Investigating The Impact Of Small Molecule Aggregation On Polaron Generation In Polymer: Y6 Organic Bulk Heterojunctions Using Optical Spectroscopy

Open Access
- Author:
- Yuan, Yue
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 16, 2024
- Committee Members:
- Seong Kim, Outside Unit & Field Member
Kenneth Knappenberger, Major Field Member
Tae-Hee Lee, Major Field Member
John Asbury, Chair & Dissertation Advisor
Philip Bevilacqua, Program Head/Chair - Keywords:
- Organic Photovoltaics
Optical spectroscopy
Time-resolved spectroscopy
Charge generation
Charge separation
Polaron - Abstract:
- Organic solar cells (OSCs) based on polymer donor and non-fullerene small molecule acceptor bulk heterojunctions have emerged as the leading candidates to replace the traditional inorganic solar cells for their flexibility, tailorability, and potential for low-cost manufacturing. Among the high performance OSCs, PM6:Y6 has gained a lot of attention in recent years because of its high power conversion efficiency (PCE) of over 15%. This extraordinary performance mainly comes from: 1, The complementary absorption of the donor and acceptor molecules. 2, The barrierless charge separation at the donor acceptor interfaces. Despite the rapid progress made in the organic photovoltaics (OPVs) field, a comprehensive understanding of how the materials morphology affects its optoelectronics properties like charge separation efficiency remains incomplete. Such information is essential for the future design of OPV molecules. In this dissertation, time-resolved mid-infrared (TRIR) spectroscopy is used to directly probe the polaron generation efficiency in a series of PM6:Y6 thin films with different donor acceptor weight ratios (DARs). The DAR induced Y6 aggregation change is monitored by photoluminescence spectroscopy and grazing incidence wide angle X-ray scattering. The similar trend between the polaron generation efficiency variation and the morphology variation against the DAR reveals the relation between molecular aggregation and charge separation. This work provides first direct experimental evidence for the aggregation effect on reducing the Coulomb attraction between the hole and electron in the charge transfers states and promoting charge separation.