Simultaneous Increase in Open-Circuit Voltage and Efficiency of Fullerene-Free Solar Cells through Chlorinated Thieno[3,4-b]thiophene Polymer Donor

The chlorinated polymer, PBTCI, has been found to be an efficient donor in nonfullerene polymer solar cells (PSCs), which showed a blue-shifted absorbance compared to that of its fluorine analogue (PTB7-th) and resulted in more complementary light absorption with a nonfullerene acceptor, such as ITIC. Meanwhile, chlorine substitution lowered the HOMO level of PBTCI, which increased the open-circuit voltage of the corresponding polymer-based devices. The 2D GIWAXS analysis illustrated that the PBTCI/ITIC blend film exhibited a face-on orientation and scattering features of both PBTCI and ITIC, suggesting that the blend of PBTCI and ITIC was phase-separated and formed individual crystalline domains of the donor and acceptor, which promoted charge transfer in the bicontinuous film and eventually elevated the solar energy conversion efficiency. The PBTCI-based nonfullerene PSC exhibited a maximum PCE of 7.57% with a V-oc of 0.91 V, which was an approximately 13% increasing in the PCE compared to that of the fluorine-analogue-based device.