How to Characterize Emerging Luminescent Semiconductors with Unknown Photophysical Properties

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How to Characterize Emerging Luminescent Semiconductors with Unknown Photophysical Properties

Alan R. Bowman and Samuel D. Stranks

PRX Energy 2, 022001 – Published 20 June 2023

Abstract

Luminescent semiconductors are the key material in a host of optoelectronic devices, including solar cells, light-emitting diodes, and x-ray scintillators, and have been discovered at an increasing rate over the last decades. To optimize any device, a luminescent semiconductor’s photophysics must be understood and its loss processes minimized. Several accessible spectroscopic techniques exist, which can together give all relevant photophysical information, namely, UV-Vis spectroscopy, photoluminescence quantum efficiency, and time-resolved photoluminescence. However, these measurements are often poorly used, incorrectly fitted, or important information is missed. Here, we present best practices in applying these techniques to characterize luminescent semiconductors with unknown photophysical properties. We highlight which information can be obtained from each measurement, when it is appropriate to apply different mathematical models, and give examples from a range of semiconductors. This work will help to standardize and streamline the characterization of luminescent semiconductors, enabling more efficient devices.

Received 19 December 2022
Revised 17 April 2023

DOI:https://doi.org/10.1103/PRXEnergy.2.022001

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)

Band gap Carrier generation & recombination Density of states Optoelectronics Photovoltaic absorbers Solar energy Solar energy harvesting devices

Spectroscopy

Energy Science & TechnologyCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Alan R. Bowman ^1,† and Samuel D. Stranks ^1,2,*

¹Cavendish Laboratory, Department of Physics, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
²Department of Chemical Engineering & Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, United Kingdom

^*sds65@cam.ac.uk
^†Current address: Laboratory of Nanoscience for Energy Technologies, EPFL, MED 1 2526 (Bâtiment MED), Station 9, CH-1015 Lausanne, Switzerland.

Popular Summary

Innovations in optoelectronic technologies, such as solar cells, light emitting diodes and x-ray scintillators, are driven by the discovery and understanding of luminescent semiconductors. As research on halide perovskites, organic crystals, bismuth-based materials and other luminescent semiconductors grows, the photophysical properties and processes should be fully understood for screening and optimization of such materials for devices. Here, the authors provide a guide for full characterization of emerging luminescent semiconductors using accessible spectroscopic techniques, namely UV-vis spectroscopy, photoluminescence quantum efficiency, and time-resolved photoluminescence, in a comprehensive and standardized way. The authors present best experimental practices and how to apply different mathematical models and extract all information from measurements using semiconductors with different photophysical properties as examples.

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Issue

Vol. 2, Iss. 2 — June - August 2023

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Images

Figure 1
(a),(b) Schematics of excitonic (i.e., bound) and free charges, respectively. (c) Typical picture for charge traps, with deep and shallow trap processes within the rapid trapping approximation (see the Supplemental Material Note 1 for definitions [10]) shown in (d),(e). (f),(g) Direct and phonon-mediated photoluminescence, respectively, and (h) Auger recombination. Here, VB and CB correspond to valence and conduction bands, respectively. In all plots, blue arrows show the change in energy level of an excited electron or hole, green arrows show photons, dashed blue arrows show phonons, and yellow arrow shows energy transfer from one excitation to a second.
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Figure 2
(a) Tauc fit plots for $Ga As$ (direct-band-gap Tauc model), silicon (indirect-band-gap Tauc model), and two fits for a low-band-gap halide perovskite (formamidinium lead tin iodide, ${FAPb}_{0.5} {Sn}_{0.5} I_{3}$ ) using direct- and indirect-band-gap Tauc models. Extracted band gaps obtained are marked on the legend. Data are scaled appropriately, so they all lie on the same plot, but we note the y axis is scaled differently for direct- and indirect-band-gap materials (see the Supplemental Material Note 2 [10]). This plot is generated using data from Papatryfonos et al. [20], Green [21], and Bowman et al. [22] for $Ga As$ , silicon, and low-band-gap halide perovskite, respectively. (b) Schematic of Stokes shift between absorption and emission, reproduced from Sobarwiki [23]. (c) Absorption (black line) and emission (red line) spectra of tetracene in solution, demonstrating the Frank-Condon principle. Reprinted from Burdett et al. [24], with the permission of AIP Publishing. (d) Absorption and emission spectra of P3HT aggregates in solution and spin coated at −80 °C. Features in the film are broadened with respect to the solution. Reprinted with permission from Ferreira et al. [25]. Copyright 2012 American Chemical Society. (e) Absorbance (dotted) and photoluminescence (solid line) of exfoliated 2D halide perovskites, ${BA}_{2} {MA}_{n - 1} {Pb}_{n} I_{3 n + 1}$ , where BA is n-butylamine, MA is methylammonium, and n is the number of metal-halide sheets (number defined in plot). Reprinted (adapted) with permission from Paritmongkol et al. [26]. Copyright 2019 American Chemical Society.
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Figure 3
Modeled photoluminescence quantum efficiency curves, as a function of the excitation rate per unit volume per unit time, for an undoped system with free charges, for a doped system with free charges, and for an excitonic system. Modeling assumptions can be found in the Supplemental Material Note 3 [10].
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Figure 4
(a) Normalized TRPL decays for a ${MAPbI}_{3}$ film at different excitation densities. Time for the decay to fall to $1 / e$ of its initial value depends on the excitation density (see legend for excitation densities). Reproduced from Nagane et al. [59] (b) TCSPC signals of a formamidinium-cesium lead iodide ( ${Cs}_{0.3} {FA}_{0.7} {Pb I}_{3}$ ) halide perovskite following excitation at 470 nm at different incident laser powers (see legend), with signals offset from each other in time. Initial history-dependent part can be seen in signals. ${Cs}_{0.3} {FA}_{0.7} {Pb I}_{3}$ samples are supplied by Rohit Prasana following standard preparation methods [60], and TCSPC measurements follow methods previously described [61]. (c) Fitting of decay rates on a log-lin plot (for two initial excitation densities), reprinted figure with permission from Bowman et al. [56]. Copyright 2022 by the American Physical Society.
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Figure 5
(a) Scaling of initial TRPL counts for a free-charge ( ${MAPbI}_{3}$ ) system with low doping and an excitonic system (PtOEP). Fits are quadratic and linear, respectively. Data are generated via ICCD using the setup we previously described. [56] (b) Typical recorded TCSPC decay, where the signal falls to the instrument background level (5 µs between laser pulses). (c) Scaling of initial counts per laser frequency of this signal (proportional to $S_{TRP L, 0}$ ), as a function of the time between incident laser pulses. It can be seen that $S_{TRP L, 0}$ continues to reduce even when the time between pulses is much longer than the decay time, due to charge traps continuing to be occupied. Data for (b),(c) are generated using a 520 nm laser (PicoQuant LDH 400) pulse and recorded by a single-photon avalanche diode (LifeSpec – ps, Edinburgh Instruments). PtOEP is deposited on glass sonicated in isopropyl alcohol and then acetone via evaporation at a pressure of 10⁻⁵ mbar and temperature of 205 °C using an evaporation rate of about 0.2 Ås⁻¹ to achieve a thickness of approximately 10 nm. ${MAPbI}_{3}$ samples in (b),(c) are prepared following Nagane et al. [59].
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