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Abstract We synthesized chalcopyrite-type (Cu 1-x Li x)GaS 2 solid solution samples with a composition of 0.00 ≤ x ≤ 0.20 by mechanochemical process and sequential heating at 550 °C in 5% H 2 S/N 2 gas atmosphere. The X-ray powder diffraction (XRD) peaks of (Cu 1-x Li x)GaS 2 shifted by substitution of Li atoms for Cu atoms in CuGaS 2. Their crystallographic parameters were refined by Rietveld analysis using XRD data. The lattice constant a of the (Cu 1-x Li x)GaS 2 solid solution increased with increasing Li content, x, while the lattice constant c decreased with increasing Li content, x. We determined band-gap energies of (Cu 1-x Li x)GaS 2 solid solution by diffuse reflectance spectra. The band-gap energy of the (Cu 1-x Li x)GaS 2 solid solution increased from 2.44 eV of CuGaS 2 (x = 0.0) to 2.54 eV of (Cu 0.8 Li 0.2)GaS 2 (x = 0.2). To understand the band diagram of (Cu 1-x Li x)GaS 2 solid solution, the energy level of the valence band maximum (VBM) was estimated from the ionization energy, which was measured by photoemission yield spectroscopy. The energy level of the conduction band minimum (CBM) was also determined by adding the band-gap energy to the VBM level. The VBM level of the (Cu 1-x Li x)GaS 2 solid solution decreased with increasing Li content, x. On the other hand, the CBM level was approximately constant. Li-doping in CuGaS 2 is effective for decreasing the VBM level without increasing the CBM level. Highlights • Crystallographic and optical properties of (Cu 1−x Li x)GaS 2 were investigated. • Chalcopyrite-type (Cu 1-x Li x)GaS 2 samples were synthesized for 0.00 ≤ x ≤ 0.20. • Band gap increased from 2.44 eV of CuGaS 2 to 2.54 eV of (Cu 0.80 Li 0.20)GaS 2. • Valence band maximum of (Cu 1-x Li x)GaS 2 decreased with increasing Li content. • Conduction band minimum of (Cu 1-x Li x)GaS 2 was almost constant with Li content. [ABSTRACT FROM AUTHOR]

Window and buffer layers in chalcopyrite devices are well known to affect junctions, conduction, and photo-absorption properties of the device. Some of these layers, particularly “buffers,” which are deposited directly on top of the absorber, exhibit metastable effects upon exposure to light. Thus, to understand device performance and/or metastability, it is sometimes desirable to selectively excite different layers in the device stack. Absorption characteristics of various window and buffer layers used in chalcopyrite devices are measured. These characteristics are compared with emission spectra of common and available light sources that might be used to optically excite such layers. Effects of the window and buffer absorption on device quantum efficiency and metastability are discussed. For the case of bath-deposited Zn(O,S) buffers, we conclude that this layer is not optically excited in research devices or modules. This provides a complimentary mechanism to the chemical differences that may cause long time constants (compared to devices with CdS buffers) associated with reaching a stable “light-soaked” state. [ABSTRACT FROM AUTHOR]

Considering the chalcopyrite solar cell fabrication process, the commonly reported substrate bending effect that is observed after thermal process, in a front side halogen lamp-based annealing configuration (Rapid Thermal Annealing –RTA- equipment), has been investigated. We first proposed a simple model explaining the substrate bending shape dependence on the initial stress state within the molybdenum (Mo) layer (free, tensile or compressive). Then, we showed that it is possible to overcome this issue by simply using a reactive (oxygen based) sputtering process for the deposition of the Mo layer. Finally, we showed that using a bilayer structure Mo(O)/Mo allows a more precise control of the flatness of the annealed samples. To understand the mechanisms governing such changes in substrate bending, structural and morphological material changes as a function of oxygen flow have been investigated by using X-ray diffraction and scanning electron microscope imaging, respectively. The elemental distribution throughout the Mo(O)/Mo bilayer structure thickness was also investigated. [ABSTRACT FROM AUTHOR]

Potential gains in short-circuit current density related to improvements in optical confinement in chalcopyrite based solar cells are studied and quantified by means of optical simulations. In the first part idealised optical conditions – anti-reflection at front interfaces, high reflection at back contact and light scattering - are introduced by simulating realistic scenarios of Cu(In, Ga)Se 2 (CIGS) solar cells with 2000 nm thick and 300 nm ultra-thin CIGS absorber, including the encapsulation at the front. For anti-reflection effect at front interfaces simulations revealed that in the photovoltaic module structure the most critical reflectance is the reflectance of the front surface of the protecting glass (possible 4.4% gain in short-circuit current density) and not the one at the front transparent conductive oxide contact, as in the case of non-encapsulated solar cell. Introduction of a highly reflective, highly diffusive back reflector is the most crucial point to improve the short-circuit current density of the ultra-thin devices. Potential for 15.8% gain in short-circuit current density related to ideal reflectance and additional 17.4% related to ideal scattering introduced at the back contact was revealed. A concrete example of light management structure was investigated in the second part by employing fully three-dimensional rigorous optical simulations. A semi-ellipsoidal texture was introduced to the substrate of the ultra-thin device. By using ZrN back reflector in simulations the gains in short-circuit current density related to the optimised size of the texture reach 10%, whereas in the case of an ideal highly reflective contact the gain is > 22% according to simulations. [ABSTRACT FROM AUTHOR]

