The junction properties of isotype and anisotype n+-ZnO/c-Si heterostructures have been studied by electrical and photoelectrical methods. We present evidence that the junction properties are strongly affected by a 10–30 nm thick ZnO layer closest to the heterointerface with distinctively different properties than those of the ZnO film bulk. This layer supports a dominant current flow via multistep tunnelling-recombination. When a 10 nm thin ZnS or ZnSe interlayer is inserted charge transport is controlled by thermionic emission. The interlayer acts as spacer and increases the band bending in the silicon absorber. However, there is still a too high trap density at the interlayer/c-Si interface, so that Voc does not exceed 0.25–0.32 V.

Cu(In, Ga)Se2 (CIGS) solar cells are under investigation for 1-sun and concentrator applications. Design criteria are examined and reveal that only grid design modifications are required. In the special case where cell width dimensions are 4–5 cm, an interdigitated design removes the back contact as a loss mechanism. Processing issues relating to the intrinsic ZnO layer are critical to optimal and reproducible cell performance. 1-sun and 20 sun performance of 17.7% are reported for different cells. The latter represents a 2.9% absolute improvement over the 1-sun control measurement. 20% performance is therefore a realistic goal. CIGS-based cells represent a viable concentrator technology.

Cu(InGa)Se2(CIGS) thin film absorbers were fabricated by a three-stage method using a coevaporation apparatus. As a Cd-free buffer layer, ZnSe, InxSe, GaxSey and ZnInxSey buffer layers have been deposited on the CIGS absorber continuously in the same apparatus. Atomic layer deposition (ALD) was employed as a growth technique for ZnSe. This technique offers a good thickness control as well as a good surface coverage. By irradiating with a solar simulator, all the solar cell parameters increased drastically for the first 50 minutes of the irradiation and then saturated at longer irradiation times. This phenomenon did not appear for the cells with a CdS buffer layer. The best efficiency of ZnO/ZnSe/CIGS thin film solar cells with about 10 nm thick ZnSe buffer layer was 11.6%. On the other hand, ZnO/InxSey/CIGS thin film solar cells showed very stable characteristics under the light illumination, and initial measurements show an efficiency of 13.0%.

Targeting large area and low cost processing of highly efficient thin film solar modules an advanced stacked elemental layer process for Cu(InGa)Se2 (CIGS) thin films is presented. Key process steps are i) barrier coating of the soda lime glass substrate combined with the addition of a sodium compound to the elemental Cu/In/Ga/Se-precursor stack and ii) rapid thermal processing (RTP) to form the CIGS compound.

By this strategy exact impurity control is achieved and the advantageous influence of sodium on device performance and on CIGS film formation is demonstrated unambiguously by means of electrical characterisation, XRD, SEM, TEM and SIMS. Sodium enriched and sodium free precursor stacks were heated to intermediate states (300°C–500°C) of the RTPreaction process. The experiment clearly reveals that on the reaction pathway to the chalcopyrite semiconductor increased amounts of copper-selenide are formed, if sodium is added to the precursor films. TEM-electron diffraction unambiguously identifies the CuSe-phase which is localised at the surface of the forming CIGS-film. These experimental findings propose a sodium assisted quasi liquid growth model for the CIS formation taking into account that sodium promotes the existence of CuSe at higher temperatures and its effect as a flux agent. The model contributes to a better understanding of the observed superior crystal qualitiy for sodium enriched in contrast to sodium free CIGS films.

Application of these experimental findings in the technique of the optimized and controlled sodium incorporation significantly improves process reproducibility, CIGS film homogenity over larger substrate areas and shifts the average efficiency of cells and modules to a significantly higher level. This is demonstrated by a 12-cell integrated series connected minimodule with an aperture area of 51 cm2 and a confirmed efficiency of 11.75 %.

The purpose of this study is to improve the reliability and reproducibility of our fabrication process for polycrystalline Cu(InGa)Se2 (CIGS) thin-film absorbers and to make a better absorber with higher efficiency. The current baseline process of selenization has been evaluated through the investigation of the formation chemistry of the device-quality CIGS thin-film absorbers with a graded band-gap structure. It has been verified that the current selenization process has been performed in a good balancing point with both Cu/III ratio and thickness of the precursor layer and the total amount of Se through H2Se gas incorporated from the surface during the selenization. A simplified model to explain the formation chemistry of the selenization in this study has been proposed.

