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The light-soaking effect in ZnO/ Cu(InGa)Se2 (CIGS) based solar cells has been studied. A CIGS thin film with Cu(InGa)(SeS)2 surface layer was obtained by selenization (H2Se)/sulfurization (H2S). A high resistively ZnO buffer layer deposited by the atomic layer deposition technique was used as a buffer layer. We found that the light-soaking effect mainly correlates with the properties of the CIGS surface, rather than with the properties of the ZnO buffer/window layer. This phenomenon can be eliminated by surface etching or doping CIGS surface with Zinc. Zinc diffusion using diethylzinc gas has been proposed in this work. To date, we have achieved efficiency of 13.9% (Voc: 560 mV, Jsc: 35.0 mA/cm2, FF: 0.71) without light soaking effect.
The measurement of quantum efficiency with bias voltage is a powerful tool to characterize CdTe/CdS solar cell. As the quantum efficiency changes drastically with bias it will be referred to as Apparent Quantum Efficiency AQE. The AQE gives insight to the spectral contents of the cell current and therefore resolves the spatial carrier collection in the cell at each working point. So it is possible to understand the influence of the junctions and changing resistances in the cell. The photoconductivity of CdS facilitates AQE well above unity, i.e. up to 100, at high forward bias. The spectral sensitivity of the CdS photoconductivity affects the cell current strongly. This can explain the dependence of fill factor and roll-over of the I-V characteristics on the spectral content of illumination. Further more back contact junction influence and defect related features, such as sub band gap generation, are evident in the AQE for high forward bias.
Solar cell devices of the structures CdS/CdTe and CdS/HgCdTe, based on II-VI semiconductor thin films have been fabricated and analyzed. CdS thin films were deposited by the recently established novel deposition technique, namely, 'Photochemical deposition' and the Cd rich HgxCd1−xTe films used for the device fabrication were deposited by the conventional ‘electrochemical deposition’ technique. First solar cell devices of the structures CdS/CdTe and CdS/HgCdTe using photochemically deposited CdS films, were fabricated and analyzed for the conversion efficiency.
The diffusion behavior and microstructural properties of the CBD-ZnS/Cu(In,Ga)Se2 (CIGS) interface of high-efficiency Cd-free CIGS thin film solar cells with efficiencies exceeding 17 % have been investigated using energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). CBD-ZnS thin layers are found to grow with small polycrystalline grains that consist of sphalerite and wurtzite phases. This is in sharp contrast to CBD-CdS layers which grow epitaxially on CIGS grains. Micro-EDX analysis revealed Zn indiffusion into the CIGS thin layer a distance of approximately 40 nm from the CBD-ZnS/CIGS interface boundary after air-annealing at 200°C, whereas no diffusion of Zn was observed before annealing. These results suggest that the main role of the air-annealing process is not substitution of Se vacancies by oxygen atoms but the diffusion of Zn into CIGS which may result in the formation of a pn homojunction. On the other hand, negligible Zn-diffusion into a CuGaSe2 thin film was found at the CBD-ZnS/CuGaSe2 interface boundary even after air-annealing. The cell performance of CBD-ZnS/CIGS thin film solar cells is discussed in connection with the Ga/(In+Ga) atomic ratio, conduction band-offset and Zn-diffusion.
This contribution compares the growth of Cu(Ga,In)S2 based thin film solar cell absorbers in rapid thermal systems using sulfur vapor Sx or H2S/Ar as reactive atmosphere, focusing on Ga-related influences on film growth and phase formation. Cu-In alloying in the precursor is kinetically hindered by the presence of Cu-Ga phases. In sulfur vapor Ga-containing samples sulfurize via an intermediate CuIn2S8 phase, thereby delaying the full sulfurization and recrystallization of the layer. In contrast, in H2S/Ar fast Ga-In interdiffusion and no intermediate chalcogenide phases are observed. The inhomogeneous Ga depth distribution usually reported for sequentially prepared Cu(In,Ga)S2 films can be assigned to the segregation of CuGaS2 prior to CuInS2.