The effects of post p-n junction formation processes, namely transparent conducting oxide (TCO) layer deposition conditions on Cu(In,Ga)Se 2 (CIGS) and CuGaSe 2 (CGS) solar cell device properties were comparatively studied. It was found that CIGS devices were relatively insensitive to the presence of oxygen and heat during TCO layer deposition, whereas CGS devices were extremely sensitive to oxygen and heat, resulting in a degradation of device performance in the presence of them. The use of the relatively low TCO deposition temperature ( T s ) of 80 °C led to significant improvements in the fill factor values of CGS devices compared to the case for T s ~ 180 °C, though the open circuit voltage was reduced. These results suggest that TCO deposition conditions can be an important parameter which critically affects ternary CGS solar cell device properties. It is, therefore, suggested that TCO layer deposition conditions for CGS devices should be optimized independently of that for conventional narrow-gap CIGS devices. [ABSTRACT FROM AUTHOR]

We compare the dopant concentration of polycrystalline Cu(In,Ga)Se 2 thin film absorbers derived from Hall and capacitance-voltage measurements. Although both measurements techniques appear to be reliable, dopant concentrations determined by capacitance-voltage analysis are significantly lower and vary with probing depth into the absorber. The doping profiles and differences between both measurement techniques are consistent with Cd in-diffusion from the CdS buffer layer during solar cell fabrication. Different doping profiles obtained after a variation of the CdS deposition process support this scenario. [ABSTRACT FROM AUTHOR]

The present contribution is a summary of an event that was organized as a special evening session in Symposium V “Chalcogenide Thin-Film Solar Cells” at the E-MRS 2016 Spring Meeting, Lille, France. The presentations in this session were given by the coauthors of this paper. These authors present retrospectives of key developments in the field of Cu(In,Ga)(S,Se) 2 solar cells as they themselves had witnessed in their laboratories or companies. Also, anecdotes are brought up, which captured interesting circumstances in that evolutionary phase of the field. Because the focus was on historical perspectives rather than a comprehensive review of the field, recent developments intentionally were not addressed. [ABSTRACT FROM AUTHOR]

To investigate the effect of a lithium doping in chalcopyrite-type CuInS 2 on its crystallographic and optical properties, we prepared (Cu,Li)InS 2 samples by a mechanochemical process and sequential heating at 550 °C in H 2 S gas. The single-phase chalcopyrite-type (Cu 1 − x Li x )InS 2 solid solution could be obtained for the samples with 0.0 ≤ x ≤ 0.10. The lattice constants a and c of tetragonal chalcopyrite-type (Cu 1 − x Li x )InS 2 solid solution increase with increasing Li content. The band gap energy of the (Cu 1 − x Li x )InS 2 solid solution linearly increased from 1.44 eV of CuInS 2 (x = 0.0) to 1.54 eV of (Cu 0.90 Li 0.10 )InS 2 (x = 0.10). We performed first-principles band structure calculations for chalcopyrite-type (Cu 1 − x Li x )InS 2 , using the HSE06 nonlocal screened hybrid density functional. Theoretically, the band-gap energies of (Cu 1 − x Li x )InS 2 increase with increasing Li content. The energy levels of the valence band maxima (VBMs) were estimated from the ionization energy measured by photoemission yield spectroscopy. The ionization energy of chalcopyrite-type (Cu 1 − x Li x )InS 2 solid solutions increased from 5.45 eV of CuInS 2 (x = 0.0) to 5.79 eV of (Cu 0.90 Li 0.10 )InS 2 (x = 0.10). The energy level of the VBM of the (Cu 1 − x Li x )InS 2 solid solution considerably decreases with increasing Li content, x. The conduction band minimum (CBM) level of the (Cu 1 − x Li x )InS 2 solid solution also decreases with increasing Li content. The Li-doping in CuInS 2 is expected to be useful for decreasing the VBM of CuInS 2 absorber and increasing the band-gap energy of CuInS 2 absorber without increasing the CBM level. [ABSTRACT FROM AUTHOR]

In order to realize the true low cost potential of Cu(In,Ga)(S,Se) 2 (CIGS) thin film solar cells, high performance needs to be combined with simple and easily controllable atmospheric-based deposition processes. A molecular solution-based approach for CIGS deposition is proposed, using metal chalcogenide precursors dissolved in an amine-thiol solvent combination. CIGS thin films were sprayed with varying Ga content and the sprayed films were incorporated into solar cells. The effect of the Ga content on the material and device properties is investigated. A champion power conversion efficiency of 9.8% (active area) was achieved, which highlights the potential of this methodology. [ABSTRACT FROM AUTHOR]

It is well established that the addition of sodium (Na) to chalcopyrite or kesterite based solar cells markedly increases the solar cell performance. In this work, we explore the effect of Na and other alkali metals like potassium (K), rubidium, caesium and lithium (Li) – on pure selenide Cu 2 ZnSnSe 4 (CZTSe) solar cells. We demonstrate the deposition of alkali metals using spin coating on e-beam evaporated metal precursors. The stack of metal precursors with alkali layer was then selenised at high temperatures to obtain a good quality CZTSe absorber. The diffusion of alkali metals into the absorber layer was confirmed using glow discharge optical emission spectroscopy. Samples doped with Na or K have shown improvement in the open circuit voltage. A maximum power conversion efficiency of 8.3% (without anti-reflection coating) and a top open circuit voltage 430 mV was achieved for combination of K and Na. Amongst the rest of alkali metals, Li looks the most promising dopant as far as optoelectronic properties are concerned. [ABSTRACT FROM AUTHOR]

