CIGS Thickness Research Articles

Efficiency enhancement of Cd-free buffer layers on CIGS solar cell performance using WxAMPS

In this paper, the impact of three buffers structures layers on the CIGS solar cell performance was numerically investigated using the one-dimensional simulation program wxAMPS. For Cd-free buffer layers, ZnS,µc_3C_SiC and Mg0.19Zn0.81O have been proposed as alternative buffers layers materials to replace the currently toxic CdS material used for the CIGS solar cells. A scrupulous optimization strategy based on the performance analysis of the thickness and doping concentration density effects was applied to each layer of the proposed stack solar cell structure ZnO/ Buffer layer/ CIGS. Moreover, the impact of the series resistance (Rs) was also examined and taken into account in the optimization process. As known, the combination of the parameter optimization (doping concentration density, thickness) with the lowest series resistance provides a minimum loss resistivity and high optical throughput. Consequently, the maximum achieved conversion efficiencies values for the three investigated solar cell structures (ZnS, µc_3C_SiC and Mg0.19Zn0.81O) were 31.16%,30.42% and 27.52%, respectively. The optimum value of 2µm was selected for the CIGS thickness while for the CIGS doping concentration density, the optimum values chosen were: 5×1018cm−3,3×1019cm−3 and 1×1018cm−3, respectively. These efficiencies parameters were obtained with a thickness absorber of 2μm, a buffer thickness layer of 40nm, and a ZnO window layer of 20nm.

Analysis and improvement of CIGS solar cell efficiency using multiple absorber substances simultaneously

In this paper, the efficiency of a CIGS solar cell was increased in several stages. The common structure and configuration of the CIGS solar cell are the ZnO:Al/ZnO/CdS/CIGS/MO combination whose efficiency is optimized approximately 20% for CIGS with thickness of 2 µm and 17% for CIGS with thickness of 1 µm. In this article, thickness of CIGS is 1 µm and the efficiency of this type of solar cell was calculated using Atlas software of Silvaco. In the first step, by applying Zn1−xMgxO material with x = 0.17 instead of ZnO material, the cell efficiency was 20.7% and then by adding GaAs as the electron reflector layer, we were able to achieve 27.1% efficiency and at the end, to increase the efficiency, one absorber layer is added under the CIGS absorber. This absorber layer is CIS (CIS is CuInSe2) that made the efficiency to become 27.9%. Indeed, CIGS absorber layer is not able to absorb all photons of the sun. So, this added absorber layer is able to absorb a part of low-energy photons, which lead to increasing the efficiency of the solar cell. It should be noted that in the whole process of this article, CIGS and CIS absorber layers are p-type.

Enhancement of the efficiency of ultra-thin CIGS/Si structure for solar cell applications

Abstract This paper describes a numerical study of ultrathin CIGS solar cell using the one-dimensional simulation program. The various properties of the absorber layer such as the band gap energy, the absorption coefficient, and the reflection coefficient are investigated. In addition, the impact of adding silicon to reduce the thickness of CIGS is also examined. We have carried out a theoretical study to show the influence of the thickness and the gallium concentration of the CIGS absorber layer on the performance of the Mo/Si/CIGS/ZnS/ZnO structure. It has been demonstrated that increasing xGa and dCIGS affect the conversion efficiency, FF, Voc, and Jsc. Finally, we have achieved a conversion efficiency η = 21.08% with an optimal value of gallium content equal to 20%when the thickness of the absorber layer has been reduced to 0.75 μm. This study allowed us to improve the performance of thin film solar cell.

