Designing a naphthyridinedione-based conjugated polymer for thickness-tolerant high efficiency polymer solar cells

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2021-05, Vol.9 (17), p.1846-1854
• Main Authors: Park, Jun-Mo, Lee, Tack Ho, Kim, Dong Won, Kim, Jae Won, Chung, Hae Yeon, Heo, Jungwoo, Park, Song Yi, Yoon, Won Sik, Kim, Jin Young, Park, Soo Young
• Format: Article
• Language: English
• Subjects: Absorption; Absorptivity; Carrier mobility; Chemical bonds; Coherence length; Comparative analysis; Crystal structure; Crystallinity; Efficiency; Energy conversion efficiency; Fluorine; Mobility; Photoelectric effect; Photoelectric emission; Photovoltaic cells; Polymers; Solar cells; Thickness
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/d1ta01362a

Conjugated polymers with a high absorption coefficient and high charge mobility are essential for high power conversion efficiency (PCE) and large area roll-to-roll processing of polymer solar cells. However, only a few conjugated polymers with both properties have been reported so far. Recently, we have reported an innovative naphthyridinedione (NTD)-based highly crystalline polymer (PNTDT-2F2T) with an exceptionally high absorption coefficient ( α = 1.6 × 10 5 cm −1 ) and charge mobility ( μ h = 8.6 × 10 −3 cm 2 V −1 s −1 ), which showed 9.6% PCE with a thick active layer (210 nm). However, the PCE decreases considerably to 7.4% with a thin active layer (80 nm) due to a reduction in the photocurrent owing to the still smaller absorption coefficient and rather larger crystalline domain size of PNTDT-2F2T. To develop a thickness-tolerant high efficiency polymer solar cell aiming at low-cost commercial manufacture, we report a newly designed NTD-based conjugated polymer (PNTD4T-2FB) with an increased absorption coefficient and controlled crystallinity that contains 1,4-difluorobenzene units, in this work. The smaller crystal size (crystalline coherence length (CCL) = 2.20 nm) and higher absorption coefficient ( α = 2.0 × 10 5 cm −1 ) of PNTD4T-2FB compared to those of PNTDT-2F2T (CCL = 3.19 nm) facilitate charge generation while maintaining a high carrier mobility ( μ h = 2.92 × 10 −3 cm 2 V −1 s −1 ). Consequently, the PNTD4T-2FB based device shows improved PCE (10.1%) with a thicker active layer (330 nm) while keeping a PCE of 9.2% for an active layer of only 70 nm thickness as well. Comparative analysis with PNTD4T-B (which does not contain fluorine) suggests that the superior characteristics of PNTD4T-2FB are attributable to the slightly twisted but rigid backbone arising from the S-F intra-molecular non-bonding interaction between 1,2-difluorobenzene and thiophene. A newly designed NTD-based polymer with an exceptionally high absorption coefficient, small crystallite size, and high charge mobility shows excellent thickness-tolerant high PCEs in fullerene PSCs.