The “roll-over” phenomenon in current-voltage (J-V) curves of CdS/CdTe devices is recognized as a result of the formation of a higher back barrier. When Cu has not been intentionally added to the back contact, roll-over is understandable. However, the mechanism was unclear for forming J-V roll-over in a CdTe cell with a back contact containing Cu. We did extensive characterizations, including XRD, XPS, SIMS, TEM, and EDS, and “recontact” experiments to understand this phenomenon. The results show that the roll-over comes from the formation of Cu-related oxides at the back side of the device during processing, rather than the diffusion of Cu to the front side of the device. Discussions related to the J-V roll-over mechanisms will also be presented.

We present new insights into inhomogeneities of Cu(In,Ga)Se2 absorbers by confocal optical transmission (OT) and spectrally resolved photoluminescence (PL) with lateral resolution in the one-micron regime. We carry out scans for both measurands at identical positions and observe explicit local variations of the PL- and OT-spectra. The PL-yield shows fluctuations in the sub- and few-µm regime. As shown in previous papers we extract local variations of the splitting of quasi-Fermi-levels of about 40-50meV, which limits the open circuit voltage of cells, and of the optical threshold of the absorber of some tens of meV related to the local optical band gap. Local transmission spectra similarly show lateral variations in the few-µm regime. From these spectra we calculate local absorption coefficients and absorbance functions which yield variations of the optical threshold of the absorber of 20-30meV, amounting to the same range as PL-results. Via Planck's generalized law we are able to compute theoretical PL-spectra from the resulting absorbance functions. The discrepancies between measured and calculated hypothetical PL-spectra are discussed with respect to diffusion of excess charge carriers and fluctuations of the band gap in the sub-µm-regime below the spatial resolution of the experiment.

A dry method for the growth of highly-structured Cu-containing chalcopyrite material on solid substrates, based on the use of metallic precursors, is described. Nanocrystals, sub-micrometer polycrystalline dots, and macroscopic clusters have be grown, either as isolated units or alternatively as embedded structures in a matrix of a binary chalcogenide compound, by adjusting processing parameters. Vapor-liquid-solid (VLS) induced growth has been used for the growth of chalcopyrite nanowires. Examples of material characterization by scanning probe techniques are shown, demonstrating the suitability of the proposed growth method.

We report photovoltaic cell structures on SnO2:F (TCO) coated glass substrates. Thin films of CdS, SnS, and CuS or PbS were deposited sequentially from chemical baths to produce the solar cell structures: SnO2:F-CdS- SnS (A)-CuS-Ag; SnO2:F-CdS- SnS (A)-PbS-Ag; and SnO2:F-CdS- SnS (B)-PbS-Ag. Heating SnS-CuS films results in the formation of Cu2SnS3, and sequential depositions of SnS and PbS to obtain solar cells produce stratified layers as required for solar cells. The photovoltaic characteristics, Voc 340 mV and Jsc 6 mA/cm2 in these structures suggest that absorber thin films based on tin sulfide are worth investigating as a relatively abundant and non-toxic material for solar cells.

Chalcopyrite-type thin films - CuInS2, CuInSe2, CuGaSe2, and Cu(In,Ga)Se2 - in various completed solar cells were studied in cross-section by means of electron-backscatter diffraction (EBSD). Valuable information on grain sizes, local grain orientations, film textures, and grain boundaries were extracted from the EBSD linescans and maps. The grain-size distributions from the chalcopyrite-type thin films can be represented well by lognormal distribution functions. The EBSD measurements on CuGaSe2 thin film reveal a <110> fiber texture, in good agreement with x-ray diffraction texture analysis performed on the same sample. The EBSD maps from all samples studied exhibit considerable twinning in the chalcopyrite-type thin films. Indeed, the most frequent types of grain boundaries in these thin films are (near) Σ3 60°-<221> and 71°-<110> twins. It is shown that rotational 180°-<221> twins (which are symmetrically equivalent to 71°-<110>) are more frequently found than anion- or cation-terminated 60°-<221> twin boundaries.

Coincidence site lattice (CSL) modelling has been applied to determine the orientations and atomic structures of first order twin boundaries in the chalcopyrite structure. A rotation of 250°32' about [110] generated a CSL similar to that for the Ó = 3 sphalerite boundary, but with one in two cation columns being metal-metal anti-sites. The models predict one coherent boundary type i.e. (112)M-(112)T, and four lateral types, (110)M-(118)T, (1110)M-(112)T, (114)M-(114)T, (001)M-(114)T. Un-relaxed atomic models for the boundaries have been constructed. The coherent boundary is shown to differ from that in sphalerite by the inclusion of one in two Se columns having alternate atoms with either three Cu and one In nearest neighbours or vice versa. All of the [110] tilt boundaries in chalcopyrite are expected to be electrically active. A tentative list of the boundaries ranked by energy, electrical and chemical activity is given, along with comments on the influence of preferred orientation in polycrystalline films in the context of solar cells.