A chemical and kinetic basis underlying processing strategies for thin film polycrystalline CdTe/CdS solar cell fabrication is presented. The processing conditions employed for moderate and high conversion efficiency CdTe/CdS solar cells fall within a consistent framework based on temperature and concentration of CdCl2 and O2 species during film deposition or thermal treatment. Detailed microstructural and compositional results are compared for thin-film CdTe/CdS structures with CdTe deposited by physical vapor deposition, electrodeposition and close-space sublimation. X-ray diffraction coupled with a model for diffusion of CdS into CdTe is used to determine the effect of treatment conditions on bulk and grain boundary diffusion coefficients. The effects of CdS diffusion, oxides, recrystallization and grain growth with respect to device efficiency, stability and process control over large area are discussed.
Copper indium gallium selenide (CIGS) solar cells on thin film stainless steel substrates were evaluated by current-voltage-temperature (IVT) from 150K-350K to determine current transport mechanisms. Both dark and photo data at reverse and low forward voltages exhibited tunneling-like behavior. At intermediate forward voltages, diffusion or thermionic emission are suggested by an ideality factor close to 1.0. At higher currents and voltages there is a trend towards recombination or space change limited behavior.
The method of diffuse laser light scattering (LLS) has been tested for process monitoring of simultaneous and sequential chalcopyrite film formation. Characteristic LLS features are observed which can be assigned to morphological and optoelectronic film modifications. The surface roughness development as a function of time of CuInS2 and CuInSe2 epilayers grown on Si(111) substrates is monitored. It is found that the critical thickness of a smooth surface without islands depends on the film composition (Cu/(Cu+In) ratio). LLS transients for sequential film formation depict a series of characteristic features which are connected with, e.g., precursor transformation, surfacial sulfurization, and secondary phase transformation.
Thin film CuIn(Ga)Se2 samples were prepared by a two-stage method starting from amorphous or nano-crystalline precursor structures. Deposition of individual Cu+Se and In+Se layers as a function of substrate temperature revealed the onset of detectable crystal structures at Tsub = 100°C and Tsub. ≥200°C, respectively. For the quaternary system the formation of CuInSe2 was observed at 400°C and evidence was found for liquid CuxSe assisted growth.
Further focus was on surface termination schemes with the objective to enhance the opencircuit voltage. Ga+Se, In+Se, Ga+Se/In+Se, and In+Se/Ga+Se terminating layers with varying amounts of Ga and In are addressed. Schemes studied to date have resulted in an increase in Voc at the expense of short-circuit current. The use of Ga containing termination layers resulted in the formation of large voids in the CIGS which could be explained by the volume changes during formation of the quaternary material and a Cu2Se free surface in these instances.
The effect of CdCl2 annealing conditions of glass/TCO/n/CdS/p-CdTe solar cell structures on the deep level density and carrier lifetime of the p-CdTe layer and correlation with the solar cell conversion efficiency was investigated. CdCl2 treatment was carried out at temperatures ranging from 370 to 460°C for 15 min. A clear correlation between trap density, carrier lifetime, conversion efficiency and the CdCl2 annealing conditions was observed. Un-annealed structures had a conversion efficiency of 5.7%, hole trap energy of EV+0.42eV, hole trap density of 8.71×1014cm−3, and decay lifetime of 0.15μs. The optimum CdCl2 annealing temperature was found to be 415°C for structures grown at a substrate temperature of 595°C, where the conversion efficiency, hole trap energy, hole trap density, decay lifetime were 13.4%, EV+0.44eV, 8.10×1012 cm−3 and 0.40μs, respectively.
The paper describes ISET's patented non-vacuum process for low cost mass production of CIGS solar cells. In this process, the water based precursor inks of mixed oxides are deposited on various conducting substrates by a variety of non-vacuum coating techniques. The oxides are converted to CIGS by annealing and the device is completed by deposition of CdS by CBD followed by ZnO deposition by MOCVD. Small area solar cells with efficiency >13% have been fabricated by this process. The advantages of this non-vacuum process are: high compositional control of the absorber layer, high materials utilization and low cost.
