Understanding and control of photoactive layer morphology in organic bulk heterojunction solar cells

이효상 Korea University 2017 국내박사

In the organic solar cell researches, a wide range of photoactive layer materials have been studied to achieve better photovoltaic properties. Even though the bulk heterojunction (BHJ) solar cells contain the same active layer materials, the device performance of solar cells significantly depends on the architecture and fabrication condition of the blend films. In this dissertation, we investigated the improvement of organic solar cell performance by a morphology control of photoactive layers. First, low band gap polymers, poly(2,5-bis(2-octyldodecyl)-3-(5-(thieno- [2,3-b]thiophen-2-yl)thiophen-2-yl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (pDPPTTi-OD) composed of thieno [2,3-b]thiophene (TTi) and diketopyrrolopyrrole (DPP) were synthesized via the Stille cross-coupling polymerization. pDPPTTi-OD showed an optical bandgap of ~ 1.57 eV, and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of -5.40 and -3.74 eV, respectively. The pDPPTTi-OD:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend films were processed at 25, 70 and 90 °C. The power conversion efficiencies (PCEs) of the devices processed at 25, 70 and 90 °C were 2.05, 2.30 and 2.90%, respectively. The 90 °C–processed devices exhibited a PCE of 2.90% with an open circuit voltage (VOC) of 0.79 V, a short circuit current density (JSC) of 9.82 mA cm-2 and a fill factor (FF) of 0.51. Second, the effective morphological control of polymer:fullerene blends was studied using chloroform (CF), CF:1,8-diiodooctane (DIO), and CF:o-dichlorobenzene (ODCB) solvent systems. The active layers are composed of low band gap polymers of poly(2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,2':5',2'':5'',2'''-quarterthiophene) (P(DPP-alt-QT)),poly(2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5'-di-(thiophen-2-yl)-2,2'-biselenophene) (P(DPP-alt-DTBSe)), and a PC71BM. Photovoltaic devices were fabricated using polymer:PC71BM blends architecture of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophen- e):poly(styrenesulfonate) (PEDOT:PSS)/Active layer/TiO¬2/Al. The highest values of PCE were obtained from the CF:ODCB solvent system (P(DPP-alt-QT: PCE = 5.4%, P(DPP-alt-DTBSe: PCE = 2.1%). Finally, 1,8-diiodooctane (DIO) solvent and poly(N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) (PCDTBT) polymerwere used in order to investigate the effect of additives in the 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2):PC71BMbased small-molecule bulk heterojunction solar cells. From the result, solar cells prepared with both PCDTBT and DIO consistently achieve high performances: a PCE of 8.13% with a VOC of 0.80 V, a JSC of 15.37 mA cm-2, and a FF of 0.66. When PCDTBT was added into the blend films, thermal and humidity stability for small-molecule solar cell were dramatically improved. In conclusion, photovoltaic properties of organic solar cells are significantly improved by an efficient morphology control of photoactive layers. To achieve the high performance of solar cells, it is necessary to find the effective processing temperature of active layer materials, choose suitable solvents in order to form the fitted domain size, and use the appropriate additives which suppress donor over-segregation. 최근 화학구조의 개선을 통해 높은 효율을 보이는 유기태양전지 광활성층 물질들이 다양하게 보고되고 있다. 하지만 동일한 광활성층 물질을 사용하는 벌크헤테로정션 태양전지라 할지라도 태양전지의 구조와 소자를 제작하는 조건에 따라 각기 다른 광전변환효율이 획득될 수 있다. 본 연구는 다양한 광활성층 몰폴로지 컨트롤을 통한 태양전지 광전변환효율의 향상을 목표로 하였다. 먼저, thieno [2,3-b]thiophene (TTi)와 diketopyrrolopyrrole (DPP)를 Stille cross-coupling을 통하여 낮은 밴드갭 고분자 pDPPTTi-OD를 중합하였다. 이 고분자는 1.57 eV의 광학적 밴드갭을 가지며, -5.40 eV의 HOMO 에너지 준위와 -3.74 eV의 LUMO 에너지 준위를 갖는다. 이 고분자를 도너로, PC71BM을 억셉터로 사용하여 각각 25 °C, 70 °C 그리고 90 °C에서 소자를 제작하였다. 각각의 소자의 효율은 2.05, 2.30, 2.90%으로 특히, 90 °C에서 만들어진 소자의 경우 0.79 V의 개방전압과 9.82 mA cm-2의 단락전류, 0.51의 충진계수로 2.90%의 광전변환효율을 획득하였다. 높은 온도의 용액으로 소자를 제작할 경우 효과적으로 PC71BM 도메인 크기를 줄이고 이를 통한 pDPPTTi-OD와 PC71BM의 효과적인 네트워크 형성으로 단략전류의 증가를 통한 광전변환효율 향상이 있었다는 것을 알 수 있었다. 다음으로, CF, CF:DIO, CF:ODCB의 세 가지 혼합용매 체계에서 고분자:풀러렌 블랜드의 몰폴로지 연구를 진행하였다. 광활성층의 도너 물질로는 P(DPP-alt-QT), P(DPP-alt-DTBSe)을 사용하였으며, 억셉터 물질로는 PC71BM을 사용하였다. 일반적인 형태의 구조로 태양전지 소자를 제작하였을 경우 CF:ODCB 혼합 용매에서 P(DPP-alt-QT)는 5.4%, P(DPP-alt-DTBSe)는 2.1%의 효율을 획득할 수 있었다. 높은 끓는점을 갖는 ODCB의 PC71BM에 대한 선택적인 용해도로 인하여 광활성층 물질이 섞일 수 있는 충분한 필름건조 시간이 확보되었으며, 잘 발달된 피브릴 네트워크 형성으로 인하여 광전변환효율의 향상을 가져온 것을 알 수 있었다. 마지막으로 DIO 용액 첨가제와 PCDTBT의 고분자 첨가제를 사용하여 p-DTS(FBTTh2)2:PC71BM 단분자 태양전지 연구를 진행하였다. 0.4% DIO와 2% PCDTBT를 함께 첨가하였을 경우 0.80 V의 개방전압, 15.37 mA cm-2의 단락전류, 0.66의 충진계수로 8.13%의 광전변환효율을 획득하였다. DIO는 단분자 도너 물질의 결정성을 증가시키고, PCDTBT는 도너의 과도한 결정성 증가에 의한 상 분리를 억제하고 원활한 네트워크를 형성한다는 것을 알 수 있었다. 또한, 110 °C 열안정성 테스트 결과 DIO만 사용하였을 경우보다 PCDTBT가 첨가되었을 경우 3시간 이후 초기효율의 26%를 나타내던 소자가 60%를 유지하는 것을 확인하였다. 65 °C/85% 상대습도 테스트결과 1000시간 이후 초기효율대비 89%의 높은 안정성을 나타내는 결과를 확인하였다. 결과적으로, 유기태양전지 광활성층의 적절한 도메인 크기를 형성할 수 있는 적합한 용매와 용해 온도를 적용하고, 적정량의 첨가제를 사용하여 몰폴로지 컨트롤을 하였을 경우 고효율과 열 안정성이 동시에 확보되는 태양전지를 제작할 수 있었다.

