Electronic Transport in Polyaniline Films and Polyaniline/Silicon Heterostructure Solar Cells. (Paperback)

Experimental studies of the electronic transport and optical absorption in polyaniline (PANI) films and PANI/silicon heterostructure solar cells are reported. Most of the emeraldine salt PANI films were prepared by spin-casting from commercially prepared, acid-doped dispersions in xylene solution. It was discovered that diluting these dispersions about tenfold with additional xylene reduced the conductivity of the PANI films cast onto glass from 102 S/cm to 10-4 S/cm or lower. This "dilution effect" is probably a variation of the well-known effect of dispersion pH upon the doping level of emeraldine salt PANI films. The optical absorption measurements on films from dispersions with varying xylene dilution were also similar to those previously reported for films prepared from dispersions with varying pH. Heterostructure solar cells were prepared by casting p-type PANI films onto n-type crystalline silicon (c-Si) and onto hydrogenated amorphous silicon (a-Si: H) n/i structures. The open-circuit voltages (VOC) of the cells measured under strong (0.5 W/cm2) white-light illumination probe the Fermi levels of the PANI films. Both the VOC measurements and temperature-dependent conductivity measurements on PANI films support a fixed bandedge model for transport. In this model, the variations in the film conductivities are determined by varying Fermi levels, and electronic transport occurs at a fixed valence bandedge level energy and bandedge conductivity (200 S/cm). We speculate that this bandedge is a mobility-edge. These conclusions differ significantly from previous models based on hopping transport or polaron bands. For the PANI/Si cells, we conclude that VOC values of 0.7 V or higher are possible in principle, which would be of technological interest. The measured VOC values saturated at about 0.5 V for the highest conductivity films spun from xylene dispersions; high conductivity, water-based dispersions yielded a lower saturated VOC. We speculate that an interface effect is limiting VOC. No saturation was found for PANI/a-Si: H cells, but the largest VOC values of around 0.7 V are not of great technological interest.