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Spectroscopy on the Dot: Photoelectron Spectroscopy and Time-Resolved Studies of Lead Sulfide Quantum Dots for Solar Cells
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0002-6469-3374
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Renewable energy is an important topic as global energy consumption continues to rise. Because the sun emits an enormous amount of energy, solar energy is a promising source. However, most of the commercial solar cell technology is manufactured in an energy demanding process and there is a need for new, easily processed materials. This thesis concerns quantum dots, which are nanoparticles that can absorb light of different energies depending on their size. They can be synthesised by solution-based chemistry and turned into solid thin films to harvest sunlight. The fundamental properties of quantum dots need to be better understood before production on large scales may commence. The aim of this thesis was to investigate the fundamental properties of lead sulfide quantum dots. 

The methods used in this thesis are based on photoelectron spectroscopy. They allowed investigation of materials as-is, but also changes upon excitation by laser or X-rays. Using a laser, dynamics on pico- to microsecond timescales were studied by time-resolved photoelectron spectroscopy. Using a range of X-rays, the probability of charge transfer in the attosecond range was investigated.  

Steady-state investigation showed that different surface treatment of the quantum dots caused different resistance towards surface oxidation and X-ray damage. 

Different layers in the structure of solar cells can influence the photovoltage, an important parameter in achieving high power conversion efficiencies. Time-resolved photoelectron spectroscopy was developed and used to investigate the contributions of the layers to photovoltage generation. We observed photovoltage dynamics on a timescale covering six orders of magnitude. 

The mechanism of charge transfer in quantum dots of different sizes was studied by core-hole clock spectroscopy in the attosecond regime. Our results show that quantum confinement affects the charge transfer only at low excitation energies. 

Abstract [sv]

Förnybar energi är viktig då den globala energikonsumtionen fortsätter öka. Solenergi är en lovande energikälla eftersom solen strålar ut otroligt mycket energi. Då kommersiell solcellsteknologi använder mycket energikrävande tillverkningsprocesser finns det behov av nya material som är enkla att tillverka. Denna avhandling handlar om kvantpunkter som är nanopartiklar vilka kan absorbera ljus av olika energi, beroende på partiklarnas storlek. 

Lösningsbaserad syntes är en teknik för att tillverka tunna filmer av kvantpunkter. Grundläggande egenskaper hos kvantpunkter behöver kartläggas ytterligare innan storskalig produktion av sådana kan påbörjas. Målet med denna avhandling var att undersöka grundläggande egenskaper hos kvantpunkter bestående av blysulfid.   

Analysmetoderna i denna avhandling baseras på fotoelektron-spektroskopi. Dessa möjliggör studier av material utan ytterligare bearbetning samt hur de förändras när de påverkas av laserljus eller röntgenstrålning. Med laserpåverkan har dynamik på pico- till mikrosekundsskala kunnat studeras med tidsupplöst fotoelektron-spektroskopi och genom att använda röntgenstrålning med olika energi har sannolikheten för laddningsöverföring studerats på attosekundtidsskalan.

Tidsoberoende studier visade att olika ytbehandlingar på kvantpunkterna gav upphov till olika motstånd mot oxidation och strålskador. 

Olika lager i solcellers struktur kan påverka fotospänningen, vilken är en viktig parameter för att uppnå hög energiomvandlingseffektivitet. Tidsupplöst fotoelektronspektroskopi utvecklades och användes för att undersöka olika lagers bidrag till fotospänningen. Vi observerade tidsberoendet hos fotospänningen på en tidskala som spände över sex storleksordningar.

Laddningsöverföringsmekanismen hos kvantpunkter med olika storlek studerades på attosekundstidsskalan med kärnhålsklocks-spektroskopi. Våra resultat visar att kvanteffekter orsakade av instängning påverkar laddningsöverföringen enbart vid låga excitationsenergier. 

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2022. , p. 107
Series
TRITA-CBH-FOU ; 2022:52
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-319933ISBN: 978-91-8040-370-2 (print)OAI: oai:DiVA.org:kth-319933DiVA, id: diva2:1702653
Public defence
2022-11-04, Kollegiesalen, Brinnelvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2022-10-11

Available from: 2022-10-11 Created: 2022-10-11 Last updated: 2022-10-11Bibliographically approved
List of papers
1. Probing and Controlling Surface Passivation of PbS Quantum Dot Solid for Improved Performance of Infrared Absorbing Solar Cells
Open this publication in new window or tab >>Probing and Controlling Surface Passivation of PbS Quantum Dot Solid for Improved Performance of Infrared Absorbing Solar Cells
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2019 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 31, no 11, p. 4081-4091Article in journal (Refereed) Published
Abstract [en]

