Modelling the simultaneous increase of the conductivity and the seebeck coefficient in highly B-doped nc-Si

A simultaneous increase of the Seebeck coefficient and the electrical conductivity resulting in a remarkable enhancement of the thermoelectric power factor was observed in heavily boron doped nanocrystalline silicon films. To understand the underlying mechanisms for this behavior, we explored the thermoelectric transport properties with a theoretical model based on Boltzmann transport equation in the relaxation time approximation. Energy filtering has been included in the model assuming an energy threshold in the transmission coefficient of holes. The relevant scattering mechanisms for holes in bulk silicon have been taken into account. It has been found that the simultaneous increase of S and σ can be interpreted by the synergy of two processes that take place upon annealing: the redistribution of ionized scatterers and the formation of energy barriers. The redistribution of ionized scatterers forms energy barriers at the grain boundaries and increases Seebeck coefficient. Moreover, it decreases the density of ionized scatterers in the grains and increases the mobility. The increase of the mobility is such that over-compensates the decrease of the conductivity due to energy filtering of carriers by the energy barriers. Hence, the conductivity increases simultaneously with the Seebeck coefficient as the annealing process progresses.