Impact of Isomer Design on Physicochemical Properties and Performance in High-Efficiency All-Polymer Solar Cells

Abstract

Combining the acceptor–donor–acceptor-type fused ring-based molecular architecture into a polymeric backbone is a promising strategy to design polymer acceptors for high-performance all-polymer solar cells (all-PSCs), and use of single isomer monomers is critical to control their physicochemical and photovoltaic properties. Here, two polymer acceptors (namely, PBI-α and PBI-β) based on two different building blocks are designed and synthesized for investigating the effect of regioisomerism of the building blocks on the physicochemical and photovoltaic properties of the polymer acceptors. The subtle difference in the thienyl-fused malononitrile group of PBI-α and PBI-β significantly influences their absorption spectra and electronic energy levels. PBI-α exhibited a lower lowest unoccupied molecular orbital level and obviously redshifted absorption spectrum compared to PBI-β, which delivered a significantly different open-circuit voltage (0.930 ± 0.004 vs 1.02 ± 0.01 V) and short-circuit current density (18.7 ± 0.3 vs 16.1 ± 0.4 mA cm–2) in the PBI-α- and PBI-β-based devices, respectively. Meanwhile, PBI-β shows a more positive molecular packing effect to enhance the π-face-on orientation backbone stacking of polymer donor PM6 than PBI-α, which is beneficial for efficient charge transport, leading to a higher fill factor of 0.684 in the PBI-β-based all-PSC compared to 0.646 for the PBI-α-based device.

Regioregularity-control of conjugated polymers: from synthesis and properties, to photovoltaic device applications

“…Thus, new D-A type polymer acceptors (PSMA) have been developed by polymerizing NFSMAs and other donating building blocks. [39][40][41]117,[166][167][168][169][170][171][172][173][174][175][176][177][178][179][180][181] Because different regio-isomers are found in the chemical structures of NFSMA molecules, controlling the RR of the PSMA is an emerging factor for their design and application in high-efficiency all-PSCs. In this section, we will review a library of examples of RR control in the photovoltaic polymers and summarize the impacts of directional, positional, and sequential RRs on the PSC performance.…”

“…The lower PCEs are due, in part, to the inhomogeneous molecular structures with low positional RR of PSMAs. 40,41,167,169,170,187 To address these issues, He and co-workers recently demonstrated the importance of controlling the positional RR of the PSMAs. They rst obtained isomerically pure monomers through recrystallization and employed them to achieve rre PBTIC-g-2F2T PSMAs by polymerization (Fig.…”

“…Also, controlling the molecular conformation by changing the positional RR of PSMAs is crucial for optimizing the properties of PSMAs and their performance in all-PSCs. 40,167,169,170,187 3.2.3 Sequential RR of D-A copolymers. The control of sequential RR of CPs is crucial for both optoelectrical properties and high-performance PSCs (Fig.…”

Controlling solid-state structure and film morphology in non-fullerene organic photovoltaic devices

“…In terms of polymer NFAs, the most reliable method of producing high performing polymeric NFAs is to take a calamitic NFA and copolymerize it with a short spacer. [65][66][67][68][69] At least one report has shown that the use of an electron-deficient spacer is beneficial. 70 Although this is a new field of study, it appears that the features that make calamitic NFAs successful are also applicable to polymeric NFAs.…”

Positional isomeric effect of monobrominated ending groups within small molecule acceptors on photovoltaic performance

“…[13][14][15][16][17] Bromination on IC has attracted increasing attention over other halogenations (F, Cl, and I) on developing novel SMAs because of the emerging strategy of polymerizing SMAs towards highly efficient polymeric acceptors. [18][19][20][21] Interestingly, the SMAs with IC-Br were found to offer a superior performance over the ones with a dibrominated IC (IC-2Br). 22,23 The asymmetric existence of a single malononitrile group on IC leads to a positional isomerization of monobromination when synthesizing IC-Br from the corresponding monobrominated indanedione and malononitrile.…”