CIGS-based Solar Cells Research Articles

All Fiber Laser Scribing of Cu(In,Ga)Se2 Thin-Film Solar Modules

Abstract State-of-the-art Cu(In,Ga)Se2 thin-film technology allows the industrial production of highly efficient solar modules. A significant growth of CIGS-based solar cell production volume can be expected for the coming years. One of the critical manufacturing steps in module production is thin-film patterning which allows the monolithic integration of cell-to-cell interconnects. Today, solar module manufacturers seek to replace sub-optimal needle scribing by suitable laser processes. It has been demonstrated, that ultra-short pulse laser scribing can reduce the overall width per interconnect and increase scribe quality. A promising tool for picosecond laser-scribing is the fiber laser. Here we present the successful implementation of all fiber laser patterning of CIGS modules.

The Effect of Sputtering Parameters on the Film Properties of Molybdenum Back Contact for CIGS Solar Cells

Molybdenum (Mo) thin films are widely used as a back contact for CIGS-based solar cells. This paper determines the optimal settings for the sputtering parameters for an Mo thin film prepared on soda lime glass substrates, using direct current (dc) magnetron sputtering, with a metal Mo target, in an argon gas environment. A Taguchi method with an L9orthogonal array, the signal-to-noise ratio, and an analysis of variances is used to determine the performance characteristics of the coating operation. The main sputtering parameters, such as working pressure (mTorr), dc power (W), and substrate temperature (°C), are optimized with respect to the structural features, surface morphology, and electrical properties of the Mo films. An adhesive tape test is performed on each film to determine the adhesion strength of the films. The experimental results show that the working pressure has the dominant effect on electrical resistivity and reflectance. The intensity of the main peak (110) for the Mo film increases and the full width at half maximum decreases gradually as the sputtering power is increased. Additionally, the application of an Mo bilayer demonstrates good adherence and low resistivity.

Selective Ablation in Cu(In,Ga)Se2 thin-film solar cells – analysis of different process regimes at sub-picosecond to nanosecond pulse duration.

Recent achievements in Cu(In,Ga)Se2 (CIGS) thin film technology allow the industrial production ofhighly efficient solar modules. A large growth of the CIGS-based solar cell production volume can beexpected for the coming years thanks to some favorable properties inherent to this absorber type. A majordrawback of CIGS is the inefficient patterning process. Since CIGS is a particularly difficult material forlaser ablation there is still no industrial all-laser scribing solution available. Manufacturers fall back onmechanical needle scribing for the P2 and P3 scribing process and have to accept substantial broadeningof the electrical interconnects due to unpredictable chipping at the scribe borders. In the present study weexplored a large variety of possible processes for the P1-P3 scribing at different wavelengths and indifferent pulse duration regimes. Beside the direct ablation of CIGS with ultrashort pulses we alsoinvestigated more exotic processes like layer side lift-off variants. The resulting scribes were analyzedusing electron microscopy (EM), laser scanning microscopy (LSM), energy dispersive X-ray spectroscopy(EDX) and electrical conductivity measurements. The most promising processes were selected forproducing functional mini-modules. Multiple optimization cycles allowed us to select the processes withthe best performance in the mini-module.

Effect of ZnO Layer Thickness on Efficiency of Cu(In,Ga)Se2 Thin-film Solar Cells

The effect of intrinsic ZnO(i-ZnO) layer thickness on the efficiency of Cu(In,Ga)Se2 (CIGS) thin film solar cells was investigated using ITO/ZnO/CdS/CIGS/Mo structures. CIGS thin films were deposited on Mo-coated soda-lime glass using co-evaporation. CdS buffer layers of about 50nm thickness were then grown by chemical bath deposition on the top of CIGS layer. Finally, the ZnO and ITO layers were deposited using rf-magnetron sputtering, resulting in solar cells with ITO/ZnO/CdS/CIGS/Mo structure. From the optical and electrical characteristics of the solar cells, we found a close relationship between the transmittance of the ZnO layer and the efficiency of the solar cells. Several characteristics improved for solar cells with a 50 nm thick ZnO layer relative to those with both 90 nm thick and no ZnO layer. Therefore, we conclude that the optimum ZnO thickness for CIGS-based solar cells is around 50 nm.

