Control of doping is crucial for enhancing the thermoelectric(TE)efficiency of a material.Whereas,doping of organic semiconductors often reduces their mobilities,making it challenging to optimize the TE performance.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.Our model shows that the high intrinsic mobility and low lattice thermal conductivity are needed to achieve the low optimal doping level and high maximum zT.To demonstrate how our model works,we investigate from first-principles TE properties of a novel class of excellent hole transport organic materials,2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene derivatives(Cn-BTBTs).The first-principles calculations show that the BTBTs indeed 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 the 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 from the simple model,using the properties of undoped BTBTs.The study has provided new insights that guide the search for organic TE materials and their optimization.