Tuning the Molecular Packing of Low-Cost Non-Fullerene Acceptors via Asymmetric Terminal Groups

Abstract

Benefiting from the simple building blocks and facile synthetic processes, IDIC derivatives based on indacenodithiophene (IDT) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene) propanedinitrile (IC) terminal groups have been widely studied since the early emerging of non-fullerene acceptors (NFAs) in organic solar cells (OSCs). Although the pristine IDIC molecule possesses a low material cost, its photovoltaic performance is unsatisfactory. Thus, this work conducts the modification of IDIC by alternating the terminal groups and the molecular symmetry. Differing from the typical NFAs in which the fluorinated terminal groups can promote photon harvest and charge mobility to optimize the photovoltaic properties, the material ID4F-C8 with the fluorine substitution exhibits inferior performance compared with the IDIC-C8 with the normal IC groups. Such unexpected results are rationalized by the varied molecular packing in the solid state, i.e., IDIC-C8 has a typical three-dimensional (3D) reticular motif and ID4F-C8 possesses a two-dimensional (2D) packing without an obvious π–π stacking. When two different groups with and without fluorine substitution are utilized as the terminals, the asymmetric ID2F-C8 retains both of the merits of the IDIC-C8 and ID4F-C8, exhibiting strong absorption, suitable energy levels, and highly ordered packing. Thus, the OSC device based on ID2F-C8 acquires the highest PCE of 11.67%. This encouraging result suggests the potential of the asymmetric strategy to develop high-performance and low-cost photovoltaic materials.

Non-covalent interactions (NCIs) in π-conjugated functional materials: advances and perspectives

“…It was found that fluorinated analogue 36c (Chart 5) displayed lower PCE (9.14%) and V oc (0.78 V) but higher J sc (15.01 mA cm 2 ) and FF (76.1) than 36a ( V oc = 0.98 V, J sc = 13.93 mA cm 2 , FF = 73.5%, and PCE = 10.31%). 80 On the other hand, 36b , because of its asymmetric structure, exhibited the best performance owing to balanced V oc and J sc ( V oc = 0.98 V, J sc = 13.93 mA cm 2 , FF = 73.5%, and PCE = 10.31%). The hole ( μ h ) and electron mobilities ( μ e ) of the fluorinated analogue were higher than for the non-fluorinated one (Table 2), explaining the higher FF value in the former.…”

Rational design of non-fullerene acceptors via side-chain and terminal group engineering: a computational study

“…28,29 In addition, end group engineering is a popular approach to enhance the charge transport in NFA molecules, as already highlighted in literature reports. 30–35 The impact of fluorination in the terminal acceptor unit was demonstrated as one of the valuable strategies to lower the molecular orbital energy levels and shift the absorption maximum towards the near-infrared region, leading to an overall improvement in the performance of the organic solar cells. 31,36,37…”

“…28,29 In addition, end group engineering is a popular approach to enhance the charge transport in NFA molecules, as already highlighted in literature reports. [30][31][32][33][34][35] The impact of fluorination in the terminal acceptor unit was demonstrated as one of the valuable strategies to lower the molecular orbital energy levels and shift the absorption maximum towards the near-infrared region, leading to an overall improvement in the performance of the organic solar cells. 31,36,37 Despite these studies, the selection, design, and fabrication of NFAs for high-performing OSCs still pose significant challenges due to the lack of precise insight into the structureproperty interplay, which prevents guided (i.e., computer-aided or in-silico) material design.…”