Final Defense: Device Physics and Applications of Ternary Organic Photovoltaics
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Organic photovoltaics (OPVs) are emerging as an attractive candidate for solar energy harvesting due to their advantages in environmental compatibility, low-cost, flexibility and transparency. The past few decades have witnessed substantial development in innovative molecular and device architecture designs to improve the performance of OPVs. The ternary bulk heterojunction (BHJ) devices stand out as one of the most widely employed strategies due to their broad absorption, efficient photocurrent generation and simple fabrication process. This thesis explores the working principles of ternary OPVs and demonstrates their application in high efficiency and scalable OPV devices.
The first part of the thesis focuses on the photogeneration process in ternary OPVs. We test the validity of the concept of “molecular alloy” by investigating the optoelectronic properties of a representative ternary system. Furthermore, we show that the end-capping exchange reaction between acceptor-donor-acceptor type of non-fullerene acceptors (NFAs) generate up to four unexpected molecular species in the ternary BHJs, leading to reduced reproducibility and reliability of OPV devices. The dipolar reaction products of the end-capping exchange are shown to reorient over time and impact the dielectric properties and device performance.
In the second part, we present the application of ternary OPVs in multi-junction devices and semitransparent modules. With the extended absorption in near-infrared (NIR) provided by a ternary sub-cell, we demonstrate high efficiency tandem OPV device. In addition, it is essential to develop a scalable fabrication process of modules to commercialize OPV technology. We study the resistance loss when scaling up semitransparent OPV (ST-OPV) devices and demonstrate ultrafine metal grid structure in a prototype module to realize negligible efficiency loss from series resistance. Moreover, we introduce a high resolution, non-destructive multilevel peel-off patterning method in the fabrication of ST-OPV modules. Combined with a NIR-absorbing ternary BHJ, the ST-OPV mini-module simultaneously achieves high light utilization efficiency and geometric fill factor. The demonstrations in this dissertation provide insight to further understand the mechanisms, and then advance the application of ternary OPVs.