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.
|Tidsskrift||ACS Applied Energy Materials|
|Status||Udgivet - 25. apr. 2022|
Bibliografisk noteFunding Information:
The authors thank the beamline specialists and staff at PM4, BESSY and TEMPO, SOLEIL for their help during beamtimes at the respective synchrotron facilities. The authors acknowledge Helmholtz-Zentrum Berlin für Materialien und Energie BESSY II Electron storage ring for time on beamline PM4 under Proposal 182-07349-ST. They also acknowledge the SOLEIL synchrotron for time on beamline TEMPO under Proposal 20180611.The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The authors also thank the Danish Agency for Science, Technology, and Innovation for providing travel funding through the instrument center DanScatt and the support from the project “SMART─Structure of MAterials in Real Time” funded by the Danish Ministry of Higher Education and Science.