Charge-collection microscopy (CCM)—commonly known as electron-beam-induced current [EBIC] microscopy—in a scanning electron microscope (SEM) was used to obtain charge-collection efficiency profiles of cleaved thin-film ZnO/CdS/CuInXGal-XSe2/Mo photovoltaic devices on glass substrates, with x varied between 0 and 100%. We observed considerable variation in the EBIC peak position, the uniformity (between and within devices), and overall charge-collection profile of the junction as a function of Ga concentration. Whereas using only CuInSe2 (CIS) absorber material results in a rather wide and buried junction in the CIS, the systematic addition of Ga to the p-CuInSe2 matrix affects the conductivity type of the material, thereby creating an increasingly abrupt, uniform, and shallow junction near the heteroface. Although the substitution of Ga for In improves overall device performance up to ∼ X=25% and the Voc continues to improve, the Jsc, FF, and device efficiency degrade with additional Ga.

Electrochemical deposition of CuInSe2 films has been explored as a low-cost alternative to vacuum deposition. A special feature of this approach is that each element or compound may be selectively deposited or dissolved at a specific potential. This approach could further provide an environmentally benign means to recycle CuInSe2 from spent or defective modules. The paper investigates the reaction mechanisms for co-deposition, as well as, removal of CuInSe2. Rotating ring-disk and photoelectrochemical methods probe the formation and dissolution of Se, Cu2Se, In2Se3 and CuInSe2 films and the corrosion of CuInSe2 single crystal. The results provide important insights into the multiple reaction paths leading to the electrosynthesis of CuInSe2 films and new directions for developing an electrochemical recycling scheme for CuInSe2 modules.

A key element of current Copper Indium Diselenide (CIS) and Copper Indium Gallium Diselenide (CIGS) thin film solar cells is the use of a Mo back contact on soda lime glass (SGL). Because of surface preparation problems, high process temperatures, and mismatch of thermal expansion coefficients, adhesion of the Mo to the soda lime glass can be variable. Also beneficial is the Na facile diffusion of the glass into the absorber layer. We report on the use of thin Cr interlayers to improve the adhesion at the Mo/glass interface. The films were subsequently annealed in vacuum under normal process conditions. Adhesion was excellent and quite uniform for Mo layers with a Cr interlayer of 50 to 800 Å compared to control samples without Cr. X-ray Photoelectron Spectroscopy (XPS) data suggests CrO bonding at the glass interface and Cr metallic bonding at the Cr Mo interface. Secondary Ion Mass Spectrometry (SIMS) data for Mo/Cr films shows diffusion of Na throughout the Mo layer identical to that for Mo alone samples. Resistivities of the films have been measured to be 11 μhms-cm, twice that for bulk material of 5.7 μohms-cm. CIGS films were then grown for comparison to films grown on Mo only substrates.

We have studied the electronic structures of CuIn(S0.875X0.125)2 (X=B, C, N, Si or P) based on the ab-initio electronic band structure calculations using the augmented spherical wave (ASW) method. We have clarified that the physical characteristics of the p-type doped CuInS2 crystals are mainly determined by a change in the strength of interactions between Cu and S atoms. On the basis of the calculated results, we discussed the material design considerations, such as controlling the strength of resistivity for p-type doped CulnS2 materials and converting the conduction type, from n-type to p-type by a codoping method.

We have measured the phototransport properties of CuGaSe2 films as a function of temperature. The simplest model which is consistent with all the experimental results consists of two recombination levels, one of which is donor-like and the other is acceptor-like. This model is similar to the symmetrical two-level model, which we have recently suggested for CuInS2 films. We thus conclude that this model, with slight variations, represents the general recombination level structure in all copper ternary chalcopyrites.

