Cu(In,Ga)Se2 Solar Cells Research Articles

Influence of alkali element post-deposition treatment on the performance of the CIGS solar cells on flexible stainless steel substrates

In this study, we studied the effect of different alkali element post-deposition treatment (PDT) on the properties of CuInGaSe2 (CIGS) solar cells deposited on flexible stainless steel (SS) substrate. It is found that a certain amount of Na or K is diffused into the surface layer and inside of the CIGS absorption layer by post-deposition treatment process, which can increase the carrier concentration of the absorption layer and effectively improve the material properties of the absorption layer material, thereby improving the open circuit voltage (VOC) and fill factor (FF) of the device. Electron beam-induced current (EBIC) and Capacitance–voltage (C–V)) measurements indicated that the length of the current collection of CIGS absorption layer treated with NaF PDT and NaF + KF PDT is much larger and the total number of net carrier concentration increases significantly. This phenomenon could be attributed to the increase in carrier lifetime after the PDT treatment.

Numerical study of a high‑performance thin film CIGS solar cell with a-Si and MoTe2 hole transport layer

Ultrathin Cu(InGa)Se2 (CIGS) material has become one of the emerging material in the research of solar cell fields due to its excellent optical, electrical and tunable bandgap properties. In this study, we have described a numerical simulation of ultrathin CIGS solar cell using SCAPS-1D program tool based on WS2 as a buffer layer, CIGS as an absorber layer with a-Si/MoTe2 as a hole transport layer (HTL). The proposed study also reveals that a-Si is better choice for HTL as compared to MoTe2 for CIGS solar cell. The cell parameters such as thickness, defect density and acceptor concentration of CIGS absorber layer have been optimized in proposed work. Based on optimization, the maximum power conversion efficiency (PCE) of 30.17% (Voc =1.093Volts,Jsc=32.41 mA/cm2 and FF=85.12%) has been reached for CIGS solar cell with a-Si HTL. The simulation further analyzed at various operating temperature and series resistance for better understanding of the solar device. All these simulation provide noteworthy manifestation for the fabrication of high efficient ultrathin energy harvesting application.

Comparative Study of Al2O3 and HfO2 for Surface Passivation of Cu(In,Ga)Se2 Thin Films: An Innovative Al2O3/HfO2 Multistack Design

In Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, interface recombination is one of the most important limiting factors with respect to device performance. Herein, metal–insulator–semiconductor samples are used to investigate and compare the passivation effects of Al2O3 and HfO2 at the interface with CIGS. Capacitance–voltage–frequency measurements allow to qualitatively and quantitatively assess the existence of high negative charge density (Q f ≈ −1012 cm−2) and low interface‐trap density (D it ≈ 1011 cm−2 eV−1). At the rear interface of CIGS solar cells, these, respectively, induce field‐effect and chemical passivation. A trade‐off is highlighted between stronger field‐effect for HfO2 and lower interface‐trap density for Al2O3. This motivates the usage of Al2O3 to induce chemical passivation at the front interface of CIGS solar cells but raises the issue of its processing compatibility with the buffer layer. Therefore, an innovative Al2O3/HfO2 multistack design is proposed and investigated for the first time. Effective chemical passivation is similarly demonstrated for this novel design, suggesting potential decrease in recombination rate at the front interface in CIGS solar cells and increased efficiency. 300 °C annealing in N2 environment enable to enhance passivation effectiveness by reducing D it while surface cleaning may reveal useful for alternative CIGS processing methods.

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.

Study of defect properties and recombination mechanism in rubidium treated Cu(In, Ga)Se2 solar cells

