Self-assembled organic molecules with a fused aromatic ring as hole-transport layers for inverted perovskite solar cells: the effect of linkers on performance

In this paper, two D-pi-A structured self-assembly monolayers (SAMs: FNE29 and DT-1) are self-assembled onto the indium tin oxide (ITO) substrates as hole-transport layers (HTLs) for inverted perovskite solar cells (PSCs). Due to the presence of a terminal -COOH group, the SAMs can be adsorbed onto the surface of the ITO substrate through the covalent self-assembly. The employed FNE29 and DT-1 SAMs differ in the pi bridge (linker); the FNE29 SAM has a hexyl-substituent terthiophene while the DT-1 SAM contains dithienopyrrolobenzotriazole as the linkers, respectively. Upon the self-assembly of ITO with the SAMs, the power conversion efficiency (PCE) is enhanced from 9.36% for the control device to 16.75% for the FNE29 modified device and further to 20.65% for the DT-1 modified PSCs. As the D-pi-A structured SAMs with a large fused pi bridge as a linker is beneficial for hole transfer, the self-assembly of DT-1 achieves a higher PCE with an improved fill factor. Moreover, due to the good hydrophobic characteristics of the SAMs, the inverted PSC devices self-assembled with these two molecules display good stability, maintaining over 80% of the original PCE after storage in air (relative humidity of 20-30%) for similar to 1000 h without encapsulation. In addition, the FNE29 and DT-1 based devices also show good photostability after operating for 1000 h. These results indicate that the D-pi-A structured SAMs with large conjugated fused pi bridges as linkers have obvious advantages as the HTLs for inverted PSCs. Two D-pi-A structured SAMs (FNE29 and DT-1) with different linkers are self-assembled onto the indium tin oxide (ITO) substrates as HTLs for inverted PSCs. The results indicate that the SAMs with large conjugated fused linkers have obvious advantages as HTLs because of its fast hole transfer.