Thermoelectric properties of nanostructured porous-polysilicon thin films

• Published in: Nano energy 2021-02, Vol.80, p.105553, Article 105553
• Main Authors: Ziouche, Katir, Bel-Hadj, Ibrahim, Bougrioua, Zahia
• Format: Article
• Language: English
• Subjects: Figure of merit; Nanostructuration; Physics; Polysilicon; Porous; Power factor; Thermal conductivity; Thermoelectricity
• ISSN: 2211-2855; 2211-2855
• DOI: 10.1016/j.nanoen.2020.105553

In this paper, we address the optimization of the electrical and thermoelectrical properties of nanostructured polysilicon material for integration in thermoelectric (TE) devices. In particular, we describe an original method to build a superficial and nanostructured porous polysilicon (POpSi) thin film on a Silicon substrate. This latter is electrically isolated from the conductive Silicon substrate by an SiO2 interlayer for Van der Pauw electric and home-made thermoelectric measurements. From the characterization data, we demonstrate that the nanostructuration brought by porosification of polysilicon layers (polySi) breaks their thermal conductivity by a factor of about 30 and has no detrimental impact on their Seebeck coefficient: a POpSi with up to 62% porosity has an experimental Seebeck value of 260 µV/K when the standard polysilicon, it is built from, is n type with a carrier density of 3.4 × 1019/cm3. On the other hand, the benefit of the thermal conductivity reduction clearly over-compensates the degradation of the experimental electrical conductivity measured in the POpSi layers: in terms of the figure of merit ZT, a 25-fold increase in a POpSi layer with 44% porosity is seen compared to the standard polySi layer (it increases from 0.004 to 0.1). Such a novel material is a viable candidate for planar thermoelectric devices using Silicon technologies. In this effort, we discuss the integration of a POpSi thin film with optimum porosity into a planar TE microgenerator (µTEG) with suspended membranes supported by a simulated conversion efficiency increased by 28% compared to a µTEG integrating the standard polySi layer counterpart. [Display omitted] •Original method to fabricate a novel nanostructured TE material : n-type porous polysilicon thin film; we name it POpSi.•Porosification of polysilicon greatly breaks its thermal conductivity with no detrimental impact on its Seebeck coefficient.•A 25-fold increase of ZT is found for POpSi with 44% porosity : ZT increases from 0.004 for standard polySi to 0.1 in POpSi.•PopSi will be a new candidate for integration into membrane-based μTEGs.•Even higher improvement expected starting the porosification process onto p-type highly B doped polySi layers.