Quantum efficiency (QE) and Incident Photon to Charge Carrier Efficiency (IPCE) indicates the ratio of the number of photons incident on a solar cell to the number of generated charge carriers. Specifically, QE is a measure of the external efficiency, while IPCE is a measure of the internal efficiency; that is, the photons reflected back from the surface of the cell are not considered. Both QE and IPCE measurements are of critical importance during the materials research and cell design stages. This is because the spectral response of the solar cell should be matched to the spectral distribution of sunlight to ensure highest efficiency in charge carrier generation.