The structural and optical properties of CdS/CdTe(S) interface region of 2-μm thick CdS/CdTe solar cells have been studied in conjunction with photovoltaic performances of the solar cells. The properties are found to be crucially influenced by the annealing temperature and oxygen concentration of the CdCl2 treatment. An increase in VOC and F.F. found in the solar cells with the CdCl2 treatment at < 360°C is interpreted as due to suppression of interdiffusion of sulfur and tellurium at the CdS/CdTe(S) interface. On the other hand, the electromodulated photoluminescence with UV light excitation (UVE-EMPL) study reveals that the increasing VOC due to increasing oxygen concentration to 5% is likely to be caused by an increase in the built-in electric field in n+-CdTe1−xSx just adjacent to the CdS/CdTe(S) interface. As a result of the modification of the process parameters, we have achieved the conversion efficiency of 13.6% (VOC: 0.817V, JSC: 23.0 mA/cm2, F.F.: 0.725) using 2.3-μm thick PV active layer without anti-reflection coating.

The microstructure of Cu(In,Ga)Se2 (CIGS) films with compositions ranging from Cu-rich to In-rich was investigated by transmission electron microscopy (TEM) and energy-dispersive Xray spectroscopy (EDS). We found that the Cu-rich samples have larger grain sizes than the Inrich sample. In the Cu-rich samples, sub-interfaces were observed. The two sides of the subinterfaces were found to have different Cu concentration. In the In-rich sample, Ga inhomogeneity across grains was observed.

In this study we report on the spatial distribution and composition of residuals on the CdTe surface following a typical wet CdCl2 treatment, and the effect that our ion-beam milling has on this residual-coated surface. Results show that residuals are spatially discrete, located primarily along grain boundaries, and are likely a cadmium oxychloride. Results also show that the residuals may penetrate deep into the CdTe surface such that typical ion-beam milling procedures do not produce complete residual removal.

Tin dioxide (SnO2) coated glass is the commonly used substrate for thin film solar cells based on CdTe absorbers. We have investigated the properties of the CdS/SnO2 interface by X-ray and ultraviolet photoelectron spectroscopy. SnO2 coated glass substrates as used for solar cell preparation were cleaned by different procedures such as derinsing, sputtering, heating and annealing in oxygen atmosphere. Different surface properties with a strongly dependent number of defects in the SnO2 band gap are identified. CdS films were deposited stepwise by thermal evaporation to determine the electronic interface properties for different surface preparation conditions. Comparative barrier heights at the CdSSnO2 contact are found for most surface pretreatments. The Fermi level position in these cases is situated in the SnO2 band gap. A different interface behaviour is determined for sputter cleaned SnO2 surfaces, which is attributed to the formation of oxygen vacancies during sputtering and subsequent formation of an interfacial SnOxSy compound.

This paper presents a new approach for relatively rapid and inexpensive synthesis of morphologically homogenous CuInSe2 absorber layers without high-temperature treatment. The approach, molecular layer electrodeposition, combines the low-cost, scale-up and environmental advantages of electrodeposition with the atomic level control of epitaxial deposition. This patented method entails building a superlattice of CuInSe2 monolayers by electro-deposition from a single electrolyte. The monolayer deposition approach extends the utility of epitaxial methods to device fabrication with simpler apparatus and practical deposition rates. The paper outlines the concepts underlying molecular layer electrodeposition, describes the special deposition apparatus and presents the results of initial investigations involving the synthesis of a ternary compound in a flow cell, controlled at molecular level.

Graded-band-gap CuIn1−xGaxSe2 (CIGS) absorbers with Ga/Ga+In value in the 20%-30% range have a demonstrated efficiency of 18.8%. For CdS-containing devices, the shortcircuit current density (Jsc) has almost reached its expected maximum. However, the open-circuit voltage of CIGS solar cells is limited by the surface microstructure and chemistry. In this work, we examine the microstructural properties and chemistry of CIGS. We also attempted to correlate the above observations and device performance.

We report on a novel ion beam selenization process. The reactive chalcogen component Se and a significant part of the thermal energy needed for CIGS formation is delivered directly into the growing surface by a low energy Se ion beam from a broad beam ion source. This highly controllable technique with respect to ion energy, dose and uniformity and with scale- up capabilities can be used in two ways either for selenization of metallic Cu/(In,Ga) thin film stacks or in co-deposition. In the case of co-deposition the CIGS growth temperature can be reduced to < 400°C

Besides the description of the method we present results of Se ion beam analysis and properties of CIGS thin films, produced by the novel selenization process.