Near infrared enhancement in CIGS-based solar cells utilizing a ZnO:H window layer

We investigated the near infrared enhancement in Cu(In,Ga)Se(2) (CIGS)- based solar cells utilizing a hydrogen-doping ZnO (ZnO:H) window layer. The results show that the carrier concentration of ZnO:H film is lower than that of AZO film which can increase the transmittance in the NIR. The advantage of ZnO:H film is higher Hall mobility than AZO film. Thus ZnO:H film has similar resistivity to AZO film. It was found that the cell efficiency was 12.4 and 13% for the AZO device and the ZnO:H device, respectively. The cell efficiency is enhanced by 4.8%. Furthermore, the results indicate that, the ZnO:H film is superior to the AZO film as the window layer for CIGS-based solar cells.

A study on composition, structure and optical properties of copper-poor CIGS thin film deposited by sequential sputtering of CuGa/In and In/(CuGa+In) precursors

Copper-poor CIGS thin films were fabricated by using two precursor of CuGa/In and In/(CuGa+In) onto the Mo coated soda-lime glass (SLG) by the sequential sputtering of CuGa and In targets. The CIG precursors were converted into CIGS absorption thin film by selenization process. The X-ray diffraction (XRD) patterns of CIGS absorber from CuGa/In precursor exhibits strong (112) preferred orientation that is more stable than (220)/(204) as compared to In/(CuGa+In) precursor. The elemental composition uniformity onto the surface and along depth were extensively analyzed with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). For as-fabricated CIGS thin films from In/(CuGa+In) and CuGa/In precursors, the atomic (at%) composition values of [Cu]//[In+Ga]/=1.14, 1.08 and [Ga]/[In+Ga]=0.33, 0.20 were observed, respectively that is well matched for highest efficient CIGS-based solar cell so far. SIMS confirmed that the Ga profile is not through the depth of CIGS thin film attributed a consistent band gap of 1.04 and 1.08eV. Further, PL spectrum of CIGS absorber formed by CuGa/In precursor exhibits relatively narrow emission peak as compared to In/(CuGa+In) precursor. It is attributed to decrease defect density with uniform composition in the CIGS absorber. The carrier concentration (Np) found to increase from 1020 to 1021cm−3 orders with the increase of Cu/(In+Ga) at.% from 0.93 to 1.02, which is related to the increasing carrier concentration for stoichiometric CIGS films.

Hydrogen effects on deep level defects in proton implanted Cu(In,Ga)Se2 based thin films

Hydrogen effects on deep level defects and a defect generation in proton implanted Cu(In,Ga)Se2 (CIGS) based thin films for solar cell were investigated. CIGS films with a thickness of 3μm were grown on a soda-lime glass substrate by a co-evaporation method, and then were implanted with protons. To study deep level defects in the proton implanted CIGS films, deep level transient spectroscopy measurements on the CIGS-based solar cells were carried out, these measurements found 6 traps (including 3 hole traps and 3 electron traps). In the proton implanted CIGS films, the deep level defects, which are attributed to the recombination centers of the CIGS solar cell, were significantly reduced in intensity, while a deep level defect was generated around 0.28eV above the valence band maximum. Therefore, we suggest that most deep level defects in CIGS films can be controlled by hydrogen effects.

Molybdenum, Molybdenum Oxides, and their Electrochemistry

The electrochemical behaviors of molybdenum and its oxides, both in bulk and thin film dimensions, are critical because of their widespread applications in steels, electrocatalysts, electrochromic materials, batteries, sensors, and solar cells. An important area of current interest is electrodeposited CIGS-based solar cells where a molybdenum/glass electrode forms the back contact. Surprisingly, the basic electrochemistry of molybdenum and its oxides has not been reviewed with due attention. In this Review, we assess the scattered information. The potential and pH dependent active, passive, and transpassive behaviors of molybdenum in aqueous media are explained. The major surface oxide species observed, reversible redox transitions of the surface oxides, pseudocapacitance and catalytic reduction are discussed along with carefully conducted experimental results on a typical molybdenum glass back contact employed in CIGS-based solar cells. The applications of molybdenum oxides and the electrodeposition of molybdenum are briefly reviewed.

(Invited) CIGS-Based Solar Cells Prepared from Electrodeposited Precursor Films

not Available.

One-step synthesis of Cu(In,Ga)Se2 absorber layers by magnetron sputtering from a single quaternary target

A one-step route was developed to fabricate Cu(In,Ga)Se2 (CIGS) absorber layers by direct magnetron sputtering from a single quaternary target with the composition of CuIn0.75Ga0.25Se2. The effects of the substrate temperature, the working pressure and the sputtering power on the morphology and phase structure of the CIGS layers were studied using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The microstructure properties of the layers, including the crystallinity, grain size, compactness and the surface evenness, were found to be strongly dependent on the deposition parameters. CIGS absorbers with compact microstructure and large grains of micrometer size were obtained at 400°C and 160W, showing a very strong (220)/(204) orientation preference when sputtered at a higher working pressure. Raman spectra indicated no precipitation of the Cu–Se binary phases, but revealed a slight difference in the Ga/(Ga+In) ratio of different layers. The overall composition of the as-sputtered CIGS film was confirmed to be in agreement with the target composition through energy dispersive X-ray spectroscopy study. In comparison with the conventional co-evaporation or post-selenization synthesis for CIGS, the one-step sputtering route is more simplified and economical, which shows great potential to reduce the production cost of CIGS-based solar cells.

