Browsing by Author "Huang, Po-Kai"

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Demonstration of Annealing-free Metal-Insulator-Semiconductor (MIS) Ohmic Contacts on a GaN Substrate using Low Work-function Metal Ytterbium (Yb) and Al2O3 Interfacial Layer
(2019-01-01)
Wu, Tian-Li
;
Tseng, Yang-Yan
;
Huang, Chih-Fang
;
Chen, Zih-Sin
;
Lin, Chih-Chien
In this letter, the Metal-Insulator-Semiconductor (MIS) contacts are fabricated on the GaN substrates and compared to the Metal-Semiconductor (MS) contacts. To alleviate the Fermi level pinning effect at the Metal-Semiconductor interface, an Al2O3 interfacial layer (IL) (1 similar to 3 nm) is used to form the MIS contacts. By comparing the electrical characteristics, the contact with Onm Al2O3 layer, i.e., MS contact, shows a lowest current compared to the contacts with an Al2O3 IL, i.e., MIS contact. Furthermore, the current at 0.5V in the fabricated Transmission Line Measurement (TLM) device with lnm Al2O3 IL is 10(3) times higher the one without an Al2O3 IL. This proves that inserting an Al2O3 IL could mitigate the Fermi level pinning, resulting in a low Schottky barrier height. Finally, based on the TLM analysis, the resistivity of 5.1x10(-4) Omega.cm(2) is demonstrated in the MIS contact with lnm Al2O3 IL.
Forced coplanarity of dithienofluorene-based non-fullerene acceptors to achieve high-efficiency organic solar cells
(2019-08-14)
Cao, Fong-Yi
;
Huang, Po-Kai
;
Su, Yen-Chen
;
Huang, Wen-Chia
;
Chang, Shao-Ling
Ladder-type structures have been the key element for high performance non-fullerene acceptors (NFAs). In this work, two dithieno[3,2-b:6,7-b ']fluorine (DTF)-based nonfullerene acceptors, non-bridged DTFT5-FIC and carbon-bridged DTFT9-FIC, with an identical conjugated backbone were designed and synthesized for investigation of the planarization effect. The chemical rigidification with forced coplanarity indeed plays a pivotal role in determining the molecular properties. Compared to the non-bridged open-chain DTFT5-FIC counterpart, the nonacyclic ladder-type DTFT9-FIC with a fully planarized backbone exhibits higher thermal stability, bathochromic absorption and higher electron mobility. The devices using PBDB-T as the p-type polymer and fully planar DTFT9-FIC as the acceptor showed a high efficiency of 9.58%, which is superior to that of the device using the partially planar DTFT5-FIC material (5.45%). By incorporating PC71BM as the second acceptor to enhance the absorption at shorter wavelengths, optimize the morphology and facilitate electron transport, the ternary-blend device using PBDB-T:DTFT9-FIC:PC71BM achieved the highest PCE of 11.82% with a V-oc of 0.88 V, a higher J(sc) of 20.59 mA cm(-2), and an FF of 65.27%.
High density GaN/AlN quantum dots for deep UV LED with high quantum efficiency and temperature stability
(2014-06-05)
Yang, Weihuang
;
Li, Jinchai
;
Zhang, Yong
;
Huang, Po-Kai
;
Lu, Tien-Chang
High internal efficiency and high temperature stability ultraviolet (UV) light-emitting diodes (LEDs) at 308 nm were achieved using high density (2.5 x 10(9) cm(-2)) GaN/AlN quantum dots (QDs) grown by MOVPE. Photoluminescence shows the characteristic behaviors of QDs: nearly constant linewidth and emission energy, and linear dependence of the intensity with varying excitation power. More significantly, the radiative recombination was found to dominant from 15 to 300 K, with a high internal quantum efficiency of 62% even at room temperature.
Highly Efficient Inverted D:A(1):A(2) Ternary Blend Organic Photovoltaics Combining a Ladder-type Non-Fullerene Acceptor and a Fullerene Acceptor
(2017-07-26)
Chang, Shao-Ling
;
Cao, Fong-Yi
;
Huang, Wen-Chia
;
Huang, Po-Kai
;
Hsu, Chain-Shu
A formylated benzodi(cyclopentadithiophene) (BD CPDT) ladder-type structure with forced coplanarity is coupled with two 1,1-dicyanomethylene-3-indanone (IC) moieties via olefination to form a non-fullerene acceptor, BDCPDT-IC. The BDCPDT-IC, as an acceptor (A(1)) with broad light-absorbing ability and excellent solution processability, is combined with a second PC71BM acceptor (A(2)) and a medium band gap polymer, PBDB-T, as the donor (D) to form a ternary blend with gradient HOMO/LUMO energy alignments and panchromatic absorption. The device with the inverted architecture using the D:A(1):A(2) ternary blend has achieved a highest efficiency of 9.79% with a superior J(sc) of 16.84 mA cm(-2).