Junction configurations and their impacts on Cu(In,Ga)Se2 based solar cells performances

Schematic representation and band diagram of two cells configuration at the equilibrium: (a) with SDL layer (conventional structure), (b) with P + layer (new structure). • We made a compared study of CIGS solar cell structure, containing SDL layer, with a new reference cell containing P+ layer. • The optimal thickness of the P+ to optimize the solar cells is 10 nm. • The P+ defect density is the critical parameter for the limitation of the performance for the CIGS solar cells. • A wide band-gap BSF layer at the CIGS/Mo interface combined with a p+ layer provides a high-performance and stable CIGS solar cells. One dimension solar cells simulator package (SCAPS) is used to study the possibility of carrying out thin CIGS solar cells with high and stable efficiency. In the first step, we modified the conventional ZnO:B/i-ZnO/CdS/SDL/CIGS/Mo structure by substituting the SDL layer with the P + layer, having a wide bandgap from 1 to l.12 eV. Then, we simulated the J-V characteristics of this new structure and showed how the electrical parameters are affected. Conversion efficiency of 18.46% is founded by using 1.1 μm of P + layer thickness. Secondly, we analyze the effect of increase thickness and doping density of CIGS, CdS and P + layers on the electric parameters of this new structure. We show that only the short-circuit current density (J SC ) and efficiency are improved, reaching respectively 34.68 mA/cm 2 and 18.85%, with increasing of the acceptors density. Finally, we introduced 10 nm of various electron reflectors at the CIGS/Mo interface in the new structure to reduce the recombination of minority carriers at the back contact. High conversion efficiency of 23.34% and better stability are obtained when wide band-gap BSF is used.

A numerical study of high efficiency ultra-thin CdS/CIGS solar cells

This work aims to improve the performance of solar cells with structure SnO2/CdS/CIGS/Mo by using the solar cell capacitance simulator (SCAPS-1D). Our idea is to insert the SnS layer between the absorber layer CIGS and the Mo (molybdenum) back contact. A maximum conversion efficiency of 24.45% (VOC = 0.78 V, JSC = 38.66 mA/cm2, FF = 0.80) was achieved with 1 μm-CIGS absorber layer, 50 nm-CdS buffer layer, 200 nm of ZnO window layer and 300 nm of SnS BSF layer. This study shows that ultra-thin CIGS solar cells with a BSF layer give better results compared to the work of Gloeckler (2005), which presented an efficiency of 19.62% (VOC = 0.61 V, JSC = 39.23 mA/cm2, FF = 0.81) with 3 µm thickness of CIGS in the structure ZnO/CdS/CIGS/Mo. In addition to this, the cells’ stability with temperature was studied and analysed.

Experimental Evidence of Multiple Diffusion Mechanisms in Thin-Film Cu(In,Ga)Se2

Lattice and grain boundary diffusions of cadmium in copper indium gallium diselenide (Cu(In,Ga)Se2 or CIGS) thin films were investigated by annealing cadmium into samples of 700-nm CIGS thickness at temperatures between 250 and 300 °C. Diffusion profiles of cadmium were analyzed by dual-beam time-of-flight secondary ion mass spectroscopy (TOF-SIMS). In addition to fast cadmium grain boundary diffusion, experiments revealed cadmium diffusion profiles with two distinct lattice diffusion stages, which could be indicative of simultaneous vacancy and dissociative diffusion mechanisms.

Technological and economical aspects on the influence of reduced Cu(In,Ga)Se 2 thickness and Ga grading for co-evaporated Cu(In,Ga)Se 2 modules

Reducing the Cu(In,Ga)Se 2 (CIGS) thickness is one way of improving the throughput and capacity in existing production, provided that the efficiency can be kept at a high level. Our experimental results from an in-line co-evaporation process show that it is possible to produce CIGS solar cells with good efficiency at a CIGS thickness of less than 1 μm. An efficiency of 14.4% was obtained for an evaporation time of 8 min and a resulting CIGS thickness of only 0.8 μm. The quantum efficiency measurements show only a minor reduction of the collection in the infrared region that can be related to losses caused by reduced absorption. Passivation of the back contact has been found to be important for thin devices and one way of obtaining good back contact properties, or to reduce the impact of back contact recombination is to use an increased Ga content near the back contact. We have found that Ga grading is feasible also in the three stage process, i.e. a Ga-rich layer near the back contact from stage one is to a high degree retained also after stages two and three. In this paper we discuss the implication of efficiency reduction for the economy of the production and how high efficiency loss that can be tolerated, provided that the output is doubled at equal production cost for the CIGS layer.

