solar cell

2015 European PV Solar Energy Conference and Exhibition

HPY Publications , , , ,

PTIR“Local measurement of photocurrent and band gap in CdTe solar cells”, Y. Yoon, J. Chae, A. Katzenmeyer, H. Yoon, J. Schumacher, S. An, A. Centrone, N. Zhitenev, 31st European Photovoltaic Solar Energy Conference and Exhibition, p. 1243, 2015. (DOI: 10.4229/EUPVSEC20152015-3DV.1.34)

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
2. Maryland Nanocenter, University of Maryland, College Park, Maryland, 20742, USA

 

ABSTRACT. Polycrystalline thin film technology has shown great promise for low cost, high efficiency photovoltaics. To further increase the power efficiency, a firm understanding of microstructural properties of the devices is required. In this work, we investigate the inhomogeneous electrical and optical properties using local excitation techniques that generate excess carriers by a near-field light illumination or by a focused electron beam irradiation. The spatially resolved photocurrent images of n-CdS / p-CdTe devices obtained by both techniques show high carrier collection efficiencies at grain boundaries. A novel and complementary technique, photothermal induced resonance (PTIR), is also used to obtain absorption spectra and maps in the near-field over a broad range of wavelengths. In PTIR a wavelength tunable pulsed laser is used in combination with an atomic force microscope tip to detect the local thermal expansion induced by light absorption. Sub-micrometer thick lamella samples of CdTe solar cells are measured, and the variation of local band-gap is analyzed. We discuss the resolution and the sensitivity of the techniques in the range of photon energies close to the band gap.

LINK

2015 Nanotechnology

HPY Publications , , , , ,

Paul_Nanotechnology“Electron beam induced current in the high injection regime”, P. M. Haney, H. P. Yoon, P. Koirala, R. W. Collins, and N. B. Zhitenev, Nanotechnology 26, 295401, 2015.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
2. Maryland NanoCenter, University of Maryland, College Park, MD 20742.
3. Dept. of Physics and Astronomy, University of Toledo, Toledo, OH, 43606.

ABSTRACT. Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of photovoltaics with sub-micron spatial resolution. The exciting electron beam results in a high generation rate density of electron–hole pairs, which may drive the system into nonlinear regimes. An analytic model is presented which describes the EBIC response when the total electron–hole pair generation rate exceeds the rate at which carriers are extracted by the photovoltaic cell, and charge accumulation and screening occur. The model provides a simple estimate of the onset of the high injection regime in terms of the material resistivity and thickness, and provides a straightforward way to predict the EBIC lineshape in the high injection regime. The model is verified by comparing its predictions to numerical simulations in one- and two-dimensions. Features of the experimental data, such as the magnitude and position of maximum collection efficiency versus electron beam current, are consistent with the  three dimensional model.

LINK

2015 IEEE Photovoltaic Specialists Conference

HPY Publications , , ,

pvsc_2015_poster“Local Photocarrier Dynamics in CdTe Solar Cells Under Optical and Electron Beam Excitations”, H. P. Yoon, P. M. Haney, Y. Yoon, S. An, J. I. Basham, and N. B. Zhitenev, 42th IEEE Photovoltaic Specialists Conference, p. 1, 2015. (DOI: 10.1109/PVSC.2015.7356020)

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
2. Maryland Nanocenter, University of Maryland, College Park, Maryland, 20742, USA

ABSTRACT. We compare local carrier dynamics in n-CdS / p-CdTe solar cells, where the electron-hole pairs are generated by either near-field optical illumination or highly focused electron beam excitation. An ion beam milling process was used to prepare a smooth surface of cross-sectional devices. The spatially resolved photocurrent images confirm high carrier collection efficiency at grain boundaries. An analytical model was used to extract material parameters at the level of single grains. We find that the minority carrier diffusion lengths extracted from both local measurement techniques are in excellent agreement, but are smaller than the values determined from macro-scale measurements.

LINK

2014 Progress in Photovoltaics

HPY Publications , , ,

pip_epi_EBIC“Comparison of thin epitaxial film silicon photovoltaics fabricated on monocrystalline and polycrystalline seed layers on glass”, C. W. Teplin, S. Grover, A. Chitu, A. Limanov, M. Chalal, J. Im, D. Amkreutz, S. Gall, H. P. Yoon, V. Lasalvia, H. M. Branz, P. Stradins, K. M. Jones, A. G. Norman, D. L. Young, B. Lee, Progress in Photovoltaics, in press (DOI: 10.1002/pip.2505), 2014.

1. National Renewable Energy Laboratory, Golden, CO, USA
2. Columbia University, New York, NY, USA
3. Helmholtz Zentrum Berlin für Materialien und Energie, Berlin, Germany
4. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

ABSTRACT:  We fabricate thin epitaxial crystal silicon solar cells on display glass and fused silica substrates overcoated with a silicon seed layer. To confirm the quality of hot-wire chemical vapor deposition epitaxy, we grow a 2-μm-thick absorber on a (100) monocrystalline Si layer transfer seed on display glass and achieve 6.5% efficiency with an open circuit voltage (V_OC) of 586mV without light-trapping features. This device enables the evaluation of seed layers on display glass. Using polycrystalline seeds formed from amorphous silicon by laser-induced mixed phase solidification (MPS) and electron beam crystallization, we demonstrate 2.9%, 476mV (MPS) and 4.1%, 551mV (electron beam crystallization) solar cells. Grain boundaries likely limit the solar cell grown on the MPS seed layer, and we establish an upper bound for the grain boundary recombination velocity (S_GB) of 1.6×104 cm/s.

