Performance Of Cu(In,Ga)Se2 Solar Cells Research Articles

Effect of solution‐processed cesium carbonate on Cu(In,Ga)Se2 thin‐film solar cells

AbstractHeavy alkali‐metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low‐cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO3) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin‐coating CsCO3 solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open‐circuit voltage. X‐ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High‐resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low‐cost route for improving the performance of CIGS solar cells by surface modification using a solution process.

Multiple Impacts of the Aluminum Oxide Passivation Layer on the Properties OF Cu(In,Ga)Se2 Solar Cells

AbstractIn this study, the origins of efficiency gains in Cu(In,Ga)Se2 (CIGS) solar cells are investigated by introducing an Al2O3 passivation layer in terms of the oxidation condition of Mo back contact, alkali‐metal diffusion, minority carrier lifetimes (τ), and charge conditions. The study reveals that introduction of an Al2O3 back‐contact passivation layer into solar cells yields multiple impacts. Al2O3 deposition enhances the oxidation of the Mo back contacts, increasing Na solubility in Mo and Na diffusion from Mo into the CIGS layer, thereby modifying the metastable properties of CIGS. The charge condition at the CIGS/Al2O3 interface is not fixed negative charge but variable, dependent on whether electrons or holes are supplied. During solar cell operation, the interfacial charge condition is expected to be neutral or positive for Al2O3 grown using plasma or thermal atomic layer deposition techniques, respectively. Moreover, the mechanical peeling off of CIGS from Mo back contact enhanced τ in a similar way as with the insertion of Al2O3. Based on this study, the enhancement of alkali metal supply and the removal of direct contact of CIGS to the metal contact (Mo) can play crucial roles in improving the performance of CIGS solar cell.

Reforming material chemistry of CIGS solar cells via a precise Ag doping strategy

The addition of small amounts of Ag to Cu(In,Ga)Se2 (CIGS) absorbers improves the device performance of CIGS solar cells by changing the film microstructure, alkali distribution, and electrostatic conditions of CIGS.

Interface passivation and band alignment of high efficiency Cu(In, Ga)Se2 solar cells: Application of Mo(OxSe1-x)2 buffer layer at Mo/ Cu(In, Ga)Se2 interface

Cu(In,Ga)Se2(CIGSe) solar cell is one of the most promising thin film solar cells with high photoelectric conversion efficiency. However, the efficiency is still far below the thermodynamic Shockley−Queisser (SQ) limit. Interfacial recombination is the main factor restricting the performance of CIGSe solar cells. In this work, oxygen-doped Mo(OxSe1-x)2 was employed to study the effect of Mo/CIGSe interfacial recombination. SCAPS simulation and first-principles calculation were used in this work. The band gap of Mo(OxSe1-x)2 was adjusted by changing the content of oxygen, the band alignment of Mo/Mo(OxSe1-x)2 interface and Mo(OxSe1-x)2/CIGSe interface were optimized. The back interface recombination was effectively reduced by using a thickness of 10–70 nm Mo(OxSe1-x)2 with an oxygen doping concentration of 12.5%–50.0% and the performance of CIGSe solar cells was improved. Finally, the conversion efficiency was improved from 15.80% to 24.35% with oxygen doping concentration of 25% and Mo(OxSe1-x)2 layer with thickness of 40 nm.

Influence of the Al-Doped ZnO Sputter-Deposition Temperature on Cu(In,Ga)Se2 Solar Cell Performance.

