Band gap engineering via controlling donor-acceptor compositions in conjugated copolymers

Area of Science:

• Organic Electronics
• Materials Science
• Computational Chemistry

Background:

• Donor-acceptor (D-A) conjugated copolymers are crucial for tuning optoelectronic properties.
• Varying the composition of π-donor and π-acceptor units is a key strategy for property modulation.
• Understanding the relationship between composition and electronic properties is vital for material design.

Purpose of the Study:

• To investigate how optoelectronic properties change with varying donor/acceptor ratios in D-A conjugated copolymers.
• To elucidate the factors governing the linearity or nonlinearity of electronic property dependence on composition.
• To provide insights for rational design of conjugated copolymers with tailored electronic properties.

Main Methods:

• First-principles density functional calculations were employed.
• Systematic variation of donor/acceptor ratios in model copolymer systems.
• Analysis of electronic properties including HOMO/LUMO levels, band gap, and bandwidth.

Main Results:

• Copolymers with similar π-electron donating/accepting moieties (e.g., thiophene-methoxythiophene) showed linear dependence of electronic properties on D/A content.
• Systems with strong D/A contrast (e.g., thiophene-thienopyrazine) exhibited nonlinear variations, but became linear when one monomer's block size was fixed.
• The strength of orbital interactions between donor and acceptor units was identified as the primary factor influencing deviations from linear composition dependence.

Conclusions:

• The composition dependence of electronic properties in D-A conjugated copolymers is influenced by the degree of electronic contrast between moieties.
• Weak orbital interactions between D and A units lead to nonlinear composition dependence.
• These findings offer valuable guidance for tuning the band gap of conjugated copolymers by adjusting D/A compositions.