Effect of copper diffusion on photovoltaic characteristics of CuGaSe 2–GaAs cells (original) (raw)
Related papers
Transport properties of CuGaSe2-based thin-film solar cells as a function of absorber composition
Thin Solid Films, 2011
The transport properties of thin-film solar cells based on wide-gap CuGaSe 2 absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E a) of approx. 0.95 eV in the dark and 1.36 eV under illumination, (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E a values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe 2 absorber independently from the absorber composition. The increase of E a with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells.
CuGaSe2-Based Solar Cells with High Open Circuit Voltage
MRS Proceedings, 2007
The objective of this work is to increase the open circuit voltage of CuGaSe2(CGS)-based solar cells without decreasing their efficiency. For that, the interface between the p-type CGS absorber and the n-type CdS/ZnO window layer is compared using three different recipes for the growth of the buffer layer. Results show the importance of the adaptation of the CdS buffer layer to the CuGaSe2 absorber film. A maximum open circuit voltage of 922 mV is achieved for the devices when using 60ºC as the chemical bath temperature and a low thiourea concentration. Drive-level capacitance profiling, external quantum efficiency and temperature dependent current-voltage measurements reveal a better quality of the CdS/CuGaSe2 interface for this buffer layer deposition conditions. Factors such as the larger depletion region width and the lower doping level, reducing the tunnelling component, are pointed out as responsible of the higher Voc.
Solar Energy Materials and Solar Cells, 1992
The growth conditions, the composition and the structural, optical and electrical properties of thin films of CuGaSe 2 and CuGaTe 2 have been studied using "flash" and "slow" evaporation in vacuum. Single phase films, when analyzing the absorption coefficient, present several energy gaps. For CuGaSe> they are 1.59, 1.66, 2.03 and 2.11 eV, for CuGaTe 2 1.23 and 1.89 eV. Both the CuGaSe~ and CuGaTe 2 evaporated films are p-type; the resistivities, carrier densities and mobilities are appropriate for thin film solar cells.
The role of the CdS buffer layer in CuGaSe 2 -based solar cells
Journal of Physics: Condensed Matter, 2007
We present some results for Ga-rich CuGaSe 2 -based solar cells using different conditions for deposition of the CdS buffer layer. Three different recipes for the growth of the CdS buffer layers at 60 and 80 • C are compared. This work shows the importance of the adaptation of the CdS growth to the absorber layer. A maximum open circuit voltage V oc of 922 mV is achieved by using 60 • C as the chemical bath temperature and lower thiourea and ammonia concentrations. Temperature-dependent current-voltage and room temperature drive-level capacitance profiling measurements are carried out to study the CdS/CuGaSe 2 interface properties. All devices are characterized by tunnellingenhanced interface recombination. However, results show that the higher V oc obtained for the low bath temperature coincides with a reduced tunnelling component. This can be attributed to a lower net doping level in the space charge region. Secondary ion mass spectrometry analysis reveals Cu diffusion into the buffer layer when CdS is made at 80 • C. This agrees with the higher charge density observed for those devices, enhancing the tunnelling.
Solar Energy Materials and Solar Cells, 2011
Current-voltage (j-V) characteristics of the record-efficiency CuGaSe 2 solar cell measured under several illumination levels are analyzed using a two-diode equation for a more accurate description of cell behavior. The contribution of each diode to the total cell j-V characteristic under illumination was estimated using the current separation method presented recently [1]. This is performed in an effort to identify the distinctive features of this record-efficiency cell, which have led to the up-to-date highest open circuit voltage of V oc ¼946 mV and fill factor of FF¼ 66.5% for CuGaSe 2 solar cells. Furthermore, the interface recombination component of the cell current under illumination is quantitatively discussed applying the interface recombination model presented earlier [2].
Thin Solid Films, 2012
CuGaSe 2 (CGS) thin films were grown on uncoated and Mo-coated soda lime glass by Pulsed Electron Deposition (PED) technique at substrate temperatures comprised between 25°C and 475°C. X-ray diffraction analysis reveals that CGS samples exhibit a noteworthy crystal quality even at low growth temperature, T g = 100°C, whereas the out-of-plane preferential orientation of CGS chalcopyrite phase switches from b 220 > to b112 > by increasing the substrate temperature. Annealing treatments seem to enhance the crystallinity of the film and to release the residual strain energy. Visible/near-infrared absorbance spectra show a monotonic decrease of CGS optical bandgap (from 1.75 to 1.65 eV) by enhancing the substrate temperature. Yet the morphology of CGS films strongly depends on T g , which promotes the formation of larger columnar grains perpendicular to the growth plane. Grain dimensions of~2 μm are achieved when CGS films are grown at high temperature (> 400°C) on Mo-coated glass. The results indicate that PED is a promising growth technique for achieving good-quality CGS that can be useful as absorber layers in thin film solar cells.
