Quantum efficiency (QE) is the ratio of the number of charge carriers collected by a photovoltaic (PV) cell to the number of photons—or packets of light—of a given energy shining on the solar cell. Quantum efficiency therefore relates to the response of a solar cell to the various wavelengths in the spectrum of light shining on the cell. The QE is given as a function of either wavelength or energy. If all the photons of a certain wavelength are absorbed and the resulting minority carriers (for example, electrons in a p-type material) are collected, then the QE at that particular wavelength has a value of one. The QE for photons with energy below the bandgap is zero.
Quantum efficiency ideally has a square shape, in which the QE value is fairly constant across the entire spectrum of wavelengths measured. However, the QE for most solar cells is reduced because of the effects of recombination, in which charge carriers are not able to move into an external circuit. The same mechanisms that affect the collection probability also affect the QE. For example, modifying the front surface of a PV cell can affect carriers generated near the surface. And because high-energy (blue) light is absorbed very close to the surface, considerable recombination at the front surface will affect the "blue" portion of the QE.
Similarly, lower energy (green) light is absorbed in the bulk of a solar cell, and a low diffusion length will affect the collection probability from the solar cell bulk, reducing the QE in the green portion of the spectrum. In somewhat technical terms, the quantum efficiency can be viewed as the collection probability due to the generation profile of a single wavelength, integrated over the device thickness and normalized to the number of incident photons.
Two types of quantum efficiency of a solar cell are often considered:
- External QE includes the effect of optical losses such as transmission through the cell and reflection of light away for the cell. However, it is often useful to look at the QE of the light that remains after the reflected and transmitted light has been lost.
- Internal QE refers to the efficiency with which light not transmitted through or reflected away from the cell can generate charge carriers—specifically electrons and holes—that can generate current. By measuring the transmission and reflection of a solar device, the external QE curve can be corrected to obtain the internal QE curve.
In addition to quantum efficiency, conversion efficiency is a measure that characterizes PV cell performance. Learn more about conversion efficiency.