Terahertz Nanoimaging of Perovskite Solar Cell Materials

Abstract

Direct visualization and quantitative evaluation of charge filling in grain boundary (GB) traps of hybrid metal halide perovskites require dynamic conductivity imaging simultaneously at the terahertz (THz) frequency and nanometer (nm) spatial scales not accessible by conventional transport and imaging methods used thus far. Here, we apply a THz near-field nanoconductivity mapping to the archetypal metal halide perovskite photovoltaic films and demonstrate that it is a powerful tool to reveal distinct dielectric heterogeneity due to charge trapping and degradation at the single GB level. Our approach visualizes the filled defect ion traps by local THz charge conductivity and allows for extracting a quantitative profile of trapping density in the vicinity of GBs with sub-20 nm resolution. Furthermore, imaging material degradation by tracking local nanodefect distributions overtime identifies a distinct degradation pathway that starts from the GBs and propagates inside the grains over time. The single GB, nano-THz conductivity imaging demonstrated here can be extended to benchmark various perovskite materials and devices for their global photoenergy conversion performance and local charge transfer proprieties of absorbers and interfaces.

Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr3 Perovskite Thin Films

“…61 The determination and quantification of the conductivity with THz-sSNOM are highly beneficial to gain more insight into the electronic structure at the GB. 60 The THz response at topographic grooves like grain boundaries is prone to probe scanning artifacts, 46 which makes correlation with other analytical techniques necessary to confirm the observation of conductivity changes, as conducted in the next section.…”

“…64 As confirmed by the correlation of THz and EDX information, we attribute the increased THz signal at the GBs (Figure 2f) to an increased local THz conductivity from strongly modified electronic characteristics induced by local carrier trapping in deep level states. 60 Trap-assisted recombination, which is a major source of nonradiative recombination, can reduce the quantum efficiency of perovskite optoelectronic devices.…”

“…The S 2 signal peaks right at the trough of the GB with a strength that exceeds that of the average grain level. Literature reports suggest an interfacial conductivity, ,− although there is no consent yet on physical origins and their role in optoelectronic device performance . The determination and quantification of the conductivity with THz-sSNOM are highly beneficial to gain more insight into the electronic structure at the GB .…”

“…However, more work is required to reduce the influence of topology on THz-sSNOM techniques, e.g., by model-based correction with the synchronously measured AFM signal, to enhance the interpretability and quantifiability of the technique. The THz analysis capabilities can be further extended by complementary spectroscopic, , (ultrafast) pump–probe, ,,− and low-temperature THz nanocapabilities. ,, …”

Recent Developments in Terahertz Nanosensors

“…[1,18,19,43] Another exciting area of research in THz nanosensing is the development of THz near-field nanosensors, which can be used to probe the local EM properties of materials with subwavelength spatial resolutions. [44][45][46][47] Combined with the recent developments of THz near-field imaging techniques, [45,[48][49][50][51][52] these sensors have the potential to revolutionize our understanding of complex nanoscale systems, including two-dimensional (2D) materials, by allowing researchers to directly observe quantum dynamics of molecules and mobile carriers in semiconductors. Furthermore, biomolecular THz near-field sensors are particularly promising for biomedical applications, such as detecting diseases and monitoring drug delivery.…”