CIGS Surface Research Articles

Impacts of surface sulfurization on Cu(In1−x,Gax)Se2 thin‐film solar cells

AbstractIn this work, the impacts of surface sulfurization of high‐quality Cu(In1−x,Gax)Se2 (CIGS) thin films deposited by three‐stage process on the film properties and the cell performance were investigated. The CIGS thin films were sulfurized at 550 °C for 30 min using H2S gas. The X‐ray photoelectron spectroscopy analysis revealed that sulfur atoms diffused into the CIGS surface layer and that the valence band minimum was lowered by the film sulfurization. The open circuit voltage (Voc) drastically increased from 0.590 to 0.674 V as a result of the sulfurization process. Temperature‐dependent current–voltage and capacitance–frequency measurements also revealed that interface recombination was drastically decreased by the lowering of the defect's activation energy level at the vicinity of the buffer/CIGS interface after the sulfurization. Copyright © 2014 John Wiley & Sons, Ltd.

Defects in Cu(In,Ga)Se2 Chalcopyrite Semiconductors: A Comparative Study of Material Properties, Defect States, and Photovoltaic Performance

AbstractUnderstanding defects in Cu(In,Ga)(Se,S)2 (CIGS), especially correlating changes in the film formation process with differences in material properties, photovoltaic (PV) device performance, and defect levels extracted from admittance spectroscopy, is a critical but challenging undertaking due to the complex nature of this polycrystalline compound semiconductor. Here we present a systematic comparative study wherein varying defect density levels in CIGS films were intentionally induced by growing CIGS grains using different selenium activity levels. Material characterization results by techniques including X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, secondary ion mass spectrometry, X‐ray photoelectron spectroscopy, and medium energy ion scattering indicate that this process variation, although not significantly affecting CIGS grain structure, crystal orientation, or bulk composition, leads to enhanced formation of a defective chalcopyrite layer with high density of indium or gallium at copper antisite defects ((In, Ga)Cu) near the CIGS surface, for CIGS films grown with insufficient selenium supply. This defective layer or the film growth conditions associated with it is further linked with observed current‐voltage characteristics, including rollover and crossover behavior, and a defect state at around 110 meV (generally denoted as the N1 defect) commonly observed in admittance spectroscopy. The impact of the (In, Ga)Cu defects on device PV performance is also established.

Photo current metastability in Cu(In,Ga)Se 2 solar cells with controlled conduction band alignment

A metastability of current density–voltage characteristics, so called a light soaking (LS) effect, in (Zn,Mg)O (ZMO)/Cu(In,Ga)Se 2 (CIGS) solar cells with the controlled conduction band offset (CBO) between window and CIGS layers was measured. The strong LS effect was observed with increasing the CBO value from +0.03 eV to +0.35 eV. Here, plus sign of CBO indicate the conduction band minimums of the window layers above that of the CIGS layers. When the CBO is positive, the CBO acts as a barrier for photo-generated electrons in the CIGS layer. On the other hand, the CIGS solar cells with Cd partial electrolyte treatment for the CIGS surface before the ZMO deposition showed the negligible LS effect. These results indicate that the LS effect would relate to the photo current which is affected by CBO and surface condition of the CIGS film, and the external quantum efficiency measurements support this hypothesis.

Effects of CdS buffer layers on photoluminescence properties of Cu(In,Ga)Se 2 solar cells

Photoluminescence (PL) have been studied on Cu(In,Ga)Se 2 (CIGS) thin films, CdS/CIGS and CIGS solar cells, to clarify the carrier recombination process. The chemical-bath deposition (CBD) of the CdS buffer layer on the CIGS thin film leads to (i) the enhancement of near-band-edge PL intensity by a factor of 2–3, (ii) change in energy of the defect-related PL and (iii) the slight change in the decay time. They are related not only to the minimization of the surface recombination but also to the modification of native defects at the Cu-poor surface of CIGS by the occupation of Cd atom at the Cu site. A donor–acceptor pair PL at low-temperature and temperature-dependent PL have been studied. They are discussed in terms of the impurity and defect levels created in the CIGS film during the CBD-CdS process.