Invited Paper: CIGS-based thin film solar cells and modules: Unique material properties

Although CIGS solar cells consist of a polycrystalline thin film grown on a glass substrate, more than 20% conversion efficiency has been achieved. The efficiency has reached the same level as polycrystalline silicon solar cells. This high efficiency has not yet been observed in other thin film solar cells including thin film Si and CdTe. Therefore, it is important to understand the mechanisms that allow CIGS solar cells to exhibit high conversion efficiencies. This paper discusses the origin of the high efficiency and demonstrates that it is caused by the unique material properties of CIGS films.

CIGS-based solar cells prepared from electrodeposited precursor films

Previously, we reported 15.4%-efficient [1] copper indium gallium diselenide (CIGS)-based photovoltaic devices from electrodeposited precursor films in which the final film composition was adjusted using the physical vapor deposition (PVD) method. At present, we are fabricating CIGS-based solar cells directly from electrodeposited precursor films, eliminating the expensive PVD step. Electrodeposited CIGS absorber layers are fabricated by a three-stage electrodeposition process in which: (a) CIGS is electrodeposited in the first stage, (b) Cu is electrodeposited in the second stage, and (c) an In layer is deposited in the final third stage. All films are electrodeposited from an aqueous-based solution at room temperature in a two-electrode cell configuration, with platinum gauze as the counter electrode and a glass/MO substrate as the working electrode. The substrate is DC-sputtered with about 1μm of Mo. The electrodeposited films are selenized at high temperature (∼550°C) to obtain a 10.9%-efficient device.

Time-Resolved Microphotoluminescence Study of Cu(In,Ga)Se2

The carrier recombination processes in Cu(In1-x,Gax)Se2 (CIGS) thin films were investigated by time-resolved microscopic-photoluminescence (µ-PL) measurement at room temperature. For films with x = 0.45, the spatial distribution of the donor–acceptor pair luminescence is much larger than the grain size. The PL decay lifetime is directly correlated with the µ-PL intensity, but the spectral shape is identical regardless of the sample position. These results suggest that the nonuniform distribution of nonradiative recombination centers mainly affects the carrier recombination in CIGS thin films. At relatively high Ga concentrations (x ≥0.7), the spatial inhomogeneity in µ-PL intensity is enhanced and decay accelerated, suggesting an increase in the density of nonradiative recombination centers. Thus, suppression of nonradiative recombination is critical to enhancing the performance of CIGS-based solar cells.

High voltage Cu(In,Ga)S 2 solar modules

Sulfurcell (SC) has been running a pilot production for thin-film solar modules using CuInS 2-chalcopyrite (CIS) as absorber material since 2004. Since then production technology has been constantly improved with module power values exceeding 64 W, corresponding to an aperture area efficiency level of about 9%. Small area (0.5 cm2) cells cut out of such CIS modules reach maximum efficiencies close to 11%. Strong efforts have been made to develop a new sequential Cu(In,Ga)S 2 (CIGS) process suitable for production of large-scale CIGS solar modules thereby enabling module efficiencies above 10%. CIGS-based solar cells are—quite similar to CIS-based modules—prepared from sputtered metals subsequently sulfurized using rapid thermal processing in sulfur vapor. Such Cu(In,Ga)S 2 solar cells reach material record efficiencies about 13%. The cells are characterized by high open-circuit voltages up to 890 mV. Based on the results of the “Helmholtz Zentrum Berlin” (HZB), Sulfurcell has successfully scaled this process to our typical module size of 125 cm × 65 cm and is currently piloting the process for mass production. This paper will give an overview of electrical and structural parameters of world's first large-scale CIGS modules. CIGS module and cell parameters will be compared with standard CIS module and cell parameters and measured CIGS efficiency temperature coefficients will be compared with typical temperature coefficients of modules based on established PV technologies.

