Organische Elektronik [.5ex] Elektronische Prozesse in ... ·...

Organische ElektronikElektronische Prozesse in organischen Halbleitern

13. Ein persönlicher Ausblick: EPR-spektroskopischeUntersuchung organischer Halbleitermaterialien

Albert-Ludwigs-Universität Freiburg

Dr. Till BiskupInstitut für Physikalische ChemieAlbert-Ludwigs-Universität FreiburgSommersemester 2018

¤ Zentrale Aspekte

¤ Elektronische Struktur und Morphologie sind entscheidendfür die Effizienz organischer Halbleiterelemente.

¤ Zur Aufklärung der Struktur-Funktions-Beziehung bedarf esspektroskopischer Methoden mit molekularer Auflösung.

¤ Paramagnetische Zustände (Exzitonen, Ladungsträger, ...)spielen eine große Rolle in organischen Halbleitern.

¤ EPR-Spektroskopie erlaubt molekulare Auflösung undeindeutige Zuordnung paramagnetischer Spezies.

¤ Triplett-Zustände sind empfindliche Sonden für die lokaleUmgebung, Orientierung und Ordnung von Polymeren.

Sommersemester 2018 T. Biskup – Organische Elektronik (13) 2 / 34

Übersicht

Grundlegende Fragestellungen: Struktur-Funktions-Beziehung

Relevante Aspekte der EPR-Spektroskopie

Paramagnetische Zustände in organischen Halbleitern

Triplett-Zustände als lokale Sonden


Organische ElektronikEin großes Versprechen mit Potential für grundlegende Veränderungen

billigleichtgewichtig

OrganischeElektronik

druckbarflexibel

OFETs OLEDs

OSCs

Synthetische Chemie:Moleküle könnenjeweils spezifisch

angepasst werden...

Thermoelektrizität


Grundlegende FragestellungenViele Aspekte organischer Halbleiter sind noch unverstanden.


Grundlegende FragestellungenZwei Strategien zur Optimierung organischer Halbleitermaterialien

I Abstimmung der Energieniveaus (HOMO und LUMO)DOI: 10.1002/adma.200501717

Design Rules for Donors in Bulk-Heterojunction SolarCells—Towards 10 % Energy-Conversion Efficiency**

By Markus C. Scharber,* David Mühlbacher, Markus Koppe, Patrick Denk, Christoph Waldauf,Alan J. Heeger, and Christoph J. Brabec

There has been an intensive search for cost-effective photo-voltaics since the development of the first solar cells in the1950s.[1–3] Among all alternative technologies to silicon-basedpn-junction solar cells, organic solar cells could lead the mostsignificant cost reduction.[4] The field of organic photovoltaics(OPVs) comprises organic/inorganic nanostructures like dye-sensitized solar cells, multilayers of small organic molecules,and phase-separated mixtures of organic materials (the bulk-heterojunction solar cell). A review of several OPV technolo-gies has been presented recently.[5] Light absorption in organicsolar cells leads to the generation of excited, bound electron–hole pairs (often called excitons). To achieve substantial ener-gy-conversion efficiencies, these excited electron–hole pairsneed to be dissociated into free charge carriers with a highyield. Excitons can be dissociated at interfaces of materialswith different electron affinities or by electric fields, or thedissociation can be trap or impurity assisted. Blending conju-gated polymers with high-electron-affinity molecules like C60

(as in the bulk-heterojunction solar cell) has proven to be anefficient way for rapid exciton dissociation. Conjugated poly-mer–C60 interpenetrating networks exhibit ultrafast chargetransfer (∼40 fs).[6,7] As there is no competing decay processof the optically excited electron–hole pair located on the poly-mer in this time regime, an optimized mixture with C60 con-verts absorbed photons to electrons with an efficiency close to100 %.[8] The associated bicontinuous interpenetrating net-work enables efficient collection of the separated charges atthe electrodes. The bulk-heterojunction solar cell has at-tracted a lot of attention because of its potential to be a truelow-cost photovoltaic technology. A simple coating or printingprocess would enable roll-to-roll manufacturing of flexible,low-weight PV modules, which should permit cost-efficientproduction and the development of products for new markets,

