Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet-Visible Absorption Spectroscopy

Fan Xie; Jin Fang; Lili Zhang; Dan Deng; Yiyao Chen; Zhixiang Wei; Fengqi Guo; Chang-Qi Ma

doi:10.1021/acsami.3c18454

Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet-Visible Absorption Spectroscopy

ACS Appl Mater Interfaces. 2024 Mar 6;16(9):11767-11777. doi: 10.1021/acsami.3c18454. Epub 2024 Feb 26.

Authors

Fan Xie^{1

2}, Jin Fang², Lili Zhang³, Dan Deng³, Yiyao Chen⁴, Zhixiang Wei³, Fengqi Guo¹, Chang-Qi Ma²

Affiliations

¹ Henan Institutes of Advanced Technology, Zhengzhou University, Zhengzhou 450052, China.
² i-Lab & Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, China.
³ CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China.
⁴ Vacuum Interconnected Nanotech Workstation (Nano-X), Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, China.

PMID: 38408283
DOI: 10.1021/acsami.3c18454

Abstract

In addition to the donor-acceptor nano phases, the intermixed phase within the organic blends is crucial for the photovoltaic performance and stability of the bulk-heterojunction organic solar cells (OSCs). Here, the intermixed phase of a representative M-PhS:BTP-eC9 all-small-molecule organic solar cell was investigated by a concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy method, where a shift of the absorption maximum wavelength was measured for the acceptor component with the increase of the acceptor concentration. The blend ratios of the acceptor to the donor in the intermixed phase, corresponding to the critical concentration for the formation of the acceptor nanophase (C_AP), were determined to be 0.35, 0.20, and 0.15 for the as-cast, thermal annealing (TA), and the combined TA and solvent vapor annealing films. These results indicated that M-PhS and BTP-eC9 are kinetically well intermixed during spin coating, whereas TA and the following solvent annealing promote the crystallization of BTP-eC9 molecules out of the intermixed phase. The photovoltaic performance of the M-PhS:BTP-eC9 cells with different blend ratios was investigated. The formation of the BTP-eC9 nano phase in the blend film leads to stable V_OC and fast increased J_SC, which can be understood by the reduction of bimolecular charge recombination and the formation of electron transporting pathways within the photoactive layer. Similarly, the critical concentration for the formation of the donor phase was estimated to be 0.15 by measuring the stabilized V_OC and increased J_SC values of the cells with different donor blending ratios. More importantly, after a fast "burn-in" thermal degradation, the M-PhS:BTP-eC9 cell showed excellent thermal stability aging at 85 °C for over 1128 h, which is in good accordance with the unchanged intermixed phases measured by the UV-vis spectra of the annealed films. The current work demonstrates the feasibility of the spectroscopy method to investigate the intermixed phases for organic bulk-heterojunction solar cells and proves that all-small-molecule solar cells can be intrinsically very stable.

Keywords: UV–vis absorption spectroscopy; intermixed phase; organic solar cells; phase separation; thermal stability.