Enhancing photovoltaic performance of GeSe thin film solar cells by photogenerated carriers redistribution via Cu doping (original) (raw)
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Thin Solid Films, 2020
In this paper, we propose a possible explanation of the limited CuIn 1-x Ga x Se 2 (CIGSe) performance at high Ga contents, related to the properties of detrimental copper selenide (Cu δ Se) secondary phases. We study CIGSe layers at different x, by means of X-ray diffraction analyses, Raman and energy dispersive X-ray spectroscopy. Our results reveal that for high Ga contents, Cu δ Se secondary phases either precipitate at the interfaces between grains or remain within the layer as intra-or inter-grain clusters, deteriorating the electronic properties of the absorber layer. On the contrary, for low Ga contents, the copper selenide segregates at the surface of the CIGSe and hence it can be easily removed by KCN surface etching. To understand the Cu δ Se precipitation within the bulk of the film for a high Ga ratio, we also investigate In-free CGSe samples at low and high Cu content. Our observations demonstrate that i) Cu δ Se has a preferential grain orientation to segregate and ii) the Cu-enriched regions within the bulk of the high Ga content films are more a kinetic than a thermodynamic issue.
Photoelectrochemical studies of copper gallium diselenide films
Solar Cells, 1990
Thin films of copper gallium diselenide of various composition were prepared by vacuum deposition. Spectral responses at several applied potentials were obtained in a photoelectrochemical cell with an acidic electrolyte (1 M HC1) and in a 0.05 M V2+/V 3÷ solution. They allowed u~ to test the intrinsic quality of the layers to establish photovoltaic applications. Effective diffusion lengths and acceptor concentrations are determined. It is shown that the samples may be classified into four groups, depending on their photoelectrochemical properties. An attempt is made to correlate behaviour to the chemical composition of the films, by defining convenient deviations from stoichiometry and from molecularity. The results suggests that in the monophasic Cu2Se-CuGaSe2 range, the deviation from stoichiometry is due to an excess of copper and to an excess of selenium, or to a defect of gallium.
Influence of post-deposition selenium supply on Cu(In,Ga)Se2-based solar cell properties
Thin Solid Films, 2015
Cu(In,Ga)Se 2 (CIGSe) layers have been deposited by the 3-stage process and once the growth completed, the structures have been kept at high temperature for 50 min with or without Se supply. Both the resulting CIGSe layer and related device properties are compared with those obtained when the substrate is cooled down right after the deposition. Moreover, such experiments have been performed with and without alkali availability. The results show that keeping the absorber at high temperature differently impacts [Ga]/([In+Ga]) atomic ratio distribution and crystalline preferential orientation depending on whether Se is supplied. Moreover, this work shows chalcogen supply can be detrimental for cells performance when the CIGSe contains alkali and in contrast be beneficial when the CIGSe is free of alkali. These observations suggest an intimate relationship between alkali and Se.
Solar Energy Materials and Solar Cells, 2014
CuInSe 2 solar cells were prepared from electrodeposited and annealed stacks of indium selenide and copper selenide on molybdenum (Mo/In 2 Se 3 /Cu-Se). In comparison to a simultaneous electrodeposition of all elements at once the electrodeposited binary selenide stack leads to larger grains and lattice coherence length if annealed under the same conditions (30 min at 550 1C). Absorbers with atomic Cu/In ratios o 1, 1 and 41 and their corresponding solar cells were characterized. An incomplete reaction in case of the Cu-poor precursor caused a Cu-deficient layer at the back contact of the absorber leading to the formation of a reverse electronic barrier reducing fill factor and short circuit current and thus the solar power conversion efficiency. Photoluminescence measurements showed a strongly compensated semiconductor for the Cu-poor absorber and an incomplete charge carrier collection was identified by quantum efficiency measurements under reverse bias. The reverse electronic barrier, the high compensation and the incomplete carrier collection could be avoided by Cu-rich growth conditions. The appearance of an excitonic transition in photoluminescence indicated a high semiconductor quality in this case. It was linked with a high quantum efficiency resulting in a local short-current density of 38.3 mA/cm 2. Temperature-dependent JV measurements identified interface recombination as the limiting recombination loss mechanism for the Cu-rich grown solar cell reducing the open-circuit potential and decreasing the conversion efficiency. The solar cell prepared from the precursor with Cu/ In E1 had similar properties as the Cu-poor device without the reverse electronic barrier at the back contact but instead dominating interface recombination. It reached a best solar power conversion efficiency of 5.5%.
Correlation between grain composition and charge carrier collection in Cu(In,Ga)Se2 solar cells
2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015
Understanding the spatial composition inhomogeneity in CIGS solar cells can provide insight into a potentially large cause of decreased cell efficiency. We used synchrotron based nano x-ray fluorescence and nano x-ray beam induduced current to investigate the correlation between grain-tograin compositional variations in Cu(In,Ga)Se2 absorber layers and variations in carrier collection efficiency. We found compositional variations as large as 5 at. % and current variations as large as 10%. Grains with higher average copper concentration and lower indium concentration showed increased carrier collection than neighboring grains with lower copper and higher indium concentration. We also investigated the overall composition dependence of carrier collection in CIGS films and show that 30% gallium content devices show a correlation between charge carrier collection, while 60% content devices do not.
