## 4 Discussion  
Since the power generation efficiency of a thermoelectric device increases with the ZT of the device, it has been generally considered that an increase of the output power is also accompanied by an increase in ZT. However, the human body applications and related thermal model are different from the general applications of a thermoelectric generator. The condition for high power density is also changed. To date, research has suggested that the power generation on the human body increases as the thermal conductivity of TE materials is reduced while the ZT is maintained. However, there has been no suggestion for the direction of material engineering to obtain higher power output in human body applications. Therefore, maximizing the material ZT is still considered the only way to raise the device performance.  
As an example of material engineering through doping, this study shows that the carrier concentration of maximum ZT can be different from the carrier concentration of the maximum power density on the human body. Since the shift direction of the power density graph is always  
Figure 7: The power density distribution of (**a**) material D and (**b**) material A based TEG with variation of the convective heat transfer coefficient between TEG and air (\(h_{air}\)).  
to the left, which means lower thermal conductivity, the cause of this difference is considered to be the high thermal conductivity of the thermoelectric material in the device. In order to confirm this, the power density was calculated for various materials with different thermal conductivities. As a result, the difference in the carrier concentration between ZT and power density was reduced for materials with lower thermal conductivity.  
In addition, it was shown that the shift of the power density curve by the change of the device parameters such as the FF and Cu thickness could be reduced when a material with low thermal conductivity was used. Environment variables such as air convection conditions can be changed. The carrier concentration of the maximum power density shifts toward that of the maximum ZT with an increase of the convective heat transfer coefficient because the influence of the thermal resistance of the device becomes large with a decrease of external thermal resistance.  
This study confirms that it is possible to improve the performance of a thermoelectric device by engineering the material in addition to the conventional method of device structure optimization. To do this, the optimal doping of the material should be determined considering a thermal resistance model, rather than merely ZT improvement. In addition, a material with low thermal conductivity can achieve maximum power density at the same carrier concentration as that yielding the maximum ZT.  
These results can be applied not only for doping but also to various material engineering methods that change the thermal conductivity and ZT simultaneously. This study emphasizes the importance of material engineering as well as the device structure in human body applications. Furthermore, while previous studies simply concentrated on the selection of the proper material for the TEG, in this paper, the improvement of the power density was studied from the perspective of material engineering. Based on these results, the researchers have suggested a direction for material engineering.  
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A1036133).  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed experiments; [PERSON] and [PERSON] contributed material data for simulation; [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] analyzed the data; [PERSON] wrote the paper.  
The authors declare no conflict of interest.