Effects of CuIn0,5Ga0,5Se2 growth by isothermal and bithermal Cu-Poor/Rich/Poor sequence on solar cells properties

AbstractCo-evaporated CuIn0,5Ga0,5Se2 thin film solar cells were grown using a sequential Cu-Poor/Rich/Poor process (CUPRO). During the growth process, the substrate temperature was either kept constant at 570 °C (iso-CUPRO) or decreased during the first step to either 360 or 430 or 500 °C (bi-CUPRO). According to atomic force microscopy (AFM) measurements, the lower the temperature is in the first step the smoother the final CIGS surface becomes. By decreasing the first step temperature, cross-section scanning electron microscopy (SEM) and θ-2θ x-ray diffraction (XRD) do not reveal clearly any important changes of morphology and crystallographic preferred orientation. SLG/Mo/CIGS/Buffer layer/i-ZnO/ZnO:Al/grid(Ni/Al/Ni) solar cells with either a chemical bath deposited CdS or an atomic layer deposited Zn(O,S) buffer layer were fabricated. For both buffer layers, the bi-CUPRO processes lead to higher efficiencies. Besides, using Zn(O,S), the electronic collection was improved for the infrared spectrum as well as for the ultraviolet spectrum. This resulted in efficiencies close to 14,5% for the Zn(O,S) cells.

Recombination kinetics and stability in polycrystalline Cu(In,Ga)Se 2 solar cells

Time-resolved photoluminescence (TRPL) measurements indicate that bare Cu(In,Ga)Se 2 (CIGS) films degrade when they are exposed to air or stored in nitrogen-purged dry boxes. The degradation significantly affects device performance and electro-optical measurements. Measuring films prior to degradation reveals long lifetimes and distinct recombination properties. For high-quality material, the surface recombination velocity at grain boundaries, bare CIGS surfaces, and CIGS/CdS interfaces is less than 10 3 cm/s, and lifetime values are often greater than 50 ns. In high injection, CIGS has recombination properties similar to GaAs. On completed devices, charge-separation dynamics can be characterized.

Two step wet surface treatment influence on the electronic properties of Cu(In,Ga)Se 2 solar cells

In this work we investigate the electronic properties of Glass/Mo/Cu(In,Ga)Se 2 (CIGS)/CdS/ZnO solar cells where the co-evaporated CIGS surfaces are subjected to a two steps chemical treatment. This two steps treatment consisting of a strong oxidative etch (bromine) of the absorber surface followed by a cyanide treatment leads to a significant (by 30%) improvement of the photovoltaic performances. The bromine etching time is used as variable in the study, while admittance spectroscopy is employed for electrically active defects characterization. Particular attention was given to the interpretation of the origin of the defects probed in the samples. One type of defects with activation energy ranging between 0.2 eV to 0.5 eV, commonly probed in the CIGS based devices and labelled N 2, are passivated by the cyanide treatment. This allowed us to propose a new interpretation concerning the nature of this type of defect.

Annealing effects on Zn 1−xMg xO/CIGS interfaces characterized by ultraviolet light excited time-resolved photoluminescence

Annealed Zn 1− x Mg x O/Cu(In,Ga)Se 2 (CIGS) interfaces have been characterized by ultraviolet light excited time-resolved photoluminescence (TRPL). The TRPL lifetime of the Zn 1− x Mg x O/CIGS film increased on increasing the annealing temperature to 250 °C, whereas the TRPL lifetime of the CdS/CIGS film had little change by annealing at temperatures lower than 200 °C. This is attributed to the recovery of physical damages by annealing, induced by sputtering of the Zn 1− x Mg x O film. The TRPL lifetime abruptly decreased with annealing at 300 °C. The diffusion of excess Zn from the Zn 1− x Mg x O film into the CIGS interface is clearly observed in secondary ion mass spectroscopy (SIMS) depth profiles. These results indicate that excess Zn at the vicinity of the CIGS surface acts as non-radiative centers at the interface. The TRPL lifetime of the Zn 1− x Mg x O/CIGS film annealed at 250 °C reached values to be comparable to that of the as-deposited CdS/CIGS film. Performance of the Zn 1− x Mg x O/CIGS cells varied with the annealing temperature in the same manner as the TRPL lifetime. The highest efficiency of the Zn 1− x Mg x O/CIGS solar cells was achieved for annealing at 250 °C. The results of the TRPL lifetime on annealing show that the cell efficiency is strongly influenced by the Zn 1− x Mg x O/CIGS interface states related to the damages and diffusion of Zn.

