**2. Polyfluorene-based type-II heterostructures**

Compared to more commonly-used π-conjugated polymer families such as the polyphenylenes (PPPs and PPVs), polythoiphenes (PTs), polypyrroles (PPYs) and polyanilines (PANIs), polyfluorenes tend to be easier to process and less sensitive to photochemical degradation while still offering very decent optoelectronic properties. This combination of facile processing and durability makes polyfluorenes an ideal case-study platform.

In particular, relatively efficient polymer light-emitting structures for the visible have been previously realized using polyfluorene-based type-II heterostructures fabricated using poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (or **TFB**) as the holetransporting material and poly(9,9'-dioctylfluorene-co-benzothiadiazole (or **F8BT**) as the electron-transporting polymer (Moons, 2002).

Meanwhile, decent photovoltaic structures have also been realized using similar polyfluorene-based type-II heterostructures. However, the best performances reported so far for polyfluorene-based photovoltaics were also using F8BT as the electrontransporting polymer but replacing the hole-transporting TFB with poly (9,9′ dioctylfluorene-*co*-bis-*N,N*′-(4-butylphenyl)-bis-*N,N*′-phenyl-1,4-phenylenediamine) (or **PFB**) (McNeill et al., 2009).

Finally, it is important to mention that while hole-transporting π-conjugated polymers with conductivities over 1000 Ω-1cm-1 have been reported, decent electron-transporting polymers are much more difficult to come by. As such, the electron-transporting material often limits the overall performances of polymer-based optoelectronic devices (Moons, 2002).

#### **2.1 Blended all polyfluorene-based type-II heterostructures for light-emitting and photovoltaic device architectures**

TFB-F8BT system provides a great material platform to understand the basic principles associated with conjugated polymer-based optoelectronic devices. For the typical lightemitting diode (LED) configuration shown in Figure 1(a), the hole-transporting TFB (labeled HTL) and electron-transporting F8BT (labeled ETL) are used to provide a type-II heterostructure. In this configuration, holes are injected from the transparent ITO anode while electrons are injected from the Aluminum cathode. To facilitate carrier injection and reduce exciton quenching at the electrode-polymer interface, optional hole- and electroninjection layers can be introduced in such device architectures. Here, a thin layer of poly(ethylenedioxythiophene):polystyrenesulphonate (or **PEDOT:PSS**) can be used to

conjugated polymers and colloidal quantum dots also raises many important fundamental questions and crucial technical challenges to address before achieving low-cost hybrid

In the long-term, we strongly believe this emerging class of hybrid polymer-based heterostructures will potentially transform the field of opto-electronics by providing lowcost and high-performance semiconductor-based nanocomposite materials and devices for applications such as light sources, biomedical & lab-on-a-chip devices, flexible and/or high-

Compared to more commonly-used π-conjugated polymer families such as the polyphenylenes (PPPs and PPVs), polythoiphenes (PTs), polypyrroles (PPYs) and polyanilines (PANIs), polyfluorenes tend to be easier to process and less sensitive to photochemical degradation while still offering very decent optoelectronic properties. This combination of facile processing and durability makes polyfluorenes an ideal case-study

In particular, relatively efficient polymer light-emitting structures for the visible have been previously realized using polyfluorene-based type-II heterostructures fabricated using poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (or **TFB**) as the holetransporting material and poly(9,9'-dioctylfluorene-co-benzothiadiazole (or **F8BT**) as the

Meanwhile, decent photovoltaic structures have also been realized using similar polyfluorene-based type-II heterostructures. However, the best performances reported so far for polyfluorene-based photovoltaics were also using F8BT as the electrontransporting polymer but replacing the hole-transporting TFB with poly (9,9′ dioctylfluorene-*co*-bis-*N,N*′-(4-butylphenyl)-bis-*N,N*′-phenyl-1,4-phenylenediamine) (or

Finally, it is important to mention that while hole-transporting π-conjugated polymers with conductivities over 1000 Ω-1cm-1 have been reported, decent electron-transporting polymers are much more difficult to come by. As such, the electron-transporting material often limits

TFB-F8BT system provides a great material platform to understand the basic principles associated with conjugated polymer-based optoelectronic devices. For the typical lightemitting diode (LED) configuration shown in Figure 1(a), the hole-transporting TFB (labeled HTL) and electron-transporting F8BT (labeled ETL) are used to provide a type-II heterostructure. In this configuration, holes are injected from the transparent ITO anode while electrons are injected from the Aluminum cathode. To facilitate carrier injection and reduce exciton quenching at the electrode-polymer interface, optional hole- and electroninjection layers can be introduced in such device architectures. Here, a thin layer of poly(ethylenedioxythiophene):polystyrenesulphonate (or **PEDOT:PSS**) can be used to

the overall performances of polymer-based optoelectronic devices (Moons, 2002).