CIGS (Cu(In,Ga)Se 2 ) absorbers are among the most efficient for photo-conversion. As part of their manufacturing cycle, absorber surface preparation is crucial for both the success of processes and the outcome of characterizations actions. This work tackles the question of an efficient deoxidation of CIGS surfaces. Chemical composition of surface is investigated thanks to XPS (X-ray Photoelectron Spectroscopy) characterizations. Present study focuses on the possible control of the CIGS absorber surface associated with simple HCl treatment and subsequent atmosphere ageing up to 20 days. Using 1.0 M HCl concentration solution, remarkable oxide elimination is achieved. Starting from an air aged as-grown surface (Cu and Se-poor), this treatment enables the generation of a deoxidized chalcopyrite reference sample. Furthermore, the reoxidation process of the treated surface at ambient atmosphere is studied considering the global evolution of the XPS signals as the best probe of the ageing process. The surface composition evolution represents a key parameter for the achievement of the following interfaces engineering. The main result is that the reoxidation process is not instantaneous (no significant oxide content before 2 days) and involves In, Ga and Se elements while the Cu seems less affected, leading again to a Cu-poor surface different from the initial one. [ABSTRACT FROM AUTHOR]

Recent studies demonstrate that air annealing can have a positive effect on the device performance of Cu 2 ZnSn(S x Se 1 − x ) 4 [CZTSSe] solar cells. In this work air annealing of the selenium containing CZTSSe is compared to the pure sulfide CZTS. It is discovered that the selenium containing absorbers benefit from air annealing at higher temperatures than selenium free absorbers. The highest efficiency obtained utilizing the air annealing treatment on selenium containing absorbers is 9.7%. We find that the band gap is narrowed when air annealing, which is partially explained by increased Cu–Zn disorder. Furthermore Zn enrichment of the surface after etching is identified as a possible cause of enhanced device performance. It is additionally observed that elemental selenium present on the CZTSSe surface is reduced in the air annealing treatment. Selenium removal is another possible explanation for the enhanced performance caused by the air annealing treatment. [ABSTRACT FROM AUTHOR]

In this work phase pure CuInSe 2 thin flms were obtained by selenization of ternary CuInSe 2 and CuInS 2 nanocrystals and mixtures of binary nanocrystals such as CuS, In 2 S 3 , Cu 2 Se and In 2 Se 3 . The temperature of the selenium source was kept at 400 °C during selenization. Monitoring the process using in-situ x-ray diffraction, the effect of selenization on the phase formation and grain growth in the precursor film was investigated. Whereas CuInSe 2 and CuInS 2 nanocrystals exhibit little grain growth, we found that mixtures of binary nanocrystals can show significant sintering depending on the reaction conditions. For the mixture of CuS and In 2 S 3 nanocrystals, the crystallinity and the morphology of the obtained fims strongly depends on the Cu/In ratio, with a Cu excess strongly promoting grain growth. With mixtures of Cu 2 Se and In 2 Se 3 nanocrystals the selenium partial pressure plays a crucial role. Selenium evaporation from the mixed compounds results in CuInSe 2 films composed of relatively small crystallites. Higher selenium partial pressures however resulted in improved sintering. Incomplete propagation of the selenization reaction through the layer was observed though, only leading to a well sintered CuInSe 2 top layer above a fine grained bottom layer. [ABSTRACT FROM AUTHOR]

Chalcopyrite Cu(InGa)Se 2 (CIGS) absorbers were fabricated by the formation of bilayer stacked glass/Mo/(InGa) 2 Se 3 /CuSe precursors followed by in-situ thermal annealing at 450 °C for approximately 10 min in a vacuum evaporator. The material properties (e.g., crystal orientation, compositional depth profile, and overall composition) and device performance of the resulting CIGS absorbers were compared with those of the CIGS absorbers formed by conventional 1-stage and 3-stage CIGS formation processes at a similar temperature. X-ray diffraction confirmed that the 1-stage co-evaporation and in-situ annealing of the bilayer precursor produced a polycrystalline CIGS absorber without a specific texture, whereas the CIGS absorber formed by the 3-stage process showed a highly (220) preferred orientation. Secondary ion mass spectrometry revealed Ga accumulation at the bottom of CIGS formed by in-situ annealing of the bilayer precursors. The cell efficiency of the device with the CIGS absorber formed by the in-situ, low-temperature (450 °C) annealing of bilayer stacked glass/Mo/(InGa) 2 Se 3 /CuSe precursors was comparable to that produced by the conventional 3-stage process at a similar temperature. [ABSTRACT FROM AUTHOR]

Silver indium sulfide (AgInS 2 ) thin films are deposited by sequential sputtering of metallic precursor [Ag/In] followed by sulfurization. Effect of substrate temperature (T sub ) during sulfurization process on the film growth is studied by varying the substrate temperature from 350 to 500 °C. Films prepared above 350 °C showed a mixture of orthorhombic and tetragonal phases of AgInS 2 with tetragonal phase being dominant. Better crystalline, nearly stoichiometric and p-type films are obtained at a substrate temperature of 500 °C. The characteristic A 1 mode of AgInS 2 chalcopyrite structure is observed in the Raman spectra at 274 cm − 1 for the films prepared above 350 °C. The grain size of the film increases from 489 to 895 nm with the increase in substrate temperature. The binding energies of the constituent elements are determined using XPS. The band gap of AgInS 2 films is in the range of 1.64–1.92 eV and the absorption coefficient is found to be > 10 4 cm − 1 . Preliminary studies on the AgInS 2 /ZnS solar cell showed an efficiency of 0.3%. [ABSTRACT FROM AUTHOR]

