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In this work,we introduced alkylthio substituent on the thiophene side chain in the polymer and synthesized a new low bandgap 2D-conjugated polymer PBDTT-S-TT(Figure 1).The alkylthio substituent increased the hole mobility of the polymer to 4.08 × 10-3 cm2 V-1 s-1 and down-shifted the HOMO energy level of the polymer by 0.11 eV with absorption of the polymer film red-shifted slightly.The PSCs based on PBDTT-S-TT as donor and [6,6]-phenyl-C7]-butyric acid methyl ester(PC70BM)as acceptor without solvent additive treatment demonstrated a high open-circuit voltage(Voc)of 0.84 V,leading to a high power conversion efficiency(PCE)of 8.42%,under the illumination of AM 1.5 G 100 mW cm-2.For comparison,the Voc and PCE of the devices based on the corresponding parent polymer PBDTT-TT with the device optimization of 3%DIO additive treatment are 0.77 V and 7.38%,respectively.The enhanced Voc value of 0.84 V for the PSC based on PBDTT-S-TT should be benefited from the down-shifted HOMO energy level of the polymer.The results indicate that the alkylthio substitution is an effective way to further improve the photovoltaic performance of the 2D-conjugated polymer donor materials in PSCs.