Time-resolved PL (TR-PL) measurements have been carried out on Cu(In1-x,Gax)Se2 (CIGS) thin films and solar cells to study the recombination of photo-generated minority carriers . Room temperature PL of both CIGS thin films and solar cells exhibited band-to-band transition for alloy composition x of 0-0.4, and PL decay curves have been utilized as a measure of carrier lifetime. The solar cell fabrication leads to the enhancement of band-to-band PL intensity, reduction of defect-related PL, and increase of PL decay time. It is noted that the increase in PL decay time has been observed in CIGS films after CBD-CdS deposition. Solar cells with high conversion efficiency are characterized by both high PL intensity and long PL decay time. The PL decay curve of CIGS solar cell is studied in terms of the alloy composition x. For small x, PL decay times of CIGS solar cells are almost independent of the emission photon energy. For large x, PL decay time depended on PL emission energy. he result suggests the change of the recombination mechanism in the CIGS solar cells with the change in the alloy composition x.

We present our work on development of Cd1-xMgxTe (CMT) polycrystalline thin film for solar cells for tandem cell applications. CMT thin films were fabricated by co-evaporation of CdTe and Mg at a substrate temperature of 400˚C. The spatial separation of the two sources resulted in a compositional gradient, which allowed the deposition of a wide range of compositions from fewer runs. Stable films for compositions up to x=0.73 were fabricated. The structural analysis showed that the as-deposited films have a sphalerite structure and are preferentially oriented in the <111> direction, and the lattice constant varies linearly with composition. Optical analysis of the samples shows that the optical bandgap varies linearly with composition. The optical absorption coefficient of the alloy films for the entire composition range (up to x=0.73) is over 6×104/cm-1 for energies above the bandgap, indicating a direct-gap semiconductor. We have fabricated solar cells using these films with the following structure: glass / SnO2 / CdS / CMT / back contact. Using the same processing conditions used for CdTe solar cells, the device efficiency was less than 2%. With modified contact processing, we were able to improve efficiencies to over 5%. Post-deposition chloride heat-treatment in oxygen ambient resulted in partial dissociation of the CMT alloy and loss of Mg. With modified chloride processing, we were able to reduce alloy decomposition and further improve the efficiencies. We have obtained devices with open-circuit voltages of up to 845 mV and efficiencies of 8% for CMT devices with a bandgap in the range of 1.6-1.62 eV which are highest reported values for CMT alloys.

In this work we investigate bias - induced metastability in CIGS solar cells. Long - term capacitance transients have been measured for two baseline CIGS devices with different efficiencies and a CGS cell in order to analyze carrier trapping processes. Based on the results we discuss hole emission process which leads to metastable increase of net acceptor density and also hole capture related to its relaxation. Time constants and activation energies for hole emission and capture have been obtained. Apart of carrier trapping processes we have also distinguished an interface-related change of capacitance. Our results indicate processes involving thermal lattice relaxation. We explain them in light of properties of (VSe+VCu) divacancy, defect with negative correlation energy, which can exist in both donor and acceptor configuration.

Solar cell structures with Sb2SxSe3-x and PbS as absorber layers were fabricated by chemical deposition on commercial transparent conductive oxide coated glass. The solid solution here was prepared by heating at 250°C a Sb2S3 thin film in contact with a chemically deposited Se-thin film. It has a graded band gap of 1-1.8 eV. A PbS thin film deposited on this layer basically fulfils the role of a p+ layer; its role as an absorber is yet to be studied. Open circuit voltage of 560 mV and short circuit current density ¡Ö 1mA/cm2under 1-3 kW/m2 tungsten halogen radiation are characteristics of these cells. Optimization of the film thicknesses and heating may offer prospects for these materials toward alternate thin film solar cell technology.

We successfully prepared (Cu1-XAgx)InSe2 solid solution with 0 ¡Ü x ¡Ü 1.0 by a mechanochemical process without any additional heating. The obtained fine powder was suitable for screen-printing. Particulate precursors were deposited in a thin layer by a screen-printing technique and then the porous precursor layer was sintered into a dense polycrystalline film by atmospheric-pressure firing. The crystal structures of the powder and the film were analyzed by x-ray diffraction and the microstructure of the film was observed in an SEM. For the (Cu1-XAgx)InSe2 films with x ¡Ü 0.2, the (Cu,Ag)InSe2 films had a good microstructure for the solar cell absorbers.

