Cu(In,Ga)Se2 Grains Research Articles

Anisotropic Charge Transport in Cu(In,Ga)Se2 by Heavy Alkali Postdeposition Treatment for Reducing Cell‐to‐Module Efficiency Loss in Monolithically Integrated Photovoltaic Modules

The recent efficiency boosting of Cu(In,Ga)Se2 (CIGS) solar cells is undoubtedly triggered by heavy alkali postdeposition treatments (PDTs). However, the effects are not obvious under monolithically integrated CIGS modules where various current‐shunting sources can deteriorate the device performance. Herein, It is reported that KF PDT can effectively suppress the major shunting sources caused by P1 and P3 laser scribing for monolithic interconnection, reducing the cell‐to‐module (CTM) efficiency gap in CIGS photovoltaics. CIGS with NaF PDT exhibits nearly isotropic and high hole mobilities, causing a large CTM efficiency loss. CIGS with additional KF PDT, on the other hand, reveals much lower in‐plane hole mobility than the out‐of‐plane component, significantly increasing the P1 shunt resistance without exacerbating the photocarrier extraction in the active area. It is suggested that such anisotropic charge transport is due to carrier scattering by low‐conductivity phases at the CIGS grain boundaries. Furthermore, passivation of the front junction by KF PDT raises the tolerance to P3 scribing‐induced damage, increasing the P3 shunt resistance while preserving the junction property unlike the NaF PDT case. The work implies that the recent trend of employing heavy alkali PDTs for a high‐efficiency cell is also crucial for designing a high‐efficiency CIGS module.

Influence of temperature and sputter source on Cu(In,Ga)Se2 SIMS depth profiles

The thin‐film photovoltaic technology based on Cu(In,Ga)Se2 (CIGS) has reached high conversion efficiencies exceeding 23%. The gallium gradient in the bulk and the amount of alkali metals in and between CIGS grains strongly affect the cell performance and therefore need to be accurately measured and quantified. ToF‐SIMS is well suited for this task, but the spatial resolution is strongly influenced by the measurement parameters, especially at the required micrometer and nanometer scales. Such highly resolved measurements on CIGS surfaces are challenging. For instance, small copper aggregations occur in SEM images recorded after a focused ion beam (FIB) cut and change the energy‐dispersive spectroscopy (EDS) results. SIMS systems that use an argon ion source generate Cu lamellas, which also influence the measurement. Both effects result from preferential sputtering. Cooling with liquid nitrogen can minimize this negative effect for both techniques. Recent developments in SIMS systems provide more options to avoid these effects. New sources, like the Bi liquid metal gun and cluster guns, are available. However, a systematic investigation on the effect of the ion sources on the sputter craters and the resulting measurement of CIGS absorber is necessary to choose an adequate setup. Thus, we performed a comparison of four different ion sources (argon ion gun, oxygen gun, cesium metal gun, and oxygen cluster gun) and measured CIGS depth profiles at three temperatures [−120°C, −50°C, and room temperature (RT)]. We investigate the influence of these conditions not only on the resulting SIMS depth profile and quantification but also on the morphology of the sputter crater.

Growth-Promoting Mechanism of Bismuth-Doped Cu(In,Ga)Se2 Solar Cells Fabricated at 400 °C.

The classical high-temperature synthesis process of Cu(In,Ga)Se2 (CIGS) solar cells limits their applications on high-temperature intolerant substrates. In this study, a novel low-temperature (400 °C) fabrication strategy of CIGS solar cells is reported using the bismuth (Bi)-doping method, and its growth-promoting mechanism is systematically studied. Different concentrations of Bi are incorporated into pure chalcopyrite quaternary target sputtered-CIGS films by controlling the thickness of the Bi layer. Bi induces considerable grain growth improvement, and an average of approximately 3% absolute efficiency enhancement is achieved for Bi-doped solar cells in comparison with the Bi-free samples. Solar cells doped with a 50 nm Bi layer yield the highest efficiency of 13.04% (without any antireflective coating) using the low-temperature technology. The copper-bismuth-selenium compounds (Cu-Bi-Se, mainly Cu1.6Bi4.8Se8) are crucial in improving the crystallinity of absorbers during the annealing process. These Bi-containing compounds are conclusively observed at the grain boundaries and top and bottom interfaces of CIGS films. The growth promotion is found to be associated with the superior diffusion capacity of Cu-Bi-Se compounds in CIGS films, and these liquid compounds function as carriers to facilitate crystallization. Bi atoms do not enter the CIGS lattices, and the band gaps (Eg) of absorbers remain unchanged. Bi doping reduces the number of CIGS grain boundaries and increases the copper vacancy content in CIGS films, thereby boosting the carrier concentrations. Cu-Bi-Se compounds in grain boundaries significantly enhance the conductivity of grain boundaries and serve as channels for carrier transport. The valence band, Fermi energy level (EF), and conduction band of Bi-doped CIGS films all move downward. This band shift strengthens the band bending of the CdS/CIGS heterojunction and eventually improves the open circuit voltage (Voc) of solar cells. An effective doping method and a novel mechanism can facilitate the low-temperature preparation of CIGS solar cells.