Conjugated Polymer-Metal Oxide Hybrid Interlayer for Improved Efficiency of Bulk-heterojunction Solar Cells : 공액 고분자-금속산화물 하이브리드 interlayer를 통한 벌크 이종접합 태양전지의 성능향상

Yu, Jiang 경북대학교 대학원 2017 국내석사

In this work, a hybrid system composed of inorganic zinc oxide nanocrystals (ZnO NCs) and the organic conjugated polymer (poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) (PFN) was utilized as an electron selective interlayer (ESIL) to improve the electro-optical characteristics of bulk-heterojunction (BHJ) polymer solar cells (PSCs). To accomplish, water/alcohol-soluble cationic polyelectrolyte, PFN, was introduced into ZnO NCs (ZnO-PFN), aiming to enhance the electron extraction capability between the photoactive layer and the metal (Al) electrode in standard geometry BHJ PSCs. Importantly, the ZnO-PFN blend system achieved a higher power conversion efficiency (PCE) than pristine (ZnO NCs) ESILs. Moreover, an optimized photovoltaic (PV) performance was obtained with a low volume of PFN incorporated into the ZnO NCs ESIL. To validate the PV performance, PSCs were fabricated based on co-polymer of thienyl substituted BDT with TT: phenyl-C71-butyric acid methyl ester (PBDTTT-C-T:PC71BM) and poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) photoactive systems and observed superior PV characteristics for ZnO-PFN hybrid ESILs. The optical transparency, microstructure, and morphological characteristics were evaluated using appropriate characterization techniques to demonstrate the superiority of the hybrid ZnO-PFN blend system. PSCs based on this ZnO-PFN composite based ESIL suggested an alternative practical approach to enhance the efficiency of the fabricated devices. 본 연구는 벌크 이종접합 기반 태양전지의 광기전력 특성을 높이기 위하여 무기 산화아연 (ZnO) 나노입자와 유기 공액고분자인PFN의 하이브리드 interlayer를 전자수송층으로 이용하였다. 전자수송층의 전자 추출 능력 및 이동도를 최적화 하기 위하여, 알코올/수용성을 가지는 양이온성 고분자전해질인 PFN과 ZnO를 다양한 혼합비율로 (1:1, 2:1, 3:1, 4:1 Vol/Vol) 에탄올에 용해하였으며, 각각을 광활성층 용액인 P3HT (poly (3-hexylthiophene-2,5-diyl)): PC61BM ([6,6]-Phenyl C61 butyric acid methyl ester)와 PBDTTT-C-T (co-polymer of thienyl substituted BDT with TT):PC71BM (phenyl-C71-butyric acid methyl ester)를 기반한 태양전지에 적용하였고 그에 따른 특성을 평가하였다. 이를 위해, 각각의 ZnO-PFN 혼합비율에 따른 전자 수송층의 투과도와 흡광도 및 표면 형태특성을 분석하였고, 제작된 유기 태양전지의 광전지 특성을 확인하였다. 결과적으로 이전에 ZnO 전자 수송층 기반의 유기 태양전지보다 ZnO-PFN 전자 수송층을 이용한 유기태양전지의 전력 변환 효율이 향상됨을 확인할 수 있었다. 또한, 광활성층 용액 종류에 따른 특성을 확인한 결과, P3HT:PC61BM의 경우 6:1의 비율로 혼합된 ZnO-PFN 전자 수송층 기반의 전력 변환 효율이 2.65%에서 2.96%로 증가였으며, PBDTTT-C-T:PC71BM의 경우 4:1의 비율로 합성된 ZnO-PFN 전자 수송층 기반의 전력 변환 효율이 3.47%에서 3.74%로 증가였다. 본 연구를 통해 공액고분자 PFN을 도핑한 ZnO를 전자 수송층으로 이용하였을 때, 전자 추출 능력 및 전자의 이동도가 향상됨을 확인하였고, 이를 활용하여 상온에서의 고효율 유기 태양전지 제작 기술을 제안하였으며, 추후, 자동차 또는 건물 등 다양한 분야의 유기 태양전지에 응용할 수 있을 것으로 전망 된다.

