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The life and death of perovskites: Interfacial function and degradation of lead halide perovskites studied by photoelectron spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Condensed Matter Physics of Energy Materials.ORCID iD: 0000-0001-7351-8183
2021 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Perovskiters liv och död : En studie av funktionen och degrationen i blyhalidperovskiters gränsskikt med fotoelektronspektroskopi (Swedish)
Abstract [en]

Lead halide perovskite solar cells are a promising new technology which could soon see widespread commercial application but is partly held back by poor long-term stability. In this thesis, photoelectron spectroscopy (PES) is used to study the dynamical processes at the surface or interfaces of lead halide perovskite materials. Some of these processes are responsible for the different types of degradation while others are essential for the function of the solar cell. The work includes a range of lead perovskite compositions with the general formula APbX3, in which A is a monovalent cation, and often organic (e.g. formamidinium or methylammonium), and X is a halide anion, typically Br- or I-. The compositions can also include mixtures of cations at the A and anions at the X site.

Part of this thesis is dedicated to investigating the degradation of the perovskite surface in response to both intense visible light and X-ray irradiation. The results show that intense illumination induces the decomposition of the perovskite into metallic lead, halide gas and organic halide salt, but also indicate how this process can be suppressed by the addition of small amounts of Cs+ ions and by adjusting the relative amounts of halides. A different process, induced by the X-ray radiolysis of the organic cation, is shown to consume rather than form metallic lead.

Another part of this thesis is dedicated to the investigation of the reactions at the interfaces between the perovskite and silver, copper or SnOx. The results show that both copper and silver react rapidly with the perovskite forming metal halides and that the metal can diffuse into the perovskite. Copper is particularly reactive, leading to the formation of two new compounds and the bulk degradation of the perovskite. The SnOx is significantly more stable but material intermixing results in the formation of a thin interface layer that may hinder charge extraction. 

Finally, a method for measuring both interfacial photovoltage and band alignment in a fully functional perovskite solar cell using hard X-ray photoelectron spectroscopy (HAXPES) is demonstrated. The results showcase the design considerations for the samples and the measurement setup and the potential of this technique. 

In summary, this thesis shows the suitability of PES for studying both the function and degradation of surfaces and interfaces of complex dynamical systems. It serves as a guide for future studies by highlighting challenges and possibilities faced when working with these systems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2021. , p. 75
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2082
Keywords [en]
lead halide perovskite solar cells, interfacial degradation, heterojunction interfaces, photoelectron spectroscopy, operando
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-454372ISBN: 978-91-513-1309-2 (print)OAI: oai:DiVA.org:uu-454372DiVA, id: diva2:1601385
Public defence
2021-11-25, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2021-11-02 Created: 2021-10-07 Last updated: 2021-11-12
List of papers
1. Experimental and theoretical core level and valence band analysis of clean perovskite single crystal surface
Open this publication in new window or tab >>Experimental and theoretical core level and valence band analysis of clean perovskite single crystal surface
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(English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095Article in journal (Other academic) Submitted
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-454337 (URN)
Available from: 2021-09-28 Created: 2021-09-28 Last updated: 2021-10-07
2. Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells
Open this publication in new window or tab >>Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells
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2017 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 40, p. 34970-34978Article in journal (Refereed) Published
Abstract [en]

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAP-bI(3))(0.85)(MAPbBr(3))(0.15)) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2017
Keywords
photoelectron spectroscopy, laser illumination, lead halide perovskite, ion migration, phase separation, stability
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-340141 (URN)10.1021/acsami.7b10643 (DOI)000413131500043 ()28925263 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 321319Swedish Research Council, 2014-6019Swedish Research Council, 2014-6463StandUpSwedish Foundation for Strategic Research , RMA15-0130
Available from: 2018-01-26 Created: 2018-01-26 Last updated: 2021-10-07Bibliographically approved
3. Effect of halide ratio and Cs+ addition on the photochemical stability of lead halide perovskites
Open this publication in new window or tab >>Effect of halide ratio and Cs+ addition on the photochemical stability of lead halide perovskites
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2018 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 6, no 44, p. 22134-22144Article in journal (Refereed) Published
Abstract [en]

