Stacking states between (f) BDT unit of PBDB-T and DCI unit of o-F-ITIC and (g) BDD unit of PBDB-T and DCI unit of m-F-ITIC. a) Radial distribution function (RDF) of the backbone of PBDB-T and F-ITIC; (b) RDF of fluorine atoms in o-F-ITIC and m-F-ITIC. TA dynamics (a) probed at 730 nm recorded from films of pure o-F-ITIC and PBDB-T:o-F-ITIC blend, and (b) probed at 714 nm recorded from films of pure m-F-ITIC and blend. a) TA signal recorded from films of PBDB-T:F-ITIC blends excited at 500 nm.
Introduction
Organic semiconductors
With the development of planar structures of molecular designs and elongated conjugated backbones, many key factors have been important related to charge carrier mobility, such as intermolecular packing states, disorder of impurities, circumstance temperatures and so on. During the past decades, many pioneering research groups have paid attention to the design and development of new organic semiconductors for next-generation optoelectronic devices due to the aforementioned advantages of organic semiconductors.
Organic optoelectronic applications
- Organic photovoltaics (OPVs)
- Organic field-effect transistors (OFETs)
Without gate-source voltage (VGS = 0 V), none of the free charge carriers exist in the conduction channel, and the source-drain current will obviously be closed. VGS – Vth |, this is called the critical point, the free charge carriers are exhausted near the drain electrode and become pinched.
Design of organic semiconductors
- Aryl-aryl coupling reactions
- Molecular backbone modification
- Side-chain engineering
- Single atoms substitution
Based on the aforementioned role of side chains, the use of side chains is mandatory for the latest organic semiconductors. However, the docking position of side chains on the conjugated backbone is one of the considerations due to the attachment of side chains which consequently raises the steric hindrance and distorts the conjugated backbone.
Toolbox of characterization of organic semiconductors
- Material characterization
- Analysis of optoelectronic properties
- Observation of thin film morphology
Together with NMR and EA measurements, MS provides complementary data of the molecular structures with basic information about target molecules. In contrast to the microscopies introduced above, the X-ray diffraction is associated with the evaluation of the intermolecular distance rather than with thin film morphologies.
Chen, C.-P.; Chan, S.-H.; Chao, T.-C.; Ting, C.; Ko, B.-T., Low-bandgap poly(thiophene-phenylene-thiophene) derivatives with broad absorption spectrum for application in high-performance bulk heterojunction polymer solar cells. Li, C., Solution-processed and high-performance organic solar cells using small molecules with a benzodithiophene unit.
TBIG based backbone modification with random polymerization
- Introduction
- Results and discussion
- Conclusion
- Experimental section
- References
UV-vis absorption spectra of TBIG-based polymers (a) in dilute chloroform solution and (b) in films; (c) Molecular energy level diagrams of the materials in this work. Modulation of the charge transport characteristics occurred in accordance with the increased proportion of the IIG segment in the polymer backbone. These results are consistent with the correlation between energy. levels of the gold electrode and HOMO levels according to the change in the ratio of TBIG to IIG. a) Normalized bias stability and (b) air stability of TBIG-based polymer OFETs;.
To gain further insight into the effects on charge transport, we performed low-temperature measurements on all series OFETs of TBIG-based polymers. To investigate the molecular arrangement and crystalline properties of the TBIG-based polymers, atomic force microscope (AFM) and X-ray diffraction with 2D grazing incidence (2D-GIXD) were performed.
CPDT based polymer and small molecules with side-chain engineering
Improved in solubility and molecular assembly of cyclopentadithiophene-benzothiadiaozle
- Introduction
- Results and discussion
- Conclusion
- Experimental section
- References
Semi-Transparent Low-Donor Content Organic Solar Cells Employing Cyclopentadithiophene-
- Introduction
- Results and discussion
- Conclusion
- Experimental section
- References
Polymer-based organic solar cells (OSCs) have attracted significant attention in academia and industry due to their potential value in the global solar energy market.1-3 Recent developments that have combined narrow band gap donor-acceptor polymer donors with soluble acceptors in a bulk heterojunction architecture has delivered energy conversion efficiency (PCE) values of more than 10%.4-. The best OSC based on 2EH-CPDT(FBTTh2)2:PC71BM without additional post-processing showed more than twice higher average PCE value than that of the 5EN-CPDT(FBTTh2)2-based OSC. J; Kim, J.; Yang, C., Dithienogermole-containing small-molecule solar cells with an efficiency of 7.3%: in-depth investigation of the effects of heteroatom substitution of Si by Ge.
J.; Li, Y., High performance organic solar cells based on a small molecule with alkylthio-thienyl conjugated side chains without extra treatments. B.; Cao, Y., Difluorobenzothiadiazole-based small-molecule organic solar cells with 8.7% efficiency by tuning π-conjugated spacers and solvent vapor annealing.
Insight into the isomeric effect of fullerene-free acceptor via fluorine substitution
Introduction
It is well documented that the favorable donor-acceptor intermolecular interactions and/or arrangements present in the mixed active layer of OSCs can play a major role in exciton dissociation, charge separation and charge recombination processes.40-45 In thread with this consideration, herein we assume that subtle changes in the location of a single atom in the ITIC (popularly called constitutional isomerism effect) can strongly affect the intermolecular interactions and/or arrangements in the OSCs, which is one of the crucial factors that govern their efficient operation. Although considerable research effort has been devoted to modifying ITIC series, including modification of core units, terminal units, heteroatoms, and the bridges between the core and terminal units within ITIC-related backbones, 46-48 the constitutional isomerism effect is in the side chains. remains unexplored. In this study, we synthesized two new ITIC-based isomeric acceptors (o-F-ITIC and m-F-ITIC) with ortho- and meta-positions of fluorine atoms in the side chains to establish the structure–property relationship induced by constitutional isomerism and made a comparison. investigation of their photovoltaic properties.