Heavy alkali-metal treatment of Cu(In,Ga)Se2 (CIGSe) absorbers has been emerging as a key process for achieving over 23% high conversion efficiencies in CIGSe solar cells. Here, we investigate the effect of rubidium fluoride post-deposition treatment (RbF-PDT) on the electronic and carrier recombination properties of narrow bandgap (narrowgap) gap and wide bandgap (widegap) CIGSe solar cells using thermal admittance spectroscopy (TAS), transient photocapacitance spectroscopy (TPC), as well as time-resolved photoluminescence (TRPL). We find that the activation energy of the main capacitance step in TAS spectra of narrowgap and widegap CIGSe solar cells reduces after RbF-PDT. On the other hand, capacitance–voltage (C–V) and temperature-dependent current–voltage (IVT) measurements demonstrate that the built-in potential, as well as the activation energy Ea, increases upon RbF-PDT both for narrowgap and widegap samples, pointing to reduced interface recombination. TPC revealed an appreciable reduction of the optical response of bulk defects in the narrowgap and widegap CIGSe, suggesting improvement of bulk properties after RbF treatment. TRPL confirmed that RbF-PDT significantly reduces carrier recombination in the bulk of narrowgap and widegap CIGSe absorbers and at the surface, leading to extended carrier lifetimes. Analysis of open-circuit voltage (VOC) losses due to nonradiative recombination in the bulk of the CIGSe showed a strong correlation between enhanced carrier lifetime and improved VOC for narrow gap CIGSe cells. In contrast, although we observed a substantial decrease of VOC losses in widegap CIGSe bulk, the analysis indicated that the key to photovoltaic performance enhancement is improved interface quality.

Effect of the Properties of Chalcopyrite Semiconductors on the Physical and Optical Parameters of Cell Layers with CIGS

In this paper, the impact of various buffers of applying components on the effectiveness of CuInGaSe2 solar cells is studied numerically. The SCAPS software is employed to achieve the investigation. The main parameters of the inspected devices are: the photovoltaic conversion effectiveness (η), the filling factor (FF), short-circuit current (Jsc), and open circuit voltage (Voc). These photovoltaic parameters are analyzed vs. the thickness in the various buffer layers under study. The numerical findings revealed that the most significant conversion effectiveness (23.4%) of the CIGS solar cell is obtained with the CdS buffer layer. An attempt is conducted to improve this efficiency by using the SCAPS and by optimizing the two electrical and technological parameters of the three layers (ZnO, CdS, CIGS).

Interface Engineering of Cu(In,Ga)Se2 Solar Cells by Optimizing Cd- and Zn-Chalcogenide Alloys as the Buffer Layer.

The optimization of band alignment at the buffer/absorber interface is realized by tuning compositions of Cd and Zn chalcogenides as the buffer layer toward high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Using the special quasi-random structure (SQS) approach, we construct randomly disordered ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys as alternatives to the traditional CdS buffer layer. The compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of ZnxCd1-xSySe1-y and ZnSxO1-x alloys is investigated by first-principles density functional theory calculations. For quaternary ZnxCd1-xSySe1-y alloys, we find that the miscibility temperatures and the bandgap bowing coefficients are proportional to the lattice mismatch between the mixing elements. The linear dependence of lattice parameters, trinomial dependence of band-gap energies and band-edge positions on the alloy-composition of ZnxCd1-xSySe1-y alloys are established. For ZnSxO1-x alloys, we find the lattice parameters also exhibit a linear dependence on its composition. Because of the large lattice mismatch and the chemical disparity between ZnO and ZnS, the bowing coefficient for the bandgap energies of ZnSxO1-x alloys is composition dependent, and is larger for dilute ZnSxO1-x alloys. With the optimization criteria of moderate spike-like conduction band offset, large valance band offset, sufficiently wide bandgap, and lattice match with respect to the CIGS absorber, we illustrate the optimal composition range of both ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys are promising buffer layers for high-efficiency CIGS solar cells.

Influence of Ga back grading on voltage loss in low‐temperature co‐evaporated Cu(In,Ga)Se2 thin film solar cells

AbstractThe performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time‐resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high‐performance CIGS solar cells and devices.

Effect of the Incoherent Encapsulation Layer and Oblique Sunlight Incidence on the Optical and Current-Voltage Characteristics of Surface-Textured Cu(In,Ga)Se2 Solar Cells Based on the Angle-Dependent Equispaced Thickness Averaging Method

In general, the optical and electrical characteristics of Cu(In,Ga)Se2 (CIGS) solar cells have been studied under the condition that sunlight is normally incident from the air to the CIGS solar cell having no thick front encapsulation layers. To obtain the calculation results in a realistic module application, we calculate the optical and current–voltage (J–V) characteristics of surface-textured CIGS solar cells by simultaneously considering the thick front encapsulation layers and oblique sunlight incidence. Using the proposed angle-dependent equispaced thickness averaging method (ADETAM), we incoherently model two successive front encapsulation layers of a cover glass layer and an ethylene vinyl acetate (EVA) layer, whose respective thicknesses are greater than the coherence length of sunlight (~0.6 μm). The angular dependences of reflectance spectrum and J–V curves are calculated and compared in a surface-textured CIGS solar cell with and without the inclusion of the two front encapsulation layers. We show that the optical absorption improvement of the surface-textured CIGS solar cell over the planar CIGS solar cell can be over-predicted when the thick front encapsulation layers are not considered in the optical modeling.

