Cerium activated ternary strontium thiogallate phosphor thin films have been deposited by multi-source evaporation of elemental strontium, gallium and sulfur as source materials. Cerium chloride (CeCl3) has been used as activator starting material. In order to avoid SrS as well as Ga phase segregations elemental In as well as compounds, such as LiF, NaF, LiCl, and SrCl2, have been investigated as possible growth enhancing agents. Coevaporation of indium resulted in an improved crystallization with the drawback of an increased reabsorption of the cerium emission. The other additives lead to improved crystal quality combined with true-blue cathodoluminescence (CL) performance (C.I.E. color coordinates of up to 0.14/0.12) which proves phosphor layers without any SrS phase segregations. Thin film electroluminescent (TFEL) devices with SrGa2S4:CeCl3 phosphor layers prepared with LiF as additive exhibit highest ever reported L40 luminances of 78 cd/m2 and luminous efficiencies of 0.017 lm/W at a transferred charge of 1 µC/cm2 with 1 kHz / 50 µs bipolar square wave excitation.

This article investigates the influence of damp heat treatments on the electronic properties of ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. We observe losses of around 20% (50%) in the fill factor and between 5% (10%) in the open circuit voltage after 100 h (1000 h) of damp heat exposure. Temperature dependent current voltage measurements point to recombination in the space charge region as the dominant loss mechanism before and after damp heat treatment. Admittance spectroscopy on the heterostructures indicates a reduction of type inversion at the CdS/Cu(In,Ga)Se2 interface as well as an increase of deep defect levels in the Cu(In,Ga)Se2 film upon damp heat tests. We ascribe the losses in the fill factor to the detected changes at the interface and the losses in the open circuit voltage to the increased defect density in the absorber.

We investigate the influence of Na on the growth of Cu(In,Ga)Se2 thin-films by three model experiments. First, we examine the influence of Na on the Se activity during selenisation of Mo films on soda-lime and borosilicate glass after growth and after thermal treatments. Second, we analyse the location and possible binding partners of Na in polycrystalline Cu(In,Ga)Se2 prior to and after air-exposure. The final experiment focuses on the identification of the chemical state of Na on the surface of as grown and air-exposed films. Our experiments demonstrate that Na influences the growth of CIGS Cu(In,Ga)Se2 due to its interaction with Se. In non-air-exposed films Na is mainly localised in the form of sodium-polyselenides (Na2Sex) at the grain boundaries. We conclude that Na2Sex acts as Se-reservoir during film formation and oxidation.

The chemical effects of oxygenation of Cu(In,Ga)Se2 (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the ‘good’ oxygenation effect from the ‘bad’ ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of CdCu interface donors and possibly a removal of OSe interface acceptors. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells.

In-rich CuInSe2 (CIS) and CuIn(Ga)Se2 (CIGS) with Ga/(Ga+In)=28% were annealed in air at 400°C. After annealing the Ga-free CIS layer, the broad photoluminescence (PL) spectrum changes to a structured spectrum which is identical to that of a Cu-rich layer. Annealing of In-rich films causes the passivation of donors and the reduction of the high compensation. The change in the PL spectrum and the clear reduction of compensation cannot be seen for the Ga-containing CIGS layers. Only a slight blue shift of the spectrum and an increase of the full width at half maximum is observed after air annealing. However, photoluminescence excitation experiments reveal a decrease of the band-tail character of the absorption edge which can be interpreted as a decrease in compensation. Air-annealing causes a strong reduction of the Ga-concentration in the CIGS layer. The Ga-loss can be detected by X-ray diffraction and secondary ion mass spectroscopy. Ga segregates as oxide phase on top of the crystal and is analysed with X-ray photoelectron spectroscopy. The thickness of that oxide layer was estimated to be 200 nm.

Post-deposition air-annealing effects of Cu(In,Ga)Se2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 °C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells shows that the Cu(In,Ga)Se2 surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 °C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent with the release of Cu from the absorber surface and its redistribution in the bulk.