Lead halide perovskite solar cells with multi-cation/mixed halide materials now give power conversion efficiencies of more than 20%. The stability of these mixed materials has been significantly improved through the addition of Cs+ compared to the original methylammonium lead iodide. However, it remains one of the most significant challenges for commercialisation. In this study, we use photoelectron spectroscopy (PES) in combination with visible laser illumination to study the photo-stability of perovskite films with different compositions. These include Br : I ratios of 50 : 50 and 17 : 83 and compositions with and without Cs+. For the samples without Cs and the 50 : 50 samples, we found that the surface was enriched in Br and depleted in I during illumination and that some of the perovskite decomposed into Pb0, organic halide salts, and iodine. After illumination, both of these reactions were partially reversible. Furthermore, the surfaces of the films were enriched in organic halide salts indicating that the cations were not degraded into volatile products. With the addition of Cs+ to the samples, photo-induced changes were significantly suppressed for a 50 : 50 bromide to iodide ratio and completely suppressed for perovskites with a 17 : 83 ratio at light intensities exceeding 1 sun equivalent.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-368425 (URN)10.1039/C8TA05795H (DOI)000456724800044 ()
Funder
Swedish Research Council, 2014-6019ÅForsk (Ångpanneföreningen's Foundation for Research and Development)Swedish Foundation for Strategic Research , RMA15-0130Swedish Energy Agency, P43294-1StandUp
Available from: 2018-12-04 Created: 2018-12-04 Last updated: 2021-10-07Bibliographically approved
4. X-ray stability and degradation mechanism of lead halide perovskites and lead halides.
Open this publication in new window or tab >>X-ray stability and degradation mechanism of lead halide perovskites and lead halides.
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2021 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 23, no 21, p. 12479-12489Article in journal (Refereed) Published
Abstract [en]

Lead halide perovskites have become a leading material in the field of emerging photovoltaics and optoelectronics. Significant progress has been achieved in improving the intrinsic properties and environmental stability of these materials. However, the stability of lead halide perovskites to ionising radiation has not been widely investigated. In this study, we investigated the radiolysis of lead halide perovskites with organic and inorganic cations under X-ray irradiation using synchrotron based hard X-ray photoelectron spectroscopy. We found that fully inorganic perovskites are significantly more stable than those containing organic cations. In general, the degradation occurs through two different, but not mutually exclusive, pathways/mechanisms. One pathway is induced by radiolysis of the lead halide cage into halide salts, halogen gas and metallic lead and appears to be catalysed by defects in the perovskite. The other pathway is induced by the radiolysis of the organic cation which leads to formation of organic degradation products and the collapse of the perovskite structure. In the case of Cs0.17FA0.83PbI3, these reactions result in products with a lead to halide ratio of 1 : 2 and no formation of metallic lead. The radiolysis of the organic cation was shown to be a first order reaction with regards to the FA+ concentration and proportional to the X-ray flux density with a radiolysis rate constant of 1.6 × 10-18 cm2 per photon at 3 keV or 3.3 cm2 mJ-1. These results provide valuable insight for the use of lead halide perovskite based devices in high radiation environments, such as in space environments and X-ray detectors, as well as for investigations of lead halide perovskites using X-ray based techniques.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2021
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-454329 (URN)10.1039/d1cp01443a (DOI)000653852800001 ()34037011 (PubMedID)
Funder
Swedish Foundation for Strategic Research , RMA15-0130Swedish Research Council, VR 2018-04125Swedish Research Council, VR 2018-04330Swedish Research Council, VR 201806465Swedish Energy AgencyGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyCarl Tryggers foundation , CTS 18:59
Available from: 2021-09-28 Created: 2021-09-28 Last updated: 2024-01-15Bibliographically approved
5. Degradation Mechanism of Silver Metal Deposited on Lead Halide Perovskites
Open this publication in new window or tab >>Degradation Mechanism of Silver Metal Deposited on Lead Halide Perovskites
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2020 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 12, no 6, p. 7212-7221Article in journal (Refereed) Published
Abstract [en]

