Ph.D Thesis


Ph.D StudentStas Obuchovsky
SubjectAtomic Layer Deposition of Metal Oxides Inside Organic
Semi-conductor Films
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professors Frey Gitti


Abstract

The heart of every polymer based organic solar cell (OSC) is the donor/acceptor bulk heterojunction (BHJ). The complex morphology of the BHJ, composed of both ordered and disordered domains, determines device performance and efficiency. In addition to challenges controlling the morphology, its characterization is also complicated, largely due to the transparency of the disordered domains to most analytical tools.

To overcome this challenge, we developed a new analytical tool to probe the complex blend morphology. Our method is based on exposing the thin organic films to sequences of ZnO atomic layer deposition (ALD). Applying ALD onto organic films, unlike conventional inorganic substrates, may lead to precursor infiltration into the sample and subsurface deposition. By varying the deposition conditions and altering the crystallinity of the nonreactive organic film, we investigated the mechanism of subsurface deposition

We find that, in contrast to polymer matrices that contain moieties which can act as lewis-bases and chemically react with ALD precursors that diffuse through the organic samples, in P3HT, a commonly used donor material in OSCs, the rate-determining step of the deposition process is the retention of the precursors inside the polymer films. The exposure of P3HT to diethyl zinc/water ALD sequencing results in the formation of nanocrystalline ZnO grains inside the amorphous domains of the organic films. Moreover, although there is more free volume for precursor diffusion in completely amorphous films, we show that the precursor retention is promoted by the presence of crystalline domains. By altering the deposition temperature we conclude that the precursors are retained inside P3HT due to physiorption. We demonstrate that the crystallinity-dependent subsurface deposition can be used as a tool to spatially direct the selective formation of hybrid materials on a nanoscale resolution.

Subsequently, we harness the subsurface ALD to study the morphology-evolution in a series of P3HT/fullerene blends commonly used for processing OSC BHJs. By following the amount and the spatial distribution of ZnO inside the BHJs, we investigate the mechanism of subsurface ALD into organic blends and map the disordered domains which are available for precursor diffusion. This mapping methodology allows us to compare the amount of intermixed domains inside BHJ films and characterize the rapid vitrification of P3HT/ICBA samples. In addition, we visually show for the first time, that during film processing fullerenes enrich the bottom interface of the BHJ, but thermal annealing homogenizes film composition.

When applied to P3HT/ICBA BHJs this methodology provided significant insights into the correlation between morphology evolution and device performance, linking between the amount of intermixed domains, electron mobility and device performance. Such insights are unattainable by conventional characterization tools. Finally, we show the feasibility of subsurface deposition as a pathway to fabricate hybrid ZnO/P3HT organic inorganic photovoltaic devices.