V. V. Evstropov

Current flow and potential efficiency of solar cells based on GaAs and GaSb p-n junctions

Dependence of the efficiency of single-junction and multijunction solar cells on the mechanisms of current flow in photoactive p-n junctions, specifically on the form of the dark current-voltage characteristic J-V, has been studied. The resistanceless J-Vj characteristic (with the series resistance disregarded) of a multijunction solar cell has the same shape as the characteristic of a single-junction cell: both feature a set of exponential portions. This made it possible to develop a unified analytical method for calculating the efficiency of singlejunction and multijunction solar cells. The equation relating the efficiency to the photogenerated current at each portion of the J-Vj characteristic is derived. For p-n junctions in GaAs and GaSb, the following characteristics were measured: the dark J-V characteristic, the dependence of the open-circuit voltage on the illumination intensity P-VOC, and the dependence of the luminescence intensity on the forward current L-J. Calculated dependences of potential efficiency (under idealized condition for equality to unity of external quantum yield) on the photogenerated current for single-junction GaAs and GaSb solar cells and a GaAs/GaSb tandem are plotted. The form of these dependences corresponds to the shape of J-Vj characteristics: there are the diffusion- and recombination-related portions; in some cases, the tunneling-trapping portion is also observed. At low degrees of concentration of solar radiation (C < 10), an appreciable contribution to photogenerated current is made by recombination component. It is an increase in this component in the case of irradiation with 6.78-MeV protons or 1-MeV electrons that brings about a decrease in the efficiency of conversion of unconcentrated solar radiation.

The dislocation origin and model of excess tunnel current in GaP p-n structures

An excess tunnel current in GaP epitaxial nondegenerate p-n junctions on GaP and Si substrates was studied. An important experimental result is that the slope of exponential current-voltage (I–V) characteristic (in lnI–V coordinates) is independent of the width of the space-charge region, i.e., on n-and p-region doping levels. This fact is unexplained by existing models. A dislocation shunt model based on multihop tunneling through a dislocation line, which may be considered as a chain of parabolic potential barriers, is proposed. The density of dislocations predicted by this model is in agreement with the transmission electron microscopy (TEM) observations.

Tunnel excess current in nondegenerate barrier (p-n and m-s) silicon-containing III–V structures

Data are scaled from a study of the forward current in three types of barrier structure: p-n homostructures p-n-GaP/n-Si, p-n-GaAs-n-GaP/n-Si, and p-n-GaAs-n-GaAs/n-Si; heterostructures n-GaP/p-Si, p-GaP/n-Si, n-GaAsP/p-Si, and n-GaAs/p-Si; and Au-n-GaP/ n-Si surface-barrier structures. Epitaxial layers of GaP and GaAs were created on Si-substrates by gaseous phase epitaxy in a chloride system. Temperature measurements show that the forward current has a tunnel character, although the width of the space charge region greatly exceeds the tunneling length. A model is proposed for nonuniform tunneling along dislocations that intersect the space charge region. This type of tunneling is taken into account by introducing a phenomenological “dilution” factor for the barrier. The model makes it possible to calculate the dislocation density in device structures from the current-voltage characteristic.

The effect of damaging radiation (p, e, γ) on photovoltaic and tunneling GaAs and GaSb p-n junctions

The properties of multiple-junction solar cells depend on the properties of the constituent photovoltaic and tunneling p-n junctions. In this study, the properties of the space-charge region for photovoltaic and tunneling p-n junctions were examined using the dark current-voltage characteristics for two semiconductors: GaSb (a narrow-gap semiconductor) and GaAs (a wide-gap semiconductor). The effects of irradiation with protons (the energy of 6.78 MeV and the maximum fluence of 3 × 1012 cm−2), electrons (the energy of 1 MeV and the maximum fluence of 3 × 1016 cm−2), and γ-ray photons (the energy of 1.17–1.33 MeV and the maximum dose of 17 Mrad) on the lifetime of charge carriers in the space-charge region of photovoltaic p-n junctions and on the peak current of connecting tunneling p-n junctions were studied. The coefficients of the damage for the inverse lifetime are determined for photovoltaic p-n junctions. The coefficients of equivalence between the used types of radiation are determined; these coefficients are found to be almost independent, on the order of magnitude, of the type and material of the p-n-junction (and nearly equal for photovoltaic GaAs p-n junctions and tunneling GaAs and GaSb p-n junctions).

Dependence of GaN photoluminescence on the excitation intensity

The dependence of the edge photoluminescence (PL) intensity on the excitation intensity in (0001) HVPE-grown GaN samples has been studied. The specific behavior found is that, at a low excitation level, the dependence is markedly superlinear, namely, superquadratic, and at high excitation levels it is nearly linear. A model accounting for the observed superquadratic behavior is proposed which is based on the identity of the recombination processes in the surface space charge region (SCR) under optical excitation with those in the SCR of the Schottky barrier or a p-n junction under current flow conditions. The superquadratic dependence is obtained analytically under the assumption that the nonradiative recombination channel is associated with multiple-hopping tunneling along a dislocation, which is formed by a chain of carrier localization centers and crosses the SCR. The experimental dependence of the PL intensity on the excitation intensity is a power-law function. The distance between the neighboring localization centers, i.e., the period of the potential along the dislocation, is determined as ∼4.1 nm.