Monolithically integrated Cu(In,Ga)Se2-mini-modules (CIGS) have been fabricated on polymer as well as on metal foils. Preferred foils with view on costs and physical properties were ferritic steel, titanium, Fe/Ni-alloys (e.g. Kovar®) and polyimide as the only appropriate low temperature candidate. The metal substrates were isolated by multiple layers of SiOx and Al2O3 which served both as diffusion barrier against substrate elements and dielectric barrier. Small area cell efficiencies of 13.8% on ferritic steel foils and 10.6% on polymer foils (both without antireflective coating) were obtained. First monolithically integrated submodules of up to 10×10cm2 substrate area were fabricated both on ferritic steel and polyimide substrates. Different patterning methods have been applied and matched to the respective substrate materials.
CuGaSe2 (CGS) is a promising material for high efficiency thin film solar cells though predicted device performance has not been realized. Understanding the difference in the chemical nature between CuInSe2 (CIS) and CGS is critical for improving Cu (In, Ga) Se2 solar cells with high Ga concentrations. In this work, we have investigated the effects of oxygen-annealing on Ga-rich CGS epitaxial films focusing on compositional changes and secondary phase formations. The photoluminescence (PL) spectrum of Ga-rich films after oxygen-annealing was observed to always change into a spectrum characteristic of CGS grown under Cu-excess conditions. Electron probe micro-analysis (EPMA) measurements indicate the formation of Ga-O after oxygen-annealing. Selective etching of the Ga-O phase showed the composition of the CGS phase became close to stoichiometric. The oxygen-annealed films showed multiple pits ∼ 100 nm in depth and ∼ 2.5 μm in width. The Ga-O phase is founded in a layer formed on the surface of the CGS phase and in a columnar form rising from the bottom of the pits to the film/substrate interface. The above results suggest that excess Ga in Ga-rich CGS tends to react with oxygen to form Ga-O, thus the composition of the remaining CGS approaches stoichiometry consistent with the changes observed in PL.
Cu(In,Ga)Se2 thin films were prepared by physical vapor deposition. The CIGS films were deposited by three kinds of method. The 1st was “2-stage process” in which (In,Ga)2Se3 precursor layer was deposited on Mo coated soda-lime glass at the 1st stage, and then exposed to Cu and Se fluxes to form CIGS films at the 2nd stage. The 2nd method was an ordinary “3-stage process”. The 3rd method was “2-stage deposition and post-annealing process” in which CIGS films were deposited at low substrate temperatures and then the obtained CIGS precursor films were annealed in Se flux at high temperatures. A solar cell using a CIGS film prepared at 400 °C by the “2-stage process” showed an efficiency of 11.8 % and that using a CIGS film deposited at 350 °C by the “3-stage process” showed an efficiency of 12.4 %. The CIGS films deposited by the “2-stage deposition and post-annealing process” have similar microstructures to the device quality CIGS films deposited by the “3-stage process” at high temperatures. The solar cell with an MgF2/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5 % (Voc=0.634 V, Jsc=36.4 mA/cm2, FF=0.756). The thin CIGS films with a smooth and flat surface can be fabricated by the “2-stage deposition and post-annealing process”. The solar cell using a 0.7μm CIGS absorber layer showed an efficiency of over 12 % and a large open circuit voltage of 0.677 V.
The observed junction between α-CuInSe2 and the In-rich compositions in the β-phase domain (e.g. CuIn3Se5) appears to play an important role in the photovoltaic process . There remain, however, inconsistencies and uncertainties about the boundary and structure of this phase. In general the structure of this phase belongs to defect tetrahedral family of structures , which can be described as normal tetrahedral structures with a certain fixed number of unoccupied structure sites. In this work, the local structures of various (Cu2Se)x(In2Se3)1−x semiconductor alloys in the β-phase domain were studied by Extended X-ray Absorption Fine Structure (EXAFS) and the results were compared to those for the α-CuInSe2 phase. The long- range order of these compositions was studied by X-Ray powder Diffraction (XRD) and electron diffraction. It was found the local structures of these compounds are well defined. These compounds, however, could not be well described by any long-range order structure model, especially the selenium position. First-principles band structure calculations were performed to assist in assigning crystal structures to CuInSe2, CuIn3Se5 and CuIn5Se8. The calculations indicated that the local environments of these compounds are well defined. Their long-range order might depend sensitively on growth history and the configurational entropies as suggested by the similar formation energies of several possible crystal structures for CuIn3Se5 and CuIn5Se8.