The paper discusses the electronic properties of Cu(In,Ga)Se2–based heterojunction solar cells with a special focus on questions which at present are not satisfactorily understood. First, we discuss an apparent quantitative contradiction between measured concentrations of recombination centers in the Cu(In,Ga)Se2 absorber material and the actual recombination rate in the solar cells. We propose, as a possible explanation for that observation, that the defect concentration in Cu(In,Ga)Se2 is spatially inhomogeneous with a systematic increase towards the heterojunction interface. Second, we address the issue of electronic metastabilities in ZnO/CdS/Cu(In,Ga)Se2 heterojunctions and, especially, in devices that use alternative buffer materials instead of CdS. Starting from a brief review of the experimentally observed types of metastabilities, we demonstrate by thermally stimulated capacitance measurements that a specific type of metastability that severely limits the performance of solar cells with non-CdS buffers is present also in high-efficiency standard devices though it has virtually no influence on the output parameters in the latter case. A possible explanation of this type of metastability points to a metastable defect reaction localized in the close to surface region of Cu(In,Ga)Se2. At the moment we cannot propose conclusive models for both open questions. However, we can localize the answers to both problems in the close-to-surface region of the Cu(In,Ga)Se2 absorber.

Thin film solar cells obtained by the “CIS on copper tape” technique are investigated. This technique promises a high throughput capability, but the efficiency of the cells is still about 5 % only. The model of the band structure of the absorber has been introduced into the model of the whole cell. Parameters of the model were determined experimentally by use of quantitative EBIC profiling, C-V doping profiling, Hall measurements, and AFM. The structure has been simulated using SCAPS-1D software. Results of the simulation show a good correlation to the measured I-V and DSR data of the cell. The benefits and drawbacks of the cell structure as well as factors limiting its efficiency are discussed.

Thin films of Zn-compounds grown in a chemical bath have been studied by Elastic Recoil Detection Analysis (ERDA). The bath contained zinc acetate, thiourea and complexing agents such as ammonia and hydrazine. Large amounts of hydrogen and oxygen are detected within all samples. A strong effect of the pH of the solution is revealed: At high pH (>11.5) mainly ZnS growth takes place, while at lower pH Zn(OH)2 is the dominant product. Hence the pH of the solution can be used to adjust the film stoichiometry. Deposition of Zn-compound films is possible without ammonia in the solution while presence of hydroxide ions and hydrazine species is necessary. It is proposed that film formation takes place via a complex combining hydrazine and hydroxide ligands.

Photoelectrochemical studies of polyacrylate-capped CdS nanoparticles embedded in a polyelectrolyte matrix on electrode surfaces are presented. Multilayered films of polyacrylatecapped Q-CdS are assembled layer-by-layer on solid surfaces modified with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). The characterization of the layer-bylayer growth by means of UV-visible absorption, photoluminescence, FT-IR spectroscopy and ellipsometry measurements revealed the incorporation of the nanoparticles in the matrix and the reproducible surface-charge reversal taking place. In this paper, we present initial photocurrentvoltage studies of films thus formed, where the feasibility of charge conduction through the polyelectrolyte matrix and the dependence of photocurrent magnitude on the number of bilayers are demonstrated. In addition, the dependence of photocurrent direction on the electrode potential, and the observation of anodic photocurrent spikes attributed to surface states in the presence of dissolved oxygen are reported.

We present an analytical model that quantitatively describes the physics behind shunting in thin-film photovoltaics and predicts size-dependent effects in the I/V characteristics of solar cells. The model consists of an array of micro-diodes and shunt in parallel between the two electrodes, one of which mimics the TCO and has a finite resistance. We introduce the concept of the screening length L, over which the shunt affects the electrical potential of the system. The nature of this screening is that the system generates currents in response to the point perturbation caused by the shunt. L is expressed explicitly in terms of the system parameters. We find the spatial distribution of the electrical potential in the system and its I/V characteristics. The measured I/V characteristics depend on the relationship between the cell size l and L, being markedly different for the cases of small (l<<L) and large (l>>L) cells. This model is verified experimentally; good agreement is obtained.

Some thin-film CIS photovoltaic devices exhibit reversible transient behavior in their electrical properties induced by modestly elevated (70 - 100 °C) temperatures. This paper evaluates changes due to light exposure, thermal exposure, and applied bias in cells fabricated by Siemens Solar Industries (SSI). When a constant bias was maintained across cells subjected to elevated temperatures in the dark, and subsequent moderate- temperature light exposure, there was little reversible transient behavior. When the bias was cycled between zero and open-circuit voltage (VOC), independent of illumination, the fill factor (FF) decreased for zero bias and increased at VOC. Hence, it is the bias rather than photon absorption that drives the transient current-voltage behavior in these cells. Investigations of the relationship between trapping mechanisms and transient behavior using the frequency and temperature dependence of capacitance showed clear cyclic behavior in the trap-response frequency. Trap density profiles were found to be relatively independent of measurement temperature, and the total trap density varied only slightly with the bias cycle.

