CdTe

2015 European PV Solar Energy Conference and Exhibition

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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.

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2015 Nanotechnology

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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.

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2015 IEEE Photovoltaic Specialists Conference

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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.

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2014 7th Annual FIB SEM Workshop

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usermeeting
“Effects of Focused-Ion-Beam Processing on Local Measurements of Semiconductor Solar Cells”, H. P. Yoon, P. M. Haney, J. Schumacher, K. Siebein, Y. Yoon, and N. B. Zhitenev, presented by H. Yoon at the 7th Annual FIB SEM Workshop in Laurel, MD, February 2014.

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. Quantitative determination of electronic properties at high spatial resolution is crucial for the development of high-efficiency solar cells. Electron beam induced current (EBIC) is a powerful technique in which electron-hole pairs are created in proximity to an exposed surface, and the carrier collection efficiency is measured as a function of excitation position. Cross-sections of device are often created by focused ion beams (FIB) due to the flexibility of the patterning and milling processes. However, the irradiating Ga ions of the FIB fabrication may introduce unintended artifacts, affecting local electronic properties. In this talk, we discuss the impact of the FIB process observed in EBIC measurements and two-dimensional finite element simulations. A series of EBIC data was obtained on a single crystalline solar cell at different electron beam voltages and beam currents to examine the depth and carrier injection level dependence inside the depletion region and away from the p-n junction. Quantitative analysis shows that the EBIC efficiency of the FIB sample is much lower (< 40 %) than that of the cleaved sample (100%) at low beam voltages (<10 keV). We discuss the effects of FIB processing of other types of photovoltaic devices including CdTe and CIGS solar cells.

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2013 Solar Energy Materials and Solar Cells

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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.

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2012 IEEE Photovoltaic Specialists Conference

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pvsc_ebic“High-resolution local-current measurement of CdTe solar cells”, H. P. Yoon, D. Ruzmetov, P. M. Haney, B. H. Hamadani, A. A. Talin, N. B. Zhitenev, 38th IEEE Photovoltaic Specialists Conference, pp. 3217-3219, 2012.

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

ABSTRACT. We investigate local electronic properties of CdTe solar cells using electron beam to excite electron-hole pairs and evaluate spatially resolved photocurrent characteristics. Standard semiconductor processes were used to fabricate Ohmic metal contacts on the surface of p-type CdTe / n-type CdS device extracted from a commercial solar panel. An ion milling process was used to prepare cross-sections of the devices. Local injection of carriers was controlled by an acceleration voltage of electron beam (1 kV to 30 kV) in a scanning electron microscope, and the
results were correlated with the local morphology, microstructure, and chemical composition of the devices.

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