Defining the buried electronic structures in solution-processed organic heterojunctions by ultraviolet photoelectron spectroscopy analysis assisted with argon gas cluster ion beam sputtering

Accurate characterization of buried electronic structures in solution-processed bulk heterojunctions (BHJs) is hindered by surface phase separation and spectral overlap between the donor and acceptor components. In this study, a depth-resolved analytical methodology is developed by integrating ultraviolet photoelectron spectroscopy (UPS) with argon gas cluster ion beam (Ar-GCIB) sputtering. This enables precise energy-level analysis of spin-coated P3HT:PCBM71 BHJ films of varying compositions (3:1, 1:1, and 1:3 by weight). Surface-sensitive UPS and reflection electron energy loss spectroscopy (REELS) characterizations reveal that the BHJ film possesses a P3HT-enriched surface that exhibits an electronic structure fully corresponding to that of P3HT but completely distinct from that of PCBM71. For the UPS depth profiles of the BHJ films obtained via Ar-GCIB sputtering, the individual HOMO levels of P3HT and PCBM71 are extracted by deconvolving the UPS spectra using a linear combination method. These values hold for different locations in the vertical direction, thus confirming the robustness of the technique. This approach facilitates the clear delineation of donor–acceptor energy-level alignments, which consistently range from 1.13 to 1.20 eV. These findings demonstrate the efficacy of UPS-Ar-GCIB depth profiling combined with spectral subtraction for non-damaging and accurate electronic structure analysis of complex BHJ systems.