Interfacial Assembly of Nanoparticles with Soft Templates for Photovoltaic Applications
Biswal, Sibani Lisa
Doctor of Philosophy
Controlling the assembly of nanoparticles into well-defined functional systems remains a challenge. The main difficulty is that the majority of bulk nanoparticle processing methods result in nanoparticle aggregation and phase separation. In this thesis, we describe a soft-templating strategy to direct the assembly of semiconducting cadmium selenide (CdSe) tetrapods (TPs) into a wire-like and network structure and their application in heterojunction photovoltaic devices. In particular, the templating occurs at the air/water interface and transferred to a substrate via the Langmuir-Blodgett (LB) technique. First we show how a small amphiphilic molecule, such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids, can serve as a soft template to control the arrangement of CdSe TPs at the air/water interface. CdSe TPs aggregate into uncontrolled disc-like structures when deposited onto the air/water interface; however, with the presence of POPC, wire-like structures are observed upon compression. Larger templating agents, such as the conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), was also shown to be able to template CdSe TPs. This conjugated polymer is of particular interest because of its relatively stability at the air/water interface and its application in organic solar cells. We also discovered a mixed solvent technique, which can manipulate the morphology of MEH-PPV formed at the air/water interface. When MEH-PPV was deposited using a mixed solvent combination of chloroform and chlorobenzene, ring-like structures formed by MEH-PPV were observed at the air/water interface. These ring structures showed very interesting optical properties such as red-shifted light absorption compared to pure solvent cases, tunable band gaps and birefringence. This unique assembly behavior is due to the difference of spreading coefficients and evaporation rates between chloroform and chlorobenzene. The soft-templating method was used to build up heterojunction photovoltaics. When CdSe TPs and MEH-PPV are mixed together in chloroform and deposited on the air/water interface, large CdSe TP network structures were formed. This structured film can be transferred onto a solid substrate multiple times via LB technique to build up the photoactive layer of a solar cell. It was observed that the power conversion efficiencies (PCEs) of the devices generated via the LB technique outperformed the conventional spin-coating method, due to the percolating network presented in the photoactive layer. We also set the best reported PCE record from the LB-aided device under the combination of CdSe TPs and MEH-PPV in this thesis. With these studies, there are now a number of possibilities of utilizing the air/water interface with soft templates and the Langmuir-Blodgett technique, which can lead to solutions for real world applications.