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Functional Materials for Perovskite Solar Cells
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-0232-9937
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Energy plays a significant role in our daily lives, but most energy provided by fossil fuels causes serious environmental problems including air pollution, global warming, and ecological damage. In addition, it has been estimated that all of our fossil fuels will run out in 2088 and thus it is highly important to study and apply renewable energy sources. Among all the alternatives, solar energy is clean, sustainable, and abundant. It is estimated that the amount of power from the sun that strikes the earth in 90 minutes is more than the entire world consumes in one year. The perovskite solar cell (PSC) is one of the strongest tools to utilize solar energy because of its high power conversion efficiency and easy fabrication process. However, the lead that is normally used in the perovskite layer is considered harmful to the environment and to human health. Moreover, the low conductivity and hole mobility of the hole-transport material (HTM) Spiro-OMeTAD and the low overall device stability against humidity are all issues that might hinder the further development of PSC technology. This thesis concerns all of these aspects, with a general focus on different functional materials.

The aim of this thesis was to develop environmentally friendly and low-cost functional materials in order to solve existing problems while at the same time revealing insights into carrier transport, molecular doping, and surface passivation.

In Chapter 1 and Chapter 2, the current status of PSCs and the experimental and theoretical methods used in this thesis are presented.

In Chapter 3, the properties of coordination complexes, including molybdenum clusters and polyiodide-linked gold complexes, and their potential application in solar cells as lead-free light absorbers are discussed.

In Chapter 4, the synthesis of four coordination complexes with different metal cores and ligands and their application as HTMs in PSCs is discussed. Their oxidation potential, hole mobility, conductivity, and packing methods are presented.

In Chapter 5, two p-type dopants – Cu(bpcm)2 and (MeO-TPD)TFSI – are introduced for the organic HTM Spiro-OMeTAD. Both of these could significantly increase the conductivity of Spiro-OMeTAD films. In addition, (MeO-TPD)TFSI could work separately without hygroscopic LiTFSI at high doping amounts thus potentially increasing the device’s stability. The structure of oxidized Spiro-OMeTAD on the base of the Spiro(TFSI)2 is also discussed.

In Chapter 6, density functional theory modeling of four different functional groups – including amino (−NH2), phosphine (−PH2), hydroxyl (−OH), and thiol (−SH) groups – in combination with polyhedral oligomeric silsesquioxane is discussed in terms of estimating the adsorption energy with respect to different perovskite surface models. The amino functional group showed the strongest adsorption energy and was further compared with the thiol group in experiments.

Abstract [sv]

Energi spelar en avgörande roll i vårt vardagliga liv. Dessvärre kommer den allra största delen av den energi som tillhandahålls från fossila bränslen, vilka orsakar allvarliga miljöproblem såsom luftföroreningar, global uppvärmning och ekologiska skador. Dessutom finns det uppskattningar som för fram påståenden om att resurserna av fossila bränslen kommer att sina kring 2088, vilket sammantaget understryker behovet av att undersöka och implementera förnybar energi. Bland alternativen återfinns solenergi, vilken ren, hållbar och rikligt tillgänglig. I jämförelse kan det konstateras att den energi som motsvarar ca 1,5 timmes solljus som träffar jordens yta motsvarar en hel årsförbrukning för hela världen. Perovskitsolceller (PSC) utgör en mycket lovande ny teknik för att kunna utnyttja solens energi grundat på deras höga omvandlingseffektivitet tillsammans med enkla tillverkningsprocesser. Dessvärre innehåller perovskitsolcellerna det giftiga grundämnet bly, vilket är skadligt för både miljö och människa. Dessutom är perovskitsolcellerna beroende av omgivande material för att de ska nå höga prestanda, och även dessa är behäftade med problem. Håltransportmaterialet (HTM) Spiro-OMeTAD är förknippat med en inneboende låg ledningsförmåga och känslighet mot fukt. Detta påverkar perovskitsolcellernas stabilitet och utgör ett hinder för vidare utveckling mot en kommersialisering. Den här avhandlingen är fokuserad mot olika funktionella material för perovskitsolceller.

Målet med denna avhandling är att utveckla miljövänliga och billiga funktionella material, vilka kan bidra till att lösa perovskitsolcellernas existerande problem och samtidigt ge insikter om effekterna av laddningstransport, dopning och ytpassivering.

I kapitel 1 och 2 presenteras en statusöversikt för PSC, liksom de experimentella och teoretiska metoder som använts i denna avhandling.

Kapitel 3 ägnas åt implementeringen av nya bly-fria koordinationsföreningar i solceller exemplifierat med två system; molybdenbaserade kluster och polyjodidlänkade guldkomplex. Målet med dessa studier var att identifiera helt nya och bly-fria ljusabsorbenter.

Kapitel 4 ägnas åt studier av fyra nya koordinationsföreningar som HTM till PSC. Dessa omfattar olika metalljoner som koordinationscentra och studerades systematiskt rörande redoxpotentialer, hål-mobilitet, ledningsförmåga samt molekylära packningsmönster.