Effect of K Doping on the Performance of Aqueous Solution‐Processed Cu(In,Ga)Se2 Solar Cell

Copper indium gallium selenide Cu(In,Ga)Se2 (CIGS) solar cells fabricated by vacuum thermal evaporation, magnetron sputtering, or organic solvent solution‐process either lead to high manufacturing cost and film nonuniformity or cause explosion, toxication and pollution. To address these issues, herein, water as a solvent to prepare the CIGS precursor solution is used and 7.04% efficient CIGS solar cells without MgF2 antireflection layer is obtained. Furthermore, for the first time, K doping on CIGS films via adding “K+” into the aqueous precursor solution is conducted. K–In–Se species is formed in the process of selenization and acts as a Se reservoir during CIGS grain growth, resulting in improved quality of the CIGS film with much larger grains. K doping can also inhibit the loss of Ga element. The high‐bandgap K–In–Se phase and higher Ga content combine to result in the increase in the CIGS bandgap. As a result, the K‐doped CIGS solar cell achieves a 33.1% enhancement in efficiency to 9.37%, with the significant increase in the open‐circuit voltage from 520.5 to 597.1 mV and fill factor from 50.77 to 55.36%. These results provide an important preliminary foundation for the development of aqueous precursor solution‐based CIGS solar cells.

Photovoltaic characteristics and computational simulation of samarium-ion doped Cu(In, Ga)Se2 thin films prepared via a non-vacuum coating process

The effects of samarium ions on the photovoltaic performance of Cu(In, Ga)Se2 (CIGS) films were investigated through experiments and computational simulation. Compared with pristine CIGS films, the conversion efficiency of the prepared solar cells increased by 26.6% (from 8.62% to 10.91%) when 0.5 mol% samarium ions were doped into CIGS films. The incorporation of 0.5 mol% samarium ions facilitated the formation of the Cu2−xSe phase during the selenization reaction. The Cu2−xSe phase as the flux agent melted at high temperatures and thus promoted the growth of CIGS grains and reduced the number of surface pores on CIGS films. The smooth morphology of 0.5 mol% samarium-ion doped CIGS films improved the coverage of the buffer layers and suppressed the formation of additional shunt paths, thereby decreasing defect density in the CIGS/cadmium sulfide (CdS) interface and the absorber layer. The simulated results revealed the incorporation of 0.5 mol% samarium ions reduced the defect density from 1 × 1011 cm−3 to 1 × 109 cm−3 in the CIGS/CdS interface and from 3 × 1017 cm−3 to 4 × 1016 cm−3 in the absorber layers. The shunt conductance and saturation current of the prepared solar cells also decreased, from 4.38 mS/cm2 to 2.60 mS/cm2 and from 7.25 × 10−3 mA/cm2 to 1.31 × 10−3 mA/cm2, respectively. Excess doping of samarium ions caused a residual Cu2−xSe phase in CIGS films and resulted in the formation of additional shunt paths to deteriorate the performance of CIGS solar cells. The incorporation of appropriate concentrations of samarium ions into CIGS films was an effective approach to improve the photovoltaic properties of CIGS solar cells.

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.

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.

Significantly Enhanced Detectivity of CIGS Broadband High-Speed Photodetectors by Grain Size Control and ALD-Al2O3 Interfacial-Layer Modification.

