flexibility, and tunable optical transparency, which are advantageous for applications in building-integrated photovoltaic (PV) and wearable electronics compared with conventional inorganic cells. [1][2][3][4][5] To achieve high efficiency, it is essential to develop suitable donor (D) and acceptor (A) materials that can be easily processed into a finely phase-separated morphology in active layer with low energy loss (E loss ) in OSCs. [6][7][8][9] Recently, the vibrant progress made on nonfullerene acceptors (NFAs), [10,11] especially the Y-series NFAs, [12] with facilely tailored chemical structures and precisely tuned bandgap, energy levels, and crystallization properties, has enabled OSCs to achieve very high power conversion efficiencies (PCEs) ≈19%. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] The bulk-heterojunction (BHJ) architecture-based devices prepared from mixing D and A in solution is the most widely adopted method for depositing photoactive layer and have successfully increased the PCE of single-junction OSCs to 19% recently. [31] Another emerging strategy is to deposit neat D and A materials A record power conversion efficiency (PCE) of over 19% is realized in planarmixed 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 (ΔE 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.