Guest—Host Cross-linked Polyimides for Integrated Optics

We report on the optical and electrical characterization of aromatic, fluorinated, fully imidized, organic soluble, thermally and photochemically, crosslinkable, guest-host polyimides for integrated optics. Refractive indices and optical losses were measured to evaluate the performance of these materials for passive applications. Materials were doped with two high temperature nonlinear optical chromophores, and poled during crosslinking to produce nonlinear optical materials. Measurements of electro-optic coefficient, macroscopic second order susceptibility, and conductivity were performed to assess these materials as potential candidates for active devices

Fundamental studies of grain boundary passivation in polycrystalline silicon with application to improved photovoltaic devices. A final research report covering work completed from February-December 1979

Several aspects of the electrical properties of silicon grain boundaries have been studied. The temperature dependence of the zero-bias conductance and capacitance of single boundaries has been measured and shown to be in good agreement with a simple double depletion layer/thermal emission (DDL/TE) model developed to predict the transport properties of such structures. In addition, it has been shown that deconvolution of the I-V properties of some boundaries via a deconvolution scheme suggested by Pike and Seager yields effective one-electron densities of trapping states which are in good agreement with estimates obtained by low temperature electron emission measurements. Experiments have also been performed which indicate that diffusion of atomic hydrogen into silicon grain boundaries greatly reduces this density of trapping states. In properly prepared, large grained polycrystalline samples all measurable traces of grain boundary potential barriers can be removed to substantial penetration depths after several hours exposure to a hydrogen plasma at elevated temperatures. Initial experiments on prototype polysilicon solar cells have shown that this passivation process can improve AM1 efficiencies. In order to more fully understand and develop this process for improving practical multigrained cells, several device research efforts with other DOE/SERI funded contractors have been initiated

Electronic properties of silicon grain boundaries

Polycrystalline silicon is a clean and relatively simple prototype of electronic ceramics. The theory of the electrostatic barriers which form at silicon grain boundaries will be discussed. The use of experimental conductance and capacitance measurements to obtain the barrier height and energy density of grain boundary states will be illustrated

HYDROGEN PASSIVATION OF POLYCRYSTALLINE SILICON PHOTOVOLTAIC CELLS

L'effet de l'hydrogène atomique sur les joints de grains dans le silicium est passé en revue ainsi que les récentes méthodes qui permettent, par balayage d'un faisceau, de mesurer la vitesse de recombinaison s, des porteurs minoritaires aux joints de grains. Les résultats obtenus sur un grand nombre de joints de grains dans du silicium sur graphite d'Honeywell sont présentés et montrent des variations de s de 2 x 105cm/s à 3 x l03cm/s, ou moins, après exposition de quelques minutes à une source intense d'hydrogène atomique. Le mode induit de la microscopie électronique à balayage a été appliqué à de nombreux joints de grains d'échantillons dont la surface a été exposée à la source externe d'hydrogène. Les résultats montrent que les différences de réduction des hauteurs de barrière suivant les joints observés sont dues à des différences dans les propriétés des joints et non à des fluctuations dans les conditions de traitement. Les résultats obtenus en appliquant la méthode de passivation par source ionique Kaufman à des cellules solaires EFG indiquent que des augmentations substantielles de rendement peuvent être obtenues dans ces dispositifs avec une exposition de seulement quelques minutes à un faisceau d'ions hydrogène.The effect of atomic hydrogen on silicon grain boundaries is reviewed along with recent scanned spot methods for measuring the minority carrier recombination velocity, s, at grain boundaries. Measurements on a large number of grain boundaries in Honeywell silicon-on-ceramic are presented which show that s typically changes from ~ 2 x 105cm/sec to 3 x 103cm/sec, or less, after a few minutes exposure to an intense atomic hydrogen source. Electron-Beam-Induced-Current data on numerous grain boundaries are discussed ; these show that the differences in barrier height reduction from boundary to boundary in response to an external hydrogen source present at the sample surface are due to differences in boundary properties and not to fluctuations in the treatment conditions. The results of applying the Kaufman ion source passivation method to Edge Fed Growth ribbon solar cells indicate that substantial efficiency increases can be achieved in these devices with only a few minutes exposure to the hydrogen ion beam