Study on Naphtho[2,1-b:3,4-b']dithiophene-based Copolymers for Bulk Heterojunction Organic Solar Cells : 벌크 이종접합 유기 태양전지를 위한 Naphtho[2,1-b:3,4-b']dithiophene을 기반으로 하는 공중합체에 관한 연구

김유진 포항공과대학교 일반대학원 2013 국내석사

Organic semiconductor materials that can be obtained from simple solution processes have attracted the attention of many research groups in recent years because of the widely increased demand for low-cost and flexible electronic products. The potential applications of these organic materials have led to intense researches in the field of organic electronics, especially in the field of organic solar cells (OSCs). OSCs are expected to be used as large scale and flexible electronic devices in various structures or buildings. However, the poor performance and stability are still serious obstacles to their commercial use. The work presented in this thesis is to study on the organic semiconductor materials of the active layer and network formation of blend films with donor polymers and acceptors. Accordingly, different NDT polymer derivatives were used and their properties and performances on OSCs were investigated. Basic background of the organic donor materials and OSCs are introduced in Chapter 1. Two novel polymer materials based on naphtho[2,1-b:3,4-b']dithiophene (NDT), PNDT-TTT and PNDT-TET, have been designed and tested in bulk heterojunction (BHJ) solar cells. Optical and electrochemical properties of these new polymers and OSC performances are studied and discussed in Chapter 2. Two donor polymers, PNDT-TTT and PNDT-TET, have tri-thiophene and bi-thiophene coupled by vinyl group, respectively, which exhibit red-shifted absorption spectra compared to PNDT-T (previously reported). These effects result in high short circuit currents in OSCs. Also, desired morphologies of these polymer:PC71BM blend films show good percolation pathways, which lead to increase device efficiency. The nano-scale network formation of donor polymer and acceptor materials are studied and dealt with in Chapter 3. Among the conjugated polymers, PNDT-T, PNDT-TT and PNDT-TTT are used for donor materials and PC71BM are used as a acceptor material. Three copolymers are NDT derivatives with different thiophene lengths. Through a combination of X-ray diffraction analysis and atomic force microscopy we see three distinct nanoscale morphologies by donor materials with different thiophene lengths. PNDT-TT presents strong intermolecular interaction, which results in the lowest efficiency of devices. However, PNDT-TTT with proper torsion structure exhibits good a mixed morphology, which leads to high OSCs performances.

Efficient small-molecular-weight organic solar cells through orientation control and nano-structuring