Kapitel 5 omfattar studier av två nya p-typ dopningsmaterial för organiska hålledare baserade på Cu(bpcm)2 och (MeO-TPD)TFSI. Båda ökar markant ledningsförmågan hos Spiro-OMeTAD. (MeO-TPD)TFSI fungerar dessutom effektivt vid höga koncentrationer utan tillsatser av det hygroskopiska saltet LiTFSI, och detta leder till en tydligt bättre stabilitet hos de resulterande solcellerna. Strukturen hos oxiderad Sprio-OMeTAD resp Spiro(TFSI)2 har analyserats.

I kapiltel 6 undersöks slutligen effekterna av polyhedrala oligomera silsesquioxaner (POSS) med en serie av funktionella sidoarmar terminerade med amin- (−NH2), fosfin (−PH2), hydroxyl (−OH), and tiol (−SH) –grupper. Dessa har undersökts både teoretiskt med hjälp av täthetsfunktionalteori (DFT) för att uppskatta de olika systemens adsorptionsenergier tillsammans med effekter på solceller. POSS-systemen terminerade med amingrupper uppvisade starkast adsorption och tydligast stabiliserande effekter.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 80
Series
TRITA-CBH-FOU ; 2020:11
Keywords [en]
perovskite solar cell, photovoltaic device, lead-free light absorbers, hole-transport materials, coordination complexes, p-type dopants, organic salts, surface passivation.
National Category
Chemical Sciences Chemical Sciences
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-267210ISBN: 978-91-7873-448-1 (print)OAI: oai:DiVA.org:kth-267210DiVA, id: diva2:1391367
Public defence
2020-02-28, K2, Teknikringen 28, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 2020-02-04

Available from: 2020-02-04 Created: 2020-02-04 Last updated: 2020-02-04Bibliographically approved
List of papers
1. Investigation of Triphenylamine (TPA)-Based Metal Complexes and Their Application in Perovskite Solar Cells
Open this publication in new window or tab >>Investigation of Triphenylamine (TPA)-Based Metal Complexes and Their Application in Perovskite Solar Cells
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2017 (English)In: ACS OMEGA, ISSN 2470-1343, Vol. 2, no 12, p. 9231-9240Article in journal (Refereed) Published
Abstract [en]

Triphenylamine-based metal complexes were designed and synthesized via coordination to Ni(II), Cu(II), and Zn(II) using their respective acetate salts as the starting materials. The resulting metal complexes exhibit more negative energy levels (vs vacuum) as compared to 2,2', 7,7'-tetrakis(N, N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD), high hole extraction efficiency, but low hole mobilities and conductivities. Application of dopants typically used for Spiro-OMeTAD was not successful, indicating a more complicated mechanism of partial oxidation besides the redox potential. However, utilization as hole-transport material was successful, giving a highest efficiency of 11.1% under AM 1.5G solar illumination.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2017
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-221019 (URN)10.1021/acsomega.7b01434 (DOI)000418744400078 ()2-s2.0-85040066582 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20180112

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2020-02-04Bibliographically approved
2. The Central Role of Ligand Conjugation for Properties of Coordination Complexes as Hole-Transport Materials in Perovskite Solar Cells
Open this publication in new window or tab >>The Central Role of Ligand Conjugation for Properties of Coordination Complexes as Hole-Transport Materials in Perovskite Solar Cells
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2019 (English)In: ACS APPLIED ENERGY MATERIALS, ISSN 2574-0962, Vol. 2, no 9, p. 6768-6779Article in journal (Refereed) Published
Abstract [en]

Two zinc-based coordination complexes Y3 and Y4 have been synthesized and characterized, and their performance as hole-transport materials (HTMs) for perovskite solar cells (PSCs) has been investigated. The complex Y3 contains two separate ligands, and the molecular structure can be seen as a disconnected porphyrin ring. On the other hand, Y4 consists of a porphyrin core and therefore is a more extended conjugated system as compared to Y3. The optical and redox properties of the two different molecular complexes are comparable. However, the hole mobility and conductivity of Y4 as macroscopic material are remarkably higher than that of Y3. Furthermore, when employed as hole-transport materials in perovskite solar cells, cells containing Y4 show a power conversion efficiency (PCE) of 16.05%, comparable to the Spiro-OMeTAD-based solar cells with an efficiency around 17.08%. In contrast, solar cells based on Y3 show a negligible efficiency of about 0.01%. The difference in performance of Y3 and Y4 is analyzed and can be attributed to the difference in packing of the nonplanar and planar building blocks in the corresponding materials.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
Keywords
ligand conjugation, coordination complex, porphyrin, hole-transport material, perovskite
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-261967 (URN)10.1021/acsaem.9b01223 (DOI)000487770000073 ()
Note

QC 20191014

Available from: 2019-10-14 Created: 2019-10-14 Last updated: 2020-02-04Bibliographically approved
3. Cu(II) Complexes as p-Type Dopants in Efficient Perovskite Solar Cells
Open this publication in new window or tab >>Cu(II) Complexes as p-Type Dopants in Efficient Perovskite Solar Cells
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2017 (English)In: ACS ENERGY LETTERS, ISSN 2380-8195, Vol. 2, no 2, p. 497-503Article in journal (Refereed) Published
Abstract [en]

