ELECTRONIC STATES INDUCED BY ION IRRADIATION IN A CONJUGATED LADDER POLYMER

Thin films of the ladder polymer poly (benzimidazobenzophenanthroline) (BBL) were irradiated with 180 keV argon ions under high vacuum condi tions. Ion-beam exposures ranging from 5 x 10(13) up to 3 x 10(18) ion s/cm(2) result in measured BBL film electrical conductivities approach ing 100 S/cm After ion irradiation and a short annealing treatment the electrical conductivity is found to be stable in air at 250 degrees C , changing less than 5% after several hundred hours. Optical absorptio n spectra measured in very thin (about 200 nm) ion-irradiated films sh ow a pronounced absorption coefficient reduction in the BBL pi-pi con jugated band transition region. In addition, increased absorption occu rs at wavelengths both above and below this band transition. As the io n-beam exposure is increased further these spectral features broaden a nd exhibit increased absorption, extending into the near-infrared spec tral range. Infrared absorption measurements indicate that structural changes begin to occur in BBL for low ion irradiation levels, although an extremely broadened infrared mode structure having characteristics reminiscent of BBL remains up to moderately high irradiation levels. These results combined with electron spin resonance data suggest that BBL films undergo heavy structural damage and atomic relaxation, along with some degree of bond reformation, causing a total loss of long-ra nge conjugation order. The irradiation-induced damage and subsequent;h ealing processes result in a stable structure, where electronic transp ort proceeds through localized midgap defect states near the Fermi lev el.