Lead halide perovskite solar cells have significantly increased in both efficiency and stability over the last decade. An important aspect of their longterm stability is the reaction between the perovskite and other materials in the solar cell. This includes the contact materials and their degradation if they can potentially come into contact through, e.g., pinholes or material diffusion and migration. Here, we explore the interactions of silver contacts with lead halide perovskites of different compositions by using a model system where thermally evaporated silver was deposited directly on the surface of the perovskites. Using X-ray photoelectron spectroscopy with support from scanning electron microscopy, X-ray diffraction, and UV-visible absorption spectroscopy, we studied the film formation and degradation of silver on perovskites with different compositions. The deposited silver does not form a continuous silver film but instead tends to form particles on a bare perovskite surface. These particles are initially metallic in character but degrade into AgI and AgBr over time. The degradation and migration appear unaffected by the replacement of methylammonium with cesium but are significantly slowed down by the complete replacement of iodide with bromide. The direct contact between silver and the perovskite also significantly accelerates the degradation of the perovskite, with a significant loss of organic cations and the possible formation of PbO, and, at the same time, changed the surface morphology of the iodide-rich perovskite interface. Our results further indicate that an important degradation pathway occurred through gas-phase perovskite degradation products. This highlights the importance of control over the interface materials and the use of completely hermetical barrier layers for the long-term stability and therefore the commercial viability of silver electrodes.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2020
Keywords
perovskite solar cells, electrode stability, X-ray photoelectron spectroscopy, interface chemistry, noble metal electrodes
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-407282 (URN)10.1021/acsami.9b20315 (DOI)000514256400040 ()31958007 (PubMedID)
Funder
Swedish Research Council, VR 2018-04125Swedish Research Council, 2018-06465Swedish Research Council, 2018-04330Swedish Foundation for Strategic Research , RMA15-0130Swedish Energy Agency, P43549-1StandUpÅForsk (Ångpanneföreningen's Foundation for Research and Development)Göran Gustafsson Foundation for Research in Natural Sciences and Medicine
Available from: 2020-03-23 Created: 2020-03-23 Last updated: 2021-10-07Bibliographically approved
6. The complex degradation mechanism of copper electrodes on lead halide perovskite
Open this publication in new window or tab >>The complex degradation mechanism of copper electrodes on lead halide perovskite
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(English)In: ACS Materials Science Au, E-ISSN 2694-2461Article in journal (Other academic) Submitted
Keywords
perovskite solar cell, X-ray photoelectron spectroscopy, stability, back contact, metal, interface chemistr
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-454366 (URN)
Available from: 2021-09-28 Created: 2021-09-28 Last updated: 2023-10-06
7. SnOx Atomic Layer Deposition on Bare Perovskite: An Investigation of Initial Growth Dynamics, Interface Chemistry, and Solar Cell Performance
Open this publication in new window or tab >>SnOx Atomic Layer Deposition on Bare Perovskite: An Investigation of Initial Growth Dynamics, Interface Chemistry, and Solar Cell Performance
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2021 (English)In: ACS Applied Energy Materials, E-ISSN 2574-0962, Vol. 4, no 1, p. 510-522Article in journal (Refereed) Published
Abstract [en]

High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C61-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnOx electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)3 and (Cs,FA)Pb(Br,I)3 and in this study on Cs0.05FA0.79MA0.16PbBr0.51I2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnOx exposure was therefore suggested to form a nonideal perovskite/SnOx interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnOx growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnOx on the perovskite. Although measurements on the perovskite bulk below and the SnOx film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2021
Keywords
perovskite solar cell, ALD, in situ QCM, HAXPES, interface, SnOx
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-438732 (URN)10.1021/acsaem.0c02405 (DOI)000613720100058 ()33615157 (PubMedID)
Funder
StandUpSwedish Research Council, VR 2018-04125Swedish Research Council, 2018-06465Swedish Research Council, 2018-04330Swedish Foundation for Strategic Research , RMA15-0130Swedish Energy Agency, P43549-1Swedish Energy Agency, 2017-004796EU, Horizon 2020, 730872
Available from: 2021-03-30 Created: 2021-03-30 Last updated: 2024-01-15Bibliographically approved
8. Direct measurements of interfacial photovoltage and band alignment in perovskite solar cells using hard X-ray photoelectron spectroscopy
Open this publication in new window or tab >>Direct measurements of interfacial photovoltage and band alignment in perovskite solar cells using hard X-ray photoelectron spectroscopy
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2023 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 15, no 9, p. 12485-12494Article in journal (Refereed) Published
Abstract [en]

A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2023
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-454370 (URN)10.1021/acsami.2c17527 (DOI)000949872700001 ()36847773 (PubMedID)
Funder
Swedish Research Council, VR 2016-04590Swedish Research Council, VR 2018-04125Swedish Research Council, VR 2018-04330Swedish Research Council, VR 2018-06465Swedish Energy Agency, P50626-1Swedish Energy Agency, P43549-1Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologySwedish Foundation for Strategic Research, RMA15-0130Carl Tryggers foundation , CTS 18:59
Available from: 2021-09-28 Created: 2021-09-28 Last updated: 2023-04-17Bibliographically approved

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