Morphological effects on the small-molecule-based solution-processed organic solar cells.

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Solution-processed organic solar cells (OSCs) offer a promising alternative to conventional photovoltaics.
• Developing efficient donor materials with controlled self-assembly is crucial for enhancing OSC performance.
• Small molecules with specific structural configurations can self-assemble into ordered structures.

Purpose of the Study:

• To investigate the self-assembly behavior of novel small donor molecules with dithiophene-phenazine-dithiophene (TH-P) and dithiophene-quinoxaline-dithiophene (TH-Q) configurations.
• To characterize the photophysical, thermal, and electrochemical properties of these new donor molecules.
• To evaluate the impact of molecular structure and self-assembly on the performance of organic solar cells.

Main Methods:

• Synthesis and characterization of TH-P and TH-Q donor molecules.
• Photophysical studies using UV-vis absorption and fluorescence spectroscopy.
• Thermal analysis including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).
• Electrochemical characterization and correlation with theoretical calculations.
• Fabrication and testing of bulk heterojunction (BHJ) and planar heterojunction (PHJ) organic solar cells.

Main Results:

• TH-P molecules formed continuous, one-dimensional nanofibers, creating a 3D network that facilitated spontaneous nanoscale phase separation.
• TH-P/PC61BM BHJ and PHJ devices achieved power conversion efficiencies (PCE) of 0.38% and 0.30%, respectively.
• TH-Q molecules formed shorter, more rigid needles with significant thickness variation.
• TH-Q/PC61BM BHJ devices achieved a PCE of 0.36%, while PHJ devices showed poor performance.
• Post-annealing affected domain size and decreased short-circuit current (Jsc).

Conclusions:

• The study demonstrates a molecular design strategy to promote one-dimensional self-assembly in small-molecule OSCs.
• Fibrillation of donor molecules is significant for achieving efficient performance in small-molecule-based OSCs.
• Controlling molecular architecture is key to directing self-assembly and optimizing nanoscale morphology for improved OSC efficiency.