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Title: PHOTO-EXCITATION ENERGY INFLUENCE ON THE PHOTOCONDUCTIVITY OF ORGANIC SEMICONDUCTORS Authors: ID Pastukhova, Nadiia (Author) ID Pavlica, Egon (Mentor) More about this mentor... Files: NPastukhova_thesis.pdf (4,56 MB) MD5: BE41CC1031372EB12759372EF0F228EE   Language: English Work type: Doctoral dissertation Typology: 2.08 - Doctoral Dissertation Organization: FPŠ - Graduate School Abstract: In this work, we experimentally studied the influence of photoexcitation energy influence on the charge transport in organic semiconductors. Organic semiconductors were small molecules like corannulene, perylene and pentacene derivatives, polymers such as polythiophene and benzothiophene derivatives, and graphene, along with combinations of these materials in heterojunctions or composites. The first part of this study is focused on the photoexcitation energy influence on the transient photoconductivity of non-crystalline curved π-conjugated corannulene layers. The enhanced photoconductivity, in the energy range where optical absorption is absent, is deduced from theoretical predictions of corannulene gas-phase excited state spectra. Theoretical analysis reveals a consistent contribution involving transitions to Super Atomic Molecular Orbitals (SAMOs), a unique set of diffuse orbitals typical of curved π-conjugated molecules. More, the photoconductivity of the curved corannulene was compared to the π-conjugated planar N,N′-1H,1H- perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN2), where the photoexcitation energy dependence of photocurrent closely follows the optical absorption spectrum. We next characterized charge transport in poly(3-hexylthiophene) (P3HT) layers deposited from solution. Our results indicate that time-of-flight (TOF) mobility depends on the photoexcitation energy. It is 0.4× 10 −3 cm 2 /Vs at 2.3 eV (530 nm) and doubles at 4.8 eV (260 nm). TOF mobility was compared to field-effect (FET) mobility of P3HT field-effect transistors (OFETs). The FET mobility was similar to the 2.3 eV excitation TOF mobility. In order to improve charge mobility, graphene nanoparticles were blended within a P3HT solution before the deposition. We found that the mobility significantly improves upon the addition of graphene nanoparticles of a weight ratio as low as 0.2 %. FET mobility increases with graphene concentration up to a value of 2.3× 10 −2 cm 2 /Vs at 3.2 %. The results demonstrate that phase segregation starts to influence charge transport at graphene concentration of 0.8 % and above. Hence, the graphene cannot form a bridged conduction channel between electrodes, which would cancel the semiconducting effect of the polymer composite. An alternative approach to enhance mobility is to optimize the molecular ordering of organic semiconductors. For that purpose, we studied an innovative nanomesh device. Free-standing nanomesh devices were used to form nanojunctions of N,N′- iiDioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires and crystalline bis(triisopropylsilylethinyl)pentacene (TIPS-PEN). We characterized the photocurrent response time of this novel nanomesh scaffold device. The photoresponse time depends on the photon energy. It is between 4.5 − 5.6 ns at 500 nm excitation wavelength and between 6.7 − 7.7 ns at 700 nm excitation wavelength. In addition, we found that thermal annealing reduces charge carrier trapping in crystalline nanowires. This confirms that the structural defects are crucial to obtaining high photon-to-charge conversion efficiency and subsequent transport from pn junction in heterostructured materials. Structural defects also influence the power conversion efficiency of organic heterostructured photovoltaics (OPVs). Anticipating that polymers with different backbone lengths produce different level of structural