Nanoparticles and nanostructures with plasmonic resonances are currently being employed to
enhance the efficiency of solar cells. Ag stripe arrays have been shown theoretically to enhance the
short-circuit current of thin silicon layers. Such Ag stripes are combined with 200 nm long and 60
nm wide “teeth”, which act as nanoantennas, and form vertical rectifying metal-insulator-metal
(MIM) nanostructures on metallic substrates coated with thin oxides, such as Nb/NbOx films. We
characterize experimentally and theoretically the visible and near-infrared spectra of these “stripeteeth”
arrays, which act as microantenna arrays for energy harvesting and detection, on silicon
substrates. Modeling the stripe-teeth arrays predicts a substantial net a.c. voltage across the MIM
diode, even when the stripe-teeth microrectenna arrays are illuminated at normal incidence.
The ability to grow large-area, large-grained polycrystalline silicon on inexpensive substrates is becoming increasingly important for photovoltaic (PV) devices. With large-grained (grain size <10 μm) 10 μm thick films it is possible with light trapping to achieve PV efficiencies exceeding 15%. If crystallites could be nucleated and grown for longer times before native nucleation occurs, then potentially these much larger grain, thin film silicon material could be produced.The interaction of sub-crystallization threshold laser fluence with hydrogenated amorphous silicon (a-Si:H) has been demonstrated on a macroscopic scale to shorten the incubation time in subsequently thermally annealed films. Further examination of crystallite laser nucleation, found that nucleation was suppressed around PECVD a-Si:H thin film(50-100nm) sample edges, and scratches, in addition to laser-ablated areas, extending as much as 100-200 μm laterally from these features. Optical microscopy and stepwise high temperature thermal annealing were used to investigate this behavior for the a-Si:H films deposited on glass substrates. The nucleation rates were measured in the treated and untreated regions. The data suggests that these features (edges, scratches, and laser ablated areas) provide stress relief by interrupting the surface connectivity. We confirm the existence of stress and stress relief by μ-Raman measurements of the crystallite transverse optical peak position relative to that of c-Si. PECVD films were annealed at temperatures between 540-600C, to enable a determination of rn at each anneal temperature. The temperature dependent measurements enabled the determination of the nucleation rate activation energies (EA), and how they are affected by film stress.
Arrays of "nanorectennas" consist of diode-coupled nanoantennas with plasmonic resonances in the visible/near-infrared
(vis/nir) regime, and are expected to convert vis/nir radiative power into useful direct current. We study plasmonic
resonances in large format (~ 1 mm2 area) arrays, consisting of electron beam-patterned horizontal (e.g., parallel to the substrate) Ag lines patterned on ultrathin (< 20 nm) tunneling barriers (NiO, NbOx, and other oxides). Our e-beam fabrication technique is scalable to large dimensions, and allows us to easily probe different antenna dimensions. These
tunneling barriers, located on a metallic ground plane, rectify the alternating current generated in the nanoantenna at
resonance. We measure the plasmonic resonances in these nanoantennas, and find good agreement with modeling,
which also predicts that the electric field driving the electrons into the ground plane (and therefore the rectification
efficiency) is considerably enhanced at resonance. Various metal-insulator-metal tunneling diodes, incorporating the
afore-mentioned barrier layers and different metals for the ground plane, are experimentally characterized and compared
to our conduction model. We observe ~ 1 mV signals from NiO-based nanorectenna arrays illuminated by 532 nm and
1064 nm laser pulses, and discuss the origin of these signals.
Next-generation photovoltaic structures require well-established deposition routes to conformal and
conducting materials with defined chemical, physical and electronic composition. This work reports on
the preliminary findings associated with conformal metal oxides on structured substrates including:
1) Discovery of sputtering process conditions that can be made semi-conformal when combined with
in-situ techniques such as ion-beam milling for honing surface structures;
2) Development of relevant ALD chemistries that are materials-properties competitive with sputtered
3) Evaluation of chemically-functionalized surface structures that maximize surface area but are
structurally tailored for efficient gas flow and to minimize line-of-sight shadowing.
The initial experiments have centered on combinations of amorphous and crystalline indium oxide,
zinc oxide, aluminum zinc oxide, indium tin oxide, fluorinated tin oxide and indium zinc oxide. This
presentation will describe these initial experiments and elucidate key physiochemical nature of the
deposited thin films.
HelioVolt Corporation is currently developing Copper Indium Gallium Selenide (CIGS) products using a solution-based
deposition of precursor films followed by rapid optical processing (ROP) to make CIGS. The ROP process takes less
than 1 minute of heating to convert the precursor stack to CIGS. Device made with ROP rival performance of device
processed using field assisted simultaneous synthesis and transfer (FASST®) processing.
