Showing papers by "Jürgen H. Werner published in 1994"

Novel optimization principles and efficiency limits for semiconductor solar cells

Abstract: Quantum efficiencies of more than merely one electron-hole pair per absorbed photon have recently been demonstrated for solar cells. The theoretical upper limit for solar cell conversion efficiency can thus be raised (from 30%) to 43% if photon absorption and electron-hole excitation follow a specific pathway. This pathway includes photon absorption at specific points in the Brillouin zone, and an inverse Auger effect mechanism for the multiplication of electron-hole pairs. We specify the principles and rules for optimal band structure tailoring.

High-quality surface passivation by corona-charged oxides for semiconductor surface characterization

Abstract: A new surface passivation method using corona-charged oxides is discussed and applied to effective lifetime measurements by microwave-detected photoconductivity decay. Three lifetime measurements are required for evaluating surface recombination velocities and semiconductor bulk lifetimes in monocrystalline silicon wafers. Surface recombination velocities lower than 1 cm/s are achieved with corona passivation.

Direct observation of a scaling effect on effective minority carrier lifetimes

Abstract: The effective minority carrier lifetime in semiconductors with recombination inhomogeneities depends on the spatial distribution of recombination sites and not only on their absolute number. We use time‐resolved microwave reflection measurements to monitor the carrier recombination in silicon wafers with a periodic metallization pattern on one surface. This contact scheme simulates the distribution of sites of enhanced carrier recombination. The experiments reveal a sensitive dependence of the effective minority carrier lifetime on the interdistance and the size—i.e., the scaling—of the metallization pattern at fixed metallization area ratio. This so‐called scaling effect occurs whenever the size and the interdistance of recombination sites are comparable to the minority carrier diffusion length. Our experiments are in good agreement with results from a new analytical three‐dimensional simulation which is based on the solution of the electronic transport equations in Fourier space. Our model is applicable...

New upper efficiency limits for semiconductor solar cells

Abstract: Quantum efficiency measurements showed that more than one electron/hole pair per absorbed photon can be created in a solar cell. Theoretical consideration of this effect leads to new upper radiative efficiency limits for photovoltaic energy conversion. More than 43% efficiency are theoretically possible for cells which are illuminated by the Sun's unconcentrated black body radiation. For sunlight of full concentration, the new limit is above 85%. These values are theoretically possible with a single semiconductor which makes efficient use of carrier multiplication. The theoretical description of radiative recombination in a cell with carrier multiplication leads us also to a novel mathematical description of the saturation current density.

Low‐temperature ohmic Au/Sb contacts to n‐type Si

Abstract: Au/Sb films annealed at temperatures as low as 370 °C yield ohmic contacts to n‐type Si. The contact formation is based on a liquid‐phase epitaxy process of Sb‐doped Si from Au solution. Measured contact resistivities range around 3×10−2 Ω cm2 and are at least one order of magnitude higher than what is expected from the solid solubility of Sb in Si. The discrepancy stems from local inhomogeneous etching and epitaxial regrowth of the (100)‐oriented Si surface by and from the Au solution. Only a small fraction of the macroscopic contact area is doped by Sb and contributes to current transport across the Au/Si(100) interface.

Polycrystalline silicon for thin film solar cells

Abstract: The authors report on the characterisation of epitaxial Si on Si substrates grown at temperatures around 450/spl deg/C as one prerequisite for crystalline Si deposition on low temperature resistant foreign substrates and describe two novel approaches aimed to produce large grained seeding films for subsequent polycrystalline Si deposition on glass substrates. A modified solution growth process at temperatures around 600-650/spl deg/C produces polycrystalline Si seeding films with grains laterally extending several 100 /spl mu/m on borosilicate glass substrates. In it second approach, the authors use polycrystalline /spl beta/-FeSi/sub 2/ films as a seed for Si deposition.

Schottky Contacts on Silicon

Abstract: Experiments on rectifying contacts started in 1874 with the pioneering work of Braun who observed asymmetries in transport of electrical current across metal/semiconductor interfaces [3.1]. The following decades brought out a variety of technical applications, but it took more than sixty years until Schottky [3.2] and, independently, Mott [3.3] gave microscopic concepts to describe these so-called Schottky contacts. Today, in 1994, these interfaces are still not completely understood.

Quantum efficiency of thin film silicon solar cells on a highly doped substrate

Abstract: The analysis of internal quantum efficiency of monocrystalline silicon solar cells is extended to thin film cells on a highly doped substrate. Our model evaluates the internal quantum efficiency of cells in the limit of strong and weak absorption, i.e. for light with absorption lengths falling below and exceeding the base width of the cells. These two regimes yield two characteristic lengths, which allow one to distinguish between recombination within the epitaxial layer and into the substrate. The implicit equations for the diffusion lengths of base and substrate are numerically and graphically solved. In all cases, the evaluation gives limiting values. For the special case of thin film silicon solar cells, we are able to determine the substrate diffusion length with high accuracy. For the base diffusion length we deduce a lower bound.