Efficiency Cu(In,Ga)Se2 Solar Cells Research Articles

Bismuth-doped Cu(In,Ga)Se2 solar cell on flexible stainless steel substrate: Examination of bismuth-doping effectiveness under different substrate temperatures on photovoltaic performances

Bismuth (Bi)-doped flexible Cu(In,Ga)Se2 (CIGS) films on stainless steel (SUS) substrates with different Bi contents are prepared, obtained by covering Bi thin layers (as Bi source) on Mo layers with Bi thickness from 0 (without Bi-doping) to 100 nm. It is disclosed that [Ga]/([Ga] + [In]) profiles are almost identical when Bi content is increased by enhancing the Bi thickness. The CIGS grain size and carrier lifetimes are improved through the proper Bi-doping (Bi thickness of 20–65 nm) under low deposition temperature of 457 °C. The 12.6%-efficient flexible Bi-doped CIGS solar cell under the deposition temperature of 457 °C is consequently obtained. Conversion efficiency of the flexible Bi-doped CIGS solar cell under the low deposition temperature of 486 °C is further enhanced to 14.8%, very close to that (15.1%) of the solar cell under convention deposition temperature (543 °C) without Bi-doping. Furthermore, the Bi-doping is effective for the enhancement of the photovoltaic performances under the deposition temperature range of 457–486 °C, whereas the Bi-doping under high CIGS deposition temperature range of 543–572 °C has the detrimental impact on the cell parameters, resulting from the sever Bi diffusion into the CIGS film.

High Efficiency CIGS Solar Cells by Bulk Defect Passivation through Ag Substituting Strategy.

Cu(In,Ga)Se2 (CIGS) is considered a promising photovoltaics material due to its excellent properties and high efficiency. However, the complicated deep defects (such as InCu or GaCu) in the CIGS layer hamper the development of polycrystalline CIGS solar cells. Numerous efforts have been employed to passivate these defects which distributed in the grain boundary and the CIGS/CdS interface. In this work, we implemented an effective Ag substituting approach to passivate bulk defects in CIGS absorber. The composition and phase characterizations revealed that Ag was successfully incorporated in the CIGS lattice. The substituting of Ag could boost the crystallization without obviously changing the band gap. The C-V and EIS results demonstrated that the device showed enlarged Wd and beneficial carrier transport dynamics after Ag incorporation. The DLTS result revealed that the deep InCu defect density was dramatically decreased after Ag substituting for Cu. A champion Ag-substituted CIGS device exhibited a remarkable efficiency of 15.82%, with improved VOC of 630 mV, JSC of 34.44 mA/cm2, and FF of 72.90%. Comparing with the efficiency of an unsubstituted CIGS device (12.18%), a Ag-substituted CIGS device exhibited 30% enhancement.

Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells

The record conversion efficiency of thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorbers has exceeded 23%. Such a high performance is currently only attainable by the incorporation of heavy alkali metals like Cs into the absorber through an alkali fluoride post-deposition treatment (PDT). As the effect of the incorporated heavy alkali metals is under discussion, we investigated the local composition and microstructure of high efficiency CIGS solar cells via various high-resolution techniques in a combinatory approach. An accumulation of Cs is clearly detected at the p-n junction along with variations in the local CIGS composition, showing the formation of a beneficial secondary phase with a laterally inhomogeneous distribution. Additionally, Cs accumulations were detected at grain boundaries with a random misorientation of the adjacent grains where a reduced Cu concentration and increased In and Se concentrations are detected. No accumulation was found at Σ3 twin boundaries as well as the grain interior. These experimental findings are in excellent agreement with complementary ab-initio calculations, demonstrating that the grain boundaries are passivated by the presence of Cs. Further, it is unlikely that Cs with its large ionic radius is incorporated into the CIGS grains where it would cause detrimental defects.

Overall Distribution of Rubidium in Highly Efficient Cu(In,Ga)Se2 Solar Cells.

Thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorbers have achieved conversion efficiencies close to 23%. Such a high performance could be reached by incorporating heavy alkali elements into the CIGS absorber using an alkali fluoride post-deposition treatment (PDT). In order to improve the understanding of the effect of the PDT, we investigated a highly efficient CIGS solar cell whose absorber was subjected to a RbF-PDT. By applying synchrotron-based X-ray fluorescence analysis in combination with scanning transmission electron microscopy and electron backscatter diffraction to a cross-sectional lamella of the whole device, we were able to correlate the local composition of the absorber with its microstructure. The incorporated Rb accumulates at grain boundaries, with a random misorientation of the adjacent grains, at the p-n junction, and at the interface between the absorber and the MoSe2 layer. The accumulation of Rb at the grain boundaries is accompanied by a reduced Cu concentration and slightly increased In and Se concentrations. Additionally, variations in the local composition of the absorber at the p-n junction indicate the formation of a secondary phase, which exhibits a laterally inhomogeneous distribution. The improved solar cell performance due to RbF-PDT can thus be expected to originate from a favorable modification of the back contact interface, the random grain boundaries, the p-n junction, or a combination of these effects.

Pre-incorporation of Na into flexible Cu(In,Ga)Se2 thin film solar cells

Incorporation of sodium into the Cu(In,Ga)Se2 (CIGS) absorber layer is a key step for the growth of high conversion efficiency CIGS solar cells. However, the doping of Na into CIGS layer on a flexible substrate, i.e. Titanium foil, is still a challenge. In this paper, we report our results on comparing two different Na pre-incorporation methods in constructing flexible CIGS solar cells on Ti substrates: i) by sputtering a soda-lime glass thin film (SLGTF) onto Ti substrates prior to the growth of CIGS and Mo back contact layers and ii) by evaporating a NaF precursor layer on the Mo layer before the CIGS growth. By carefully optimizing the growth conditions, the conversion efficiencies of 13.47% and 15.02% are achieved for the SLGTF sample and the NaF sample, respectively. The NaF precursor method shows a better effect. The diffusion of Na and the gradient of Ga play important roles in the performance of the solar cells.

Development of flexible Cd-free Cu(In,Ga)Se2 solar cell on stainless steel substrate through multi-layer precursor method

Cu(In,Ga)Se2 (CIGS) absorbers on flexible stainless steel (SUS) substrates in thin-film solar cells are deposited by multi-layer precursor method with different [Ga]/([Ga]+[In]) grading profiles, thus yielding different band-gap energy (Eg) grading profiles. In Eg grading profiles, near-surface Eg is defined as average Eg within 200 nm from CIGS surface, and minimum Eg is the lowest Eg. It is disclosed that near-surface Eg and minimum Eg have impacts on open-circuit voltage and short-circuit current density, respectively. 17.6%-efficient CIGS solar cell on flexible SUS substrate using CdS buffer is obtained with the suitable Eg grading profile consisting of near-surface Eg and minimum Eg of 1.26 and 1.08 eV, respectively. Flexible Cd-free CIGS solar cell on SUS substrate using ZnS(O,OH) buffer is next fabricated with relative low conversion efficiency of 8.5%, attributed to a relatively low Zn diffusion into CIGS and/or the large conduction band offset (CBO) at ZnS(O,OH)/CIGS interface. However, all photovoltaic parameters of the Cd-free CIGS solar cell are improved after light-soaking process owing to improvement of the CBO at ZnS(O,OH)/CIGS interface. Ultimately, 14.8%-efficient Cd-free CIGS solar cell on flexible SUS substrate utilizing ZnS(O,OH) buffer is obtained, thereby demonstrating the ability for portable power source.