In-situ characterization of composition, electronic structure and their depth profiles of surface of Cu(In1-xGax)Se2 (CIGS) film grown by three stage co-evaporation has been carried out by means of photoemission and inverse photoemission spectroscopy (PES/IPES), for the purpose of investigating the formation mechanism of the CIGS-side wide band-gap region adjacent to CBD-CdS/CIGS interface in cell structure. Sample-transportation in vacuum below 1 x 10-8 Torr yielded almost contamination-free feature of the CIGS surface. The as-transferred surface of Cu0.93(In0.65Ga0.35)Se2 grown at the identical condition for the high performance solar cell exhibited seriously Cu and Ga deficient composition. Chemical formula of this region was inbetween Cu : (In+Ga): Se = 1 : 3 : 5 and 1 : 5 : 8. In-situ UPS/IPES measurements CIGS showed that the as-grown surface region of the CIGS already had expanded band gap energy up to 1.4 eV and n-type character. A gradual decrease of band energy and a rise of valence band maximum as a function of depth from the original surface were observed. These results have revealed that the surface of CIGS by the three stage method already has the wide band gap, which might originate in so-called Cu-vacancy ordered phase.

We report the preparation of Cu(In,Ga)Se2 (CIGS) thin-films using the electrodeposited (ED) CuGaSe2 (CGS)/CuInSe2 (CIS) bilayers. CGS/CIS bilayers were prepared on soda-lime glass /Mo substrates to realize the controlled Ga/(Ga+In) ratios and smooth layers of CIGS thin-films. It was found that the composition and morphology of CGS films was highly dependent on the composition of the bath. Crack-free and morphological CGS thin-films were obtained by the addition of supporting electrolyte and brightener. For the morphology and the crystallization of the ED-CIGS films, the best electric charges of CGS and CIS films were 1.0 C and 6.0 C respectively and the films were annealed at 600 oC for 60 min. However, the interface of the ED-CGS/CIS film had some voids and interdiffusions of Ga and In did not take place by annealing. The CIGS solar cell using ED-CGS/CIS films as an absorber exhibited diode behavior.

We present a compensated donor-acceptor conversion model to explain the metastable light-induced changes in the performance of CIGS solar cells. In this model, compensating donors plays the role of recombination channel. Modeling using SCAPS-1D yielded reasonable fits to the I-V curves in different metastable states, matching the experimentally observed decreases in short circuit current and fill factor as well as the lack of change in open circuit voltage. Comparison of the experimental results from bifacial solar cells and SCAPS simulations strongly supports the compensated donor-acceptor conversion model both qualitatively and quantitatively.

Conducting and semiconducting indium oxide thin films have been grown at room temperature by oxygen ion beam assisted e-beam evaporation. We studied an influence of the deposition conditions on thin film properties including the crystal structure, resistivity, optical transmittance, stoichiometry, morphology, and intrinsic stress. X-ray diffraction analysis shows that the crystal structure of indium oxide films changes from amorphous to polycrystalline with preferred (222) orientation when the discharge current increases from 0.5 A to 2.0 A. Film resistivity is tailored in a wide range from 3E-4 ohm-cm to 2E9 ohm-cm by modifying both the evaporation rate of indium and the discharge current of the oxygen ion source. X-ray photoelectron spectroscopy data reveal that the highly-resistive films are more oxygen-enriched than the highly-conductive samples due to electrical activity of the oxygen vacancies. Morphological properties are evaluated using an optical profiler and a measured root-mean-square (RMS) roughness is ~1 nm for produced indium oxide films. All films being studied in this work have compressive stress ranging from 0.4 Gpa to 1.8 Gpa. They are highly transparent with a transmittance up to 90%.

Thus, high-performance indium oxide films can be engineered by the reactive ion beam assisted deposition to meet the requirements of different applications such as solar cells, photodetectors, OLEDs, transparent TFTs, and optical coatings.

Chalcopyrite CuInS2 and the structurally related kesterite Cu2ZnSnS4 are known as photovoltaic absorber materials. In this study different precursor thin films of the quaternary Cu-Zn-Sn-S system (stacking: Mo/CuS/ZnS-SnS) and of the pentenary Cu-In-Zn-Sn-S system (stacking: Mo/CuIn/ZnS-SnS) were annealed in sulfur atmosphere. The predominant crystalline phases were detected by in-situ energy dispersive X-ray diffraction (EDXRD). Additionally the X-ray fluorescence signals of the film components were recorded to detect diffusion effects. For the quaternary system we found ZnS, CuS, Cu2-xS, Sn2S3 and SnS as main binary phases during annealing. The Sn2S3-SnS phase transition had a significant impact on the later formation of ternary/quaternary phases. A high diffusivity of copper can explain the little influence of the precursor stacking on the reaction path and may also be responsible for the poor adhesion of the films. For annealing temperatures above 450°C Cu2ZnSnS4 can be identified clearly by XRD. The incorporation of indium in the system leads to new diffraction peaks which can be explained by the formation of solid solutions in the system CuInS2-Cu2ZnSnS4.

