Understanding the relationship between frontier orbital level offsets to optoelectronic and electronic properties of doped P3HT-based composites

One of the greatest advantages of organic/polymeric semiconductors is their ease of tunability of optoelectronic, electronic, and optical properties pre- and post-synthesis. Chemical doping has numerous advantages in comparison to the other routes due to being quick, cost-effective, and highly controllable. For dopants to ionize the host material, the frontier orbital offset should be carefully engineered to enable efficient charge transfer between dopant and host material. This work directly studies different frontier orbital levels between poly(3-hexylthiophene-2,5-diyl) (P3HT) and a variety of molecular dopants. Results show that the electrical conductivity decreases as the LUMO of the dopant increases; this could be explained by the increased energy barrier for electron transfer from the HOMO of the P3HT to the LUMO of the dopant acceptors. Among the several dopants investigated, this study reveals that the Seebeck coefficients decrease as the frontier orbital offsets (between P3HT HOMO and acceptor LUMO) decrease and the electrical conductivity increases. Optimal offsets are also identified where certain parameters exhibit the highest or lowest values.