defects, we examined charge transport dependence on the molecular weight of poly[4,8-bis(5-(2- ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2- ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th) from 50 kDa to 300 kDa. We found p-type hopping transport in PTB7-Th, characterized by 0.1 – 3× 10 −2 cm 2 /Vs mobility, which increases with temperature and electric field. The polymer molecular weight exhibits a non-trivial influence on charge transport. FET mobility in the saturation regime increases with molecular weight. A similar trend is observed in TOF mobility and FET mobility in the linear regime, except for the 100kDa polymer, which manifests in the highest mobility due to reduced charge trapping. The lowest trapping at the dielectric interface of OFET is observed at 200 kDa. In addition, the 200 kDa polymer exhibits the lowest activation energy of the charge transport. Although the 100 kDa polymer indicates the highest mobility, OPVs using the 200 kDa polymer exhibit the best performance in terms of power conversion efficiency. Keywords: organic semiconductors, optical absorption spectroscopy, time-of-flight photoconductivity, transient photocurrent spectroscopy, organic thin film transistors, atomic force microscopy, superatomic molecular orbitals, pn heterojunction, organic nanowires, graphene, composites, charge mobility, charge trapping, temperature dependence, photodetector, photovoltaic, solar cell, organic electronics Place of publishing: Nova Gorica Publisher: Nadiia Pastukhova Year of publishing: 2018 Number of pages: 130 PID: 20.500.12556/RUNG-4116-d026b030-1ecd-21f1-40d9-88d444161064  COBISS.SI-ID: 5239035  NUK URN: URN:SI:UNG:REP:TTUX0W0V Publication date in RUNG: 08.10.2018 Views: 6684 Downloads: 170 Metadata:    Cite this work Plain text BibTeX EndNote XML EndNote/Refer RIS ABNT ACM Ref AMA APA Chicago 17th Author-Date Harvard IEEE ISO 690 MLA Vancouver : Copy citation

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Secondary language

Language:	Slovenian
Title:	VPLIV ENERGIJE FOTONOV NA TOK FOTOVZBUJENIH NOSILCEV NABOJA V ORGANSKIH POLPREVODNIKIH
Abstract:	V disertaciji so predstavljeni rezultati eksperimentalnih raziskav o vplivu energije fotonov na fotovzbujeni transport naboja v organskih polprevodnikih. Organski polprevodniki, ki smo jih preučevali so bili v obliki majhnih molekul (koranulen, perilen in pentacenski derivati), polimerov (derivati politiofena in benzotiofena), grafena, ter kombinacije teh materialov v heterostrukturah in kompozitih. Prvi del je osredotočen na študij vpliva energije fotonov na optično absorpcijo in časovno-odvisni tok fotovzbujenih nosilcev naboja v tankih plasteh koranulena. Struktura molekule koranulena je ukrivljena in zaradi π-konjugacije izkazuje prevodnost fotovzbujenih nosilcev naboja. V energijskem območju, kjer koranulen nima optične absorpcije, smo opazili povečano prevodnost fotovzbujenih nosilcev naboja. Ta pojav smo razložili s pomočjo teoretičnih izračunov optičnih ekcitacij v molekuli koranulena, ki se nahaja v plinasti fazi. Analiza meritev je pokazala, da k povečani prevodnosti prispevajo fotovzbujeni nosilci naboja, ki so zaradi ustrezne energije fotonov vzbujeni v t.i. super atomske molekulske orbitale (SAMO). Te orbitale so značilne za ukrivljene π-konjugirane molekule. V molekulah, ki niso ukrivljene, teoretični izračuni ne napovedujejo SAMO orbital. Posledično, kot smo tudi pokazali s študijo prevodnosti fotovzbujenih tokov v derivatu molekule perilena, energijska odvisnost prevodnosti fotovzbujenih nosilcev naboja dosledno sledi optični absorpciji. V nadaljevanju so prikazani rezultati študije transporta naboja v plasteh poli(3- heksiltiofena) (P3HT), ki so bili nanešeni iz raztopine na steklene podloge. Rezultati nakazujejo, da je gibljivost fotovzbujenih nosilcev, ki je izmerjena preko metode časa preleta (TOF), odvisna od energije absorbiranih fotonov. TOF gibljivost znaša 0,4 × 10 -3 cm 2 /Vs pri energiji fotonov 2,3 eV (530 nm) in se podvoji pri 4,8 eV (260 nm). TOF gibljivost smo primerjali z gibljivostjo nosilcev naboja izmerjeno v P3HT tranzistorjih na poljski pojav (FET). FET gibljivost je podobna TOF gibljivosti pri energiji fotonov 2,3 eV. Pokazali smo, da se gibljivost poveča z dodatkom grafenskih nanodelcev. Nanodelce grafena smo dodali v raztopino P3HT v različnih masnih razmerjih pred nanosom na stekleno podlogo. Pokazali smo, da se gibljivost bistveno poveča že pri masnem razmerju grafena 0,2%. Z višanjem koncentracije se gibljivost veča. Pri najvišji koncentraciji 3,2%, ki smo jo pripravili, doseže FET gibljivost vrednost 2,3 × 10 -2 cm 2 /Vs. Ugotovili smo, da pri koncentracijah nad 0,8% pride do ivzbiranja grafena v skupke, kar prepreči, da bi grafen tvoril prevodni most med elektrodama in s tem izničil polprevodne lastnosti polimerskega kompozita. Alternativni pristop za povečanje gibljivosti nosilcev naboja v organskih polprevodnikih je povezan z urejanjem molekul v molekulskih kristalih. S tem namenom smo uporabili inovativno večplastno nanostrukturo elektrod, ki temelji na nanomrežici. Prostostoječo nanomrežico smo uporabili za tvorbo nano-stikov med nanožicami N, N'-dioktil-3,4,9,10-perilendikarboksimida (PTCDI-C8) in kristali bis(triizopropilsililetinil)pentacena (TIPS-PEN). Preučevali smo odzivni čas toka fotovzbujenih nosilcev naboja. Ugotovili smo, da je odzivni čas odvisen od energije fotonov. Pri vzbujanju z valovno dolžino 500 nm znaša odzivni čas 4,5 - 5,6 ns, pri 700 nm pa 6,7 - 7,7 ns. Poleg tega smo pokazali, da s termičnim popuščanjem izboljšamo odzivni čas, kar je posledica višje urejenosti molekul v nanožicah. S tem smo pokazali, da so strukturni defekti ključni dejavnik za učinkovitost pretvorbe fotonov v naboj na stiku med dvema polprevodnikoma in nadaljni transport po organskem polprevodniku. Strukturni defekti vplivajo tudi na učinkovitost pretvorbe svetlobne energije v električno energijo v organskih heterostrukturnih fotovoltaikih (OPV). Polimeri z različnimi dolžinami osnovne verige se različno uredijo in s tem tvorijo različno stopnjo strukturnih defektov. To smo dognali s tem, ko smo preučili odvisnost transporta nabojev od molekulske mase v poli[4,8-bis(5-(2-etilheksil)tiofen-2- il)benzo[1,2-b,4,5-b']ditiopen-2,6-diil-alt-(4-(2-etilheksil)-3-fluorotieno[3,4-b]tiofen- 2-karboksilat-2-6-diil] (PTB7-Th). Molekulska masa PTB7-Th je znašala od 50 kDa do 300 kDa. Pokazali smo, da so vrzeli glavni nosilci naboja v PTB7-Th. Njihova gibljivost znaša 0,1 - 3 × 10 -2 cm 2 /Vs. Gibljivost narašča s temperaturo in z električnim poljem, kar je značilno za transport naboja s poskakovanjem. Molekulska masa polimera ima netrivialen vpliv na transport naboja. FET gibljivost v nasičenem režimu se povečuje z molekulsko maso. Podoben trend smo opaziti tudi pri meritvu TOF gibljivosti, razen pri polimeru velikosti 100 kDa. Pri tem polimeru smo opazili, da je gibljivost največja zaradi najnižje gostote pasti, ki lovijo nosilce naboja. Poleg tega polimer z velikostjo 200 kDa kaže najnižjo aktivacijsko energijo transporta nosilcev naboja. Čeprav ima polimer velikosti 100 kDa največjo gibljivost, OPV-ji z uporabo polimera velikosti 200 kDa kažejo najboljšo učinkovitost v smislu učinkovitosti pretvorbe moči.
Keywords:	Organski polprevodniki, optična absorbcijska spektroskopija, čas preleta, tok fotovzbujenih nosilcev naboja, časovno-odvisni električni tok, organski tankoplastni tranzistorji, mikroskopija na atomsko silo, superatomske molekulske orbitale, pn heterostructure, organske nanožice, grafen, kompoziti, gibljivost nosilcev naboja, pasti naboja, temperaturna odvisnost, fotodetektor, fotovoltaika, sončna celica, organska elektronika

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