Control of doping is crucial for enhancing the thermoelectric efficiency of a material. However, doping of organic semiconductors often reduces their mobilities, making it challenging to improve the thermoelectric performance. Targeting on this problem, we propose a simple model to quantitatively obtain the optimal doping level and the peak value of thermoelectric figure of merit (zT) from the intrinsic carrier mobility, the lattice thermal conductivity, and the effective density of states. The model reveals that high intrinsic mobility and low lattice thermal conductivity give rise to a low optimal doping level and a high maximum zT. To demonstrate how the model works, we investigate, from first-principles calculations, the thermoelectric 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 BTBTs exhibit high 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 thermoelectric applications. Moreover, the maximum zT predicted from the simple model agrees with that observed from the first-principles calculations. This study has provided new insights to guide the search for organic thermoelectric materials and their optimization.