Tandem Organic Photovoltaics.

Abstract

The unique properties of organic semiconductors have led to significant scientific and commercial interest in organic electronics over the last ten years. During that time, these devices have gone from being a laboratory curiosity to being in hundreds of millions of pockets around the world. Going forward, there are opportunities for organic photovoltaics (OPVs) to provide carbon-neutral energy production due to the potential for flexible, low-cost, and large-scale production. In the first part of this thesis, we demonstrate techniques for depositing and controlling the morphologies of organic thin films. Organic vapor phase deposition (OVPD) is utilized to demonstrate a method to deposit organic thin films efficiently over large areas. An inverted architecture for OPVs is presented, which presents the possibility of depositing devices directly onto low-cost metal foils. We also explore the mechanisms and effects of structural templating in OPVs, where the molecular orientation of the active materials is controlled. This results in an improvement in power conversion efficiency of over 50% compared to untemplated devices. Additionally, we introduce new buffer layers in OPVs which lead to significant improvements in the device fill factor, resulting in an increase in power conversion efficiency of more than 25%. In the second part of this thesis, we present developments in tandem OPVs for high-efficiency photovoltaics. By incorporating multiple sub-cells into a multi-junction OPVs architecture, the losses inherent in all OPV devices can be reduced significantly. Two of the works presented incorporate one solution-processed and one vacuum-processed sub-cell, resulting in efficiencies as high as 8.3 +/-0.3% power conversion efficiency. We have also developed new techniques to utilize two solution-processed sub-cells into a monolithic tandem architecture, leading to a > 10% increase in power conversion efficiency compared to an optimized single-cell device.