The Cu(In,Ga)Se2 (CIGS) thin film has been commercialized as solar cells with great success, but its application for photodetectors still faces some practical challenges, including low detectivity and long response time. In this paper, the structure of the Mo/CIGS/CdS/ZnO/ITO heterojunction has been fabricated, and satisfactory performances of high detectivity and fast response time have been achieved by suppressing the dark current and enhancing the carrier mobility. The controllable growth of CIGS grains is accomplished through optimizing the selenization process, demonstrating that bigger grain sizes resulted in higher carrier mobility and better response characteristics. Particularly, the high rise/decay speed of 3.40/6.46 μs is achieved. Furthermore, the interface of the CIGS/CdS heterojunction has been modified by the Al2O3 layer via the atomic-layer deposition (ALD) process. The dark current of the device is effectively suppressed by the ALD-Al2O3 layer, which remarkably drops from ∼10-7 to ∼10-9 A. As a consequence, the detectivity rises from 3.08 × 1011 to 1.84 × 1012 Jones. In addition, the ALD-Al2O3 layer shows a protective effect as well, which is positive for photoelectrical conversion. Besides, the wide linear dynamic range of 102.1 dB and large -3 dB bandwidth of 78 kHz are acquired. This work suggests that the CIGS-based heterojunction has great potential for high-performance thin-film photodetectors.

Thermodynamic limitations for alkali metals in Cu(In,Ga)Se2

Abstract

Investigation on Sb-doped induced Cu(InGa)Se2 films grain growth by sputtering process with Se-free annealing

In this study, the influences of doping with antimony sulfide (Sb2S3) and metallic Sb on the properties of Cu(InGa)Se2 (CIGS) films and devices have been investigated. The incorporation of metallic Sb in CIGS films has little effect on grain growth, while the incorporation of Sb2S3 could significantly increase CIGS grain size and alter the preferential orientation of CIGS films. Photoluminescence spectra reveal that the pre-deposited Sb2S3 is beneficial to reduce or passivate defects in CIGS layer. Furthermore, the effects of annealing temperature on grain size and phase structure of Sb-doped CIGS films have been explored and Sb distribution profiles in CIGS film are elucidated. These results suggest that Sb2S3 could take effects on enhancing grain growth beyond a certain critical temperature when the generated antimony selenide (Sb2Se3), rather than Sb2S3, decomposed into some volatile phases. These volatile phases could facilitate the diffusion of the atoms in grain boundaries when they pass through from the bottom to outside of CIGS films. Finally, the efficiency of solar cells increases from 8.4% for un-doped sample to 11.9% for Sb2S3-doped sample. This study provides a new approach for fabricating CIGS thin film with large grains and a potential way for non-hazardous mass production application.

Si-Doping Effects in Cu(In,Ga)Se2 Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells.

We found that elemental Si-doped Cu(In,Ga)Se2 (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance.

A study on mechanisms of Sb-doping induced grain growth for Cu(InGa)Se2 absorbers deposited from quaternary targets

In order to investigate the effects of Sb-doping induced grain growth of Cu(InGa)Se2 (CIGS) films processed from quaternary target, thin metallic Sb layers have been deposited in the bottom, in the middle and on the top of CIGS layer, respectively. The effects of temperature on Sb-doped CIGS grain size are explored. These results demonstrate that the incorporated Sb in the bottom and middle of CIGS films could significantly increase the grain size of films at selenization temperature of 500 °C, while the incorporated Sb on the top of CIGS takes little effect on grain growth. The phase and morphology variation of metallic Sb and the phase evolution of Sb-doped Mo-CIGS interface with selenization temperature are investigated. These results suggest that the metallic Sb might be selenized into Sb2Se3. As soon as Sb2Se3 generates, it might decompose into volatile phases at 500 °C. In the migration of these volatile phases through CIGS films, the grain growth is enhanced.

