Scanning Kelvin Probe Microscopy of CdTe Solar Cells Measured Under Different Bias Conditions
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1165-M08-28
Scanning Kelvin Probe Microscopy of CdTe Solar Cells Measured Under Different Bias Conditions H.R. Moutinho, R.G. Dhere, C.-S. Jiang, and M.M. Al-Jassim National Renewable Energy Laboratory, Golden, CO 80401
ABSTRACT We have investigated different methods for preparing CdTe/CdS cross sections for electrical measurements, including the following: cleaving; using GaAs substrates; and sandwiching the structure between the substrate and a glass slide, and polishing with diamond discs and alumina suspension. The latter method proved to be the most reliable, with a success rate of over 90%. We investigated cross sections of CdTe/CdS samples with scanning Kelvin probe microscopy (SKPM) using two different methods: applying the alternate bias with a frequency equal to 18.5 kHz, or equal to the frequency of the second cantilever resonance peak. The results showed that using the second resonance frequency produced a smoother signal, allowing the calculation of the electric field inside the device using just the raw SKPM data. We were able to measure the distribution of the electrical potential inside working devices. Then, by taking the first derivative of the potential, we calculated the electric field and determined the location of the p-n junction.
INTRODUCTION Cadmium telluride (CdTe) and cadmium sulfide (CdS) solar cells [1] are among the best candidates to replace the conventional silicon solar cells used today. The main advantages of these devices are the use of less material and energy during manufacturing, which translates into lower costs. However, despite successful use in commercial solar cells, there are still many unknowns about the operation of these devices, and the efficiencies achieved are still well below the theoretical values. In this work, we apply scanning Kelvin probe microscopy (SKPM) [2,3] to investigate the distribution of the electrical potential inside CdTe solar cells. The SKPM data are generated by the electrical interaction between the surface of the sample and a conductive tip subjected to ac and dc voltages. The electrostatic force between the tip and sample is given by: F =−
1 dC (Vdc − Vs )2 + 1 Vac2 − dC (Vdc − Vs )Vac sin(ωt ) − 1 dC Vac2 cos(2ωt ) , 2 dz 2 dz 4 dz
(1)
where C is the capacitance, z is the tip–sample distance, Vdc is the applied dc bias, Vs is the surface potential between tip and sample, Vac is the applied ac bias, and ω is the frequency of the ac bias. By using a lock-in amplifier, we isolate the part of the force with the frequency of the ac bias. Then, by using a feedback loop while varying the dc bias, we measure the surface potential when the electrostatic force is equal to zero. Quantitative measurements are very difficult because the surface potential is also a function of the work function of the tip, which, in general, is not precisely known.
SKPM is a relatively new technique, and only a few applications to CdTe/CdS have been reported [4–6]. SKPM is performed in an atomic force microscope (AFM), and information on surface potential
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