David Elam

High Efficiency Hybrid Silicon Nanopillar–Polymer Solar Cells

Recently, inorganic/organic hybrid solar cells have been considered as a viable alternative for low-cost photovoltaic devices because the Schottky junction between inorganic and organic materials can be formed employing low temperature processing methods. We present an efficient hybrid solar cell based on highly ordered silicon nanopillars (SiNPs) and poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS). The proposed device is formed by spin coating the organic polymer PEDOT:PSS on a SiNP array fabricated using metal assisted electroless chemical etching process. The characteristics of the hybrid solar cells are investigated as a function of SiNP height. A maximum power conversion efficiency (PCE) of 9.65% has been achieved for an optimized SiNP array hybrid solar cell with nanopillar height of 400 nm, despite the absence of a back surface field enhancement. The effect of an ultrathin atomic layer deposition (ALD), grown aluminum oxide (Al2O3), as a passivation layer (recombination barrier) has also been studied for the enhanced electrical performance of the device. With the inclusion of the ultrathin ALD deposited Al2O3 between the SiNP array textured surface and the PEDOT:PSS layer, the PCE of the fabricated device was observed to increase to 10.56%, which is ∼10% greater than the corresponding device without the Al2O3 layer. The device described herein is considered to be promising toward the realization of a low-cost, high-efficiency inorganic/organic hybrid solar cell.

Ultrathin, flexible organic-inorganic hybrid solar cells based on silicon nanowires and PEDOT:PSS.

Recently, free-standing, ultrathin, single-crystal silicon (c-Si) membranes have attracted considerable attention as a suitable material for low-cost, mechanically flexible electronics. In this paper, we report a promising ultrathin, flexible, hybrid solar cell based on silicon nanowire (SiNW) arrays and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The free-standing, ultrathin c-Si membranes of different thicknesses were produced by KOH etching of double-side-polished silicon wafers for various etching times. The processed free-standing silicon membranes were observed to be mechanically flexible, and in spite of their relatively small thickness, the samples tolerated the different steps of solar cell fabrication, including surface nanotexturization, spin-casting, dielectric film deposition, and metallization. However, in terms of the optical performance, ultrathin c-Si membranes suffer from noticeable transmission losses, especially in the long-wavelength region. We describe the experimental performance of a promising light-trapping scheme in the aforementioned ultrathin c-Si membranes of thicknesses as small as 5.7 μm employing front-surface random SiNW texturization in combination with a back-surface distribution of silver (Ag) nanoparticles (NPs). We report the enhancement of both the short-circuit current density (JSC) and the open-circuit voltage (VOC) that has been achieved in the described devices. Such enhancement is attributable to the plasmonic backscattering effect of the back-surface Ag NPs, which led to an overall 10% increase in the power conversion efficiency (PCE) of the devices compared to similar structures without Ag NPs. A PCE in excess of 6.62% has been achieved in the described devices having a c-Si membrane of thickness 8.6 μm. The described device technology could prove crucial in achieving an efficient, low-cost, mechanically flexible photovoltaic device in the near future.

Radial junction nanopillar arrays textured n-type silicon solar cells

We report the radial p-n junction nanopillar arrays textured n-type silicon solar cells, passivated by ALD deposited Al2O3. Low cost solution processible metal assisted chemical etching (MACE) and spin on dopant (SOD) techniques were used for the fabrication of proposed solar cells. The nanopillar arrays textured surface exhibits the excellent broadband omnidirectional antireflection properties, leading to AM1.5G spectrum weighted reflectivity as low as 4.37%. Despite having the high surface to volume ratio of the nanopillar arrays textured surface, we observed an open circuit voltage (VOC) and the short circuit current density (JSC) as high as 572mV and 29.9mA/cm2 respectively, which leads to the power conversion efficiency (PCE) in excess of 11.30%, for the optimized structure of the solar cells described herein. External quantum efficiency (EQE) measurements conforms the photocurrent enhancement contributed by the enhanced optical absorption in the optimized nanopillar arrays textured surface.

Temperature dependence of ZnO thin film growth and its application to self-powered fabrics

Zinc oxide thin films are becoming increasingly popular for their wide range of properties and the ability to deposit these films on organic substrates for applications such as biotemplating, OLEDs, or deposition on fabrics for functionalization purposes including power generation from the flexing and deformation of wearable textiles. However, since many fabrics and other organic substrates can not survive the typically high temperatures of thin film growth, it is important to characterize and understand the dependence of the properties of these films as a function of temperature deposition and subsequent thermal treatments. We report on the properties of zinc oxide thin films grown by Atomic Layer Deposition (ALD) and the dependence of surface roughness, film stress, surface energy and crystalline structure on deposition temperature.

Pulsed laser deposition (PLD) employed in the fabrication of low temperature barium titanate (BTO) films on carbon fibers

In order to explore the functionalization of wearable fabrics for power generation, barium titanate (BTO) was deposited on nickel tape and on carbon fiber fabric employing pulsed laser deposition (PLD) methods under conditions normally not considered ideal to produce optimum ferroelectric properties, namely, at temperatures as low as 100 °C and under various oxygen partial pressures. The remnant charge polarization and film resistance properties were evaluated to determine the effect of the aforementioned deposition conditions. The C-V characterization indicates that the BTO films still retained ferroelectric properties in films produced at temperature as low as 200 °C, below which, the behavior was only paraelectric.