A series of Cu-rich CuInSe2 epitaxial thin films were grown by molecular beam epitaxy on GaAs(001) substrates from elemental sources at a growth temperature of 450 °C. All samples were grown with an excess of Cu. Electron microprobe analysis (EPMA) indicated a Cu/ In ratio of about 2.1–2.6 in the as-grown films. Additionally, the Se/ (In+Cu) ratio was observed to be ∼0.95 indicating that the films were slightly Se poor. These Cu-rich samples were etched in a KCN solution for periods ranging from 30 seconds to 3 minutes. EPMA measurements indicated that the bulk Cu/ In ratio was reduced to ∼0.92 in all films regardless of etching time. Atomic force microscopy (AFM) was used to characterize the topology of each sample before and after etching. These measurements indicated that the precipitates present on the as-grown films were removed and large nearly isotropic holes were etched into the sample to a depth of over 1000 Å even for etching times as short as 30 seconds. The samples were also evaluated both before and after etching using a Phillips MRD diffractometer with parallel beam optics and a 18,000 watt Cu rotating anode X-ray source in the chalcopyrite [001], [101], and [112] directions. A peak was observed at ∼15 degrees in the [001] scan after etching consistant with the presence of the ordered vacancy compound, CuIn3Se5. Additionally the integrated intensity ratios of the chalcopyrite (202) reflection to the chalcopyrite (101) reflection ∝(fCu-fIn)2 along the [101] direction indicated the presence of a near-surface region containing cation sublattice disorder that was subsequently removed by the etching process.

A multilayer synthesis of polycrystalline CuInSe2 has been performed by vacuum evaporation with repeat layers thinner than 225Å. Samples deposited with different elemental sequences and repeat layer thicknesses were analyzed by x-ray diffraction as a function of annealing temperature. Samples with 225Å layers produce multiple phases in the synthetic pathway. In samples with thinner layers small amounts of In2Se3 were detected but CuInSe2 predominated. Diffraction and calorimetry data indicate that CuInSe2 and In2Se3 crystallites begin growth at room to moderate temperatures. Layering of samples was probed by grazing angle x-ray diffraction and similarly indicates interdiffusion of layers at room to moderate temperatures.

Chemical-bath-deposited CdS thin films from an ammonia-thiourea solution have been studied by x-ray diffraction, surface profilometry, ellipsometry, and other techniques. The compactness of the CdS films, structural properties of the films, and the growth mechanism have been investigated. For the deposition conditions used, we found that the film compactness reaches its maximum at a deposition time of 35 minutes. Films grown at longer deposition times are less compact, consistent with the CdS duplex layer structure proposed previously. This transition from compact layer growth to porous layer growth is important for depositing CdS films in solar cell applications. Based on x-ray diffraction (XRD) studies, we were able to determine the crystal phase, lattice constant, and other structural properties.

CuInSe2 (CIS) epitaxial layers have been grown on both GaAs (001) and In0.29Ga0.71 As pseudo lattice-matched substrates by molecular beam epitaxy, and characterized for device applications. Despite a large lattice mismatch of Δa/a˜2.2%, epitaxial growth of CuInSe2 has been demonstrated on GaAs (001) showing their film properties strongly dependent on the Cu/In ratio. In-rich films had a large number of twins on {112} planes, and were found to be heavily compensated. On the other hand, Cu-rich films showed distinct photoluminescence emissions indicating significantly higher film quality in comparison with In-rich films. Two dimensional reciprocal x-ray intensity area mapping and cross-sectional transmission electron microscopy showed the formation of an interfacial layer in the vicinity of the CuInSe2/GaAs interface resulting from the strain-induced interdiffusion between CuInSe2 and GaAs. Reduction in lattice mismatch to Δa/a˜0.2% by using In0.29Ga0.71As pseudo lattice-matched substrates made possible the growth of high quality CuInSe2 with predominant free exciton emissions in their photoluminescence spectra and with residual defect densities of as low as p˜l×1017cm-3 implying the growth of device quality CuInSe2 epitaxial films.

The defect chalcopyrite material CuIn3Se5 has been identified as playing an essential role in efficient photovoltaic action in CuInSe2-based devices; it has been reported to be of n-type conductivity, forming a p-n junction with its p-type counterpart CuInSe2. Because the most efficient cells consist of the Cu(In1-xGax)Se2 quaternary, knowledge of some physical properties of the Ga-containing defect chalcopyrite Cu(In1-xGax)3Se5 may help us better understand the junction phenomena in such devices.

