Direct observation of charge accumulation in quantum well solar cells by cross-sectional Kelvin probe force microscopy

We report here the direct observation of charge accumulation in GaAs/AlGaAs multiple quantum well (MQW) solar cells by employing cross-sectional Kelvin probe force microscopy (KPFM). This sample is characterized by thin barrier layers that enable miniband formation. The contact potential difference, or potential between the tip and the semiconductor sample, was measured along the p–i–n junction. We observed, under illuminated conditions, a change in the potential gradient, or bending, at a position of the MQW layer, but not in the reference sample without quantum well. This clearly shows that charge is accumulated in the MQW region. We also found that electron accumulation in the MQW layer and the density measured on the surface is about 1 × 1011 cm−2. Our experimental results show that KPFM is a powerful way of understanding the device physics of nanostructure-based solar cells.