e.g., in the field of portable electronics. One major obstaclefor the commercialization of bulk-heterojunction solar cells isthe relatively small device efficiencies that have been demon-strated up to now.[5] The best energy-conversion efficienciespublished for small-area devices approach 5 %.[9–11] A de-tailed analysis of state-of-the-art bulk-heterojunction solarcells[8] reveals that the efficiency is limited by the low open-circuit voltage (Voc) delivered by these devices under illumi-nation. Typically, organic semiconductors with a bandgap ofabout 2 eV are applied as photoactive materials, but the ob-served open-circuit voltages are only in the range of 0.5–1 V.There has long been a controversy about the origin of the Voc

in conjugated polymer–fullerene solar cells. Following theclassical thin-film solar-cell concept, the metal–insulator–met-al (MIM) model was applied to bulk-heterojunction devices.In the MIM picture, Voc is simply equal to the work-functiondifference of the two metal electrodes. The model had to bemodified after the observation of the strong influence of thereduction potential of the fullerene on the open-circuit volt-age by introducing the concept of Fermi-level pinning.[12] Ithas also been shown that the Voc of polymer–fullerene solarcells is affected by the morphology of the active layer[13] andthat it can be influenced by the electrochemical potential ofthe cathode (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS).[14] However, even an extendedMIM model was found to be insufficient for a complete expla-nation of the Voc of bulk-heterojunction solar cells. Recently,Gadisa et al.[15] studied the correlation of electrochemicalproperties of six similar polythiophene derivatives and theopen-circuit voltage of polymer–fullerene solar cells using[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an ac-ceptor and the same polythiophenes derivatives as donors.The authors attributed the variation of the photovoltage tovariation of the oxidation potential of the conjugated poly-mer. Concluding the summary of the recent literature, there isno general consistent understanding of the origin of the open-circuit voltage of the bulk-heterojunction solar cell. In thiswork, the relation between the energy levels of the donor–ac-ceptor blend and the open-circuit voltage of 26 different bulk-heterojunction devices is studied. A simple relation betweenthe energy level of the highest occupied molecular orbital(HOMO) of the polymer and the Voc is derived, which is usedto estimate the maximum efficiency of bulk-heterojunctionsolar cells. Based on the model, the ideal material parametersfor a conjugated polymer–PCBM device are determined. Theresults presented here can be used as a guideline for the selec-

COM

MUN

ICATIO

NS

Adv. Mater. 2006, 18, 789–794 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 789

–[*] Dr. M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk,

C. Waldauf, Dr. C. J. BrabecKonarka AustriaAltenbergerstrasse 69, A-4040 Linz (Austria)E-mail: [email protected]. A. J. HeegerDepartment of Materials ScienceBroida Hall 6125University of California at Santa BarbaraSanta Barbara, CA 3106-5090 (USA)

[**] This work was partially supported by the Commission of the Eur-opean Community (Project MOLYCELL Contract No. 502783) andthe BMBF project EKOS: Förderkennzeichen 03N2023.

Advanced Materials 18:789, 2006

I Morphologie und ProzessierungThe Next Breakthrough for Organic Photovoltaics?Nicholas E. Jackson,*,† Brett M. Savoie,† Tobin J. Marks,* Lin X. Chen,* and Mark A. Ratner*

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

*S Supporting Information

ABSTRACT: While the intense focus on energy level tuning in organic photovoltaicmaterials has afforded large gains in device performance, we argue here that strategiesbased on microstructural/morphological control are at least as promising in any rationaldesign strategy. In this work, a meta-analysis of ∼150 bulk heterojunction devicesfabricated with different materials combinations is performed and reveals strongcorrelations between power conversion efficiency and morphology-dominated properties(short-circuit current, fill factor) and surprisingly weak correlations between efficiency andenergy level positioning (open-circuit voltage, enthalpic offset at the interface, opticalgap). While energy level positioning should in principle provide the theoretical maximumefficiency, the optimization landscape that must be navigated to reach this maximum isunforgiving. Thus, research aimed at developing understanding-based strategies for moreefficient optimization of an active layer microstructure and morphology are likely to be atleast as fruitful.

