(Invited) Plasmonic Composites of Semiconducting Polymers for Optoelectronic Applications

Downscaling metallic fillers in polymer composites to nanometers opens a new field of applications as, for example, in photochemical catalysis, optical sensing or nonvolatile rewritable memories. Combination of plasmonic properties of NPs with semiconducting properties of p-conjugated polymers brings a new functionality for advanced optoelectronic applications due to the synergy effect of surface plasmons and delocalized p-electrons in the polymer backbone. It can be used e.g. for better harvesting of absorbed light for photovoltaic energy conversion by the enhancement of charge transfer near the NP surface.¹ Various procedures of plasmonic composites preparation will be shown on the example of poly(3-alkylthiophene), P3AT and the electrical and optical properties of these composites will be discussed.

By mixing negatively charged Ag or Au NPs with cationic derivatives of P3AT in a composition ratio near to the charge balance between NPs and cationic chains aggregates were formed, in which interparticle distances of NPs enable strong interactions of their plasmons, as it was demonstrated by their optical extinction and high SERS activity, which is even preserved in dry thin solid films prepared from such sol. The surface plasmon resonance of metal nanoparticles contributed markedly to the overall extinction of the composite showing a red shift of optical absorption with increasing number of layers due to an increased electronic coupling of plasmons.

For understanding phenomena that accompany the effect of plasmonic nanoparticles on the optoelectronic behavior of the composites we prepared a more defined system consisting of a metal NPs layer prepared by physical vapor deposition onto surface of spin-cast thin films of poly(3-hexylthiophene). Gold or silver deposited in a very thin layer formed a nanoparticles-like island structure with the morphology depending on the effective thickness of the deposited layer and on its subsequent thermal treatment. Particularly, we focused on the changes of the electrical conductivity of the polymer/NPs composite system measured during temperature cycling (annealing), and on its relation to morphology of the same composite structures monitored by TEM and extra-high resolution SEM microscopy. Comparison to the analogous polystyrene/AuNP layers was made in order to distinguish the role of the polymer support on the morphology and electrical properties of the nanoparticles assembly. The electrical conductivity and optical properties were found to be strongly related to the morphology of the Au NP surface layer, reflecting the thermodynamical behavior of the polymer during thermal annealing, namely the glass transition and melting of the polymer structure. A stabilizing effect of the thiophene-gold interaction on the nanoparticles morphology was observed.

By embedding the Au or Ag NPs, having strong optical extinction in the wavelength interval from 450 nm to 570 nm, in the active layer of P3HT-fullerene bulk heterojunction solar cell, we changed the photovoltaic spectrum of incident photon to collected charge ratio. Besides that, it was shown that in the cells with Au NPs the power conversion efficiency under white light illumination was improved by about 25%. By careful control of the nanoparticles formation we were able to exclude the effects of variation of the plasmonic resonance frequency and possible effects of incident light scattering that would increase the interaction pathway of light within the active polymer. We attributed the observed increase in photovoltaic conversion efficiency to a strong local field enhancement around metal nanoparticles that locally increases optical absorption in a surrounding semiconducting polymer. Figure 1 shows the influence of the Ag and Au NPs on the IPCE and volt-amp characteristics of P3HT/PCBM solar cells. In the case of Au nanoparticles, a clear increase in the power conversion efficiency under white light illumination was detected. The observed increase in the photovoltaic conversion efficiency was attributed to a strong local optical field enhancement around metal nanoparticles that locally increases light absorption in the surrounding semiconducting polymer. The maximum of this optical absorption was found at the position of the respective plasmon resonance peak. It indicates that the enhancement of the solar cells efficiency occurred due to the localized surface plasmon phenomena.²

Acknowledgement: This work was funded by the Czech Science Foundation under the project No. P108/12/1143 and the Ministry of Education, Youths and Sports of the Czech Republic (COST LD 14011).

References

[1] P.V. Kamat, J.Phys. Chem. C 111 (2007) 2834.

[2] G.D. Spyropoulos et.al, Photonics and Nanostructures - Fundamentals and Applications 9, 2 (2011) 184.