The effect of Ga-grading in CIGS thin film solar cells

In this paper the effect of an in depth variation of the Ga/(In+Ga) ratio in CIGS based thin film solar cells is presented. The conclusions made are based on a review of earlier publications, theoretical considerations and results from a large set of new devices. For standard devices with normally thick CIGS films (1.5–2 μm) deposited at a relatively long deposition time (60 min) an improved efficiency of around 0.4% units for the devices with an increased Ga/(In+Ga) ratio towards the back contact is observed. This improvement is due to a field assisted carrier collection resulting in an improved QE response at long wavelengths. When the CIGS thickness is reduced the importance of the increased Ga/(In+Ga) ratio towards the back contact is enhanced and at a CIGS thickness of 0.5 μm a gain of 2.5% units is obtained. The gain is due to an improved V oc and FF. The main reason for the improvement is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. Also for pure CIS a significant improvement of the device performance is obtained by introducing an increased Ga concentration towards the back contact. This improvement is, however, more related to the introduction of Ga itself than the gradient of the Ga-concentration. From many simulations the largest gain is predicted for an increased Ga/(In+Ga) ratio towards the CIGS surface. However, neither in the literature nor from our own experiments we can find evidence for an improved device performance due to an increased Ga-concentration towards the CIGS surface.

Rapid growth of thin Cu(In,Ga)Se 2 layers for solar cells

In this study, we have decreased the deposition time from 60 min down to 3.75 min for three sets of Cu(In,Ga)Se 2 (CIGS) layers: baseline CIGS (≈2 μm thick and homogeneous composition), Ga-graded CIGS (≈2 μm thick) and Ga-graded thin CIGS (≈1 μm thick). The CIGS layers were fabricated with co-evaporation and analysed with a scanning electron microscope and X-ray diffraction. The complete devices were analysed with I– V and quantum efficiency. By reducing the deposition time from 60 to 3.75 min, the efficiency was reduced from 14.7% down to 12.3% (for the baseline CIGS). This reduction is explained by increased recombination, which correlates with decreased grain size for CIGS layers with shorter deposition times. In order to decrease the deposition time, with a maintained high efficiency, a 1.8–2 μm thick CIGS layer with a Ga-gradient is shown to be the best alternative down to 7.5 min deposition time, at which a CIGS layer, resulting in a 14.6% efficient solar cell, was fabricated. At 3.75-min deposition time the efficiency was improved when the CIGS thickness was reduced from 2 μm down to 1 μm.

Characterization of the Cu(In,Ga)Se 2/Mo interface in CIGS solar cells

In a high efficiency CIGS solar cell, we observed a MoSe 2 layer at the interface by SIMS, TEM and X-ray diffraction. MoSe 2 had a layer structure and the layers were oriented perpendicular to the Mo layer. The MoSe 2 contributes to the improvement of adhesion at the CIGS/Mo interface. The effects of the MoSe 2 layer on the electrical and photovoltaic properties of CIGS solar cells were investigated. The CIGS/Mo heterocontact, including the MoSe 2 layer, is not Schottky-type, but a favorable ohmic-type by the evaluation of dark I– V measurement at low temperature. A characteristic peak at 870 nm is observed in the differential quantum efficiency of a solar cell with a CIGS thickness of 500 nm. This peak relates to the absorption of the MoSe 2 layer. The band gap of MoSe 2 is calculated to be 1.41 eV from the absorption peak.

Electrical properties of the Cu(In,Ga)Se 2/ MoSe 2/Mo structure

We investigated the electrical properties of the Cu(In,Ga)Se 2/MoSe 2/Mo structure. CIGS/Mo heterocontact including the MoSe 2 layer is not Schottky-type but a favorable ohmic-type contact by the evaluation of dark I– V measurement at low temperature. A characteristic peak at 870 nm is observed in differential quantum efficiency of a solar cell with a CIGS thickness of 0.5 μm. This peak is considered with relating to the absorption of the MoSe 2 layer. The band gap of MoSe 2 is calculated to be 1.41 eV from the absorption peak. The band diagram is discussed on the basis of the electrical point of view.