Since the turn of the last century, organic photovoltaic(OPV) devices have occupied a prominent place in both

fundamental and applied photovoltaic research, with publishedpower conversion efficiencies (PCEs) of single-junction devicesnow approaching 10%1 and industry reports exceeding 11%.2

By any metric, the pace of these advances is remarkable. OPVshave achieved an approximately 10-fold increase in PCE over a15-year interval (albeit starting from a low base of ∼1%) andexhibit one of the steepest growth curves of any photovoltaictechnology.

OPVs are based on several materials approaches in which atleast two organic semiconductors are blended or layered for thepurpose of producing electricity from sunlight. This definitionis necessarily broad because unlike most other PV technologies,OPVs are not restricted to a specific identity of activecomponents, and these are continuously being modified andimproved. OPVs are thus distinguished from other technologies(e.g., c-Si, a-Si, CdTe, CIGS, etc.) in that the principal means ofincreasing efficiency over most of their development has beento tailor, via molecular/macromolecular architecture modifica-tion, the intrinsic semiconducting and optical properties of theactive layer components. For instance, the optical band gap andvalence/conduction band edges are designable componentsthat are routinely altered to better conform to optimum values

predicted by device modelsa process referred to as energylevel tuning. Probably the best known design model based onenergy level tuning is a study by Scharber et al., which extendsthe standard Shockley-type band gap analysis3 to includeefficiency losses associated with the enthalpic offset at thedonor−acceptor heterojunction (ΔECS; see Figure 1).4 Notethat, as of 11/13/2014, the Web of Science indicates that thisreport has been cited 2075 times over the past 8 years.5 It isdifficult to overstate the centrality of energy level tuning in thedesign of OPV materials; hardly any report of a new OPVmaterial in the past decade has failed to cite the importance ofcarefully positioning the energy levels.6−11

Given the centrality of energy level tuning in OPV materialsdesign and after approximately a decade of concerted effortacross the field to synthesize new materials with tailored energylevels, it is fair to ask how well these design rules predict theexperimental performance of OPVs in experimental practice. Tothis end, we report here a meta-study of ∼150 independentdonors paired with fullerene acceptors. Our analysis is limitedto conjugated polymers, the ionization potentials (IPs) ofwhich are typically known from cyclic voltammetry (CV) or, insome cases, photoemission experiments, and published UV−visabsorption spectroscopic data that enable extraction of theoptical band gap (Eg,opt). In all cases, IPs are used as reported,Eg,opt data are extracted from the onset of the lowest energyabsorption feature, and only the highest PCE devices usingeither PC60BM or PC70BM acceptors are included. AveragePCEs are always used when available, although in some cases,the batch size was not reported. Polymers are only considereddistinct if they possess differentiable conjugated structures.

Received: October 21, 2014Accepted: December 11, 2014Published: December 11, 2014

Energy level alignment deter-mines the maximum efficiency

under no-loss conditions, but themajority of materials are limitedby practical morphological and

processing concerns.

Perspective

pubs.acs.org/JPCL

© 2014 American Chemical Society 77 dx.doi.org/10.1021/jz502223t | J. Phys. Chem. Lett. 2015, 6, 77−84

Journal of Physical Chemistry Letters 6:77, 2015

~ Energy level alignment determines the maximum efficiencyunder no-loss conditions, but the majority of materials arelimited by practical morphological and processing concerns.


