DC ELQE was measured by a calibrated silicon photodiode (Thorlabs, FDS10X10), which was directly attached to the device. Two separate source measure units (SMUs) were used for applying voltage to the device and reading photocurrent from the photodiode, respectively. For AC measurement, a pulse generator (HP 8114A) applied voltage pulses (2 Hz, 500 ns duration) with a load resistance (RL) of 47 Ω. EL was measured using a biased Si photodiode (Thorlabs, DET100A2, rise time of 35 ns), connected to a current amplifier (FEMTO, DHPCA‐100) with a set gain of 103 V A−1 and bandwidth of 200 MHz. Considering a limited diameter of photodiode (9.8 mm) and its distance to the emissive device (4.4 mm), a correction factor was applied to the measured EL flux (assuming Lambertian emission). Voltages applied to before and after RL (V1 and V2, respectively) and output of the current amplifier were read by an oscilloscope (Rohde&Schwarz HMO3004). To avoid the permanent damage to the device, the measurement with a photodiode was performed over the limited V2 range of 0–30 V. Then, the device was moved to the spectrometer setup and V2 was swept over 0–40 V (V1 of 0–70 V). EL spectrum was measured at each voltage using a spectrometer equipped with an array of cooled charge‐coupled devices (CCDs), with a long integration time of 20 s to compensate the low duty cycle of the pulse. The photon flux at V2 > 30 V was plotted using the area of EL spectrum, which was calibrated with the data using a photodiode at the same condition. The SMUs and oscilloscope were controlled with the software SweepMe! (sweep‐me.net).
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