Sodium doped CuIn (1 − x) Ga x (Se (1 − y) Te y ) 2 (CIGST) thin films were grown on Mo-coated soda lime glass substrates by a two-stage technique. The effects of Te content and annealing temperature on the microstructural characteristics and phase segregation in CIGST thin films were studied. It was observed that presence of Ga in these films affected the kinetics of the reaction and phase formation and largely eliminated the separation of the Te-rich phase towards the Mo surface, which was previously observed under the two-stage processing conditions in films containing only Cu, In, Se and Te. Microprobe measurements showed that the smaller grain material visible through the gaps within the larger grain material had higher Ga and Se content compared to larger grains, which were richer in Te and In. For CIGS film formation, an annealing regime that involved a low temperature partial reaction step followed by a high temperature reaction step yielded better films compared to a process with a single high temperature step. However, for films containing both Se and Te, the two temperature processing approach caused excessive phase separation. [ABSTRACT FROM AUTHOR]

ZnSnP 2 is a promising candidate as a solar absorbing material consisting of earth-abundant and low-toxic elements. In this study, the phosphidation method, where co-sputtered Zn–Sn thin films react with phosphorus gas, was adopted for fabricating ZnSnP 2 thin films. To establish the conditions for producing ZnSnP 2 thin films, we investigated the influence of phosphidation temperature on the product phases, and interpreted the experimental results using chemical potential diagrams of the Zn–Sn–P system. ZnSnP 2 thin films with a single phase were obtained by phosphidation at 500 °C under a phosphorus vapor pressure of 10 − 2 atm. However, formation of ZnSnP 2 protrusions was observed on the surface of the thin films. Based on the experimental results and the chemical potential diagrams, it is indicated that un-reacted liquid Sn particles reacted with Zn and phosphorus gas to form ZnSnP 2 protrusions in a manner similar to the vapor-Liquid-Solid growth mode. [ABSTRACT FROM AUTHOR]

The microstructure characteristics of the soda-lime-glass/Cu(In,Ga)Se 2 (SLG/CIGS) interface in Cu-poor CIGS films are investigated by transmission electron microscopy and selected area electronic diffraction (SAED). The SAED patterns show very sharp and strong spots, indicating the main structure of CIGS chalcopyrite. Small dispersed crystals with size distribution from 2 to 5 nm seem to be embedded in amorphous matrix, and additional spots indicate the presence of an ordered vacancy compound (OVC). This observation is consistent with the Raman results, and the OVC phase with the nanoclusters exists in the CIGS matrix, instead of layer structure. Lattice distortion results in local changes in contrast. Some pseudo-disordered structure is observed, however, the structure is actually the chalcopyrite CIGS structure. 180° rotation twins are also observed at the SLG/CIGS interface. Lattice distortion is widely observed at the interface of the Cu-poor CIGS films, and the extra spots could be caused by different lattice orientations. [ABSTRACT FROM AUTHOR]

We have investigated a specially grown Cu(In,Ga)Se 2 (CIGSe) absorber, which was deposited by co-evaporation of Cu, In, Ga, and Se using a modified three stage process. Prior to the growth, the molybdenum-coated glass substrate was covered by a bent shroud made from tantalum (Ta), leading to a wedged absorber structure with a width of about 2 mm where the film thickness varies from 0 to 2 μm. In this region of interest the thickness dependency of morphology, concentration ratios and electronic properties was studied with secondary electron microscopy (SEM), X-ray fluorescence (XRF) and hard X-ray photoelectron spectroscopy (HAXPES), probing the CIGSe sample along the thickness gradient. The evidence of the thickness gradient itself was proven with SEM measurements in cross section geometry. By using XRF it was found that with decreasing film thickness the Cu concentration decreases significantly. This finding was also verified by HAXPES measurements. Furthermore, an enrichment of Ga towards the Mo back contact was found using the same technique. Besides these results the formation of a molybdenum selenide (MoSe) phase was observed on the fully covered part of the Mo coated substrate indicating a high mobility of Se on Mo under the given temperature conditions of the modified three stage deposition process. [ABSTRACT FROM AUTHOR]

In this work an all nanoparticulate precursor for application in Cu(In 1 − x Ga x )Se 2 solar cell absorbers is presented. Binary Cu–In nanoparticles, Cu–Ga powder and elemental Se nanoparticles were mixed in dispersion and deposited on Mo-coated substrates. Research was focused on Cu(In 1 − x Ga x )Se 2 layer formation kinetics, phase composition characterised by differential scanning calorimetry and in-situ X-ray diffraction (XRD). Furthermore phase composition and morphology were studied by ex-situ XRD, Raman spectroscopy and scanning electron microscopy. The results revealed a fast consumption of the precursor and the formation of CuInSe 2 below 340 °C. Binary secondary phases were not observed at any temperature. [ABSTRACT FROM AUTHOR]

Cu-poor and Cu-rich Cu(In,Ga)Se 2 (CIGSe) absorbers were used as substrates for the chemical bath deposition of ultrathin CdS buffer layers in the thickness range of a few nanometers in order to make the CIGSe/CdS interface accessible by hard X-ray photo-emission spectroscopy. The composition of both, the absorber and the buffer layer as well as the energetics of the interface was investigated at room temperature and after heating the samples to elevated temperatures (200 °C, 300 °C and 400 °C). It was found that the amount of Cd after the heating treatment depends on the near surface composition of the CIGSe absorber. No Cd was detected on the Cu-poor surface after the 400 °C treatment due to its diffusion into the CIGSe layer. In contrast, Cd was still present on the Cu-rich surface after the same treatment at 400 °C. [ABSTRACT FROM AUTHOR]

