Craig M. Johnson

Slow-light-enhanced upconversion for photovoltaic applications in one-dimensional photonic crystals.

We present an approach to realizing enhanced upconversion efficiency in erbium (Er)-doped photonic crystals. Slow-light-mode pumping of the first Er excited state transition can result in enhanced emission from higher-energy levels that may lead to finite subbandgap external quantum efficiency in crystalline silicon solar cells. Using a straightforward electromagnetic model, we calculate potential field enhancements of more than 18× within he slow-light mode of a one-dimensional photonic crystal and discuss design trade-offs and considerations for photovoltaics.

Passivation effects in B doped self-assembled Si nanocrystals

Doping of semiconductor nanocrystals has enabled their widespread technological application in optoelectronics and micro/nano-electronics. In this work, boron-doped self-assembled silicon nanocrystal samples have been grown and characterised using Electron Spin Resonance and photoluminescence spectroscopy. The passivation effects of boron on the interface dangling bonds have been investigated. Addition of boron dopants is found to compensate the active dangling bonds at the interface, and this is confirmed by an increase in photoluminescence intensity. Further addition of dopants is found to reduce the photoluminescence intensity by decreasing the minority carrier lifetime as a result of the increased number of non-radiative processes.

Limiting efficiency of generalized realistic c-Si solar cells coupled to ideal up-converters

The detailed balance model of photovoltaic up-conversion is revised for the specific case of a c-Si solar cell under the AM1.5G solar spectrum. The limiting efficiency of an ideal solar cell with a band gap of 1.117 eV may be increased from approximately 33% to 40% with ideal up-conversion. However, real solar cells do not demonstrate the step-function absorption characteristic assumed in the standard detailed balance model. Here, we use tabulated Si refractive index data to develop a generalized model of a realistic conventional c-Si solar cell. The model incorporates optical design and material parameters such as free carrier absorption that have a non-trivial impact on the operation of the up-conversion layer. While these modifications are shown to decrease the absolute limiting efficiency, the benefit of up-conversion is shown to be relatively greater.

Limiting Efficiency of Erbium-Based Up-Conversion for Generalized Realistic c-Si Solar Cells

Up-conversion of sub-band-gap near-infrared light to absorbable wavelengths has been suggested as a means of greatly improving the efficiency of standard silicon solar cells. Most theoretical assessments of the potential of such designs have utilized a detailed-balance approach that invokes vastly idealized models of both the solar cell and the up-converting layers. In this study, we consider the limiting efficiency of an up-converting photovoltaic device based on more realistic models for the silicon solar cell and an up-conversion layer approximating the performance of an erbium-doped phosphor. While the generalized silicon cell has been previously shown to demonstrate good performance (a relative efficiency enhancement of 23%-26%) when coupled to an ideal up-converter, the modifications considered here result in drastically diminished device performance overall (a maximum relative enhancement of about 7%). This reduction may be compensated to some extent by the sensitization of erbium absorption by the addition of a suitable species to the up-converting layer; in this case, the relative limiting efficiency is pushed back up to 15%. Based on these results, we draw conclusions regarding device engineering that should guide future work in this area.

Enhanced Broadband Light Trapping in c-Si Solar Cells Using Nanosphere-Embedded Metallic Grating Structure

A hexagonal nanosphere (NS)-embedded back plasmonic grating structure is proposed to improve the light absorption of crystalline silicon (c-Si) solar cells. These structures are simple, can be deposited toward the final stages of device processing, and involve no increase in the surface area of the semiconductor layer. Experimental fabrication of this structure on a 200-μm c-Si wafer has been realized using silver, resulting in broadband absorption enhancement in the near-infrared region. In corresponding to the optical measurement, a maximum potential photocurrent density enhancement of around 2.23 mA/cm2 has been predicted, compared with the reference with a planar metal reflector. Three-dimensional finite-element method numerical simulations were also performed on smooth and irregular surface geometries of the NSs. Our results demonstrate that while both configurations perform better than a planar reflector, the irregular nanofeatures on the gratings can adjust the optical resonance to ensure that the light is more efficiently scattered into the Si, which would significantly improve its optical absorption. These structures have the potential to be used as rear metal contacts, in addition to performing the function of a light-trapping layer without increasing the fraction of the metal component.

Empirical and Quokka simulated evidence for enhanced V OC due to limited junction area for high efficiency silicon solar cells

A new design for high efficiency silicon solar cells with the use of limited p-n junction area and point Al rear contact is proposed. In this work, planar test devices with different emitter area and spacing are fabricated to demonstrate the limited junction area (LIA) design can improve VOC for a well-designed cell structure. Interim lifetime and PL measurements during fabrication confirm lower dark saturation current (I0) and higher implied VOC (iVOC) in the LIA samples with a robust surface passivation scheme. Higher VOC and efficiency are also simulated with Quokka on some LIA devices compared to the full area emitter control sample.

Porous Silicon Omnidirectional Bragg Reflector for Si Solar Cells

A Bragg reflector model based on multiple porous silicon layers is proposed to address the poor absorption of Si at longer wavelength and also to address the issue of absorption from metal back reflectors.

Plasmonic rear reflectors for thin-film solar cells: design principles from electromagnetic modelling

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/cm2 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.

Hot carrier solar cells from group III-V quantum well structures

To circumvent Shockley-Queisser Limit whilst utilizing thin film deposition, we intend construction of a hot carrier solar cell (HCSC). This device would challenge a fundamental assumption of Shockley-Queisser: that all energy of incoming photons in excess of the acceptance threshold of the cell material is lost as heat. If “excess” energy charge carriers are tapped before they thermalize with the matrix, theoretical cell efficiency (66%) under one sun is twice that of a single-junction silicon cell. In this pursuit, two principal tasks await: actual retardation of carrier thermalization by preventing the decay of accompanying optical phonons, and collection of the carriers via devices known as “Energy Selective Contacts” (ESCs), which withdraw only carriers possessing a narrow range of energies to prevent entropic losses. We propose construction of a Hot Carrier Solar Cell utilizing elemental group III Nitrides for ESC and absorber. Indium Nitride, with its large phononic band gap and small electronic band gap, can provide a suitable absorber, whereas alloys of In(x)Ga(1-x)N can form complementary and lattice-matched ESCs.

Up-conversion luminescence enhancement in erbium-doped porous silicon photonic crystals for photovoltaics

Solar cell efficiency can be increased by adding a rear layer that captures unabsorbed low-energy photons and combines their energy to emit higher-energy photons. This concept has been demonstrated for silicon solar cells using erbium-doped phosphors. Here we investigate the possibility of enhancing intra-4f up-conversion processes within band-edge slow light modes in photonic crystals. We discuss the potential efficiency enhancement realizable one-dimensional erbium-doped porous silicon photonic crystals and present preliminary investigations into these interactions in a real structure.