Plasma-Enhanced Atomic Layer Deposition of TiN Thin Films as an Effective Se Diffusion Barrier for CIGS Solar Cells.

Plasma-enhanced atomic layer deposition (PEALD) of TiN thin films were investigated as an effective Se diffusion barrier layer for Cu (In, Ga) Se2 (CIGS) solar cells. Before the deposition of TiN thin film on CIGS solar cells, a saturated growth rate of 0.67 Å/cycle was confirmed using tetrakis(dimethylamido)titanium (TDMAT) and N2 plasma at 200 °C. Then, a Mo (≈30 nm)/PEALD-TiN (≈5 nm)/Mo (≈600 nm) back contact stack was fabricated to investigate the effects of PEALD-TiN thin films on the Se diffusion. After the selenization process, it was revealed that ≈5 nm-thick TiN thin films can effectively block Se diffusion and that only the top Mo layer prepared on the TiN thin films reacted with Se to form a MoSe2 layer. Without the TiN diffusion barrier layer, however, Se continuously diffused along the grain boundaries of the entire Mo back contact electrode. Finally, the adoption of a TiN diffusion barrier layer improved the photovoltaic efficiency of the CIGS solar cell by approximately 10%.

High‐Performance and Industrially Viable Nanostructured SiOx Layers for Interface Passivation in Thin Film Solar Cells

Herein, it is demonstrated, by using industrial techniques, that a passivation layer with nanocontacts based on silicon oxide (SiOx) leads to significant improvements in the optoelectronical performance of ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. Two approaches are applied for contact patterning of the passivation layer: point contacts and line contacts. For two CIGS growth conditions, 550 and 500 °C, the SiOx passivation layer demonstrates positive passivation properties, which are supported by electrical simulations. Such positive effects lead to an increase in the light to power conversion efficiency value of 2.6% (absolute value) for passivated devices compared with a nonpassivated reference device. Strikingly, both passivation architectures present similar efficiency values. However, there is a trade‐off between passivation effect and charge extraction, as demonstrated by the trade‐off between open‐circuit voltage (Voc) and short‐circuit current density (Jsc) compared with fill factor (FF). For the first time, a fully industrial upscalable process combining SiOx as rear passivation layer deposited by chemical vapor deposition, with photolithography for line contacts, yields promising results toward high‐performance and low‐cost ultrathin CIGS solar cells with champion devices reaching efficiency values of 12%, demonstrating the potential of SiOx as a passivation material for energy conversion devices.

Diffusion transport over grain-boundary barriers as the origin of N1 deep level transient spectroscopy signal in [formula omitted] solar cells

In this work, we explore the hypothesis that the N1 peak in Cu(In,Ga)Se2 (CIGS) solar cells is caused by secondary barriers. We indicate, through numerical modelling of the corresponding Deep Level Transient Spectroscopy (DLTS) signal, that these barriers may be located at grain boundaries. Moreover, the proper reconstruction of the N1 signal using the double-barrier model requires an assumption that corresponding capacitance transients are due to the diffusion limited transport of holes over grain boundaries. Only then we were able to achieve sufficiently small values of the saturation current for realistic values of material parameters. We show that the position of the N1 DLTS peak and its activation energy depends on the effective density and the effective mobility of holes which can be calculated based on grain size and the properties of grain boundaries. This interpretation opens a room for the explanation of the unusual properties of the N1 signal in CIGS solar cells.