CuInSe 2 (CIS) and Cu(In,Ga)Se 2 (CIGS) thin films were prepared by selenizing stacked Cu–In and In–CuGa metal layers, respectively, with selenoamide in a tube furnace at 400 °C at atmospheric pressure for solar cell fabrication. Chalcogen amides can allow safe transportation and handling of chalcogen sources that can produce chalcogen hydrides at reasonably low temperature. Selenoamide used in this study is stable at room temperature and decomposes into H 2 Se below 150 °C. Towards the bottom of the CIS thin films, In segregation was observed particularly during selenization and it was concluded to be due to diffusion of released liquid In towards the bottom of the films and upward diffusion of Cu driven by Cu 2 − x Se formation on the surface during the process. Homogenizing In in CIS thin films was realized by carrying out post-selenization annealing without the presence of Se at 500 °C for various time periods. On the other hand, Ga accumulated to the bottom of the CIGS thin films upon selenization creating a bandgap profile, which benefits collection of the minority carriers. The post-selenization annealing applied to CIGS thin films favored further intermixing between In and Ga while some degree of the bandgap profile was preserved. Solar cells fabricated with a soda–lime glass/Mo/CIS/CdS/ZnO/ZnO:Ga/Al structure exhibited power conversion efficiencies up to 1.6% without any further optimization. [ABSTRACT FROM AUTHOR]

Near stoichiometric chalcopyrite Cu-rich and In-rich CuInSe 2 thin films have been simultaneously grown on GaN (0001) by molecular beam epitaxy. Microstructure analysis of the Cu-rich and In-rich CuInSe 2 showed that the rotation twin was formed at the interface of CuInSe 2 /GaN, and no interface reaction occurs between CuInSe 2 and GaN. Our experimental results show that GaN is stable for the epitaxial growth of CuInSe 2 thin film, which exhibits a promising potential for optoelectronic applications. [ABSTRACT FROM AUTHOR]

We report the simultaneous deposition of two different metal precursors dissolved in liquid carbon dioxide (l-CO2), aiming to the synthesis of ternary chalcopyrite (e.g. CuInS2) nanoparticles on a mesoporous TiO2 film. The l-CO2-based deposition of Cu and In precursors and subsequent reaction with a dilute H2S gas resulted in CuxInySz nanoparticles uniformly deposited across the entire thickness of a mesoporous TiO2 film. Further heat treatment (air annealing and sulfurization) led to the formation of more stoichiometric CuInS2 nanoparticles. The formation of CuInS2 on TiO2 was confirmed by scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. The crystal growth of CuInS2 was also found to be controllable by adjusting the number of coating cycles of the l-CO2-based deposition. [ABSTRACT FROM AUTHOR]

Abstract: Sulfurization growth of single-phase chalcopyrite CuInS2 (CIS) thin films was demonstrated using less hazardous liquid metal–organic ditertiarybutylsulfide [(t-C4H9)2S: DTBS]. The effect of sulfurization temperature and DTBS flow rate on the structural and optical properties of CIS was analyzed by scanning electron microscope, X-ray diffraction, energy dispersive X-ray spectroscopy, and photoluminescence spectra. The measurement results indicated that the CIS film formed by this method was suitable as a photo-absorbing layer for CIS-based solar cells. [Copyright &y& Elsevier]

Abstract: Chalcopyrite Cu(In,Al)Se2 (CIAS) thin films are grown on stainless steel substrate through one-step electrodeposition at room temperature. Indium is partially replaced with aluminum to increase the band gap of CuInSe2 without creating significant change in the original structure. The deposition potential is optimized at −0.8V (vs. SCE) and annealing of the films is performed in vacuum to remove binary phases present in the as-deposited films. In/Al ratio is varied from 1/9 to 8/2, to find the suitability for solar cell fabrication. For In/Al ratio of less than 8/2, CuAlSe2 phase is formed in the film in addition to the CIAS phase. Depth profile X-ray photoelectron spectroscopy analysis of the CIAS sample prepared with In/Al ratio of 8/2 in the precursor solution confirmed the existence of single phase CIAS throughout the film. This film showed p-type conductivity while the rest of the samples with In/Al ratio less than 8/2 showed n-type conductivity. The band gap of the film varied from 1.06 to 1.45eV, with variation in deposition potential. Structural, optical, morphological, compositional and electrical characterizations are carried out to establish the suitability of this film for solar cell fabrication. [Copyright &y& Elsevier]

Abstract: CuInSe2 (CIS) films on Mo/soda-lime glass were prepared by selenization of sputtered precursors from Cu2Se and In2Se3 binary targets. Cu–In–Se precursors were sequentially sputtered with double-layered structure of both Cu2Se/In2Se3 and In2Se3/Cu2Se. The structural, compositional and electrical properties of annealed Cu–In–Se precursors were investigated for solar cell applications. Better adhesion between CIS and Mo back contact was observed in the double-layered structure of Cu2Se/In2Se3 at all working pressures. Less MoSe2 phase on both structures was generated at a lower working pressure of 0.67Pa. A CIS solar cell with Al/Al-doped ZnO/i-ZnO/CdS/CIS (900nm, Cu2Se/In2Se3 at 0.67Pa)/Mo/soda-lime glass was continuously fabricated, and its conversion efficiency was 4.6%, with 346mV of open circuit voltage, 30.5mA/cm2 of short circuit current density and 43.5% fill factor. [Copyright &y& Elsevier]

