High Efficiency Single and Multijunction Organic Photovoltaics

Abstract

Organic photovoltaics (OPVs) offer a lightweight and potentially cost-effective approach for solar energy harvesting. The first OPV heterojunction device was reported in 1985 with 1% efficiency. There has been a rapid development of small molecule and polymer materials, as well as different growth techniques such as vapor-deposition and solution-processing over the past 30 years. With recent emergence of the non-fullerene acceptors (NFAs), the efficiency has been improved to 15%; a benchmark for OPV commercialization. The flexible and semitransparent form factors of OPVs also lead to unique applications such as flexible electronics and building integrated photovoltaics (BIPV), showing considerable market potential.

This dissertation will focus on high efficiency single and multijunction OPVs. Following the background introduction of organic materials and solar cells, the simulation methods based on transfer matrix is discussed. The rest of the work can be divided into two parts. The first part studies a group of dipolar donor molecules with donor-acceptor-acceptor' structures, either with the propeller or coplanar donor unit. The molecular conjugation length and side chain configuration are adjusted to better understand the structure-property-performance relationships. In the second part, two multijunction structures will be discussed, both of which employ subcells with the d-a-a' donors. The first structure focuses on the fully vacuum-deposited tandem and triple junction cells with efficiencies of 10-11%, delivering several important multijunction design principles. It is followed by discussion of a tandem cell combining the vacuum-deposited and solution-processed NFA-based subcells, achieving a record 15% efficiency with close to 100% fabrication yield. A variety of characterization methods including crystallography, photoluminescence, external quantum efficiency and current density-voltage measurements, optical simulation etc. will be presented along with these results. With improved donor and NFA materials along with their inherent structural design flexibility, further improvement of OPV performance is expected to be achieved in the near future with plenty of market potential.