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Elimination of interface energy barriers using dendrimer polyelectrolytes with fractal geometry

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In this work we study conjugated polyelectrolyte (CPE) films based on polyamidoamine (PAMAM) dendrimers of generations G1 and G3. These fractal macromolecules are compared to branched polyethylenimine (b-PEI) polymer using methanol as the solvent. All of these materials present a high density of amino groups, which protonated by methoxide counter-anions create strong dipolar interfaces. The vacuum level shift associated to these films on n-type silicon was 0.93 eV for b-PEI, 0.72 eV for PAMAM G1 and 1.07 eV for PAMAM G3. These surface potentials were enough to overcome Fermi level pinning, which is a typical limitation of aluminium contacts on n-type silicon. A specific contact resistance as low as 20 mΩ·cm² was achieved with PAMAM G3, in agreement with the higher surface potential of this material. Good electron transport properties were also obtained for the other materials. Proof-of-concept silicon solar cells combining vanadium oxide as a hole-selective contact with these new electron transport layers have been fabricated and compared. The solar cell with PAMAM G3 surpassed 15% conversion efficiency with an overall increase of all the photovoltaic parameters. The performance of these devices correlates with compositional and nanostructural studies of the different CPE films. Particularly, a figure-of-merit (Vσ) for CPE films that considers the number of protonated amino groups per macromolecule has been introduced. The fractal geometry of dendrimers leads to a geometric increase in the number of amino groups per generation. Thus, investigation of dendrimer macromolecules seems a very good strategy to design CPE films with enhanced charge-carrier selectivity.

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Articles publicats en revistes (Física Aplicada)

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Elimination of interface energy barriers using dendrimer polyelectrolytes with fractal geometry