The drive-level capacitance profiling technique has been applied to ZnO/CdS/CuIn1−xGaxSe2/Mo solar cell devices, in order to study properties of defects in the CuIn1−xGaxSe2 film. Properties studied include the spatial uniformity, bulk defect response, carrier density, and light-induced metastable effects. These results indicate that previous estimates of carrier densities, from C–V profiling, may be significantly overestimated. In addition, a defect response previously thought to be located at the interface is observed to exist throughout the bulk material. Finally, an infrared light-soaking treatment is demonstrated to induce metastable changes in the bulk CuIn1−xGaxSe2 film. Hence, the drive-level capacitance profiling technique provides valuable insights into these films. Herein, the technique itself is fully explained, compared to other junction capacitance methods, and its utility is demonstrated using numerical simulation.
Defects in the band gap of CuIn1−xGaxSe2 have been characterized using transient photocapacitance spectroscopy. The measured spectra clearly show response from a band of defects centered around 0.8 eV from the valence band edge as well as an exponential distribution of band tail states. Despite Ga contents ranging from Ga/(In+Ga)=0.0 to 0.8, the defect bandwidth and its position relative to the valence band remain constant. This defect band may act as an important recombination center, contributing to the decrease in device efficiency with increasing Ga content.
We calibrate the secondary electron signal from a standard scanning electron microscope to voltage, yielding an image of the surface or near-surface potential. Data on both atomically abrupt heterojunction GaInP/GaAs and diffused homojunction Si solar cell devices clearly show the expected variation in potential with position and applied bias, giving depletion widths and locating metallurgical junctions to an accuracy better than 10 nm. In some images, distortion near the p-n junction is observed, seemingly consistent with the effects of lateral electric fields (patch fields). Reducing the tube bias removes this distortion. This approach results in rapid and straightforward collection of near-surface potential data using a standard scanning electron microscope.
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