CdS is an important part of CIGS-based photovoltaics as a buffer layer between the absorber and the window layer. Chemical Bath Deposition (CBD) is the most common way of depositing CdS. Due to the nature of the CBD process CdS forms not only on the absorber layer surface but also in the bulk solution. In order to control the deposition of CdS at the surface and to avoidCdS formation in the bulk solution the kinetics of these reactions must be understood. In this work we study the formation of CdS in the chemical bath by using in situ spectroscopic extinction measurements. By monitoring the absorbance of the solution we are able to observe the bulk phase reaction (chemical formation of CdS in solution). Deposition of CdS (growth of the CdS layer at the surface) is investigated by measuring the CdS layer thickness after the reaction. The kinetics of both reactions were investigated by a systematic variation of temperature, initial reactant concentration, Reynold number as well as time. Finally the results of our study suggest a model of growth behaviour based on molecule-by-molecule deposition, which explains especially the dependency on temperature and hydrodynamics.

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

Tin oxide (SnO2) is a durable, inexpensive transparent conducting oxide (TCO) material used for thin-film photovoltaic devices. However, the optical properties of conducting SnO2:F are generally not as good as in other conducting TCO materials such as ITO and ZnO:Al. Our previous analyses indicate that for thin-film solar cells, improving the optical properties of SnO2-coated glass could enhance photon collection and gain up to 10% additional photocurrent. Previously, we showed that some commercial SnO2 samples could have much higher optical absorption than others [2]. In this work, we continue our study on causes that could contribute to the high optical absorption of SnO2 films. The SnO2:F samples are fabricated by low-pressure metal-organic chemical vapor deposition or atmospheric-pressure chemical vapor deposition with tin precursors that includes different amounts of chlorine. Optical, electrical, and compositional analyses were performed. In addition to the free-carrier-introduced optical absorption, the non-active dopant also impacts the optical absorption. Among the SnO2 films fabricated with different precursors, the optical properties show a relationship based on the level of chlorine in the precursors and films. With a low-optical-absorption SnO2 layer, the solar cell could have better photon collection and a higher short-circuit current density.

Cu(In,Ga)S2 thin films prepared by rapid thermal sulfurization of metallic precursors yielded solar cells with efficiencies reaching 12.9% [1]. A good short circuit current density was observed together with open circuit voltages up to 850 mV. However, the fill factor was close to, but typically did not exceed 70%. In this contribution we report on the role of junction formation by chemical bath deposition on these parameters. Concentrations in the bath and deposition times were varied. A comparison is made between CdS and Zn(S,O) buffer layers. The influence of the incorporated gallium on surface properties was investigated by ultraviolet photoelectron spectroscopy (UPS) for the valence band edge and near edge X-ray absorption fine structure (NEXAFS) for the conduction band edge. Even in our best cell (13.1%) the activation energy of the saturation current is found to be still smaller than the band gap. High diode ideality factors and voltage dependent current collection prevent higher fill factors.

We report solvothermal preparations of nanocrystalline CuIn1-xAlxSe2 materials prepared from the reaction of Se, CuX2 (X = Cl− or stearate), InCl3, and Al(oleate)3 in refluxing oleylamine for 30 minutes to 3 hours. Scanning electron microscopy (SEM) images reveal morphologies consisting of hexagonal plates (100-400 nm diameter) with smaller isomorphic nodules. Micro-Raman spectroscopy, x-ray diffraction, and optical bandgap data are consistent with Al3+ incorporation into the chalcopyrite structure. For aluminum-containing reactions, product Al/(In+Al) ratios are estimated to be between 0.15 and 0.35 regardless of the indium-aluminum stoichiometry employed in the reaction. When Se is added to the reaction last, the reaction pathway involves an early-formed Cu2-xSe(s) intermediate that appears to react with In- and Al-containing species simultaneously. This intermediate is avoided when heating InCl3, Al(oleate)3, and Se together prior to Cu addition, but the final product includes Se contamination that must be removed or reacted by annealing.

