We demonstrate optical and electrical property enhancement of solar cells using a variety of dielectric nano-resonator array coatings. First, we study close-packed silicon dioxide (SiO<sub>2</sub>) nano-resonator arrays on top of silicon (Si) and gallium arsenide (GaAs) solar cells. From macroscale measurements and calculations, we find that absorptivity of solar cells can be improved by 20 % due to the resonant couplings of excited whispering gallery modes and the thin-film antireflection effect. Next, we image photocurrent enhancement at the nanoscale via near-field scanning photocurrent microscopy (NSPM). Strong local photocurrent enhancement is observed over each nano-resonator at wavelengths corresponding to the whispering gallery mode excitation. Finally, for better optical coupling to solar cells, we explore hybrid nano-resonator arrays combining multiple materials such as silicon dioxide, silicon nitride, and titanium dioxide. Due to higher number of photonic modes within such hybrid coatings, absorptivity is enhanced by more than 30 % in a Si solar cell.
Photocurrent generation of methylammonium lead iodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) hybrid perovskite solar cells is observed at the nanoscale using near-field scanning photocurrent microscopy (NSPM). We examine how the spatial map of photocurrent at individual grains or grain boundaries is affected either by sample post-annealing temperature or by extended light illumination. For NSPM measurements, we use a tapered fiber with an output opening of 200 nm in the Cr/Au cladded metal coating attached to a tuning fork-based atomic force microscopy (AFM) probe. Increased photocurrent is observed at grain boundaries of perovskite solar cells annealed at moderate temperature (100 °C); however, the opposite spatial pattern (i.e., increased photocurrent generation at grain interiors) is observed in samples annealed at higher temperature (130 °C). Combining NSPM results with other macro-/microscale characterization techniques including electron microscopy, x-ray diffraction, and other electrical property measurements, we suggest that such spatial patterns are caused by material inhomogeneity, dynamics of lead iodide segregation, and defect passivation. Finally, we discuss the degradation mechanism of perovskite solar cells under extended light illumination, which is related to further segregation of lead iodide.
Relative surface topography of metal electrodes is one of the subtle issues determining the electrical performance of molecular devices. Systematic conductivity measurements of nanoscale junctions containing self-assembled monolayer of conjugated molecules are reported for a variety of metal electrodes. The monolayer is assembled on 25-100 nm electrode. Another 10-100 nm electrode is defined on top of the monolayer by metal evaporation. The characteristic energy scales are determined from the temperature dependence of conductance and from the non-linear current-voltage characteristics. Unexpectedly, the energy scales of the dominant conductance channels are small in comparison with the molecular level spacing. In all cases, the dominant room temperature conductance is hopping with characteristic energy of the order of 10-100 meV determined by the nature of metal contacts. Relative contribution of tunneling conductance strongly depends on the surface topography of the metal electrodes.