Although halide perovskite materials hold great promise for optoelectronics, defect-assisted recombination still limits their efficiency. In the April issue of Matter, Fassl et al. present an open-source model for analyzing the photoluminescence spectra of perovskite films, showing a much lower internal quantum efficiency than previously thought in the field

Hutter, Eline M.

Utrecht University Repository

llPreviewsPerovskite escape room:Which photons leave the film,and which are trapped inside?Eline M. Hutter1,*Although halide perovskite materials hold great promise for opto-electronics, defect-assisted recombination still limits their effi-ciency. In the April issue of Matter, Fassl et al. present an open-source model for analyzing the photoluminescence spectra ofperovskite films, showing a much lower internal quantum efficiencythan previously thought in the field.1Inorganic Chemistry and Catalysis, Departmentof Chemistry, Utrecht University, Princetonlaan 6,3584 CB Utrecht, the Netherlands*Correspondence: e.m.hutter@uu.nlhttps://doi.org/10.1016/j.chempr.2021.03.017Solar cells employing halide perovskitesemiconductors as light absorbers havebeen rapidly developed, resulting in re-cord power conversion efficiencies largerthan 25%.1 Combining these perovskitesolar cells with commercial silicon solarcells into tandem cells has even led to re-cord efficiencies ofmore than 29%. Addi-tionally, halide perovskites have beensuccessfully used as photoactive mate-rials in other applications such as light-emitting diodes and radiation detectors,as well as in photocatalysis.2,3 The excel-lent performance of halide perovskites inthese applications is mainly due to thefact that the densities of detrimental de-fects are relatively low (~1015 cm3).4 Asa result of these low defect densities,light-generated charges canbeefficientlycollected. Vice versa, injected chargesefficiently emit photons because of thehigh yield of radiative recombinationcompared with non-radiative defect-as-sisted recombination. In order to reachthe thermodynamic efficiency limit forthe above-mentioned applications, it willbe essential to eliminate all defect-assis-ted recombination pathways. Ideally, thephotoluminescence quantum efficiency(QE) is unity at the relevant charge den-sities, meaning that every electron-holepair can only recombine radiatively.Consequently, photoluminescence mea-surements are widely used for assessingthe quality of halide perovskite samplesand studying the effect of the synthesismethod, composition, or post-synthetictreatments. Although measuring thephotoluminescence is experimentallyone of the most straightforward charac-terization methods, interpretation ofphotoluminescence spectra and/or life-times is not that trivial and requiresdetailed analysis.5 This is especially aconcern when dealing with thin films,where a substantial part of the emittedphotons will be internally reflected andhence not detected in a photolumines-cence measurement. In halide perov-skites, the chance that an emitted photonwill leave the film (escape probability)6 istypically quite low because the refractiveindex (n ~ 2.6) is relatively high withrespect to air. The escape probabilityvaries with the surface roughness7 andcan change when a passivating orcharge-transport layer with a differentrefractive index is deposited on top ofthe perovskite. Variations in the escapeprobability lead to differences in the in-tensity and/or spectral shape of the exter-nally measured photoluminescence.Therefore, in order to use photolumines-cence measurements to fully understandhow an experimental variable, such asthe synthesis method, affects thedefect-assisted recombination, it iscrucial to understand whether it affectsthe escape probability.In the April issue of Matter, Fassl et al.present a rigorous method to analyzethe externally measured photolumines-Chem 7, 8cence spectra of methylammoniumlead iodide (MAPbI3) perovskites.8Measuring the photoluminescence in-side an integrating sphere enables theauthors to detect photons emitted inall directions, in contrast to measuringthe photoluminescence at a singlespot or angle. An important observa-tion is that the photoluminescencespectra collected inside an integratingsphere are much broader and morered shifted than spectra collected froma 6 mm spot. In addition, collecting allemitted photons by using an inte-grating sphere shows that the spectralshape depends on the film thickness.Although the spectra collected fromthe top are similar, the integratingsphere measurements show that 260-nm-thick films emit more low-energyphotons than 80-nm-thick films. Theseobservations are interpreted from thereabsorption of emitted photons, whichoccurs when the wavelength of an inter-nally reflected photon overlaps the ab-sorption spectrum of the perovskite(see Figure 1). Because low-energyphotons have less overlap with the ab-sorption spectrum, these will eventuallyhave a higher chance of leaving the filmthan high-energy photons. Fassl et al.refer to these non-absorbed low-en-ergy photons (that leave the perovskitefilm after several internal reflections) asscattered photons.8 The emission spec-trum of these scattered photons is redshifted with respect to directly emittedphotons, which leads to an additionalpeak in the photoluminescence spec-trum. Because the ratio between thetwo varies from sample to sample,they argue that this combination ofscattered and directly emitted photonscould explain the wide variation in pho-toluminescence spectra reported in theliterature.34–848, April 8, 2021 ª 2021 Elsevier Inc. 845Figure 1. Schematic of the