ZnO is a wide band gap and transparent semiconductor with good chemical stability and peculiar opto-electronic properties that motivated its wide use in technological applications as optical waveguides, chemical and gas sensors, UV emitters and solar cells.1 Such versatility continues to inspire experimental and theoretical studies that aims at unveiling the structure-property relationships in ZnO nano-structures.2 In last generation solar cells based on hybrid bulk heterojunction, n-type ZnO nanoparticles (NPs) have been exploited as functional constituents in composite materials with conjugated polymers and have showed promising performances, thanks to their transparency in the visible spectra, high electron mobility, stability and easy processabillity.3 Within this context, we applied state-of-the-art first-principles methods to study the ZnO nonpolar surfaces (10-10 and 11-20). We discuss, from a quantum-mechanical perspective, the interactions between these surfaces and common capping agents used in ZnO NP synthesis, namely tri-butil-phosphonic acid and long-chain aliphatic amines. These molecules have different polarity and they affect the electronic structure of ZnO in opposite ways. In particular, we found that the specific molecule-surface interactions can modulate the ZnO NP opto-electronic properties that are relevant for solar energy conversion processes (e.g. band gap, work function, band edge absolute positions). We evaluated these properties when the NP surfaces are covered with different coverage and with mixed phosphonic acid and amine adsorbed layer, account also for the polymer matrices via implicit solvation. Direct comparison with experimental data provided a reliable validation of our ab initio results. From analysis of these data, we derive new general design principles toward better performing nano-structured devices for solar energy conversion. 1. (a) M. H. Huang et al., Science 2001, 292, 1897-99; (b) S. Polarz et al., Adv. Func. Mater. 2007, 17, 1385-91; (c) J. F. Gnichwitz et al., J. Am. Chem. Soc. 2010, 132, 17910-20; (d) A. Manekkathodi et al., Nanoscale 2013, 5, 12185-91; (e) H. Wang et al., J. Mat. Chem. A 2013, 1, 6622. 2. (a) B. Meyer et al., Phys. Rev. B 2003, 67, 035403; (b) A. Iglesias-Juez et al., J. Mater. Chem. A 2015, 3, 8782-92. 3. (a) W. J. Beek et al., J. Phys. Chem. B 2005, 109, 9505-16; (b) Y. Lin et al., J. Am. Chem. Soc. 2011, 133, 6513-16.

Tuning the electronic properties of ZnO nonpolar surfaces for solar energy conversion devices: New insights from a first-principles study

251st National Meeting (2016)

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