Status of flexible CIS research at ISET

Polycrystalline thin film solar cells fabricated on light-weight, flexible substrates are very attractive for space applications. In this work CulnSe2 (CIS) based thin film devices were processed on metallic foil substrates using the selenization technique. CIS deposition method involved reaction of electron-bean evaporated Cu-In precursor layers with a selenizing atmosphere at around 400 C. Several metallic foils such as Mo, Ti, Al, Ni, and Cu were evaluated as possible substrates for these devices. Solar cells with AM1.5 efficiencies of 9.0-9.34 percent and good mechanical integrity were demonstrated on Mo and Ti foils. Monolithic integration of these devices was also demonstrated up to 4 in x 4 in size

Efficient CuInSe 2 Solar Cells Fabricated by a Novel Ink Coating Approach

A novel technique is developed for the deposition of CuInSe 2 (CIS) thin films for solar cell applications. The technique uses an ink formulation that contains Cu-In metallic pigments. A precursor layer is first formed coating this ink onto the selected substrate. The precursor film is then reacted with Se to form the CIS compound. Solar cells were fabricated on CIS absorber layers prepared by this low cost ink coating approach and devices with a conversion efficiency of over 9.0% were demonstrated. © 1998 The Electrochemical Society. S1099-0062(98)08-063-8. All rights reserved. Manuscript submitted August 14, 1998; revised manuscript received September 9, 1998. Available electronically October 1, 1998. Group I-III-VI materials are considered to be highly promising as absorber layers in high-efficiency thin film solar cell structures. In fact, the highest efficiency thin film device to date was produced on a Cu(In,Ga)Se 2 (CIGS) absorber film grown by a vacuum evaporation technique. The demonstrated conversion efficiency of 17.7% confirmed the capability of this material to yield highly efficient active devices when employed in thin film solar cell structures. High-efficiency solar cells have commonly been fabricated on CuInSe 2 (CIS) or CIGS absorbers deposited by costly vacuum deposition techniques such as coevaporation 1 and two-stage processes utilizing evaporation or sputtering. 2 There is presently great interest in the development of new lower cost processing methods for the growth of high quality CIS-type absorbers for thin film solar cell applications. Slurry or ink deposition by large area nonvacuum coating methods such as screen printing, spraying, curtain coating, roll coating, or doctor blading are attractive low-cost approaches for the growth of thin film solar cell absorbers, provided that the precursor layers obtained by these deposition techniques can be converted into high quality semiconductor films that are required for solar cell fabrication. There have been several attempts to deposit CIS absorbers using the screen printing technique. For example, Arita et al. described a method that involved (i) mixing pure Cu, In, and Se powders in the compositional ratio of 1:1:2, (ii) milling these powders in a ball mill and forming a screen printable paste, (iii) screen printing the paste on a substrate, and (iv) sintering this precursor film to form the compound layer. As can be seen from the review of previous work, the nature of the ingredients in the formulation of a paste or an ink is very important for the formation of a precursor layer which can later be converted into a high quality CIS-type compound film with properties that are desirable for solar cell applications. In this article we report a low-cost ink coating technique that was successfully employed for the deposition of CIS absorbers that could be used for the fabrication of over 9% efficient thin film solar cells. Experimental The general steps of the low-cost process used in this work for the growth of thin film CIS absorbers are schematically shown in The source of Cu and In in this work was a Cu-In alloy powder with a preselected and fixed Cu/In stoichiometric ratio. The Cu-In alloy powder was obtained by the melt atomization technique. To prepare the powder, 99.99% pure Cu and 99.99% pure In were melted under a hydrogen curtain at above 900°C. The Cu/In ratio of the melt corresponded to the targeted value range of 0.87-0.9. The melted alloy was transformed into powder in a gas atomizer employing Ar as the quenching gas. Quenched powder was collected at the bottom of the reactor and sieved to separate the particles that were smaller than 20 µm in size which were used in this work as the pigment. About 10 g of the Cu-In pigment was mixed with 23 g of water. A small amount (about 1.5 wt %) of a wetting agent and dispersant were added to this aqueous formulation. The mixture was milled in a ball mill for 42 h. The resulting metallic ink was water-thin. Particle size analysis was done on a sample of this ink using

Application of CIS of high-efficiency PV module fabrication. Annual technical progress report, April 1, 1996--March 31, 1997

