peaks, in chalcopyrite material suggest that the tetragonal phase maintains a near-stoichiometric composition. Overall Cu-poor film compositions may therefore be obtained by adjustment within the cubic phase which implies compositional segregation between the phases.