Heterojunction Cu(In,Ga)Se2 (CIGS) solar cells comprise a substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al. Here, Al-doped zinc oxide (AZO) films were deposited by magnetron sputtering, and the substrate temperature was optimized for CIGS solar cells with two types of CIGS light absorbers with different material properties fabricated by three-stage co-evaporation and two-step metallization followed by sulfurization after selenization (SAS). The microstructure and optoelectronic properties of the AZO thin films fabricated at different substrate temperatures (150–550 °C) were analyzed along with their effects on the CIGS solar cell performance. X-ray diffraction results confirmed that all the deposited AZO films have a hexagonal wurtzite crystal structure regardless of substrate temperature. The optical and electrical properties of the AZO films improved significantly with increasing substrate temperature. Photovoltaic performances of the two types of CIGS solar cells were influenced by changes in the AZO substrate temperature. For the three-stage co-evaporated CIGS cell, as the sputter-deposition temperature of the AZO layer was raised from 150 °C to 550 °C, the efficiencies of CIGS devices decreased monotonically, which suggests the optimum AZO deposition temperature is 150 °C. In contrast, the cell efficiency of CIGS devices fabricated using the two-step SAS-processed CIGS absorbers improved with increasing the AZO deposition temperature from 150 to 350 °C. However, the rise in AZO deposition temperature to 550 °C decreased the cell efficiency, indicating that the optimum AZO deposition temperature was 350 °C. The findings of this study provide insights for the efficient fabrication of CIGS solar cells considering the correlation between CIGS absorber characteristics and AZO layer deposition temperature.

Efficiency improvement of electrodeposition-processed Cu(In,Ga)Se2 solar cell with widen surface bandgap by spin-coating In2S3 thin film

The Cu(In,Ga)Se2 (CIGSe) -based solar cell is a promising candidate in the photovoltaic market due to its excellent performance. It was reported that the surface bandgap of CIGSe is crucial to improve the device properties. However, in the post-selenization process of precursor layers, Ga tends to accumulate at the back contact leading to a narrow surface bandgap. In this work, the surface bandgap of CIGSe film was successfully widened by a spin-coating In2S3 thin film on the CIGSe surface. After annealing treatment of the CIGSe/In2S3 stack, sulfur incorporates into the absorber layer and a Cu-poor surface composition is formed. Thus, a surface inversion of CIGSe from p-type to n-type occurs leading to formation of a buried homojunction. The effect of different In2S3 thicknesses on device performance were investigated. A champion efficiency of 12.88 % for CIGSe solar cell was obtained with 40 nm In2S3 compared to 10.03 % for the reference device without In2S3. However, the performance of CIGSe solar cells were deteriorated with In2S3 film thicknesses over 40 nm. These results demonstrate that a suitable In2S3 film thickness is significant to increase the surface bandgap and promote the efficiency.

Silver-assisted optimization of band gap gradient structure of Cu(In,Ga)Se2 solar cells via SCAPS

Recently, silver (Ag) substitution for copper (Cu) into Cu(In,Ga)Se2 (CIGS) films has attracted widespread attention due to the lower synthesis temperature and the enlarge band gap. In this paper, SCAPS simulation is employed to investigate the influence of Ag doped CIGS absorber layer on the lattice defect and band gap gradient. First, the impact of the [Ga]/([In] + [Ga]) (GGI) and [Ag]/([Ag] + [Cu]) (AAC) on the defect concentrations is calculated systematically. It is demonstrate that the compression (GGI > 0.25) is more likely to cause the tetragonal distortion compared with the tension (GGI < 0.25) on the unit cell. And appropriate substitution of Cu with Ag can reduce the defect concentrations. Further investigations on the variation of the lattice structure indicate the ideal band gap can be enlarged to the range of about 1.20–1.29 eV by silver. With the ideal lattice structure, the efficiency of solar cells has a relative increase of about 48 % compared with that of the reference solar cell. This study offers a new sight for the incorporation of Ag into CIGS films from the perspective of lattice structure, which should be further improve device performance of CIGS solar cells at low temperature deposition process.

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.

Understanding deposition temperature dependent photovoltaic characteristics of Cu(In,Ga)Se2 solar cells: A study with thermally stable alkali aluminosilicate glass substrates

