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

다국어 초록 (Multilingual Abstract)

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.

국문 초록 (Abstract)

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.

목차 (Table of Contents)

• Chapter 1. Introduction ........................................................ 1
• 1.1 Organic Photovoltaics ............................................................. 1
• 1.1.1 Background ........................................................................... 1
• 1.1.2 Device architecture ............................................................... 3
• 1.1.3 Operating principle ............................................................... 5
• 1.2 Photoactive materials for organic solar cells .......................... 14
• 1.2.1 Electron donors ................................................................... 14
• 1.2.1.1 Polymer donors ........................................................ 14
• 1.2.1.2 Small-molecule donors ............................................ 17
• 1.2.2 Electron acceptors ............................................................... 20
• 1.3 Morphology control in the bulk-heterojunction active layer 22
• 1.3.1 Effect of processing solvent ................................................ 23
• 1.3.2 Solvent additive .................................................................. 25
• 1.3.3 Thermal annealing .............................................................. 28
• 1.3.4 Solvent vapor annealing ...................................................... 31
• 1.4 Stability ................................................................................. 32
• Chapter 2. Experimental Section ....................................... 36
• 2.1 Synthesis and characterization .............................................. 36
• 2.1.1 Materials ............................................................................. 36
• 2.1.2 Synthesis of pDPPTTi-OD.................................................. 37
• 2.1.3 Characterization methods .................................................... 40
• 2.2 Device fabrication and measurements ..................................... 41
• 2.2.1 Materials ............................................................................. 41
• 2.2.2 Solar cell device fabrication ................................................ 42
• 2.2.3 Solar cell performance measurements ................................ 43
• Chapter 3. Processing temperature control of a diketopyrolo
• pyrrole-alt-thieno [2,3-b]thiophene
• polymer for organic solar cells ...................... 45
• 3.1 Synthesis and characterization .................................................. 45
• 3.2 Optical and electrochemical properties ...................................... 49
• 3.3 Computational simulation ........................................................ 54
• 3.4 Photovoltaic properties ............................................................ 55
• 3.5 Molecular orientation ............................................................... 60
• 3.6 Morphologies of active layers ................................................... 64
• 3.7 Summary ................................................................................ 68
• Chapter 4. Effect of asymmetric solubility in a binary
• solvent system on the performance of bulk
• heterojunction solar cells ............................... 69
• 4.1 Chemical structure and characterization .................................... 69
• 4.2 Photovoltaic properties ............................................................ 71
• 4.3 Morphologies of active layers ................................................... 77
• 4.4 Solubility of materials .............................................................. 83
• 4.5 Mobility measurement ............................................................. 89
• 4.6 Summary ................................................................................ 92
• Chapter 5. Synergistic effects of solvent and polymer
• additives on photovoltaic performance and stability of small molecule bulk
• heterojunction solar cells ............................... 93
• 5.1 Chemical structures and device architecture .............................. 93
• 5.2 Photovoltaic properties ............................................................ 96
• 5.3 Morphologies of active layers ................................................. 102
• 5.4 Molecular orientation ............................................................. 107
• 5.5 Mobility measurement ........................................................... 115
• 5.6 Comparison of properties with other polymers ......................... 118
• 5.7 Thermal stability ................................................................... 123
• 5.8 Summary .............................................................................. 125
• Chapter 6. Conclusions & Future directions .................. 126
• Bibliography ...................................................................... 129
• Korean Abstract ................................................................ 141