Optimization of molybdenum thin films for electrodeposited CIGS solar cells

Molybdenum thin films are widely used as back contact for CIGS-based solar cells. In this paper, the properties of Mo layers deposited by DC and RF sputtering are investigated in view of a specific optimization of electrodeposited CIGS solar cells. In the first part of the paper RF and DC films are grown on soda lime glass, in a pressure range from 2 to 20 mTorr, and for various RF power and DC current. They are then characterized by optical, electrical and structural methods. It appears that the films deposited by RF mode sputtering are more reflective, conductive and adherent than those obtained by DC mode. Structurally, they present different behaviors with respect to nucleation and growth of CIGS precursor layers by electrodeposition. A large difference is observed in the photovoltaic properties of completed cells, with much better performances obtained with DC Mo layers.

Dependence of Se beam pressure on defect states in CIGS-based solar cells

The influence of Se beam pressure on defect properties in Cu(In 1– x ,Ga x )Se 2 (CIGS)-based solar cells fabricated by the three-stage process was investigated by admittance spectroscopy. The Se beam pressure was varied from 1.3×10 −3 to 4.4×10 −3 Pa, at the position of the samples, by varying the Se cell temperature. The spectra for all samples show three distinct peaks denoted as α, β, and ζ with activation energies of 20, 150, and 300 meV, respectively. The trap density of ζ increased from 5×10 14 to 4×10 15 cm −3 with decrease in Se beam pressure; the trap densities of α and β did not change. These results suggest that the origin of ζ is related to the Se deficiency. The density of ζ seems to correlate with the cell efficiency. Therefore, it is important to control the Se beam pressure in order to obtain highly efficient solar cells.

Optimization of CIGS-Based PV Device through Antimony Doping

This communication discusses grain size enhancement in CIGS thin films induced by Sb-doping and demonstrates consequent photovoltaic (PV) device performance improvement in CIGS-based solar cells. The chemical-doping approach also shows promise in enabling low-temperature (≤ 400°C) processing of CIGS-based PV technology.

Atmospheric pressure synthesis of In2Se3, Cu2Se, and CuInSe2 without external selenization from solution precursors

In2Se3, Cu2Se, and CuInSe2 thin films have been successfully fabricated using novel metal organic decomposition (MOD) precursors and atmospheric pressure-based deposition and processing. The phase evolution of the binary (In-Se and Cu-Se) and ternary (Cu-In-Se) MOD precursor films was examined during processing to evaluate the nature of the phase and composition changes. The In-Se binary precursor exhibits two specific phase regimes: (i) a cubic-InxSey phase at processing temperatures between 300 and 400 °C and (ii) the γ-In2Se3 phase for films annealed above 450 °C. Both phases exhibit a composition of 40 at.% indium and 60 at.% selenium. The binary Cu-Se precursor films show more diverse phase behavior, and within a narrow temperature processing range a number of Cu-Se phases, including CuSe2, CuSe, and Cu2Se, can be produced and stabilized. The ternary Cu-In-Se precursor can be used to produce relatively dense CuInSe2 films at temperatures between 300 and 500 °C. Layering the binary precursors together has provided an approach to producing CuInSe2 thin films; however, the morphology of the layered binary structure exhibits a significant degree of porosity. An alternative method of layering was explored where the Cu-Se binary was layered on top of an existing indium-gallium-selenide layer and processed. This method produced highly dense and large-grained (>3 µm) CuInSe2 thin films. This has significant potential as a manufacturable route to CIGS-based solar cells.

Electrodeposited Cu-In-Ga-Se Thin Films for CIGS-based Solar Cells

AbstractCyclic voltammogram studies were performed on H2SeO3, CuSO4, In2(SO4)3, GaCl3, H2SeO3 + CuSO4 + In2(SO4)3 and H2SeO3 + CuSO4 + In2(SO4)3 + GaCl3 to understand the electrodeposition mechanism. The reduction potential from the cyclic voltammogram studies indicates that the first deposited layer is Cu from the Cu-In-Se and Cu-In-Ga-Se solution mixture. The subsequent deposition of the In and Ga layer is more favorable on the first-deposited Cu layer.

Copper indium gallium diselenide thin films for sun angle detectors in space applications

This work reports on processing, analysis and characterization of copper indium gallium diselenide (CIGS) used as a photosensitive layer for sensors such as sun angle detectors in space applications. CIGS-based solar cell devices with different CIGS layer thicknesses and the pn-junction located on the opposite side of the incidence of light were illuminated through their ultra-thin transparent molybdenum back contacts. The results from the current density versus voltage and quantum efficiency measurement indicate that the CIGS absorber layer may not exceed 750 nm at backside illumination, due to the limited CIGS diffusion length.