Thermal stability is important for many thin film organic semiconductor devices but is challenging due to their weakly Van der Waals-bonded nature. Here, we show that diluting common small molecule hole transport materials through co-evaporation with the amorphous fluoropolymer Teflon AF leads to a dramatic improvement in their thermal and morphological stability without sacrificing electrical performance. Blend films with 25 vol. % Teflon decrease the drive voltage of single layer hole-only devices by more than 30% and dramatically increase their operating temperature limit to over 250 °C. The stability improvement appears to result from a nanoscale network of Teflon chains that repolymerize throughout the blend film following evaporation and inhibit gross movement of the organic semiconductor molecules. These results open up a pathway to stabilize the morphology of small molecule organic semiconductors and point to a more general opportunity to exploit semiconductor dilution to systematically vary thermal, optical, and other material properties without compromising electrical transport.