High-Efficiency Small Molecule-Based Bulk-Heterojunction Solar Cells Enhanced by Additive Annealing

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Solvent additive processing is key to optimizing active layer morphology in organic solar cells (OSCs).
• The processing method significantly impacts the performance of bulk heterojunction (BHJ) OSCs.
• Porphyrin small molecules are promising materials for OSCs.

Purpose of the Study:

• To investigate the critical role of 1,8-diiodooctane (DIO) additive removal timing on the morphology and performance of inverted BHJ OSCs.
• To compare the effects of different additive removal methods (quick vs. slow annealing) on device performance.
• To understand the structure-property relationships influenced by additive processing.

Main Methods:

• Fabrication of inverted BHJ OSCs using a porphyrin small molecule active layer.
• Investigated additive removal via quick vacuuming, methanol washing, and additive annealing (1-hour dwelling).
• Characterized film morphology using UV-vis absorption spectroscopy, grazing incidence X-ray diffraction (GIXRD), resonant soft X-ray scattering (RSoXS), and atomic force microscopy (AFM).

Main Results:

• Quick removal of DIO (vacuuming or methanol washing) resulted in poorer device performance.
• Additive annealing (1-hour dwelling) enhanced power conversion efficiency to 7.78% with a short-circuit current of 19.25 mA/cm(2).
• Additive annealing led to smaller phase separation and improved structural order in the active layer.
• Quick removal methods resulted in larger-scale phase separation.

Conclusions:

• The method and timing of additive removal critically influence the morphology and performance of small molecule-based inverted OSCs.
• Additive annealing is a superior processing strategy compared to rapid additive removal for achieving high-performance OSCs.
• Optimized morphology through controlled additive removal is essential for advancing small molecule solar cell technology.