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

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, largely due to its facile solution-based processing.

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 organized monolayers.

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

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

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 consequences on the performances of the polymer host itself.

Hybrid Polyfluorene-Based Optoelectronic Devices 185

Fig. 9. (a) 2D X-ray diffraction pattern of pristine PEO polymer nanofibers (b) Schematic (not drawn to scale) showing oriented crystallite arrangement within the fibers. The white spaces between the polymer crystallites represent regions of amorphous polymer domains. (c,d) Results for PEO fibers incorporated with nanoparticles. Adapted from (Sharma et al., 2010)

**3.4 The incorporation of PbS nanocrystals in polyfluorene-based bilayered type-II** 

To alleviate those two fundamental problems, a better option explored more recently consists in depositing a thin self-assembled monolayer of quantum dots directly at the junction of the organic heterostructure between the hole- and electron-transporting organic materials (Steckel et al., 2003). However, this approach renders the whole fabrication sequence significantly more complex and usually involves the thermal evaporation of short-

With long-molecule organics, this approach is even more delicate. One might suggest that simply spin-coating a pure F8BT layer atop a pure TFB layer or vice-versa would work. However, conjugated polymers from a same family tend to dissolve in similar solvents. As a consequence, the solvent of the second layer will significantly deteriorate the first layer. As shown in Figure 10, a viable approach to fabricating decent polyfluorene-based bilayered heterostructures consists in lifting-off a F8BT layer from one substrate and re-depositing

Fig. 10. Electroluminescence of blended (◦) and bilayered (•) TFB-F8BT light-emitting diodes

As shown in Figure 11, this approach can allow the incorporation of a thin monolayer of PbS quantum dots directly at the junction between the TFB and F8BT. Still, their performances

with permission. Copyright 2010 American Chemical Society.

molecule organic semiconductors atop the quantum dots.

directly on a TFB layer previously spin-coated on another substrate.

**heterostructures** 

under 19V forward bias.

Fig. 8. The consequences of quantum-dot incorporation on blended polyfluorene-based film structures. Confocal fluorenscence mapping of the domain structure for the same tolueneblended TFB:F8BT (a) without PbS quantum dots and (b) impregnated with PbS quantum dots. The scale bars are 3 µm.

To verify the consequences of the PbS quantum dots incoporation, we used confocal fluorescence mapping of blended TFB:F8BT films with and without the colloidal quantum dots. The visible emission intensity maps (collected with a silicon detector) shown in Figure 8 reveal the domain structure for the blended films with and without quantum dots. There, the bright regions are F8BT-rich domains, while the dark regions represent TFB-rich domains. Without colloidal dots, Figure 8(a) shows significantly larger domains compared with the same blended film impregnated with near-infrared colloidal quantum dots (Fig. 8b).

Another factor to consider is the potential consequences of the nanoparticles incorporation on the crystalline phase of both polymers. Indeed, most polymers are known to consist of crystalline domains surrounded by amorphous chains. Indeed, the degree of crystallinity and the organization of those domains will also significantly impact the optoelectronic properties of the polyfluorenes (X. Ma et al., 2010). To study the consequences of nanoparticle incorporation on the structural organization of the polymers, we used electrospinning to pull polymer nanofibers (Sharma et al., 2010). Using a split collector during the electrospinning, it is possible to align those polymer fibers across the collector gap as shown in Figure 9(a). Using the 2D X-ray diffraction facility at the DND-CAT Synchrotron Research Center (AdvancedPhoton Source at the Argonne National Laboratory), we were able to observe the clear diffraction orders indicating that the crystallites in pure polymer fibers favor an alignment along the fibers such as shown in Figure 9(b). However, the incorporation of inert silica nanoparticles inside the polymer fibers disrupts this orientational ordering and yields a ring-like diffraction pattern shown in Figure 9(c), now suggesting a randomized polycrystalline structure such as shown in Figure 9(d). Indeed, by integrating for all azymuthal angles, we can confirm that the degree of crystallinity remains similar while the crystallites no longer show preferred alignment along the polymer fibers. This effect can also have very detrimental consequences of the optoelectronic properties of the polyfluorene-based device architectures incorporated with high concentrations of quantum dots.

Fig. 8. The consequences of quantum-dot incorporation on blended polyfluorene-based film structures. Confocal fluorenscence mapping of the domain structure for the same tolueneblended TFB:F8BT (a) without PbS quantum dots and (b) impregnated with PbS quantum

To verify the consequences of the PbS quantum dots incoporation, we used confocal fluorescence mapping of blended TFB:F8BT films with and without the colloidal quantum dots. The visible emission intensity maps (collected with a silicon detector) shown in Figure 8 reveal the domain structure for the blended films with and without quantum dots. There, the bright regions are F8BT-rich domains, while the dark regions represent TFB-rich domains. Without colloidal dots, Figure 8(a) shows significantly larger domains compared with the same blended film impregnated with near-infrared colloidal quantum

