**1. Introduction**

176 Advanced Photonic Sciences

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> It is now well established that controlled delocalization in π-conjugated chains can lead to unique optoelectronic properties in polymer materials (MacDiarmid, 2001). While still being in a relatively early developmental stage, conjugated-polymer systems with a myriad of optoelectronic properties can now be synthesized at relatively low costs. Albeit a very promising technology, there remains some key challenges to address before efficiently integrating these conjugated-polymer systems into large scale applications including displays, biomedical imaging & sensing, lab-on-a-chip, solid-state lighting and photovoltaic devices and architectures (Arias et al., 2001; Moons, 2002; Morteani et al., 2003).

> Pending material issues still limit the functionality and the overall performances of these emerging material systems, while photo-chemical degradation can severely restrict their lifetimes. Since the main photo-chemical degradation process is usually a photo-oxidation reaction that truncates the conjugation length of the polymer chain to reduce the π-electron delocalization, this undesireable process can be significantly suppressed by simply permeating the structure with a transparent dielectric coating. Using such coating technologies, lifetimes of 20 years have now been demonstrated for commercial polymerbased photovoltaic devices and displays.

> In the last few years, the hybrid integration of semiconductor nanocrystals within conjugated polymer host systems has grown into a very active research area as it provides a new pathway of (1) improving the conjugated polymers' optoelectronic properties and/or (2) providing added functionality to the conjugated polymer-based structures.

> This chapter will present a general overview of hybrid polymer-nanocrystal material systems and their application as low-cost optoelectronic devices. Using a device-engineering perspective, we will focus our attention on the synthesis & processing, structural and optoelectronic properties of polyfluorene-based systems interfaced with lead-sulfosalt (PbS) semiconductor nanocrystals grown by hot-colloidal methods. Using this specific hybrid material system as our case study, we will begin by providing a general understanding of pure polymer-based type-II heterostructures and their limitations. Then, we will demonstrate how the incorporation of lead-chalcogenide quantum dots can be used to (1) add new functionality and (2) improve the performances of those all polyfluorene-based optoelectronic devices. Most importantly, we will also see that the hybrid integration of

Hybrid Polyfluorene-Based Optoelectronic Devices 179

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

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

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

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

**2.2 The importance of the domain sizes and the crystalline phase in polyfluorene-**

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

generates the emission.

or solar cell device structure.

**based thin-film structures** 

previously used to facilitate carrier injection.

ETL-rich domains such as shown in Figure 2 (Moons, 2002).

conjugated polymers and colloidal quantum dots also raises many important fundamental questions and crucial technical challenges to address before achieving low-cost hybrid optoelectronic devices with superior performances.

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 highperformance optoelectronics platforms and photovoltaics.
