Fabrication and characterization of�ber-like nanostructured polymer �lmsfor organic photovoltaic applications

David Moerman, Olivier Douhéret, Roberto Lazzaroni

Laboratory for Chemistry of Novel MaterialsUniversity of Mons - Materia-Nova

I. Introduction

The active layer of organic based solar cells (OPV) is usually made of abulk heterojunction (BH) active layer consisting of a blend of a polymer aselectron donor and a fullerene derivative as electron acceptor.

Fig. 1 � Schematic bulk-heterojunction solar cell and elec-tronic transfer between the donor and the acceptor.

The molecular order in BH layers is shown to play a key role in the transportof the electronic excitations towards the donor/acceptor interfaces wheredissociation into free charges takes place (Fig. 1). It also ensures thecharge transport towards the electrodes via appropriate percolation paths.Further, control of the polymer material ordering at the nanoscale will in-�uence transport and also contribute to ultimately adjust the blend mor-phology, hence leading to improvement of the device e�cency.

In this poster we present how to control the generation of nanostructuredpoly(3-hexylthiophene) (P3HT), an electron donor polymer, in solution tofabricate fully �brillar thick �lms by spin-coating. The stability of �brillarP3HT in solution is demonstrated when the acceptor material ([6,6]-phenyl-C61-butyric acid methyl ester or PCBM) is added in solution. This enablesfabrication of thick �lms of OPV blends with controlled morphology. Thecontribution of these nanostructures to the photovoltaic performances is �-nally pointed out. The molecular structures of P3HT and PCBM and theillustration of the �ber-like P3HT are given in Fig. 2.

Fig. 2 � Molecular structure and schematic self-assembledstructure of P3HT and molecular structure of PCBM.

II. Results

Solution process to promote P3HT organization

• The organization of the polymerchains leads to a red-shift in theUV-vis absorption spectrum:−H− Molecularly dissolved P3HT(polymer chains are free to moveand twist)−�−: Partial organization in so-lution(interacting polymer chains lead-ing to a bandgap decrease)

⇒ Evidence of organized and non-organized P3HT in solution

Fig. 3 � UV-vis. absorption spectra.

• Extraction of organizated P3HT by centrifugation.

⇒ Three absorption bands typical of the organized polymer, also calledvibronic structure, dominate the spectrum − J −

Spin-coating the organized P3HT (− J −)

FIBER-LIKE MORPHOLOGY

Film and solution have the samevibronic structure ⇒ P3HT chainsorganized in both �lm and solutionpresent �brillar like morphology

Fig. 4 � Tapping-mode phase AFM imageof a thick �lm (≈100 nm) of organized

P3HT and its UV-vis absorption spectrum.

FIBRILS COMPOSED OF 3MOLECULAR LAYERS

The distance a between twopolymers chains within a �bril isdetermined by X-ray di�raction

(see Fig.5).

Fig. 5 � Schematic structure of a P3HT �bril.

Blends of FIBRILLAR P3HT and PCBMThe vibronic structure attests ofthe stability of FIBRILLAR P3HT

when mixed with PCBM

Fig. 6 � UV-vis absorption spectra of solutionand �lm composed of FIBRILLAR P3HT:PCBM.

Fibrillar structure of P3HTevidence in a blend

Fig. 7 � Tapping Mode phase AFM imageof FIBRILLAR P3HT:PCBM blend.

Fabrication and characterization of photovoltaic devices

The power e�ciency is given by

η = FF JSCVOC

PL/A

where FF = JmVm

JSCVOC

(FF for �ll-factor, PL = incident lightpower and A = area)

Fig. 8 � Ideal current density�voltagecharacteristic in the dark (dotted

line) and under illumination (full line).

NON-ORGANIZED P3HT:PCBM FIBRILLAR P3HT:PCBM

Fig. 9 � Current density�voltage characteristics of non-organized and �brillar P3HT:PCBM blend solar cells inthe dark and under calibrated A.M. 1.5 white light illumi-

nation, 100 mW.cm−2.

Blends based on FIBRILLAR P3HT lead to a DOUBLING of the powere�ciency through JSC increase, i.e. better transport of photocreated charges.

III. Conclusion

By tuning standard solution processing, we promote the organization of P3HT. Films for photovoltaicdevices were obtained from this organized phase, which is found to have a �brillar morphology. This�brillar structure is stable when blended with PCBM and the photovoltaic performances of the relateddevices are shown to double with the use of �brillar P3HT.

Acknowledgements

This work is supported by the Fonds pour la for-

mation à la Recherche dans l'Industrie et dans

l'Agriculture (FRIA).

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