Another factor to consider is the potential consequences of the nanoparticles incorporation on the crystalline phase of both polymers. Indeed, most polymers are known to consist of crystalline domains surrounded by amorphous chains. Indeed, the degree of crystallinity and the organization of those domains will also significantly impact the optoelectronic properties of the polyfluorenes (X. Ma et al., 2010). To study the consequences of nanoparticle incorporation on the structural organization of the polymers, we used electrospinning to pull polymer nanofibers (Sharma et al., 2010). Using a split collector during the electrospinning, it is possible to align those polymer fibers across the collector gap as shown in Figure 9(a). Using the 2D X-ray diffraction facility at the DND-CAT Synchrotron Research Center (AdvancedPhoton Source at the Argonne National Laboratory), we were able to observe the clear diffraction orders indicating that the crystallites in pure polymer fibers favor an alignment along the fibers such as shown in Figure 9(b). However, the incorporation of inert silica nanoparticles inside the polymer fibers disrupts this orientational ordering and yields a ring-like diffraction pattern shown in Figure 9(c), now suggesting a randomized polycrystalline structure such as shown in Figure 9(d). Indeed, by integrating for all azymuthal angles, we can confirm that the degree of crystallinity remains similar while the crystallites no longer show preferred alignment along the polymer fibers. This effect can also have very detrimental consequences of the optoelectronic properties of the polyfluorene-based device architectures incorporated with

dots. The scale bars are 3 µm.

high concentrations of quantum dots.

dots (Fig. 8b).

Fig. 9. (a) 2D X-ray diffraction pattern of pristine PEO polymer nanofibers (b) Schematic (not drawn to scale) showing oriented crystallite arrangement within the fibers. The white spaces between the polymer crystallites represent regions of amorphous polymer domains. (c,d) Results for PEO fibers incorporated with nanoparticles. Adapted from (Sharma et al., 2010) with permission. Copyright 2010 American Chemical Society.

#### **3.4 The incorporation of PbS nanocrystals in polyfluorene-based bilayered type-II heterostructures**

To alleviate those two fundamental problems, a better option explored more recently consists in depositing a thin self-assembled monolayer of quantum dots directly at the junction of the organic heterostructure between the hole- and electron-transporting organic materials (Steckel et al., 2003). However, this approach renders the whole fabrication sequence significantly more complex and usually involves the thermal evaporation of shortmolecule organic semiconductors atop the quantum dots.

With long-molecule organics, this approach is even more delicate. One might suggest that simply spin-coating a pure F8BT layer atop a pure TFB layer or vice-versa would work. However, conjugated polymers from a same family tend to dissolve in similar solvents. As a consequence, the solvent of the second layer will significantly deteriorate the first layer. As shown in Figure 10, a viable approach to fabricating decent polyfluorene-based bilayered heterostructures consists in lifting-off a F8BT layer from one substrate and re-depositing directly on a TFB layer previously spin-coated on another substrate.

Fig. 10. Electroluminescence of blended (◦) and bilayered (•) TFB-F8BT light-emitting diodes under 19V forward bias.

As shown in Figure 11, this approach can allow the incorporation of a thin monolayer of PbS quantum dots directly at the junction between the TFB and F8BT. Still, their performances

Hybrid Polyfluorene-Based Optoelectronic Devices 187

Fig. 12. Self-assembled PbS nanocrystalline film structure. (a) Cross-sectional SEM

the nanocrystalline film showing a good conductivity uniform across the surface.

the hole-mobility for the dithiol-treated nanocrystalline films.

while the hole mobility µh is calculated using (Juška et al., 2001):

measurement, such as shown in Figure 13(b,c).

*h*

micrograph of a self-assembled film of PbS colloidal quantum dots formed by dithiol ligandexchange chemistry. The inset shows a typical AFM mapping of the nanocrystalline PbS film surface revealing a 2 nm roughness parameter. (b) Cross-sectional map of the intensity of the 451 cm-1 Raman line associated with the 2LO-phonon vibration of the PbS film structure measured by confocal micro-Raman spectroscopy. (c) Conductive AFM (TUNA) mapping of

p-type doping of the PbS nanocrystalline films, the CELIV measurement provides us with

For CELIV measurements, the samples consist of a 250~350 nm-thick nanocrystalline films fabricated using the layer-by-layer spin coating method and sandwiched between ITO and Al contacts such as shown in Figure 13(a). As shown in Figure 13(b,c), a linearly-increasing bias is then applied across the sample while the transient currents are measured through the voltage drop across a 200 Ω load. The conductivity σ can then be obtained using (Juška et al., 2001):

max

2

2

*d <sup>j</sup> At*

3 1 0.36

The parameters tmax, j0 and Δj in those equations can be obtained directly from the CELIV

In contrast, the time-of-flight (TOF) method can be used to measure the minority carrier mobility. For TOF measurements, the samples have the same structure as for the CELIV measurements but require a much thicker nanocrystalline layer (1.0 ~ 1.5 µm) formed by depositing multiple layers of quantum dots. The 5 ns pulses of a Q-switched Nd:YAG laser are then used to excite the sample from the ITO side, while a reverse-bias is applied on the sample to extract the photo-generated minority carriers. In this case, the transient currents can also be recorded using the voltage drop across a 200 Ω load as shown in Figure 13(d).

2

*e d <sup>V</sup>* 

0

*j*

*d j t A* 

(1)

(2)

(3)

3 2

2 max

The electron mobility µe is then directly obtained as (Tiwari & Greenham, 2009):

greatly suffer from poor injection efficiencies and from significant carrier losses into the organic layers (Konstantatos et al., 2005), most especially in the electron-transporting layer.

Fig. 11. Incorporation of PbS nanocrystals in polyfluorene-based bilayered type-II heterostructures. (a) Device schematics. (b) Self-assembled monolayer of PbS quantum dots. (c) Cross-sectional TEM of the structure after lift-off and deposition of the polymer layer atop the structure. (d) Cross-sectional confocal fluorescence intensity mapping showing the thin quantum-dot layer at the interface.
