R. Matson

Structure, chemistry, and growth mechanisms of photovoltaic quality thin‐film Cu(In,Ga)Se2 grown from a mixed‐phase precursor

The formation chemistry and growth dynamics of thin‐film CuInSe2 grown by physical vapor deposition have been considered along the reaction path leading from the CuxSe:CuInSe2 two‐phase region to single‐phase CuInSe2. The (Cu2Se)β(CuInSe2)1−β (0<β≤1) mixed‐phase precursor is created in a manner consistent with a liquid‐phase assisted growth process. At substrate temperatures above 500 °C and in the presence of excess Se, the film structure is columnar through the film thickness with column diameters in the range of 2.0–5.0 μm. Films deposited on glass are described as highly oriented with nearly exclusive (112) crystalline orientation. CuInSe2:CuxSe phase separation is identified and occurs primarily normal to the substrate plane at free surfaces. Single‐phase CuInSe2 is created by the conversion of the CuxSe into CuInSe2 upon exposure to In and Se activity. Noninterrupted columnar growth continues at substrate temperatures above 500 °C. The addition of In in excess of that required for conversion produce...

Na in selenized Cu(In, Ga)Se2 on Na-containing and Na-free glasses : distribution, grain structure, and device performances

Abstract We examined the effect of deposition of Na on Mo-coated glasses and the Na content of the substrate glass on standard production Cu(In 1− x Ga x )Se 2 (CIGS)-based solar cells fabricated by selenization of Cu-Ga-In precursor thin films. Under optimal conditions, net Na content has a larger effect on the films than does the choice of substrate glass. Device performances improved with modest amounts of added Na on borosilicate glass. Device performances on soda-lime glass were not improved by adding Na. The supply of Na appears to have been adequate from the glass itself. A peak in device performance was found as a function of integrated Na in the CIGS layer as determined by secondary ion mass spectrometry (SIMS). The Na is found primarily in the areas of decreased grain size in the selenized CIGS where Ga is also found. S, deposited with the Na does not end up in the same place as does the Na. Rather, it tends to move toward the surface and accumulate in a buried layer. This is probably due to the reaction process rather than to the microstructure. Oxygen has no apparent effect on Na behavior in the CIGS.

Studies on sulfur diffusion into Cu(In,Ga)Se2 thin films

A systematic study was carried out to investigate the distribution of sulfur (S) in CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) absorbers which were exposed to an H2S atmosphere at elevated temperature. Results demonstrated that S diffusion into CIS layers was a strong function of the original stoichiometry of the absorber before sulfurization. Sulfur inclusion into Cu-rich CIS films was much more favorable compared to S diffusion in Cu-poor layers. The sulfur distribution profile was also strongly influenced by the micro-structure of the original CIS and CIGS layers, with sections of the films with smaller grains accommodating more S. Copyright

Na incorporation in Mo and CuInSe2 from production processes

Abstract Results of characterization of thin films of Mo deposited by DC magnetron sputtering on soda-lime glass (Mo/SLG) and CuInSe 2 (CIS) on Mo/SLG are presented. The primary objective of the work was to clarify the factors determining the concentration of Na in commercial-grade CIS. Mo films were deposited by three laboratories manufacturing CIS thin film solar cells. Analysis was by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffraction. Changes in Mo deposition parameters in general affected the Na level but there was no obvious link to any single Mo deposition parameter. Oxygen content directly affected the Na level. The Na behavior was not obviously connected to film preferred orientation. Selenization of the Mo layers was also examined. Elemental Se vapor was found to produce significantly less selenization than H 2 Se. The amount of selenization was also strongly dependent upon Mo deposition conditions, although a specific source of the change in reaction rate was not found. Na distributions in the CIS deposited on the Mo were not limited by the diffusivity of the Na. The Na concentration in the CIS was increased by annealing the Mo films both with and without intentionally added Na. The Na level in the CIS appears to be set more by the CIS deposition process than by the Na concentration in the Mo so long as the Mo contains sufficient Na to saturate the available sites in the CIS.

CuIn(Ga)Se2-based devices via a novel absorber formation process

Abstract A novel pathway for the formation of copper–indium (gallium) diselenide has been developed. This two-stage process consists of (a) the formation of Cu–In–(Ga)–Se precursors, and (b) subsequent thermal treatment to form CuIn(Ga)Se2. The morphology, structure and growth mechanism for several different precursor structures prepared under various conditions were studied and correlated to the deposition parameters as well as the structure and morphology of the annealed films. Photovoltaic devices prepared from CuInSe2 and CuIn0.75Ga0.25Se2 resulted in efficiencies of 10% and 13%, respectively.

