Er-Chien Wang

Resonant enhancement of dielectric and metal nanoparticle arrays for light trapping in solar cells

We numerically investigate the light trapping properties of two-dimensional diffraction gratings formed from silver disks or titanium dioxide pillars, placed on the rear of Si thin-film solar cells. In contrast to previous studies of front-surface gratings, we find that metal particles out-perform dielelectric ones when placed on the rear of the cell. By optimizing the grating geometry and the position of a planar reflector, we predict short circuit current enhancements of 45% and 67% respectively for the TiO₂ and silver nanoparticles. Furthermore, we show that interference effects between the grating and reflector can significantly enhance, or suppress, the light trapping performance. This demonstrates the critical importance of optimizing the reflector as an integral part of the light trapping structure.

Effect of Nanoparticle Size Distribution on the Performance of Plasmonic Thin-Film Solar Cells: Monodisperse Versus Multidisperse Arrays

The effect of the silver nanoparticle size distribution on the performance of plasmonic polycrystalline Si thin-film solar cells is studied. Monodisperse particle arrays are fabricated using nanoimprint lithography. Multidispersed particle arrays are fabricated using thermal evaporation followed by annealing. The short-circuit current enhancement for the cells without a back reflector is 24% and 18% with the multidisperse array and the monodispersed array, respectively. For the cells with a back reflector, the current enhancement increases to 34% and 30%, respectively, compared with 13% enhancement due to the reflector alone. Better performance of multidisperse Ag nanoparticle arrays is attributed to a broader scattering cross section of the array owing to a broad particle size distribution and a higher nanoparticle coverage.

Rounded rear pyramidal texture for high efficiency silicon solar cells

Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies in the range 24.4%-25.6% As high as these efficiencies are, there are opportunities to increase them further by improving on the light trapping. Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping than cells with texture on just the front. One of the principle losses of double-sided pyramidal texture is the light that escapes after a second pass through the cell when the facet angles are the same on the front and rear. This contribution investigates how this loss might be reduced by changing the facet angle of the rear pyramids. A textured pyramid rounding is introduced to improve the light trapping. The reduction in surface recombination that rounding the facets introduces is also evaluated. With confocal microscopy, spectrophotometry and ray tracing, the rounding etch time required to yield the best light trapping is investigated. With photoconductance lifetime measurements, the surface recombination is found to continue to decrease as the rounding time increases. The spectrophotometry and ray tracing suggests that the double sided textured samples featuring rounded rear pyramids have superior light trapping to the sample with a planar rear surface. The high-efficiency potential of rounded textured pyramids in silicon solar cells is demonstrated by the fabrication of 24% efficient back-contact silicon solar cells.

Contact Resistivity of Evaporated Al Contacts for Silicon Solar Cells

The contact resistivity of evaporated Al on doped silicon is examined for a range of process conditions common to the fabrication of laboratory silicon solar cells. The effects of silicon surface preparation prior to evaporation, sintering temperature, the use of a shutter, and evaporation power are investigated. The presented evaporation conditions yielded the lowest published contact resistivity between Al- and phosphorus-doped Si over a large range of doping concentration. It is also demonstrated that a contact resistivity below 10-6 Ω·cm2 can be achieved without sintering. Three-dimensional simulations are utilized to compare the obtained results for evaporated Al contacts with those for passivated contacts.

Etch-back simplifies interdigitated back contact solar cells

The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel simplified etch-back technique, while aiming for no compromise on high-efficiency potentials. Simplified etch-back creates localized heavy and light phosphorus and boron diffusions simultaneously. This process also leaves localised heavy diffusions to be approximately a micron higher than neighbouring light diffusion regions. In comparison to the IBC solar cells that ANU developed to date [1], key advantages of this technique feature reduction in cell process steps; requires only two diffusions to create p, p+, n and n+ diffusions; no high-temperature oxidation masking steps required as diffusion barriers; independent optimization of contact recombination, lateral carriers transport and surface passivation; and potential higher silicon bulk lifetime and reduced contamination due to low thermal budget. Based on the etch-back technique, the total saturation current density deduced from the test structures for the IBC cell is below 30 fA/cm2.

High-resolution photocurrent imaging of light trapping by plasmonic nanoparticles on thin film Si solar cells

We study Ag nanoparticles on thin-film Si solar cells using reflection and photocurrent mapping. We observe increased reflectance at wavelengths below the plasmon resonance and increased absorption at longer wavelengths due to light-trapping effects.