Grundlegende FragestellungenAspekte der Struktur-Funktions-Beziehung

Morphologiein Lösung

Flexibilität derKonformation

LokaleOrdnung

ElektronischeStruktur

?Struktur-Funktions-Beziehung

Morphologieim Film

Ursprung vonTripletts


Grundlegende FragestellungenGesucht: Verständnis der Zusammenhänge auf molekularer Ebene

ZielI Verständnis der Zusammenhänge auf molekularer EbeneI gezielte Synthese und Modifikation von Molekülen

Voraussetzungen

I Sonde mit molekularer AuflösungI interdisziplinärer Ansatz (Synthese, Experiment, Theorie)

mögliches Werkzeug

I der (ungepaarte) ElektronenspinI EPR-Spektroskopie


Grundlegende FragestellungenDer Elektronenspin als Sonde seiner lokalen Umgebung

Stabilradikal

Triplett Radikalpaar

N

O

S

R

n


Wechselwirkungen des ElektronenspinsDer Elektronenspin als Sonde seiner lokalen Umgebung

Zeeman-WWHEZ = µBBgSäußeres Magnetfeld

Hyperfein-WWHHF = SA0I + aSIumgebende Kerne

Dipolare WWHZFS = SDSzwischen Elektronen

Austausch-WWHEX = JS1S2

zwischen Elektronen


Systeme mit zwei ElektronenEine Frage des Abstands zwischen den beiden Spins

Wechselwirkung

A or BAAbstand r

(8 nm)

r < 5 Å

r > 80 Å

5 Å < r < 80 Å

→ 2 Radikale

→ Radikalpaar

→ Kopplung zu Singulett- oder Triplett-Zustand


Systeme mit zwei ElektronenEine Frage des Abstands zwischen den beiden Spins

S

T+

0T

-T

2JEnergie

r

|J| ≫ |D|

|J| ≈ |D|

Triplett

Radikalpaar


Grundlegender Aufbau eines EPR-SpektrometersGeringfügige Modifikationen je nach Anwendungsfall

TREPR

MW-Quelle Detektor

Resonator

Zirkulator

Magnet

Rekorder

Trigger

gepulste Lichtquelle


Paramagnetische Zustände in org. HalbleiternEin einfaches Bild der Prozesse in einer organischen Solarzelle

Organische Solarzelle

LUMOs

HOMOs

LUMOs

HOMOs

Donator Akzeptor

CTC


Paramagnetische Zustände in org. HalbleiternEin einfaches Bild der Prozesse in einer organischen Solarzelle

S0

S1

T1

CS3CT 1CT

ExzitonenLadungstransferkomplex (CT)Polaronen

43

3

5

Ladungstransfer

Ladungstrennung

Singulett-Triplett-Mischung

1 2

4 Interkombination (ISC)

5 Rücktransfer von Elektronen

1

2


Paramagnetische Zustände in org. HalbleiternWas sich mit Hilfe der EPR-Spektroskopie detektieren lässt

ExzitonenSingulett- oder Triplett-Zustände à TREPR

Ladungstransfer-KomplexeSpinkorrelierte Radikalpaare à TREPR

„Freie“ Polaronenungepaarte Elektronen, Duplett-Zustände à LEPR

Defekte, dotierte MoleküleStabilradikale, Duplett-Zustände à cw-EPR

* Jede Spezies hat ihren charakteristischen„Fingerabdruck“ in der EPR-Spektroskopie.


Triplett-Zustände als lokale SondenDie Erzeugung paramagnetischer Zustände in Polymeren

N

S

NSN S

C8H17C8H17

n

diamagnetisch

Singulett-Grundzustand

kein EPR-Signal

N

S

NSN S

C8H17C8H17

n

paramagnetisch

angeregter Triplett-Zustand

EPR-Signal


Triplett-Zustände als lokale SondenEinige für die EPR-Spektroskopie wichtige Eigenschaften

θ

BR

I Wechselwirkung abhängig vomAbstand (R) zwischen den Spins

I Wechselwirkung abhängig vomWinkel (θ) zum Magnetfeld (B).

I Wechselwirkung charakterisiertdurch zwei Parameter (D und E).

I Optische Anregung führt zuNicht-Boltzmann-Populationen

* Mittels TREPR-Spektroskopie an Triplett-Zuständen lassensich Delokalisation und Orientierung untersuchen.