Chemistry of co-evaporated CIGS surfaces submitted to chemical treatments relevant to fabrication steps were investigated by XPS and admittance spectroscopy. A Se XPS signal specific of the CIGS surfaces was identified. Surface states seen by Admittance and surface chemistry are seen to change significantly during the elaboration steps. Consequences for device elaboration are briefly discussed.
Cu(In,Ga)Se2 (CIGS) thin films were deposited using the three-stage process. At the third stage, an amount of Indium was added to the CIGS that is greater than the standard used in processing high-efficient CIGS solar cells. The effects of Indium excess and substrate temperature were then investigated by electron-beam-induced-current (EBIC) and cathodoluminescence (CL). The addition of more indium compared to the standard noticeably affects the ZnO/CdS/CIGS heterojunction. On the other hand, the substrate temperature primarily affects the luminescence behavior of these films. It is suggested than In enrichment and Na incorporation play a main role in the electronic properties of the film. From these results, the efficiencies obtained for this set of CIGS cells are finally understood.
Transparent conducting oxides (TCOs) are becoming a more critical element in thin-film photovoltaic devices. In the continued drive to increase efficiency and stability while reducing cost and optimizing performance, the optical, electrical, and materials properties of TCOs gain increasing importance. TCOs can perform a variety of important functions, including contacts, antireflection coatings, and chemical barriers. In this paper, we will review some of the current advances in the field of transparent conductors and, where possible, will relate these advances to thin-film photovoltaic devices. Highlights will be on the rapidly growing collection of new n- and p-type materials; the implications of these materials on PV have not been fully assessed.
High efficiency CdTe solar cells are typically grown with CdTe thicknesses from 3 to 15 μm, although the thickness required for 90% absorption of the incident irradiation at 800 nm is only ∼1 μm. In this paper, we present the effect of CdTe thickness reduction on the performance of CdS/CdTe solar cells in which both the CdS and CdTe films were grown by sputtering. We produced a series of cells with different CdTe thickness (from 0.5 to 3.0 μm), and held the CdS thickness and back-contact-processing constant. The effect of CdTe thickness reduction on the diffusion of CdS into CdTe was studied using optical absorption and x-ray diffraction techniques. Only slight decreases occur in open-circuit voltage, short-circuit current, and fill factor with decrease in CdTe film thickness to 1.0 μm. Almost 10% efficient cells were obtained with 1 μm CdTe. Below 1 μm, all cell parameters decrease more rapidly, including the red quantum efficiency.
We present a systematic study on the polycrystalline Cu(In,Ga)(S,Se)2 alloys with a gallium to indium ratio of Ga/(Ga+In)<0.3 and a sulfur to selenium ratio varying in the range between 0<S/(S+Se)<1. All samples were grown by coevaporation of the elements at constant rates under high vacuum conditions. The formation of island-like (Cu,S,Se) segregations correlate with the sulfur to selenium ratio in the layer and are found in the growth region near the copper rich phase boundary. These segregations are related to a preferred incorporation of sulfur in the copper rich growth mode. We obtain solar cell grade material from an indium rich growth mode up to a sulfur to selenium ratio of S/(S+Se)=0.9. A detailed analysis of the electronic and optical properties of Mo/CIGSSe/CdS/ZnO:Al heterojunctions allows us to determine the energetic position of the bands within the Cu(In,Ga)(S,Se)2 alloy system. We find that contrary to the Cu(In,Ga)(Se)2 alloy system the valence band position is significantly lowered with increasing bandgap.