Nanometre-scale optical property fluctuations in Cu2ZnSnS4 revealed by low temperature cathodoluminescence

Abstract

A B S T R A C TBand tailing is a major contributing factor to the large open circuit voltage (V oc ) deficit that is currently limiting Cu 2 ZnSnS 4 (CZTS) photovoltaic devices. It occurs in highly doped, highly compensated semiconductors and gives rise to a non-uniform electronic band structure. Here we report spatially resolved fluctuations in CZTS optical properties using low temperature cathodoluminescence (CL) in a scanning electron microscope (SEM). Principal component analysis reveals three CL peaks whose relative intensity vary across domains~100 nm in size. It is not known whether the non-uniform optical properties are due to changes in composition or due to structural order-disorder at constant composition. Measurement of composition with energy dispersive X-ray (EDX) analysis in an SEM and ordering with Micro-Raman mapping revealed CZTS to be uniform within the spatial resolution (estimated at~0.4 µm and 1.1 µm respectively) and sensitivity of the two techniques. The CL results are consistent with the presence of band tailing in CZTS.

Voids in Kesterites and the Influence of Lamellae Preparation by Focused Ion Beam for Transmission Electron Microscopy Analyses

“…Hence, the observation of voids in kesterite samples using TEM analysis has to be taken with care if the preparation of the lamellae is done using FIB. In fact, several works found voids and local excess of Zn(S, Se) [57]- [61], which are the main indications of the artifacts found in our study. TEM studies have to be paired with other analysis in order to have complementary data.…”

Mapping Shunting Paths at the Surface of Cu2ZnSn(S,Se)4 Films via Energy-Filtered Photoemission Microscopy

“…Recently, we have shown the presence of Sn antisite domain boundaries in CZTS nanostructures employing atomic-resolution transmission electron microscopy ( Kattan et al., 2016 ). Nanoscale domains of secondary phases have also been observed employing atom-probe tomography ( Tajima et al., 2014 ) and high-resolution cathodoluminescence ( Mendis et al., 2018 ). Systematic approaches to reduce bulk composition disorder via controlled annealing and the introduction of additives (e.g., alkali metal, Ag, Cd, Ge, and Sb) have led to improvement in device efficiency, yet V OC values remain in the 500–600 mV range ( Johnson et al., 2014 , Kumar et al., 2015 , Su et al., 2015 , Tiwari et al., 2016 , Qi et al., 2017 , Giraldo et al., 2018 , Sai Gautam et al., 2018 ).…”

Planck's generalised radiation law and its implications for cathodoluminescence spectra

“…Equation (11) indicates that the steady-state CL emission rate is proportional to the maximum photon density nγ max , which increases exponentially with γ (Equation 8). The physical origin of this effect is illustrated schematically in Figure 1.…”

“…Cathodoluminescence (CL) is a highly versatile electron microscopy technique for analysing the optical properties of inorganic semiconductors [1][2] as well as plasmonics [3][4]. The incoherent luminescence produced by electron-hole pair recombination provides important information on the electrical activity of defects, such as grain boundaries [5][6][7][8], as well as doping heterogeneities [9][10][11]. Furthermore, the development of time resolved CL at high spatial and temporal resolution by pulsed laser excitation of a photocathode [12][13] has enabled carrier lifetime measurement in localised regions of the specimen [14][15].…”