Abstract

Organic photovoltaics (OPVs) are intended to be low-cost, long-lived devices with an array of possible applications. While record device power conversion efficiencies continue to improve, developments have largely been achieved with increasingly synthetically complex, high-cost molecules. Meanwhile, a deeper understanding of underlying molecular excited states and active layer structures influencing long-term performance is often neglected. To improve understanding of what drives efficiency in OPVs, Squaraine, a synthetically simple, low-cost organic molecule with a diverse array of excited state species and blending properties, is considered here. Focus is placed on the Squaraine charge transfer H aggregate (HCT) and how active layer morphology influences the remarkable energy transfer properties of this species. First, by quantifying the contribution to OPV efficiency of a diverse array of Squaraine excited state species, a clear dependence is observed on the extent of mixing between Squaraine and PCBM. Some mixing is shown to be beneficial for molecular devices, shortening the distance for exciton diffusion. However, improved crystallinity within Squaraine domains is observed to overcome any stifling of exciton diffusion associated with concomitant phase separation. Thus, since crystal packing could improve device efficiency, questions are raised about the efficiency contributions of HCTs which form in pure, crystalline Squaraine domains. To address the concerning phase separation, incorporation of two different Squaraine donors in a two donor, one acceptor ternary blend is identified as a pathway for improved control over aggregation and phase separation within OPV active layers. Limited energy transfer across diverse excited state populations and broad distribution of states is observed to have a dramatic negative effect on device efficiency. Yet, isolated HCT formation, or crystallization, is identified as not inherently bad for device performance when extensive phase separation is restricted. The typical negative impact of HCT formation on device efficiency in organic photovoltaics is determined to be primarily due to larger domain sizes and diverse excited state populations. Finally, with sub picosecond transient absorption measurements we directly observe energy transfer kinetics of HCTs and identify morphological features that drive their contribution to device efficiency. Sub-picosecond energy transfer from HCTs to monomers is facilitated by the Dexter energy transfer mechanism. Furthermore, formation of a Squaraine:PCBM charge transfer complex is rapid following HCT excitation and energy transfer to monomers, demonstrating the potential of HCTs for OPV efficiency. However, excessive domain enlargement is found to slow down exciton motion and hinder energy transfer, emphasizing the importance of small domains in HCT-based OPV devices. This dissertation thus highlights the importance of well-mixed materials and the diverse array of active layer morphologies that Squaraine derivatives may provide. Rapid energy transfer observed in small domains of Squaraine charge-transfer H-aggregates is a key advantage for improving OPVs and driving research towards technological improvement and commercialization.

Library of Congress Subject Headings

Organic photovoltaic cells; Charge transfer devices (Electronics); Energy transfer

Publication Date

6-2023

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Christopher J. Collison

Advisor/Committee Member

Santosh Kurinec

Advisor/Committee Member

Parsian Mohseni

Campus

RIT – Main Campus

Plan Codes

MCSE-PHD

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