Formation of Long-Range Ordered Organic Thin Films and their Impact on Solar Cell Energy Loss

Abstract

With a growing global population and increasing demand for modern technologies from de- veloping countries, energy production is set to be one of the most important issues in the twenty-first century; one that must be met in an environmentally responsible way for the sake of present and future generations. Toward this end, renewable energy has come to the forefront of the discussion. In particular, solar energy, due to the vast energy potential that the sun provides our planet every day, is the most promising candidate. Though solar pho- tovoltaic (PV) technology has been around for decades, it is usually asserted that adoption will grow when the price of obtaining a PV system comes down. Increasingly, however, the cost of a PV system installation is dominated by costs other than the modules themselves (i.e., installation, permitting costs, etc.). To address this issue, researchers have sought to replace the bulky, heavy, and expensive-to-install PV modules based on silicon today with lightweight, flexible PVs based on abundant and inexpensive organic materials (i.e., OPVs).

While these are extraordinary promises, OPVs today have not yet been able to achieve the stable lifetimes and power conversion efficiencies necessary to make them commercially viable, primarily since most organic thin films today are disordered. While the practical aspi- rations of OPVs dictate that thin films be utilized, organic bulk single crystals have revealed remarkable optical and electrical properties that show dramatically improved performance compared to their disordered counterparts.

The primary thrust of this thesis is to unite the enhanced performance of crystalline organic materials with the practicality of thin films. We introduce a facile fabrication tech- nique to turn as-deposited amorphous rubrene (an archetypal small molecule) thin films into ones that are highly crystalline with macroscopic grain sizes. We then study the practical benefit of thickness tunability these crystalline films possess via an investigation of rubrene homoepitaxy, as well as heteroepitaxy with the common fullerene acceptor C60. Finally, we show how an OPV based on the highly crystalline rubrene/C60 heterojunction reveals high-performance physics akin to inorganic solar cells and discuss how to unlock these performance enhancements in other materials systems.