Wei Gao

Abstract A record power conversion efficiency (PCE) of over 19% is realized in planar‐mixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe‐4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/BS3TSe‐4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe‐4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (Δ E 2 ) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe‐4F achieve a PCE of 18.48%. By incorporating another mid‐bandgap acceptor Y6‐O into D18/BS3TSe‐4F to form a ternary PMHJ, a higher open‐circuit voltage ( V OC ) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

In this work, an effectual strategy of constructing polar small molecule acceptors (SMAs) to promote fill factor (FF) of nonfullerene polymer solar cells (PSCs) is first reported. Three asymmetrical SMAs of IDT6CN, IDT6CN-Th, and IDT6CN-M, which own large dipole moments, are designed and synthesized. The PSCs based on three polar SMAs exhibit apparently higher FFs compared with their symmetrical analogues. The asymmetrical design strategy accompanied with side chain and end group engineering makes IDT6CN-Th- and IDT6CN-M-based nonfullerene PSCs achieve high power conversion efficiency with FFs approaching 77%.

Abstract A novel small molecule acceptor MeIC with a methylated end‐capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520–740 nm. The MeIC‐based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short‐circuit current ( J SC ) of 18.41 mA cm −2 , significantly higher than that of the device based on J71:ITCPTC (11.63% with a J SC of 17.52 mA cm −2 ). The higher J SC of the PSC based on J71:MeIC can be attributed to more balanced μ h /μ e , higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X‐ray diffraction and resonant soft X‐ray scattering results. It is worth mentioning that the as‐cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as‐cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

Ternary OSCs fabricated with two acceptors with similar absorption spectra achieved the best PCE of 14.13% with an impressive FF of 78.2%.