Nanosolar, Inc. (San Jose, CA) earned U.S. Patent 7,663,057 for its low-cost, nanoparticle solution based, high-volume production method for large area CIGS photovoltaic devices. The quantum dot inks are compatible with roll-to-roll manufacturing of photovoltaic cells and modules that can be readily scaled up to high production volumes say inventors Dong Yu, Jacqueline Fidanza, Martin R. Roscheisen and Brian M. Sager.
The liquid nanoparticle ink (aqueous or otherwise) may be spread in a thin film over a substrate using solution based coating techniques such as web coating, spray coating, spin coating, doctor blade coating, printing techniques including contact printing, gravure printing, microgravure printing, ink-jet printing, jet deposition, and the like. Such a film can be deposited on a flexible substrate, in a roll-to-roll manner using a commercially available web coating system. The film may then be heated to remove the solvent and to sinter the nanoparticles together to form a layer of a IB-IIIA-VIA alloy.
The majority of the mass of the particles range in size from no more than about 40% above or below an average particle size of about 5 nanometers, so no more than about 2 nanometers above or below the average particle size. The use of such ink avoids the need to expose the material to an H2Se gas during the construction of a photovoltaic cell and allows more uniform melting during film annealing, more uniform intermixing of nanoparticles, and allows higher quality absorber films to be formed.
The patent covers several key features of a liquid ink that impact cell structure and function. The liquid ink includes particles contain elements of groups IB, IIIA and (optionally) VIA, e.g., copper and indium (with or without gallium) and selenium or sulfur. The particles may be between about 0.1 and about 500 nanometers (nm). The decreased particle size can significantly lower both the melting point and the sintering temperature required
Generally, reduction in the melting point is inversely proportional to the particle radius, i.e., the smaller the nanoparticles, the lower the melting point. Smaller particles also tend to pack closer together and make better contact with each other. Reduction in size from bulk material to particles in about the 500 nm regime can already show significant differences in melting point and other altered physical and chemical properties.
With much smaller particle sizes, e.g. in the nanometer size range, the surface area of particles will increase and nanoparticles will be in intimate contact with each other. In addition, in the nanometer size range, the reactivity of the particles and interaction between nanoparticles will be enhanced. This will help particles fuse together much easier thus enhancing the cohesion of the resulting CIGS layer. This promotes coalescence between neighboring particles during sintering.
Quantum nanoparticles are an important class of nanoparticles that can be used to make inks according to embodiments of the present invention. The electronic and optical properties of metals and semiconductors can change dramatically as the particulates of the materials are reduced to approach the nanometer size range of dimensions. At such size levels, the physical dimensions of the material generally impact the electronic, optical, chemical, and/or physical behavior of the material.