Dopant interdiffusion effects in n-i-p structured spiro-OMeTAD hole transport layer of organometal halide perovskite solar cells

In this work we performed dopant diffusion experiments on the hole transport material spiro-OMeTAD. Pure spiro-OMeTAD/F4-TCNQ doped spiro-OMeTAD and pure spiro-OMeTAD/DMC doped spiro-OMeTAD thin films were deposited on Au/Si(100) substrates. The energy levels, chemical states, and surface morphologies of formed films were studied by ultraviolet photoemission spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy as a function of the storage time in ultrahigh vacuum. We found only the pure spiro-OMeTAD/DMC doped spiro-OMeTAD film showed the n-type doping with the Fermi level of the film shifting away from the HOMO edge by 0.3 eV after 36 h. We propose that the diffusion of DMC dopant leads to the increase of the efficiencies of perovskite solar cell with the pin-hole free n-i-p structured hole transport layer several days after fabrication.