Browsing by Author "Tajima, Keisuke"

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Effects of end- on oriented polymer chains at the donor/ acceptor interface in organic solar cells
(2018-12-07)
Wang, Fanji
;
Nakano, Kyohei
;
Yoshida, Hiroyuki
;
Hashimoto, Kazuhito
;
Segawa, Hiroshi
Molecular alignment at the interface between the electron donor and acceptor (D/A) materials in thin films may have a large effect on the charge generation process in organic solar cells (OSCs). In this study, the effects of the polymer chain orientations at the D/A interface were investigated in OSCs with planar heterojunction (PHJ) structures. Poly(3-alkylthiophene) films with edge-on (main chain parallel to the interface) and end-on (main chain vertical to the interface) orientations at the surface were used to construct PHJs with a fullerene derivative. Ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy revealed that the difference in polarization energy through the charge-permanent quadrupole interaction causes a rigid energy shift of 0.5eV between the two orientations. The PHJ device with the end-on oriented polymer chains at the D/A interface showed a smaller photovoltage loss expected from the energy structure and a more efficient charge separation than the device with the edge-on oriented polymer chains. These improvements were attributed to charge delocalization along the end-on oriented polymer chains normal to the D/A interface, through which the effective binding energy of the charge pairs in the interfacial charge-transfer state was reduced. These results provide important information about molecular arrangements at the D/A interface that may enable further improvements in OSC performance.
Face-on reorientation of pi-conjugated polymers in thin films by surface-segregated monolayers
(2020-04-07)
Wang, Wei-Chih
;
Chen, Sheng-Yuan
;
Yang, Yaw-Wen
;
Hsu, Chain-Shu
;
Tajima, Keisuke
A new pi-conjugated polymer based on naphthalenediimide with a branched semifluoroalkyl side chain (FNDIT2) was synthesized and used to change the polymer orientations in thin films. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy revealed that FNDIT2 spontaneously forms surface-segregated monolayers in blend films with other pi-conjugated polymers to adopt a preferred face-on orientation, with the pi-plane of the polymer aligned more parallel to the surface. Thermal annealing of the films induced further reorientation of the polymers inside the films closer to the face-on orientation, as shown by two-dimensional grazing-incidence wide-angle X-ray scattering. The induced face-on orientations in the films led to higher charge mobility in the vertical direction. This study provides a general and versatile strategy to control the molecular orientation in thin films and enhance the performance of thin-film organic electronics.
Haptacyclic Carbazole-Based Ladder-Type Nonfullerene Acceptor with Side-Chain Optimization for Efficient Organic Photovoltaics
(2017-12-06)
Hsiao, Yu-Tang
;
Li, Chia-Hua
;
Chang, Shao-Ling
;
Heo, Soowon
;
Tajima, Keisuke
In this research, a haptacyclic carbazole-based dithienocyclopentacarbazole (DTCC) ladder-type structure was formylated to couple with two 1,1-dicyanomethylene-3-indanone (IC) moieties, forming a new nonfullerene acceptor DTCCIC-C17 using a bulky branched 1-octylnonayl side chain at the nitrogen of the embedded carbazole and four 4-octylphenyl groups at the sp(3)-carbon bridges. The rigid and coplanar main-chain backbone of the DTCC core provides a broad light-absorbing window and a higher-lying LUMO energy level, whereas the bulky flanked side chains reduce intermolecular interactions, making DTCCIC-C17 amorphous with excellent solution processability. The DTCCIC-C17 as an acceptor is combined with a medium band gap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-y1)-benzo[1,2-b:4,5-bldithiophene))-alt-(5,5-(1',3'-di-2-thieny1-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) as the donor in the active layer to obtain suitable highest occupied molecular orbital/lowest unoccupied molecular orbital energy alignments and complimentary absorption. The devices with an inverted configuration (ITO/ZnO/active layer/MoO3/Ag) without using an aqueous poly(3,4-ethylenedioxythiophene) polystyrene sulfonate layer were fabricated for better device stability. When the diiodooctane-treated PBDB-T:DTCCIC-C17 active layer was thermally annealed at 50 degrees C for 10 min, the device achieved the highest efficiency of 9.48% with a high V-oc of 0.98 V, a J(sc) of 14.27 mA cm(-2), and an FF of 0.68.
Porphyrin-Containing Polymer as a Superior Blue Light-Absorbing Additive To Afford High-J(sc) Ternary Solar Cells
(2019-01-09)
Li, Chia-Hua
;
Chang, Chia-Chih
;
Hsiao, Yu-Hsuan
;
Peng, Shih-Hao
;
Su, Yi-Jia
Integrating an additional component featuring complementary light absorption into binary polymer solar cells is a superior tactic to ameliorate solar cell efficiency and stability. An appropriate additive not only extends the absorption range but may also facilitate charge separation and transport processes. In this work, we elucidate the effects of incorporating a porphyrin-containing conjugated polymer (PPor-1), which displays absorption in 350-500 nm, into binary PTB7-Th:4TIC and PTB7-Th:ITIC blends, affording devices with an average power conversion efficiency approaching 9%. We successfully demonstrate that PPor-1 can be incorporated as an additive to impart improved J(sc) (up to 19.1 mA cm(-2)).