Transport in Charged Defect-Rich π-Conjugated Polymers

Some models of charge transport in pi-conjugated polymers treat these materials as if they were electrical insulators. Although this may be appropriate for a few materials, many polymers are effectively doped p-type by a high density of charged defects. Herein, limits are estimated for the charged defect density above which the resulting electrostatic fluctuations may govern transport and for the corresponding free hole density above which space-charge-limited currents should not occur. These limits are lower than the experimentally observed values in many pi-conjugated polymers, suggesting that these materials are more accurately described by models of doped semiconductors. This analysis also provides an explanation for two otherwise puzzling experimental observations, the low-field Poole-Frenkel mobility and the correlated energetic disorder.