김지환 서울대학교 대학원 2013 국내박사

Due to the fast growing energy consumption and the growing environmental concerns over the climate change risks such as global warming, use of solar energy is one of the promising candidates for renewable sources of electricity. Among various solar cells, organic solar cells (OSCs) are considered to have large potential due to the fact that organic materials are cheap and easy to form a film with inexpensive process even in the large area. Unfortunately, OSCs have relatively low power conversion efficiency compared to other kinds of solar cells. Therefore the most important issue in OSCs is how to improve the power conversion efficiency (PCE). This thesis reports a couple of methods to improve the PCE of small molecular weight OSCs; nano-structuring and orientation control of active materials. Small molecular weight OSCs have advantages of easy purification, good reproducibility, easy fabrication of multi-layered structure and so on over polymer based solar cells. One drawback of small molecular weight OSCs is located on the difficulties to form interpenetrating network between donor and acceptor molecules in nanometer scale because of the large entropy of mixing in the amorphous structure. A new method to form small-molecular based bulk heterojunction (BHJ) through alternating thermal deposition (ATD) is proposed, which is a simple modification of conventional thermal evaporation. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C60) systems is confirmed by atomic microscopy (AFM), grazing incidence X-ray small angle scattering (GISAXS), and absorption measurements. From the analysis of the data, CuPc|C60 films fabricated by ATD are composed of nanometer sized disk-shape CuPc nano grains and aggregated C60, which explains the phase separation of CuPc|C60. Compared with co-deposited OSCs, the ATD OSCs show significant enhanced performance. However ZnPc, which has the same crystalline structure with CuPc, did not show the improvement by ATD due to the initial growth difference. To understand the mechanism of ATD, the initial growth of CuPc on different substrate condition is monitored using GISAXS. Disk-type nano grains of CuPc were observed in an ultrathin CuPc layer evaporated on a hydrophilic Si surface. The disk type grains consisted of a crystalline part and a non- crystalline part. The disk type grains were smaller in the case of CuPc on hydrophobic Si surface, which showed lower crystallinity with random distribution. Despite regularly distributed CuPc grains the mobility was lower in a thin film transistor device fabricated on a hydrophilic surface than on a hydrophobic surface due to the lower average density of the molecules relating to porous molecular packing between nanograins on a hydrophilic surface. Improvement of the performance of OSCs was also achieved by controlling the molecular orientation. A highly efficient planar heterojunction OSC based on zinc phthalocyanine (ZnPc)/C60 is obtained by controlling the orientation of the ZnPc using copper iodide (CuI) as the interfacial layer. The proportion of face-on ZnPc molecules was increased significantly on the CuI layer compared to the layer without the CuI layer analyzed with wide angle X-ray scattering (WAXS) and optical absorption. The PCE of the orientation controlled planar heterojunction OSC was remarkably enhanced to 3.2% compared with 1.2% without the control of the molecular orientation. The mechanism of formation of the face-on molecular orientation of ZnPc on CuI layer is proposed. Using the two different incidence angles in the glazing incidence wide angle x-ray scattering (GIWAXS), it turns out that the ZnPc layer on the (111) gamma phase CuI is the beta phase ZnPc with (-101) orientation which is transformed to (313) gamma phase ZnPc as the film growth. These phase transitions can be explained with quasi epitaxial growth.

Nanoscale Interface Control for High Efficiency Polymer/Fullerene Solar Cells : 고효율 고분자/풀러렌 태양전지를 위한 나노단위 계면 제어에 대한 연구

