Unveiling the energy alignment across ultrathin 4P-NPD hole extraction interlayers in organic solar cells

Molecular thin films of N,N′-di-1-naphthalenyl-N,N′-diphenyl [1,1′:4′,1″:4″,1'-quaterphenyl]-4,4'-diamine (4P-NPD) have been demonstrated to function as efficient exciton blocking layers in organic solar cell devices, leading to improved device performance by minimizing exciton losses and by providing hole extraction selectivity. However, the exact mechanisms have been debated, as ultrathin thicknesses of less than 1 nm are required to observe optimized device performance improvements. In this work, we conduct photoelectron spectroscopy to gain information about core levels, HOMO/LUMO levels, and work functions for the hole extraction side of an organic solar cell device consisting of the small molecule tetraphenyldibenzoperiflanthene (DBP) as an electron donor and 4P-NPD for exciton blocking/hole extraction, the latter being in contact with the hole transport layer MoO _x. Using in situ deposition and characterization, we demonstrate that a negative HOMO energy offset increases with 4P-NPD thickness on the DBP donor layer, which cannot account for the improvement observed in device performance. Investigation of the 4P-NPD/MoO _xinterface, on the other hand, reveals shifts of the electronic levels in 4P-NPD and a band alignment that favors hole extraction while blocking for exciton/electron leakage. This appealing behavior is enhanced for ultrathin 4P-NPD films of less than 1 nm. Thus, the exciton blocking/hole extraction behavior of 4P-NPD interlayers in organic solar cell devices is confirmed and understood from the detailed energy level alignment across both interfaces, as extracted from the in situ photoelectron spectroscopy studies.