The quantum efficiency describes the ratio between the number of photons that are emitted by an OLED and the number of electrons that were injected into the device. It depends on the emitters, stack structure, and substrate.
Quartz Crystal Microbalance
A piezoelectric quartz crystal is used to determine mass changes as a result of eigenfrequency change of the crystal. Material is coated onto the crystal that attracts a certain other material. When this other material becomes attached, it lowers the crystal frequency, the amount being directly related to its mass. The mass change can be in the nanogram range.