Modulation of Molecular Stacking via Tuning 2-Ethylhexyl Alkyl Chain Enables Improved Efficiency for All-Small-Molecule Organic Solar Cells

Abstract

There is always a dilemma between strong π−π stacking/ crystallinity and suitable domain size for all-small-molecule organic solar cells (ASM-OSCs), which puts forward higher requirements for the design of molecular donors. In this work, a series of novel molecular donors with different positional 2-ethylhexy (EH) attachments are designed and synthesized, named SM-R, SM-REH, SM-EH-R, and SM-EH-REH. It is found that EH-substitution on end groups (SM-REH) enables improved π−π interaction and crystallinity but with decreased solubility and phase size, leading to the improved efficiency of 15.6% as compared to 14.0% of SM-R. In contrast, EH-substitution on the πbridge (SM-EH-R) significantly suppresses π−π stacking and increases the solubility, resulting in the lower efficiency of 11.9%. The further EHsubstitution on end-groups of SM-EH-R, namely, SM-EH-REH, recovers the π−π stacking strength and obtains a moderate efficiency of 14.4%. Despite the higher crystallinity and increased π−π stacking in some molecules, the blend films show the gradually decreased domain size in the sequence of SM-R, SM-REH, SM-EH-R, and SM-EH-REH owing to the steric hindrance of the EH-chain. Overall, this work indicates that obtaining the higher π−π stacking/crystallinity and decreased domain size is achievable by tuning the EH-chain substitution, which paves the way to further improve the photovoltaic performance of ASM-OSCs.

Tackling Efficiency Challenges and Exploring Greenhouse-Integrated Organic Photovoltaics

“…A shorter alkylthio group displayed a strong inclination toward crystallization, while a longer alkylthio group exhibited diminished crystallinity. Ge et al [94] also optimized the molecular stacking by 2-ethylhexyl alkyl chains in a series of donor materials. It has been demonstrated that the side chain length can significantly affect the PCE of OSCs.…”

The central core size effect in quinoxaline-based non-fullerene acceptors for high VOC organic solar cells

“…[1][2][3][4][5][6] With material innovation, device optimization, and mechanism studies, OSCs have witnessed significant progress in the past decade and achieved power conversion efficiencies (PCEs) of over 19% and 20% for single junction and tandem devices, respectively. [7][8][9][10][11][12][13][14][15][16] However, it is still a challenge to comprehend the relationship between the molecular structure of NFAs and their photovoltaic performance and to achieve a balance among the various parameters influencing the PCE of OSCs. [17][18][19] As one of the most successful NFAs, Y6 possesses a unique banana-like configuration with an A-DA′D-A architecture and achieved a PCE breakthrough of 15.7%.…”