PhD Defense

Congratulations to Maged Abdelsamie on his successful PhD Defense!

11/16/2016

​Defense Title: Solution-Processing of Organic Solar Cells: From In Situ Investigation to Scalable Manufacturing

ABSTRACT: Solution processable organic photovoltaics (OPVs) have attracted attention in the last decade because of the promise of low-cost manufacturing of sufficiently efficient devices at high throughput on large-area rigid or flexible substrates with potentially low energy and carbon footprints. In OPVs, the photoactive layer is made of a bulk heterojunction (BHJ) layer and is typically composed of a blend of an electron-donating (D) and an electron-accepting (A) materials which  phase separate at the nanoscale and form a heterojunction at the D-A interface that plays a crucial role in the generation of charges. Despite the tremendous progress that has been made in increasing the efficiency of organic photovoltaics over the last few years, there have been numerous debates on the mechanisms of formation of the crucial BHJ layer and few clues about how to successfully transfer these lessons to scalable processes where fluid dynamics and process kinetics can be very different. We took the view that understanding the formation of the solution-processed bulk heterojunction (BHJ) photoactive layer is one of the most crucial steps to developing strategies towards the implementation of organic solar cells with higher efficiency and scalable manufacturability. In this thesis, we have investigated the mechanism of the BHJ layer formation during solution processing from common lab-based processes, such as spin-coating, with the aim of understanding the roles of materials, formulations and processing conditions, and subsequently used this insight to enable the scalable manufacturing of organic solar cells by wire-bar and blade-coating methods while achieving performance parity with spin coating. To do so, we have developed in situ and in-line diagnostics techniques to provide us with insight into the ink drying and thin film formation processes. As a first step, we have developed a modified spin-coater, which allows us to perform in situ UV-visible absorption measurements during spin coating. This has the ability to provide key insights into the formation and evolution of polymer aggregates in solution and during the transformation to the solid state. We have investigated the roles of aggregation and processing kinetics on the formation of organic BHJs made of a blend of poly(3-hexylthiophene) (P3HT) and a soluble fullerene. We have then moved on with investigating the role of formulation and crystallization dynamics on the solidification pathway of small-molecule donors. Our investigations have revealed the reasons behind the need for the empirically-adopted solvent additives, or alternatively thermal or solvent vapor annealing, to achieve good performance in solar cells.  The study has also provided a new perspective on material design showing a way to avoid using solvent additives by designing materials with a greater ease of crystallization. Our work has also touched upon modern polymers, such as PBDTTPD, where we have found the choice of additives impacts the formation mechanism of the BHJ. Finally, we have performed a comparative study of the BHJ film formation dynamics during spin coating versus wire-bar coating of p-DTS(FBTTh2)2: fullerene blends that has helped in curbing the performance gap between lab-based and scalable techniques. This was done by implementing a new apparatus that combines the benefits of rapid thin film drying common to spin coating with the scalability of the wire-bar coating. Using the new apparatus, we successfully attain similar performance of solar cell devices to the ones fabricated by spin coating with dramatically reduced material waste