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 conditions. Ion-beam exposures ranging from 5 × 1013 up to 3 × 1018 ions/cm2 result in measured BBL film electrical conductivities approaching 100 S/cm. After ion irradiation and a short annealing treatment the electrical conductivity is found to be stable in air at 250 °C, changing less than 5% after several hundred hours. Optical absorption spectra measured in very thin (about 200 nm) ion-irradiated films show a pronounced absorption coefficient reduction in the BBL π-π* conjugated band transition region. In addition, increased absorption occurs at wavelengths both above and below this band transition. As the ion-beam exposure is increased further these spectral features broaden and exhibit increased absorption, extending into the near-infrared spectral 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-range conjugation order. The irradiation-induced damage and subsequent healing processes result in a stable structure, where electronic transport proceeds through localized midgap defect states near the Fermi level.