The band energy diagram of PCBM–DH6T bulk heterojunction solar cells: synchrotron-induced photoelectron spectroscopy on solution processed DH6T:PCBM blends and in situ prepared PCBM/DH6T interfaces
Applying high resolution synchrotron-induced photoelectron spectroscopy (SXPS), the electronic properties of a bulk heterojunction (BHJ) model solar cell consisting of phenyl-C61-butyric-acid-methyl-ester (PCBM) as an acceptor and α,ω-dihexylsexithiophene (DH6T) as a donor are investigated. This donor material can be prepared via UHV thermal evaporation and solution based techniques. Samples prepared by either technique show identical core levels and valence band spectra proving the equivalency of the resulting electronic properties. The formation of the PCBM/DH6T interface is studied in an in situ experiment based on stepwise evaporation of DH6T onto PCBM. The deposition of donor–acceptor mixed solutions leads to phase separated bulk heterojunction layers with a donor cap. The combination of SXPS measurements on a series of ex situ prepared blend films from solutions with varying donor–acceptor concentrations with in situ interface formation experiments enables deriving the band diagram across the bulk heterojunction and into the donor cap. Band bending of up to 0.3 eV is induced in the DH6T cap layer and a dipole of 0.26 eV forms at the PCBM:DH6T bulk heterojunction. The direction of the band bending leads to hole accumulation on the donor side of the interface, which may increase recombination with transferred electrons in the acceptor and thereby negatively affects the device performance.