Silver Indium Di-sulfide (AgInS2) thin films are deposited using ultrasonic spray pyrolysis technique and the effect of substrate temperature (Ts) on film growth is studied by varying the temperature from 250 to 400°C. From the structural analysis, orthorhombic AgInS2 phase is identified with preferential orientation along (002) plane. Further analysis with Raman revealed the coexistence of Cu–Au ordered and chalcopyrite structures in the films. Stoichiometric films are obtained at Ts of 300°C. Above 300°C, the film conductivity changed from p to n-type and the grain size decreased. The band gap of AgInS2 films varied from 1.55 to 1.89eV and absorption coefficient is found to be >104 cm−1. The films have sheet resistance in the range of 0.05 to 1300Ω/□. Both p and n type films are prepared through this technique without any external doping. [ABSTRACT FROM AUTHOR]

Abstract: Chalcopyrite CuIn1−xGaxSe2 (CIGS; x<0.3) nanoparticles were synthesized by reacting CuCl, InCl3, GaCl3 and Se in oleyl amine over 250°C. Depending on the reaction temperature and duration, the obtained nanoparticles had sizes of less than 100nm and different chemical compositions. Because the atomic percentage of Se decreased with increasing reaction time, the proportion of Se and Ga content also changed. Furthermore, the lattice parameters, a and c, changed with increasing reaction temperature and time. Lastly, the bandgap energies of the CIGS films coated on glass plates were~0.98eV and did not significantly change with increasing heat-treatment temperature. [Copyright &y& Elsevier]

Abstract: We report a study of CuGa–In metallic precursors for chalcopyrite thin film. CuGa and In thin films were prepared by DC sputtering at room temperature. Due to low melting point of indium, the sputtering power on indium target was optimized. Then, CuGa and In multilayers were annealed at low temperature. At 120°C, the annealing treatment could enhance diffusion and alloying of CuGa and In layers; however, at 160°C, it caused a cohesion and crystalline of indium from the alloy which consequently formed irregular nodules on the film surface. The precursors were selenized to form copper indium gallium selenide (CIGS) thin films. The morphological and structural properties were investigated by scanning electron microscopy, X-ray diffraction and Raman spectra. The relationships between metallic precursors and CIGS films were discussed in the paper. A smooth precursor layer was the key factor to obtain a homogeneous and compact CIGS film. [Copyright &y& Elsevier]

The roughness and the morphology of chalcopyrite Cu(In,Ga)S2 (CIGS) films on different substrates prepared by dc reactive magnetron co-sputtering with thicknesses ranging from 10 to 1000nm were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy (AFM). One-dimensional power spectral density (1DPSD) functions, derived from the AFM profiles, were used to compare the microstructure scaling behavior of the thin films. For CIGS films on Mo coated glass substrates, the variation of the grain size (dg) obtained from the AFM measurements, with film thickness (df) in the columnar grained films was dg(AFM)~df 0.33. The growth exponent β, characterizing the roughness evolution of the films, changes with the film thickness. The root-mean-square roughness (Rrms) increases only slightly as Rrms ~df β (β=0.13±0.09) in the initial growth stage, while Rrms increases with β=0.56±0.03, when the film thickness is larger than 50nm. 1DPSD analysis showed three spatial-frequency regimes of the roughness evolution during room temperature deposition, where the intermediate region disappears when the substrate temperature is increased. The nucleation and growth mechanisms are discussed in terms of surface diffusion, grain growth and shadowing effects. The substrate morphology does not significantly affect CIGS growth, which was proved for TiN and float glass. From the viewpoint of surface roughness and film growth, TiN is comparable to molybdenum and is suited as a potential back contact for chalcopyrite solar cell applications. [ABSTRACT FROM AUTHOR]

Abstract: A photo-assisted one-step electrodeposition has been applied to help in forming smooth and dense CuInSe2 (CIS) films. The difference in surface morphology and crystalline quality between CIS films with and without photo-assistance has been investigated. In the photo-assisted electrodeposition process, a halogen lamp providing maximum light intensity at about 0.6μm–1.0μm was used as light source to be irradiated onto the surface of Mo-coated soda lime glass substrates. Electrodeposited CIS thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectroscopy. SEM and AFM results show a smoother film with lower roughness by photo-assisted electrodeposition at lower deposition potential. From the XRD patterns, it was found that photo-assistance enhanced the crystalline quality, and the enhancement remained after annealing at 500°C for 120s. The analysis of Raman spectra indicated a reduction in secondary phases after applying photo-assistance. These results suggested effects of photo-assistance including activating surface diffusion and growing high-crystalline quality films with reduced defects during electrodeposition. [Copyright &y& Elsevier]

Abstract: Metastable behaviour in chalcopyrite-based solar cells is often related to the presence of intrinsic defects in the absorber layer which can change configuration through thermally activated transitions. The facilities to model these defects have recently been introduced in the software SCAPS (a one-dimensional solar cell simulator developed at the University of Gent and freely available to the research community). The presence of these defects leads to strongly non-uniform defect densities which can explain admittance spectroscopy measurement results under different bias conditions. In particular the appearance of a peak with a characteristic energy of about 0.13eV which only appears when the spectrum is measured under forward bias conditions can be related to the presence of the IIICu-defect. [Copyright &y& Elsevier]

