External Quantum Efficiency of Electroluminescence (EQE ) Measurement system
A light emitting diode (LED) and a photovoltaic (PV) device have a complementary physical action, one transforming electricity to light and the other transforming light to electricity. The detail balanced principle between light absorption and emission has been shown that in an ideal solar cell electrons and holes only recombine radiatively. Thus, in theory, a solar cell with maximum power conversion efficiency will also act as an LED with the maximum possible luminescence efficiency (Shockley–Queisser limit).(1) In reality, though, the non-radiative recombination (losses) are unavoidable in solar cells. The reciprocity relation between the photovoltaic quantum efficiency EQE and the luminescent quantum efficiency EQE of a solar cell enables us to calculate the non-radiate losses in open circuit voltage V . (2)
In Our lab we use the HAMAMATSU C9920-12 system to measure (EQ ) of the solar cell devices. The sample is placed in an integrating sphere in order to count for emission in different direction. A source meter (Keithley 2400) provide constant current or voltage to derive the device electroluminescence. The electroluminescence is detected via a photonic multichannel analyser (PMA_12). The PMA software, provided by HAMAMATSU, controls the measurement parameters such as exposure time and the applied current/voltage. It also allows for scanning through sequential current or voltage steps. At the end of the measurement the software displays the EL spectrum, and automatically calculates EQ from the EL flux [lm] and plot it versus applied current/voltage. The EQ is then used to determine non-radiative voltage losses in thin film organic solar cells.
Figure 1: Overview of the EQ measurement setup, including the Keithley Source Measure Unit, HAMAMATSU PMA, and integrating sphere.
(1) W. Shockley, H. J. Queisser, Detailed balance limit of efficiency of p‐n junction solar cells. Journal of applied physics 32, 510-519 (1961).
(2) U. Rau, Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells. Physical Review B 76, 085303 (2007).