Oled uygulamaları için ditiyenotiyofen (DTT) temelli floresans moleküllerin hazırlanması ve özelliklerinin incelenmesi
Syntheses and investigation of fluorescence dithienothiophene (DTT) based materials for oled applications
Yazar: İPEK ÖSKEN
Danışman: PROF. DR. TURAN ÖZTÜRK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı
Derece: Doktora, 2014
Konu: Kimya = Chemistry
Dizin: Organik materyaller = Organic materials

Özet:
Organik moleküllerin sahip oldukları fonksiyonel gruplar ya da bu grupların birbiri ile etkileşimi sonucunda gösterdikleri farklı özellikler onların günlük hayattaki uygulamalarda kullanılmasında esas önemin bir parçasıdır. Ne var ki uzun zaman bu moleküllerin metaller ya da yarı iletkenler gibi nasıl kullanılabileceği merak konusu olmuşsa da, 2000 yılı Nobel Kimya Ödülü bu yolda bir çığır açmış ve bu moleküllerin kullanılabileceği yeni bir sınıf, iletken polimerler, ortaya çıkmıştır. Böylece başlayan serüven, günümüzde birçok farklı alanda etkisini göstermeye devam etmektedir. İletken polimerler, kapasitörlerden biosensörlere, güneş pillerinden LED'lere birçok alanda kullanılmakta hatta özelliklerinin geliştirilmesi için hala yoğun araştırmalar yapılmaktadır.
Günümüz teknolojisinin ilerlemesi ile arzu edilen özelliklere sahip monomerleri sentezlemek, karakterize etmek ve alternatif uygulamalarını hazırlamak daha kolaylaşmıştır. Her geçen gün yeni birçok yöntem ve bu yöntemlerin iyileştirilmesi ve geliştirilmesine ait araştırmalar yapılmaktadır. Organik moleküllerin içinde kolay sentezlenebilen ve türevlendirilebilen bileşikler ve uygulamaları giderek daha da önem kazanmakta ve ilgi çekmektedir. Bu konuda ditiyenotiyofenler (DTT) geleceğin uygulamaları için kullanılan  bir bileşik olup, fonksiyonlandırılmış bir sentez yöntemi de grubumuz tarafından geliştirilmiştir. Elektrokromik cihaz uygulamaları, biosensörler, güneş pilleri, fluoresans etiketleme bileşikleri olarak kullanımları giderek artmakta ve bu bileşiklerin önemi her geçen gün ortaya çıkmaktadır.
Tüm bu bilgiler ışığında bu tez kapsamında DTT ve DTT-S,S-dioksit türevli bileşikler ve bunların fluoren ile farklı yüzdelerdeki kopolimerleri sentezlenmiştir. Sentezlenen monomerlere ait 1H-NMR, 13C-NMR, kütle spektroskopisi ve erime noktası tayinleri ile karakterizasyon yapılmıştır. Ayrıca monomerlerin optik özellikleri de incelenmiştir. Bu bileşiklerin kuantum verimleri 0.58 ile 0.03 arasında değişirken, optik bant aralıkları da 2.44 ile 3.57 eV arasında bulunmuştur.
Fluoren ile kenetleme reaksiyonları kullanılarak farklı yüzdelerde ve farklı DTT çekirdeklerine sahip 3 alternatif kopolimer grubu, P1, P2 ve P3 hazırlanmıştır. Polimerlerin molekül ağırlıkları ışık saçınım cihazı ile tayin edilmiştir ve değerler 9500 ile 860000 arasında değişmektedir. Bu kopolimerlere ait optik ve elektronik bant aralıkları hesaplanmış ve değerlerin 2.17 eV ile 3.57 eV arasında olduğu tespit edilmiştir. Bu kopolimerlerin kuantum verimleri de standartlar aracılığıyla hesaplanmış, 0.66 ile 0.03 arasında sonuçlar elde edilmiştir. Döngülü voltametri yardımı ile kopolimerlerin elektriksel davranışları da incelenmiş HOMO ve LUMO bant değerleri hesaplanmıştır.
