  Bulk-heterojunction (BHJ) non-fullerene organic solar cells prepared from
sequentially deposited donor and acceptor layers (sq-BHJ) have recently been
promising to be highly efficient, environmentally friendly, and compatible with
large area and roll-to-toll fabrication. However, the related photophysics at
donor-acceptor interface and the vertical heterogeneity of donor-acceptor
distribution, critical for exciton dissociation and device performance, are
largely unexplored. Herein, steady-state and time-resolved optical and
electrical techniques are employed to characterize the interfacial trap states.
Correlation with the luminescent efficiency of interfacial states and its
non-radiative recombination, interfacial trap states are characterized to be
about 50% more populated in the sq-BHJ than as-cast BHJ (c-BHJ), which probably
limits the device voltage output. Cross-sectional energy-dispersive X-ray
spectroscopy and ultraviolet photoemission spectroscopy depth profiling
directly vizualize the donor-acceptor vertical stratification with a precision
of 1-2 nm. From the proposed "needle" model, the high exciton dissociation
efficiency is rationalized. Our study highlights the promise of sequential
deposition to fabricate efficient solar cells, and points towards improving the
voltage output and overall device performance via eliminating interfacial trap
states.