NREL CIGS devices with up to 20% efficiency are prepared using a three-stage process for the CIGS layer with the last
step of an intrinsic ZnO and conductive ZnO:Al bilayer. This work outlines the efficiency and performance parameters
for these CIGs devices when this bilayer is replaced with indium zinc oxide (a-InZnO), an amorphous metal oxide. It is
well known that metal oxides can serve a variety of important functions in thin film photovoltaics such as transparent
electrical contacts (TCO's), antireflection coatings and chemical barriers. In the case of a-InZnO, we have reported on
the determination of the relative roles of metals and oxygen stoichiometries on the opto-electronic properties of a-InZnO
thin films as well as the stability of those films in damp heat. Since InZO has a tunable conductivity based on the
amount of oxygen introduced during deposition, it can be used as both the intrinsic and TCO layers. We were able to
establish preliminary metrics for an all InZnO bilayer whose performance was comparable to a common CIGs device.
Organic light emitting devices (OLEDs) are projected to provide a
low-cost, long-lived, and efficient wide area lighting
solution if challenges in reliability, cost, and efficiency can be overcome. Development of new transparent conducting
oxides (TCOs) that do not contain indium for use as the anode in bottom-emitting OLEDs can lead to cost savings and provide longer device lifetimes. Indium-free TCOs need to meet or exceed performance targets including high
conductivity and visible light transmission, acceptable stability and, for blue or white OLEDs, a high work function to
match the deep HOMO of the hole transport material. In this work, we report results from our efforts to scale up sputter deposition on large area substrates (up to hundreds of cm2) of a
Ga-doped ZnO TCO having a composition identified
using combinatorial methods. We present the results of initial
scale-up efforts and evaluate relevant properties for these
films. Finally, we have incorporated these materials in the production of OLEDs, and show performance comparisons
between devices fabricated on the scaled-up GZO and commercial indium tin oxide (ITO). The results demonstrate that
we are able to generate substrates with the appropriate work function to reduce the operating voltage of blue
phosphorescent OLEDs compared to commercial ITO. This work
function-HOMO energy matching leads to more efficient charge injection into the device hole transport layer.
The work reported here explores the impact of polymer morphology on the physics and performance of perylene benzimidazole/poly(3-hexylthiophene) bilayer photovoltaic devices. By varying both the annealing temperature and the solvent used for polymer deposition, we demonstrate control of the polymer chain morphology. An increase in the relative ordering of the polymer chain conformation is observed through a shift in the absorption onset and absorption spectral shape, and results in improved photovoltaic performance.
We have explored the use of polymer / small molecule organic composites in the form of a polymer / perylene diimide heterojunction bilayer in order to combine the advantageous properties of both materials. Using the electron transporting perylene benzimidazole (PBI) and the hole conducting polymer poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene (M3EH-PPV), we have achieved increased power conversion efficiencies for a planar device of up to 0.71% under 80 mW/cm2 white illumination. By varying the order of the photoactive layers, we have probed the mechanisms creating the photovoltage and found that the photovoltage is not determined by the difference in electrode work functions alone. In addition to the internal field, the interfacial chemical potential gradient, caused by exciton dissociation at the polymer / perylene diimide interface, appears to contribute to the photovoltage. We also discuss why, contrary to some expectations, the polymer / perylene diimide devices are more efficient than the analogous pure small molecule perylene diimide / phthalocyanine cells.
The main factors inhibiting higher conversion efficiencies in plain polymer layer sandwich photovoltaic devices are the low exciton dissociation efficiency and the low carrier mobilities in the polymer. We consider two different blend approaches for increasing these qualities. NiO (or LiNiO) hole transporting nanoparticles are blended into the photoactive polymer MEH-DOO-PPV in an attempt to increase hole mobility across the device. Improvements to device performance were not significant at these blend concentrations. Devices made using blends of hole and electron transporting polymers M3EH-PPV and CN-ether-PPV showed increased dissociation efficiency and gave power conversion efficiencies of up to 0.6% with stable electrodes.
TlCaBaCuO films using e-beam evaporation with subsequent sintering and annealing that have critical current densities of at least several hundred kA/sq cm, critical temperatures over 100 K, and surface resistance better than that of gold at 77 K and 8 GHz. Processing techniques have been developed for making microwave passive elements and a four-terminal active device called the superconducting flux flow transistor (SFFT) for microwave applications. The techniques include contacting, dielectric, normal layer deposition, and controlled HTS film etching. The SFFT shows promise as a microwave amplifier, oscillator, and active impedance converter and may also have many other applications. The advantages of the device include high speed, potentially low noise, and, for some applications, useful impedance levels.