This is the Phase II Annual Technical Report of the subcontract titled {open_quotes}Application of CIS to High Efficiency PV Module fabrication.{close_quotes} The general objectives of the program are the development of a novel, non-vacuum process for CIS film deposition, optimization of the various layers forming the CIS device structure, and fabrication of high efficiency submodules. The specific goals of the project are the development of 13% efficient small area cells and 10% efficient submodules using a novel, low-cost CIS deposition approach. During this research period, the authors concentrated their efforts on three different areas of research. Within the National CIS Partnership Program, they participated in the {open_quotes}substrate/Mo interactions{close_quotes} working group and investigated issues such as Na diffusion from the soda-lime glass substrate into the Mo layers and CIS films. It was determined that the Na content within the Mo layers was not a strong function of the nature of the Mo film. However, diffusion through the Mo layers was found to be a function of the Mo film characteristics as well as a very strong function of the CIS growth process. Na was found to be on the grain boundaries in Mo and CIS layers

Application of CIS to high-efficiency PV module fabrication. Annual technical progress report, April 1, 1995--March 31, 1996

The authors investigated the interactions between the soda-lime glass substrate, the Mo contract film and the CIS absorber layer. Excessive Na diffusion through the Mo layer was found to be the reason for excessive interaction between the substrate and the CIS layers obtained by the H{sub 2}Se selenization technique. This chemical interaction influenced the stoichiometric uniformity of the absorbers. Addition of Ga into the CIS layers by the two-stage selenization technique yielded graded absorber structures with higher Ga content near the Mo/absorber interface. Gallium was later diffused through the absorber film by a high-temperature annealing step, and large bandgap alloys were obtained. Solar cells with active-area efficiencies of close to 12% were fabricated on these CIGS layers. Sulfur addition experiments were also carried out during this period. By controlling the Se and S availability to the precursors during the reaction step of the process, various S profiles were obtained in high-bandgap absorber layers. The highest-efficiency cell made on S-containing absorbers was about 10% efficient. A low-cost, non-vacuum technique was successfully developed for CIS film growth. Layers prepared using this novel approach were used for solar-cell and submodule fabrication. Solar cells with active-area efficiencies around 13% were demonstrated; submodules with efficiencies above 8% were also fabricated. These results represent the best PV devices ever produced on CIS layers obtained by a non-vacuum technique

Final Report: Sintered CZTS Nanoparticle Solar Cells on Metal Foil; July 26, 2011 - July 25, 2012

This is the final report covering 12 months of this subcontract for research on high-efficiency copper zinc tin sulfide (CZTS)-based thin-film solar cells on flexible metal foil. Each of the first three quarters of the subcontract has been detailed in quarterly reports. In this final report highlights of the first three quarters will be provided and details will be given of the final quarter of the subcontract

Final Report: Sintered CZTS Nanoparticle Solar Cells on Metal Foil; July 26, 2011 - July 25, 2012

This is the final report covering 12 months of this subcontract for research on high-efficiency copper zinc tin sulfide (CZTS)-based thin-film solar cells on flexible metal foil. Each of the first three quarters of the subcontract has been detailed in quarterly reports. In this final report highlights of the first three quarters will be provided and details will be given of the final quarter of the subcontract

Novel two-stage selenization methods for fabrication of thin-film CIS cells and submodules. Final subcontract report, March 1, 1993--March 31, 1995

This is the Phase 11 Final Technical Report of the subcontract titled {open_quotes}Novel Two-Stage Selenization Methods for Fabrication of Thin Film CIS Cells and Submodules.{close_quotes} The general objectives of the program are the development of a cost-effective, large-area process for CIS film deposition, optimization of the various layers forming the CIS device structure, and fabrication of high efficiency submodules using these optimized device components. During this research period, growth parameters of ZnO window layers were varied to optimize their electrical and optical properties. Investigation of the chemical interactions between the glass substrates, Mo layers and the selenization atmosphere revealed that the nature of the glass/Mo substrate greatly influenced the quality of the solar cells fabricated on them. Moderate amounts of sodium diffusing from the soda-lime glass substrate into the CIS film improved the efficiencies of the solar cells fabricated on such films. Mo layers allowing excessive Na diffusion through them, on the other hand, reacted excessively with the H{sup 2}Se environment and deteriorated the solar cell performance. Addition of Ga into the CIS layers by the two-stage selenization technique yielded graded absorber structures with higher Ga content near the Mo/absorber interface. Cu-rich CIS layers were grown with grain sizes of larger than 5 {mu}m. In the Phase I Annual Report large area CIS submodules with efficiencies of about 3% were reported. During the present Phase II program 1 ft{sup 2} size CIS submodule efficiency was improved to 7%. Smaller area submodules with efficiencies as high as 9.79% were also fabricated using CIS layers obtained by the H{sub 2}Se selenization method. The processing yield of the devices based on a non-vacuum CIS deposition approach was improved and solar cells with efficiencies greater than 10% were fabricated

