Efficient conventional- and inverted-type photovoltaic cells using a planar alternating polythiophene copolymer

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Developing efficient and stable organic photovoltaic (OPV) materials is crucial for renewable energy.
• Low-band-gap polymers are essential for absorbing a broad solar spectrum.
• Molecular design influences polymer properties like ordering and energy levels.

Purpose of the Study:

• To synthesize and characterize a novel low-band-gap alternating copolymer, poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT).
• To investigate the photovoltaic performance of PTBT when blended with PC(61)BM.
• To explore the relationship between molecular structure, film morphology, and device efficiency.

Main Methods:

• Synthesis of PTBT copolymer.
• Fabrication and testing of conventional and inverted photovoltaic devices using PTBT:PC(61)BM blends.
• Characterization of film morphology using Atomic Force Microscopy (AFM).
• Measurement of charge carrier mobility using Space-Charge-Limited Current (SCLC) analysis.
• Analysis of molecular ordering using Selected-Area Electron Diffraction (SAED).

Main Results:

• PTBT exhibits a minimized backbone torsion angle and pronounced intermolecular ordering due to octyloxy side chains.
• The polymer possesses a deep Highest Occupied Molecular Orbital (HOMO) level at -5.41 eV.
• Power-conversion efficiencies (PCEs) of 5.9% (conventional) and 5.3% (inverted) were achieved.
• High open-circuit voltage (V(OC) ≈ 0.85-0.87 V) was observed, surpassing many thiophene-based polymers.
• AFM, SCLC, and SAED indicated ideal film morphology with bicontinuous pathways.

Conclusions:

• The molecular design strategy yields processable, planar donor-acceptor (D-A) polymers with deep HOMO levels.
• PTBT is a promising material for high-performance organic photovoltaic applications.
• Optimized film morphology is critical for achieving high device efficiencies.