Surface and ultrathin-layer absorptance spectroscopy for solar cells

The optical and opto-electrical methods Photothermal Deflection Spectroscopy and Fourier Transform Photocurrent Spectroscopy, were originally designed to measure bulk material properties. Here, we modify these methods in order to measure defect absorptance at the semiconductor surfaces and in ultra-thin layers. Additionally, we present a new method of sample preparation for Attenuated Total Reflection Fourier Transform Infrared spectroscopy, allowing us to routinely probe the hydrogen content and micro structure of similar surfaces and ultrathin layers. These methods were used to study the effects of sample storage, annealing and light soaking on defect density and the hydrogen content. Surface-defect layers, present on 350 nm thick hydrogenated amorphous silicon, were studied and correlated to behavior of only 10 nm thick films of the same material. The characteristics of hydrogen-terminated crystalline silicon surfaces were studied too. Interestingly, these three distinct structures all exhibited similar behavior: loss of hydrogen due to <200°C annealing, practically no increase of defect density by light soaking, reduction of defect density just by storage in air. The observed behavior of the ultrathin layers is diametrically different from the usual behavior of bulk hydrogenated amorphous silicon.