At present, the soft processing of materials attracts considerable interest. Chemical bath deposition (CBD) effects film formation by means of a controlled chemical reaction. Adherent, uniform and reproducible films of zinc sulfide (ZnS) have been deposited, upon low iron-content microscope slides, under acidic conditions, from a solution containing zinc chloride, urea and thioacetamide. Scanning electron micrographs of the deposited layers suggest that, as the reaction proceeds, uniform film growth is associated with increasing particle size, rather than continuous nucleation and deposition of new primary particles. Energy Dispersive Analysis by X-Rays (EDAX) spectra are typical of CBD-deposited films of ZnS; signals attributable to elements within the glass substrate are also detected, an observation consistent with the thin nature of the films. Grain size distributions have been investigated using computational image analysis, and an average increase in the diameter of the deposited particles of 33.7 nanometres per hour has been calculated.

In this work, Cu and In thin films, as precursors for CuInSe2 (CIS) formation, have been deposited on glass substrate up to 30 × 30 cm2 area using an electron beam evaporator in sequential processes. In order to obtain a similar global composition, three types of sequential processes of evaporation: A) Cu/In/Cu/In, B) Cu/In/Cu/In/Cu/In and C) In/Cu/In/Cu have been tested. As-grown thin films were studied at room temperature and after 120° C annealing. XRD analysis of these films showed mainly the CuIn2−x (0≤ × ≤ 1) phase at room temperature, and Cu11In9 after annealing at 120° C. After alloying, the films were selenized at temperature between 250° and 400° C in vacuum using elemental selenium vapour. XRD of the selenized thin films corresponding to In/Cu/In/Cu sequence and previously annealing at 120° C, showed the major presence of the polycrystalline chalcopyrite structure CuInSe2 with preferential orientation (112) plane at temperature as low as 250° C. From SEM studies and profilometer measurements a decrease in the mean roughness could be observed after annealing at 120° C. In contrast the resistivity of the films increased.

Improvement of electric back contact formation is one of the major issues of the CdTe thin film solar cell research. Chemical etching of CdTe before metallization is accepted to improve contact formation. It is believed that a CdTe/Te contact is formed by this procedure leading to a Fermi level position in the CdTe close to the valence band maximum for low contact resistance. We have studied the electronic properties of chemically etched CdTe surfaces with photoelectron spectroscopy. Etching of the samples was performed in air (“ex-situ“) as well as in an electrochemical setup directly attached to the UHV system (“in-situ“). The formation of a Te layer is clearly shown by (S)XPS. In contrast to previous studies we could not detect the formation of a p-CdTe surface for different experimental conditions. The detected Fermi level position indicates still band bending and hence a blocking Schottky barrier.

Measurements of film impedance in a wide frequency range (impedance spectroscopy, IS) were used to evaluate the grain boundary (GB) and the bulk material (B) resistance. Undoped films demonstrated very low carrier concentration and negligible GB resistance, both due to depletion of the bulk material by the GB deep states. Doping the film with Cu led to a significant increase in the GB resistance, in addition to the expected increase in carrier concentration. Illumination of doped films reduced GB resistance by orders of magnitude. This effect was used for the analysis of Hall concentration and Hall mobility data, aimed at exclusion of GB influence on measured parameters, and was used to determine the carrier concentration and mobility in the grain bulk. Hall studies on undoped films under illumination were also used to estimate the lifetime of the photogenerated carriers.

In this paper the electronic properties of the different interfaces of CdTe thin film solar cells will be analysed by using a surface science approach. Experimental basis for the experiments is an integrated UHV systems which allows to prepare and analyse real solar cells as well as appropriate model interfaces. Recently obtained data on the ITO surface, the ITO/SnO2/CdS front contact, the CdS/CdTe heterojunction and the CdTe/Te back contact will be presented. In addition, bulk properties as doping and lateral inhomogeneities will be addressed. For all these interfaces experimentally determined band energy diagrams will be given and discussed in relation to solar cell performance. Finally, the sum of the results will be used to propose a modified band energy diagram of the complete CdTe thin film solar cell and its implication for further cell improvement will be presented.

The influence of non-equilibrium space charge distributions on the fill factor of ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices has been investigated. Metastable changes of the amount of charge captured in deep levels have been produced by red or white illumination. Subsequent characterization by current-voltage, capacitance-voltage and admittance spectroscopy has been conducted at low temperature. The fill factor increases after a white light soaking and decreases after illumination of the reverse-biased device with red light at low temperature. This effect has been attributed to the metastable change of net doping concentration in the p+ layer of absorber close to interface. Blue light, absorbed in CdS provides holes to that region neutralizing the negative charge and thus improving the fill factor. The defects responsible for the interface-related metastability are in our opinion the same as those causing the persistent increase of the net doping concentration in the bulk of Cu(In,Ga)Se2 and seem to be related to selenium vacancies.

State of the art CIGS thin films have been studied by means of the semiconductorelectrolyte junction. They appear as chemically robust allowing to use aggressive electrolyte compositions as for instance more acidic pH, down to 0 in sulfuric acid environment. In these electrolytes, reliable capacitance and photo-current related characterization techniques have been used. It has been shown that a short treatment in a gold (III) solution can facilitate the characterizations, and stabilize the surface composition. These results tend towards settling a standardized electrochemical testing procedure for CIGS layers