Direct Quantification of Quasi-Fermi-Level Splitting in Organic Semiconductor Devices

Open Access

Direct Quantification of Quasi-Fermi-Level Splitting in Organic Semiconductor Devices

Drew B. Riley, Oskar J. Sandberg, Nora M. Wilson, Wei Li, Stefan Zeiske, Nasim Zarrabi, Paul Meredith, Ronald Österbacka, and Ardalan Armin

Phys. Rev. Applied 15, 064035 – Published 15 June 2021

Abstract

Nonradiative losses to the open-circuit voltage are a primary factor in limiting the power-conversion efficiency of organic photovoltaic devices. The dominate nonradiative loss in the bulk is intrinsic to the active layer and can be determined from the quasi-Fermi-level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin-film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electromodulated photoluminescence, which can be used to directly measure the intrinsic nonradiative loss to the open-circuit voltage, thereby quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related nonradiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the nonradiative losses of the open-circuit voltage. The reported method will be useful not only in characterizing and understanding losses in organic solar cells but also in other device platforms such as light-emitting diodes and photodetectors.

Received 5 March 2021
Accepted 7 May 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.064035

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electronic structure Optoelectronics Photovoltaic absorbers Solar cells

Organic LEDs Organic semiconductors Organic sensors

Electroluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Drew B. Riley ^1,*, Oskar J. Sandberg ^1,†, Nora M. Wilson ², Wei Li¹, Stefan Zeiske¹, Nasim Zarrabi¹, Paul Meredith¹, Ronald Österbacka ², and Ardalan Armin^1,‡

¹Sustainable Advanced Materials Programme (Sêr SAM), Department of Physics, Swansea University, Singleton Park, Swansea SA2 8PP, United Kingdom
²Faculty of Science and Engineering, Åbo Akademi University, 20500 Turku, Finland

^*1915821@swansea.ac.uk
^†o.j.sandberg@swansea.ac.uk
^‡ardalan.armin@swansea.ac.uk

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand

Issue

Vol. 15, Iss. 6 — June 2021

Subject Areas

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) The electroluminescence schematic. (b) The simulated conduction ( $E_{C}$ ) and valence levels ( $E_{V}$ ) (solid lines), electron ( $E_{F n}$ ) and hole ( $E_{F p}$ ) quasi-Fermi levels (dashed lines), and QFLS under the following conditions: one-sun open-circuit (blue) and dark injected current of one-sun short-circuit current (red). (c) The electromodulated-photoluminescence schematic. (d) The simulated conduction, valence, electron and hole quasi-Fermi levels, and QFLS under the following conditions: one-sun open-circuit (blue) and one-sun $V_{off} \pm Δ V$ (red and green). An, anode; Cat, cathode; FG, function generator; Amp, current amplifier; OSC, oscilloscope; ND, neutral-density wheel.
Reuse & Permissions
Figure 2
The simulated (a)–(c) conduction ( $E_{C}$ ), valence ( $E_{V}$ ), electron ( $E_{F n}$ ) and hole ( $E_{F p}$ ) quasi-Fermi levels for devices with (a) no injection barrier, (b) a medium injection barrier, and (c) a high injection barrier. (d) The simulated injection barrier dependence of $V_{OC}$ (black circles), QFLS (green squares), and nonradiative losses measured by simulating electroluminescence (blue circles) and electromodulated-photoluminescence (blue squares) experiments.
Reuse & Permissions
Figure 3
The current-density–voltage curves at one-sun illumination for devices with various cathode materials. All devices have a structure ITO/PEDOT:PSS/PM6:Y6/cathode (cathode material shown in the legend), except for the pink curve, where the device structure is ITO/PM6:Y6/ $Ag$ .
Reuse & Permissions
Figure 4
(a)–(g) The device structures with an increasing injection barrier. (h)–(n) The LED external quantum efficiency as measured by electroluminescence ( $η_{EL}$ ) as a function of the dark injected current (blue) and the electromodulated photoluminescence ( $η_{EMPL}$ ) as a function of the short-circuit current (red) and $J_{SC}^{AM1.5}$ (gray dashed line). PCE, power-conversion efficiency; FF, fill factor. The values of the device parameters and the measurements are taken from the top-performing pixel. The error bars are calculated from measurement errors in the oscilloscope and the lock-in amplifier (for details, see Sec. SIII of the Supplemental Material [47]).
Reuse & Permissions
Figure 5
(a) The QFLS measured by electromodulated photoluminescence (blue squares) and the expected $q V_{OC}$ as measured by $η_{EL}$ (blue circles) for PM6:Y6 systems with increasing electrode-induced nonradiative V_OC losses. The size of the various nonradiative losses to the $V_{OC}$ is indicated on the plot. The upper axis lists cathode materials while the lower axis lists measured $V_{OC}^{AM1.5}$ . (b)–(d) The device structure designed to have (b) low, (c) medium, and (d) high nonradiative losses at the cathode.
Reuse & Permissions

Physical Review Applied