Impact of Surface Roughness in Measuring Optoelectronic Characteristics of Thin-Film Solar Cells

TL;DR

This study demonstrates that reducing surface roughness of CdTe thin-film solar cells via FIB polishing improves nanoscale optoelectronic measurements, enabling clearer insights into their properties while highlighting potential ion-beam damage risks.

Contribution

The paper introduces a FIB polishing method to minimize surface roughness in thin-film solar cells, enhancing measurement accuracy of nanoscale optoelectronic properties.

Findings

FIB polishing reduces surface roughness from ~600 nm to ~20 nm.

Polished surfaces show CPD independent of topography.

High-resolution PL imaging is enabled on smoother surfaces.

Abstract

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.