Mapping the Side-Chain Length of Small-Molecule Acceptors towards the Optimal Hierarchical Morphology in Ternary Organic Solar Cells

Zichun Zhou; Shengjie Xu; Xiaozhang Zhu

doi:10.1055/a-1472-7302

Organic Materials, Table of Contents

CC BY-NC-ND 4.0 · Organic Materials 2021; 03(02): 191-197
DOI: 10.1055/a-1472-7302

Focus Issue: Peter Bäuerle 65th Birthday

Short Communication

Mapping the Side-Chain Length of Small-Molecule Acceptors towards the Optimal Hierarchical Morphology in Ternary Organic Solar Cells

Zichun Zhou

^aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

^bSchool of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China

,

Shengjie Xu

^aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

,

Xiaozhang Zhu

^aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

^bSchool of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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Abstract

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Abstract

Using multiple light-absorbing materials to realize a broader and better absorption spectrum in multi-component organic photovoltaics has achieved significant success to obtain high power conversion efficiency. Meanwhile, the good materials combinations with matched electronic structure and proper blend morphology for charge generation and transport are of primary importance for implementation of the multi-component strategy. Hierarchical morphology has been clearly demonstrated to improve all performance parameters in ternary organic photovoltaics but shows strong dependence on the molecular structures. Here we develop four small-molecule electron acceptors with different alkyl chain lengths to find the optimal solution of alkyl chain towards the defined hierarchical morphology and carry out a clear and comprehensive investigation of the alkyl chain length effects on the structure–morphology–device performance relationships in ternary blends. There is a positive correlation between the power conversion efficiencies of the four ternary systems and their short-circuit current density parameters, manifesting the significance of distinguishing optimal alkyl side chain length of small-molecule electron acceptors for defined hierarchical morphology to afford efficient carrier generation. The non-optimal side chains would retard the BTR crystallization and make the PC₇₁BM domain sizes incontrollable, leading to a morphology without a defined hierarchy. Such a detailed mapping of the alkyl side chain length of small-molecule electron acceptors provides new insight into the materials combinations for the next-step high-performance multi-component organic photovoltaics.

Key words

ternary organic solar cells - hierarchical morphology - side-chain modulation - small-molecule acceptors

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References

References and Notes
Fabrication of organic solar cells: The devices were developed with a conventional structure of ITO/PEDOT:PSS/active layer/PNDIT-F3N/Al. The ITO-coated glass substrates were cleaned with sequential ultrasonication in a soap–deionized water mixture, deionized water, acetone and isopropanol. The washed substrates were further treated with oxygen plasma for 10 min to eliminate any remaining organic components. A thin layer (approximately 30 nm) of PEDOT:PSS (Clevios P VP 4083) was first spin-coated on the ITO substrates at 3,000 r.p.m. and baked at 150 °C for 5 min under ambient conditions. The substrates were then transferred into a nitrogen-filled glove box. Subsequently, the active layer was spin-coated on the PEDOT:PSS layer via spin-coating from a chloroform solution of BTR, NITI and PC₇₁ BM at various spin rates. The best PCE was achieved via spin coating 25 mg BTR, 10 mg NITI and 25 mg PC₇₁BM (1:0.4:1) in 1 ml chloroform at 1,500 r.p.m. The resultant film thickness was approximately 300 nm and obtained via a surface profilometer (Dektak XT, Bruker). Here, SVA with THF was used to optimize the blend morphology and promote device performance. SVA was conducted in a 60-mm glass Petri dish containing 200 µL THF at various durations in a glove box. The optimal duration in this study was 60 s. Then, PNDIT-F3N (as the electron transport layer) was spin-coated on the active layer at 3,000 r.p.m. from the alcohol solution. In the final stage, aluminium (100 nm) was thermally evaporated onto the active layer as the top electrode. Shadow masks were used to define the device area (0.03262 cm²) of the devices.
Device characterization: The current density–voltage (J–V) characteristics of unencapsulated photovoltaic devices were measured under N₂ using a Keithley 2400 source meter. A 300 W xenon arc solar simulator (Oriel) calibrated by a standard Si diode was used to simulate the AM 1.5 G solar irradiation. The EQE was performed using certified IPCE equipment.
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