Cascade Organic Photovoltaics.

Abstract

Within the last decade, organic photovoltaics have emerged as a potentially viable option for carbon-neutral energy production due to their low cost, flexibility, and compatibility with roll-to-roll processing. However, while the maximum theoretical efficiency of OPVs is only slightly below that of their inorganic counterparts, demonstrated OPV efficiencies have still only reached ~11%. In this thesis, we present our work on the relatively new class of cascade organic photovoltaics and through that work we discover some critical factors that must be resolved to enable significant further gains in OPV efficiencies. In Chapters 1 and 2, we focus on the tradeoff between absorption and exciton diffusion efficiencies in organic heterojunction solar cells. Working with planar devices, we employ rigorous modeling and experiments to demonstrate the physical mechanisms by which energy can be lost in OPVs and detail the ways in which the absorption/diffusion tradeoff has been previously addressed. We show that MoO3, a common anode buffer layer, quenches excitons. We propose a new type of buffer layer to prevent quenching at the anode, which we term an exciton dissociation layer (EDL). By inserting an EDL into a single heterojunction (SHJ) device, an additional heterojunction (subjunction) is created, converting the device into a cascade heterojunction (CHJ) structure. We establish that the multiple heterojunctions in CHJs are operating electrically in parallel and develop a model that can predict their external quantum efficiency. In the Chapter 3, we develop practical design rules for CHJ devices, requiring that charge injection barriers be minimized and the maximum power point voltage of each subjunction be closely matched. Applying these design rules, we demonstrate a 40% improvement in efficiency. In Chapter 4, we develop a new model for interlayer Förster resonant energy transfer (FRET) in OPVs and show that the FRET process can actually significantly hinder device efficiencies. With this new model, we propose specific material and device design rules to achieve major efficiency enhancements via FRET. Using these design rules, we are able to demonstrate a 93% improvement in efficiency for a CHJ with 4 absorbers (up to 7.3% demonstrated here) compared to an optimized SHJ device.