During 3-stage thermal co-evaporation of Cu(In,Ga)Se2 (CIGSe), alkali aluminosilicate (AAS) glass has proven to be stable at process temperatures Tmax of up to 650 °C. This is considerably higher than the maximum endurable Tmax for soda-lime glass (SLG), which is in the range of 530–550 °C. As the Na-content of the AAS glass used here is comparable to that of SLG, growing CIGSe at elevated Tmax is expected to promote Na diffusion from the glass substrate through the Mo back contact (BC) into the absorber layer. An increased Na concentration in the CIGSe absorber would be expected to result in an improved device performance as it is mostly associated with an increased open-circuit voltage (Voc) and fill factor (FF). This paper discusses the influence of varying Tmax from 530 to 650 °C during the 2nd and the 3rd stage of the 3-stage process on the morphological, electrical, and photovoltaic performance of CIGSe solar cells. Upon raising Tmax from 530 to 650 °C, the short-circuit current density (Jsc) decreased due to bandgap (Eg) widening. Glow discharge optical emission spectrometry analysis reveals that the Na concentration in the absorber is gradually decreased when elevating Tmax. As will be seen, we attribute this in part to a densification of the Mo BC during the high temperature process. A maximum conversion efficiency (η) was realized at Tmax = 600 °C. An increased Voc at Tmax = 600 °C is due to the wider Eg and to an increased carrier concentration despite the fact that the Na concentration in the CIGSe thin film was low compared to lower Tmax. Admittance spectroscopy analysis is performed to access information on defect energy level and density in the finished devices. In the light of present findings, ways to further improve η at elevated Tmax are suggested.

Optimal CdS Buffer Thickness to Form High-Quality CdS/Cu(In,Ga)Se2 Junctions in Solar Cells without Plasma Damage and Shunt Paths.

CdS has been known to be one of the best junction partners for Cu(In,Ga)Se2 (CIGS) in CIGS solar cells. However, the use of thick CdS buffer decreases the short-circuit current density of CIGS solar cells. There are two obstacles that limit the use of ultrathin CdS. The first is plasma damage to CIGS during the preparation of transparent conducting windows and the second is a low shunt resistance due to the direct contact between the window and CIGS via pinholes in the thin CdS buffer. In other words, to avoid plasma damage and shunt paths, we may have to use a CdS buffer that is thicker than necessary to form a high-quality CdS/CIGS junction. This work aims to determine how thin the CdS buffer can be employed without sacrificing device performance while also eliminating the above two obstacles. We investigate the effect of CdS thickness on the performance of CIGS solar cells with silver nanowire-based window layers, which can eliminate both obstacles. An approximately 13 nm thick CdS buffer allows us to achieve high short-circuit current density and fill factor values. To attain an even high open-circuit voltage, an additional CdS buffer with a thickness of 13 nm is needed. The data from this study imply that an approximately 26 nm thick CdS buffer is sufficient to form a high-quality CdS/CIGS junction.

Performance enhancement of CIGS thin-film solar cell

In the present study, A thin-film solar cell based on Cu(In,Ga)Se2 (CIGS) is carried out using two-dimensional device simulator called Silvaco–Atlas. A basic CIGS solar cell was simulated under the room temperature of 298 k. It is found that the obtained simulation results agree very well with recent published experimental results, which validate our used model. The aim of this study is to enhance the CIGS solar cell performance by optimizing its parameters. For this purpose, the CIGS cell layer thicknesses and doping densities have been optimized. With this optimization process, the cell efficiency increases from 22.9 to 27.5%. In several research studies, the CIGS solar cells were tested under the room temperature but the realistic operating temperature is varied. In order to optimize the operating temperature and study its effect on the CIGS cell performance, the operating temperature was varied. The results show that as the temperature decreases, the cell performance increases. At the optimum temperature of 240 k, the CIGS cell achieves a very important efficiency of 32.45%.

Recombination Channels in Cu(In,Ga)Se2 Thin Films: Impact of the Ga-Profile

Depth bandgap profiles via a [Ga]/([Ga]+[In]) variation in the Cu(In,Ga)Se2 (CIGS) absorber layer have been implemented as a strategy to enhance the performance of CIGS solar cells. Since the [Ga]/...

Improvement of Cu(In,Ga)Se2 solar cell performance by thiourea treatment

We investigated the effects of a thiourea treatment performed to the absorber of Cu(In,Ga)Se2 (CIGS) solar cells. The thiourea treatment successfully improved the open-circuit voltage, fill factor, and conversion efficiency of the solar cells. Reduced ideality factor and reverse saturation current density demonstrated that the suppression of carrier recombination contributed to the improvement in solar cell performance. Increased intensity in cross-sectional electron-beam-induced current measurements confirmed the improved film quality with the thiourea treatment. Additionally, an enhanced carrier density observed with the treatment suggests the passivation of donor-type defects. These results indicate that the thiourea treatment is promising to improve the absorber quality and enhance the performance of CIGS solar cells.