Cu2ZnSnS4 (CZTS) thin films were fabricated by using three RF co-sputtering continued with sulfurization method. The new type of thin film solar cells using CZTS as an absorber consists of buffer-layer and window-layer on CZTS films that were fabricated on a Mo-coated Soda Lime Glass (SLG) substrate. It was confirmed that CZTS solar cells with high conversion efficiency existed in a relatively narrow composition region. In this paper, the fabrication method of CZTS-based thin film solar cells in our laboratory was stated briefly and the influence of the composition ratio on the photovoltaic properties were presented. Furthermore, the properties of a genuine non-toxic solar cell using a Cd-free buffer-layer were introduced.

Cadmium telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal band gap of 1.45 eV and its high optical absorption coefficient. The typical CdTe thin film solar cell is of the superstrate configuration where a window layer (CdS), the absorber (CdTe), and a back contact are deposited onto a glass slide coated with a transparent electrode. Substrate CdTe solar cells where the above listed films are deposited in reverse order are not common. In this study, the growth of CdTe thin films deposited on foil substrates by the close-spaced sublimation (CSS) has been investigated for the purpose of fabricating substrate based CdTe solar cells. The CdTe films were deposited at substrate temperatures (TSUB) in the range of 300 to 600°C, and source temperatures (TSRC) in the 600 to 650°C range. The effect of the substrate-source temperature variations on the growth rate, film structure and morphology were studied using XRD and SEM. It was found that for low substrate temperature and as the growth rate increases, grain size was the same but the films appeared to be more uniform and more densely packed with less or no pinholes. The growth rate increased as the source temperature increased. The substrate temperature clearly influences the grain growth and the preferred orientation. As the substrate temperature increased the growth rate decreased and the grain size varied from 2 to 6 μm. XRD analysis showed that with the increase in substrate temperature film orientation changes from preferential along the (111) direction to a mix of (111) (220) and (311).

Non-stoichiometry is a characteristic feature of ternary chalcopyrites like Cu-III-VI2 (III=In,Ga; VI=S,Se). The results of a comparative study of structural trends within the homogeneity region of the chalcopyrite type α-phase of the Cu2Se(S)-In2Se3(S) and Cu2Se(S)-Ga2Se3(S) quasibinary phase diagrams are presented. Powder samples of Cu-rich and Cu-poor [Cu2Se(S)]1-y-[In2Se3(S)]y as well as [Cu2Se(S)]1-y-[Ga2Se3(S)]y alloys were prepared (0.4<y<0.6) by solid state reaction of the elements (T=850°C) and investigated by X-ray powder diffraction and electron microprobe analysis. It was shown that the grain size depends on composition and structural parameters. The tetragonal distortion η=c/2a has been determined for the different trivalent cations and influences the microstructure in Cu-poor Cu1-xIII1+x/3VI2 samples. In Cu-rich samples the Cu-content is in all cases the driving force for the formation of the homogeneous microstructure observed.

Polycrystalline Cd1-xMgxTe (CMT) thin films are a potential absorber material for two-junction thin-film tandem solar cell applications because the desired top cell bandgap range of 1.6 to 1.8 eV is readily obtained using CMT with only small resultant changes in the lattice constant from that of CdTe. Tandem devices require the top cell to have a transparent back contact to transmit the sub-bandgap spectrum to the bottom cell. Sputtered Cu-doped ZnTe (ZnTe:Cu) thin films, which offer potential as a transparent back contact interface layer, have been used successfully in CdTe devices. We apply ZnTe:Cu back contacts to CMT devices to continue development toward a transparent top cell. We describe the effects of depositing ZnTe:Cu at different temperatures and with different Cu contents on the net acceptor concentration, minority carrier lifetime, and device performance. We present here the highest reported CMT device efficiency of 9.6% at a bandgap of 1.57 eV.