Structural peculiarities of CCSVT-grown CuGaSe2 thin films
Thin Solid Films, 2005
The microstructure of the CuGaSe 2 (CGSe) thin films deposited by a novel chemical close-spaced vapour transport (CCSVT) technique on clean and Mo-coated soda lime glass (SLG) substrates has been investigated by transmission electron microscopy (TEM). The CGSe bulk and the interface between the CGSe and Mo films have been investigated. The as-grown CGSe films possess high bulk crystalline quality. At the CGSe/Mo interface, a MoSe 2 interfacial layer (~20 -40 nm) has been observed and also an excess of Ga. Additionally, composition measurements and depth profiling of the elements were performed by elastic recoil detection analysis (ERDA). It has been found that the CGSe constituent elements are homogeneously distributed in the bulk, whereas the surface composition is influenced by the [Ga]/[Cu] ratio in the film. With the [Ga]/[Cu] ratio increase, the CGSe surface composition changes from Ga-and Cu-poor, and Se-rich to Cu-poor, and Gaand Se-rich. Photoluminescence (PL) spectroscopy has been used as a complementary technique to study the defect profiles at the CGSe front and rear sides as a function of the [Ga]/[Cu] ratio. The PL data support the results of structural investigations, pointing out higher Ga concentration at the films rear side. D
Defect levels in the epitaxial and polycrystalline CuGaSe2 by photocurrent and capacitance methods
Journal of Applied Physics, 2011
The defect levels in epitaxial and polycrystalline wide bandgap chalcopyrite CuGaSe 2 with various stoichiometry deviations were investigated using modulated photocurrent spectroscopy. The results were analyzed as a function of light intensity and Fermi-level position. Comparison of the results from epitaxial and polycrystalline material distinguished levels belonging to intrinsic defects and their correlation with the material stoichiometry. We also compared the fingerprints of defect levels by MPC to the results derived from capacitance spectroscopy performed on Schottky diodes fabricated on both epitaxial and polycrystalline layers. This allowed us to attribute unambiguously levels observed in the capacitance response to bulk point defects. In the final conclusions we provide information on the electronic parameters of nine defect levels observed in CuGaSe 2 and their correlation with the material stoichiometry. These results should help to identify intrinsic defects that are important for the photovoltaic performance of solar cells based on chalcopyrites.
Surface and bulk properties of CuGaSe2 thin films
Journal of Physics and Chemistry of Solids, 2003
Using complementary techniques, namely X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS), we present a comparative study of the bulk and surface composition in device grade CuGaSe 2 (CGSe) thin films. The films were deposited in two stages by an open-tube chemical vapor deposition (CVD) process. The first stage leads to a nearly stoichiometric polycrystalline CGSe film of approximately 1.5 mm thickness. During the second stage the film is annealed in a Ga-and Se-rich atmosphere. While the XRF-data show a nearly stoichiometric integrated film composition, the surface composition, as determined by XPS analysis, is Cu-poor, pointing towards a highly non-stoichiometric surface layer. In addition, sodium was found at the film surfaces. The data are discussed in the framework of an ordered defect compound formation and the formation of a (Cu,Na) -Ga-Se compound at the surface of the CuGaSe 2 films. Complementary ultraviolet photoelectron-and inverse photoelectron spectroscopy investigations of the film surface derive a widening of the surface energy band gap up to 2.2 eV in comparison with a bulk energy band gap around 1.65 eV (obtained by optical transmission analysis). The observed data are consistent with our model of a two layer film structure containing a defect-rich near-surface region and a defect-poor bulk. q
Effect of absorber surface modification on the optoelectronic properties of Cu2CdGeSe4 solar cells
Thin Solid Films, 2020
High quality Cu 2 CdGeSe 4 micro-crystalline powder has been synthesized by molten salt method at 700°C in closed quartz ampoules using elemental Ge, binary CdSe and CuSe as precursor materials and KI as flux material. The effect of initial Cu and Cd content on the bulk composition of grown crystals was investigated. According to energy dispersive X-ray spectroscopy results, the two types of Cu 2 CdGeSe 4 powders, with nearly stoichiometric and with Cd-rich composition were synthesized. X-ray diffraction and Raman analyses confirmed that all studied Cu 2 CdGeSe 4 crystals had orthorhombic crystal structure. It was essential to chemically and thermally modify the surface of crystals before implementing the powder crystals as absorber materials in monograin layer solar cells. Results showed that both Br 2 −MeOH and HCl combined with KCN etching were effective to remove secondary phases on the crystal surface. Raman and X-ray photoelectron spectroscopy analyses revealed that after annealing at 400°C the crystal surface is covered by Ge x Se 1-x phase, which was effectively removed by KCN etching. Using this approach, we achieved Cu 2 CdGeSe 4 monograin layer device conversion efficiency of 5.7%.