LINK

2013 Solar Energy Materials and Solar Cells

HPY Publications , , ,

solmat_CdTe“Local electrical characterization of cadmium telluride solar cells using low-energy electron beam”, H. P. Yoon, P. M. Haney, D. Ruzmetov, H. Xu, B. H. Hamadani, A. A. Talin, and N. B. Zhitenev, Solar Energy Materials and Solar Cells 117, 499-504, 2013.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899.
2. Energy and Environment Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
3. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA

ABSTRACT. We investigate local electronic properties of cadmium telluride solar cells using electron beam induced current (EBIC) measurements with patterned contacts. EBIC measurements are performed with a spatial resolution as high as ≈20 nm both on the top surface and throughout the cross-section of the device, revealing an enhanced carrier collection in the vicinity of grain boundaries. Furthermore, we measure local current-voltage characteristics using contacts with dimension both larger (≈5 µm × 10 µm) and smaller (≈1 µm × 1 µm) than the device thickness (≈4 µm), finding that the value of local open-circuit voltage is also larger near grain boundaries.

LINK

2011 Nanotechnology

HPY Publications , , ,

NT_SiNW“Single nanowire radial junction solar cells fabricated using Al catalyzed Si nanowires”, Y. Ke, X. Wang, C. E. Kendrick, Y. A. Yu, S. M. Eichfeld, H. P. Yoon, J. M. Redwing, T. S. Mayer, and Y. M. Habib, Nanotechnology 22, 445401, 2011.

1. Department of Materials Science and Engineering, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
2. Department of Electrical Engineering, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
3. Illuminex Corp., Lancaster, PA 17601, USA

ABSTRACT. Single nanowire radial junction solar cell devices were fabricated using Si nanowires synthesized by Al-catalyzed vapor–liquid–solid growth of the p+ core (Al auto-doping) and thin film deposition of the n+-shell at temperatures below 650 ◦C. Short circuit current densities of ∼11.7 mA /cm2 were measured under 1-sun AM1.5G illumination, showing enhanced optical absorption. The power conversion efficiencies were limited to <1% by the low open circuit voltage and fill factor of the devices, which was attributed to junction shunt leakage promoted by the high p+ /n+ doping. This demonstration of a radial junction device represents an important advance in the use of Al-catalyzed Si nanowire growth for low cost photovoltaics.

LINK

2010 Applied Physics Letters (NW)

HPY Publications , , , , ,

apl_nwPV“Radial junction silicon wire array solar cells fabricated by gold-catalyzed vapor-liquid-solid growth“, C. E. Kendrick, H. P. Yoon, Y. A. Yuwen, G. D. Barber, H. Shen, T. E. Mallouk, E. C. Dickey, T. S. Mayer, and J. M. Redwing, Applied Physics Letters 97, 143108, 2010.

1. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
2. Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

ABSTRACT. The fabrication of radial junction silicon Si solar cells using Si wire arrays grown by Au-catalyzed vapor-liquid-solid growth on patterned Si substrates was demonstrated. An important step in the fabrication process is the repeated thermal oxidation and oxide etching of the Si wire arrays. The oxidation cleaning process removes residual catalyst material from the wire tips and exposes additional Au embedded in the material. Using this cleaning process and junction formation through POCl3 thermal diffusion, rectifying p-n junctions were obtained that exhibited an efficiency of 2.3% and open circuit voltages up to 0.5 V under Air Mass 1.5G illumination.

LINK

2010 Applied Physics Letters (Pillars)

HPY Publications , , , ,

apl_pillar array“Enhanced conversion efficiencies for pillar array solar cells fabricated from crystalline silicon with short minority carrier diffusion lengths”, H. P. Yoon, Y. A. Yuwen, C. E. Kendrick, G. D. Barber, N. J. Podraza, J. M. Redwing, T. E. Mallouk, C. R. Wronski, and T. S. Mayer, Applied Physics Letters 96, 213503, 2010.

1. Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
2. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

ABSTRACT. Radial n+/p+ junction solar cells composed of densely packed pillar arrays, 25 μm-tall and 7.5 μm in diameter, fabricated from p-type silicon substrates with extremely short minority carrier diffusion lengths are investigated and compared to planar cells. To understand the two times higher AM 1.5 efficiencies of the pillar array cells, dark and light I-V characteristics as well as spectral responses are presented for the two structures. The higher pillar array cell efficiencies are due to the larger short-circuit currents from the larger photon absorption thickness and the shorter carrier collection length, with a significant additional contribution from multiple reflections in the structure.

LINK