Thin film flexible solar cell

#The present invention relates generally to solar or photovoltaic cells and, more particularly, relates to thin film solar cells deposited onto plastic substrates at low temperatures.

#The conversion of the sun's electromagnetic energy into electricity occurs in a wide variety of semiconductor materials. Single element semiconductors, such as silicon (Si) and germanium (Ge), as well as some compound materials, such as gallium arsenide (GaAs), cadmium telluride (CdTe) and indium phosphide (InP), are capable of forming junctions that create a built-in electric field in the material. These materials are further capable of absorbing photons of energy sufficient to create electron-hole pairs, thereby creating an electrical current. This phenomenon, which is known as the photovoltaic (PV) effect, is the scientific and theoretical foundation for today's solar energy market.

#Electricity produced by solar cells would have a bigger impact were it not so expensive. The cost of the raw materials, equipment, and energy required to manufacture crystalline and polycrystalline solar cells is prohibitive for wide-scale market penetration. Presently, it is cheaper to produce electricity with coal fired or nuclear plants. As much as half the cost of solar cells lies in an expensive substrate such as crystalline silicon. A less expensive substrate is desirable.

#The details of the present invention, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts, and in which:

#FIG. 1 is an energy band diagram of a typical pn-homojunction solar cell under light incidence.

#FIG. 2 is a perspective view of a typical pn-homojunction solar cell driving a load.

#A homojunction solar cell is a semiconductor pn-junction constructed to absorb a determinable range of wavelengths from the sun's electromagnetic spectra and operable an electrical bias. “Homojunction” means that the same semiconductor material is used for both the n-layer and the p-layer. FIG. 1 is an energy band diagram 100 of a typical pn-homojunction solar cell 105 under light incidence. If the photon energy (hν) of the light incident upon the cell is greater than the semiconductor material's band gap energy, (Ec−Eν), electron-hole pairs are generated in depletion region 130, n-layer 110 and p-layer 115. While some recombination of carriers occurs, some of the carriers 108 generated drift across the pn-junction depletion region resulting in photocurrent.

#FIG. 2 is a perspective view of a typical “thick-film” crystalline pn-homojunction solar cell 200 having the energy band diagram shown in FIG. 1. Junction 225 is formed by joining an n-layer 215 and a p-layer 210 made from the same semiconductor material but having different carrier types and concentrations. Solar cell 200 is the product of the same ingot growth and wafer fabrication technology used in the first steps of modern integrated circuit manufacture. Thick crystal p-layer 210 (often millimeters in thickness) provides cell 200 with structural support as well as photon absorption capability. The resulting junction 225 provides an electrical field sufficient to force a photoelectric current I through a current collecting circuit 235.

#N-layer 215 is a thin layer and typically comprises an extrinsic donor-doped semiconductor material, such as phosphorous-doped silicon. P-layer 210 is comparatively thicker and typically comprises an extrinsic acceptor-doped semiconductor that exhibits superior absorption in a known range of solar spectra, such as aluminum or boron doped silicon. N-layer 215 is deposited onto p-layer 210 using any of a variety of known pn-junction fabrication techniques, including formation of an alloyed junction, epitaxial growth, and thermal diffusion.