Interfacial engineering of P3HT/ZnOhybrid solar cells using phtalocyanines

P. Giannozzi

Dip. Chimica Fisica Ambiente, Universita di Udine, Italy

20 Avril 2015, IMPMC, Universite 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 FoilTEX –

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 isa high-mobility, wide-gap (3.4 eV) material withwurtzite structure.On the right, the (1010) surface of ZnO, the mostcommon 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 IBM-SP6 machine 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)

(two different ZnPc layer thickness, 4 nm and 15 nm, yield similar results)

Experiments: optical spectra, P3HT/ZnPc/ZnO

ZnPc film thickness: blue dots 4 nm, black 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:

Voc Jsc PCEno ZnPc 0.71 0.17 0.064 nm ZnPc 0.61 0.26 0.0915 nm ZnPc 0.60 0.07 0.07

Open-circuit voltage Voc in V,short-circuit density current Jscin mA/cm2, Power ConversionEfficiency (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 Voc < 0.48 V, is a

measure of recombination in the heterostructure region, showing improved lifetime

for P3HT/ZnPc/ZnO.

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 areduction of Voc of ∼ 0.1 V.

Experimental data on actual samples, produced and measured at CNRS Marseille,confirm all of the above findings.