Anode interlayer in organic photovoltaics: Narrow bandgap small molecular materials as exciton‐blocking layer

Six materials were used as an interlayer at the anode side (anode interlayer [AIL]) of an archetypical planar heterojunction organic solar cell (OSC). In addition to two conventional wide bandgap hole transport materials (HTMs), tris(4‐carbazol‐9‐ylphenyl)amine ( TCTA ) and trans ‐4,4′‐bis[ N ‐(naphthalen‐1‐yl)‐ N ‐phenylamino]stilbene ( NPAE ), we explore four narrow bandgap materials, bis(biphenylaminospiro)‐fumaronitrile ( PhSPFN ), bis( N ‐(naphthalen‐1‐yl)‐ N ‐phenylamino)anthraquinone ( NPAAnQ ), bis‐(di(2‐fluorophenyl)aminospiro)‐fumaronitrile ( FPhSPFN ), and bis[4‐( N ‐(pyren‐1‐yl)‐ N ‐phenylamino)phenyl]fumaronitrile ( PyPAFN ), the energy levels of which essentially align with the ones of SubPc, the active light‐absorbing material of the OSC study herein. By using a narrow bandgap AIL, universally enhanced short‐circuit current density and power conversion efficiencies (PCEs) have been achieved. In addition, one of these materials, FPhSPFN , results in a PCE of 5.13%, which is the highest reported value for SubPc solar cells with a similar architecture. This is ascribed to the formation of an otherwise passive exciton‐blocking interface. Furthermore, this demonstrates that charge selectivity by way of a high‐lying lowest unoccupied molecular orbital (LUMO) energy level is not a prerequisite for successful AIL design. As such, in terms of energy level alignment and bandgap energies, we establish a viable alternative approach toward interface and interlayer material design.