The Molecular Ordering and Double‐Channel Carrier Generation of Nonfullerene Photovoltaics within Multi‐Length‐Scale Morphology

Abstract

and low energy loss that deliver high opencircuit voltage (V OC ) and short-circuit current (J SC ) in solar cell devices. [1,2] An ideal BHJ thin film requires a bicontinuous network morphology of tens of nanometers of domain size to ensure efficient exciton splitting and charge transport, [3] leading to an improved fill factor (FF). The crystallization of NFA molecule and conjugated polymer in mixture plays an important role in determining the thin-film morphology, [4,5] where the π-π stacking is considered as the primary driving force. [6] This process results in crystallites buried in donor-acceptor mixture, [7] which significantly influences the optoelectronic process in OPV.NFA molecules crystallize through backbone and side-chain interactions. The classic fused ring NFAs arouse great interest currently, [8] whose single crystal is taken as a platform to investigate the specific intermolecular interactions that govern the crystallization. NFA molecules are much larger compared with conventional organic semiconductors such as pentacene or its soluble analogs, which leads toThe success of nonfullerene acceptor (NFA) solar cells lies in their unique physical properties beyond the extended absorption and suitable energy levels. The current study investigates the morphology and photophysical behavior of PBDB-T donor blending with ITIC, 4TIC, and 6TIC acceptors. Single-crystal study shows that the π-π stacking and side-chain interaction dictate molecular assembly, which can be carried to blended films, forming a multi-length-scale morphology. Spontaneous carrier generation is seen in ITIC, 4TIC, and 6TIC neat films and their blended thin films using the PBDB-T donor, providing a new avenue of zero-energy-loss carrier formation. The molecular packing associated with specific contacts and geometry is key in influencing the photophysics, as demonstrated by the charge transfer and carrier lifetime results. The 2D layer of 6TIC facilitates the exciton-to-polaron conversion, and the largest photogenerated polaron yield is obtained. The new mechanism, together with the highly efficient blending region carrier generation, has the prospect of the fundamental advantage for NFA solar cells, from molecular assembly to thin-film morphology.

Side Chain Length and Interaction Mediated Charge Transport Networks of Non-Fullerene Acceptors for Efficient Organic Solar Cells

“…Different to the ball-like structured fullerene electron acceptors, the state-of-the-art NFAs (e.g., Y6 series) are featured with a large planar structure, with two electron-withdrawing end groups sandwiching an electron-donating backbone core where two upper and lower alkyl chains are attached on . Although this unique structure offers high flexibility in the molecular design and renders tunable optical and electron properties for NFAs, it also leads to variable aggregation behavior of NFAs, in which NFAs can form H-, J-, and X- aggregations between two molecules, which appear as dimers with different shapes (e.g., S, W) in a three-dimensional (3D) space, further form 3D lamella or honeycomb networks for efficient charge transport, and ultimately determining the photovoltaic performance of OSCs. , Dedicated molecular design efforts including core engineering , and terminal modification , as well as alkyl chain substitution − have been performed not only to optimize the optoelectronic properties of NFAs but also to regulate their aggregation behavior to further advance the PCE of their OSCs.…”

Photoinduced intra- and inter-molecular charge transfer dynamics in organic small molecules with an intra-molecular push–pull electronic structure

“…Recently, by separately adding 1,8-diodoctane (DIO) into ITIC, 4TIC, and 6TIC neat films, Xu et al obtained more A-to-A type J-aggregation in these molecular films, where they reported a stronger inter-CT TA signal than their films without DIO. 19 On the other hand, Fig. 4e shows that the NFA molecular aggregation mode has a weak impact on the time scale of the inter-CT relaxation process.…”

“…15 To date, there has been growing recognition that an optimized aggregation mode of NFA molecules can significantly improve their photoelectric performances, such as the light absorption, 26 charge dynamics, 27 as well as the inter-CT effect. 19 In view of this, by fixing the intra-molecular electronic push-pull potential as D on = 0.6 eV, we further consider the inter-CT effect between neighboring NFA molecules with different aggregation modes: A-to-A type J-aggregation, A-to-D type J-aggregation, and D-to-D type H-aggregation (see the inset of Fig. 3a-c).…”

“…18 Recently, Xu et al employed transient absorption (TA) spectroscopy and observed inter-CT signals in other typical NFA molecules (ITIC, 4TIC, and 6TIC). 19 In an earlier work, 16 from the static energetic viewpoint, we theoretically clarified that intra-and inter-CT effects intrinsically coexist in NFA molecules, by which the excited state binding energy can be significantly reduced and close to the thermal energy at room temperature.…”

“…[12][13][14] With these optimizations, intra-and inter-molecular charge transfer (CT) effects in NFA molecules have been successively reported. [15][16][17][18][19][20][21] A basic consensus is that the NFA molecular push-pull electronic structure first induces an intra-CT effect. However, in NFA molecules with only the intra-CT effect considered, their excited state binding energy is still as high as 0.2 eV.…”