We report here the synthesis via Suzuki polymerization of fluorene-based copolymers containing various energy bandgap dyes (R1, R2, R3, R4, CAR, and, PZB) and their application into electronic devices such as EL devices and photovoltaic cells. The UV-vis. absorption and PL emission spectra of the polymers were measured to investigate their optical properties and EL spectra and IVL characteristics were measured to show how they had the possibility for commercializing. Conventional PLED devices were fabricated in the configuration [ITO/PEDOT-PSS/polymer/Ca/Al] using the polymers as emitting layers. Also, we applied some of polymers into photovoltaic cells because they had low energy bandgap below 2.1 eV.
Firstly, four novel fluorene-based light-emitting polymers (PFR1, PFR2, PFR3, and PFR4) containing different comonomers have been designed, synthesized, and characterized. The single layer LED devices fabricated from these polymers emitted light from bluish green to pure red. The results show that the color of the light emitted by the homopolymer, poly(9,9-bis(2'-ethylhexyl)fluorene), could be tuned by incorporating comonomers (R1, R2, R3, and R4) with narrower band-gaps. The energy band-gap of the copolymers with those comonomers varied according to the position of the cyano group in the vinylene unit and the type of aromatic group (thiophene or phenylene) incorporated. Notably, PFR4 showed pure red emission (chromaticity values x = 0.67, y = 0.34) that is almost identical to the standard red (0.66, 0.34) demanded by the national television system committee (NTSC) and also exhibited a low turn-on voltage. And then, using new polymerization methods, Suzuki coupling reaction, four fluorene-based alternating polymers (PFR1-S, PFR2-S, PFR3-S, and PFR4-S) containing different comonomers (R1, R2, R3, and R4) have been designed, synthesized, and characterized. Single layer LED devices fabricated from these polymers emitted bluish green to pure red light. The absorption and emission maxima of the copolymers varied according to the position of the cyano group in the vinylene unit (α- or β-position) and the type of incorporated aromatic group (thiophene or phenylene). Notably, PFR4-S showed pure red emission (CIE coordinate values x = 0.66, y = 0.33) that is almost identical to the standard red (0.66, 0.34) demanded by the National Television System Committee (NTSC). PFR3-S also exhibited pure red emission (chromaticity values x = 0.63, y = 0.38) and its maximum luminance and maximum external quantum efficiency.
Secondly, a novel series of soluble conjugated copolymers comprised of 9,9'-bis(2-ethylhexylfluorene) and comonomer included carbazole moiety, 3,6-bis[1-{4-bromobenz(1-cyanovinylene)}1-9-(2-ethylhexyl)-carbazole (BCEC), were synthesized by nickel-catalyzed Yamamoto coupling reaction. By means of the feed ratio, the electronic and optical properties of the polymers were tuned. The UV-vis spectra of polymers exhibit absorption band at about 382 nm. These polymers showed photoluminescence (PL) maxima at 466 and 479 nm, respectively. The EL spectrum results were similar to the PL spectra. However, the EL spectra showed the possibility of the white light emission through the electroplex emission around 700nm from the single polymer device. The EL spectra of the devices were little changed with the various current densities.
Thirdly, low energy band-gap and soluble conjugated polymers (PFRs-S) were synthesized from 9,9-dioctylfluorene and low band-gap comonomers (R3 and R4) through alternating copolymerization, Suzuki coupling reaction were used for the donor materials in bulk heterojunction polymer photovoltaic cells (PPVCs) and C60 was used as an acceptor material. The devices made from PFR3-S:C60 (1:1) and PFR4-S: C60 (1:1) were constructed in a sandwich structure with indium tin oxide (ITO)/PEDOT:PSS acting as an anode and Ca/Al as a cathode. In PPVCs made with PFR3-S : C60 and PFR4-S : C60 blends, their spectral responses were extended up to 700 nm and showed improved spectral match to solar radiation better than compared to PPVCs with MEH-PPV:C60 (1:1) blends as reference. The energy conversion efficiencies (he) in the devices of PFR3-S and PFR4-S reached 0.98 and 1.02 % under AM 1.5 solar simulator (100 mW/㎠). Compared to the PPVC of MEH-PPV:C60 (1:1) blend, the devices of PFR3-S and PFR4-S retained high energy conversion efficiencies. Also, their short cirucuit current Jsc were better than the PPVC of MEH-PPV:C60 (1:1) blend, 3.18 and 3.09 mA/㎠, respectively. Also, their open-circuit voltage ($V_oc$) were shown 0.85 V and 0.81 V in PFR3-S and PFR4-S devices.