Large area low cost processing for CIS photovoltaics. Final technical report

An ink coating method was developed for CIS absorber deposition. The technique involves four processing steps: (1) preparation of a Cu-In alloy powder, (2) preparation of an ink using this powder, (3) deposition of the ink on a substrate in the form of a precursor layer, and (4) selenization to convert the Cu-In precursor into a fused CIS film. Absorbers grown by this low-cost, large-area method were used in the fabrication of 10.5% efficient solar cells

CIS-Type PV Device Fabrication by Novel Techniques; Phase I Annual Technical Report, 1 July 1998 - 30 June 1999

This report describes work performed by International Solar Electric Technology, Inc. (ISET) during phase I of the R&D partnership subcontract titled ''CIS-Type PV Device Fabrication by Novel Techniques.'' The objective of this program is to bring ISET's novel non-vacuum CIS technology closer to commercialization by concentrating on issues such as device-efficiency improvement, larger-bandgap absorber growth, and module fabrication. Advances made in CIS and related compound solar cell fabrication processes have clearly shown that these materials and device structures can yield power conversion efficiencies in the 15%-20% range. However, many of the laboratory results on CIS-type devices have been obtained using relatively high-cost vacuum-based deposition techniques. The present project was specifically geared toward developing a low-cost, non-vacuum ''particle deposition'' method for CIS-type absorber growth. There are four major processing steps in this technique: (i) preparation of a starting powder containing all or some of the chemical species constituting CIS, (ii) preparation of an ink using the starting powder, (iii) deposition of the ink on a substrate in the form of a thin precursor layer, and (iv) conversion of the precursor layer into a fused photovoltaic absorber through annealing steps. During this Phase I program, ISET worked on tasks that were geared toward the following goals: (i) elimination of back-contact problems, (ii) growth of large-bandgap absorbers, and (iii) fabrication of mini-modules. As a result of the Phase I research, a Mo back-contact structure was developed that eliminated problems that resulted in poor mechanical integrity of the absorber layers. Sulfur inclusion into CIS films through high-temperature sulfurization in H{sub 2}S gas was also studied. It was determined that S diffusion was a strong function of the stoichiometry of the CIS layer. Sulfur was found to diffuse rapidly through the Cu-rich films, whereas the diffusion constant was at least three orders of magnitude smaller in Cu-poor layers. Additionally, S profiles in sulfurized CIS films were correlated with the distribution of the grain size through the film. Absorbers containing large concentrations of Ga near the Mo contact interface also had large S content in that same region due to the small grain size of the Ga-containing material. New work on monolithic integration procedures overcame the problem of low shunt resistance and yielded CuIn(S,Se){sub 2} (CISS) mini-modules of about 64-cm{sup 2} area with close to 7% efficiency

Technical efficiency measurement within the manufacturing sector in Cote d'Ivoire: A stochastic frontier approach

This article analyses the productive performance in four manufacturing sectors of the Ivorian economy: textiles and garments, metal products, food processing, wood and furniture. To appraise the productive performance, econometric production frontier models are estimated, illustrating the maximum output attainable from a given quantity of inputs. The frontier and firm efficiency scores are derived from stochastic production functions estimated on cross-sectional data. The stochastic specification of the models allows for the decomposition of the error term into two components, one the normal random effect and the other to account for technical inefficiency that we explain by various exogenous variables describing the economic and institutional environment. Firm size proves to be a statistically significant determinant of the productive performance. Across the four sectors, the positive impact of being large compensates the negative effect of a formal institutional status in an environment where government regulations still prevail.