The present investigation deals with a two-junction device having GaAs as the top cell and Ge for the bottom cell on a Ge substrate. Compared with the conventional two-terminal device configuration, three terminals avoid the loss due to current mismatching between the cells, and the resistance loss originating from the tunnel junction between the cells. Device structures were investigated and optimized with regard to the thicknesses and doping levels of both top and bottom active junctions that lead to the highest device performance. Due to the split of the incident solar spectrum between GaAs and Ge cells, the latter only receives the light to which the former is transparent (mainly in the near infrared) and therefore behaves differently from a single-junction Ge cell. Optimal current-voltage and power-voltage characteristics were generated for individual cells together with the corresponding device PV parameters. The predictions show that an extended spectral coverage is achieved leading to an enhanced overall power output from the devices. The potential applications of these devices in conventional as well as concentrator PV were assessed and discussed as a function of the simulated concentration ratio of the incident light under AM1.5 illumination conditions. We have shown that a relatively thin double-junction GaAs/Ge device can achieve a remarkably high power output.

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.

The microstructural changes during heating of bi-layers of Cu-poor CuInS2 and CuS under different sulfur excess conditions were studied. This was done by means of energy dispersive X-ray diffraction of polychromatic synchrotron radiation in a vacuum setup where the sulfur pressure conditions can be controlled. Understood as the formation of a new microstructure, the recrystallization of the Cu-poor CuInS2 phase was characterized by a change in the reflection profile (from Cauchy-type to Gauss-type), the reduction of the breadth and a subsequent normalized-intensity increase of the 112 reflection. The Cauchy component of the breadth was used to monitor the recrystallization under different sulfur and heating rate conditions. The main results are: a) Cu availability for the consumption of the CuIn5S8 phase is a pre-requisite for recrystallization, b) in presence of the Cu2-xS phase, increased sulfur pressure enhances recrystallization.

The co-sputtered Cu-In precursor layers were characterized by bi-layer surface structure in which island-type crystals were formed in a small-crystalline matrix layer. The elemental composition of the island-type crystals corresponds to the compound CuIn2 and the matrix (area) consists of copper-rich Cu11In9 phase. The surface morphology of sequentially evaporated Cu-Zn-Sn precursor layers is determined by the deposition order of stacked consistent metal layers. Precursor Mo-Sn-Zn-Cu films exhibit a well-formed “mesa-like” structure of the surface in which larger crystals (about 1,5 μm) are located on a “small-crystalline” valley. For films with other sequences of metallic layers, the mesa like structure is not so well exposed and well formed flat precursor layers were produced replacing separate metallic Cu and Sn layers with Cu/Sn alloy layer. Selenization of both Cu-In and Cu-Zn-Sn precursor layers begins with the formation of binary Cu-selenides with compositions varying with the temperature. At temperatures higher than 3700C the selenization of Cu-In results in single-phase CuInSe2 films in contrast to the selenization of Sn-Zn-Cu films that results always in multi-phase films consisting of high quality Cu2ZnSnSe4 crystals and of separate small-crystalline phase of ZnSe.

This work reports the surface and in-depth resolved Raman scattering analysis of polycrystalline CuInSe2 layers grown with different chemical composition, as function of the Cu to In content ratio (0.48 ≤ × ≤ 1.14). Measurements performed at the surface of the Cu-poor layers corroborate the formation of both Cu-poor Ordered Vacancy Compound (OVC) and CuAu ordered CuInSe2 phases for compositions corresponding to x ≤ 0.82 (OCV) and x ≤ 0.66 (CuAu), respectively. In-depth resolved measurements performed on the layers with lower Cu content have allowed observing a strong decrease with the depth of the intensity of the Raman peak from the chalcopyrite ordered CuInSe2 phase. This suggests a preferential formation of both OVC and CuAu ordered phases at the region close to the interface with the back Mo layer. On the other hand, the comparison between the spectra directly measured on the front and back surfaces of the layers -after removal of the layers from the Mo coated glass substrates- suggests also a worsening of the crystalline quality at the back region of the layers in the whole range of compositions.

