Aromaticity is an important concept to understand the stability and physical properties of π-conjugated molecules. Recent studies on pro-aromatic and anti-aromatic molecules revealed their irresistible tendency to become diradicals in the ground state. Diradical character thus becomes another very important concept and it is fundamentally correlated to the physical (optical, electronic and magnetic) properties and chemical reactivity of most of the organic optoelectronic materials. Molecules with distinctive diradical character show unique properties which are very different from those of traditional closed-shell π-conjugated systems, and thus they have many potential applications in organic electronics, spintronics, non-linear optics and energy storage. This critical review first introduces the fundamental electronic structure of Kekulé diradicals within the concepts of anti-aromaticity and pro-aromaticity in the context of Hückel aromaticity and diradical character. Then recent research studies on various stable/persistent diradicaloids based on pro-aromatic and anti-aromatic compounds are summarized and discussed with regard to their synthetic chemistry, physical properties, structure-property relationships and potential material applications. A summary and personal perspective is given at the end.

Two cheliform non-fullerene acceptors, DTPC-IC and DTPC-DFIC, based on a highly electron-rich core, dithienopicenocarbazole (DTPC), are synthesized, showing ultra-narrow bandgaps (as low as 1.21 eV). The two-dimensional nitrogen-containing conjugated DTPC possesses strong electron-donating capability, which induces intense intramolecular charge transfer and intermolecular π-π stacking in derived acceptors. The solar cell based on DTPC-DFIC and a spectrally complementary polymer donor, PTB7-Th, showed a high power conversion efficiency of 10.21% and an extremely low energy loss of 0.45 eV, which is the lowest among reported efficient OSCs.

mobility. [18] However, P-OSCs have a drawback in batch-to-batch reproducibility of donor polymers, which potentially limits the mass deployment of OSCs. [19] Compared to P-OSCs, small-molecule based OSCs (SM-OSCs) are more attractive in commercialization because of well-defined molecular structures, [20][21][22] simpler synthesis and purification, [23][24][25] and low batch-to-batch variations. [26][27][28][29] With the SM donors developed, the state-of-the-art SM-OSCs show similar PCEs as those obtained for P-OSCs (over 11%) using fullerene derivative, PCBM, as the electron acceptor. [19,[30][31][32][33] However, when the NFSM acceptors were used in nonfullerene-based small molecule organic solar cells (NFSM-OSCs) their PCE can only reach slightly over 10% [34][35][36][37] which is much lower than those obtained for nonfullerene polymer solar cells (NFP-OSCs) with usually PCE over 13%. [38,39] The progress of NFSM-OSCs is strongly lagged behind their polymer counterparts.The PCE of an OSC is determined by three parameters, opencircuit voltage (V oc ), short-circuit current density (J sc ), and fill factor (FF). The main reason for the low PCE in NFSM-OSCs is due to their relative low J sc and FF. [40,41] As shown in Table S1 (Supporting Information), all of the efficient NFP-OSCs with PCE over 14% show high J sc (>20 mA cm −2 ) and high FF In this paper, two near-infrared absorbing molecules are successfully incorporated into nonfullerene-based small-molecule organic solar cells (NFSM-OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP-TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP-TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and J sc . These result in a very respectable PCE, which is the highest among all NFSM-OSCs and all small-molecule binary solar cells reported so far.

A fused-ring thiophene-thieno­[3,2-b]­thiophene-thiophene (4T)-based low-band gap electron acceptor, 4TIC, has been designed and synthesized for non-fullerene solar cells. The utilization of the 4T center core enhances the charge mobility of 4TIC and extends its absorption band edge to ∼900 nm, which facilitates its function as a very efficient low-band gap electron acceptor. The rigid, π-conjugated framework of 4T also offers a lower reorganization energy to facilitate lower V OC energy loss. Femtosecond transient spectroscopy showed a level of polaron generation in 4TIC results in the more efficient transfer of energetic carriers higher than that seen with the benchmarked molecule, ITIC. Film morphology analysis has also shown that 4TIC has structural order that is more prominent than that of ITIC with a multiscale phase separation in the blend with donor polymer PTB7-Th. As a result, solar cells based on PTB7-Th and 4TIC exhibit a high power conversion efficiency of 10.43% and a relatively low non-ideal photon energy loss of 0.33 V. The low band gap and small energy loss make 4TIC suitable for tandem solar cells as a back-cell to reduce the transmission loss. As a demonstration, we fabricated series connection tandem solar cells incorporating 4TIC, which exhibts a high device performance of 12.62%.

Two novel non-fullerene acceptors, SN6IC and SN6IC-4F, based on an S,N-heteroacene backbone were designed and synthesized. The cyclopentadiene fragments of commonly used acceptors were replaced with pyrrole rings to improve the electron-donating ability to increase the energy levels of the molecules. Both acceptors match well with the absorption and energy levels of the polymer donor PBDB-T, and PBDB-T:SN6IC-4F-based solar cells showed an excellent power conversion efficiency of 13.2%, with a relatively small V OC loss of 0.54 eV. This study proves that the introduction of a nitrogen atom to replace the sp 3 -hybridized carbon in the fused ring is very effective for making highly efficient NFAs to further improve the performance of organic solar cells.