SLIDE 1 Interfacial engineering of P3HT/ZnO hybrid solar cells using phtalocyanines
- P. Giannozzi
- Dip. Chimica Fisica Ambiente, Universit`
a di Udine, Italy
20 Avril 2015, IMPMC, Universit´ e Paris VI Work done in collaboration with
- G. Mattioli, P. Alippi, F. Filippone, A. Amore Bonapasta (ISM-CNR, Rome) for
ab-initio simulations
- M.I. Saba, G. Malloci, C. Melis, A. Mattoni (IOM-CNR, Cagliari) for classical
MD simulations
- S. Ben Dkhil, A. Thakur, M. Gaceur, O. Margeat, A. K. Diallo, Ch. Videlot-
Ackermann, J. Ackermann (CNRS Marseille) for the experimental part
– Typeset by FoilT EX –
SLIDE 2
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 TiO2, 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
SLIDE 3 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
SLIDE 4 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 IBM-SP6 machine at Cineca) using the parallel algorithms of the QUANTUM Espresso distribution.
SLIDE 5
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)
SLIDE 6
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
SLIDE 7
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
SLIDE 8 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
ZnPc/ZnO peaks and
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)
SLIDE 9
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)
(two different ZnPc layer thickness, 4 nm and 15 nm, yield similar results)
SLIDE 10 Experiments: optical spectra, P3HT/ZnPc/ZnO
ZnPc film thickness: blue dots 4 nm, black dots 15 nm. Up: The spectrum
- f 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.
SLIDE 11
Experiments: current density-voltage curves
Measured performances: Voc Jsc PCE 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 Voc in V, short-circuit density current Jsc in mA/cm2, Power Conversion Efficiency (PCE) in %
SLIDE 12 Experiments: transient open circuit voltage decay
Blue: P3HT/ZnPc/ZnO, Red: P3HT/ZnO. Illumination is suppressed with circuit
- pen (no current flowing) and the decay time of carriers is measured.
Carrier lifetime as a function of the open circuit voltage, in the region Voc < 0.48 V, is a measure of recombination in the heterostructure region, showing improved lifetime for P3HT/ZnPc/ZnO.
SLIDE 13 Discussion and conclusions
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 Voc of ∼ 0.1 V. Experimental data on actual samples, produced and measured at CNRS Marseille, confirm all of the above findings.
