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The accurate control of doping is the key to enhanced efficiency of a thermoelectric(TE)material.However,doping of organic semiconductors often reduces their mobilities,which makes optimization of TE materials challenging.Targeting on this problem,we propose a simple model to quantitatively obtain the optimal doping level and the peak value of TE figure of merit,zT,from the intrinsic carrier mobility,the lattice thermal conductivity,and the effective density of states.The model shows that high intrinsic mobility and low lattice thermal conductivity together give rise to low optimal doping level and high maximum zT value.Considering the doping dilemma for enhanced efficiency,we propose to search for TE materials with high intrinsic mobility and low lattice thermal conductivity,which tend to acquire high zT at low optimal doping level.The first-principles calculations show that BTBTs exhibit high hole mobilities,extremely low thermal conductivities(~0.2 W m-1 K-1)and large Seebeck coefficients(~0.3 mV K-1),making them ideal candidates for TE applications.When estimated with the single-crystal mobility of C8-BTBT,31.3 cm2 V-1 s-1,the peak value of zT reaches 0.73 at the optimal doping level of 2.2×1019 cm-3.These values are consistent with those predicted by the simple model,based on the properties of undoped BTBTs.