Abstract: Electrical properties of thin film photovoltaic modules exhibit metastable behavior when exposed to light, when kept in the dark after light exposure and as a result of external bias. These effects vary in magnitude and expression and can mainly be observed in CdTe and chalcopyrites. The light-soaking induced effect in these technologies was analyzed for electrical module parameters such as open circuit voltage (Voc), short circuit current, fill factor (FF), maximum power point (Pmpp) and the current–voltage characteristic slope at Voc. The influence of the light-soaking irradiance and the duration of the illumination at constant temperature was investigated. Furthermore, light-soaking with additional external bias was examined. Overall, all modules showed metastable behavior with increases in the FF, the Voc and the Pmpp after light-soaking. The magnitude varied for different technologies and was largest for the investigated CdTe modules. Differences among the technologies could be observed for the change in Voc. While CdTe exhibited a significant increase in both FF and Voc, Cu(In,Ga)Se2 (CIGS) modules mainly showed an increase in the FF. Differences in metastable behavior were also observed for relaxation times in the dark. While CdTe modules returned to the initial levels in several hours, it took several weeks for the CIGS modules to relax. The presented experiments confirm that metastabilities reported on cell behavior also occur at the module level. [Copyright &y& Elsevier]

Abstract: To enhance the conversion efficiency of thin film photovoltaics, chalcopyrite based absorber materials are usually substituted in the cation and anion lattice to yield an absorber with a graded bandgap composed of mixed pentanary crystals Cu(In,Ga)(Se,S)2. Applying in-situ investigations during the crystallisation of the chalcopyrite is a prerequisite to understand the formation of inhomogeneities and elemental gradients caused by the growth process. The variation of anions yields a combined crystallisation path due to metal sulphoselenides with chalcogen exchange reactions upon heating, i.e. copper sulphoselenides act as a chalcogen buffer and afford substitution of S with Se during annealing. The extended chalcogen substitutions occur due to the complete solid solution of S and Se in Cu(S,Se) and Cu2−x(S,Se). On the contrary, different reaction paths are found for the cations. Although the metals show a good intermixture within the intermetallic alloys featuring In–Ga substitutions, no formation of a mixed or ternary (In,Ga)-chalcogenide is observed. This can be attributed to a narrow solid solution range of the (In,Ga)-chalcogenides. As a consequence the preceding formation of an In-rich and the delayed crystallisation of a Ga-rich chalcopyrite cause a vertical elemental gradient with Ga-accumulation near the back electrode. An interdiffusion of the In- and Ga-rich chalcopyrites is observed for increasing annealing time and temperature. The presented study is focused on quantification of the phase evolution, variation of lattice parameter and diffusion of elements as obtained by Rietveld refinements. [Copyright &y& Elsevier]

Abstract: The following paper presents a processing route for Cu(In,Ga)Se2 absorber layers that is based on nanoparticle dispersions which are applied by doctor blade deposition and converted with elemental selenium vapors. In particular, the preparation of the precursor layers is investigated by systematically assessing the influence of the stacking sequence of mono- and multi-metallic layers on sintering, elemental distribution and solar cell efficiency. By applying suitable stacking sequences, precursor layers with both local Cu-rich and over-all Cu-poor stoichiometry could be prepared that allowed improved sintering properties and modifications of the gallium gradient. Despite the still prevailing porosity of the absorber layer, solar cells with efficiencies exceeding 5% could be obtained. [Copyright &y& Elsevier]

Abstract: In this work CuInSe2 (CISe) thin films were fabricated by rapid thermal processing of printed CuIn nanoparticles and thermally evaporated selenium as precursor layer. Research is focused on real-time investigations such as in-situ X-ray diffraction and dynamic scanning calorimetry. These measurements show CISe formation starting at 300°C. Significant amount of intermediate phases as reported for state-of-the-art rapid thermal processing of stacked elemental layer was not observed. The morphology of the nanoparticulate layers was examined by field emission scanning electron microscopy. Grain size developed from nanosized binary CuIn nanoparticles to microsized CISe grains. Porosity decreases in the temperature range from 340°C to 540°C. [Copyright &y& Elsevier]

Abstract: Laser scribing of the Mo back electrode is commonly applied to define the cell structure of Cu(In,Ga)(Se,S)2 (CIGSSe) thin film solar cells. The patterning process was performed on laboratory samples using ns and ps pulse length laser processes. After structuring, CIGSSe absorbers were processed by rapid thermal processing (RTP) of stacked elemental layer precursors. Microscopic inhomogeneities were investigated on different sample positions. For samples structured with ns pulse, the absorber morphology in the laser line vicinity is different as compared to the morphology in the unstructured cell area. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show significant changes in the absorber grain size and chemical composition. Close to the laser line, the typically observed Ga accumulation on the back contact is less pronounced and more Ga is incorporated closer to the surface leading to a smaller grain size. The observed changes are attributed to partial damaging of a diffusion barrier between glass and Mo induced by the ns laser process, which allows diffusion of sodium from the glass substrate into the absorber during RTP. The enhanced Ga incorporation closer to the surface is an indication for the influence of sodium on the local phase development during RTP. The damages of the diffusion barrier can be effectively prevented by the application of a ps laser scribing process. CIGSSe absorbers processed on samples structured with ps pulse length do not show the described microscopic inhomogeneities around the laser line. [Copyright &y& Elsevier]

