Key roles of metallo-organic complexes: from photovoltaics materials to enzymatic structures P. Giannozzi Dip. Chimica Fisica Ambiente, Universit` a di Udine, Italy, and IOM-Democritos, Trieste ISM Montelibretti, 12 Novembre 2013 Work done in collaboration with a lot of people (see next slide) – Typeset by Foil T E X –
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Key roles of metallo-organic complexes:from photovoltaics materials
to enzymatic structures
P. Giannozzi
Dip. Chimica Fisica Ambiente, Universita di Udine, Italy, and
IOM-Democritos, Trieste
ISM Montelibretti, 12 Novembre 2013
Work done in collaboration with a lot of people (see next slide)
– Typeset by FoilTEX –
About this talkThe two subjects presented here:
1. Hybrid heterostructures for photovoltaic applicationsin collaboration with: G. Mattioli, P. Alippi, F. Filippone, A. Amore Bonapasta(ISM); M.I. Saba, G. Malloci, C. Melis, A.Mattoni, (IOM Cagliari) S. Ben Dkhil,A. Thakur, M. Gaceur, O. Margeat, A. K. Diallo, Ch. Videlot-Ackermann, J.Ackermann (CNRS Marseille)
2. Metal-induced aggregation processes in β-amyloids peptidesin collaboration with K. Jansen (DESY), G. La Penna (ICCOM), V. Minicozzi,S. Morante, G. C. Rossi, F. Stellato (Roma II)
are quite different but they have something more in common than Zn atoms, DFTsimulations, and a collaboration with people from Rome:
• both are joint experimental and theoretical investigations, and
• on the theory side, in both cases complementary theoretical techniques: classicalor tight-binding MD + first-principle DFT, have been used.
New hybrid materials for solar cells
Hybrid photovoltaic cells: organic molecule or π−conjugated polymer acting asdye (light absorber) and electron donor, on inorganic substrate acting as acceptor.Hold great promises for the realization of cheap and high-yield solar cells.
Good dye and donor candidates:(on the right) polymers such as P3HT,poly(3-hexylthiophene-2,5-diyl);Phtalocyanines (Pc) (on the left, ZnPc)
Good substrate candidate: metal oxide nanoparticles,typically TiO2, with ZnO emerging as alternativematerial (both are cheap and nontoxic). ZnO is a highmobility wide gap (3.4 eV) material with wurtzitestructure. On the right, the (1010) surface of ZnO,the most common surface in ZnO nanoparticles
Model systems
In the past, both P3HT/ZnO and ZnPc/ZnO hybrid systems have been proposedand studied. In this work, the idea is to increase the efficiency of such systemsby introducing ternary heterostructures such as P3HT/ZnPc/ZnO. Hopefully, theymay provide better efficiency via
• Increased optical absorption over a wider spectrum, and
• Reduced electron-hole recombination
Problems for a first-principle theoretical approach:
• Very large supercells (hundreds of atoms) even for simplest model structures(few layers of a surface, or a very small nanoparticle): big calculations!
• Hard problem in a Density-Functional Theory (DFT) framework, due to
– Long-range dispersion (van der Waals) interactions– Strongly correlated 3d states in Zn (correct energy level alignement is crucial)– Need for reliable (or not too wrong) excited states: band gap, optical spectra
Theoretical Methods
Theoretical solutions adopted:
• Model Potential Molecular Dynamics allows relatively quick selection ofpotentially stable structures, followed by Density-Functional Theory refinements
• Usage of advanced DFT functionals:
– DFT+U corrects the worst failures of DFT in correlated materials– vdw-DF allows to include van der Waals forces– tests with hybrid functionals to gain confidence in the results
• Usage of Time-Dependent DF(P)T for calculation of optical spectra (good formolecules, much less so for solids)
DFT calculations performed on HPC machines (mostly on the SP at Cineca) using
the parallel algorithms of the QUANTUM Espresso distribution.
Model P3HT/ZnPc/ZnO: structure, stability
ZnPc on (1010) ZnO surface forms stable layer (Eb = 2.2 eV/molecule)
8-unit P3HT binds with Eb = 0.6 eV/unit to ZnPc/ZnO (vs 0.4 eV/unit to ZnO)
Electronic states, energies
CS (charge-separated) states: e− is in ZnO CBM (Conduction Band Minimum),
h+ is in molecular HOMO. The ZnPc layer raises P3HT LUMO to a more favorable
position for e− transfer to ZnPc and ZnO, improving charge separation at interface
Electronic states, localization in space
Electron-hole recombination made less likely by ZnPc layer: e− and h+ densities
in charge-separated state are more spacially separated and have smaller overlap
Simulated TD-DFPT optical spectra
A. ZnPc/ZnO absorption: split Q-bandsat 1.7 and 1.9 eV, Soret band at 3.1 eV.
B. P3HT/ZnPc/ZnO: superpositionof ZnPc/ZnO peaks and of theblue-shifted (2.3 eV) peak of P3HT.
C. 4-unit P3HT on ZnO: absorptionpeak at 2.15 eV.
(Contribution from ZnO substrate is subtracted out)
Experiments: optical spectra, ZnPc on ZnO
ZnPc on glass: two peaks (Q bands) at 622 nm and 711 nm
ZnPc on ZnO: additional peaks due to molecule-substrate interactions
appear at 674 nm (blue arrow) and at 742 nm (light blue arrow)
Experiments: optical spectra, P3HT/ZnPc/ZnO
ZnPc film thickness: black dots 4 nm, blue dots 15 nm. Up: The spectrum
of P3HT/ZnPc/ZnO exhibits absorption peaks of P3HT and of ZnPc, plus the
new optical features of ZnPc/ZnO interface. Down: External Quantum Efficiency
(EQE) shows that the new band at 674 nm contributes additional photocurrent.