Cross-Section AFM and EFM Examination of Thin-Film Solar Cells

ABSTRACT We demonstrated the feasibility of analyzing cross sections of thin-film CdTe/CdS and CIGS/CdS solar cells using atomic force microscopy (AFM). The AFM images were compared with images obtained with the scanning electron microscopy (SEM), and the correspondence was very good. We also used electrostatic force microscopy (EFM) [1] to image the distribution of the electrical potential on cross sections of CdTe/CdS solar cells biased at different conditions. We were able to follow the potential drop at the junction, and the changes in the depletion width for different bias values. EXPERIMENTAL We used the following structures in this work: glass/SnO 2 /CdS/CdTe and glass/Mo/CIGS/CdS/ZnO. The CdTe was grown by close-spaced sublimation, the CdS by chemical-bath deposition, the SnO 2 by chemical-vapor deposition, the CIGS by physical evaporation of Cu, In, and Ga in Se vapor, and the Mo and ZnO by sputtering. We prepared the cross sections by scratching one side of the structure (in general, the film side) and applying a pressure on the other side until the structure broke. All the devices were still working during the EFM analysis. The AFM and EFM analyses were performed in an Autoprobe CP Research system, from ThermoMicroscopes. We used doped Si tips for the AFM and EFM analyses, and Co coated tips for some of the EFM analysis. We obtained the EFM images by applying ac and dc voltages between the tip and the sample surface, and then monitoring the electrostatic force induced on the tip. To each EFM image, a simultaneous topographic image was also generated. The electrostatic force between the tip and the sample is given by the equation: where V s is the the surface potencial, C the capacitance, and z the distance between the sample and the tip. V dc is the dc bias, and V ac and ω are the amplitude and frequency of the ac bias, respectively In this work we used a lock-in amplifier to analyze only the ω component (16 kHz) of the electrostatic force. The pside of the device was grounded during analysis and the dc bias was applied to the n-side. From equation RESULTS AND DISCUSSION Cross-Section AFM In These features indicate intragranular fracture. The cross section of a CIGS/CdS solar cell is seen in We have also analyzed the cross-section structures with SEM. We were able to locate the same areas in the AFM 500 n