Effects of annealing under various atmospheres on electrical properties of Cu(In,Ga)Se 2 films and CdS/Cu(In,Ga)Se 2 heterostructures

The effects of annealing under various atmospheres on the electrical properties of Cu(In,Ga)Se 2 (CIGS) films and CdS/CIGS heterostructures were investigated. For CIGS films without CdS, the electrical properties of CIGS degraded under vacuum and O 2 annealing, although such degradations were not observed under N 2 annealing. For the CdS/CIGS heterostructures, the electrical properties of the junctions improved after annealing under all gas ambients. Therefore, CdS films prevent the chemical reactions at the CIGS surfaces and are necessary for effectively annealing the CIGS film. We observed a distinct correlation between the degradation of the electrical properties and increase in the defect density. Finally, we discussed the origin of the defect states.

Effect of Se beam flux rate on photovoltaic properties of CIGS cells fabricated by three‐stage co‐evaporation process

The conversion efficiency of CIGS cells fabricated with three stage co-evaporation process was improved by regulating the Se beam flux rate. When the Se beam flux rate was 15 A/s or less, the surface of CIGS was uniform and it showed tightly connected structure and facet shape structure. However pits and crevices were found on the surface in the condition of Se beam flux rate above 15 A/s, which caused an increase in the leakage current. The best CIGS solar cell was obtained at the Se beam flux rate of 15 A/s and it had the following photovoltaic parameters: conversion efficiency of 17.57%, Jsc = 36.48 mA/cm2, Voc = 0.655 V and FF = 73.5% in an active area of 0.45 cm2. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Film quality improvement of CIGS thin film grown by mechanochemical process

We have demonstrated a selenium treatment to enhance the film quality of CIGS films prepared by a mechanochemical process (MCP). It was confirmed that the selenium treatment was useful process in order to improve the film quality by enhancing the sintering and the grain growth. Although the film quality was improved, a selenium layer was deposited on CIGS surface during the selenium treatment. Consequently we developed a selenium removal technique using a Hot Filament Melting (HFM) process, and we succeeded in removing the remained selenium layer on the surface. These results suggested that the selenium treatment followed by the HFM process was useful to improve the film quality grown by the MCP. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ monitoring of the growth of Cu(In,Ga)Se2 thin films

Abstract Thin films of Cu(In,Ga)Se 2 were grown by co-evaporation process in a conventional vacuum coating system at the base pressure of 10 −5 mbar. The controllable of the two-stage growth process was developed. During the growth process, substrate temperature, graphite heater temperature, heating output power and temperature of the CIGS surface were monitored simultaneously. In this setup, we use the change in the thermal behavior of the substrate due to the variations in emissivity of CIGS film during the transition of Cu-rich to Cu-poor in the second stage corresponding to the change of power fed into the substrate heater as the control signal. By observing the variation of control signals, the desired final composition of the film can be obtained. Using our device fabrication, Al/ZnO(Al)/CdS/CIGS/Mo/SLG solar cells with efficiency over 14% (without AR) were achieved.

The effect of Na in polycrystalline and epitaxial single-crystal CuIn 1− xGa xSe 2

Na is found to improve the performance of Cu(In,Ga)Se 2 (CIGS) solar cells although the mechanism is not clear. This paper briefly reviews some of the observations on Na in CIGS polycrystalline and epitaxial films. Experiments suggest weak electrical effects of Na within grains, primarily by reducing compensation and in some cases by enhancing acceptor concentrations. As it segregates to surfaces, it has been suggested that Na acts through passivation of grain-boundary defects. However, its main effect is on device open-circuit voltage (and somewhat on fill factor), which does not correlate with grain size but rather with bulk grain defects. The Na concentration scales somewhat with grain boundary density averaged over large areas of film, suggesting that it may be active there. Modest Na concentrations often increase grain size in polycrystals, although not when the grain size is already large, and often changes preferred orientation. Na segregates to the surfaces of CIGS grains. These results suggest that it may act at the surface, modifying growth mechanisms or defect organization during growth. TEM evidence shows that strong concentration of Na in the grain boundaries, sufficient to passivate surface defects by itself, is unlikely to occur. Finally, Na is removed from the surface of CIGS by aqueous solutions, such as those used to form the heterojunction. It is concluded that Na acts at the surface during growth to organize point defects, probably including by reduction of vacancy populations, within the bulk grains but that it has no residual effect once growth is complete.

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.

