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Enhanced Photocurrent With Hexagonal Cu2Te In Polymers Thin Films

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Hexagonal-shaped Cu2Te nanodisks self-organize into a network of ribbons fully embedded in rr-P3HT thin films.


Hybrid nanocrystal-polymer materials are of great importance for the solution-based fabrication of thin films with tailored optical and electronic properties for sensing, light emission, and photovoltaic applications. The excellent control over size, shape, and composition of inorganic nanocrystals (NCs) allows tuning of their physical properties and makes them interesting materials for devices, ranging from optoelectronics to medicine. On the other hand, polymers can also be synthesized with customized properties to fulfil a wide range of functionalities. Organic electronics has indeed seen a significant progress in the last years thanks to the synthesis of highly performing polymer semiconductors and conductors.

One important aspect of polymer technology that it shares with NCs is low-cost, solution-based processing techniques, such as spin-coating or drop-casting. The main approaches to prepare hybrids that combine the two materials are either:

  • linking of the NCs and polymer constituents by specific interactions at the nanoscale (i.e., by controlling their interface properties)
  • or by simple mixing of the two components in a blend that can be further processed by standard solution-based fabrication methods.

 

Improving compatibility between NCs and polymers is desired to avoid NC aggregation during film formation that can be detrimental for the functionality of the hybrid material.  This issue is in part alleviated when working with NCs of certain shapes, such as rods, tetrapods, octapods, and nanodisks, which have the ability to self-segregate into chainlike assemblies.  Aggregation of NCs into well-dispersed linear arrays in the polymer film may help to prevent the local clustering of particles in regions of the mixture that otherwise would be nearly polymer free, and therefore improves the overall miscibility of the two components. In addition to that, the presence of networks of linear and branched arrays of NCs in a polymer can provide pathways for charge transport.

In this context, copper chalcogenide $latex \mathbf{Cu_{2}Te}$ nanocrystals are of particular interest due to their low toxicity compared to cadmium or lead based systems, and because their optical and electrical properties can be tuned by varying their chemical composition. Furthermore, shape control can significantly enhance their ability to aggregate into ordered linear arrays.

Synthesis and Characterization

Recently, Milena P. Arciniega et al.,(1) have  reported NC-polymer hybrid films formed by linear stacks of Cu2Te hexagonal-shaped nanodisks (NDs) in  poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) thin films, prepared by drop casting of the blends, followed by controlled evaporation of the solvent on solid substrates. The assembly process of the NDs in the polymer solution was studied at different concentrations of NDs:

  • In hybrid films prepared with a low ND-volume fraction (≈1%), the particles were randomly dispersed and oriented preferentially with one of the two larger facets towards the substrate.
  • The formation of ND ribbons was observed at higher ND-volume fraction (≈2%), although most of them remained in the same orientation observed at a lower volume fraction.
  • Then, at fraction of ≈5%, one has a network of ribbon structures (consisting of face-to-face stacks of NDs) randomly dispersed in the film, with mutually orthogonal orientation in the denser regions of disks that resemble a square labyrinth pattern (Fig. 1)

 

Considering the widespread use of P3HT in solar cells and photodetectors, the obtained hybrids were studied through optical absorption and ultraviolet photoelectron spectroscopy, and characterized by photocurrent measurements to elucidate the impact of the ordered structures on the polymer performance.

Fig.1. (Left). High angle annular dark-field-scanning transmission microsopy: closer view of a ribbon formed by short segments highlighted by different colors bars. (Right). Sketches illustrating the three different observed ribbon configurations:

  • R1 (nanodisks with a side-facet perpendicular to the the substrate)
  • R2 (tilted and shifted nanodisks)
  • R3 (misaligned nanodisks forming a bend ribbon)

 

Experimental

Considering the widespread use of P3HT in solar cells and photodetectors, the obtained hybrids were studied through various experimental techniques:

  • HAADF-STEM: high angle annular dark-field scanning electron microscopy
  • Grazing incidence X-Ray scattering (GIWAXS/GISAXS) done at the XRD2 beamline of the Brazilian Synchrotron (LNLS, in Campinas)
  • X-Ray diffraction
  • XPS and VUV photoemission spectroscopy
  • Photoresponse Measurements: a primary photocurrent of $latex \rm I_{pr} = 21 mA.cm^{-2}$ was obtained giving a responsivity of 65A/W in the UV for an active layer of $latex \rm 0.06 cm^{2}$. For comparison, early hybrid photodetectors based on PbSe, CdS, or CdSe nanocrystals reported a responsivity of 0. 2 A/W.

 

The authors have demonstrated the ability of hexagonal-shaped $latex \rm Cu_{2}Te$ nanodisks to self-organize into a network of ribbons fully embedded in rr-P3HT thin films. The most remarkable feature of these blends is that even at high nanocrystal content the ribbons do not self-segregate into isolated “pockets” of densely packed nanocrystals, from which the polymer is excluded, but instead form with the polymer an interpenetrating nanostructure, similar to that observed in spinodal decomposition processes. The homogenous structure of the blend,  extending at least over several microns, can be exploited for several applications. For example, the measured photoresponse of layered vertical device architectures with films of the blend displays an enhanced photocurrent compared to devices in which the active film is made by rr-P3HT alone. Such behavior can be explained by the bulk heterojunction interface area, and the more facile dissociation of the excitons genereated in the $latex \rm Cu_{2}Te$ hexagonal nanodisks. This concept has the potential to significantly increase the performance of composite materials for photodetection and solar energy harvesting that so far were limited by low loading fractions of nanoparticles.

Source: Milena P. Arciniegas, Francesco Di Stasio, Hongbo Li, Davide Altamura, Luca De Trizio, Mirko Prato, Alice Scarpellini, Iwan Moreels, Roman Krahne, and Liberato Manna (2016), Self-Assembled Dense Colloidal Cu2Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater., 26: 4535–4542. doi:10.1002/adfm.201600751