Hazırlanan materyallerin diyot uygulamaları da ITO/ PEDOT:PSS/PX (60 nm)/Ca/Al diyot konfigurasyonu temel alınarak yapılmış, etkinlik, parlaklık, elektroluminesans ve dış kuantum verimleri hesaplanmış, ayrıca CIE renk koordinatları belirlenmiştir.
Elde edilen diyotlar içinde P1-25 en yüksek etkinlik 0.129 (Cd/A), en yüksek parlaklık (Cd/m2) ve en yüksek dış kuantum verimine %0.042 sahip olarak bulunmuştur. Hazırlanan diyotlar içerisinde beyaz sınırda olan türevler gözlenmiş, bunların geliştirilmesi için farklı film kalınlıklarından diyot hazırlanması, kopolimerlerin "blend" halinde filmlerinin hazırlanması ve farklı diyot konfigurasyonlarının denenmesi  gibi yöntemler kullanılmıştır.
Böylece DTT-S,S-dioksit çekirdeğine sahip birçok kopolimerin özellikleri incelenmiş, yüksek verim ve ışıma gücüne sahip alternatif diyot uygulamaları için geniş ve detaylı bir çalışma gerçekleştirilmiştir.

Abstract:
One of the important aim of chemistry is the synthesis and purification of new compounds. After hte discovery of atoms and their relations in between, many synthetic approach was developed to achive compounds with different atoms attached together or atoms attached together with different order. This difference between the coherence of atoms that are attached to each other leads these compounds, reveal different properties such as boiling points,vapour and osmotic pressure. Many alternative applications are based on  these distinctive characteristics.
The properties of organic molecules can vary by depence on their functional groups, which are obtained by altered binding of atoms. Functional groups are specific groups of atoms or bonds that are responsible for the characteristic behaviour in reactions. Each functional group with their ability of absorption, emission or capacitive behaviour will conclude as a material for daily applications. Depending on the characteristics, these materials can be classifed as capacitors, transistors, solar cells, etc. Synthesis of monomers with functional groups was the issue in earlier progress of chemistry. Over time functionalization of monomers and their purification was not atriumph but a general pathway to obtain new materials. The new era was the usage of these monomers and polymers where many efforts have been done, but none was quite the solution. However, after the 2000 Nobel Prize in Chemistry announced, a new area of materials, conducting polymers, was arised. This new class was the solution of how to use organic compounds in technological applications. This new class of materials had conjugated double bond where electrons can flow on a backbone. Comparing to their undoped form, their doped behaviour, with iodine, bromine etc., revealed a higher conductivity. Conducting polymers can be used in capacitors, biosensors, photovoltaics or LED's, which there is still an entensive research based on their improvement.
By the development of technology it has been easier to synthesise and characterize new compounds with additonal groups attached to extend their usage. Also, improvement of the technology provided us to achieve devices with alternative configurations, as a result, effective and practical usage in daily life. 
Many monomers such as thiophenes, carbazoles, phenyl rings, conjugated double bonds and their co-monomers are new eras of the earlier century. Their synthesis led many groups to investigate their charcterictics in multi-disciplanary areas. Each monomers or their coupled dimers revealed great attention. Beyond other significant and applicable monomers, dithienothiophenes (DTT) are now being used extensively in many device techniques. DTT's are three fused thiophene rings which are similarly having donor character such as thiophene. Our group have been focused on a different synthetic procedure which concludes with extended conjugations. This new synthetic approach was first published in literature in 2004 by our group and since then our main topic was to increase the yield and so, the usage of these monomers in technological applications. Electrochromic device applications, biosensors, photovoltaics and fluorescence biolabeling are some of the examples that these class of compounds can be used in. Nowadays, because of their donor character they are mainly used in Donor-Acceptor type copolymer or oligomer synthesis. By changing the acceptor group or oxidation of the central sulfur atom, the features of the compounds obtained differs. As an example, if the resulting monomer or polymer absorbs a broad band of su light then they are used in solar cell fabrication, if the compounds emit light with a high quantum yield especially in solid state, they become candidates for organic light emitting diode (OLED) applications.
