671314 event 1701360293 1701360337 <![CDATA[Ph.D. Dissertation Defense - Yi-Chien Chang]]> Title:  Application of solution-based electrical doping to non-fullerene organic solar cells

Committee:

Dr. Bernard Kippelen, ECE, Chair, Advisor

Dr. Ajeet Rohatgi, ECE

Dr. Azadeh Ansari, ECE

Dr. Natalie Stingelin, MSE

Dr. Juan-Pablo Correa-Baena, MSE

]]> Solution-based electrical doping of conjugated polymer films using 12-molybdophosphoric acid hydrate (also known as phosphomolybdic acid or PMA) via post-process immersion was previously shown to lead to p-type electrical doping over a limited depth from the polymer film's surface. Such doping approach enables the fabrication of hole-collecting layer-free organic solar cells (also referred to as organic photovoltaics or OPVs), which greatly simplifies the device architecture and fabrication complexity of organic solar cells. Later on, it was found out that the available volume of polymer films indeed plays a critical role in allowing infiltration of PMA nanoclusters and p-type doping of certain state-of-the-art polymers. Accordingly, variations of the original PMA doping technique based on film volume expansion were demonstrated. However, the reported techniques are currently not compatible with the fabrication of organic solar cells due to degradation of the bulk heterojunction films during the doping step. Building on our previous findings, we first introduce a modified PMA doping technique that incorporates a combination of treatments (i.e., solvent vapor pre-treatment and solvent swelling) to vary the polymer film volume for effective electrical p-doping. At the same time, this new doping technique is applicable to the fabrication of single-junction organic solar cells with a photoactive layer comprising an emerging polymer donor and a non-fullerene acceptor. The fabricated PMA-doped non-fullerene OPV devices exhibit comparable photovoltaic performance to the reference devices with a commonly used and thermally evaporated MoO3 hole-collecting layer. Secondly, we present a novel charge recombination stack that can be applied to organic tandem solar cells. Here, PMA doping is utilized to embed the hole-collecting component of the charge recombination stack into the photoactive layer of the bottom sub-cell. In addition, reducing the solvent drying time of the solution-processed top photoactive layer was found to prevent possible intermixing between the top and bottom photoactive layers. Consequently, we demonstrate inverted organic tandem solar cells that feature the proposed charge recombination stack, which yield an open-circuit voltage that is close to the sum of open-circuit voltages of the individual sub-cells, and a fill factor that matches closely with the better fill factor of the two sub-cells.

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