Simulation of double buffer layer on CIGS solar cell with SCAPS software

In a CIGS thin film solar cell, the buffer layer is interposed between the absorber layer and the window layer, which plays an important role in interface electricity. CdS is often used as a buffer layer in CIGS cells. Beside the toxicity of Cd, the main drawback of CdS is its relatively narrow band gap (2.4 eV), which can lead to the current loss due to parasitic absorption. In order to reduce the use of CdS, we try other buffer layers instead of CdS and optimize buffer layer structure. Numerical simulation has been used to explore the effect of different buffer layer materials (CdS and ZnSe) on the performance of CuInGaSe2 solar cells with SCAPS-1D software. The main photovoltaic parameters of the analog device: open circuit voltage, short circuit current density, fill factor and conversion efficiency (η) were compared and analyzed. The results show that different buffer materials and thickness have great influence on the performance of the cell. In this simulation, CdS buffer layer was replaced by ZnSe material, which has a wider band gap. However, the efficiency of this kind of cell is lower than that of CIGS device with CdS buffer layer because of the bad energy band alignment. In order to improve the short-wave absorption and the efficiency of CIGS cell, we simulated a novel CdS/ZnSe double buffer layer structure. The efficiency of the optimized cell with double buffer layer increases from 18.47% (conventional cell with the same CdS buffer layer thickness) to 19.01%.

Improvement of the CIGS solar cell performance: structure based on a ZnS buffer layer

Cu(In,Ga)Se2 (CIGS) based thin film solar cells are the most efficient thin-film solar cells today. The non-toxic and wide band-gap zinc sulphide (ZnS) is a promising material to replace the cadmium sulfide (CdS) as the buffer layer in CIGS based solar cells. In this work we present a simulation study of a CIGS based solar cell with a buffer layer of ZnS, using the simulator Silvaco-Atlas. Our primary simulation shows a 22.6% efficiency of the CIGS solar cell with the CdS buffer layer which is comparable to reported and highest experimental results. However, the simulated efficiency of the CIGS solar cell with the ZnS buffer layer as high as 23.54% was achieved. The effects of layer parameters like the thickness, the acceptor and donor densities of the CIGS absorber and ZnS buffer layers and the CBO on the photovoltaic parameters of the ZnS/CIGS solar cell are optimized in order to improve the performance of the ZnS/CIGS solar cell. The highest efficiency of 27.33% is achieved when the ZnS buffer and the CIGS absorber layers have thicknesses of 0.025 µm and 4 µm with acceptor and donor densities of 6 × 1017 cm−3 and 1018 cm−3, respectively and a CBO in the range − 0.05 to 0.05 eV. The present results of simulation can help the development of the solar cells with higher conversion efficiency and low cost.

Growth improved of CIGS thin films by applying mechanical perturbations to the working electrode during the electrodeposition process

Cu(In,Ga)Se2 (CIGS) is a semiconductor which has been extensively studied for photovoltaic applications. With this semiconductor, efficiencies of solar cell greater than 20% have been achieved by physical vapor deposition. There are different techniques to make the CIGS absorber layer. However, electrodeposition is the most adequate technique to make it for different technical and economic reasons. In this paper, the performance of CIGS solar cells are compared using different strategies to synthesize the CIGS films on Mo-coated glass substrates by one-step electrodeposition employing an electrolytic cell with horizontal electrodes. For the above, two strategies are presented, these are: (i) DC electrodeposition and (ii) DC electrodeposition plus mechanical perturbations to the working electrode during the CIGS film growth. In the as-electrodeposited CIGS film a grain growth in selenium atmosphere process was applied and photovoltaic devices were completed with CdS, ZnO and ZnO:Al layers. The photovoltaic devices that were made with the CIGS films obtained by DC electrodeposition plus mechanical perturbations exhibit better absorber morphology with respect to that where only DC electrodeposition was used. The short circuit current density obtained with the absorber synthetized only by DC electrodeposition was of 7.35 mA/cm2 while for the absorber synthetized by DC electrodeposition plus mechanical perturbations was of 28.38 mA/cm2, both under 1000 W/m2 (AM 1.5).

