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 EX –
About this talk The two subjects presented here: 1. Hybrid heterostructures for photovoltaic applications in 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 peptides in 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, DFT simulations, 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: classical or 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 as dye (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 TiO 2 , with ZnO emerging as alternative material (both are cheap and nontoxic). ZnO is a high mobility wide gap (3.4 eV) material with wurtzite structure. 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 proposed and studied. In this work, the idea is to increase the efficiency of such systems by introducing ternary heterostructures such as P3HT/ZnPc/ZnO. Hopefully, they may 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 of potentially 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 for molecules, 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 ( E b = 2 . 2 eV/molecule) 8-unit P3HT binds with E b = 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-bands at 1.7 and 1.9 eV, Soret band at 3.1 eV. B. P3HT/ZnPc/ZnO: superposition of ZnPc/ZnO peaks and of the blue-shifted (2.3 eV) peak of P3HT. C. 4-unit P3HT on ZnO: absorption peak 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.
Experiments: current density-voltage curves Measured performances: PCE V oc J sc no ZnPc 0.71 0.17 0.06 4 nm ZnPc 0.61 0.26 0.09 15 nm ZnPc 0.60 0.07 0.07 Open-circuit voltage V oc in V, short-circuit density current J sc in mA/cm 2 , Power Conversion Efficiency (PCE) in %
Experiments: transient open circuit voltage decay Blue: P3HT/ZnPc/ZnO, Red: P3HT/ZnO. Illumination is suppressed with circuit open (no current flowing) and the decay time of carriers is measured. Carrier lifetime as a function of the open circuit voltage, in the region V oc < 0 . 48 V, is a measure of recombination in the heterostructure region, showing improved lifetime for P3HT/ZnPc/ZnO.
Discussion and conclusions (1) Theoretical predictions on the ternary P3HT/ZnPc/ZnO system: • The system is thermodynamically stable • Light absorption from both P3HT and ZnPc covering a wide spectrum • Increased charge separation due to ZnPc layer reduces recombination • The P3HT HOMO is shifted by the ZnPc layer to higher energies, leading to a reduction of V oc of ∼ 0 . 1 V. Experimental data on actual samples, produced and measured at CNRS Marseille, confirm all of the above findings.
Aggregation of peptides induced by metal ions Very nasty degenerative illnesses are caused by aggregation of naturally present proteins or peptides into toxic amyloid fibrils and plaques In Alzheimer disease, the main components of plaques are β -amyloids peptides (A β ): chains of 39 to 43 aminoacids, obtained by cleavage of a precursor protein (in the figure: A β 40 peptide in water) There is experimental evidence that transition metal ions Cu, Zn, Fe play a role in the processes of A β aggregation and plaque formation The details of the metal-A β binding are thus subject of intense study
β -amyloids binding with Cu and Zn: state of the art • The structure of A β binding with Cu is relatively well characterized, with Cu having a stable intra-peptide coordination • A β binding with Zn is not as clear. Competing structural models – from XAS: inter-peptide Zn 2+ bridge between three or more histidines (His) belonging to different peptides. Rather peculiar and infrequent: hallmark of peptide aggregation? – from NMR: intra-peptide binding to three His and either the N-terminus or a residue (Glu 11 ) • Competition for peptide binding between Cu and Zn ions likely Goal of this work: to find, using numerical simulations, realistic configurations for A β chains coordinated by Zn 2+ , fitting XAS results
Simulation procedure • Initial configurations generated with graphical tools (VMD), optimized with Amber force fields and Monte Carlo Random Walk • Selected configurations truncated (aminoacids 1-10 removed), optimized, set into an orthorhombic cell filled with water molecules, thermalized with classical MD, optimized with Tight-Binding MD • Finally, first-principle (i.e. from electronic structure) Car-Parrinello Molecular Dynamics runs are performed to check the stability and refine the structure of the various binding configurations The last step is by far the most time-consuming, requiring parallel execution on big computer facilities, including the BG/P (courtesy of DEISA DECI and of John von Neumann Institute for Computing)
Choosing the starting configurations Four good starting models (generated for A β 16 ) compatible with XAS data (many more turned out to be bad and were discarded): • S1: Zn bound to four histidines • S2: Zn bound to three histidines • S3: two Zn ions, bound to four histidines • S4: two Zn ions, bound to three peptides
Car-Parrinello Molecular Dynamics Introduce fictitious dynamics on the electronic orbitals φ v : r + 1 � | ˙ � � r ) | 2 d� M I ∇ 2 L = µ φ v ( � R I − E [ φ, R ] � 2 v I ( µ = fictitious electronic mass), subject to orthonormality constraints on the orbitals, implemented via Lagrange multipliers Λ ij . The above Lagrangian generates the following equations of motion: φ i = − δE M I ¨ µ ¨ � � + Λ ij φ j R I = −∇ � R I E [ φ, R ] δφ i ij (nuclear motion is classical). These equations can be integrated (i.e. solved) for both electrons and nuclei using classical Molecular Dynamics algorithms. The combined electronic and nuclear dynamics keeps electrons close to the ground state.
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