A detailed simulation analysis demonstrates different donor-acceptor intermolecular complexes formed between the F-ITIC acceptors and PBDB-T donor, which not only supports our hypothesis but also provides a possible explanation for the significant difference between the OSCs based on the two isomers. The results obtained from a combination of theoretical and experimental studies here advance the understanding of the role of constitutional isomerism in intermolecular interactions, thereby facilitating a molecular design of non-fullerene acceptors.
Results and discussion
As shown in Figure 4.1b, cyclic voltammetry (CV) highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies show little change for o-F-ITIC eV) and m-F-ITIC eV. Photovoltaic parameters of devices based on PBDB-T:o-ITIC and PBDB-T:m-F-ITIC with optimal fabrication conditions.a. The insets show the structure of the fluorine atoms in o-F-ITIC and m-F-ITIC. a) Probability distribution of backbone dihedral angles of F-ITIC.
Electron density distributions of the HOMO and LUMO for optimized (b) PBDB-T:o-F-ITIC and (c) PBDB-T:m-F-ITIC. In contrast, some m-F-ITIC aggregates appear as dark spots in the TEM image of the m-F-ITIC-based blend.
Conclusion
Note that PBDB-T:o-F-ITIC exhibits a higher intensity ESA signal than that of PBDB-T:m-F-ITIC, which is consistent with the trend of the bleaching signals induced by the mixtures excited at 710 nm. Furthermore, both blends showed a shorter bi-exponential lifetime of the ESA signals (approximately 1200 nm ps) than that of the pure PBDB-T donor film (1.52 ps) (Figure 4.18b), which is again similar to the above . TA data showing the excitation at 710 nm. However, a very small difference between the lifetimes of two mixtures makes it difficult for us to conclude that the higher PCE of PBDB-T:o-F-ITIC comes from this process alone from our fitted electron transfer rate results.
Therefore, we further analyzed the polaron lifetimes of both blend films from the polaron ESA band, examined at 930 nm, using the nanosecond-resolved TA measurement (Figure 4.18c). Here, compared to the m-F-ITIC based sample (10.95 ns and 214.68 ns), the o-F-ITIC-based mixture clearly showed a longer polaron lifetime (20.88 ns and 257.23 ns), which increased implied polaron generation in this device due to the reduced recombination mechanism, which may explain the reason why the EQE curve of PBDB-T:o-F-ITIC exceeds that of PBDB-T:m-F-ITIC around 500 nm.
Experimental section
The resulting crude mixture was purified by column chromatography using hexane as eluent to give a colorless oil (2.0 g, 69%). A solution of compound 2 (1.6 g, 3.2 mmol) in THF (40 mL) was added and then the reaction mixture was warmed to room temperature and stirred overnight. The resulting mixture was stirred at 85°C overnight, the reaction was quenched with water and then extracted with chloroform.
The resulting crude mixture was purified by column chromatography using hexane/ethyl acetate as eluent to give a yellow solid (0.62 g, 59%). ITIC.51 Coarse-grained mappings of PBDB-T and F-ITIC beads were performed to produce a similar atomic model structure (Figure 4.8).
R.; McCulloch, I., Highly efficient and air-stable polymer solar cells based on P3HT with a novel non-fullerene acceptor. Cui, Y.; Yang, C.; Yao, H.; Zhu, J.; Wang, Y.; Jia, G.; Gao, F.; Hou, J., Efficient color-tunable semi-transparent organic solar cells enabled by a non-fullerene ultra-low band gap receiver. A.; Huang, F.; Cao, Y., Nonfullerene acceptor with high dielectric constant for efficient bulk heterojunction organic solar cells.
G.; Wu, K.; Qiu, B.; Yang, C.; Li, Y.; Yang, C., Side-chain impact on molecular orientation of organic semiconductor acceptors: high performance non-fullerene polymer solar cells with thick active layer over 400 nm. Li, C.; Liu, F.; Zhang, H.; Chen, Y., An ADA-type small molecule electron acceptor with end-extended conjugation for high-performance organic solar cells.
Acknowledgements
우선, 이 글을 쓰는 동안에도 박사과정 내내 밤낮없이 많은 가르침과 지도를 해주신 지도교수님 양창덕 교수님께 감사의 말씀을 전하고 싶습니다. 다음으로, 제가 졸업기간을 무사히 마칠 수 있도록 가장 가까운 곳에서 도움과 지지를 보내주신 우리 연구실 구성원들에게 감사의 말씀을 전하고 싶습니다. 박사님께도 감사의 말씀을 전하고 싶습니다. Shanshan Chen 박사 낯선 나라에 살면서도 학위 과정 내내 저를 지지해주고 아낌없는 노력을 아끼지 않은 타냐 쿠마리.
많은 조언과 학문적 지도를 해주신 김봉수 교수님께 감사드리며, 많은 것을 배웠다고 전하고 싶습니다. 마지막으로, 졸업하는 동안 저를 지지해준 가족과 친구들에게 감사 인사를 전하고 싶습니다.