Semitransparent and bifacial ultrathin Cu(In,Ga)Se2 solar cells via a single-stage process and light-management strategy

Semitransparent and bifacial photovoltaic (PV) technology has received considerable attention recently owing to its potential for energy-harvesting window applications. Ultrathin Cu(In,Ga)Se2 (CIGS) solar cells with transparent conducting front and rear electrodes, which enable partial visible light transmission, are considered promising candidates for this purpose. However, to realize improved performance, a variety of challenges, including fabrication processes, material selection, and device structure design, remain to be addressed. Here, the material properties and performance characteristics of semitransparent ultrathin CIGS solar cells prepared on two types of transparent conducting oxide (TCO) rear contacts, namely, textured SnO2:F and flat In2O3:Sn films, using a single-stage co-evaporation method, are reported. Comparative performance evaluations of devices with the different TCO rear contacts reveal that the single-stage process is a very effective method for fabricating high-performance semitransparent CIGS solar devices, and that light management plays an important role in devices with ultrathin CIGS absorbers. A textured polydimethylsiloxane (PDMS) layer with light-scattering and antireflection properties enhances the light absorption properties of the devices. The device (absorber thickness: 322 nm) prepared on the flat In2O3:Sn rear contact with the textured PDMS layer exhibits superior performance as compared to other state-of-the-art solar devices, with a conversion efficiency of 10.5% and an average visible transmittance of 12.3%.

Cross-sectional Kelvin probe force microscopy on Cu(In,Ga)Se2 solar cells: Influence of RbF and KF post-deposition treatment on the surface potential of the absorber layer

Post-deposition treatment (PDT) of the absorber layer with alkali fluorides has led to a significant increase in the efficiency of Cu(In,Ga)Se2 (CIGS) thin-film solar cells. In this contribution, we investigate the influence of alkali PDTs on the absorber's surface potential by means of Kelvin probe force microscopy (KPFM). To this end, we perform KPFM on cross sections of complete CIGS solar cells. To improve the reliability of the measurement procedure, we deposit a gold layer on top of the solar cell as a reference layer. Using this approach, we study the influence of RbF and KF PDT on the absorber's surface potential for CIGS solar cells with different absorber and buffer compositions. In all cases, an increased surface potential of the cross section of the absorber layer is measured for the cells with PDT.

Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells

AbstractSolution processing of Cu(In,Ga)Se2 (CIGS) absorber makes it cost‐competitive in the photovoltaic market. It is reported that copper‐poor ordered vacancy compound (OVC) is crucial for high performance CIGS solar cells. However, in solution process method, controllable formation of OVC is unavailable and limited research has been carried out. In this work, the controllable formation of the OVC phase on the CIGS surface is successful by controlling the selenization temperature and intentional variation of Cu/(In+Ga) stoichiometry in precursors for top layers and bulk layers deposition. The effects of OVC contents on the device performance are investigated. The CIGS thin film with OVC phase exhibits a lower valence band position. Meanwhile, the CIGS devices with optimized OVC content show decreased interface defects density and better carrier collection ability. The above advantages translate into a champion PCE of 16.39% for CIGS device with OVC phase, which is the champion performance among non‐hydrazine solution‐processed CIGS solar cells. The results demonstrate that the controllable formation of OVC phase approach should make a significant contribution to the efficiency promoting of solution processed CIGS solar cells.

Failure and Recovery Modes of Submicron Cu(In,Ga)Se2 Solar Cells with High Cu Content.

As an emerging route to further lower the production cost, reducing the thickness of Cu(In,Ga)Se2 (CIGS) absorber has drawn substantial attention and has been intensively studied in recent years. However, thickness-induced change still limits the device performance of thin CIGS solar cells. Herein, by examining a series of submicron CIGS solar cells with varied Cu content through their photovoltaic (PV) performance, the optimal Cu content in these submicron CIGS devices is found to be lower than that of the normal thickness CIGS devices. Electrical and compositional characterizations reveal that reduced thickness makes absorber vulnerable to the shunt paths formed with high Cu content. By intentionally lowering the Cu content in submicron CIGS, shunt resistance (Rsh) of the devices is significantly improved and therefore a high fill factor (FF) is achieved. Moreover, RbF postdeposition treatment (PDT) can passivate the shunt paths in the high Cu content samples to a very good extent, manifesting by the significantly improved FF.

