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Evaporation induced self-assembly and characterization of nanoparticulate films: a new route to bulk heterojunctions

Abstract
Polymer-based semiconducting materials are promising candidates for large-scale, low-cost photovoltaic devices. To date, the efficiency of these devices has been low in part because of the challenge of optimizing molecular packing while also obtaining a bicontinuous structure with a characteristic length comparable to the exciton diffusion length of 10 to 20 nm. In this dissertation we developed an innovative evaporation-induced nanoparticle self-assembly technique, which could be an effective approach to fabricate uniform, densely packed, smooth thin films with cm-scale area from home-made P3HT nanoparticles. Unlike the previous reports of nanoparticle-based film formation, we use a mixture of two solvents so that the solvent quality slowly decreases over time and particles aggregate at the air-liquid interface. The charge mobility of P3HT nanoparticle film fabricated using this technique is very similar to that of drop-cast P3HT films. Sintering (i.e. formation of contact discs between particles) and the effect of different size distribution of nanoparticle dispersion on film charge mobility were also studied. Binary films composed of P3HT and PCBM can be obtained using the same method from a suspension containing both P3HT and PCBM nanoparticles. The existence of PCBM in binary films was confirmed, and the relative composition of the two types of particles as a function of film-formation time was measured. Moreover, we show how to control the internal, molecular-scale structure within the P3HT nanoparticles and the absorption spectra by slowing the process of particle formation. These results provide an example of manipulating one phase of the active layer of OPV devices independently of the other phase.
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