We compare the consequences of two different approaches for post-deposition treatments of ZnO/CdS/Cu(In,Ga)Se-2 heterojunction solar cells on their electronic properties: (i) air annealing of the bare, polycrystalline absorber and (ii) air annealing of the completed heterostructure. both in dry air at 200 degrees C. We use temperature dependent admittance spectroscopy to gain information on the position of the electron Fermi level at the CdSiCu(In,Ga)Se-2 interface and on the width of the space charge region within the Cu(In,Ga)Se-2 absorber layer of the differently oxidized samples. Changes which occur after annealing of the bare polycrystalline absorbers in dry air at 200 degrees C can be explained by assuming passivation of donor-like states at grain boundaries with oxygen. Annealing of the completed heterostructure has: a complementary impact on the electronic properties of the devices. Here, we find that annealing leads to a decrease of the space charge density. Field- and temperature-enhanced Cu-migration from the surface into the bulk of the absorber material has to be invoked to explain this result.

The oxidation of Cu-poor Cu(In,Ga)Se-2 thin films in air is investigated using x-ray photoelectron spectroscopy and secondary ion mass spectrometry. The thermal oxidation in air at 200 degrees C leads to the formation of In2O3, Ga2O3 and SeO2 at the surface. The oxidation at room temperature is strongly influenced by the air humidity and leads additionally to the formation of free selenium. Furthermore, the results show that the short-term thermal oxidation affects mainly the surface and the long-term room temperature oxidation affects the bulk as well. The oxidation of the absorber layer in humid air has a negative effect on the performance of Cu(In,Ga)Se-2/CdS/ZnO solar cells.

Semiconductor quantum dots and wires are the subject of great interest, mainly due to their size-dependent electronic structures, in particular increased band gap and therefore optoelectronic properties. We have electrodeposited films of size-quantized CdS (∼4 to 5 nm cross section by 15 nm height) as a buffer layer on CuInS2. The resulting CuInS2/CdS thin-film solar cells gave increased photocurrents and higher light-to-electricity conversion efficiencies (>11%) than those made with conventional nonquantized CdS films. This was due mainly to the increased band gap of the quantized CdS, allowing more light to reach the active CuInS2 layer.

Heterojunctions based on CuGaSe2 are under investigation for photovoltaic applications since 1977, but their efficiency was limited mainly due to low open circuit voltages with respect to the band gap. In order to characterize this junction more detailed we present in this paper the analysis of the bandoffset between CuGaSe2 and CdS deposited by chemical bath deposition. X-ray photoelectron spectroscopy measurements reveal a valence band offset of Delta E-v = 0.9 eV between the absorber and the buffer layer. We also correlate those measurements to investigations of the dominant recombination mechanism in dependence of the substrate material and the Chemical bath CdS recipe. Ultraviolet Photoelectron Spectroscopy measurements exhibit an energetic difference of 0.8 eV between the Fermi-level and the valence band maximum indicating no conduction type inversion at the surface. Temperature and intensity dependent current-voltage measurements prove that junction related recombination processes are responsible for the voltage limitation.

The conversion efficiency of thin film solar cells based on Cu-rich grown CuInS2 is limited due to the electrically active absorber/buffer interface as well as due to the grain boundaries. The investigations carried out show that low admixtures of Zn to CuInS2 result in an enhanced Zn-concentration at the surface compared to the bulk of the film. This Zn-incorporation leads to an improved open circuit voltage exceeding 800 mV. Moreover, thermally activated recombination processes become dominant over a wider temperature range and the influence of the photosensitivity of the CdS buffer layer decreases. The conversion efficiency (eta=10.4%) is limited by inferior transport properties till now.

CuInS2 thin films were prepared by sulfurization of sequentially deposited CuIn stacks with elemental sulfur. Precursors as well as reacted films are characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). Excess copper is required for optimum properties and leads to a CuS secondary phase segregated at the surface. Stoichiometry is regained by etching after which heterojunctions are formed by deposition of a CdSZnO window layer. An active area efficiency of 10.4% has been achieved.

