The mobility-lifetime products ( μτ) and interface property have been examined through the photovoltaic studies in actual hydrogenated amorphous silicon (a–Si:H) p–i–n junction solar cells. A small amount of boron atoms included in a–Si:H enhances the μτ products of both electrons and holes up to the order of 10−7 cm2/V, which corresponds to the carrier diffusion length in excess of 5000 Å. The doped window layer possessing inferior photoelectric property works as the recombination region for photocarriers generated in the active i layer, and practically dominates the interface property together with the surface recombination velocity S0 at the electrode/doped layer interface. The S0 at the SnO2/p a–Si:H interface is estimated to be about 3×102 cm/s with an assumption of the electron mobility at 0.1 cm2/Vs. Prolonged light exposure causes a reversible change of the μτ products in every layer composing the p–i–n junction. These experimental results are discussed in connection with photovoltaic performances.
We have theoretically studied the electronic and optical properties of a quantum well (QW) in which the well region is constructed from a binary alloy semiconductor A 1--x B x in the coherent potential approximation (CPA). A tight binding model is used for a single particle (electron, hole, Frenkel exciton) in the well composed by a rectangular array of N x  N y  N z sites. The effect of the diagonal randomness is reduced to the coherent potential S(E), which is assumed to be the same for all sites, and is selfconsistently determined with the average Green's function. For a slab (1, 1, N z ) and wire (1, N y , N z ), the density of states r(E) is composed of N z (or N y  N z ) subbands, each shows the two (one)-dimensional van-Hove singularity. When x (or 1 --x) is small, a B (A) impurity-band always appears at the lower (higher) energy side of the lowest (highest) host-band. As the well width becomes narrower and/or the dimensionality decreases, the boundary for D/t decreases which separates the amalgamation type and the persistence type.Introduction Alloy semiconductors are important materials for giving us a variety of electronic and optical properties. For example, a convenient way to produce an optical device with a desirable wavelength is by using an alloy semiconductor A 1--x B x from two semiconductors A and B which have different band gap energies. This method is called band-gap engineering, which now is a guiding principle in the semiconductor technology.Theory for random alloy systems has been developed in the late 1960s, and the coherent potential approximation (CPA) was proposed first for diagonal randomness [1-4] and then for off-diagonal randomness [5]. The theory of CPA succeeds in explaining two types of electronic properties in alloy systems. If the physical parameters of A and B are not very different, the electronic properties of A 1--x B x change continuously in response to the change in the concentration x. This is called the amalgamation type. On the other hand, if the differences are large, the properties show two components originated from each constituent although the system is homogeneous. This is called the persistence type. In covalent semiconductors, most of bulk alloy systems are found to be of the amalgamation type, which then shows the band-gap engineering is applicable. If an alloy semiconductor is used for a quantum well region of superlattice structures, however, there is no guarantee for being the amalgamation type, because the effect of randomness is considered to be enhanced by the low dimensionality and the confinement of the wave function. The purpose of the paper is to extend the CPA theory to quantum well structures of various dimensionalities and geometrical size, and give a theoretical guiding principle for the electronic properties of alloy quantum well systems.
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