**2.1 Blended all polyfluorene-based type-II heterostructures for light-emitting and** 

optoelectronic devices with superior performances.

performance optoelectronics platforms and photovoltaics.

**2. Polyfluorene-based type-II heterostructures**

electron-transporting polymer (Moons, 2002).

**PFB**) (McNeill et al., 2009).

**photovoltaic device architectures** 

platform.

facilitate the hole injection and improve the structural quality of the TFB film by alleviating the surface roughness of the indium-tin oxide (ITO) substrate. In contrast, a thin layer of low work-function metal such as Calcium can be used to facilitate the electron-injection on the other side of the junction. In such a case, the Aluminum contact remains necessary to prevent oxidation of the low work-function metal. In this system, injected carriers bind into an exciton at the TFB-F8BT interface. Due to the band alignment, this exciton is much more likely to migrate to the electron-transporting F8BT until it recombines radiatively and generates the emission.

Fig. 1. Energy diagrams and schematics of TFB-F8BT polyfluorene-based type-II heterostructures. (a) For use as visible light-emitting diode. (b) For use as a photodetector or solar cell device structure.

In contrast, Figure 1(b) illustrates how a similar heterostructure can be used as a photovoltaic device. There, the exciton is photo-generated in the hole-transporting TFB and dissociates upon meeting the energy barrier at the TFB-F8BT interface to allow carrier extraction. Of course, this structure does not require the PEDOT:PSS and Ca layers previously used to facilitate carrier injection.

Due to the very low mobilities in conjugated polymers compared with conventional semiconductors, it is clear that the bulk of these devices' optoelectronic properties stem from the interface between the hole- and electron-transporting polymers. In the case of polymer-based LED structures, the exciton will usually recombine within tens of nanometers from the ETL-HTL interface. Meanwhile, any exciton generated more than tens of nanometers from the ETL-HTL interface in photovoltaic device structures will recombine radiatively before reaching the surface and those carriers will be lost. To enable an easy processing, these all polyfluorene-based type-II heterostructures are usually formed using a **blended** precursor solution containing both polymers dissolved in a given solvent. When this blend is deposited on the ITO substrate by spin- or dip-coating, phaseseparation occurs as the solvent evaporates. This leads to the formation of HTL-rich and ETL-rich domains such as shown in Figure 2 (Moons, 2002).

#### **2.2 The importance of the domain sizes and the crystalline phase in polyfluorenebased thin-film structures**

Based on the previous discussion, we now understand that the optoelectronic properties of those conjugated polymer-based heterostructures will depend largely on the interface between the hole-transporting and electron-transporting polymers. As such, an intuitive

Hybrid Polyfluorene-Based Optoelectronic Devices 181

coating a blended 1:1 toluene-based precursor atop a thin PEDOT:PSS hole-injection layer previously spin-coated directly on a commercial ITO substrate. After annealing at 160ºC, the

Fig. 4. Blended TFB-F8BT polyfluorene-based light-emitting diode. (a) Typical

photoluminescence and electroluminescence from the TFB:F8BT blend at 19 V. The inset shows the typical domain structure obtained by phase separation between the hole-

**3. Migrating the emission of polyfluorene-based LEDs towards the near** 

**infrared using lead-sulfosalt (PbS) colloidal quantum dots**

transporting TFB (pink) and the electron-transporting F8BT (green). (b) The current-voltage characteristics for this 520 nm LED structure. The inset shows a typical small-area device

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

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).

top electrode was evaporated through a shadow mask.

and the arrow points to the active device area.

et al., 2003; X. Zhang et al., 2007).