OLEDs are electroluminescent display devices made out of stacked organic thin films sandwiched between two metal electrodes. The total thickness of an OLED is less than 500 nm (for a typical OLED architecture) making it the most compact flatpanel display on the market. Unlike LCD displays, OLED displays do not requireany back lighting and can operate in broader temperature ranges. OLED displaysare lighter, brighter, have wider viewing angles (160º), faster response time, andlow turn on voltages compared to the LCD displays. The operation of OLEDs relies on molecular excitation and radiative relaxation (or decay) of conjugated organic molecules (chromophores). When a voltage is applied across the OLED, the luminescent (fluorescent or phosphorescent chromophores) within the emissive layer get promoted to the excited state of the specific chromophore by recombination of opposite charge carriers. Radiative relaxation of the excited chromophore back to the ground state results in the generation of photons with energies defined by the chromophores electronic structure. An efficient pathway of obtaining pure colored emission in OLEDs, is accomplished by choosing the correct host – dopant system, where a proper energy match between the host and dopant allows for energy transfer to the emissive species thus allowing for pure RGB, white or IR emission.
Hole injection occurs when one electron is removed from the HOMO of hole-transporters, resulting in a positively charged radical cation. Similarly, the electron transport layer is expected to transport electrons from the cathode. The electron transport process adds one electron into the LUMO of electrontransporters, making the ETL molecule a negatively charged radical anion. The emission from this OLED device comes from the green emitter, Alq3. This is due to the fact that Alq3 has a smaller HOMO-LUMO energy gap compared with diammine, resulting in preferential energy transfer from diammine to Alq3. Additionally, diammine transports holes faster than Alq3 transports electrons, so the excitons (a loosely bound hole and electron pair) are generated within the Alq3 layer, near the diammine/Alq3 interface. One of the drawbacks of using Alq3 neat film as emissive layer, is the severe self-quenching of excitons in the neat film. This problem can be overcome by doping an emissive dye into the ETL, thus creating a new layer called the light emitting layer (EML). Inside of the EML, energy will transfer readily from the host to a dopant with a smaller optical gap resulting in efficient emission from the dopant. The self-quenching of excitons is also suppressed by lowering the concentration of excitons and thus device efficiency is improved. An additional benefit of doping is to control emission colors of OLEDs by doping different emissive dyes.
With all the information obtained by the literature research and the synthetic experience that we have focused on, some DTT and DTT-S,S-dioxide monomers, oligomers and their alternative copolymers varying different amount of DTT cores with fluoren have been synthesized and investigated in this thesis. Monomers and oligomers were characterised by 1H-NMR, 13C-NMR, mass spectroscopy and their melting points were found. Their optic behaviours were examined and quantum yields varied in between 0.58 and 0.03 where their optical band gaps were found to be 2.44 and 3.57 eV.
There were three different types of alternative copolymers depending on their geometry and difference DTT cores, P1, P2 and P3. The molecular weight distributions were examined by light scattering instrumentation and the average molecular weights were found to be in between 9500 ile 860000. Their optical and electronical band gaps were investigated and varied from 2.17 and 3.57 eV. Copolymers quantum yields were also measured via standarda and varied in between 0.66 – 0.03. To obtain the electronic properties, cyclic voltammetry was carried out and HOMO, LUMO and electrical band gaps were calculated.
Diode applications of the copolymers were maintained by ITO/ PEDOT:PSS/PX (60 nm)/Ca/Al diode configuration, efficiency, luminous, electroluminesans and external quantum efficiencies were measured.
Comparing within all the diodes, P1-25 has the most efficient value 0.129 (Cd/A), most lumionus 697 (Cd/m2) and best external quantum efficiency 0.042%. Here are some promising materials found for white OLED applications so further investigations were carried out such as preparing "blend" of copolymers or performing different diode configuration.
As a result, an extensive research based on  synthesis of DTT-S,S-dioxide containing copolymers and their diode applications were carried out to perform high efficiency, luminous and external quantum efficicncies.