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Pinning energies of organic semiconductors in high-efficiency organic solar cells
Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-9174-7593
Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering. (Wallenberg Wood Science Center)ORCID iD: 0000-0002-0300-8089
Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-3190-2774
Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering. (Wallenberg Wood Science Center)ORCID iD: 0000-0001-9879-3915
2023 (English)In: JOURNAL OF SEMICONDUCTORS, ISSN 1674-4926, Vol. 44, no 3, article id 032201Article in journal (Refereed) Published
Abstract [en]

With the emergence of new materials for high-efficiency organic solar cells (OSCs), understanding and finetuning the interface energetics become increasingly important. Precise determination of the so-called pinning energies, one of the critical characteristics of the material to predict the energy level alignment (ELA) at either electrode/organic or organic/organic interfaces, are urgently needed for the new materials. Here, pinning energies of a wide variety of newly developed donors and non-fullerene acceptors (NFAs) are measured through ultraviolet photoelectron spectroscopy. The positive pinning energies of the studied donors and the negative pinning energies of NFAs are in the same energy range of 4.3-4.6 eV, which follows the design rules developed for fullerene-based OSCs. The ELA for metal/organic and inorganic/organic interfaces follows the predicted behavior for all of the materials studied. For organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer, the pinning energies often underestimate the experimentally obtained interface vacuum level shift, which has consequences for OSC device performance.

Place, publisher, year, edition, pages
IOP Publishing Ltd , 2023. Vol. 44, no 3, article id 032201
Keywords [en]
organic semiconductors; organic solar cells; pinning energies; integer charge transfer; interface dipoles
National Category
Computer Systems
Identifiers
URN: urn:nbn:se:liu:diva-192672DOI: 10.1088/1674-4926/44/3/032201ISI: 000949633500001OAI: oai:DiVA.org:liu-192672DiVA, id: diva2:1746431
Note

Funding Agencies|Swedish Research Council [2016-05498, 2016-05990, 2020-04538]; Swedish Energy Agency [45411-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoeping University [2009 00971]; Wallenberg Wood Science Center (WWSC)

Available from: 2023-03-28 Created: 2023-03-28 Last updated: 2024-05-17
In thesis
1. Interfaces in Organic Solar Cells
Open this publication in new window or tab >>Interfaces in Organic Solar Cells
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic semiconductors (OSs), the promising candidates for the next generation electronic devices, have shown their advantages of light weight, flexibility, semi-transparency, tunable optical gaps, and energy levels, in the development history of over 70 years. OSs have been widely used in versatile applications such as organic light-emitting diodes, organic field-effect transistors, organic photodetectors, and organic solar cells (OSCs) which are mainly studied in this thesis. Nowadays, OSCs have been developed rapidly and reached a new era of high efficiencies with new records close to 20%, since the development of non-fullerene-based donor (D) -acceptor (A) systems. Despite the impressive progress in the field of OSCs, fundamental understandings on the interface energetics are lagging far behind the development of materials and device engineering, which dampens the further material design, device optimization, and scalable production.

Interface energetics take charge of many key electronic processes in organic electronic devices, such as charge injection or extraction at the electrode/ OS interface, charge generation or recombination at the D/A interface, and ambient optoelectronic stability resulted from the OS/air interface, all of which significantly affect the device performance. In this thesis, we systematically study the energy level alignment (ELA) at interfaces of inorganic/organic, organic/organic, and organic/air in OSCs, correlate the investigated energetic landscape with the device performance, and provide new understandings on the optoelectronic process in organic electronic devices.

Firstly, we determine the pinning energies of a wide variety of newly developed donors and non-fullerene acceptors (NFAs) through ultraviolet photoelectron spectroscopy, to provide the critical characteristics of the material for ELA prediction at either electrode/OS or D/A interfaces. The ELA for inorganic/organic interfaces follows the predicted behavior based on integer charge transfer model, but for organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer, the pinning energies often underestimate the experimentally obtained interface vacuum level (VL) shift. To explore the origin of the extra VL shift, we map the ELA at a range of D/NFA interfaces by fabricating and characterizing D-A bilayer heterojunctions monolayer-by-monolayer with the Langmuir-Schäfer technique. We find that the abrupt and significant VL shifts at the D-A interfaces are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of D-NFA systems. Furthermore, we investigate the influence of H2O and O2 molecules from ambient air on the work functions (WFs) of OS films. We find that OS films generally show higher WFs measured in ambient air, but lower WFs measured in high vacuum, compared to the WFs measured in ultrahigh vacuum. Two mechanisms are proposed to explain this phenomenon: (1) Competition between p-doping induced by O2 or H2O/O2 complexes, and n-doping induced by H2O clusters; (2) Polar H2O molecules preferentially modifying the ionization energy of one of the frontier molecular orbitals over the other. Finally, we fabricate the charge-transport-layer (CTL) free OSCs based on a newly developed NFA molecule with minimum performance degradation. Based on the determined D-A composition and ELA at the Anode/OS and the Cathode/OS interface, we propose several interface design rules for the efficient CTL-free devices, shedding new light on the simplified device structure for achieving more efficient optoelectronic applications.

