Amal K. Ghosh

Photovoltaic and rectification properties of Al/Mg phthalocyanine/Ag Schottky‐barrier cells

The photovoltaic and rectification properties of Al/Mg phthalocyanine/Ag sandwich cells are reported. At low voltages, the current in the forward direction varies exponentially with voltage. A charge density of [inverted lazy s]1018/cm3 is estimated from C‐V measurements. The short‐circuit photocurrent JscαFm (mu2009[inverted lazy s]0.5), where F is the incident light intensity. The open‐circuit photovoltage Vocα logF as expected for a Schottky barrier or p‐n junction. The J‐V curve in the photovoltaic mode is characteristic of a cell with large series resistance. From the photovoltaic action spectra, the electron diffusion length is estimated to be [inverted lazy s]1.5×10−6u2009cm. The action spectrum is dependent on the direction of the incident radiation. A theory is presented which explains the results. The junction is attributed to a Schottky barrier of Vd ∼ 0.6 eV and width ∼ 2.5 × 10−6 cm estimated from C‐V measurement. The values determined from photovoltaic measurements are in agreement. The lifetime of ...

Theory of the electrical and photovoltaic properties of polycrystalline silicon

Grain boundary states play a dominant role in determining the electrical and photovoltaic properties of polycrystalline silicon by acting as traps and recombination centers. The recombination loss at grain boundaries is the predominant loss mechanism in polycrystalline solar cells. Cell parameters are calculated based on a transformation of grain boundary recombination centers to a uniform distribution of such states throughout the grain. Effective carrier lifetime is expressed in terms of grain size, allowing calculation of short‐circuit current, open‐circuit voltage, and fill factor. Excellent agreement is observed between theory and experiment for almost all device parameters. It is indicated that one could fabricate 10% efficiency polycrystalline solar cells from 20‐μm‐thick material if the grain size exceeds 500 μm.

Rectification, space‐charge‐limited current, photovoltaic and photoconductive properties of Al/tetracene/Au sandwich cell

Tetracene thin films sandwiched between two evaporated metal electrodes show rectification and photovoltaic effects. The rectification does not follow the J‐V equation for Schottky barriers or p‐n junctions. A theory is presented to explain the rectification and photovoltaic response. The forward current of the diode (Au positive) is space‐charge‐limited current (SCLC) and is attributed to hole injection from the gold electrode. The forward current varies as Vl+1/d2l+1, where V is the voltage, d the thickness and 1=(Tc/T)∼3. The SCLC is related to an exponential distribution of traps ∼7×1019/cm3. Other results related with trapping are also presented. No variation of capacitance with voltage is found, indicating that the tetracene film is completely depleted. In the photovoltaic mode, the Au electrode is positive and the Al negative. The photovoltaic current and voltage are related by the relation V=V0(1−e−aJ). The short‐circuit photocurrent varies as Fn where n varies between 0.6 and 1 and F is the incid...

Hall mobility of polycrystalline silicon

Hall mobility of polycrystalline silicon was measured in the dark and under illuminated conditions. Grain boundary potential barriers present in the dark can be eliminated with light. When the barriers are removed, the mobility between 200 and 400 K is found to vary as T−2, which is the dependence observed in single crystals for the same order of magnitude of doping. The free‐carrier concentration of 5×1015 cm−3 was not affected by illumination, and the room temperature mobility in 1‐mm grain size material after barrier elimination with light was 900 cm2/Vu2009sec. A phenomological theory of Hall mobility in polycrystallllne silicon which explains these observations is presented.

Angle‐of‐incidence effects in electron‐beam‐deposited SnO2/Si solar cells

The power‐conversion efficiency of SnO2/Si solar cells fabricated by electron‐beam evaporation of SnO2 is strongly dependent on the angle of incidence of the SnO2 vapor stream on silicon. The optimum angle of incidence is between 50° and 70°. Solar cells with power‐conversion efficiencies exceeding 10% have been reproducibly fabricated by depositing the SnO2 at angles of incidence in this optimum range.

Excitation mechanism of the Cr3+ luminescence center in SrTiO3

From thermoluminescence (TL), thermally stimulated currect (TSC), and photoluminescence and photoconductivity kinetic response measurements, the excitation mechanism of the Cr3+ luminescence center in SrTiO3 is explored. Based primarily on the strong correlation between TSC and TL, it is concluded that the dominant excitation mechanism is a charge‐transfer process, although a contribution from a resonance‐transfer process cannot be completely ruled out. The superficial differences which exist between photoluminescence and photoconductivity kinetics can be accounted for completely within the framework of a charge‐transfer model by recognizing that the photocurrent is controlled primarily by the electron traffic through the conduction band, while the photoluminescence is controlled primarily by the hole traffic through the luminescence center.

Properties of the Room‐Temperature Electron Trap in Rutile

By a combination of thermoluminescence, thermally stimulated current, and photocurrent measurements, the electron trapping parameters of the room‐temperature trap in rutile were obtained. The photon‐capture cross section and the electron‐frequency factor were determined. The values of the electron‐capture cross section, the trap density, and the ratio of optical to thermal ionization energies of the trap were calculated. The relation of the trapping center to the photocurrent decay is discussed.

Luminescence characteristics of epitaxially grown cubic ZnS

The luminescence properties of cubic ZnS grown epitaxially on GaAs by both vapor‐transport and vacuum‐evaporation techniques were investigated. Emission spectra, excitation spectra, temperature dependence of emission, and optical bleaching of thermoluminescence were measured. The results indicate that the dominant defect in this material is the self‐activated (SA) center. The strong blue emission exhibited the known behavior of the emission from SA centers, namely, the shift of the emission peak to shorter wavelengths with increasing temperature and the 0.9‐μm peak in the optical bleaching of this emission in thermoluminescence. However, the excitation specturm for this emission and the absence of pronounced thermal quenching are different from previously reported results. Only the higher‐energy peak at 330 nm of the two excitation peaks previously reported is observed. Evidence is presented which suggests that the difference is due to differences in crystalline perfection. The present material exhibits a...