Hydrogenated micro-/nano-crystalline silicon thin films for thermoelectrics

Abstract

Hydrogenated microcrystalline silicon thin films (µc-Si: H) deposited by plasma-enhanced chemical vapour deposition represent a relatively low cost material to process at commercial scale in mature sectors, i.e. photovoltaics and thin film transistors. This, accompanied by low intrinsic thermal conductivity (k) (~ 1W/m K) [1] and the ability to control the microstructure and doping, make µc-Si: H a promising candidate for thermoelectric (TE) applications. Hence, this study reports on a comprehensive evaluation and optimisation via thermal annealing of the TE properties of p- and n-doped films deposited on rigid and flexible substrates (glass and Kapton). The initial focus is on the behaviour of TE properties with annealing of n-doped films. An increase of the thermoelectric power factor (PF) up to 2.08 × 10-4 W/m K2 at 500 °C for rigid, and up to 8.6 × 10-5 W/m K2 at 200 °C for flexible samples was achieved, along with a reduction of the thermal conductivity (k) down to 2.9 ±0.4 W/m K in rigid samples. Then, the same study was performed for p-doped samples finding an increase in PF up to ~3 × 10-4 W/m K2 at 500 °C for rigid, and up to 1.2 × 10-4 W/m K2 at 350 °C for flexible samples, with a decrease of k to 1.5 ±0.4 W/mK in rigid samples. Irrespective of the doping type, samples deposited on Kapton substrates demonstrated to have a lower thermal stability. Subsequently and for the first time, the behaviour of µc-Si: H’s TE properties as a function of uniaxial mechanical strain, and its combined effect with temperature are presented. Due to compensation effects between the electrical conductivity and the Seebeck coefficient, the PF was not enhanced, but overall was largely preserved. Finally, a thermoelectric generator design was proposed to illustrate the suitability of the deposition method for direct translation into practical device. It consists of twelve thermocouples with dimensions (n: 20 x 10; p: 20 x 9)mm, electrically connected in series in a lateral/lateral configuration to provide a power output of 1.04 µW, and a voltage ~ 0.15 V under a temperature difference of 60 °C. These findings demonstrate a means of optimising thermoelectric performance, provide an insight into the physics at work, and underline the clear potential for application of µc-Si: H thin films in thermoelectric generators.