Abstract: Although kesterite solar cells show the same range of band gaps as the related chalcopyrites, their efficiencies have so far reached only 10%, compared with 20% for the chalcopyrites. A review of the present literature indicates that several non-ideal recombination channels pose the main problem: (i) recombination at the interface between the kesterite and the CdS buffer. This is very likely due to an unfavourable cliff-like band alignment between the absorber and the buffer. However, for pure selenide absorbers, this recombination path is not dominating, which could be due to a spike-like band alignment at the absorber–buffer interface. (ii) A second major recombination becomes obvious in a photoluminescence maximum well below the band gap, even in record efficiency absorbers. This is either due to a very high density of defects, comparable to the density of states in the band, or to stannite inclusions. In view of the phase diagram, secondary phases are not likely the source of the low energy emission. Only in sulphide kesterite a non-stoichiometric SnS phase could also cause this low energy radiative recombination. [Copyright &y& Elsevier]

Abstract: CuInSe2 (CISe) is a prototype material for the I–III–VI chalcopyrites such as Cu(In,Ga)(S,Se)2 used as absorber layers in thin film photovoltaic cells. Carefully-controlled pulsed-laser annealing (PLA) is a unique annealing process that has been demonstrated to improve the device performance of chalcopyrite solar cells. Here, we investigate the changes in defect populations after PLA of electrochemically-deposited CISe thin films previously furnace annealed in selenium vapor. The films were irradiated in the sub-melting regime at fluences inducing temperatures up to 840±100K. Deep-level transient spectroscopy on Schottky diodes reveals that the activation energy of the dominant majority carrier trap changes non-monotonically from 215±10meV for the reference sample, to 330±10meV for samples irradiated at 20 and 30mJ/cm2, and then back to 215±10meV for samples irradiated at 40mJ/cm2. A hypothesis involving competing processes of diffusion of Cu and laser-induced generation of In vacancies may explain this behavior. [Copyright &y& Elsevier]

Abstract: The Cu2Se–In2Se3 system comprises several copper indium selenide (CIS) compounds with solar-matched bandgaps along with the optimum properties of the CuInSe2 compound. This work investigates electrochemical growth of CIS films under various conditions, initially identified with cyclic voltammetry. The film growth, monitored with X-ray fluorescence analysis, shows excellent composition and thickness uniformity. The results agree with secondary ion mass spectroscopy profiles and X-ray diffraction data, indicating the conversion of initially formed binary phases to homogenous ternary compound. Deposition potential and substrate/electrolyte interface control the film formation mechanism and hence its composition. Electrolyte composition and agitation influence the film thickness. Judicious combination of process parameters is essential to obtain CIS films with optimum properties. [Copyright &y& Elsevier]

Abstract: CuGaSe2 thin films have been prepared by one-step electrodeposition and rapid thermal annealing process. According to composition and morphology analysis, deposition potential of −0.6V vs. SCE is considered to be optimum for electrodeposition. From the X-ray diffraction and Raman studies, the as-deposited film exhibits poor crystallinity without the evidence of CuGaSe2 or other Ga-containing phases, while the rapid thermal annealing-treated film shows chalcopyrite structure CuGaSe2 phase containing MoSe2 phase between the Mo substrate and the absorber and minor second phase Cu2− x Se. The obtained CuGaSe2 thin film has a band gap of about 1.68eV and p-type conductivity. [Copyright &y& Elsevier]

Abstract: In an effort to eliminate the CdS buffer layer and its costly preparation process we are considering sputtered buffer layers. In particular, we report in this contribution on the reactive sputtering of wide gap Zn(O,S) compound semiconductors and their application in solar cells with different types of chalcopyrite absorbers. While we were able to freely adjust the composition through the oxygen partial pressure, the structural and optical properties are superior when the composition is close to the ternary endpoints. Open circuit voltage and short circuit current density as a function of Sulphur content in the buffer show opposite trends. Working cells were achieved with low band gap as well as wide band gap absorbers, however, their performance is so far inferior to that of the standard stacks. [Copyright &y& Elsevier]

Abstract: Reactive sputtering is an option to further reduce costs associated with the deposition of the transparent front contact for chalcopyrite-based solar modules. Our approach here is to develop a proof of concept for a ZnO window, where the i-ZnO and the doped ZnO:Al are both sputtered in a reactive process. It is shown, that on cell level the device performance is preserved when replacing the standard RF processes with the fully reactive process. It is also shown, that the series resistance of module test structures increases with reactively sputtered ZnO. This finding, as well as a reduced damp heat stability of mini modules without encapsulation are tentatively assigned to an increased contact resistance and corrosion at the Molybdenum/ZnO interface within the interconnects. [Copyright &y& Elsevier]

Abstract: Properly encapsulated chalcopyrite-based solar modules from a number of manufacturers have passed the accelerated ageing tests in damp heat. Non-encapsulated modules after damp heat exposure show an increased series resistance which is contributed significantly from the increase in lateral sheet resistance of the transparent Al-doped zinc oxide (ZAO) front contact. We have shown previously, that a rough substrate morphology (on a μm-scale) is a major influence in the ZAO degradation mechanism, due to local perturbations of the ZAO growth (extended grain boundaries). To further model and examine the extended grain boundaries we present a study of the electrical properties of RF-sputtered ZAO films before and after damp heat, grown on rough quartz glass as well as on polished, texture-etched and patterned silicon. The pattern consisted of equidistant trenches and have a periodical two-dimensional geometry. The strongest decrease of ZAO conductivity occurs on the patterned silicon substrate perpendicular to the trenches. The conductivity parallel to the trenches, however, had the same trend as the conductivity of ZAO on smooth silicon. Measured activation energies of the conductivity of 40meV or less after damp heat are not sufficient to explain the drastic decrease of conductivity in terms of a grain-boundary-barrier theory (as described, e.g., by Seto). We will summarise our current understanding of the extended grain boundaries and present a refined barrier model to explain our experimental observations. [Copyright &y& Elsevier]