우성호 경북대학교 대학원 2014 국내박사

Polymer solar cell (PSC) technology has been believed to be a promising solution to the energy crisis, which we inevitably encountered in the near future, and also to be a useful energy source in upcoming ubiquitous society due to the possibility of producing solar cells with lightweight, flexible, inexpensive, and easily manufactured using low temperature and/or roll-to-roll processes. There are two kinds of important interfaces in PSCs. One is the bulk heterojunction (BHJ) interface between donor polymer domain and fullerene acceptor domain after phase separation, the other is charge transport interfaces between active layer and two electrodes in both normal and inverted structure PSCs. In this thesis, we studied on 1) how to improve morphology and BHJ interface property using phenyl compound additive; 2) the effect of Au, Ag nanoparticles (NPs) on hole transporting buffer layer (PEDOT:PSS) in normal structure; 3) the effect of self-assembled monolayer (SAM) and 4) the effect of electron-rich polymer nanolayer for electron transporting buffer layer (ZnO) in inverted structure. The addition of 4-fluorobenzonitrile (4-FBN) into poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends enhanced not only the crystallinity of the P3HT domains but also the absorption property. The in-situ synthesized Au, Ag NPs with a size of 20~40 nm were well dispersed in PEDOT:PSS media and increased the device efficiency due to the increased surface roughness and surface area of the PEDOT:PSS layer. The performance of inverted PSCs was improved through a modified interface of ZnO/active layer with a series of carboxylic acid-functionalized SAMs due to the enhancement in the FF and JSC values. Finally, the efficiency of the inverted PSCs based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and PC71BM showed up to 9% by placing a 2-nm-thick poly(ethylenimine) nanolayer on the ZnO surface owing to the induced surface dipole between ZnO and PEI. These nanoscale interface optimizations can be key technologies to achieve high efficiency PSCs without change of established structures, processes and facilities. 인구증가와 경제발전에 따른 지속적인 화석연료 사용량 증가는 화석 연료의 고갈이라는 위험과 함께 그에 따른 이산화탄소 배출, 환경오염, 온실효과, 오존층파괴등과 같은 심각한 기후환경변화의 문제를 초래함에 따라, 전세계적으로 신재생에너지개발에 대한 관심이 고조되고 있다. 고분자태양전지는 저가격과 함께 유연하고 가벼우며 롤투롤(roll-to-roll) 제조공정을 이용해 대량 생산이 가능하다는 장점으로 인해 최근 많은 연구가 진행되고 있다. 고분자태양전지에는 크게 두 종류의 계면이 존재하는데, 하나는 광활성층의 상분리후에 생성된 고분자 도우너 영역과 풀러렌 억셉터 영역간의 벌크헤테로접합 계면이고, 다른 하나는 광활성층과 두 전극층 사이의 전하이동과 관련된 계면이다. 본 논문에서는 1) 페닐화합물을 첨가제로 사용하여 벌크헤테로접합 계면특성과 모폴로지를 향상시키는 방법; 2) 정상구조(normal structure)에서 정공전달층인 PEDOT:PSS층 내부에 존재하는 금속나노입자의 영향; 3) 인버트구조(inverted structure)에서 전자전달층인 ZnO표면을 카르복실산 작용기를 가지는 단분자 화합물로 표면처리 한 효과; 4) ZnO표면을 비공유전자를 많이 가지고 있는 폴리에틸렌이민(poly(ethyleneimine), PEI)으로 표면처리 한 효과에 대하여 살펴보았다. 4-fluorobenzonitrile을 첨가제로 사용한 경우 P3HT고분자 영역의 결정성 증가 및 이로 인한 광흡수 증가가 관찰되었다. PEDOT:PSS용액내에서 직접 합성된 20~40 nm크기의 금, 은 나노입자는 PEDOT:PSS층의 표면조도 및 표면적을 증가시켜 생성된 홀이 전극부로 효과적으로 추출될 수 있도록 하여 효율증가에 기여하였다. 또한, 인버트구조의 소자에서 카르복실산 작용기를 가진 단분자층으로 ZnO표면처리를 실시한 결과, 접촉각과 암전류 특성 개선 및 FF와 JSC 증가로 인해 효율이 향상됨을 확인하였다. 끝으로, PTB7:PC71BM 광활성층을 적용한 인버트구조 고분자태양전지에서 2 nm두께의 PEI로 ZnO를 표면처리한 결과 약 9%의 고효율 소자를 얻을 수 있었으며, 이는 ZnO와 PEI간에 형성된 표면쌍극자효과에 기인한다. 본 연구결과에 의해 고분자/풀러렌 태양전지의 나노단위 계면최적화를 통해 소자 특성이 크게 개선되는 것을 확인하였으며, 본 연구에서 제시하는 기술들은 향후 고분자 태양전지가 상용화 될 경우 주어진 공정과 장비의 변경 없이 고효율화를 달성할 수 있는 핵심기술이 될 것으로 판단된다.

Design and Synthesis of Conjugated Polymers with Efficient Light Absorption for High Performance Polymer Solar Cells

이종원 서울대학교 대학원 2016 국내박사

Until now regioregular poly(3-hexylthiophene) (P3HT) has been studied as an electron donor material for organic photovoltaics, and bulk heterojunction photovoltaic devices based on P3HT and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) have been reported to have power conversion efficiency (PCE) as high as 5%. However for devices utilizing, the main problem of P3HT is its quite large bandgap (1.92.0 eV) which limits the light absorption from the solar spectrum. To overcome the absorption limitation, extensive effort has focused on developing donor–acceptor (D–A) conjugated polymers. One unique feature of these polymers is that their HOMO and LUMO energy levels are governed by the HOMO energy level of the donor and the LUMO energy level of the acceptor, respectively. Therefore, the energy levels of polymers can be tuned by choosing appropriate combination of D and A unit. Among various semiconducting polymers for polymer solar cells (PSCs), a copolymer consisting of fluorene and dithienyl benzothiadiazole (PFDTBT) exhibits a high VOC of around 1 V because of its deep HOMO energy level, but it shows relatively low PCE with moderate JSC due to a large bandgap of 1.8 eV, which is the same problem of P3HT. For the purpose of reducing the bandgap of PFDTBT, we introduced a strong electron-donating oxygen atom into fluorene to synthesize benzochromene (BC), which was then polymerized with electron deficient dithienyl benzothiadiazole (DTBT) to afford two low-bandgap polymers, PBCDTBT. The PBCDTBT-based solar cell exhibits a high power conversion efficiency (PCE) of 5.74%, which is much higher than that of the reference fluorene-based polymer (PFDTBT) (2.56%). The higher PCEs of PBCDTBT is mainly attributed to higher JSC, which arises from stronger and broader absorption, higher exciton generation rate, more effective charge separation and higher hole mobility as compared to PFDTBT, although the VOC is sacrificed to some extent. Then, to enhance the JSC without VOC loss, we introduce ternary blend strategy. Ternary blends incorporating multiple donor absorbers with complementary absorption into active layers provide an opportunity to enhance the PCE of organic solar cells. In addition to complementary absorption between the donors, ternary blends exhibit improved internal morphology by choosing appropriate third component. For this purpose, in this study we design a ternary blend solar cell with two donors composed of diketopyrrolopyrrole-based polymer (PTDPP2T) and small molecule ((TDPP)2Ph), and one acceptor (PC71BM). Consequently, the PCE of ternary blend solar cell is increased from 6.60% (PTDPP2T:PC71BM) to 7.48% (PTDPP2T:(TDPP)2Ph:PC71BM), which is attributed to complementary absorption behavior between PTDPP2T and (TDPP)2Ph. Also, when the small amount of (TDPP)2Ph (<10 wt%) is added to PTDPP2T:PC71BM, a narrower fibril size with face-on orientation in qz direction on the substrate is obtained, which facilitates charge transport. Another effective strategy to further enhance JSC of the PSCs is to synthesize random copolymers composed of one electron donating unit and two different electron accepting units, if the absorptions of two electron accepting units are complementary to each other. To this end, we synthesized a new series of conjugated random copolymer composed of bithiophene (electron donating unit) with TDPP and pyridine-capped diketopyrrolopyrrole (PyDPP) (co-electron accepting units). The random copolymers show broad light absorption and face-on orientation on the substrate, which is beneficial to achieving high short circuit current. The VOC of the random copolymer can also be controlled systematically by varying the ratio of PyDPP to TDPP in the copolymer, since the HOMO energy level becomes deeper as the PyDPP content in the random copolymer is increased. Consequently, the solar cell device made of the random copolymer with the ratio of 3:1 (TDPP:PyDPP) shows higher PCE (8.11%) than those made of corresponding homopolymers, PTDPP2T (6.70%) and PPyDPP2T (4.14%). From these results, it can be concluded that the absorption properties of the active layers are influenced by chemical modification of polymer and third component in ternary blend. Also these absorption properties are important factors to design high performance PSCs.

