Crystallinity and morphology effects on a solvent-processed solar cell using a triarylamine-substituted squaraine

Area of Science:

• Organic electronics
• Materials science
• Photovoltaics

Background:

• Organic photovoltaic cells (OPVs) offer potential for low-cost, flexible solar energy conversion.
• Developing efficient active layer materials is crucial for advancing OPV technology.
• Solution processing enables scalable and cost-effective manufacturing of OPVs.

Purpose of the Study:

• To investigate the performance of 2,4-Bis[4'-(N,N-di(4″-hydroxyphenyl)amino)-2',6'-dihydroxyphenyl]squaraine (Sq-TAA-OH) as an active layer in bulk-heterojunction organic photovoltaic cells.
• To optimize the morphology of the active layer through solvent engineering and thermal annealing for enhanced power conversion efficiency (PCE).
• To correlate the morphological characteristics of the active layer with photovoltaic performance.

Main Methods:

• Fabrication of bulk-heterojunction organic photovoltaic cells with the configuration ITO/PEDOT:PSS/Sq-TAA-OH:PC71BM/LiF/Al.
• Utilizing a solvent mixture of good and poor Sq-TAA-OH solubilizing solvents with diiodooctane (DIO) additive for bulk heterojunction layer formation.
• Employing thermal annealing post-deposition to optimize film morphology.
• Characterizing film morphology using X-ray diffraction and scattering studies.

Main Results:

• Achieved power conversion efficiencies (PCEs) up to 4.8% with optimized V(OC) = 0.84-0.86 V, J(SC) = 10 mA cm(-2), and FF = 0.53.
• Sq-TAA-OH films exhibited d-spacing features similar to single-crystal packing.
• DIO additive broadened crystallite size distribution in pristine films, while thermal annealing narrowed it, leading to improved phase separation and formation of ~48 nm crystallites in the best-performing active layers.

Conclusions:

• Sq-TAA-OH is a promising material for solution-processed organic photovoltaics.
• Morphological control through solvent mixtures and thermal annealing is critical for achieving high PCEs.
• Optimized phase separation and crystallite formation in the active layer directly correlate with enhanced device performance.