铜铟镓硒薄膜太阳能电池新型图形化透明前电极研究

铜铟镓硒薄膜太阳能电池新型图形化透明前电极研究

Effects of Ammonia‐Induced Surface Modification of Cu(In,Ga)Se2 on High‐Efficiency Zn(O,S)‐Based Cu(In,Ga)Se2 Solar Cells

Post‐treatment of Cu(In,Ga)Se2 (CIGS) surfaces, as an efficient way to improve the performance of CIGS solar cells, has received increasing attention in recent years. To alleviate the limitations of the as‐grown CIGS thin films, such as oxidation, Na accumulation, and microstructure of CIGS surface, ammonia (NH3) etching solution was introduced to post‐treat the CIGS surface to improve its quality in this study. To investigate the mechanisms of NH3 etching effects on the CIGS surfaces and the resulted devices, a series of NH3 solution treatments, of different concentrations, on CIGS films is carried out and the resulted surfaces and devices are carefully examined. It is demonstrated that proper concentration of NH3 solution could not only result in a new microstructure between CIGS and Zn(O,S) thin films but also improve the performance of the Zn(O,S) based CIGS solar cells, especially in terms of the device fill factor. As a result, a Zn(O, S) based CIGS solar cell with a conversion efficiency of over 20% is obtained with application of NH3 treatment together with MgF2 anti‐reflective coating. Furthermore, it is found that NH3 treatment could effectively reduce the undesired light soaking effect, which often appears in the Zn(O,S) based CIGS solar cells.

Improvement in the performance of CIGS solar cells by introducing GaN nanowires on the absorber layer

Cu(In, Ga) Se2 (CIGS) thin-film solar cells are currently the fastest growing photovoltaic technology due to the higher performance ratio and lower energy payback time compared to the silicon, and good stability. Despite of these interesting properties, CIGS solar cells suffer from poor spectral response at short wavelengths particularly in the range of 300–400 nm. In the present work, we have introduced GaN nanowires in CIGS solar cells in order to improve the performance of photovoltaic device in the short wavelength region (300–400 nm). To evaluate the influence of GaN nanowires, the morphological, structural and electrical properties have been studied. These studies have shown the introduction of GaN nanowires not only improve the cell efficiency but also enhance the spectral response in the wavelength range 300–400 nm. The CIGS solar cells fabricated with GaN nanowires displayed power conversion efficiency of 10.15% with a fill factor of 65%, which are substantially higher than that of reference cell (without GaN nanowires).

Effect of Selenization Processes on CIGS Solar Cell Performance.

Cu(In, Ga)Se2 (CIGS) films were fabricated by a two-step process method using sputtering from Cu0.7Ga0.3 and In targets. The metallic precursor structures of In/CuGa/In were prepared, and CuGa film was adjusted to the thicknesses of 150, 200, 250 and 300 nm, in order to optimize the CIGS film. After selenization, three independent CIGS (112), CIGS (220/204) and CIGS (312/116) began to crystallize at ~280 °C and phase peaks continued growing until 560 °C. Experimental results showed that with a single stage selenization method, the excessive stoichiometry of the CIGS films was obtained. Using three sequential stages for the selenization process, with a annealing time of 20 min, the stoichiometry of the CIGS absorbers with the Cu/(In + Ga) and Ga/(In + Ga) showed atomic ratios of 0.94 and 0.34, respectively. The intensity of the (112) XRD diffraction peak became stronger, indicating an improvement in the crystallinity. Raman spectra of CIGS absorbers showed a main peak (174 cm-1) and two weak signals (212 and 231 cm-1). TEM image for electron diffraction pattern showed that the grains were randomly oriented. CIGS solar cell device prepared with a proper selenization, a maximum efficiency of 12.45% was obtained.