Abstract [sv]

Organiska halvledare (OS), de lovande kandidaterna för nästa generations elektroniska enheter, har visat sina fördelar med låg vikt, flexibilitet, halvtransparens, avstämbara optiska luckor och energinivåer, i en utvecklingshistoria på över 70 år. OS har använts i stor utsträckning i mångsidiga tillämpningar såsom organiska lysdioder, organiska fälteffekttransistorer, organiska fotodetektorer och organiska solceller (OSC) som huvudsakligen studeras i denna avhandling. Nuförtiden har OSC:er utvecklats snabbt och nått en ny era av hög effektivitet med nya rekord nära 20 %, sedan utvecklingen av icke-fullerenbaserade donator (D) -acceptor (A) system. Trots de imponerande framstegen inom OSC:er släpar grundläggande förståelse för gränssnittsenergin långt efter utvecklingen av material och enhetsteknik, vilket dämpar ytterligare materialdesign, enhetsoptimering och skalbar produktion.

Gränssnittsenergi tar hand om många viktiga elektroniska processer i organiska elektroniska enheter, såsom laddningsinjektion eller extraktion vid elektroden/OS-gränssnittet, laddningsgenerering eller rekombination vid D/A-gränssnittet och omgivande optoelektronisk stabilitet som ett resultat av OS/ luftgränssnitt, som alla väsentligt påverkar enhetens prestanda. I denna avhandling studerar vi systematiskt energinivåanpassningen (ELA) vid gränssnitt mellan oorganiskt/organiskt, organiskt/organiskt och organiskt/ luft i OSC:er, korrelerar det undersökta energiska landskapet med enhetens prestanda och ger nya förståelser om den optoelektroniska processen i organiska elektroniska apparater.

För det första bestämmer vi gränssnittsenergierna för en mängd olika nyutvecklade donatorer och icke-fullerenacceptorer (NFA) genom ultraviolett fotoelektronspektroskopi, för att tillhandahålla de kritiska egenskaperna hos materialet för ELA-förutsägelse vid antingen elektrod/OS eller D/A gränssnitt. ELA för oorganiska/organiska gränssnitt följer det förutsagda beteendet baserat på heltalsladdningsöverföringsmodellen, men för organisk-organiska heteroövergångar där både donatorn och NFA har stark intramolekylär laddningsöverföring, underskattar gränssnittsenergierna ofta den experimentella erhållen gränssnittsvakuumnivå (VL) förskjutning. För att utforska ursprunget till det extra VL-skiftet, kartlägger vi ELA vid en rad D/NFA-gränssnitt genom att tillverka och karakterisera D-A-dubbelskiktsheteroövergångar monolager-per-monolager med Langmuir-Schäfer-tekniken. Vi finner att de abrupta och signifikanta VL-skiftningarna vid D-A-gränssnitten tillskrivs gränssnittsdipoler inducerade av D-A-elektrostatiska potentialskillnader. VL-skiftningarna resulterar i minskade gränssnittsenergiförskjutningar och ökade laddningsöverföringstillståndsenergier (CT) som förenar de motstridiga observationerna av stora energinivåförskjutningar som härleds från homogena filmer och stora CT-energier i D-NFA-system. Vidare undersöker vi påverkan av H2O- och O2- molekyler från omgivande luft på arbetsfunktionerna (WF) hos OS-filmer. Vi finner att OS-filmer generellt visar högre WF mätt i omgivande luft, men lägre WF mätt i högvakuum, jämfört med WF som mäts i ultrahögt vakuum. Två mekanismer föreslås för att förklara detta fenomen: (1) Konkurrens mellan p-doping inducerad av O2- eller H2O/O2-komplex och n-doping inducerad av H2O; (2) Polära H2O-molekyler modifierar företrädesvis joniseringsenergin hos en av gränsmolekylorbitalen framför den andra. Slutligen tillverkar vi laddningstransportlager (CTL) fria OSC:er baserade på en ny NFA-molekyl med minimal prestandaförsämring. Baserat på den bestämda D-A-sammansättningen och ELA vid anod/OS och katod/OS-gränssnittet, föreslår vi flera gränssnittsdesignregler för de effektiva CTL-fria enheterna, vilket kastar nytt ljus över den förenklade enhetsstrukturen för att uppnå mer effektiva optoelektroniska applikationer.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2023. p. 97
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2305
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-193170 (URN)10.3384/9789180751292 (DOI)9789180751285 (ISBN)9789180751292 (ISBN)
Public defence
2023-05-22, K2, Kåkenhus, Campus Norrköping, Norrköping, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilSwedish Energy Agency
Available from: 2023-04-18 Created: 2023-04-18 Last updated: 2023-04-19Bibliographically approved

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