**3. Migrating the emission of polyfluorene-based LEDs towards the near infrared using lead-sulfosalt (PbS) colloidal quantum dots**

Due to their relatively large HOMO-LUMO separations, conjugated polymer-based lightemitting diodes are perfectly suited for operation in the visible but their potential for nearinfrared operation remains limited. As we mentioned previously, the hybrid integration of semiconductor nanocrystals and conjugated polymer material systems can provide an easy pathway for (1) improving the conjugated polymer-based devices' optoelectronic properties and/or (2) providing added functionality to the conjugated polymer-based device structures.

Indeed, semiconductor quantum dots have been recently used to controllably-alter the optoelectronic properties of a wide variety of host systems for biosensing, light-emitting or photovoltaic applications (Bakueva et al., 2003; Liu et al., 2009; McDonald et al., 2005; Steckel et al., 2003; X. Zhang et al., 2007).

Owing to low bandgaps, ultrafast recombination processes and large nonlinear coefficients, crystalline lead-salt chalcogenides (a sub-group of the IV-VI semiconductor family) have been one of the basic materials used in modern infrared light sources & lasers, photodetectors and high-performance thermoelectric for the last 50 years (Klann et al., 1995; Preier, 1979). Bulk lead sulfosalt (PbS) is well-suited for infrared optoelectronics, having a direct 0.41 eV bandgap and uncommonly large exciton binding energy (close to 300 meV).

Meanwhile, the first colloidal synthesis of chalcogenide semiconductor nanocrystals (CdS) in the mid-1980's (now referred-to as *colloidal quantum dots*) has provided a new pathway to producing low-cost optoelectronic materials with novel physical properties (Brus, 1984; Rossetti et al., 1983; Steigerwald et al., 1988). Then, it was only a matter of time before leadsalt nanocrystals were synthesized using the colloidal method (Hines & Scholes, 2003; I. Kang & Wise, 1997; Machol et al., 1993; Wang et al., 1987; Yang et al., 1996).

Hybrid Polyfluorene-Based Optoelectronic Devices 183

More recently, the self-assembly of lead-salt nanocrystals into more complex nanowire (1D) (Cho et al., 2005; Jang et al., 2010; Koh et al., 2010), monolayer (2D) (Anikeeva et al., 2007, 2008; Coe-Sullivan et al., 2003; Konstantatos et al., 2005; Steckel et al., 2003; X. Zhang et al., 2007) and nanocrystalline films and superlattices structures (3D) (Hanrath et al., 2009; Klem et al., 2007, 2008; Luther et al., 2008; Talapin et al., 2005) with a wide range of most promising optoelectronic properties has rapidly become a very active field of research,

Fig. 7. (a) PbS nanowires formed by oriented-attachment of colloidal nanocrystals and (b) PbS nanocrystal films obtained by directed self-assembly. The inset shows the top-view of the self-assembled film. (c) This assembly process can be controlled down to reasonably-well

Recently, exciting reports such as the observation of superb multiple-exciton generation efficiencies (Sargent, 2009; Sukhovatkin et al., 2009), highly-efficient hot-electron injection (Tisdale et al., 2010), and cold-exciton recycling (Klar et al., 2009), have propelled nanocrystalline lead-chalcogenide film structures to the forefront of cutting-edge research (M. S. Kang et al., 2009; W. Ma et al., 2009; Sambur et al., 2010; Steckel et al., 2003). Figure 7 shows typical examples of nanowires (1D), monolayers (2D) and films (3D) fabricated via

Due to their band-structure alignment, we have shown that such PbS nanocrystals would be ideal for hybrid integration into the TFB:F8BT heterostructure to help migrate its operation towards the near-infrared. The most intuitively-obvious thing to do would be of course to simply mix colloidal quantum dots within the blended precursor prior to deposition. While this approach does work, threshold voltages and currents are generally very high, while quantum efficiencies and net output powers tend to be very low (Choudhury et al., 2010; Konstantatos et al., 2005). Indeed, this approach suffers from major fundamental drawbacks. Assuming that the quantum dots are distributed homogeneously in the blended film, we also know that only the quantum dots located within tens of nanometers for the interfaces will be active. As such, this approach requires very large concentrations of quantum dots while most of them remain inactive. Moreover, it is likely that the incorporation of such large concentrations of quantum dots in the polymer host will have detrimental

the directed self-assembly of PbS nanocrystals synthesized by hot-colloidal method.

**3.3 The incorporation of PbS nanocrystals in polymer-based host systems** 

consequences on the performances of the polymer host itself.

**3.2 Directed self-assembly of lead-salt nanocrystals** 

largely due to its facile solution-based processing.

organized monolayers.

As shown in Figure 5, the TFB:F8BT conjugated polymer-based heterostructures also constitute an ideal host system for PbS semiconductor nanocrystals to provide low-cost and high-performance hybrid heterostructures for key applications such as lighting & displays, biomedical devices, lab-on-a-chip, flexible optoelectronics, night-vision and solar-energy harvesting device architectures.

Fig. 5. Hybrid polyfluorene-based light-emitting heterostructures. (a) The TFB:F8BT system provides an ideal host system for PbS nanocrystal incorporation. (b) The incorporation of PbS nanocrystals can migrate their operation to the near-infrared (between 900 - 1600 nm), depending on the their size.
