Fine-Tuning Semiconducting Polymer Self-Aggregation and Crystallinity Enables Optimal Morphology and High-Performance...

Area of Science:

• Materials Science
• Organic Electronics
• Polymer Chemistry

Background:

• Polymer aggregation and crystallization are critical for all-polymer solar cells (all-PSCs) performance.
• Controlling polymer self-assembly for optimal active layer morphology in all-PSCs is challenging.
• Naphthalene diimide (NDI)-based acceptor polymers, like N2200, often exhibit strong self-aggregation.

Purpose of the Study:

• To develop a strategy for modulating the self-aggregation and crystallinity of NDI-based acceptor polymers.
• To investigate the impact of controlled aggregation on the morphology and performance of all-PSCs.
• To establish structure-property relationships for designing high-performance all-PSC materials.

Main Methods:

• Synthesis of random copolymers (PNDI-CBS) by replacing alkyl side-chains with compact bulky side-chains (CBS) in NDI polymers.
• Characterization of solution-phase aggregation and solid-state crystallinity using UV-vis absorption, PL spectroscopy, thermal analysis, and GIWAXS.
• Fabrication and photovoltaic performance testing of all-PSCs blending PNDI-CBS with a donor polymer (PBDB-T).
• Morphological analysis using PL quenching and R-SoXS.

Main Results:

• Increasing the molar fraction of CBS units progressively suppressed polymer aggregation and crystallinity.
• All-PSCs fabricated with more amorphous acceptor polymers achieved significantly increased power conversion efficiencies (PCE) up to 8.5%.
• Reduced acceptor aggregation led to smaller phase-separation domains and higher short-circuit current density (Jsc).
• Lower crystallinity made active layers less sensitive to film deposition methods, enabling easier processing.
• Excessive reduction in acceptor aggregation increased donor phase-separation domain sizes and donor polymer crystallites, reducing performance.

Conclusions:

• Systematic modulation of acceptor polymer aggregation via side-chain engineering is an effective strategy for enhancing all-PSC performance.
• Optimal balance in aggregation strength between donor and acceptor polymers is crucial for achieving high PCE and desired active layer morphology.
• The developed PNDI-CBS copolymers offer a pathway to processable and high-performance all-PSCs.