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.