A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics
Joel C. Weber, Kristine A. Bertness, John B. Schlager, Norman A. Sanford, Atif A. Imtiaz, Thomas M. Wallis, Pavel Kabos, Kevin J. Coakley, Victor Bright, Lorelle M. Mansfield
We present a near field scanning microwave microscope (NSMM) optimized for imaging photovoltaic samples. Our system incorporates a cut Pt-Ir tip inserted into an open ended coaxial cable to form a weak resonator, allowing the microwave reflection S11 signal to be measured across a sample over the 1 5 GHz range. A phase tuning circuit increases impedance measurement sensitivity by allowing for precise tuning of the S11 minimum down to -78 dBm. Integration of a bias tee and preamplifier enables the simultaneous acquisition of the DC tip-sample current measurement while a tuning fork feedback system provides simultaneous topographical measurement. Light-free tuning fork feedback enables accurate characterization of photovoltaic samples under both dark and illuminated conditions. The NSMM is demonstrated on an inhomogeneous, third generation Cu(In,Ga)Se2 (CIGS) 17 % efficient sample in the dark state as well as when illuminated by a 405 nm diode laser. In the illuminated state, the S11 and DC current features are found to spatially broaden around grain boundaries. The broadening is attributed to the presence of additional minority charge carriers under illumination. These images demonstrate our NSMMs ability to more completely characterize and resolve nano-scale electromagnetic features of novel photovoltaics at microwave frequencies.