TTF 액정 유도체를 이용한 유기태양전지 제조 및 분석

박수연 전북대학교 대학원 2011 국내석사

Bulk-heterojunction (BHJ) solar cells comprising interpenetrating networks of an organic donor and a fullerene derivative acceptor such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) constitute a promising technology because they are easy to fabricate by solution processing and are predicted to yield power conversion efficiencies (PCEs) of up to 10%. The high efficiency of P3HT?fullerene devices can be explained by the ability of the blend to phase separate and crystallize into desirable BHJ morphologies after processing, allowing for efficient charge separation and transport. The majority of research relating to BHJ solar cells has focused on polymeric donor materials, since they generally have better film-forming properties than non-polymeric materials. Small-molecule donor materials can also form useable BHJ solar cells by solution processing, although it is more challenging to obtain high-quality films. Small-molecule materials offer advantages over polymeric materials in terms of ease of synthesis and purification, which greatly improve fabrication reproducibility, as well as possessing a greater tendency to self assemble into ordered domains, which leads to high charge carrier mobilities. Small molecules do not suffer from batch to batch variations, broad molecular-weight distributions, end-group contamination, or difficult purification methods, which can be significant problems for polymeric materials. These considerations make small molecules a promising class of donor material for BHJ solar cell applications. In this study, the bulk-heterojunction solar cell was fabricated from a discotic liquid crystal based on tetrathiafulvalene (TTF) structure as an electron donor and PCBM as an electron acceptor. Discotic liquid crystalline materials have been studied extensively as promising organic semiconductor candidates in optoelectronic devices. This material exhibited highly ordered oblique columnar LC phases in a wide temperature range including room temperature and displayed novel functional properties as a donor material.

Bulk Heterojunction Solar Cells based on modified-Polyethylenimine Ethoxylated Cathode Interfacial Layers

