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Bian, Qingzhen
Publications (2 of 2) Show all publications
Bian, Q. (2020). Excitonic and charge carrier transport in organic materials and device applications. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Excitonic and charge carrier transport in organic materials and device applications
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the potential for future commercial use, organic electronics have been intensively studied for the last few decades. To exploit the next generation of high-performance devices, detailed study of the underlying physics is essential. Excitonic and charge carrier transport plays a critical role in device performance and related studies have attracted a lot of attention in recent decades. This thesis particularly focused on excitonic and charge carrier transport in organic materials and related device applications.

In natural light harvesting systems, such as the reaction centers of purple bacteria, quantum coherence has been proposed to be present as a contributor to the related charge and energy transport processes, and almost 100% charge conversion is present in these efficient biological systems. This high energy conversion efficiency inspires the idea that if a similar strategy was used in artificial energy conversion devices such as organic photovoltaics, etc., this could significantly enhance the device’s performance. In the first study, the charge separation process in some donor/acceptor blends was investigated. The contribution of quantum coherence to device performance was studied in detail using several steady state and ultrafast transient techniques. In one efficient donor/acceptor blend, a pronounced coherence of charge separation was identified, which contributed to the enhancement of the photocurrent generation, which finally resulted in efficient device performance.

For the light emitting diodes, triplet excitons harvesting plays a critical role in device performance. In the thermally activated delayed fluorescence (TADF) materials, due to an efficient reverse intersystem process from triplet excitons to singlet excitons, the losses due to triplet excitons were suppressed. As a result, a desired high quantum yield has been achieved. To enhance device efficiency, the detailed study of the upconversion physics between triplet and singlet is needed. Previous studies have proposed some physical models to explain this efficient upconversion process, while the nature of this physical process is still under debate and unclear. In my second work, we studied the exciton kinetics in two different TADF materials. These TADF materials were inserted in a protein fibril host, and the resulting protein scaffold was able to modify the geometric configuration of the related TADF molecule. As a result, an enhancement of the photoluminescence quantum yield was achieved.

To achieve efficient device performance in organic electronics, the physical processes at the metal/material interface and charge carrier injection/extraction, also play a critical role. Efficient charge injection can be achieved by Ohmic contact, and charge injection/extraction of metal/organic materials has been intensively studied in the last few decades. In my third study, an efficient hole transport material based on the biopolymer DNA was introduced. A hole doping process was found in the hybrid materials and contributes to the Ohmic contacts. The hybrid material can be used in different organic electronics devices, such as field effect transistors, light emitting diodes and solar cells, and thus demonstrates a general application capability.

In organic photovoltaics, the loss from the open circuit photovoltages has been an Achilles’ heel for further enhancement of device performance. The voltage loss includes the radiative and non-radiative value, and intensive studies have focused on how to suppress losses from the non-radiative channel. In my fourth study, the non-radiative voltage loss was studied in a series of terpolymer blends and ternary blends. Compared to the ternary blends, a decreased nonradiative loss was found in the terpolymer blends. 

Abstract [sv]

Organisk elektronik har studerats intensivt de sista decennierna för att nå praktisk användning. För nästa generations högpresterande komponenter krävs studier av de bakomliggande fysikaliska mekanismerna. Exciterade tillstånd och laddningar transporteras i dessa komponenter och studier av dessa är mycket aktuella. Denna avhandling fokuseras på omvandlingen mellan exciterade tillstånd och laddningar i material och komponenter.

I naturliga fotosyntetiska system, som gröna växter och röda bakterier, har kvantkoherens föreslagits bidra till energiomvandlingen, som har en nästan 100% kvantverkningsgrad i omvandlingen från excitoner till laddningar. Om liknande mekanismer kunde användas i artificiella material och komponenter kanske detta kunde bidra till förbättrad funktion i organiska solceller. Med användning av många typer av transienta optiska och elektriska mätningar har vi visat att kvantkoherens förekommer i en speciell blandning av tre material, med högre fotoström som resultat, och som ger goda prestanda i organiska solceller.

I organiska lysdioder spelar excitoner av tripletttyp en kritisk roll, då sådana tillstånd är mörka och ger en förlust i prestanda. Med nya material, där termiskt assisterad långsam fotoluminiscens har etablerats, genom termisk konvertering av triplett till singlett och ett litet energigap mellan singlett och triplett, minskar förlusterna drastiskt. Fysikaliska modeller för denna process är fortfarande oklara. I mitt arbete har två molekyler av detta slag självmonterats i proteintrådar, och konsekvensen är att deras geometri förändras. Detta leder till en ökning av fotoluminiscensen.

I organisk komponenter, dioder och transistorer, spelar injektion av laddningar över gränsen mellan metallisk elektrod och halvledare en kritisk roll. Med ohmska kontakter uppnås god injektion. I mitt arbete används ett tunt skikt av en blandning med en elektronisk polymer och DNA, som håltransportlager för injektion av hål till halvledaren. Den elektroniska polymeren blir håldopad i detta blandmaterial, och ger en ohmsk kontakt till organiska halvledare, i dioder, fälteffekttransistorer och solceller, med möjlighet till många tillämpningar.

För organiska solceller har förlust av fotospänning varit en Akilles häl. Denna förlust av fotospänning orsakas båda av icke-strålande och strålande rekombination till grundtillståndet från det exciterade tillståndet, och har studerats i detalj de senaste åren. I mitt arbete studerades fotospänningen och den icke-strålande rekombinationen i ett antal polymerblandningar med tre komponenter, jämförda med en terpolymer, där polymererna sammanfogats till en polymerkedja. Den icke-strålande rekombinationen är svagare i terpolymeren, än i blandningarna.  

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 53
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524
National Category
Condensed Matter Physics
urn:nbn:se:liu:diva-164233 (URN)9789176850268 (ISBN)
Public defence
2020-04-02, Nobel (BL32), B-building, Campus Valla, Linköping, 10:15 (English)
Biomolecular and Organic Electronics (BiOrgEl) group
Available from: 2020-03-11 Created: 2020-03-11 Last updated: 2020-03-11Bibliographically approved
Bian, Q., Ma, F., Chen, S., Wei, Q., Su, X., Buyanova, I. A., . . . Inganäs, O. (2020). Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes. Nature Communications, 11(1), Article ID 617.
Open this publication in new window or tab >>Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes
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2020 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 11, no 1, article id 617Article in journal (Refereed) Published
Abstract [en]

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial ~200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.

Place, publisher, year, edition, pages
Nature Publishing Group, 2020
National Category
Other Physics Topics
urn:nbn:se:liu:diva-164232 (URN)10.1038/s41467-020-14476-w (DOI)000524950500001 ()32001688 (PubMedID)2-s2.0-85078713267 (Scopus ID)

Funding agencies: Knut and Alice Wallenberg Foundation (KAW) through a Wallenberg Scholar grant; Crafoord Foundation; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2014.0041]; China Scholarship Council (CSC)China Scholarship Council [201508320

Available from: 2020-03-10 Created: 2020-03-10 Last updated: 2020-04-27Bibliographically approved

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