Fourthly, a series of novel organic semiconducting polymers composed of fluorene and bis(2-phenyl-2-cyanovinyl)-10-hexylphenothiazine) (PZB) comonomer was synthesized and characterized. These polymers were found to be thermally stable and readily soluble in common organic solvents. Our results showed that the color of the light emitted by the homopolymer, poly(9,9-dioctylfluorene-2,7-diyl) (PDOF), could be easily tuned by incorporating PZB comonomer, which has a narrower band gap. The photoluminescence (PL) emission spectra of the copolymers were highly red-shifted as the fraction of PZB in the copolymer was increased. Light-emitting devices were fabricated in the ITO/PEDOT:PSS/polymer/Ca/Al configuration using the polymers as the emitting layer. The electroluminescence (EL) device based on PF-PZB50 showed almost pure red emission [CIE coordinate values (x, y) - (0.63, 0.36)] that was very close to the standard red (0.66, 0.34) demanded by the National Television System Committee. In addition, bulk-hetero junction polymeric photovoltaic cells (PPVCs) fabricated from a thin film composed of a blend of PF-PZB50 and the fullerene derivative [6, 6]-phenyl C61 butyric acid methyl ester (PCBM), in a ITO/PEDOT:PSS/(PF-PZB50:PCBM=1:3)/LiF/Al configuration, showed promising performance. Specifically, under 100 mW/㎠ of AM 1.5 white light illumination, the device produced an open-circuit voltage of 0.78 V and a short-circuit current of 2.38 mA/m2. The fill factor of the device was 0.29, and the power conversion efficiency was 0.53%.
9,9-다이옥틸플루오렌과 다양한 낮은 에너지 밴드갭을 갖는 단량체(R1, R2, R3, R4, CAR, and PZB)를 공중합을 통해 고분자를 합성하였다. 합성된 고분자들은 일차적으로 물리적 성질 - 열분석, 분자량 분석, 원소분석-을 통해 기초적인 성질을 조사하고 광학적 성질-자외선 흡수 스펙트럼, IR 스펙트럼, PL 스펙트럼, 사이클로볼타메트리-을 통해 성공적으로 합성된 고분자를 확인하였다. 마지막으로 전기 발광소자를 제작하여 전기발광 특성과 물질의 전기 발광 성능 평가를 수행하였다. 특히 낮은 에너지 밴드갭을 갖는 고분자 물질에 대해서는 전자 받게 성질이 강한 C60 유도체를 도입하여 새로운 전기 발광 고분자의 응용예를 확인하였다.
첫번째로 네가지의 구조변화를 가지는 단량체(R1, R2, R3, and R4)를 합성하였다. 이들 단량체는 두가지의 변화요소를 도입하여 녹색에서 적색 발광까지의 다양한 색 변이를 관찰할 수 있었다. 페닐렌기 또는 띠오펜기의 변화와 -CN기의 비닐렌 위치에서 알파와 베타위치로의 변화를 통해 최종적으로 R4에서는 띠오펜기와 -CN기가 베타위치에 있는 경우로 최대의 장파장 영역으로의 이동을 보였다. 이 단량체를 이용하여 기존에 청색발광 물질로 잘 알려진 폴리플루오렌 고분자의 주쇄에 랜덤한 방법으로 15%를 도입하였을 때 순수한 적색발광을 구현할 수 있었다. NTSC(National Television System Committee)에서 규정하는 순수한 적색발광의 좌표인 (0.66, 0.34)에 아주 근접하는 (0.67, 0.34)를 보였다. 이렇게 합성된 단량체 네가지(R1, R2, R3, and R4)를 이용하여 좀 더 개선된 전기발광 성능을 구현하고자 새로운 중합방법인 스즈키 합성법을 이용하여 얼터네이팅 고분자 재료(PFR1-S, PFR2-S, PFR3-S, and PFR4-S)를 합성하였다. 합성된 고분자들중에 PFR3-S와 PFR4-S에서 가장 좋은 전기발광 성능을 보였다. 이 두물질은 색순도면에서 (0.63, 0.38 : PFR3-S)과 (0.66, 0.33 : PFR4-S)의 값으로 표준 적색좌표에 아주 근접하는 값을 보였다. 또한 PFR3-S의 경우는 최대밝기가 3500cd/㎡ 으로 좋은 전기발광 특성도 나타내었다.
두번째로 새로운 플루오렌계 적색 발광 고분자를 위해 단량체(PZB)를 설계하고 합성하였다. 이는 페노싸진을 기본으로 하여 위에서 도입된 -CN기와 페닐렌 링을 확장시킨 구조로 합성하였다. 이 단량체를 플루오렌 고분자 주쇄에 도입하여 적색 발광을 이루도록 합성하였다. 합성된 고분자들은 PZB 단량체를 다양한 비율로 포함하도록 합성하였는데 그중 50%가 도입된 PF-PZB50에서 가장 좋은 특성을 나타내었다. 색순도면에서 뿐만 아니라 전기발광 특성 모두에서 좋은 성질을 보여 상업화에 좋은 후보군으로 PFR3-S와 함께 제시될 수 있을것이다.
마지막으로 합성된 고분자들 중에 낮은 에너지 밴드갭을 가지는 PFR3-S, PFR4-S, 그리고 PF-PZB50 고분자들을 이용한 새로운 응용예인 유기 태양전지에의 적용 가능성을 실험하였다. 현재 보고되어지는 에너지 전환효율은 3~5%대의 값을 보이는데 위의 세 고분자들은 모두 이 값에 근접하는 좋은 성질(0.53 ~ 1.00%)을 보였다.