different pathways after a radiative recombination event in a thinperovskite filmThe photons within the escape cone will be emitted externally (detectable), whereas the ones out ofthe escape cone will be reflected internally (not detectable). These internally reflected photons canbe reabsorbed by the perovskite if their energy matches the absorption spectrum of the perovskite.Photons with lower energy are more likely to leave the film after several internal reflections(scattered; detectable in an integrating sphere).llPreviewsFassl et al. also present a fitting proced-ure to determine both the escape prob-ability and the internal QE from thenumber of externally emitted photonsper absorbed photon (the externalQE). A high internal QE is associatedwith low defect densities and is there-fore an important parameter for pre-dicting material quality and device per-formance. Previous assumptions in theliterature on the escape probabilityranged from 5% to 13% and have ledto estimated internal QEs of around90%. However, Fassl et al. show thatthe escape probability is actually muchhigher and that consequently, the inter-nal QE is more likely close to 80% (at~1017 absorbed photons cm2s1).This means that the chance that a pho-togenerated electron-hole pair will re-combine non-radiatively is approxi-mately 20%: twice as large aspreviously estimated. Therefore, animportant implication of this higherescape probability is that defect-assis-ted recombination in MAPbI3 perov-skites plays a larger role than what wasthought before. In addition, the authorsshow how the internal reflection de-pends on both the wavelength ofemitted photons and the film thickness,so that the internal QE cannot be ob-tained if one assumes a single valuefor the escape probability. Finally,MAPbI3 on indium tin oxide (ITO) is846 Chem 7, 834–848, April 8, 2021used as a showcase for developing afitting procedure that analyzes theparasitic absorption, i.e., the numberof emitted photons that are reabsorbedby a contact layer. The authors predictthat increasing the internal QE whileminimizing the parasitic absorptionwill be necessary for solar cells to reachtheir thermodynamic efficiency limitbecause this will maximize the open-cir-cuit voltage (VOC).The authors have provided the fullcurve-fitting model as an open-sourceMATLAB app, making it an easily acces-sible fitting procedure for others toanalyze similar data without the needto reproduce the code. Although thisinitial work started with MAPbI3 perov-skites, the same approach can in factbe extended to any other thin-filmsemiconductor. Therefore, the articleand freely available fitting procedureby Fassl et al. could find use in a muchbroader research community thanthose working on perovskite solar cells.Still, performing the full analysis mightnot be straightforward for everyonegiven that it still requires access to anintegrating sphere. However, theobservation that both the intensity andspectral shape of photoluminescenceare very sensitive to the measurementconditions, as shown by Fassl et al.,stresses the importance of carefullyconsidering parameters such assurface roughness and film thicknesswhen comparing photoluminescencespectra. In addition, one should takeinto account that contacting the perov-skite with a passivating layer or charge-transport material could affect thephotoluminescence intensity throughchanges in the escape probabilityand/or the parasitic absorption. Hence,although measuring the photolumines-cence is relatively simple in comparisonwith other spectroscopic techniques,drawing conclusions on how an experi-mental variable affects parameterssuch as defect densities might requireextensive data analysis or complemen-tary measurement techniques.1. National Renewable Energy Laboratory.http://www.nrel.gov.2. Huang, H., Pradhan, B., Hofkens, J., Roeffaers,M.B.J., and Steele, J.A. (2020). Solar-drivenmetal halide perovskite photocatalysis:design, stability, and performance. ACSEnergy Lett. 5, 1107–1123.3. Jeong, D.-N., Yang, J.M., and Park, N.-G.(2020). Roadmap on halide perovskite andrelated devices. Nanotechnology 31, 152001.4. Brenner, T.M., Egger, D.A., Kronik, L., Hodes,G., and Cahen, D. (2016). Hybrid organic-inorganic perovskites: low-costsemiconductors with intriguing charge-transport properties. Nat. Rev. Mater. 1, 15007.5. Hutter, E.M., Kirchartz, T., Ehrler, B., Cahen, D.,and Von Hauff, E. (2020). Pitfalls and prospectsof optical spectroscopy to characterizeperovskite-transport layer interfaces. Appl.Phys. Lett. 116, https://doi.org/10.1063/1.5143121.6. Kirchartz, T., Krückemeier, L., and Unger, E.L.(2018). Research update: recombination andopen-circuit voltage in lead-halideperovskites. APL Mater. 6, https://doi.org/10.1063/1.5052164.7. Richter, J.M., Abdi-Jalebi, M., Sadhanala, A.,Tabachnyk, M., Rivett, J.P.H., Pazos-Outón,L.M., Gödel, K.C., Price, M., Deschler, F., andFriend, R.H. (2016). Enhancingphotoluminescence yields in lead halideperovskites by photon recycling and light out-coupling. Nat. Commun. 7, 13941.8. Fassl, P., Lami, V., Berger, F.J., Falk, L.M.,Zaumseil, J., Richards, B.S., Howard, I.A.,Vaynzof, Y., and Paetzold, U.W. (2021).Revealing the internal luminescence quantumefficiency of perovskite films via accuratequantification of photon recycling. Matter 4,https://doi.org/10.1016/j.matt.2021.01.019.

Perovskite escape room: Which photons leave the film, and which are trapped inside?

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Perovskite escape room: Which photons leave the film, and which are trapped inside?

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