Fig. 3: Kelvin probe force microscope and electrostatic potential fluctuation.
From: Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells
a,b, Topology image measured with AFM (a) and the corresponding KPFM image (b) of a fresh cleaved CZTSe cell. The blue dashed lines labelled with numbers indicate where the grain boundary profiles were extracted. The yellow (horizontal) dashed line in b indicates where the line scan was performed. c, Contact potential difference (CPD) and topology profiles (Z) extracted along the yellow (horizontal) dashed line in b. Here CPD = (ΦP − ΦS) / q, where ΦP and ΦS are the work function of the probe and the measured sample, respectively, meaning CPD increases linearly with the decrease of work function of the measured sample. The inset shows the statistical CPD distribution and the Gaussian fit of the horizontal CPD scan, showing a mean CPD of 237.2 mV and a standard deviation of 5.4 mV. d, The CPD and topology profiles of grain boundary (GB) 1 (indicated in a and b). The blue shade is a guide to the eye showing where the grain boundary is located. Each dashed black line is a guide to the eye roughly indicating the mean CPD around the grain boundary, which is used to define the ΔCPD. The band bending at GB1 is about −8 meV (upward). The CPD and topology profiles of GB2–GB5 are shown in Supplementary Fig. 10. e, IQE spectrum (black line), first derivative of IQE (red dashed line) and normalized PL spectrum (blue line) of the high efficiency CZTSe cell. The vertical dashed blue and red lines indicate the PL peak position and the peak position of −dIQE / dλ, respectively. λ is the wavelength. f, Plot and fitting of electrostatic potential fluctuation, γopt (red) using the Shklovskii and Efros model below the bandgap energy and the fitting of the Urbach tail energy, Eu (blue). Detailed fitting models are described in Supplementary Discussion 1.
