Low-dimensional semiconductors such as quantum dots and quantum wires can be used to enhance the efficiency of photocurrent generation in solar cells. For example, a carbon nanotube (CNT) photodiode can generate multiple electron-hole pairs from a single high-energy photon. We have recently built a scanning photocurrent microscope capable of probing CNT photodiodes with high spatial and spectral resolution. I will present our prelminary measurements of CNT and graphene photodiodes and discuss the remarkable optoelectronic properties of CNTs that will be the focus of future experiments.