Surface properties of colloidal quantum dots (CQDs) are critical for the transportation and recombination of the photoinduced charge carrier in CQD solar cells, therefore dominating the photovoltaic performance. Herein, PbS CQD passivated using liquid-state ligand exchange (LSLX) and solid-state ligand exchange (SSLX) strategies are in detail investigated using photoelectron spectroscopy (PES), and solar cell devices are prepared to understand the link between the CQD surface properties and the solar cell function. PES using different energies in the soft and hard Xray regime is applied to study the surface and bulk properties of the CQDs, and the results show more effective surface passivation of the CQDs prepared with the LSLX strategy and less formation of lead-oxide. The CQD solar cells prepared with LSLX strategy show higher performance, and the photoelectric measurements suggest that the recombination of photoinduced charges is reduced for the solar cell prepared with the LSLX approach. Meanwhile, the fabricated solar cells exhibit good stability. This work provides important insights into how to fine-tune the CQD surface properties by improving the CQD passivation, and how this is linked to further improvements of the device photovoltaic performance.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2019
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-254499 (URN)10.1021/acs.chemmater.9b00742 (DOI)000471728200026 ()2-s2.0-85067114703 (Scopus ID)
Note

QC 20190715

Available from: 2019-07-16 Created: 2019-07-16 Last updated: 2024-03-15Bibliographically approved
2. The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials
Open this publication in new window or tab >>The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials
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2022 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 24, no 20, p. 12645-12657Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Royal Society of Chemistry (RSC), 2022
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-319922 (URN)10.1039/d2cp01050j (DOI)000796667900001 ()35579959 (PubMedID)2-s2.0-85131425424 (Scopus ID)
Funder
Swedish Research Council, 2018-04125Swedish Foundation for Strategic Research, RMA15-0130Carl Tryggers foundation , CTS 18:59EU, Horizon 2020, 730872
Note

QC 20221011

Available from: 2022-10-11 Created: 2022-10-11 Last updated: 2024-03-18Bibliographically approved
3. A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots
Open this publication in new window or tab >>A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots
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2020 (English)In: Scientific Reports, E-ISSN 2045-2322, Vol. 10, no 1, article id 22438Article in journal (Refereed) Published
Abstract [en]

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

Place, publisher, year, edition, pages
Springer Nature, 2020
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-289492 (URN)10.1038/s41598-020-79792-z (DOI)000605618400009 ()33384445 (PubMedID)2-s2.0-85098490097 (Scopus ID)
Note

QC 20210203

Available from: 2021-02-03 Created: 2021-02-03 Last updated: 2022-10-11Bibliographically approved
4. Photovoltage Generation across Different Interfaces in a PbS QuantumDot Solar Cell Investigated by Time-Resolved PhotoelectronSpectroscopy
Open this publication in new window or tab >>Photovoltage Generation across Different Interfaces in a PbS QuantumDot Solar Cell Investigated by Time-Resolved PhotoelectronSpectroscopy
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Quantum dot solar cells have not yet achieved optimal device performances and to direct development there is thereforea need to understand the device function of present solar cell structures in more detail. Understanding where photovoltage isgenerated in a device and where energy losses occur is a key aspect of this. We have previously shown that time-resolved core levelphotoelectron spectroscopy can be used to follow the photovoltage rise and decay at a specific interface from pico- to microsecondtimescales. Here, we extend this study and investigate the photovoltage generation in different parts of a PbS quantum dot solar cellthrough sample design. We show that thick absorbing quantum dot layers are required for generating a high photovoltage at theinterface between n-type PbS quantum dots and p-type quantum dots. Furthermore, we show that the full photovoltage is only generatedwhen a gold contact is deposited on the quantum dots and that the presence of this contact also leads to significantly slowercharge recombination.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-319924 (URN)
Note

QC 20221011

Available from: 2022-10-11 Created: 2022-10-11 Last updated: 2022-10-11Bibliographically approved
5. Unravelling the ultrafast charge dynamics in PbS quantum dotsthrough resonant Auger mapping of the sulfur K-edge
Open this publication in new window or tab >>Unravelling the ultrafast charge dynamics in PbS quantum dotsthrough resonant Auger mapping of the sulfur K-edge
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they arepromising for application in photovoltaics and other optoelectronic devices. The ultrafastcharge transport is intriguing, offering insight into the mechanism of electron tunnelingprocesses within the material. In this study we investigated the charge transfer times of PbSquantum dots of different sizes and non-quantized PbS reference materials by comparing thepropensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays.We show that charge transfer times in PbS quantum dots decrease with excitation energy andare similar at high excitation energy for quantum dots and non-quantized PbS. However, atlow excitation energies a distinct difference in charge transfer time is observed with thefastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots.Our observations can be explained by iodide ligands on the quantum dots creating a barrierfor charge transfer, which reduces the probability of interparticle transfer at low excitationenergies. The probability of intraparticle charge transfer is limited by the density of availablestates which we describe according to a wavefunction in a quantum well model. The strongerquantum confinement effect in smaller PbS quantum dots is manifested as longer chargetransfer times relative to the larger quantum dots at low excitation energies.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-319925 (URN)
Note

QC 20221011

Available from: 2022-10-11 Created: 2022-10-11 Last updated: 2023-12-04Bibliographically approved

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