Nanoscopic study of the compositions, structures, and electronic properties of grain boundaries in Cu(InGa)Se2 photovoltaic thin films

Polycrystalline Cu(InGa)Se2 (CIGS) thin film is a well-known light absorber material for photovoltaic solar cells. However, the mechanism of the benignity of its grain boundary (GB) remains in dispute. By using a combination of experimental techniques, we have systematically investigated the compositions, structures, and properties of the grain interior (GI) and GB of the polycrystalline CIGS films at nanometer scale. A definitive Cu deficient layer with thickness depending on the overall Cu content of the films was observed near the non Σ3 GBs with distinctly different characteristics from GIs. The band alignment between GB and GI was discovered to be of type II with downward offset for both conduction and valence bands at GB, well correlating to the local copper deficiency and structure. Our findings expressively support the type inversion and large hole barrier in this GB layer, and establish a comprehensive mechanism for the benignity of the CIGS grain boundaries towards carrier recombination.

Band gap widening at random CIGS grain boundary detected by valence electron energy loss spectroscopy

Cu(In,Ga) Se2 (CIGS) thin film solar cells have demonstrated very high efficiencies, but still the role of nanoscale inhomogeneities in CIGS and their impact on the solar cell performance are not yet clearly understood. Due to the polycrystalline structure of CIGS, grain boundaries are very common structural defects that are also accompanied by compositional variations. In this work, we apply valence electron energy loss spectroscopy in scanning transmission electron microscopy to study the local band gap energy at a grain boundary in the CIGS absorber layer. Based on this example, we demonstrate the capabilities of a 2nd generation monochromator that provides a very high energy resolution and allows for directly relating the chemical composition and the band gap energy across the grain boundary. A band gap widening of about 20 meV is observed at the grain boundary. Furthermore, the compositional analysis by core-loss EELS reveals an enrichment of In together with a Cu, Ga and Se depletion at the same area. The experimentally obtained results can therefore be well explained by the presence of a valence band barrier at the grain boundary.

Effects of the incorporation of alkali elements on Cu(In,Ga)Se2 thin film solar cells

This study describes in detail the effects of sodium and potassium on Cu(In,Ga)Se2 (CIGS) absorbers and solar cells. We report on the influence of these species on the surface and bulk composition as well as bulk defect structure of CIGS films as revealed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and photoluminescence (PL). From the XPS studies it is found that Na and K promote oxygen absorption onto the CIGS films. Furthermore, potassium accelerates the formation of indium and gallium oxides on the film surface, making the surface Cu-deficient. Low temperature PL studies suggest that (i) Na and K help passivate non-radiative recombination centers, presumably at the grain boundaries, and (ii) Na further impacts the bulk defect structure inside of CIGS grains, which is not observed with K. This change in bulk defect structure is attributed to the greater diffusivity of Na in CIGS relative to K due to the smaller atomic size. This in-depth study (integration of XPS, SIMS, PL, and device characteristics) reveals that the surface chemistry and the grain boundary passivation have stronger influences on the device performance than the bulk defect structure.

Metal precursor with bi-layer indium for Cu(In,Ga)Se2 thin film preparation

A novel process was developed to prepare a sequentially stacked CuGa/In precursor using DC magnetron sputtering to make absorber for Cu(In,Ga)Se2 (CIGS) solar cell. Compared with precursor made of normally used single-layer In, CuGa/In film with bi-layer In was found to be with smoother surface and better coverage of the CuGa film by In islands. CIGS absorber prepared by selenizing the precursor with bi-layer In film showed a surface with less roughness, which indicated strong influence of the surface texture of the precursor on the morphology of the absorber. Furthermore, the thickness inhomogeneity of the In islands on the CuGa layer in the precursor was found to induce different degree of mixing for Ga and In, resulting in a lateral composition fluctuation among CIGS grains in addition to the normally observed vertical Ga grading in each grain. By using a precursor with bi-layer In film, an improvement of cell efficiency is always obtained resulting from the increased open circuit voltage (Voc) and larger fill factor (FF).

Improved growth of solution-deposited thin films on polycrystalline Cu(In,Ga)Se2

CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se2 (CIGS) solar cells. As recently reported, a non-contiguous coverage of CBD buffers on CIGS grains with {112} surfaces can be detected, which was explained in terms of low surface energies of the {112} facets, leading to deteriorated wetting of the chemical solution on the CIGS surface. In the present contribution, we report on the effect of air annealing of CIGS thin films prior to the CBD of CdS and Zn(O,S) layers. In contrast to the growth on the as-grown CIGS layers, these buffer lay- ers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different growth behavior by increased surface energies of CIGS grains due to the annealing step, i.e., due to oxidation of the CIGS surface. Reference solar cells were processed and completed by i-ZnO/ZnO:Al layers for CdS and by (Zn,Mg)O/ZnO:Al for Zn(O,S) buffers. For solar cells with both, CdS and Zn(O,S) buffers, air-annealed CIGS films with improved buffer coverage resulted in higher power-conversion efficiencies, as compared with the devices containing as-grown CIGS layers. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Assessment of off-axis and in-line electron holography for measurement of potential variations in Cu(In,Ga)Se2 thin-film solar cells

