Upon using thermoelectric generators (TEGs) as an energy harvester for wearable or room ambient electronics, mechanical flexibility, lightness, and low thermal conductivity are major issues. Because natural air cooling is assumed for such applications, very low thermal conductivity and millimeter thicknesses are required to obtain sufficient temperature difference between the front and back sides of the TEG. In this repot, two approaches in the authors’ group are introduced. The first one is a novel device structure and fabrication method using CNT yarn with p/n doping and a fabric substrate. It is highly durable against bending or stretching because of the mechanical isolation of the thermoelectric components from the substrate. A key issue is how to suppress the high thermal conductivity of CNTs. A protein molecule is used as a gate for the phonon propagation and as a tunneling junction for electrons. The second one is to use molecular solids of organic semiconductors. Giant Seebeck coefficients, > 0.1 V/K, are found to appear in such materials near room temperature range. Owing to the extremely large Seebeck coefficient, the device structure can be ultimate simple without connecting many p/n semiconductor blocks as in conventional TEGs.