CIS Thin Films Research Articles

CIS and CIGS based photovoltaic structures developed from electrodeposited precursors

In the present study we report the electrodeposition and characterization of CIS and CIGS thin films and a post-deposition thermal processing in vacuum to improve the film stoichiometry by incorporating additional In, Ga and Se. Different kinds of analyses showed that CIS as well as CIGS possess a very thin In-rich surface n-layer. The formation and characterization of solar cell structures from the electrodeposited precursor with the configuration glass/Cr/Mo/CIS(CIGS)/CdS/ZnO/MgF 2 is also reported. The optoelectronic properties such as Voc, Isc, FF, η etc. of the cells are presented.

Temperature dependence of photoacoustic spectra in CuInSe 2 thin films grown by molecular beam epitaxy

CuInSe 2 (CIS) thin films, the thickness of about 1.0 μm and composition of Cu In ratio (γ = 1.79), were grown on (0 0 1)-oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of T s, = 450°C. The samples have been characterized by means of piezoelectric photoacoustic (PPA) measurements between liquid helium (4.2 K) and room temperature (300 K). Two distinct PPA signals due to band gap of CIS and GaAs are observed in the whole temperature range from 4.2 to 300 K and the PPA signals of CIS decrease curvilinearly at the temperature range. Since the PPA signals of CIS thin films can be obtained up to room temperature, the PPA measurements are quite effective to obtain the optical characterizations, especially for the nonradiative recombination processes.

Evolution of the phases and chemical composition during the formation of CIS thin films prepared by interdiffusion process in selenides layers

CuInSe 2 (CIS) thin films were prepared using a novel procedure involving the interdiffusion of the species present in the bilayer system In x Se y Cu z Se y assisted by a post deposition annealing in a high-pressure atmosphere of elemental selenium. The influence of the annealing temperature on the phase and chemical composition of both the single precursor layers and the bilayer system was established through X-ray diffraction (XRD) using a specially developed computer programme and energy dispersive spectroscopy (EDS) measurements. The chemical composition homogeneity in the bulk of the resulting samples was also studied by (Auger electron spectroscopy) AES depth profile measurements. The possible chemical reactions which give rise to the formation of a range of binary and ternary compounds are given on the basis of EDS measurements and the identification, with a high degree of reliability, of the phases present in both the bilayer system and the selenide precursor layers. A special methodology was developed for the determination of the crystallographic phases which combined commercial software criteria (in this case APD (automated powder diffraction)) together with a new criterion, which takes into account the reflections in all parallel planes (when they exist) of each of the phases present in the sample studied.

Characterization of co-electrodeposited and selenized CIS (CuInSe 2) thin films

The structural, morphological, compositional and photoelectrochemical properties of co-electrodeposited and selenized CIS thin films were characterized using various techniques. As-deposited as well as selenized films exhibited a compact or a granular morphology depending on the composition. The film stoichiometry was improved after selenization at 550 °C in a tubular furnace. The film conductivity type, carrier concentration and flat-band potential were determined from the photoelectrochemical studies. The films are formed with a mixed composition of the binary as well as the ternary phases. From the results of these studies, a model is suggested for the superficial as well as the bulk conductivity types of the film. © 1997 Elsevier Science S.A. All rights reserved.

Surface microstructure of CIS thin films produced by rapid thermal processing

The surfaces of polycrystalline CuInSe2 thin films produced by rapid thermal processing (RTP) have been analyzed by scanning tunnelling microscopy and spectroscopy in ambient air. Deviating from standard measurement techniques the tunnelling microscope is driven by an AC sample voltage for surface morphology mapping in the constant current mode. Additionally, a Fermi energy mapping of the semiconductor surface is performed by mapping significant features of the I–V tunnelling characteristic. The polarity of the tunnelling current proves to be a reliable measure of the conductivity type of the material (n- or p-type); the observation of leakage currents at small bias voltages allows the identification of gap states around the Fermi level or metallic phases. Current-voltage curves taken at positions of different conduction type verigy the spectroscopic information in the maps. Typical areas imaged are (1.5 μm)2. Intra- and inter-granular nonuniformities of the conduction type are observed. Although the bulk material of all samples investigated is p-conductive, abrupt changes of the conductivity type of the surfaces from p- to n-type are observed as a function of the overall copper-to-indium ratio. The dominant current flow direction in slightly Cu-rich thin film bulk material is associated with p-type conduction, whereas In-rich samples exhibit largely n-type conductivity at the surface. Surfaces of copper-rich bulk materials show Fermi level pinning. The spectroscopic results do not depend on material and geometry of the tunnelling tip.