Cerium activated ternary strontium thiogallate phosphor thin films have been deposited by multi-source evaporation of elemental strontium, gallium and sulfur as source materials. Cerium chloride (CeCl3) has been used as activator starting material. In order to avoid SrS as well as Ga phase segregations elemental In as well as compounds, such as LiF, NaF, LiCl, and SrCl2, have been investigated as possible growth enhancing agents.Coevaporation of indium resulted in an improved crystallization with the drawback of an increased reabsorption of the cerium emission. The other additives lead to improved crystal quality combined with true-blue cathodoluminescence (CL) performance (C.I.E. color coordinates of up to 0.14/0.12) which proves phosphor layers without any SrS phase segregations. Thin film electroluminescent (TFEL) devices with SrGa2S4:CeCl3 phosphor layers prepared with LiF as additive exhibit highest ever reported L(40) luminances of 78 cd/m(2) and luminous efficiencies of 0.017 1m/W at a transferred charge of 1 mu C/cm(2) with 1 kHz/50 mu s bipolar square wave excitation.

The goal of the project was the development of thin film solar cells with a small consumption of semiconductor material and at the same time high efficiency. Thin films of the compound semiconductors CuInSe{sub 2}, CuGaSe{sub 2}, CuInS{sub 2} and their alloys were prepared by simultaneous evaporation of the elements. These films were analysed in view of the use as absorber layers in thin film solar cells. Heterojunctions for solar cells were fabricated with ZnO as window layer. A main emphasis was the possibility to transfer the deposition process to large areas. The development of thin film solar cells reached a very high level also compared to international activities: Solar cells with the conventional structure Cu(In, Ga)Se{sub 2}-CdS-ZnO reached efficiencies of 16-17% (active area 0,5 cm{sup 2}). With minimodules (100 cm{sup 2}) with 15 monolithically series connected cells 13,9% (aperture area) were realised. Structures with Cd-free window layers could be fabricated with an efficiency of over 15%. Using CuInS{sub 2} as absorber material efficiencies of 12% and an open circuit voltage of more than 0.8 V were reached. With CuGaSe{sub 2} as absorber layer the efficiency could be increased to 9,3% at an open circuit voltage of 0.87 V. These results provide the prerequisites for the upscaling of the technology to large area production as it was demonstrated by the successful transfer to the Centre for Solar Energy and Hydrogen Research (ZSW). Furthermore, these results are an excellent basis for the further development of this type of solar cells. (orig.) [Deutsch] Ziel des Vorhabens war die Entwicklung von Duennschichtsolarzellen mit geringem Aufwand an Halbleitermaterialien und gleichzeitig hohem Wirkungsgrad. Hierzu wurden Duennschichten aus den Verbindungshalbleitern CuInSe{sub 2}, CuGaSe{sub 2}, CuInS{sub 2} und deren Legierungen als Absorberschicht vor allem mittels simultaner Verdampfung der Elemente hergestellt und untersucht. Heterouebergaenge fuer Solarzellen wurden mit ZnO als Fensterschicht realisiert. Ein wesentlicher Gesichtspunkt war die Umsetzbarkeit der Abscheideverfahren fuer die Halbleiterduennschichten auf grosse Flaechen. Bei der Entwicklung von Solarzellen konnte ein, auch im internationalen Vergleich, sehr hoher Stand erreicht werden: Solarzellen mit der konventionellen Struktur Cu(In, Ga)Se{sub 2}-CdS-ZnO erreichen auf kleiner Flaeche (aktive Flaeche ca. 0,5 cm{sup 2}) Wirkungsgrade von 16-17%. Mit Kleinmodulen (100 cm{sup 2}) mit 15 monolithisch in Serie verschalteten Zellen konnte bis auf 13,9% (bezogen auf die Aperturflaeche) realisiert werden. Strukturen ohne Cd-haltige Fensterschicht konnten mit einem Wirkungsgrad von ueber 15% hergestellt werden. Mit CuInS{sub 2} als Absorbermaterial wurden Wirkungsgrade von 12% und Leerlaufspannungen von mehr als 0,8 V erreicht. Mit reinem CuGaSe{sub 2} als Absorberschicht war eine Steigerung auf 9,3% bei einer Leerlaufspannung von 0,87 V moeglich. Diese Ergebnisse liefern die Voraussetzungen fuer die Umsetzung der Technologie auf die Herstellung grossflaechiger Module, wie der erfolgreiche Transfer an das Zentrum fuer Sonnenenergie- und Wasserstoff-Forschung (ZSW) zeigte. Ferner bilden die Ergebnisse eine sehr gute Basis fuer die Weiterentwicklung dieses Solarzellentyps. (orig.)

