Thermoelectric (TE) energy conversion is based on electronic transport, and high TE performance requires both high electrical conductivity and high thermopower (or Seebeck coefficient). High electrical conductivity can be achieved by increasing the electronic carrier concentration but this diminishes thermopower, which has been one of the major hurdles in further improving TE performance. Here we propose an effective method to avoid the undesirable behaviors by engineering the interface of conjugated carbon materials with facile gas treatment methods for selective doping. To demonstrate the feasibility, carbon nanotube (CNT) and undoped polyaniline (PANI) were made into a composite. After HCl gas doping of the composite, similar to 700% increase in electrical conductivity was observed but thermopower was maintained to be similar to 90% value prior to the doping. We believe that the undoped PANI between CNTs was minimally doped due to their pi-pi interactions, maintaining an energy barrier to have a high thermopower. Electrical transport in the CNT network with PANI interfaces was also theoretically studied, suggesting the power factor can be remarkably improved by imposing an energy barrier at the interface of conjugated carbon structures. Our approach to selectively dope specific sites in the composites can be further extended to other composites to obtain desired transport properties. (C) 2016 Elsevier Ltd. All rights reserved.