Frederick E. Knier

Manifestation of glass transition in electronic charge transport in Si and Ge Backbone polymers

Abstract Electronic transport of photoinjected holes in high molecular weight linear polymers with silicon and germanium backbone is electric field dependent and thermally activated. At fixed electric fields, during both rate and step heating, the mobility activation energies (ϵ) in all these polymers change abruptly and reversibly at critical temperatures (ifT c ) without concurrent abrupt changes in mobilities. These critical temperatures coincide with the respective glass transition temperatures. In all cases, ϵ at T > T g is lower than ϵ at T T g , typically by a factor of 2 to 3. In polymers with aliphatic side groups ϵ ( T > T g ) is extremely low, practically approaching zero. The change of activation near T g is similar to that observed in amorphous charge-transporting polymers and other glasses, including selenium and its alloys. The universality of this phenomenon is emphasized.

Electronic Transport in Si and Ge Backbone Polymers—Effect of Thermal Transitions

Abstract Time-of-Flight hole drift mobility measurements have been carried out on representative aromatic and aliphatic pendant group containing silicon and germanium backbone polymers over a wide range of electric field and temperature. The temperature range is broad enough in each case to incorporate the region of glassy solidification. For the specific case of poly(di-n-hexylsilylene), experiments have been extended to incorporate the side chain melting phenomenon identified with the bathochromic shift in the polymer. Mobility changes associated with this transition are analyzed. Specific features observed in Si and Ge backbone polymers, which appear to be a general characteristic of a wide range of disordered solids, are highlighted.

Chemical modification of charge transport in silicon backbone polymers

Abstract Charge transport in highly insulating Si backbone polymers has been studied by the Time Of Flight technique. Transit pulse shapes and the field and temperature dependence of the hole drift mobility (for many systems about 10 −4 cm 2 /Vs at 295K and E = 10 5 V/cm) have been characterized for a prototypical example, poly(methylphenylsilylene), PMPS. The effect of dopants, which differ in their relative oxidation potentials, on the drift mobility and the transit pulse shape have also been studied. Comparison of the overall results with other glassy polymeric media suggest that transport proceeds by hopping among chain backbone derived domain like segments.

Reversible chemical modification of the electrical behavior of a-Se

Abstract a -Se films prepared from either high purity (99.999%) source material which has undergone hydrazine reduction or from high purity Se powder surface treated with hydrazine exhibit enhanced electron and diminished hole mobility lifetime (μτ) products relative to source material. The effect of this chemical treatment is stable enough to survive repeated distillation of the selenium. A history of hydrazine reduction or surface treatment, however, has no effect on hole or electron drift mobility. The effect of hydrazine exposure can be completely reversed by oxidation of selenium oxide with nitric acid followed by reprecipitation of selenium with SO 2 . The effect of hydrazine on the photoelectronic behavior of a -Se is restricted to changes in the near midgap density of states and may involve, at least in part, chemical modification of pre-existing native defects.

Transport Behavior Associated with Bathochromic Shift in Si and Ge Polymers

Abstract Time-of-flight drift mobility experiments have been carried out on poly(di-n-hexylsilylene) (PDHS) and poly(di-n-butylgermylene)(PDBG) over a wide range of temperature and electric field. A change in transport behavior in PDHS and PDBG associated with the conformational transitions which are responsible for the bathochromic shift in these polymers, is unambiguously demonstrated.

Chemical process to normalize the electrical properties of a-Se

The influence of specific chemical dopants on the electrophotographic behavior of selenium and its alloys has been established in prior work. This communication describes a chemical procedure that has been found effective in removing electronically active impurities from amorphous selenium. The methodology involves converting contaminated selenium into a chemical intermediate that is separated by selective alcoholic dissolution and then reduced to high-purity selenium. The electrical characteristics of the amorphous films obtained by vacuum evaporation of the latter are determined directly from analysis of xerographic potentials.

Effect of Thermally Induced Transitions on Electronic Transport in Aliphatic Polysilylenes

Electronic transport of photoinjected holes in high molecular weight linear polymers with silicon and germanium backbone displays a familiar pattern of electric field and temperature dependence similar to behavior already reported in a very diverse collection of glassy molecular solids. At fixed electric fields, during both rate and step heating, the mobility activation energy e in all these polymers changes abruptly and reversibly at a temperature Tc, which is specific for each polymer, without concurrent abrupt changes in mobilities. These temperatures coincide with the respective glass transition temperatures Tg. In all cases, the below-Tg activation is higher than above Tg. In polymers with aliphatic side groups, e(t > t g) is extremely low, practically approaching zero. The change of activation near Tg is similar to that observed in amorphous charge-transporting polymers and other glasses including selenium and its alloys. Polymers such as poly(di-n-hexylsilylene) (PDHS) which undergo conformational transitions at temperatures TTr due to side chain crystallization and melting, also experience a change in mobility and its activation energy at TTr. The change in the transport behavior and the association with the bathochromic shift in the uv spectra is demonstrated.