Correlations of Optical Absorption, Charge Trapping, and Surface Roughness of TiO₂Photoanode Layer Loaded with Neat Ag-NPs for Efficient Perovskite Solar Cells

Abstract

We systematically investigated the effect of silver nanoparticles (Ag-NPs) on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Neat, spherical Ag-NPs at loading levels of 0.0, 0.5, 1.0, and 2.0 wt % were embedded into the titanium dioxide (TiO2) photoanode layer. The plasmonic effect of the Ag-NPs strongly enhanced the incident light absorption over a wide range of the visible wavelength region in addition to the inherent absorbance of the perovskite sensitizer. The low conduction energy level of the Ag-NPs compared to that of TiO2 provides trap sites for free charge carriers. Thus, the correlation between the enhancement of the optical absorption and the number of charge traps provided by the Ag-NPs is critical to determine the device performance, especially current density (Jsc) and PCE. This is confirmed by the quantitative comparison of the incident light absorption and the time-resolved photoluminescence decay according to the loading levels of the Ag-NPs in the TiO2 layer. The absorption enhancement from 380 to 750 nm in the UV-visible spectrum is proportional to the increase in the loading levels of the Ag-NPs. However, the Jsc increases with the device with 0.5 wt % Ag-NPs and gradually decreases with increases in the loading level above 0.5 wt % because of the different contributions to the absorbance and the charge trapping by different Ag-NP loading levels. In addition, the suppression of the surface roughness with dense packing by the Ag-NPs helps to improve the Jsc and the following PCE. Consequently, the PCE of the PSC with 0.5 wt % Ag-NPs is increased to 11.96%. These results are attributed to the balance between increased absorbance by the localized surface plasmon resonance and the decreased charge trapping as well as the decreased surface roughness of the TiO2 layer with the Ag-NPs.

Plasmonics in Organic and Perovskite Solar Cells: Optical and Electrical Effects

“…However, recent demonstrations in the literature have surprisingly been related to optical enhancements, such as enhanced light absorption with corresponding increase in EQE as well as enhanced photon recycling . There is the potential for morphological and electrical effects contributing to improved device performance. Of interest, the electrical effects include enhanced charge collection, increased exciton dissociation, improved internal resistance and hot electron transfer .…”

“…Depending on the plasmonic nanostructure and device structure employed, it can be necessary to optimize the integration of plasmonic nanostructures into perovskite solar cells. As shown in Figure , the optimum device performance is achieved when the increase in light absorption is balanced against an increase in charge trapping . It is interesting to note that “plasmon‐electrical effects”, which have been demonstrated in OPV devices, are not currently investigated in perovskite solar cells, where the effect can find use in thicker absorber layers that have relatively less charge extraction efficiency, or to boost light absorption and carrier extraction concurrently in thinner absorber layers .…”

“…The highest PCE achieved was 17.7%, which is impressive considering the relatively thin (240 nm) perovskite absorber layer. Recently, Yang et al investigated the effects of Ag NPs that were embedded into a TiO 2 photoanode layer of perovskite solar cells . In the study, it was found that the surface roughness of the photoanode layer was reduced due to the dense packing of the photoanode resulting from electrostatic interactions between negatively charged TiO 2 and positively charged Ag NPs.…”

“…AFM topography images of TiO 2 photoanode layers with Ag NPs loading levels (in wt %) of a) 0.0, b) 0.5, c) 1.0, and d) 2.0. Reproduced with permission . Copyright 2016, American Chemical Society.…”

To Be Higher and Stronger—Metal Oxide Electron Transport Materials for Perovskite Solar Cells

“…Au decorated TiO 2 could enhance the light absorption and reduce the charge recombination in PSCs owing to the strong near‐field effects of localized surface plasmon resonance (LSPR) and related spatial charge separation, thus improving the J sc and FF ( Figure a,b) . The enhancement of J sc was also observed in Ag embedded TiO 2 based PSCs . It is found that proper loading level of Ag could increase the absorbance in the long wavelength region and charge transport capability for the better device performance.…”