  Microstructural properties of thin-film absorber layers play a vital role in
developing high-performance solar cells. Scanning probe microscopy is
frequently used for measuring spatially inhomogeneous properties of thin-film
solar cells. While powerful, the nanoscale probe can be sensitive to the
roughness of samples, introducing convoluted signals and unintended artifacts
into the measurement. Here, we apply a glancing-angle focused ion beam (FIB)
technique to reduce the surface roughness of CdTe while preserving the
subsurface optoelectronic properties of the solar cells. We compare the
nanoscale optoelectronic properties before and after the FIB polishing.
Simultaneously collected Kelvin-probe force microscopy (KPFM) and atomic force
microscopy (AFM) images show that the contact potential difference (CPD) of
CdTe pristine (peak-to-valley roughness of approximately 600 nm) follows the
topography. In contrast, the CPD map of polished CdTe (roughness of
approximately 20 nm) is independent of the surface roughness. We demonstrate
the smooth CdTe surface also enables high-resolution photoluminescence (PL)
imaging at a resolution much smaller than individual grains (< 1 micrometer).
Our finite-difference time-domain (FDTD) simulations illustrate how the local
light excitation interacts with CdTe surfaces. Our work supports low-angle FIB
polishing can be beneficial in studying buried sub-microstructural properties
of thin-film solar cells with care for possible ion-beam damage near the
surface.
