The quantum efficiency is the ratio of the number of carriers that are collected by the solar cell to the number of photons incident on the solar cell and can be expressed as a function of the wavelength or the photon energy. For example, when all photons of a specific wavelength and are absorbed and the generated electron-hole pair are collected, the quantum efficiency at this wavelength is unity. The quantum efficiency is affected by both photo-generation of electron-hole pairs as well the collection efficiency of these electron-hole pairs. The quantum efficiency of photons with an energy below the silicon bandgap is zero due to a lack of photogeneration. The quantum efficiency of high energy photons is typically below 1 due to recombination at the front surface of the solar cell and this is referred to the blue response of a solar cell. Near-infrared light is absorbed throughout the solar cell and the quantum efficiency in this region is affected by both collection losses (e.g. recombination in the bulk and at the rear surface) as well as optical losses due to parasitic absorption at the rear of the solar cell as well as light that escapes from the solar cell without being absorbed.
The quantum efficiency measurement principle is schematically shown in Figure 1. A solar cell is placed on a temperature controlled stage and is kept in short-circuit conditions. Subsequently, the solar cell is exposed to monochromatic light and the incident light intensity and photogenerated current are measured at the same time. If desired, a bias light can be applied during the measurement to ensure that the solar cell is closer to operating conditions. The monochromatic light is typically modulated with a certain frequency to allow for a better signal to noise ratio by using lock-in measurements.
Figure 1: Schematic of a quantum efficiency measurement.
There are two types of quantum efficiency: internal and external. The external quantum efficiency (EQE) includes the reflection losses of the solar cell. The internal quantum efficiency (IQE) is corrected for the optical losses due to reflection at the front of the solar cell.
A video of a spectral response measurement at UNSW is shown below.