When 3:00 PM - 5:00 PM May 11, 2015
Where 1005 H.H. Dow Building
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Cascade Organic Photovoltaics

Adam Barito
Thesis Defense

Max Shtein, advisor


Within the last decade, organic photovoltaics (OPVs) have emerged as a potentially viable part of the solution for carbon-neutral energy production due to their low cost, flexibility, and compatibility with large-scale, 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 ~12%. While the cost and energy required to fabricate OPVs is lower than inorganic PVs, practical efficiency is a primary driver of adoption in the marketplace and OPV efficiencies must approach 15-20% before having a chance to become commercially competitive. 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 the first part of the thesis, we focus on the tradeoff between photo-absorption and exciton diffusion efficiencies in organic heterojunction solar cells. Working with planar (as opposed to bulk) heterojunction device architectures, we employ rigorous modeling and experiments to demonstrate the physical mechanisms by which energy can be lost in OPVs (namely non-radiative recombination of excitons, either via bulk recombination or parasitic quenching at traps or conductive interfaces) and detail the ways in which this tradeoff has been previously addressed. We show that MoO3, a material frequently used in OPV cells as an anode buffer layer and work function modifier, quenches excitons. We propose a new type of anode 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 is created, converting the device into a multiple (cascade) heterojunction (CHJ) structure. We establish that the multiple heterojunctions (termed “subjunctions”) in CHJs are operating electrically in parallel and develop an optical and diffusion based model that can predict their external quantum efficiency. In the second part of the thesis, through a systematic combinatorial study, 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 PCE by introducing a thin transparent EDL into a SHJ device. In the third and final part of the thesis, we develop a new model for interlayer Förster resonant energy transfer (FRET) in OPVs and show that the FRET process (present in the vast majority of OPVs) can actually hinder device efficiencies under certain circumstances. With this new model, we propose specific material and device design rules that if employed, can prevent any efficiency losses due to FRET and instead achieve major efficiency enhancements. While the specific materials following those design rules do not yet exist, we are still able to optimize devices using the new model and pre-existing materials, demonstrating a >140% improvement in power conversion efficiency for a CHJ with 4 absorbers (up to 7.3% demonstrated here) compared to its optimized SHJ counterpart (3.0%).