Wook Jun Nam

Modeling of bulk and bilayer organic heterojunction solar cells

Solar cells require symmetry-breaking features such as built-in electrostatic fields and/or effective fields. We examine an organic heterojunction structure with no built-in field and explore the origins of its current-voltage characteristics and open circuit voltage ( V OC ) . Two behaviors are found: (1) V OC = V I + m [(HOMO(D) - (LUMO(A)] where m ≈ 1 , the intercept ( V I ) is determined by interface recombination kinetics, HOMO(D) is the donor highest occupied molecular orbital, and LUMO(A) is the acceptor lowest unoccupied molecular orbital; (2) if interface recombination is suppressed, V OC is controlled by bulk/contact recombination and is not dependent upon HOMO(D) - LUMO(A).

Gold Nanowires for the Detection of Elemental and Ionic Mercury

We demonstrate for the first time the use of Au nanowires for the electrical detection of elemental and ionic mercury. By monitoring changes in resistance upon exposure to Hg vapor, concentrations as low as 6.25 μg/m 3 (2 X 10 -10 M or 5 ppb) were detected. Similarly, we used HgCl 2 in an aqueous solution as a source for Hg ions and were able to detect Hg 2+ concentrations as low as 10 -8 M. We also demonstrate the impact of the sensor surface to volume ratio, substantiate the model of a surface origin for the sensitivity, and underscore the advantages of the Au nanowire structure over the thin-film structure for Hg sensing.

Incorporation of a light and carrier collection management nano-element array into superstrate a-Si:H solar cells

Superstrate a-Si:H solar cells incorporating a nano-column array for light and photocarrier collection have been fabricated and evaluated. It is found that the short circuit current density (JSC) is significantly increased while the open circuit voltage and fill factor are not detrimentally affected by this architecture. Numerical analysis of JSC matches experiment and shows that the enhanced JSC observed is due to both effective absorber thickness and photonic-plasmonic effects. Further analysis shows that this nano-column architecture can lead to a 42% increase in conversion efficiency over that of the planar control for a 200 nm absorber thickness cell.

Exploration of nano-element array architectures for substrate solar cells using an a-Si:H absorber

Architectures involving Ag and transparent conducting oxide (TCO) nano-element arrays for light and photocarrier collection management in substrate solar cells are numerically explored and compared. Some architectures with TCO nano-elements are shown to perform better than the best reported Ag arrays and (1) increase JSC at least 57% over that of a planar 200 nm a-Si:H control, (2) attain absorber utilization <7 mg/W, and (3) have only 224 nm as the longest collection length. Photonic effects are the cause of the light trapping enhancement in these devices. While the computations were done for a-Si:H, the insight provided is equally applicable to other absorbers.

Education and Training Approach for the Future Nanotechnology Workforce

The Pennsylvania Nanofabrication Manufacturing Technology (NMT) Partnership was formed in 1998 to create and nurture a skilled nanofabrication workforce in the State of Pennsylvania. The approach laid out emphasizes developing a strong nanotechnology workforce education effort that both opens the door to more high-tech jobs for Pennsylvanians and also strengthens PA industry. The partners in this endeavor are 29 Pennsylvania colleges and universities, Penn State University, industry, the State of Pennsylvania, and the National Science Foundation. The activities the partnership has developed in its approach include three-day Nanotech Camps for secondary school students, three-day Nanotechnology Workshops for secondary school teachers, Nanotechnology Utilization Workshops tailored for industry, Nanotechnology Associate (2-year) Degree programs, Baccalaureate Degree programs with concentrations in Nanotechnology, and Nanotechnology Experiment Kits to bring nanotechnology into existing chemistry, physics, and biology courses.

Nano- and microchannel fabrication using column/void network deposited silicon

Nano- and microchannels are fabricated using a novel deposited column/void network silicon film as a sacrificial material. This nanostructured silicon consists of nanometer-sized columns defined normal to the substrate in a void matrix, where the voids are continuously connected with each other, forming a network. The void network structure results in a high sacrificial layer etch rate due to the void network-enhanced transport of reactant and reaction products during the etching process, and high effective surface area. The use of our unique deposited column/void network material coupled with lift-off processing results in a manufacturable process for nano- and microchannel and nano- and microcavity fabrication. The approach provides extremely flat surfaces without a chemical–mechanical polishing process, and allows for multiple layers of channel or cavity structures with crossovers.

30% Increase in Available Photons per Cell Area Using Nanoelement Array Light Trapping in 700-nm-Thick nc-Si Solar Cells

We fabricated nc-Si light trapping cells that produced photocurrents 30% higher than those of controls. Computer studies using experimentally determined, self-consistent TCO and nc-Si thicknesses showed this nanodome design is capable of producing

Application of the AMPS computer program to organic bulk heterojunction solar cells

J_{{\rm sc}}\sim30

Highly ordered nano-cone back reflector arrays for ultra-thin high performance CIGS cells

mA/cm 2 with only 10% of the nc-Si absorber volume needed by the corresponding planar cell. Interestingly, these experimental and computer modeling results were both attained with a nanodome spacing of 1250 nm, a value almost twice that suggested in other studies of similar structures. The fabricated cells had photocurrents that were reverse bias dependent. FESEM studies showed this effect correlated with the presence of a curtain of nc-Si defects surrounding each nanodome. These defects are similar to those seen earlier by other workers using nc-Si cells with larger feature sizes. A model for this voltage dependent photocurrent behavior is presented. This model and a discussion of the Voc performance origins show that deposition procedures and material defects are holding this architecture back from its potential.

Fabrication and evaluation of highly manufacturable nanoscale flow-through parallel electrode structures

Numerical modeling of organic bulk heterojunction (BHJ) solar cells has been undertaken using the AMPS computer code and material parameters representative of the P3HT/PCBM cell. These simulations show that the VOC and thus the efficiency of these BHJs is currently controlled by interface recombination. These results show VOC tracks with the HOMO (D) − LUMO (A) difference with a relationship of the form VOC = VI + m[HOMO (D) − LUMO (A)], as is seen experimentally. The AMPS simulations show that the built-in potential and the contact barrier heights surprisingly do not affect VOC in these cells. These do, however, affect the cell fill factor FF and efficiency. If interface recombination is suppressed, then VOC values larger than HOMO (D) − LUMO(A) difference are possible since band bending must occur at the heterojunction to sustain the bulk and contact recombination that balances generation at open circuit. With interface recombination suppressed and contacts chosen for optimum FF, this modeling shows BHJs are capable of power conversion efficiencies of about 10% using material parameters corresponding to a P3HT/PCBM cell.