We have shown that variation in the real part of the dielectric permittivity of typical transparent conducting oxide (TCO) films can have a profound effect on the optical properties of the material. This has been demonstrated by adding small amounts of Zr to an ITO ceramic sputtering target and analyzing the resulting ITO and ITO:Zr (ITZO) films. Comparative electrical and optical analyses of the films show that, although the carrier concentration and mobility do not change appreciably by adding 1 wt.% ZrO2 to the ITO sputtering target, the plasma wavelength increases significantly for the ITZO film. We believe that the underlying physics of these results can be exploited in designing future TCO films for photovoltaic (PV) applications—especially those that embody industrial advantages but remain limited by low mobility.

CuInS2 has emerged during recent years as a good candidate to substitute CuInSe2 as polycrystalline absorber in thin film solar cells, mainly due to its direct band gap energy of 1.5 eV. In this study, absorber layers of both Cu-rich and Cu-poor types have been grown on soda-lime glass substrates by proper selection of the deposition parameters. The morphology and the optical properties of the resulting CuInS2 films were studied in dependence of the deposition order of the elemental constituents: alternate evaporation of the precursors, simultaneous deposition of the three constituents and sequential modulation of the evaporation fluxes.

The effect of the amount of Na present during the 3-stage growth of CIGS at very low temperature T2 on polyimide (PI) foils is studied. While at higher growth temperatures Na seems to impede In-Ga interdifussion, at very low temperatures it appears to further the process. An increase in Voc for a higher Na concentration can be explained by a higher net carrier concentration as measured by drive level capacitance profiling. Admittance spectroscopy measurements show shallow defects when the Na concentration increases. These results suggest that the main role of Na could be the passivation of InCu donor deep defect, in agreement with Wei's theory. Efficiencies of up to 15.1 % (0.5 cm2 active area with antireflection coating) and 13.6%, 14.1% (1 cm2 total and active area respectively without antireflection coating) for nominal T2=420° C were achieved on PI substrates so far.

Short-bandgap group II-VI compound cadmium telluride is widely used for the infrared optics, radiation detectors, and solar cells where p-type CdTe is needed. p-type conductivity of CdTe is mainly caused by the chlorine-based A-centers, and in part, by the less stable copper-oxygen complexes. As a rule, CdTe films are recrystallized by the help of a cadmium chloride flux that saturates CdTe with chlorine. In chlorine-saturated CdTe A-centers are converted to isoelectronic complexes that cause resistivity increasement of CdTe up to 9 orders of magnitude. Excess copper and oxygen or group I elements as sodium also deteriorate the p-type conductivity of CdTe like excess chlorine. p-type conductivity of CdTe can be restored e.g. by the vacuum annealing which removes excess chlorine from the film. Unfortunately, treatment that betters p-type conductivity of the CdTe film degrades the junction of the superstrate configuration cells. In this work we investigate possibilities to prepare p-type CdTe films on the molybdenum coated glass substrates. Samples were prepared by the vacuum evaporation and dynamic recrystallization of 6N purity CdTe on the top of Mo-coated glass substrates. Then samples were recrystallized with cadmium chloride flux under tellurium vapour pressure. Results of the test studies on the structure and electronic parameters of samples are presented and discussed.

Electroluminescence images gained from Cu(In,Ga)Se2 mini-modules under different voltage bias conditions are investigated. The mini-modules of area 20 × 20 cm2 with 42 cells exhibit typically 10-20 localized shunts. The consequences of these shunts on the performance of the individual cells and of the entire module are analyzed quantitatively by evaluating the electroluminescence images. Our evaluation method uses the fact that the electroluminescence intensity at each position in each cell within the module depends on the actual voltage drop over the junction at this specific location. Thus, the analysis of the electroluminescence intensity allows us to reconstruct the current/voltage characteristics of all individual cells in the module. In addition, we provide first simulations using a distributed diode network model to quantitatively explain the experimental results.