PRASETIO ADI 부경대학교 대학원 2020 국내석사

저비용 고분자 전해질 polyethylenenimine 80 % ethoxylated (PEIE)는 유연한 유기 발광 다이오드 및 유기 태양 전지의 계면 개질 층으로서 널리 사용되어왔다. 그러나, PEIE는 금속 산화물에 비해 전하 이동이 더 낮다. 이 연구에서, 벌크 이종 접합 태양 전지의 소자 성능은 소수성 분자 스테아르 산을 사용하여 PEIE를 변형시킴으로써 현저하게 개선된다. 그 결과, stearic-acid 변성 PEIE (m-PEIE로) 장치는 강성 장치에 대해 9.84 % (FF 0.67, JSC 18.78 mA cm-2, VOC 0.79 V)의 향상된 전력 변환 효율 (PCE)을 달성했습니다. 플렉서블 디바이스의 경우 8.39 % (FF 0.66, JSC 16.65 mA cm-2, VOC 0.78 V). 한편 PEIE pristine 장치는 PCE는 8.45 % (FF 0.62, JSC 17.55 mA cm-2, VOC 0.77 V), 7.55 % (FF 0.64, JSC 15.53 mA cm-2, VOC 0.76 V)에 불과했습니다. ZnO 장치는 PCE가 9.02 %로 딱딱하고 PCE는 5.32 % 만 보이는 유연한 장치에서 눈에 띄게 감소했습니다. 또한, 장기 공기 저장 안정성이 실질적으로 개선되어 (10 일 후 보유율 87.75 %) PEIE 깨끗한 장치보다 우수합니다. 소자 안정성 및 광기 전 성능에서의 이러한 개선은 cathode interfacial layer (CIL)의 소수성의 증가에 기인한다. Atomic force microscopy (AFM) 특성화는 m-PEIE 기반 장치가 캐소드 / 광활성 층 계면에서 더 나은 전하 전달을 용이하게하는보다 부드러운 거칠기를 갖는다는 것을 보여준다. 또한, m-PEIE 소자는 광도 의존, 암색 J-V 및 electrochemical impedance spectroscopy (EIS) 분석에 따라 PEIE 소자에 비해 전하 재결합이 적다. 또한, m-PEIE 장치는 장치를 습기로부터 보호하는 스테아르 산의 소수성 장 탄화수소 사슬로 인해 주변 조건에서 더 우수한 안정성을 갖는다. m-PEIE 기반 장치는 10 일 후에도 초기 효율의 87 % 이상을 유지했습니다. 더 중요한 것은, m-PEIE 층은 유연한 장치에 유리한 추가적인 열 어닐링 처리를 필요로하지 않는다. A low-cost polyelectrolyte, Polyethylenimine 80% ethoxylated (PEIE) has been widely used as an interface modified layer in flexible organic light-emitting diodes and organic solar cells. However, the PEIE has lower charge transfer compare to metal oxides. In this study, the device performance of bulk heterojunction solar cells is remarkably improved by modifying the PEIE using a hydrophobic molecule stearic acid. As a result, the stearic acid-modified-PEIE (denoted as m-PEIE) device achieved an improved power conversion efficiency (PCE) of 9.84% (FF 0.67, JSC 18.78 mA cm-2, VOC 0.79 V) for the rigid device and 8.39% (FF 0.66, JSC 16.65 mA cm-2, VOC 0.78 V) for the flexible device. Meanwhile, the PEIE pristine device only obtained a PCE of 8.45% (FF 0.62, JSC 17.55 mA cm-2, VOC 0.77 V) for rigid and 7.55% (FF 0.64, JSC 15.53 mA cm-2, VOC 0.76 V) for the flexible device. The ZnO device achieved a PCE of 9.02% for rigid and noticeably decreased on a flexible device which only shows a PCE of 5.32%. Furthermore, the long-term air–storage stability is substantially improved (87.75% retention after 10 days) which is superior to PEIE pristine device. These improvements in device stability and the photovoltaic performance are attributed to the increase of hydrophobicity of the cathode interfacial layers (CILs). Atomic force microscopy (AFM) characterization shows that m-PEIE based device has more smooth roughness which facilitates better charge transfer at the cathode/photoactive layer interface. Moreover, the m-PEIE device has less charge recombination compared to PEIE device according to the light intensity-dependent, dark J-V, and electrochemical impedance spectroscopy (EIS) analysis. Besides, the m-PEIE device has better stability in ambient conditions due to its hydrophobic long hydrocarbon chain of stearic acid which protects the device from moisture. The m-PEIE based device retained over 87% of the initial efficiency after 10 days. More importantly, the m-PEIE layer does not require a further thermal annealing treatment which is beneficial for flexible devices.

Design and synthesis of fluorinated conjugated polymers based on 3,3’-difluoro-2,2’-bithiophene for high performance polymer solar cells