Growth of silicon thin layers on cast MGSi from metal solutions for solar cells

Abstract In pursuit of device-quality layer formation on cast, metallurgical-grade silicon (MG Si) substrates for solar cells, the growth kinetics of silicon liquid phase epitaxy (LPE) from metal solutions was studied. We found an ideal solvent system, Cu Al, for growth of Si layers with thicknesses of tens of microns on cast MG Si substrates by LPE at temperatures near 900°C. This solvent system utilizes Al to ensure good wetting between the solution and substrate by removing silicon native oxides, and employs Cu to control Al doping into the layers. Isotropic growth is achieved because of a high concentration of solute silicon in the solution and the resulting microscopically rough interface. As a result, macroscopically smooth Si layers have been grown on cast MG Si that are suitable for device fabrication. With the microscopically rough interface, the growth rate has been studied with a diffusional model involving a boundary layer that takes the melt convection into account. The model was found to be in good agreement with experimental results, indicating only a small boundary layer (∼ 0.1 cm) and a silicon diffusivity of ∼ 2 × 10−4 cm2 s−1 in the liquid. The thin layer (∼ 30 μm) grown on the MG-Si substrate has a minority-carrier diffusion length greater than the layer thickness.

Defect Chalcopyrite Cu(In 1-x , Ga x ) 3 Se 5 Polycrystalline Thin-Film Materials

The defect chalcopyrite material CuIn 3 Se 5 has been identified as playing an essential role in efficient photovoltaic action in CuInSe 2 -based devices; it has been reported to be of n-type conductivity, forming a p-n junction with its p-type counterpart CuInSe 2 . Because the most efficient cells consist of the Cu(In 1-x Ga x )Se 2 quaternary, knowledge of some physical properties of the Ga-containing defect chalcopyrite Cu(In 1-x Ga x ) 3 Se 5 may help us better understand the junction phenomena in such devices. Polycrystalline Cu(In l-x Ga x ) 3 Se 5 (with O 2 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.

Junction formation and characteristics of CdS/CuInSe2/metal interfaces

Polycrystalline thin films of CuInSe2 (CIS) were prepared by galvanic electrochemical (EC) and physical vapor deposition (PVD) methods and were characterized using high resolution photoluminescence at low temperatures to study defect states, scanning electron microscopy to study surface morphology, and transmission electron microscopy to determine the grain size and individual crystallographic orientation of the grains for possible correlation between the properties of the two films. Metal contacts, Schottky devices in the form of Al/p-CIS, and CdS/p-CIS heterostructures were also prepared. The electrical properties of the resulting interfaces were investigated using current-voltage (I–V) and capacitance-voltage (C–V) characteristics, and by electron-beam-induced current measurements. Devices prepared from PVD films exhibited a higher generation factor G, sharp interfaces and the lowest density of interface states. On the contrary, devices of low G values (made from EC films) showed a much higher density of interface states with a high density of both shallow and deep traps, as detected by deep level transient spectroscopy. The results were used to correlate the resulting variation in the heterojunction characteristics and back contact behavior with the corresponding defect states dominating the CIS.

The role of oxygen in CdS/CdTe solar cells deposited by close-spaced sublimation

The presence of oxygen during close-spaced sublimation (CSS) of CdTe has been previously reported to be essential for high-efficiency CdS/CdTe solar cells because it increases the acceptor density in the absorber. The authors find that the presence of oxygen during CSS increases the nucleation site density of CdTe, thus decreasing pinhole density and grain size. Photoluminescence showed that oxygen decreases material quality in the bulk of the CdTe film, but positively impacts the critical CdS/CdTe interface. Through device characterization, they were unable to verify an increase in acceptor density with increased oxygen. These results, along with the achievement of high-efficiency solar cells (13% AM1.5) without the use of oxygen, led them to conclude that the use of oxygen during CSS deposition of CdTe can be useful but is not essential.

Fundamental thermodynamics and experiments in fabricating high efficiency CuInSe2 solar cells by selenization without the use of H2Se

Selenization is the current process by which state‐of‐the‐art CuInSe2 polycrystalline thin‐film photovoltaic modules are industrially fabricated. The distinguishing characteristic of this approach is that material deposition is separate from compound formation. In conventional selenization, In‐Cu layers, often referred to as precursors, are deposited on molybdenum‐coated glass substrates and subsequently transformed into CuInSe2 following exposure to a selenium‐containing environment. Although the highly toxic gas, H2Se, has been considered a necessary component of selenization, recent safety concerns have accelerated the development of Se vapor as a possible substitute for H2Se. In more recent variations of the process, solid selenium is incorporated during the precursor fabrication step, and subsequent thermal annealing is used to form compounds among the three elements. In this paper, we discuss the thermodynamic fundamentals of selenization using elemental Se as an alternative to H2Se. This discussion...