MICROWAVE PHOTOELECTRIC EXAMINATION OF PHOTOVOLTAIC DEVICES

Potential-dependent, simultaneous measurements of the photocurrent and microwave reflection due to photoinduced excess conductivity in solid state devices, such as p-n junctions (photodetectors) or metal-oxide-semiconductor structures are presented. The two quantities provide different information about the photogenerated charge carriers. They allow a more detailed insight into the kinetic processes at the interface than measurements of the photocurrent alone, which cannot distinguish whether photocurrent quenching is caused by a rise of surface recombination, or by a drop of charge transfer due to kinetic barriers. The combination of both may help to evaluate the quality of an energy-converting device. The results show that the dependence of the microwave reflection on the applied potential varies much more significantly with device parameters, such as substrate thickness or the width of the space-charge region, than the photocurrent. A rise of the reflected microwave power indicates an increasing average concentration of photogenerated carriers over the whole thickness of the substrate, whereas a decay occurs if this concentration is diminished due to losses by photocurrent and recombination. Under constant illumination and absorption conditions these loss processes can be varied with the applied potential over a wide range of magnitude. In addition a theoretical approach is outlined to approximate the measured microwave curves for p-n junctions and to determine device parameters such as diffusion length and back-surface recombination velocity.