Fig. 2. Example of a large domain structure in a polyfluorene-based type-II heterostructures using xylene-based precursor measured using AFM. Adapted from (Moons, 2002) with permission.

way to controllably alter the optoelectronic properties of those structures would be to control the domain size. One relatively simple way of doing this is by changing the solvent used in the precursor solutions. Since the domains form by phase-separation, faster evaporations rates (using solvents such as chloroform, acetone or hexane) will generally yield significantly smaller domains compared with lower evaporation rates (using solvents such as toluene or xylene). As an example, Figure 3 shows a comparison of the photoluminescence from films of pure TFB, pure F8BT and 1:1 blends of TFB:F8BT obtained both from toluene- and chloroform-based precursors.

Fig. 3. Blended TFB-F8BT polyfluorene-based type-II heterostructures using different solvents. (a) Fluorescence emission from pure TFB, pure F8BT, and 1:1 ratio TFB:F8BT blended films obtained from toluene-based precursor solutions. (b) Same measurements obtained from chloroform-based precursors. The insets shows confocal fluorescence images of the domain structures in blended films. The insets are Adapted from (Moons, 2002) with permission.

In the toluene-based blend, the quenching of the F8BT emission by the TFB is much less pronounced compared with the chloroform-based blend. Indeed, the larger domains in the toluene-based blend provide more leasure for the photo-generated excitons to recombine radiatively before reaching another domain interface. In the chloroform-based blend, the probability for radiative-recombination is much lower since the photo-generated excitons are more likely to hit another domain interface and dissociate before they can recombine radiatively. This results precisely in the severely quenched F8BT fluorescence seen in Figure 3(b). Based on this result, it is obvious that larger domains will generally be preferable for light-emitting diode structures while smaller domains will be more desirable for photo-detector or photovoltaic device architectures.

For example, Figure 4 shows the optimal visible light-emitting diode architecture we obtain using a blended TFB:F8BT type-II heterostructure. Here, the structure is formed by spin-

Fig. 2. Example of a large domain structure in a polyfluorene-based type-II heterostructures using xylene-based precursor measured using AFM. Adapted from (Moons, 2002) with

way to controllably alter the optoelectronic properties of those structures would be to control the domain size. One relatively simple way of doing this is by changing the solvent used in the precursor solutions. Since the domains form by phase-separation, faster evaporations rates (using solvents such as chloroform, acetone or hexane) will generally yield significantly smaller domains compared with lower evaporation rates (using solvents such as toluene or xylene). As an example, Figure 3 shows a comparison of the photoluminescence from films of pure TFB, pure F8BT and 1:1 blends of TFB:F8BT obtained

Fig. 3. Blended TFB-F8BT polyfluorene-based type-II heterostructures using different solvents. (a) Fluorescence emission from pure TFB, pure F8BT, and 1:1 ratio TFB:F8BT blended films obtained from toluene-based precursor solutions. (b) Same measurements obtained from chloroform-based precursors. The insets shows confocal fluorescence images of the domain structures in blended films. The insets are Adapted from (Moons, 2002) with permission.

In the toluene-based blend, the quenching of the F8BT emission by the TFB is much less pronounced compared with the chloroform-based blend. Indeed, the larger domains in the toluene-based blend provide more leasure for the photo-generated excitons to recombine radiatively before reaching another domain interface. In the chloroform-based blend, the probability for radiative-recombination is much lower since the photo-generated excitons are more likely to hit another domain interface and dissociate before they can recombine radiatively. This results precisely in the severely quenched F8BT fluorescence seen in Figure 3(b). Based on this result, it is obvious that larger domains will generally be preferable for light-emitting diode structures while smaller domains will be more desirable

For example, Figure 4 shows the optimal visible light-emitting diode architecture we obtain using a blended TFB:F8BT type-II heterostructure. Here, the structure is formed by spin-

both from toluene- and chloroform-based precursors.

for photo-detector or photovoltaic device architectures.

permission.

coating a blended 1:1 toluene-based precursor atop a thin PEDOT:PSS hole-injection layer previously spin-coated directly on a commercial ITO substrate. After annealing at 160ºC, the top electrode was evaporated through a shadow mask.

Fig. 4. Blended TFB-F8BT polyfluorene-based light-emitting diode. (a) Typical photoluminescence and electroluminescence from the TFB:F8BT blend at 19 V. The inset shows the typical domain structure obtained by phase separation between the holetransporting TFB (pink) and the electron-transporting F8BT (green). (b) The current-voltage characteristics for this 520 nm LED structure. The inset shows a typical small-area device and the arrow points to the active device area.