Influence of minimum position in [Ga]/([Ga]+[In]) profile of Cu(In,Ga)Se2 on flexible stainless steel substrate on its photovoltaic performances

Cu(In, Ga)Se2 (CIGS) solar cells on stainless steel (SUS) substrates are fabricated with various [Ga]/([Ga]+[In]), GGI, profiles in their absorbers. From GGI profiles, near-surface GGI is defined as average GGI within 200nm from CIGS surface, mini GGI value is the lowest GGI, and mini GGI position is a distance between the lowest GGI position and CIGS surface. These are used as representatives of the GGI profile. Their impacts on cell performances are quantitatively examined. It is revealed that near-surface GGI is optimized in a range of approximately 0.40–0.45 (or band-gap energy of 1.28–1.30eV), resulting in improvement of conduction band offset at CdS/CIGS interface. From simulation, optimized mini GGI position is in a range of 250–350nm, well consistent with space charge region width. According to experimental results, fill factor and open-circuit voltage are increased when mini GGI position is decreased from 1.1 to 0.4µm, while short-circuit current density is increased with decreasing mini GGI value. Ultimately, 17.3%-efficient CIGS solar cell on flexible SUS substrate with thin CIGS thickness of 1.5µm is achieved, where near-surface GGI, mini GGI position, and mini GGI value are 0.42, 0.5µm, and 0.06, respectively.

Simulation of High Efficiency CIGS Solar Cells with SCAPS-1D Software

In this paper, the high efficiency Cu(In,Ga)Se2 (CIGS)-based solar cells solar cells was analyzed and designed by SCAPS-1D software. This paper deals with the influence of a buffer layer on the performance of the CIGS-based solar cells. The photovoltaic parameters have been calculated in different buffer layer materials (CdS, ZnS, ZnSe), we give great alternative for Cadmium sulphide (CdS). Starting with a good structure, we simulated the J-V characteristics and showed how the absorber and buffer layers thickness, defect density influence the short-circuit current density (Jsc), open-circuit voltage (Vco), fill factor (FF), and efficiency (η) of solar cell. The optimized solar cell shows an efficiency of > 22% under the AM1.5G spectrum and one sun.

Controlled back slope of Ga/(In+Ga) profile in Cu(In,Ga)Se2 absorber fabricated by multi layer precursor method for improvement of its photovoltaic performance

Cu(In,Ga)Se2 (CIGS) absorbers with several Ga/III, Ga/(In+Ga) ratio, profiles were deposited by the so-called “multi layer precursor method” consisting of Ga-Se/In-Se/Cu-Se stacks. The effects of average Ga/III ratios and back slopes of the Ga/III profiles on the photovoltaic performance were investigated. The optimum average Ga/III ratio of approximately 0.36 leads to the appropriate average band-gap energy as well as sufficiently long carrier lifetime in CIGS absorber, thus enhancing open-circuit voltage. In addition, the back slope of 0.50μm−1, defined as the ratio of the change in Ga/III ratio to the change in the depth range from 1 to 2μm from CIGS surface, induces back surface field of 0.34V/μm, thereby increasing short-circuit current density. Ultimately, the CIGS absorber on flexible stainless steel substrate with the double graded Ga/III profile (the average Ga/III of 0.36 and the back slope of 0.50μm−1) was fabricated by the multi layer precursor method. This results in the 14.36%-efficient CIGS solar cell on the flexible substrate without an anti-reflective layer.

Compositional investigation of potassium doped Cu(In,Ga)Se2 solar cells with efficiencies up to 20.8%

AbstractWe report on the interaction between intentional potassium doping of thin film Cu(In,Ga)Se2 (CIGS) solar cells, CIGS absorber composition, and device efficiency. Up to now high efficiency CIGS solar cells could not be produced with a gallium/(gallium + indium) ratio higher than 35%. The new doping process step does not only increase solar cell conversion efficiencies up to 20.8%, but also allows a shift in the CIGS absorber composition towards higher gallium content whilst maintaining this high efficiencies level. We find that the saturation of the open circuit voltages for higher gallium content that is normally observed can partially be overcome by the new doping procedure. This observation leads us to the conclusion that even on this high performance level CIGS solar cells still hold a potential for further development beyond the record values reported here. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diffusion barrier properties of molybdenum back contacts for Cu(In,Ga)Se2 solar cells on stainless steel foils

