Cu(In,Ga)Se2 (CIGSe) layers have been deposited by the 3-stage process and once the growth was completed, the structures have been kept at high temperature for 50min with or without Se supply. Both the resulting CIGSe layer and related device properties are compared with those obtained when the substrate is cooled down right after the deposition. Moreover, such experiments have been performed with and without alkali availability. The results show that keeping the absorber at high temperature differently impacts [Ga]/([In+Ga]) atomic ratio distribution and crystalline preferential orientation depending on whether Se is supplied. Moreover, this work shows that chalcogen supply can be detrimental for cell performance when the CIGSe contains alkali and in contrast be beneficial when the CIGSe is free of alkali. These observations suggest an intimate relationship between alkali and Se.

We report on the characterization of the chemical and electronic interface structure of the heavily intermixed In 2 S 3 /Cu(In, Ga)Se 2 (CIGSe) interface. Discussing our findings inferred from direct and inverse photoemission as well as soft x-ray emission spectroscopy, we particularly focus on the impact of the interfacial intermixing/interdiffusion processes on the electronic band alignment at the interface. Furthermore, we present deposition-temperature-dependent data on the (In, Al) 2 S 3 /CIGSe interface. We find that the chemical processes at the buffer/absorber interface are thermally activated with a threshold temperature of approximately 200°C.

Recent results have revealed that the low deposition time issue of chemical bath deposited (CBD) Zn(S,O,OH) buffer layer used in Cu(In,Ga)Se2 (CIGSe) solar cells could be resolved using H2O2 as an additive in the chemical bath solution. Although the use of this additive does not hinder the electrical properties of the resulting Zn(S,O,OH)-buffered CIGSe solar cells, the impact of H2O2 on the Zn(S,O,OH) properties remains unclear. The present contribution aims at determining the chemical composition and the microstructure of Zn(S,O,OH) film deposited by CBD using the alternative deposition bath containing the standard zinc sulfate, thiourea, ammonia but also H2O2 additive. Both X-ray photoemission spectroscopy and energy dispersive X-ray spectroscopy analyses reveal higher sulfur content in alternatively deposited Zn(S,O,OH), since the first step growth of the layer. According to transmission electron microscopy analyses, another consequence of the higher deposition rate achieved when adding H2O2 in the bath is the modification of the absorber/buffer interface. This could be explained by the enhancement of the cluster growth mechanism of the layer.

The present work aims at investigating the microstructure of Zn(O,S) buffer layers relative to their deposition route, namely either chemical bath deposition (CBD) or RF co-sputtering process (PVD) under pure Ar. The core of the study consists of cross-sectional transmission electron microscopy (TEM) characterization of the differently grown Zn(O,S) thin films on co-evaporated Cu(In,Ga)Se2 (CIGSe) absorbers. It shows that the morphology of Zn(O,S) layer deposited on CIGSe using CBD process is made of a thin layer of well oriented ZnS sphalerite-(111) and/or ZnS wurtzite-(0002) planes parallel to CIGSe chalcopyrite-(112) planes at the interface with CIGSe followed by misoriented nanometer-sized ZnS crystallites in an amorphous phase. As far as (PVD)Zn(O,S) is concerned, the TEM analyses reveal two different microstructures depending on the S-content in the films: for [S]/([O]+[S])=0.6, the buffer layer is made of ZnO zincite and ZnS wurtzite crystallites grown nearly coherently to each other, with (0002) planes nearly parallel with CIGSe-(112) planes, while for [S]/([O]+[S])=0.3, it is made of ZnO zincite type crystals with O atoms substituted by S atoms, with (0002) planes perfectly aligned with CIGSe-(112) planes. Such microstructural differences can explain why photovoltaic performances are dependent on the Zn(O,S) buffer layer deposition route.

In order to decrease the deposition time of chemical bath deposited (CBD) Zn(O,S) buffer layers in CIGSe solar cell, the alternative CBD route using H2O2 as additional oxygen source has been investigated. The morphology and the optical properties of the Zn(O,S) thin films grown with and without additive have been compared through scanning electron microscopy (SEM) observations and UV–visible transmission T(λ) and reflectivity R(λ) measurements, respectively. It is observed that deposition time shorter than 5min is sufficient to achieve films with similar properties to those deposited following the standard recipe in 15min. The characteristics of CIGSe/Zn(O,S) structures for which the Zn(O,S) growth has been interrupted after different bath immersion durations have been investigated by XPS measurements. The evolution of the In3d and Zn2p3/2 signals reveals that after 2min of deposition, the Zn(O,S) layer grown by the alternative process completely covers the CIGSe and suggests that the increase of the Zn(O,S) growth rate is most probably due to the acceleration of cluster mechanism growth. A comparative study of devices buffered with the so-called fast and standard Zn(O,S) shows similar efficiencies in either case after light soaking.

