Nanoparticles

The charge separation and transport in the conjugated polymer/semiconductor nanocrystals composites were first studied by Greenham et al. in 1996 [8]. A HSC with ITO/MEH:PPV-CdSe/Al structure was fabricated, after incorporating 90 wt% of CdSe into the polymer matrix the device gave a 0.6 % PCE and a 12 % quantum efficiency at 514 nm, while the PCE could only approached 0.1 % under 80 mW/cm2 white light. After the breakthrough in control the shape of CdSe nanocrystals, CdSe quantum dots (QDs), nanorods, and tetrapods have been extensively studied [49–51]. Compared to the earlier CdSe QDs based HSCs, the PCEs have been improved by an order of magnitude, through the modification of CdSe QDs and the use of new polymers [52-54], however, the elongated CdSe nanocrystals were found to have better charge transport properties than nanopar­ticles, consequently drew more attention [9, 10, 12, 48].

Triggered by the research on CdSe nanocrystals, charge transport properties in TiO2/polymer heterojunction were also studied and photoinduced charger transfer from conjugated polymers to TiO2 was confirmed to be possible [55-57]. Mesoporous TiO2 film with a uniform pore distribution was prepared with the block polymer template (Fig. 9.5) and used for preparing HSCs by infiltrating P3HT into the pores, a monochromatic PCE of 1.5 % was obtained at 33 mW/cm2 514 nm illumination and an efficiency of 0.45 % was estimated under AM 1.5 condition [16,17]. Besides, many

other efforts have also been done to prepare HSCs based on porous TiO2 film [18, 58-60]; however, the PCEs are typically lower than 0.5 % except for the cases when interface modifications are applied [61, 62]. However, it is difficult to optimize the polymer/TiO2 interface and the complete infiltration of polymers into TiO2 net­work is also a challenge [63]. These problems could be overcome by simultaneous deposition of the blend of polymer and TiO2 particles and a 0.42 % PCE has been achieved under AM 1 irradiation by Kwong et al. [20, 21], but there are also some disadvantages that could limit further improvement of the device performance, namely, for the different nature of the two distinct components which cannot be simply dissolved in the same solvent. Direct blending will cause unavoidable aggregation of TiO2 particles resulting in poor organic-inorganic interface, and therefore low effi­ciency [34]. Capping TiO2 nanocrystals with amphiphilic molecules, e. g., oleic acid or trioctylphosphine oxide could improve their solubility in organic solvents, but the long alkyl chain of these molecules might hinder the charge transfer process, sometimes ligand exchange is needed for better charge transfer, all of this will complicate the fabrication of such devices and it is still difficult to control the morphology and dispersion of TiO2 particles in polymer matrix [64, 65].

The aforementioned problem could be avoided by using Ti precursor instead of TiO2 particles; unfortunately, a new problem arose, i. e., the low crystallinity of TiO2 due to the absence of high temperature sintering, which could also lead to poor device performance [19].

All these drawbacks might be overcome by using ZnO nanocrystals, since soluble ZnO nanoparticles could be synthesized without using any surfactant. Uniform ZnO/conjugated polymer composite films have been easily prepared [34] and ZnO-MDMO:PPV-based devices gave PCEs of about 1.6 % [23, 25, 66] while the ZnO:P3HT gave 0.9 % [26] under AM 1.5, 1 sun condition. Both devices show much higher efficiencies than that of TiO2: polymer-based devices owing to the improved polymer/nanocrystal interface. Besides, crystallized ZnO nanoparticles could be prepared under low temperature by using a highly reactive ZnO precursor

(see Fig. 9.6); namely, Diethyl Zinc which has been used to fabricate HSCs with MDMO-PPV [24] and P3HT [27] and 1.1 % and 1.4 % conversion efficiencies had been achieved, respectively. Great breakthrough has been made in 2009, Oosterhout et al. [30] were able to study the 3D morphology of the device prepared from Diethyl Zinc and P3HT via electron tomography. The authors provided a detailed insight into the role of 3D morphology in charge generation and trans­porting. The hybrid device showed a PCE up to 2 % when the nanoscale morphology of the active layer was optimized, Fig. 9.7 shows the structure and J-V characteristics of the device.