The performance of CuInSe2/CdS solar cells fabricated by the sandwich structure technique

The solar cell structure is based upon a heterojunction of p-type CulnSe2 (CIS) and n-type CdS or mixed CdZnS. The use of a mixed CdZnS layer is reported to increase the open circuit voltage. The cells have been fabricated using the stacked elemental layer (SEL) technique for the fabrication of CIS thin films. The effect of layer resistivities on the performance of the device has been studied. The photoresponse was also determined and an essentially flat region between 0.6 and 1.0 mu m was found. Dark and light I-V characteristics in ambient and different solar cell temperatures have been investigated. Capacitance-voltage-frequency relationships were determined. A photovoltaic effect was observed with an ELH illumination (85 mW cm-2). Conversion efficiencies up to 5.2% were obtained. The results are discussed and recommendations for future work are suggested.

Laser ablation deposition as a preparation method for sensor materials

Abstract In the development of new materials for sensors, laser ablation deposition (LAD) offers a relatively easy technique to realise complicated material compositions and multilayer structures. In this paper, LAD of three different thin films for sensor and actuator materials was carried out using a Q-switched ND:YAG laser and an XeCl excimer laser. CuInSe 2 (CIS) and Cu(In, Ga)Se 2 (CIGS) are well-established as exceptionally efficient semiconductors with potential applications in the fields of solar cells, infrared radiation monitors and fibre optic infrared detectors. CIS and CIGS thin films were deposited by an in situ process on fused silica and single crystal silicon substrates. The composition of the target material was largely maintained in the thin films. Lead zirconium titanate (PZT) materials are widely used in sensor and actuator applications. Laser ablation has been used to produce piezoelectric thin films for use in a low voltage cantilever bimorph actuator structure. Nd-doped PZT films were deposited on to Ag-coated oxidised silicon substrates. Displacements in the order of microns were obtained from a 15 mm long element. Diamond-like carbon thin films also have potential in sensor applications. By varying the peak power density of the laser beam and the deposition temperature, it was found to be possible to tailor the electrical and optical properties of the deposited films between the extremes of diamond-like and graphite-like carbon.

Characterization of CuInSe2 Thin Films by Photoluminescence Measurements

PL measurement has been clearly demonstrated to be an effective technique not only to evaluate the quality of CIS-based thin films but also to investigate the CIS formation process during the selenization stage. The studies investigating the dependence of the PL spectrum on the excitation intensity and the effect of post-annealing in air clarified that the PL spectrum of CIS thin films was dominated by a donor-acceptor (D-A) pair emission and even in a 5 min post-annealing, the PL peak of as-deposited CIS films drastically shifted from 0.93 eV (1340 nm) to about 0.89 eV (1400 nm).

LOW PRESSURE MOCVD OF COPPER BASED COMPOUNDS FOR PHOTOVOLTAIC APPLICATIONS

In recent years copper based compounds such as copper indium diselenide (CuInSe,, CIS) have emerged as the most promising candidates for high efficiency stable solar cells. To date research has been centred around the standard thin film deposition techniques i.e. evaporation, sputtering and electroplating. While these techniques give good films it has proved difficult to gain reproducible results. The production of devices based on CIS thin films has now progressed to a point where, to achieve further advances in device efficiency, more research into alternative deposition techniques and the basic deposition parameters is required. The major problems encountered with the standard technique are poor step coverage and lack of control over the stoichiometry. A low pressure MOCVD reactor has been constructed to investigate the growth of thin film CIS and the factors affecting deposition. The considerations involved in the design of such a system are discussed. A detailed study of reactor parameters affecting low pressure MOCVD of copper, copper indiurn and copper indium diselenide is being carried out. Mass spectrometric analysis of the decomposition products has been undertaken and the growth mechanisms and kinetics involved in these deposition processes are being investigated.