Dependence of band alignments at interfaces between CdS by chemical bath deposition and Cu(In1-xGax)Se2 by conventional 3-stage co-evaporation on Ga substitution ratio x from 0.2 to 1.0 has been systematically studied by means of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). For the specimens of the In-rich CIGS, conduction band minimum (CBM) by CIGS was lower than that of CdS. Conduction band offset of them was positive about +0.3 ~ +0.4 eV. Almost flat conduction band alignment was realized at x = 0.4 ~ 0.5. On the other hand, at the interfaces over the Ga-rich CIGS, CBM of CIGS was higher than that of CdS, and CBO became negative. The present study reveals that the decrease of CBO with a rise of x presents over the wide rage of x, which results in the sign change of CBO around 0.4 ~ 0.45. In the Ga-rich interfaces, the minimum of band gap energy, which corresponded to energy spacing between CBM of CdS and valence band maximum of CIGS, was almost identical against the change of band gap energy of CIGS. Additionally, local accumulation of oxygen related impurities was observed at the Ga-rich samples, which might cause the local rise of band edges in central region of the interface.

The rapid thermal processing (RTP) of Cu-rich Cu/In precursors for the synthesis of CuInS2 thin films is possible within a broad processing window regarding leading parameters like top temperature, heating rate, and Cu excess. The key reaction pathway for the CuInS2 phase formation has already been investigated by in-situ energy dispersive X-ray diffraction (EDXRD) for various precursor stoichiometries, heating rates and top temperatures at sulphur partial pressure conditions which are typical for physical vapour deposition processes. According to the phase diagrams of the binary sulphide phases, the sulfur partial pressure strongly determines the occuring crystalline phases. However, a controlled variation of the maximum sulphur partial in a typical RTP experiment has not been carried out yet. In order to study the influence of this parameter a special RTP reaction chamber was designed suitable for in-situ EDXRD experiments at the EDDI beamline at BESSY, Berlin. In a typical in-situ RTP/EDXRD experiment sulphur and a Cu/In/Mo/glass precursor are placed in an evacuated graphite reactor. The amount of sulphur determines the maximum pressure available at the top temperature of the experiment. As the RTP process proceeds a complete EDXRD spectrum is acquired every 10 seconds and thus the various stages of the reaction path and the crystalline phases can be monitored. The first experiments show already a significant change in the reaction pathway and the secondary Cu-S phases which segregate on top of the CuInS2 thin film during the reaction.

Powder samples of (ZnX)2x(CuBX2)1-x mixed crystals (B=Ga,In; X=S,Se,Te) were prepared by solid state reaction of the elements and annealed with cooling rates between 2 - 42 K/h in the entire composition range. Structural parameters (e. g. lattice parameter, tetragonal deformation η) were investigated by X-ray and neutron diffraction studies. The microstructure was studied by transmission electron microscopy (TEM) and HRTEM. The chemical composition was determined by EDX analysis on the transmission electron microscope.

The chemical disorder process in the 2(ZnX)x(CuBX2)1-x solid solution series leads to a phase separation, i. e. in a certain composition range (2-phase field) two phases, tetragonal domains and a cubic matrix, coexist. Its width depends on the three-valent cation only and is independent from the size of anion. The habit of the tetragonal domains is like a flat discus, its short extension lies always parallel to their tetragonal c-axis. They are simultaneously arranged on the (100),(010) and (001) planes within the cubic matrix. After nucleation of the domains, their growth obeys t1/2, i.e. it is controlled by diffusion. In the sulfides and selenides systems with In as three-valent cation the tetragonal domains are 'infected' by stannite type (or CuAuI type) cation ordering independently of their orientation, whereas in the tellurides system CuPt type cation ordering was obtained. The tetragonal (ZnX)2x(CuGaX2)1-x mixed crystals exhibit only chalcopyrite type structure and orientation domains without another type of cation ordering.

In the single phase regions the lattice parameter follows Vegrads rule, whereas in the 2-phase field the both phases try to match in the a-b-plane (atetr∼acub), causing an increase of the tetragonal lattice parameter c and herewith a strong increase of the tetragonal deformation η.

The effects of irradiation with high-energy protons (3 MeV, up to a fluency of 1013 protons/cm2), on structural, electrical and optical properties of polycrystalline CdS and CdTe thin films have been investigated. XRD investigation has revealed that the films contain wurtzite-type CdS, (001) preferentially oriented in the growth direction, and cubic phase CdTe, respectively. The defects induced by ionizing radiations have been studied by thermally stimulated currents spectroscopy (TSC). Parameters of identified defect levels were determined and their possible origin is discussed.