Nicholas P. Sergeant

Design of Transparent Anodes for Resonant Cavity Enhanced Light Harvesting in Organic Solar Cells

Organic solar cells offer an attractive approach to low-cost solar energy conversion, due to a combination of abundant materials and high throughput fabrication processes.[1] However, organic semiconductors suffer from short exciton diffusion lengths and low charge carrier mobility, which necessitates the use of thin photoactive films and intercalated networks of donor and acceptor molecules in a so-called bulk heterojunction.[2,3] On the other hand, thinner photoactive regions in organic solar cells cause a reduction in optical absorption. This leads to the well known tradeoff in organic solar cells between internal quantum efficiency (IQE) and absorption efficiency. Several approaches have been previously used to enhance absorption in organic solar cells,[4] including plasmonics,[5] photonic crystal approaches[6] and external coatings.[7] Moreover, in recent years significant research efforts have been directed towards replacing the tin-doped indium oxide (ITO) transparent electrode in optoelectronic devices, owing to its poor mechanical flexibility, the necessary performance enhancing thermal treatment unsuitable for low temperature substrates, as well as the increasing cost of indium.[8] Potential alternative transparent conductors include high conductivity polymers,[9] unpatterned metal films,[10] patterned metal grids,[11] random metal nanowire meshes,[12] graphene,[13] and random carbon nanotube meshes.[14] In addition, several groups have shown promising results for tri-layer dielectricmetal-dielectric electrodes.[15–18] Here, building on the latter approach, a tri-layer electrode is proposed composed of a thin film of silver (Ag) sandwiched between two layers of molybdenum trioxide (MoO3). The MoO3/Ag/MoO3 transparent electrode is ITO-free, compatible with low-temperature substrates, and capable of alleviating the absorption-IQE trade-off by creating a resonant optical cavity to coherently trap light in the photoactive absorber. The thin Ag film dominates the lateral conductive properties of the electrode and therefore provides a means to obtain a sheet resistance below 10 Ω per square. Silver is known to prefer 3D island growth and therefore the percolation threshold of Ag layers

Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks

Spectral control of the emissivity of surfaces is essential in applications such as solar thermal and thermophotovoltaic energy conversion in order to achieve the highest conversion efficiencies possible. We investigated the spectral performance of planar aperiodic metal-dielectric multilayer coatings for these applications. The response of the coatings was optimized for a target operational temperature using needle-optimization based on a transfer matrix approach. Excellent spectral selectivity was achieved over a wide angular range. These aperiodic metal-dielectric stacks have the potential to significantly increase the efficiency of thermophotovoltaic and solar thermal conversion systems. Optimal coatings for concentrated solar thermal conversion were modeled to have a thermal emissivity <7% at 720K while absorbing >94% of the incident light. In addition, optimized coatings for solar thermophotovoltaic applications were modeled to have thermal emissivity <16% at 1750K while absorbing >85% of the concentrated solar radiation.

Three-dimensional self-assembled photonic crystals with high temperature stability for thermal emission modification

Selective thermal emission in a useful range of energies from a material operating at high temperatures is required for effective solar thermophotovoltaic energy conversion. Three-dimensional metallic photonic crystals can exhibit spectral emissivity that is modified compared with the emissivity of unstructured metals, resulting in an emission spectrum useful for solar thermophotovoltaics. However, retention of the three-dimensional mesostructure at high temperatures remains a significant challenge. Here we utilize self-assembled templates to fabricate high-quality tungsten photonic crystals that demonstrate unprecedented thermal stability up to at least 1,400 °C and modified thermal emission at solar thermophotovoltaic operating temperatures. We also obtain comparable thermal and optical results using a photonic crystal comprising a previously unstudied material, hafnium diboride, suggesting that refractory metallic ceramic materials are viable candidates for photonic crystal-based solar thermophotovoltaic devices and should be more extensively studied.

High performance solar-selective absorbers using coated sub-wavelength gratings

Spectral control of the emissivity of surfaces is essential for efficient conversion of solar radiation into heat. We investigated surfaces consisting of sub-wavelength V-groove gratings coated with aperiodic metal-dielectric stacks. The spectral behavior of the coated gratings was modeled using rigorous coupled-wave analysis (RCWA). The proposed absorber coatings combine impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. The aspect ratio of the V-groove can be tailored in order to obtain the desired spectral selectivity over a wide angular range. Coated V-groove gratings with optimal aspect ratio are predicted to have thermal emissivity below 6% at 720K while absorbing >94% of the incident light. These sub-wavelength gratings would have the potential to significantly increase the efficiency of concentrated solar thermal systems.

Design of selective coatings for solar thermal applications using sub- wavelength metal-dielectric structures

Spectral control of the emissivity of surfaces is essential in applications such as solar thermal energy and thermophotovoltaic energy conversion in order to achieve the highest conversion efficiencies possible. We investigated surfaces consisting of periodic, nanoscale V-grooves coated with aperiodic metal-dielectric stacks. This approach combines impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. We explain how changes in the angle of the V-grooves can be used to tailor the spectral selectivity over a wide angular range to significantly increase the efficiency of thermophotovoltaic and solar thermal systems. Optimal coatings for concentrated solar power are predicted to have thermal emissivity below 5% at 450°C while absorbing >90% of the incident light.

Resonant cavity enhanced light harvesting in flexible thin-film organic solar cells

Dielectric/metal/dielectric (DMD) electrodes have the potential to significantly increase the absorption efficiency and photocurrent in flexible organic solar cells. We demonstrate that this enhancement is attributed to a broadband cavity resonance. Silver-based semitransparent DMD electrodes with sheet resistances below 10 ohm/sq. are fabricated on flexible polyethylene terephthalate (PET) substrates in a high-throughput roll-to-roll sputtering tool. We carefully study the effect of the semitransparent DMD electrode (here composed of Zn(x)Sn(y)O(z)/Ag/In(x)Sn(y)O(z)) on the optical device performance of a copper phthalocyanine (CuPc)/fullerene (C60) bilayer cell and illustrate that a resonant cavity enhanced light trapping effect dominates the optical behavior of the device.

Enhancing far-field thermal emission with thermal extraction

We show that the thermal emission of a finite-size blackbody emitter can be enhanced. We experimentally observe a four-fold enhancement of the far-field thermal emission of a carbonblack emitter having an emissivity of 0.85.