Flexible Cu(In,Ga)Se2 (CIGS) solar cells on stainless steel foils face the problem of efficiency deterioration when iron impurities diffuse into the absorber layer. The influence of the magnetron sputtering conditions and the design of Mo-based back contacts on the property of the diffusion barrier against iron is reported here for high efficiency CIGS solar cells grown at low substrate temperatures (Tmax = 475 °C). The overall material density of the Mo back contact was identified as the dominant parameter for the impurity diffusion barrier performance. It was found that this is also true for Mo bilayer contacts, which show enhanced film densities at low residual stress. The iron diffusion profile in the back contact and CIGS was measured by secondary ion mass spectroscopy, where a linear decrease in the iron impurity concentration in the CIGS towards the CdS buffer layer was found. Furthermore, this iron distribution in CIGS and its consequences on the solar cell efficiency is discussed, supported by defect analysis measurements and photovoltaic device simulations. With a stress-free ∼500 nm thick Mo bilayer back contact, best solar cell efficiencies above 15% were achieved with antireflection coating.

Buffer-less Cu(In,Ga)Se2 solar cells by band offset control using novel transparent electrode

Cu(In,Ga)Se2 (CIGS) solar cells without buffer layers have been demonstrated. Currently, CdS, Zn(O,S,OH), ZnS, or InS buffer layers are used in high efficiency CIGS solar cells to suppress interface recombination. One of the important parameters to reduce the recombination is the conduction band offset (CBO) between the buffer and CIGS layers. In this study, we have proposed the use of a novel transparent conductive oxide (TCO) which can control the CBO to reduce interface recombination and eliminate the buffer layers. The device simulation was used to verify the effect of CBO control theoretically. Then, the novel TCO material of ZnO1−xSx:Al prepared by co-sputtering of ZnO:Al2O3 and ZnS targets was fabricated to verify the CBO effect experimentally. The efficiency of a CIGS solar cell with a ZnO:Al/CIGS/Mo/soda-lime glass structure, i.e. buffer-less structure using a conventional TCO, was significantly low because of severe shunting. In contrast, the use of ZnO1-xSx:Al instead of ZnO:Al increased the shunt resistance of the CIGS solar cell, resulting in higher open-circuit voltage and efficiency. The result is the first proof of the concept of the buffer-less CIGS solar cells.

The role of growth temperature and Se flux on Cu(In,Ga)Se2 thin film deposited on a stainless steel substrate and solar cell

The role of growth temperature and Se flux on Cu(In,Ga)Se2 (CIGS)/Mo deposited on stainless steel substrates without a barrier is investigated by combined use of secondary ion mass spectroscopy, scanning electron microscopy and Raman spectroscopy. Fe atoms diffuse into the CIGS film with an increasing slope toward the CIGS front surface and the CIGS/Mo interface at 500 °C. High Se flux can increase the grain size and depress the formation of group III-rich phases on the surface and grain boundaries of the CIGS film, reducing Fe content in the surface region of the CIGS film and grain boundaries, which results in the increase of open circuit voltage (Voc) and fill factor (FF). Meanwhile, Jsc also significantly increases for high Se flux. In this case, a 12.1%-efficient CIGS solar cell on stainless steel without a diffusion barrier and Na supply is obtained. The influence of diffusing Fe on a solar cell with different Se fluxes is further discussed in detail.

Characterization of Cu(In,Ga)Se2/Mo Interface in CIGS Solar Cells

AbstractThe interface between a Cu(In,Ga)Se2 (CIGS) and an underlying Mo layer was studied by X-ray diffraction and high resolution transmission electron microscopy. The CIGS layer was deposited onto Mo coated soda-lime glass using the “3-stage” process. A MoSe2 layer found to form at the CIGS/Mo interface during the 2nd stage of the “3-stage” process. The thickness of the MoSe2 layer depended on the substrate temperature used for CIGS film deposition as well as the Na content of the CIGS and/or Mo layers. For higher substrate temperatures, thicker MoSe2 layers were observed. The Na in the CIGS and/or Mo layer is felt to assist in the formation of MoSe2. Current-Voltage measurements of the heterojunction formed by the CIGS/Mo interface were ohmic even at low temperature. The role of the MoSe2 layer in high efficiency CIGS solar cells is discussed.