In this work, we investigate the influence of gallium content on the defects properties of co-evaporated CuInxGa1−xSe2 by sub gap modulated photocurrent spectroscopy and admittance spectroscopy techniques. A series of CuInxGa1−xSe2 based solar cells with different gallium content in the range from 0% to 33%, and with the same CdS buffer layer have been investigated. On one hand, photocurrent spectroscopy results show 2 types of defects named D1 and D2, and on the other hand, admittance spectroscopy results exhibit only one type of defect. I–V curves show that one of the two defects probed by photocurrent is responsible of the dominant recombination mechanisms next to the heterointerface, between the absorber and the buffer layers. Moreover, I–V curves under AM 1.5 conditions show that the cell with no probed D2 defect presents the best photovoltaic performances.

The present contribution deals with the rather longstanding issue of the preferential orientation of Cu(In,Ga)Se-2 polycrystalline thin films. We investigate both the influence of the growth process parameters and that of the presence of Na on the competition between [112] and [220] orientations. The influence of the presence of Na is studied through the comparison of CIGSe layers co-evaporated on our laboratory standard Mo-coated soda lime glass (SLG/Mo) and on substrates with a sodium diffusion barrier (SLG/barrier/Mo); the process dependence of the orientation is evaluated through the comparison of films grown by the standard bithermal three-stage (400-630 degrees C) and the derived isothermal three-stage process (620 degrees C). For all the process/substrate combinations, the properties of the films (preferential orientation, grain size and morphology) have been determined at key steps of the growth. From these experimental results, it can be concluded that, as already suggested in the literature, the final layer orientation is strongly related to the texturation of the (In,Ga)(2)Se-3 precursor; however, the amount of Na available when the film becomes Cu-rich (recrystallization at the end of the 2nd-stage) can also strongly impact the film orientation. Such a phenomenon is herein interpreted by mean of the grain boundary migration model of recrystallization. In agreement with this new interpretation of the experimental data, processes have been designed in order to grow [220] textured CIGSe layers. Copyright (C) 2011 John Wiley & Sons, Ltd.

In order to reduce the co-evaporation time of Cu(In,Ga)Se2 (CIGSe) thin film absorber, a sequential approach has been investigated. CIGSe layers have been grown using the three-step based CUPRO (Cu-Poor/Rich/Off) process at substrate temperature of 600 and 500°C. The first step consists in the growth of Cu-poor ([Cu]/[In+Ga]=0.9) precursor layers. This paper aims at investigating the impact of this layer deposition duration on the CIGSe and respective solar cell properties. It is observed that for the two substrate temperatures investigated, the morphological and structural properties of the CIGSe layers do not change with increasing precursor deposition speed, even when it is increased by ten. Furthermore, the respective device performance also appears not affected by this reduction of the precursor growth time; all cells demonstrate 15% efficiency. From this work, the duration of our standard deposition process could be decreased from 23 to 14min without performance loss independently of the substrate temperature.

Electrodeposited thin film cells have been fabricated with record-breaking efficiencies of 11.4%. This presentation examines conversion mechanisms in cells with a focus on the effect of CdS buffer layers using a range of complementary tools. Dark currents (IVs) are well described by series and parallel resistances, and two dominant recombination mechanisms represented by parallel diodes. Measurements of IV as a function of temperature (IVT) allow extraction of activation energies corresponding to these processes and indicate their spatial position. Admittance spectroscopy (AS) gives an independent estimate of the same energies, and yields values of the defect densities of states in the forbidden gap. Two dominant levels are apparent, confirming the validity of the IV analysis. Spectral response (QE) measurements are presented, yielding information on minority carrier collection efficiency. The different methods of parameter extraction are correlated and indicate recombination levels some hundreds of meV above the valence band and below the conduction band. Bias dependence of admittance spectroscopy gives indications on the localisation of defect centres with one defect situated at the CdS heterointerface and the other in the bulk of the depletion region. The dark current analysis indicates that photogenerated minority carrier collection is the limiting factor in these cells at the operating bias.

The present paper aims at stating when and why small grains transform to large grains during Cu(In,Ga)Se2 (CIGSe) film growth following three-step processes. Experimental observations revealed that such recrystallization is achieved when the nominal composition of the films is close to a 1:1:2 stoichiometry. A new model based on grain boundary migration theory is proposed in order to establish a causal relationship between such a composition threshold and grain boundary motion yielding large grain formation. This model is related to some of the experimental observations related to CIGSe layer growth that have previously been difficult to explain.

The chemical structure of the interface between a nominal In2S3 buffer and a Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorber was investigated by soft x-ray photoelectron and emission spectroscopy. We find a heavily intermixed, complex interface structure, in which Cu diffuses into (and Na through) the buffer layer, while the CIGSe absorber surface/interface region is partially sulfurized. Based on our spectroscopic analysis, a comprehensive picture of the chemical interface structure is proposed.

