Fig. 1. (Color online) (a) J−V characteristic, (b) C−V characteristic, which reflects (c) the charge dynamics in QLED, (d) Nyquist plot at the selected working voltage, (e) an equivalent circuit model can be built up according to the Nyquist plot, (f) C−f characteristic at the selected working voltage, (g) the trap distribution can be calculated if the capacitance rise is induced by carrier trapping.

Fig. 2. (Color online) Nyquist plot and its equivalent circuit model of a blue QLED.

Fig. 3. (Color online) Equivalent circuit model of QLEDs based on (a) device structure, (b) the function of each circuit element. Modified with permission from (a) Ref. [45] Copyright 2022 Springer Nature, (b) Ref. [47] Copyright 2022 American Chemical Society.

Fig. 4. (Color online) (a) Equivalent circuit model of QLED based on CPE circuit element, (b) Nyquist plot of QLED with two semicircles and its equivalent circuit model. Modified with permission from (a) Ref. [54] Copyright 2020 American Chemical Society, (b) Ref. [55] Copyright 2022 Springer Nature.

Fig. 5. (Color online) Capacitance−frequency characteristics of (a) blue QLED at 1.7 V and 2.3 V, (b) purple QLED within the voltage range from 0 to 10 V. Modified with permission from (a) Ref. [41] Copyright 2022 AIP Publishing, (b) Ref. [60] Copyright 2019 AIP Publishing.

Fig. 6. (Color online) Capacitance−frequency characteristic of the QLED. Modified with permission from Ref. [60] Copyright 2019 AIP Publishing.

Fig. 7. Energy level of a p−i−n heterojunction. Modified with permission from Ref. [37] Copyright 1992 AIP Publishing.

Fig. 8. (Color online) Capacitance−frequency characteristics of OPV with two-step capacitance rise. Modified with permission from Ref. [64] Copyright 2016 American Physical Society.

Fig. 9. (Color online) Capacitance−voltage characteristic of a blue QLED.

Fig. 10. (Color online) (a) Capacitance−voltage characteristics of (a) pristine device and UV-ozone treated device, (b) pristine device and CF₄-treated device. Modified with permission from (a) Ref. [75] Copyright 2019 RSC Pub, (b) Ref. [45] Copyright 2022 Springer Nature.

Fig. 11. (Color online) (a) Capacitance−voltage characteristics of (a) the inverted QLED, (b) normal QLED with ZnO and TPBi ETLs. Modified with permission from (a) Ref. [53] Copyright 2022 Elsevier Ltd and (b) Ref. [20] Copyright 2021 Wiley-VCH Verlag GmbH & Co. KGaA.

Fig. 12. (Color online) (a) Capacitance−voltage characteristics of (a) pristine QLED and MoO₃-modified QLED, (b) blue QLED at different degradation periods. Modified with permission from (a) Ref. [87] Copyright 2022 AIP Publishing, (b) Ref. [21] Copyright 2019 Springer Nature.

Fig. 13. (Color online) Capacitance−voltage and dC/dV−V characteristics of a red QLED. Modified with permission from Ref. [86] Copyright 2022 IOP Publishing.

Fig. 14. (Color online) 1/C²−V characteristics of the HOD (a) and EOD (b). Modified with permission from Ref. [6] Copyright 2020 Springer Nature.

Table 1. The applications and limitations of the impedance measurement.