조제웅 서울대학교 대학원 2015 국내박사

Electronic energy level engineering of conjugated polymers including low-bandgap for harvesting a wide range of solar spectrum, deep highest occupied molecular orbital (HOMO) energy level for high open circuit voltage (VOC) and sufficient offset of lowest unoccupied molecular orbital (LUMO) energy levels between polymer donor and fullerene acceptor is a key strategy to achieve high performance polymer solar cells (PSCs) and several modification methods of chemical structure have been proposed to control energy levels of conjugated polymers. Among these methods, introduction of fluorine atom has attracted much attention for the past few years because high power conversion efficiencies (PCEs) over 7% have been reported with fluorine substituted polymer-based solar cells. However, the effects of fluorination on optoelectrical and photovoltaic properties of polymers have not been revealed clearly and have been studied in only few polymer systems. For further investigation, more fluorinated monomers and polymers for PSCs should be designed and synthesized. In this work, we synthesized difluoro-bithiophene as a new fluorinated building block in conjugated polymers for PSCs and various fluorinated copolymers based on difluoro-bithiophene are successfully designed and synthesized in order to clarify the effect of fluorination on the properties of polymers and its device performance of PSCs. First, fluorinated poly(3,4-dialkylterthiophenes) (PDATs) composed of difluoro-bithiophene and 3,4-dialkylterthiophene are synthesized. Fluorination on polymer backbone changes its electronic structure, leading to deeper HOMO energy level and enhances molecular packing of the polymers as evidenced by strong vibronic shoulder in UV-Vis absorption spectrum and pi-pi stacking pattern in GIWAXS. When bulky side chain (ethylhexyl) are introduced as a solubilizing group, fluorinated polythiophenes develop finer fibril structure and exhibit a high PCE of 5.12% with a VOC of 0.87 V and a short circuit current density (JSC) of 9.82 mA/cm2. Fluorinated DA type polymer, copolymerized by diketopyrrolo[3,4-c]-pyrrole (DPP) as an A unit and difluoro-bithiophene as a D unit, is also synthesized for investigating the fluorination effect on D-A type polymer. After introduction of fluorine atoms on DPP-based polymer, deeper HOMO energy level of polymer is observed without significant change of optical properties. Although fluorination increases fibril size and lowers JSC of DPP-based polymer, fluorinated DPP-based polymer exhibits higher PCE of 6.39% than non-fluorinated DPP-based polymer (PCE = 5.47%) due to large improvement of VOC. Furthermore, for investigating the effect of fluorination position on the properties of D−A type polymer, two types of fluorinated polymers are synthesized, HF with fluorination on D unit and FH with fluorination on A unit. Compared to non-fluorinated polymer, fluorinated polymers exhibit deeper HOMO energy levels without change of bandgap and stronger vibronic shoulder in UV−Vis absorption spectra, indicating that fluorination enhances intermolecular interaction. HF exhibits a high PCE of 7.10%, which is higher than the PCE (6.41%) of FH, with well-developed fibril network, low bimolecular recombination and high hole mobility. Finally, D−A polymers with different degree of fluorination are synthesized by using 2,1,3-benzothiadiazole (BT) unit substituted by different number of fluorine atoms. 3F with mono-fluorinated BT and 4F with di-fluorinated BT have deeper HOMO energy levels, stronger vibronic shoulder in UV-Vis absorption spectra, and narrower size of fibril in blend film with PCBM than 2F with non-fluorinated BT. Among fluorinated polymers, 3F exhibits the highest PCE of 7.92% with low bimolecular recombination, high hole mobility and well-developed interconnected network with nanoscale fibril. From these results, it can be concluded that the optoelectrical and photovoltaic properties of conjugated polymer are significantly influenced by introduction of fluorine atoms and fluorinated conjugated polymers are promising materials for achieving high performance PSCs.

Design and Synthesis of Donor-Acceptor Type Low-Bandgap Polymers for High Performance Polymer Solar Cells

이윤규 서울대학교 대학원 2012 국내박사

Currently, BHJ solar cells fabricated by simple blending of regioregular poly(3-hexylthiophene) (P3HT) as an electron-donating polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have reached the power conversion efficiency (PCE) as high as 5 %. Although possessing several advantageous properties, P3HT has a relatively high bandgap of 1.9 eV and thus harvests photons only up to 22 % of available photons. To overcome the inherent weakness of the conducting polymers, we have designed and synthesized several kinds of donor-acceptor type alternating copolymers. In the design of low-bandgap polymers, some important aspects should be considered. If the bandgap of the donor polymer is too narrow, that is, the HOMO level of polymer is high-lying and LUMO level is low-lying, there exist opposite effects. Since the open circuit voltage (VOC) of polymer solar cell linearly depends on the difference between the LUMO level of acceptor (PCBM) and HOMO level of donor polymer, the high-lying HOMO level of polymer can result in reduced VOC. Secondly, since a LUMO-LUMO offset of 0.3 ~ 0.4 eV is necessary for efficient electron transfer from the donor polymer to PCBM, the small offset between the LUMO of the polymer and the LUMO of PCBM caused by low-lying LUMO level of polymer prevents electron transfer from polymer to PCBM. In this aspect, we have successfully controlled the molecular energy levels of D-A type copolymers by introducing different substituent on [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) unit which is used as an acceptor moiety in D-A type copolymer. By fine-tuning of the molecular energy level of copolymers, the photovoltaic performance was achieved up to 2.17% which is the highest value among the TQ-based solar cells. Along with the low-bandgap nature, high charge mobility of conjugated polymers is also important for improving the performance of polymer solar cells. In order to exhibit good charge transport property, the polymer chains are required to pack closely each other facilitating charge transport through intermolecular hopping. For the purpose of inducing denser packing of polymer chains, we have synthesized low-bandgap polymers with fine-tuned number of solubilizing side groups and low-bandgap polymers incorporated with planar component. In both cases, the molecular packing characteristics was greatly improved which resulted in enhanced charge carrier mobility. The power conversion efficiency was largely increased up to 3.8% when planar quinoxaline derivative was introduced in D-A type alternating copolymer.