Rear reflectors for solar cells comprised of metal films with periodic arrays of nanoscale features on their surface can provide significantly enhanced light trapping in the absorber layer. However these structures can also result in significantly increased parasitic absorption into the metal layer at various wavelengths of light. Conversely these highly absorbing resonances can also coincide with the wavelengths which display the largest enhancement to the cell’s photocurrent. As such it is important to understand the underlying causes for such photocurrent enhancements and losses in the metal in order to design the optimum structure for use. 3D Finite-difference-time-domain simulations have been used to model a variety of structures and analyze the spatial distribution of absorption within different materials which make up the structure, the angles at which light will be scattered from the rear surface, as well as the idealized short circuit current from each structure integrated across the AM1.5 spectrum. These reveal the properties of these modes at resonant wavelengths at which absorption into both materials is enhanced. Despite the enhanced coupling of light into the metal at these wavelengths, the amount of light scattered back into the absorber at large angles is also significantly boosted. For a large variety of geometries, the impact of this large angle scattering dominates leading to significant increases to a cell’s photocurrent. Our simulations allow us to understand the contributions of multiple plasmonic effects occurring in such structures, allowing selection of the most suitable geometries to achieve large-angle scattering in a desired wavelength range.
Over the last decade, plasmonic nanoparticle arrays have been extensively studied for their light trapping potential in thin film solar cells. However, the commercial use of such arrays has been limited by complex and expensive fabrication techniques such as e-beam lithography. Nanosphere lithography (NSL) is a promising low-cost alternative for forming regular arrays of nanoscale features. Here, we use finite-difference time-domain (FDTD) simulations to determine the optical enhancement due to nanosphere arrays embedded at the rear of a complete thin film device. Array parameters including the nanosphere pitch and diameter are explored, with the FDTD model itself first validated by comparing simulations of Ag nanodisc arrays with optical measurements of pre-existing e-beam fabricated test structures. These results are used to guide the development of a nanosphere back-reflector for 20 μm thin crystalline silicon cells. The deposition of polystyrene nanosphere monolayers is optimized to provide uniform arrays, which are subsequently incorporated into preliminary, proof of concept device structures. Absorption and photoluminescence measurements clearly demonstrate the potential of nanosphere arrays for improving the optical response of a solar cell using economical and scalable methods.
The use of plasmonic structures to enhance light trapping in solar cells has recently been the focus of significant research, but these structures can be sensitive to various design parameters or require complicated fabrication processes. Nanosphere lithography can produce regular arrays of nanoscale features which could enhance absorption of light into thin films such as those used in novel solar cell designs. Finite-difference-time-domain simulations are used to model a variety of structures producible by this technique and compare them against the use of mirrors as rear reflectors. Through analysis of these simulations, sensitivity of device performance to parameters has been investigated. Variables considered include the feature size and array period, as well as metal and absorber materials selection and thickness. Improvements in idealized photocurrent density are calculated relative to the use of rear mirrors that are a standard for solar cells. The maximum simulated increase to photocurrent density was 3.58mA/cm<sup>2</sup> or 21.61% for a 2μm thick Si cell relative to the case where a silver mirror is used as a rear reflector. From this, an initial set of design principles for such structures are developed and some avenues for further investigation are identified.
Metal nanoparticles are known for their unique optical properties due to surface plasmon excitations. The far field and
near field effects from these metal particles have been captured to enhance efficiency of thin film solar cells by way of
light trapping. Our group has extensively studied the different design parameters for a plasmon enhanced solar cell like
effect of metal size/shape, location, effect of dielectric layer thickness and also the effect of plasmons on the electrical
properties like passivation of cells. Whilst identifying and minimising parasitic absorption losses in these metal particles
are important and is attracting lot of attention, we choose to look at a more interesting issue of ageing effects. Plasmonics
at the moment promises efficiency enhancements exceeding 30% including associated losses in metals. However as is
needed for solar cells, the technologies incorporated have to stand the test of time. In this work we look at the age effects
on the plasmon performance by analysing our cells over time. Our preliminary results show that plasmons supported by
silver metal nanoparticles can degrade by upto 10% with time. Metal nanoparticles when exposed to air can get tarnished
easily causing degradation of the plasmonic properties. This will result in weakening of the scattering and reduce light
trapping effects. We also look at ways of minimizing the ageing losses by overcoating. MgF<sub>2</sub> is used as the dielectric
film to overcoat metal nanoparticles preventing degradation and also to isolate MNP layer from the back surface
reflector of cells. Our results show that such a rear scheme brings an additional current enhancement over interested
wavelength region improving over time.
Scattering from metal nanoparticles via excitation of surface plasmon (SP) resonances has the potential to dramatically increase the emission of light-emitting devices. A further redshift in the plasmon resonance is possible by overcoating the metal nanoparticles with a high refractive index medium. In this paper we report a red shift in the emission enhancement peak from Silicon on Insulator (SOI) light emitting diodes (LEDs) by overcoating the metal particles with ZnS, as determined by the electroluminescence (EL) spectra. We demonstrate a 7 fold increase in the electroluminescence at 970nm with an evident redshift from 900nm for the uncoated case.
Localized surface plasmons on metallic nanoparticles can be surprisingly efficient at coupling light into or out of a silicon waveguide. We have previously reported a factor of 7 times enhancement in the electroluminescence from a silicon-on-insulator light-emitting diode with silver nanoparticles at a wavelength of 930nm. In this paper we model the scattering enhancement for metal particles on a silicon-on-insulator substrate and show that the shape of the spectrum is well predicted using the scattering cross-section and angular dependence of emission of an ideal dipole on a layered
substrate. This indicates that the scattering and absorption enhancement at long wavelengths is mainly a single-particle effect, in contrast to previous suggestions that it is a waveguide-mediated multi-particle effect. In particular we show that the particle-waveguide interaction leads to a dramatic enhancement of scattered light at long wavelengths compared with the light scattered by metal islands on glass.