Polycrystalline Cu(Inl-xGax)3Se5 (with O<x<l) thin films have been grown on 7059 Coming glass, soda-lime silica glass, n-type (100)Si, and Mo-coated soda-lime glass by coevaporation from elemental sources. In general, optical data show direct optical bandgaps that range from 1.20 eV for x=0 to ∼1.85 eV for x=l (this represents ∼200 meV higher bandgaps than the Cu(In, Ga)Se2 counterparts). Micrographs of the thin films show a substantial change in morphology as the Ga content is increased—for identical conditions of growth rate and substrate temperature. X-ray diffraction patterns agree with previously publish data for the ternary case (x=0), where these materials have been referred to as ordered vacancy compounds. Pole figures confirm a high degree of texturing in the films and a change in preferred orientation as Ga content is increased.

CuInSe2 thin films grown on Corning 7059 glass substrates were prepared by molecular beam deposition method with various Cu/In ratios in order to study the relationship between composition and material properties. Under a fixed copper beam flux intensity and overpressure selenium environments, the composition of films was modulated by changing the indium beam flux: intensity to obtain Cu-rich or In-rich films. The X-ray diffraction patterns show that the epitaxy films grown onto glass substrates are the polycrystalline structure. The grain size of polycrystalline films indicate that the surface morphology of the films are varied as a function of Cu/In ratio.

High quality epitaxial CuInSe2 (CIS) films with a range of Cu/In ratios (γ) = 0.80∼2.24 grown by molecular beam epitaxy (MBE) have been post-annealed at temperatures of TA=200∼400°C in both dry-air and Ar atmospheres. Changes in the structure and composition due to annealing have been investigated. The only oxide observed experimentally for both the In-rich, and the Cu-rich CIS films was In2O3. This is consistent with equilibrium thermodynamic calculations which indicate that In2O3 is the mnst stable solid oxide phase. During annealing some reac-tions arc probably kinetically limited making the annealing process a function of time and temperature. but the equilibrium thermodynamic results reported here simplify interpretation of phase space.

Three real-time composition monitoring methods in physical vapor deposition process of CIGS thin films were proposed. One is the detection of near infrared (NIR) transmittance through the CIGS films from the IR source such as a heater back side of the substrate. One is the detection of NIR reflectance of the incident light from the films. The other is the measurement of the substrate temperature during the preparation of the CIGS films. These methods utilize the difference of the carrier density between Cu-rich (∼ 1020/cm3) and (In+Ga)-rich ( > 1016/cm3) films. The NIR transmittance through and reflectance from the Cu-rich film decrease due to high free carrier absorption. The temperature of the Cu-rich films was lower than that of the (In+Ga)-rich ones under the constant heating power because the Cu-rich films have larger radiation in proportion to absorption. Therefore, the change of Cu/(In+Ga) ratios at stoichiometry can be monitored by the variation of the NIR transmittance, NIR reflectance or the film temperature in real time. These composition monitoring methods are very useful and easily applied to the physical vapor deposition process of the CIGS films for photovoltaic application. The solar cell with active area of about 0.5 cm2, fabricated by obtaining the CIGS film using the latter monitoring method, showed an efficiency of 17.0%.

Deep levels in chalcopyrite based heterojunctions are investigated by capacitance techniques. A method is presented which allows the determination of defect distributions from admittance measurements. For CuInSe2 a defect level at 280 meV above the valence band with an attempt to escape frequency of about 1011 s-1 is detected. A characteristic defect structure consisting of a shallow defect level and a broad deep structure is observed for Cu(In, Ga)Se2. The depth of the shallow defect is affected by annealing treatments which are necessary to achieve the optimum performance of the devices. CulnS2grown under an excess of CuS exhibits a high density of shallow defects whereas for Cu-poor CuInS2 defects close to midgap position with a large capture cross section are observed. These defects are metastable with respect to current injection and illumination relaxing to the equilibrium state around room temperature. The increase of the defect density after illumination results in a reduced bucking current. CV measurements can be interpreted in terms of a high density of deep states.

Cu-rich Cu(In, Ga)Se2 (CIGS) films were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray fluorescence spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). The Cu-rich CIGS film were treated in KCN and NH3 solutions. In the as-deposited Cu-rich CIGS film, the cation ratio of Cu/(In+Ga) at the surface exceeded the bulk value. Cross-sectional TEM of the KCN-treated film suggested that Cu2-xSe existed both at the grain boundaries and on the grains near the surface of the Cu-rich CIGS film. The Cu2-xSe was completely removed by the treatment in the KCN solution and was removed only at the front surface by treatment in the NH3 solution.