Acceptor-donor-acceptor small molecules based on indacenodithiophene for efficient organic solar cells

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Organic solar cells (OSCs) offer a promising alternative to traditional silicon-based photovoltaics due to their potential for low-cost, flexible, and large-area applications.
• Developing efficient and stable small molecule electron donors is crucial for advancing OSC performance.

Purpose of the Study:

• To synthesize and characterize novel A-D-A type small molecules for use as electron donors in solution-processed OSCs.
• To investigate the impact of molecular structure, including π-bridge length and end acceptor groups, on the optoelectronic and photovoltaic properties.

Main Methods:

• Synthesis of four A-D-A small molecules featuring a 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b']dithiophene core, bithiophene or terthiophene π-bridges, and alkyl cyanoacetate or rhodanine end groups.
• Characterization of thermal stability, UV-Vis absorption, and energy levels (HOMO).
• Fabrication and testing of OSC devices using blends of the synthesized molecules and PC71BM, followed by thermal annealing.

Main Results:

• The synthesized molecules exhibited excellent thermal stability (>360 °C) and low HOMO levels (-5.18 to -5.22 eV).
• Strong optical absorption was observed in the 350-670 nm range with high molar extinction coefficients (1.1–1.6 × 10^5 M⁻¹ cm⁻¹).
• OSCs achieved power conversion efficiencies ranging from 2.32% to 5.09%, with the best device reaching 5.32% after annealing.

Conclusions:

• The A-D-A small molecules are suitable electron donors for solution-processed OSCs.
• Molecular design, specifically the choice of π-bridge and end acceptor, significantly influences the optoelectronic properties and device performance.
• Further optimization of these molecular structures holds potential for enhancing OSC efficiency.