A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics

We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S(11) signal to be measured acro...

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Bibliographic Details
Published in:Review of scientific instruments 2012-08, Vol.83 (8), p.083702-083702
Main Authors: Weber, J C, Schlager, J B, Sanford, N A, Imtiaz, A, Wallis, T M, Mansfield, L M, Coakley, K J, Bertness, K A, Kabos, P, Bright, V M
Format: Article
Language:English
Subjects:
BOUNDARIES
CAPACITANCE
CIGS
COPPER INDIUM SELENIDE
COPPER SELENIDE
Feedback
Grain boundaries
Illumination
MICROSCOPY
MICROSTRUCTURES
MICROWAVES
Nanostructure
Photovoltaic cells
Solar cells
Tuning
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Summary:We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S(11) signal to be measured across a sample over a frequency range of 1 GHz - 5 GHz. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S(11) minimum down to -78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current, and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination at 405 nm. NSMM measurements were obtained on an inhomogeneous, third-generation Cu(In,Ga)Se(2) (CIGS) sample. The S(11) and DC current features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically generated charge that becomes trapped and changes the local depletion of the grain boundaries, thereby modifying the local capacitance. Imaging provided by the NSMM offers a new RF methodology to resolve and characterize nanoscale electrical features in photovoltaic materials and devices.
ISSN:0034-6748
1089-7623
DOI:10.1063/1.4740513