Solar RRL, 2019
Thin-film solar cells based on Cu(In,Ga)Se2 absorber layers have reached conversion efficiencies of well above 20%. One key of this success is the incorporation of alkali metals such as Na and K into the surface and the volume of the Cu(In,Ga)Se2 thin film. The present work discusses the impact of Na and K on the grain-boundary (GB) properties in Cu(In,Ga)Se2 thin films, i.e., on the barriers for charge carriers, Φb, and on the recombination velocities at the GBs, sGB. The authors first revise the physics connected with these two quantities as well as their impact on the device performance, and then provide values for the barrier heights and recombination velocities from the literature. The authors measured sGB values by means of cathodoluminescence analysis of Na-/K
Formation and characterisation of MoSe2 for Cu(In,Ga)Se2 based solar cells
Thin Solid Films, 2005
The reaction kinetics of the MoSe 2 formation have been investigated by selenizing Mo layers in Se vapor at different temperatures and for different durations. The samples were characterized by means of Rutherford backscattering spectrometry, X-ray diffraction (XRD), electron diffraction, and by bright-field and high-resolution transmission electron microscopy. It was found that in all samples, a homogeneous MoSe 2 layer is formed on top of the Mo layer. The temperature dependence shows that the MoSe 2 layer thickness increases strongly for temperatures higher than ca. 550 8C. At a substrate temperature of 450 8C, no difference in the MoSe 2 thickness was detected for samples selenized for different durations. The diffusion constant of Se in MoSe 2 is estimated from the selenization duration dependence of MoSe 2 layer thicknesses at 580 8C. Finally, Cu(In,Ga)Se 2 (CIGS) solar cells in substrate configuration were developed on indium tin oxide (ITO) transparent back contacts. An intentionally grown MoSe 2 intermediate layer on ITO, prior to CIGS deposition, causes a significant efficiency improvement, suggesting that MoSe 2 can facilitate a quasi-ohmic contact. Solar cell efficiencies of up to 11.8% are obtained using an ITO/MoSe 2 back contact.
Cogent Engineering
Thin films of CuGaSe were deposited on n-Si (1 0 0) by rf magnetron sputtering from a stoichiometric CuGaSe 2 target. The objective of this study was to characterize the thin film/Si heterojunction for potential photovoltaic applications, evaluate possible candidates for metal contacts and to establish whether heteroepitaxial growth could be achieved, particularly as the mismatch of lattice parameters corresponding to the base of the copper gallium selenide (CGS) tetragonal cell is quite close to that of Si, with a 2.9% mismatch. For this study, Si substrates were prepared by the standard Radio Corporation of America (RCA) cleaning procedure immediately followed by the deposition of CGS by sputtering at a substrate temperature of 600°C. The deposited thin-film stoichiometry and morphology were characterized by Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). Rutherford back scattering (RBS) analysis indicated a thin-film composition of Cu 1 Ga 1 Se 1 indicating that the films were Se deficient, although channeling was not observed. The polycrystalline nature of the deposited M. A. Awaah ABOUT THE AUTHORS The research team works on a number of semiconducting material systems for high speed and high efficiency device application. The groups current interests are investigating improved Si Solar Cell efficiency and Next Generation High Efficiency and low-cost Photovoltaics materials. Thin film photovoltaic materials are currently being sought as replacement to current GaAs solar arrays for space applications and as replacements for current silicon based terrestrial technology. Systems based on polycrystalline CuInGaSe2 (CIGS) absorbers have achieved efficiencies above 20%. The current research focuses on the growth and characterization of CuInSe2 (CIS), CuGaSe2 (CGS) material, and contacts studies to these films. The authors would like to express their sincere gratitude to Drs. S. Jeelani and P. K. Ray, Tuskegee University, for their keen interest in the work and the encouragement they provided, J. Callaway for technical assistance and Tuskegee Center for Advanced Materials for providing experimental facility.
Solar RRL, 2019
Thin-film solar cells based on Cu(In,Ga)Se2 absorber layers have reached conversion efficiencies of well above 20%. One key of this success is the incorporation of alkali metals such as Na and K into the surface and the volume of the Cu(In,Ga)Se2 thin film. The present work discusses the impact of Na and K on the grain-boundary (GB) properties in Cu(In,Ga)Se2 thin films, i.e., on the barriers for charge carriers, Φb, and on the recombination velocities at the GBs, sGB. The authors first revise the physics connected with these two quantities as well as their impact on the device performance, and then provide values for the barrier heights and recombination velocities from the literature. The authors measured sGB values by means of cathodoluminescence analysis of Na-/K