Nano-scale near-field imaging for solar cell coatings
A solar cell is a device that directly converts light energy into electrical energy through a photoelectric effect or a photochemical effect, wherein a crystalline silicon solar cell operating with a photoelectric effect is the mainstream. Although the efficiency of the crystalline silicon solar cell can be improved by doping and surface covering the anti-light reflective layer, the light reflection exceeding the band gap and some specific wavelengths causes a huge loss of optical energy, which in turn limits the efficiency of the crystalline silicon solar cell. YH Wang et al. used organometallic tribromide nanoparticles (CH3NH3PbBr3) to absorb some of the short-wavelength sunlight and convert it into a polarized electric field. The polarized electric field can enhance the asymmetry of the organic-crystalline silicon heterojunction solar cell by promoting molecular rearrangement, thereby increasing the surface active carrier density, and finally the efficiency of the organic-crystalline silicon heterojunction solar cell is from 12.7. % increased to 14.3%.
Professor QL Bao from Suzhou University and others have made outstanding contributions in the fields of photoelectric conversion ion transport behavior and carrier concentration distribution of perovskite structure micro/nanowires. In 2016, in the study of the photoelectric conversion ion transport behavior of perovskite-structured micro-nanowires published on ACS Nano, the authors used neaspec's near-field optical microscope neaSNOM to find out: 1. When no external field voltage is applied, the micro-nanowire The carrier density (Fig. 1 g. s-SNOM amplitude signal) and the light refractive index (Fig. 1 g. s-SNOM phase signal) are relatively uniform in the region; 2. When the external field positive voltage is applied, the carrier density in this region follows The migration of I-ion (Br−) to the right (Fig. 1 h. s-SNOM amplitude signal), its refractive index shifts to the left with the migration of MA+ ions (CH3NH3+) (Fig. 1 g. s-SNOM The phase signal is relatively uniform; 3. When the external field negative pressure is applied, the situation is exactly the opposite of when a positive voltage is applied (Fig. 1 i). This study shows that understanding the ion transport behavior in inorganic-organic perovskite structures is important for understanding the special photoelectric behavior of perovskite-based structures, and thus lays a solid foundation for the application of optoelectronic devices for inorganic-organic perovskite materials.
Figure 1. SNOM measures the ion transport behavior of the photoelectric conversion of perovskite-structured micro-nanowires.
Df. Schematic diagram of ion mobility measurement; gi, corresponding s-SNOM optical signal amplitude and phase diagram
In 2017, Prof. QL Bao and others published in the AdvanceMaterials article again using neaspec's near-field optical microscope neaSNOM to study the carrier density of perovskite nanoparticle coatings on solar cell surfaces for the first time. The results show that the near-field signal intensity of the perovskite nanoparticle coverage area is higher than that of the Si/SiO2 region (see Figure 2b below; Figure 2a shows the morphology of the corresponding region). In addition, the authors also studied the effect of increasing the time of illumination (see Figure 2 c, d below). The results show that the near-field signal intensity increases with light time from 12.5 μV (yellow, 0 min) to 14.4 μV (red, 60 min), and the near-field signal reflects the change in the movable free carrier density. Finally, the results of infrared neaSNOM study show that the perovskite nanoparticle coating on the surface of the solar cell enriches and captures a large amount of electrons as the illumination time increases.
Figure 2. SNOM measures the carrier density of a perovskite nanoparticle coating. a. AFM topography; b, s-SNOM optical signal diagram - no illumination;
c, s-SNOM optical signal diagram - illumination 30min; d, s-SNOM optical signal diagram - illumination 60min
The authors foresee that this research is of great significance for designing new solar cells and improving their conversion efficiency. At the same time, the research also proposes a research method that combines perovskite structural materials with crystalline silicon solar cells, and provides new ideas for later research and application.
Related references
1.Zhang YP; et. al. Reversible Structural Swell−Shrink and Recoverable Optical Properties in Hybrid norganic−OrganicPerovskite. ACS Nano 2016,10, 7031−7038.
2.Wang YH; et. al. The Light-InducedField-Effect Solar Cell Concept - Perovskite Nanoparticle Coating Introduces Polarization Enhancing Silicon Cell Efficiency. AdvancedMaterial 2017, First published: 3 March 2017; DOI: 10.1002/adma.201606370.
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