In this work, the nucleation and growth of the CdS buffer layer (chemical bath deposition) on Cu(In,Ga)Se2. (CIGS) thin films was observed using the surface-specific methods of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). The AFM images show islands at short deposition times (<30 s) whereas XPS measurements do not register any sulfur on these surfaces. The authors postulate that Cd(OH)2 precipitates out of the basic deposition solution while rinsing it off the CIGS surface, thus creating the islands. All methods used here indicate the onset of bulk CdS growth after about 30 s deposition time. According to the AFM images, the entire CIGS surface seems to be covered within 1 min deposition. This is confirmed by SE, which shows bulk layer growth after 1 min; it increases at about 0.4 nm/s after the induction phase

The conversion efficiency of thin film solar cells based on CuInS 2 (η=12%) is mainly limited by a moderate open circuit voltage (≈720 mV). This limitation can be overcome by modifying the absorber/buffer interface leading to open circuit voltages exceeding 800 mV. The addition of ZnS to the CuInS2 as well as adjusting the preparation conditions for the CdS-buffer leads to an increased V oc. The coevaporation of ZnS or CdS additives and diffusion from precursor layers for two types of fabrication processes has been examined: codeposition of the elements and diffusion of Cu and S into In xSy layers. The addition of ZnS leads to Zn-rich segregations on the CuInS2 surface. No shift of the bandgap due to the Zn incorporation could be measured. Additionally, an improved sulfur incorporation using the binary In2S3 as the In and S source was found

The luminescence properties of lead ions and their sensitizing effect on cerium activators in strontium sulfide thin-film electroluminescence (TFEL) devices have been investigated. Polycrystalline SrS:Pb and SrS:Ce,Pb thin active layers for such TFEL devices have been prepared by multi-source deposition. Emission spectroscopy under application of a high electric field (EL), under e-beam excitation [cathodoluminescence (CL)], and under CL conditions with additional EL drive has been carried out. It has been found that the luminescence of SrS:Pb under EL drive is very weak, while the CL signal is considerably higher. This CL signal has shown strong quenching effects upon application of an additional ac voltage. The Pb2+ emission dropped to some 5% at an applied voltage of 100 V0p, being roughly 40 V below EL threshold. More than half of this drop occurred between 0 and 30 V0p. A remarkably lower quenching effect on the Ce3+ activator luminescence has been observed. Such luminescence quenching is attributed to the field induced ionization of the luminescent ions. From the obtained results it is concluded that the optical energy transfer from Pb2+ sensitizers to Ce2+ activators in SrS TFEL devices is low.

Sequential processes for the fabrication of polycrystalline Cu(In,Ga)(S,Se)(2) thin films using (In,Ga)(x)(S,Se) precursors are presented and discussed. Depending on the deposition conditions and the source material, different InxS precursor films for the formation of CuInS2 can be obtained. For In/(In + Cu)>0.5 the reaction pathway of CuInS2 thin films follows the Cu2S-In2S3 pseudobinary tie line after the indiffusion of Cu and S independent of the composition of the InxS precursor. A Cu-rich step resulting in the segregation of CuS is accompanied by a recrystallization of the entire film. The coverage of CuInS2 with CuS is more complete for the sequential process as compared to codeposited thin films. The device performance of devices based on Cu(In,Ga)(S,Se)(2) grown by codeposition and by sequential processes is comparable.

The flux-assisted deposition of the active phosphor layer in the highest efficiency, true blue (CIE coordinates of x = 0.14y = 0.12) electroluminescent (EL) devices based on SrGa2S4:Ce,Cl is described. Luminances of 78 cd/m2 luminous efficiencies of 0.017 lm/W at a transferred charge of 1 μC/cm2 with 1 kHz/50 μs bipolar square wave excitation were achieved. The influence of the coevaporated fluxes LiF and NaF in addition to the elements Sr, Ga, S, and the activator starting material CeCl3, on film growth is examined. The relation between GaSr, SSr, fluxSr rates, and substrate temperature (TSub) on phase homogeneity due to flux, crystal quality, film thickness, and device performance is described.