Schottky Barrier Width Research Articles

Phthalocyanine organic solar cells: Indium/x-metal free phthalocyanine Schottky barriers

A thin organic film of polycrystalline particles of x-metal free phthalocyanine (x-H2Pc) dispersed in a polymer binder, when sandwiched between tin oxide (NESA) and indium electrodes, is shown to exhibit a strong photovoltaic effect. The photovoltaic and rectification properties of In/x-H2Pc/NESA sandwich cells are reported. From the photovoltaic action spectra, the active region responsible for electric power generation was found to be confined to the metal/semiconductor interface. A Schottky barrier width of 300 Å was determined, which allows the capture of 30% of solar irradiance. An electron trap density of 3×1017/cm3 and a Schottky barrier built-in potential of 0.63 V are estimated from C-V measurements. At low voltage, the dark current in the forward direction varies exponentially with voltage: from this dependence values of 2×10−9 A/cm2 and 1.3–2.6 for the saturation current J0 and diode quality factor n are determined. At higher voltage, a super quadratic dependence of forward current on voltage indicated that current conduction is limited by an exponentially decreasing distribution of traps. At peak solar power (135 mW/cm2), a power conversion efficiency (η′) of 1.2% to transmitted light has been obtained. The transmittance of the indium electrode approached 2%. The devices exhibit open circuit voltages Voc of 0.45 V and short-circuit currents of 0.2 mA/cm2 at Air Mass Zero (AMO) sunlight. Therefore, the engineering efficiency of our device approached 0.03%. The monochromatic quantum efficiency of free-carrier generation approached 75% at low light level; this diminished to 30% at solar intensities, characteristic of a cell with large series resistance. The effect of pigment loading, cell thickness, light intensity, binder material, dye sensitization, and the nature of barrier electrode has been studied and optimized.

Minority carrier diffusion length measurements in CdTe by a photocurrent technique

A simple technique for measuring the minority carrier diffusion length in semiconductors is demonstrated. The technique does not require the knowledge of any parameter of the investigated material. It consists, basically, in measuring the photocurrent of a semiconductor-electrolyte Schottky barrier, as a function of applied reverse bias. Weakly absorbed light is used, so that the light penetration depth is much larger than the Schottky barrier width. The diffusion length of n-type CdTe doped at a concentration of 8×1014/cm3 is found to be 1.75 μm.

Hole-Trap Depletion-Layer Formation in CdS

The formation of a depletion layer associated with a hole trap in sensitive CdS platelet crystals with blocking contacts has been studied. By monitoring the current associated with the emptying of optically filled traps in the depletion region as a function of applied voltage and temperature, only one trap species was observed, the properties of which are a density of at least 4.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$/${\mathrm{cm}}^{3}$; ionization energy, 0.14\ifmmode\pm\else\textpm\fi{}0.02 eV; frequency factor, 2\ifmmode\times\else\texttimes\fi{}${10}^{7}$/sec; capture cross section, 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$. The carrier range was 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{2}$/V, and this was determined to be the recombination range. The zero-voltage Schottky-barrier width at the gold contact is found to be 0.20 \ensuremath{\mu}. It is argued that the trap is a hole trap, and it is shown to be similar to the trap level postulated to explain the CdS green emission.