AbstractElectron holography is employed to study variations of the electrostatic crystal potential in Cu(In,Ga)Se2 (CIGS) thin-film solar cells at different length scales: Long-range potential variations across the layer structure of the solar cell as well as inhomogeneities within the layers are analyzed by off-axis holography. In-line holography is applied to examine the local potential variation across a CIGS grain boundary. The phase reconstruction from a focal series is performed by a modified transport of intensity equation (TIE) which is optimized to reduce common artifacts. For comparison, three different microscopes of different optical configurations were used for in-line holography. Based on the results, the impact of the used microscope as well as further acquisition parameters on the in-line holography measurement is assessed. The measured potential variations are discussed considering the effect of different possible sources that may cause potential fluctuations. It is found that most of the variations are best explained by mean inner potential fluctuations rather than by inhomogeneities of the electronic properties. Finally, the present resolution limit of both methods is discussed regarding the feasibility of future electronic characterization of CIGS by holography.

Physical properties of Cu(In,Ga)Se2 film on flexible stainless steel substrate for solar cell application: A multi-layer precursor method

The process named “multi-layer precursor method”, consisting of Ga–Se/In–Se/Cu–Se stacked precursors, is proposed to prepare Cu(In,Ga)Se2 (CIGS) films on flexible stainless steel (SUS) substrates. In this work, precursor deposition temperature (TPRE), varying from about 250 to 430°C, is first scrutinized. From Raman scattering measurement, it is suggested that the increase in the TPRE enhances Cu2−xSe secondary phase during film deposition, thus enlarging CIGS grain size, while decreases ordered vacancy compound (OVC) near surface of the finished film, caused by desorption of In2Se during the precursor deposition. Suitable TPRE is in a range of approximately 370–400°C, resulting in long carrier lifetime. Next, according to the examinations of material compositions of CIGS absorbers, the optimized [Cu]/([In]+[Ga]) contributes to the absorbers with large grain size due to the help of Cu2−xSe phase during film deposition and OVC near their surfaces for pn homo-junction. The appropriate [Ga]/([In]+[Ga]) leads to optimized band-gap energy with high CIGS quality implied by long carrier lifetime. Ultimately, the effective [Cu]/([In]+[Ga]) and [Ga]/([In]+[Ga]) ratios for the flexible CIGS solar cell with high conversion efficiency under the so-called “multi-layer precursor method” are in ranges of about 0.74–0.84 and 0.28–0.35, respectively.

Bismuth‐doped Cu(In,Ga)Se2 absorber prepared by multi‐layer precursor method and its solar cell

AbstractBismuth (Bi)‐doped Cu(In,Ga)Se2 (CIGS) films were prepared by the so‐called “multi‐layer precursor method”, obtained by depositing them onto Bi layers with various thicknesses on Mo‐coated soda‐lime glass (SLG) substrates. Material composition (Cu, In, Ga, and Se) profiles of the CIGS films are almost identical, whereas sodium (Na) is reduced, when Bi thickness is increased. Moreover, the incorporation of Bi into the CIGS film is enhanced with thicker Bi layer. With Bi thickness from 0 to 70 nm, the 2.4‐μm‐thick CIGS absorbers demonstrate the increase in CIGS grain size, carrier lifetime, and carrier concentration, thus improving their cell performances, especially open‐circuit voltage (VOC). With further increase in Bi thickness of above 70 nm, the CIGS films show the deterioration of CIGS film quality owing to the formation of Bi compounds such as Bi, BiSe, and Bi4Se3. Consequently, Bi‐doped CIGS absorber with thickness of 2.4 μm, prepared with the 70‐nm‐thick Bi layer on Mo‐coated SLG substrate, gives rise to the improvement of photovoltaic performances, especially VOC. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)