π-π Stacking Distance and Phase Separation Controlled Efficiency in Stable All-Polymer Solar Cells.

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• The morphology of the active layer is critical for organic solar cell performance and stability.
• All-polymer solar cells (All-PSCs) offer improved thermal stability over fullerene-based devices, but challenges remain in achieving both high efficiency and stability in polymer blends.
• Factors limiting thermal stability and methods to enhance efficiency without compromising it in All-PSCs are not fully understood.

Purpose of the Study:

• To investigate the morphological factors affecting thermal stability in all-polymer solar cells (All-PSCs).
• To understand the relationship between molecular packing, phase separation, and device performance under thermal stress.
• To identify strategies for achieving high efficiency and thermal stability in All-PSCs through ternary blending.

Main Methods:

• Studied the morphology of poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1)/poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]] (PCE10)/PNDI-T10 blend systems.
• Analyzed the impact of thermal annealing on molecular packing, π-π stacking distance, and phase separation.
• Evaluated device performance (PCE) and hole conductivity before and after thermal annealing.

Main Results:

• Rearranged molecular packing and phase separation were identified as key factors in the poor thermal stability of devices containing PCE10.
• TQ1/PNDI-T10 devices showed improved power conversion efficiency (PCE) with decreased π-π stacking after annealing.
• PCE10/PNDI-T10 devices, while having good initial PCE, suffered from increased π-π stacking and reduced hole conductivity upon annealing, leading to lower PCE.
• A TQ1/PCE10/PNDI-T10 (1/1/1) ternary system achieved maximum PCE after annealing due to favorable molecular interactions and a balanced variation in molecular packing.

Conclusions:

• Ternary blends can provide a pathway to simultaneously achieve high efficiency and thermal stability in All-PSCs.
• Combining materials with excellent pristine efficiency with those that improve under thermal annealing is a viable strategy.
• Optimizing molecular interactions and managing molecular packing variations through ternary blending is crucial for robust All-PSC performance.