Low-cost CIGS solar cells by paste coating and selenization

A simple process for the deposition of Cu(In,Ga)Se 2 (CIGS) absorber layers is described. A low-cost CIGS precursor paste deposited by simple and fast doctor blade technique is subsequently selenized under selenium vapour in a quartz tube at 10 mbar (10 min at 550 °C). The precursor paste is prepared with metal chlorides and nitrates dissolved in alcohol. The solution is then mixed with a cellulose solution to adjust the viscosity for optimal deposition. The conversion of the precursor to the CIGS phase was confirmed by X-ray diffraction (XRD). Grain size and morphology were characterised with electron microscopy. A double-layer structure formed during selenization, with a CIGS layer on top of an amorphous carbon layer. Auger electron spectroscopy (AES) shows a decreasing Ga/In ratio from the carbon–CIGS interface towards the CIGS surface. The layer structure grown on Mo-coated glass substrates (conventional dc-sputtering) was processed to solar cells by depositing a CdS buffer layer (chemical bath deposition) and ZnO/ZnO:Al front contacts (conventional rf-sputtering). A maximum efficiency of 6.7% was achieved with approximately 0.5-μm-thick absorber layers. Quantum efficiency measurements reveal photon absorption losses for the longer wavelengths, which are attributed to the thin layers.

Chemical elaboration of well defined Cu(In,Ga)Se 2 surfaces after aqueous oxidation etching

The present work is an attempt to prepare well defined surfaces of Cu(In,Ga)Se 2 (CIGS) thin films in order to answer to basic questions about the relationship between bulk and surface composition. The approach is to use an oxidative etch with an aqueous bromine solution, known to lead to specular surfaces. The CIGS surface is then analyzed by mechanical profilometry, SEM and XPS, allowing for determination of the surface roughness and the nature of surface species. After short time bromine etch, a Se 0 film is formed on the CIGS surface which can be completely removed by KCN treatment, leaving a CIGS specular surface. An highlight result is that under specific conditions, the surface composition is close to the stoichiometry of the Cu(In,Ga) 3Se 5 copper deficient phase. This is the first time that such a study is conducted on technology relevant thin polycrystalline CIGS film. It is expected that the method described will help conducting experiments (e.g. Angle resolved XPS, SIMS, etc.) with an improved resolution.

Properties of CuInGaSe 2 solar cells based upon an improved three-stage process

Recently, we have developed an improved three-stage growth method by simultaneously using a pyrometer and a thermocouple, to accurately control thickness and composition during growth of CuInGaSe 2 (CIGS). As a result, we have obtained solar cells that show preliminary efficiencies up to 16.4% without anti-reflection coating. Using the technique, we have investigated possible mechanism behind non-optimal cell efficiency, focusing on the effects of the molybdenum contact layer. Some additional advantages of our pyrometer monitoring technique are described. Optimizing the properties of Mo layer leads to suppression of Na segregation and distinctive damage to the CIGS surface, improving both cell performance and reproducibility.

Studies of buried interfaces Cu(In,Ga)Se 2/CdS XPS and electrical investigations

The formation of interface Cu(In,Ga)Se 2/CdS in solar cells is not yet well understood but seems to be the key to further improvements of their performance. This interface depends on many parameters such as the initial chemical state of the CIGS surface or the chemical bath deposition conditions used to grow the CdS layer. In this paper, we focus our attention on the CIGS/CdS hetero-interface at different stages of its formation using mainly XPS studies of buried interfaces which were studied after gradual sputtering. We have investigated interfaces on CIGS submitted to various surface treatments, analogue to those involved in fabrication steps (NH 3 dipping…). Kelvin probe and admittance spectroscopy measurements have been also performed on several interfaces prepared in the same conditions to correlate the chemical composition with the electrical response of the buried interfaces.

High-efficiency CIGS solar cells with modified CIGS surface

High-efficiency CIGS solar cells with modified CIGS surface

High-efficiency Cu(In,Ga)Se thin-film solar cells with a novel In(OH):Zn buffer layer

We propose the inclusion of a novel In(OH) 3 :Zn 2+ buffer layer for fabricating high-efficiency CIGS solar cells. This buffer layer was deposited using a solution consisting of ZnCl 2 , InCl 3 ·4H 2 O, and thiourea. The In(OH) 3 :Zn 2+ films showed high resistivities of 2.1×10 8 Ω cm and transmittance of above 95% in the visible range. We expected two effects due to this new buffer layer: first is the formation of a passivation layer on the CIGS surface and the second is Zn-doping into CIGS layer, resulting in the formation of a buried junction. A cell efficiency of 14.0% ( V oc : 0.575 V, J sc : 32.1 mA/cm 2 , FF: 0.758) was achieved by using an In(OH) 3 :Zn 2+ buffer layer, without the light soaking effect.