Morphologie von Polymerfilmen auf SubstratOrientierung des Rückgrats und Grad der Ordnung

Ordering of PCDTBT Revealed byTime-Resolved Electron Paramagnetic Resonance

Spectroscopy of Its Triplet ExcitonsTill Biskup,* Michael Sommer, Stephan Rein, Deborah L. Meyer,

Markus Kohlstädt, Uli Würfel, Stefan Weber

Angew. Chem. Int. Ed. 54:7707, 2015



1. Drop-cast auf PET 2. Orientierungsabhängige TREPR



SimulationenI Globale Anpassung

aller SpektrenI Gauß-Verteilung

der Orientierungen

ErgebnisseI PCDTBT-Filme zeigen

starke OrdnungI Polymerrückgrat liegt

auf dem Substrat

-π -π/2 ππ/20

σ ≅ 26° ±σ

68%S = 0.75


Triplett-EntstehungswegeDirekte S0→T-Anregung: lange vergessen und doch real

Direct S0→T Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions

Probed by Time-Resolved EPR SpectroscopyDeborah L. Meyer, Florian Lombeck, Sven Huettner, Michael Sommer, Till Biskup

J. Phys. Chem. Lett. 8:1677, 2017



400 450 500 550 600 650 700Wellenlänge / nm

400 450 500 550 600 650 700Wellenlänge / nm

S0 S1 Tn

ISCS0 Tm+x

S1

S0

Tn

S0

S1

Tn Tm+bTm+aTm

AbsorptionTriplettausbeute

Energie



S0

S1

T1

3CT 1CT CS

S0

S1

T1

ExzitonenLadungstransferkomplex (CT)Polaronen

12

3

3

1 Interkombination (ISC)

2 Rücktransfer von Elektronen

3 Singulettspaltung

4 Direkte S0→T-Anregung4


Elektronische Struktur konjugierter PolymereVom Baustein zum Polymer: EPR-Spektroskopie und DFT-Rechnungen

TBT Entirely Dominates the Electronic Structureof the Conjugated Copolymer PCDTBT:

Insights from Time-Resolved EPR SpectroscopyClemens Matt, Deborah L. Meyer, Florian Lombeck, Michael Sommer, Till Biskup

...-TBT-Cbz-TBT-Cbz-TBT-Cbz-TBT-Cbz-...

ADN

C8H17C8H17

NSN

S

S

Macromolecules 51:4341, 2018



S S

NS

N

S

SC8H17C8H17

N

NSN

S

SC8H17

C8H17

N

NSN

n

TBT CbzTBTCbz

CbzTBT PCDTBT

S

N

C8H17

C8H17

N

C8H17C8H17

S

NSN



300 350 400 450 500 550 600 650 700 750 800Wellenlänge / nm

TBT

CbzTBT

CbzTBTCbz

PCDTBT

no

rma

lisie

rte

In

ten

sitä

t



280 300 320 340 360 380 400Magnetfeld / mT

TBT

CbzTBT

CbzTBTCbz

PCDTBT

A

E



TBT

CbzTBT

CbzTBTCbz

PCDTBTSommersemester 2018 T. Biskup – Organische Elektronik (13) 32 / 34

Wiederholung: Grundlegende FragestellungenEPR-Spektroskopie kann Beiträge zu allen Aspekten leisten

Morphologiein Lösung

Flexibilität derKonformation

LokaleOrdnung

ElektronischeStruktur

?Struktur-Funktions-Beziehung

Morphologieim Film

Ursprung vonTripletts


¤ Zentrale Aspekte

¤ Elektronische Struktur und Morphologie sind entscheidendfür die Effizienz organischer Halbleiterelemente.

¤ Zur Aufklärung der Struktur-Funktions-Beziehung bedarf esspektroskopischer Methoden mit molekularer Auflösung.

¤ Paramagnetische Zustände (Exzitonen, Ladungsträger, ...)spielen eine große Rolle in organischen Halbleitern.

¤ EPR-Spektroskopie erlaubt molekulare Auflösung undeindeutige Zuordnung paramagnetischer Spezies.

¤ Triplett-Zustände sind empfindliche Sonden für die lokaleUmgebung, Orientierung und Ordnung von Polymeren.


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