Back-contact barrier analysis to develop flexible and bifacial Cu(In,Ga)Se2 solar cells using transparent conductive In2O3: SnO2 thin films

The presence of a back-contact (BC) barrier disturbs the current density-voltage (J-V) characteristics of solar cells by limiting the majority carrier transport, which is commonly referred as “rollover effect”. In this study, the majority barrier height as high as 131 meV at the Cu(In,Ga)Se2 (CIGS)/Mo-BC interface was calculated using the temperature dependent J-V characteristics. Subsequently, the lift-off process was applied to peel off the substrate-type solar cells from Mo-BC to investigate the origin of the rollover effect. The MoSe2 layer was detected on lift-off Mo samples using energy dispersive X-ray spectroscopy and the grazing-incidence X-ray diffraction measurements. Finally, optically transparent In2O3:SnO2 (ITO) thin films were deposited on the lift-off CIGS samples to fabricate flexible and bifacial solar cells. The solar cell with ITO-BC showed improved fill factor and efficiency without rollover effect, owing to highly Ohmic-contact at CIGS/ITO interface. In this contribution, it has been demonstrated that MoSe2 layer, i.e. with unreferred crystallographic orientation, can be removed by lift-off process to develop bifacial CIGS solar cells on flexible substrates. The bifacial cell with ITO-BC showed efficiencies of 10.1% and 2.8% for frontside and backside illuminations, respectively.

Sn doped ZnO thin films as high resistivity window layer for Cu(In,Ga)Se2 solar cells

For high efficiency Cu(In,Ga)Se2 (CIGS) solar cell, the high-resistivity layer with high optical transmittance has to be adopted between the buffer layer and the high-conductivity window layer. In this paper, we propose Sn doped ZnO (ZTO) film, instead of the traditional intrinsic ZnO (i-ZnO) film, as an alternative n-type high-resistivity window layer for CIGS solar cell. In this experiment, both ZTO and i-ZnO films are strong (002) oriented, and the surface morphologies of the two films are almost the same. The statistical roughnesses of i-ZnO film and ZTO film are 0.58 nm and 0.63 nm, respectively. However, the optical transmittance of ZTO film is higher than that of i-ZnO film with the same thickness. The efficiency of ZTO based CIGS cell was 14.24%, which is almost the same as the efficiency of i-ZnO based CIGS cell. These results fully suggest that it is very feasible to replace i-ZnO with ZTO as the high resistant window layer.

Superstrate-type flexible and bifacial Cu(In,Ga)Se2 thin-film solar cells with In2O3:SnO2 back contact

Bifacial and flexible Cu(In,Ga)Se2 (CIGS) thin-film solar cells with superstrate-type structure were fabricated. For this fabrication, the substrate-type solar cells were first transferred from Mo/Glass substrates to 50-µm-thick flexible fluorinated ethylene propylene (FEP) copolymer films as superstrates using a lift-off process. The 300-nm-thick In2O3:SnO2 (ITO) thin-films were subsequently deposited on the rear sides of the CIGS absorber layers as back contacts (BC) of the finished bifacial devices with the structure of FEP/Epoxy/Al/Ni/Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CdS/CIGS/ITO/Ni/Al. Particular attention was paid to maintain the high optical and electrical transport properties of ITO-BC as well as to improve the Ohmic-like contact behavior at the CIGS/ITO interface to improve cell performances. Under the optimized ITO preparation, the resulting ITO thin film possesses the average optical transparency of ITO (>77%) in the wavelength range of 400–1100 nm, and the resistivity of 6.1 × 10−4 Ωcm with corresponding high carrier mobility of 42.8 cm2/Vs, as well as Ohmic-like characteristics at the CIGS/ITO interface can be realized with shunt resistance as low as 5.5 Ωcm2. Consequently, the flexible and bifacial CIGS thin-film solar cell was fabricated with efficiencies of 9.1% and 2.4% for frontside and backside illuminations, respectively. In this study, highly transparent and conductive ITO thin films have been successfully used as BC in flexible and bifacial CIGS solar cells.

Real-Time Optimization of Anti-Reflective Coatings for CIGS Solar Cells.

A new method combining in-situ real-time spectroscopic ellipsometry and optical modeling to optimize the thickness of an anti-reflective (AR) coating for Cu(In,Ga)Se2 (CIGS) solar cells is described and applied directly to fabricate devices. The model is based on transfer matrix theory with input from the accurate measurement of complex dielectric function spectra and thickness of each layer in the solar cell by spectroscopic ellipsometry. The AR coating thickness is optimized in real time to optically enhance device performance with varying thickness and properties of the constituent layers. Among the parameters studied, we notably demonstrate how changes in thickness of the CIGS absorber layer, buffer layers, and transparent contact layer of higher performance solar cells affect the optimized AR coating thickness. An increase in the device performance of up to 6% with the optimized AR layer is demonstrated, emphasizing the importance of designing the AR coating based on the properties of the device structure.