High-performance fullerene-free polymer solar cells with solution-processed conjugated polymers as anode interfacial layer

A series of conjugated polymers based on PFS derivatives with π-conjugated 5-(9H-fluoren-2-yl)-2,2′-bithiophene (fluorene-alt-bithiophene) backbones, namely PFS-3C, PFS-4C and PFS-6C, were synthesized for their use as the anode interfacial layers (AILs) in the efficient fullerene-free polymer solar cells (PSCs). Alkyl sulfonate pendants with different lengths of alkyl side chains were introduced in the three polymers in order to investigate the effect of the alkyl chain length on the anode modification. The obtained three polymers exhibited similar absorption bands and energy levels, indicating that changing the length of the alkyl side chains did not affect the optoelectronic properties of the conjugated polymers. Based on the PBDB-T:ITIC active layer, we fabricated the fullerene-free PSCs using the three polymers as the AILs. The superior performance of the fullerene-free PSC device was achieved when PFS-4C was used as the AIL, showing a power conversion efficiency (PCE) of 10.54%. The high performance of the PFS-4C-modified device could be ascribed to the high transmittance, suitable work-function (WF) and smooth surface of PFS-4C. To the best of our knowledge, the PCE obtained in the PFS-4C-modified device is among the highest PCE values in the fullerene-free PSCs at present. These results demonstrate that the PFS derivatives are promising candidates in serving as the AIL materials for high-performance fullerene-free PSCs.