In this work, two Cu(II) complex compounds are designed and synthesized for applications as p-type dopants in solid-state perovskite solar cells (PSCs). Through the characterization of the optical and electrochemical properties, the complex Cu(bpcm)(2) is shown to be eligible for oxidization of the commonly used hole-transport material (HTM) SpiroOMeTAD. The reason is the electron-withdrawing effect of the chloride groups on the ligands. When the complex was applied as p-type dopant in PSCs containing Spiro-OMeTAD as HTM, an efficiency as high as 18.5% was achieved. This is the first time a Cu(II) pyridine complex has been used as p-type dopant in PSCs.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2017
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-204091 (URN)10.1021/acsenergylett.6b00691 (DOI)000394080000031 ()2-s2.0-85034060563 (Scopus ID)
Note

qc 20170329

Available from: 2017-03-29 Created: 2017-03-29 Last updated: 2020-02-04Bibliographically approved
4. Organic Salts as p-Type Dopants for Efficient LiTFSI-free Perovskite Solar Cells
Open this publication in new window or tab >>Organic Salts as p-Type Dopants for Efficient LiTFSI-free Perovskite Solar Cells
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Despite the ubiquity and importance of organic hole-transport materials in photovoltaic devices, their intrinsic low conductivity remains a drawback. Thus, chemical doping forms an indispensable solution always required. The most widely used p-type dopant, FK209, is a cobalt coordination complex. By reducing Co(III) to Co(II), Spiro-OMeTAD becomes partially oxidized and the film conductivity is initially increased. To further increase the conductivity, the hygroscopic co-dopant LiTFSI is typically needed. However, lithium salts are normally quite hygroscopic, and thus water absorption has been suggested as a significant reason for perovskite degradation and therefore limited device stability. In this work, we report a LiTFSI-free doping process by applying organic salts in relatively high amounts. Film conductivity and morphology are studied at different doping amounts. The resulting solar cell devices show comparable power conversion efficiencies (PCEs) as those based on conventional LiTFSI-doped Spiro-OMeTAD but considerably better long-term device stability in ambient atmosphere.

Keywords
p-type dopant
National Category
Chemical Sciences
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-267208 (URN)
Note

QC 20200205

Available from: 2020-02-04 Created: 2020-02-04 Last updated: 2020-02-05Bibliographically approved
5. Single crystal structure and opto-electronic properties of oxidized Spiro-OMeTAD
Open this publication in new window or tab >>Single crystal structure and opto-electronic properties of oxidized Spiro-OMeTAD
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2020 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548XArticle in journal (Refereed) Published
Abstract [en]

Single crystals of Spiro(TFSI)2 were grown, the optical and electronic properties were characterized and compared with neutral Spiro-OMeTAD. Density-functional theory was used to get insights into binding and band structure properties. The flat valence bands indicate a rather limited orbital overlap in Spiro(TFSI)2.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2020
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-267203 (URN)10.1039/C9CC09270F (DOI)
Note

QC 20200204

Available from: 2020-02-04 Created: 2020-02-04 Last updated: 2020-02-04Bibliographically approved
6. Mechanistic Insights from Functional Group Exchange Surface Passivation: A Combined Theoretical and Experimental Study
Open this publication in new window or tab >>Mechanistic Insights from Functional Group Exchange Surface Passivation: A Combined Theoretical and Experimental Study
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2019 (English)In: ACS APPLIED ENERGY MATERIALS, ISSN 2574-0962, Vol. 2, no 4, p. 2723-2733Article in journal (Refereed) Published
Abstract [en]

Four different functional groups including amino (-NH2), phosphine (-PH2), hydroxyl (-OH), and thiol (-SH) were combined with POSS (polyhedral oligomeric silsesquioxane) molecules to investigate how functional groups affect the surface passivation of POSS systems. Results from density-functional theory (DFT) calculations indicate that functional group amino (-NH2) with adsorption energy 86 (56) kJ mol(-1) is consistently better than that of thiol (-SH) with adsorption energy 68 (43) kJ mor(-1) for different passivation mechanisms. Theoretical studies on the analogous POSS-OH and POSS-PH2 systems show similar adsorption energies. Two of the systems were also investigated experimentally; aminopropyl isobutyl POSS (POSS-NH2) and mercaptopropyl isobutyl POSS (POSS-SH) were applied as passivation materials for MAPbI(3) (MA = methylammonium) perovskite and (FA)(0.85)(MA)(0.15)Pb(I-3)(0.85)(Br-3)(0)(.15)(FA = formamidinium) perovskite films. The same conclusion was drawn based on the results from contact angle studies, X-ray diffraction (XRD), and the stability of solar cells in ambient atmosphere, indicating the vital importance of choice of functional groups for passivation of the perovskite materials.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
Keywords
perovskite solar cells, polyhedral oligomeric silsesquioxane (POSS), passivation, DFT calculation, stability
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-251284 (URN)10.1021/acsaem.9b00050 (DOI)000465644600047 ()
Note

QC 20190517

Available from: 2019-05-17 Created: 2019-05-17 Last updated: 2020-02-04Bibliographically approved

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