An asymmetrical building block-TBF modified with alkylthio phenyl as the side chain, polymerization with 4, 7-di (5-bromothiophen-2-yl)-5, 6-dioctyloxybenzo[c] [1,2,5] thiadiazole (BT), a new polymer-TBFPS-BT was obtained. As described in the UV-Vis spectrum, TBFPS-BT exhibited good absorption covering from 300 to 750 nm. Benefited from the alkylthio phenyl substitution, the polymer possessed deep HOMO level of 5.39 eV. The photoluminescence (PL) spectra indicated the generated excitons could be almost completely dissociation at the polymer/PC71BM interface. Furthermore, the high and balanced mobility (1.49 x 10(-4) cm(2) V-1 s(-1) for hole and 1.07 x 10(-4) cm(2) V-1 s(-1) for electron) greatly contributed to enhance the fill factor (FF) and short-circuit current density (J(sc)) of device. PSC with different electron transporting layers (ETLs) were investigated, the results showed that when using perylene diimide (PDI) derivative with amino terminal substituent (PDIN) as ETL companied with 2% 1,8-Diiodooctane (DIO), the device delivered a best PCE of 6.55% (V-oc = 0.8 V, J(sc) = 11.93 mA cm(-2) and FF = 0.69). It is worth noting that all of the devices with ETLs and 2% DIO have shown high FFs over 0.65. So, the suitable modification on TBF and selection of ETLs were valid methods to improve device efficiency. (C) 2017 Elsevier Ltd. All rights reserved.