ELECTRICAL AND OPTICAL CHARACTERIZATION OF GRAIN BOUNDARIES IN POLYCRYSTALLINE CADMIUM-TELLURIDE (SOLAR CELL, THIN FILM)

The optical and electrical properties of grain boundaries in n- and p-type CdTe have been investigated by analysis of single boundaries within bicrystals and of polycrystalline thin films. Bicrystals were cut from semi-crystalline boules with random orientation between grains. Films (15-25 (mu)m in thickness) were deposited in a hot wall vacuum evaporation system onto graphite and glass substrates.;Derived boundary properties include the conductivity activation energy in light and dark (0.18 - 0.67 eV), the boundary diffusion potential (0.14 - 0.67 eV), the energy distribution of defect states (10('14)-10('17) m('-2)-eV('-1) over the range measured), the minority-carrier recombination velocity (10('4)-10('5) cm/s), the majority carrier capture coefficient (6 x 10('-10)-10('-8) cm('3)/s), and optical cross section of defect states (3-8 x 10('-16) cm('2)).;Bicrystal conductivity measurements indicate carrier transport across boundaries is dominated by thermionic emission over a potential barrier, with any voltage applied across the boundary being substantially supported by the reverse biased portion of the boundary depletion layer. Capacitance measurements indicate that the dopant impurity density decreases near the boundary interface. The density of interface states is largest near either band edge, with a tailing off into the band gap.;Conductivity measurements of the CdTe thin films indicate a majority carrier mobility three to five orders of magnitude below crystalline valves, primarily due to grain boundary effects. Films exhibit a strong photoconductivity (three to four orders of magnitude at solar intensities) due to filling of boundary defect states by minority carriers.;Experimental techniques included x-ray analysis of bicrystals to determine misorientation between grains, current versus voltage and capacitance versus voltage characteristics of single boundaries in the dark and under illumination, spectral response of boundary photoconductivity and photocapacitance, temperature dependence of boundary conductivity and capacitance, and response of boundary conductivity to time varying light sources.;Passivation experiments were performed on bicrystals and thin films. Heat treatment in H(,2) or Li provide effective boundary passivation in p-type material, but effects were transient, decaying in a few weeks. Heat treatment in air provided effective temporary passivation for boundaries in n-type material.