Hole transport layer selection toward efficient colloidal PbS quantum dot solar cells.

The effect of energy level alignment between the hole transport layer (HTL) and active layer in PbS quantum dot (QD) solar cells was investigated. Here, a great variation in device performance was observed when employing different hole transporting materials. Devices using HTLs that could not block electrons only show poor device behaviors, while those employing wide band-gap hole transporting materials with shallow lowest unoccupied molecular orbital (LUMO) energies to block electrons exhibit reduced dark currents as well as enhanced device efficiencies. A power conversion efficiency of 4.4% was obtained by utilizing Poly-TPD as the HTL due to the optimized energy level alignment. These improvements were realized by preventing current leakage and consequent counter diode formation. The efficiency can be further improved to 4.9% by inserting EDT-treated PbS QD film (PbS-EDT) hole transporting materials with higher hole mobility as well as suitable energy levels that can increase the collection efficiency.