This article reports the first investigations of CuIn1−xGaxSe2 (CIGSe) polycrystalline thin films by means of atom probe tomography. Attention is focused on the distribution of Na atoms within the films. Both Na-containing and Na-free CIGSe thin films have been investigated. When Na is available during the CIGSe coevaporation, it is observed to be mainly segregated at the grain boundaries of the films; however, it is also detected within the grains of CIGSe at very low concentration.

The amount of copper excess provided during the Cu(In,Ga)Se2 (CIGSe) 3-stage co-evaporation process is among the most operator subjective. In the present paper the influence of this parameter on the properties of the CIGSe films as well as on the behaviour of the related solar cells is investigated. It is observed that both the In/Ga lateral intermixing and the grain size are enhanced when the excess of copper is increased. Contrary to what could be expected, these changes only weakly affect the performance of the solar cells. Increasing the copper excess also yields a rougher CIGSe morphology. This latter evolution is observed to be the most important factor influencing the device behaviour. Through accurate analysis of quantum efficiency, it is concluded that, in the case of the standard cell structure, there exists a threshold in copper excess, beyond which the cell performance is significantly reduced.

Inorganic semiconductors have properties that are notoriously difficult to control due to the deleterious impact of crystalline imperfections, and this is especially so in solar cells. In this work, it is demonstrated that materials grown using wet chemistry processes for the preparation of nanocristalline precursors can achieve the same performance as the best state of the art, namely conversion efficiencies above 11% with CuInS2. Interestingly, due to the growth process, the active material inherit a porous morphology that is shown to play a part in the performance and functionality of the active material. The new device morphology leads to a device operation closer to that of nanoscale organic interpenetrated solar cells or dye sensitized solar cells than to those of standard polycrystalline ones.

In this paper, it is shown that (In1−xAlx)2S3 thin films can be grown through the co-evaporation of elemental indium, aluminum and sulfur. It is nevertheless observed that the introduction of aluminum within the indium sulfide thin films hinders the crystallites size and even yields almost amorphous films when x is 0.2. The investigations of the optical properties of the films reveal that contrary to what could be expected, the band gap increase is low; the highest values measured do not exceed 2.2eV. However, as suggested by X-ray photoelectron spectroscopy measurements, such widening most probably affects the lower conduction band states.

Sub-Cap Modulated Photo Current Spectroscopy (SGMPCS) is an excellent tool in order to investigate the band gap defect density of the absorber layer, directly on Cu(In-x,Ga1-x)(Se-y,S1-y)(2) (CIGSS) based solar cells. This technique is essentially sensitive to defect states located in the absorber layer, which has the lowest band gap of the heterojunction solar cell. It allows the determination of the sigma-N(E) product, where sigma is the defect Optical Cross Section (OCS) and N(E) is its Density Of States (DOS).We have developed an analytical model, allowing to derive the above product from the imaginary part of the ac photocurrent of the solar cell, under reverse applied dc bias. We have then applied this model to study the defect density of the co-evaporated CIGS (i.e. y = 1) absorber layer of a heterojunction solar cell. Two different defect distributions have been exhibited by SGMPCS, the properties of which vary with thermal annealing. Correlation with Admittance Spectroscopy allows us to derive an estimation of the defect OCS. (C) 2008 Elsevier B.V. All rights reserved.

There is agreement in the literature that Cu(In1-xGax)Se-2 (CIGSe) absorber used in solar cells has an optimum composition (x approximate to 0.3) corresponding to a band gap (1.1-1.2 eV) far below the theoretical value giving the maximum (1.4-1.5 eV). This paper presents a re-investigation of the crystal structure of bulk CIGSe compounds for both stoichiometric and Cu-poor compositions. Regardless of the gallium content, all the stoichiometric compounds are found to adopt the well-known chalcopyrite structure (space group 1-42d) while a modification of the structure is evidenced for the high Ga-content Cu-poor compounds. The X-ray diffraction analyses demonstrate that the crystal structure Of Cu0.743In0.543 Ga0.543Se2 is derived from that of the stannite structure (space-group 1-42m). Ab-initio calculations show a strong dependence of the electronic structure near the Fermi level with the copper content. Such modifications are expected to significantly change the optical properties of Cu-poor CIGSe materials. (C) 2008 Elsevier B.V. All rights reserved.

CuInS2 (chalcopyrite structure) thin films were synthesized at 250°C using a two-stage process consisting firstly in the co-evaporation of a large grain In2S3 (defect spinel structure) precursor layer followed by the addition of copper and sulfur. The crystalline properties of the resulting films are similar to those leading to high efficiency solar cells. An energy conversion efficiency of 6.7% has been attained with a 1.5 μm thick CuInS2 layer and a standard CdS buffer layer/ZnO window structure. Improved performances can be expected through the growth of thicker absorbers.