Organic/polymeric materials are of emerging importance for thermoelectric conversion. The soft nature of these materials implies strong electron-phonon coupling, often leading to carrier localization. This poses great challenges for the conventional Boltzmann transport description based on relaxation time approximation and band structure calculations. In this work, combining Kubo formula with finite-temperature time-dependent density matrix renormalization group (FT-TD-DMRG) in the grand canonical ensemble, we developed a nearly exact algorithm to calculate the thermoelectric power factor PF=α^2 σ, where α is the Seebeck coefficient and σ is electrical conductivity, and apply the algorithm to organic materials modeled by Holstein Hamiltonian with electron-phonon coupling. Our algorithm can provide a unified description covering the weak coupling limit described by bandlike Boltzmann transport to the strong coupling hopping limit.