Isomeric effect of fluorene-based fused-ring electron acceptors to achieve high-efficiency organic solar cells
(2020-03-14)
Xue, Yung-Jing
;
Cao, Fong-Yi
;
Huang, Po-Kai
;
Su, Yen-Chen
;
Cheng, Yen-Ju
Acceptor-donor-acceptor (A-D-A) non-fullerene electron acceptors (NFEAs) using ladder-type donor structures have become the dominant n-type materials for achieving high-efficiency OSCs. In this work, two isomeric fluorene-based ladder-type structures FCTT (TT-C-F-C-TT) and FTCT (T-C-TFT-C-T) have been designed and synthesized. These two isomeric donors with the different fused-ring arrangement, molecular geometry, and side-chain placement were end-capped with the FIC acceptors to form two NFEAs FCTT-FIC and FTCT-FIC isomeric materials. Compared to FTCT-FIC using the thiophene (T)-terminal donor, FCTT-FIC with the thienothiophene (TT)-terminal donor has more evenly distributed side chains on both sides of the backbone and less steric hindrance near the FIC acceptors, which enables stronger antiparallel pi-pi packing among the end-groups to create a channel for efficient electron transport, as evidenced by the thin-film GIWAXS measurements. FCTT-FIC displayed a larger optical bandgap and deeper-lying energy levels than its FTCT-FIC isomer. Compared to the PBDB-T:FTCT-FIC device, the PBDB-T:FCTT-FIC device showed a higher PCE of 10.32% with an enhanced J(sc) of 19.63 mA cm(-2) and an FF of 69.14%. A PM6:FCTT-FIC device using PM6 as a p-type polymer achieved the highest PCE of 12.23%. By introducing PC71BM as the second acceptor to enhance the absorption at shorter wavelengths, optimize the morphology and facilitate electron transport, the ternary-blend PM6:FCTT-FIC:PC71BM (1 : 1 : 0.5 in wt%) device yielded the highest PCE of 13.37% with a V-oc of 0.92 V, a higher J(sc) of 19.86 mA cm(-2), and an FF of 73.2%. This result demonstrated that the TT-terminal ladder-type donor is generally a better molecular design than the corresponding T-terminal ladder-type isomer for the development of new A-D-A NFEAs.
New Thieno[3,2-b]thiophene-Based Acceptor: Tuning Acceptor Strength of Ladder-Type N-Type Materials to Simultaneously Achieve Enhanced V-OC and J(SC) of Nonfullerene Solar Cells
(2018-07-01)
Chang, Shao-Ling
;
Cao, Fong-Yi
;
Huang, Wen-Chia
;
Huang, Po-Kai
;
Huang, Kuo-Hsiu
A new thieno[3,2-b]thiophene-incorporated acceptor TTC has been developed. The TTC acceptor was installed in a haptacyclic ladder-type core (BDCPDT) to furnish an n-type BDCPDT-TTC. The standard PBDB-T:BDCPDT-IC device showed a PCE of 9.33% with a V-OC of 0.86 V and a J(SC) of 16.56 mA/cm(2). By molecular engineering of the acceptor unit, the BDCPDT:PBDB-T-based device delivered an enhanced efficiency of 10.29% with a simultaneously enhanced V-OC of 0.94 V and J(SC) of 17.72 mA/cm(2). Incorporation of the electron-donating thieno[3,2-b]thiophene unit into the acceptor moiety decreases the electron accepting strength, thereby upshifting the HOMO/LUMO energy levels to decrease the Delta E-HOMO and E-LOSS achieving a larger Voc. Second, the extended conjugated bicyclic thieno[3,2-b]thiophene ring beneficially induces an additional optical transition at short wavelengths, leading to improvement of J. Alternatively, BDCPDT-FIC installed with the fluorinated acceptor shows more red-shifted absorption to achieve a high J of 19.12 mA/cm(2).
Side-chain modulation of dithienofluorene-based copolymers to achieve high field-effect mobilities
(2017-04-01)
Lee, Chia-Hao
;
Lai, Yu-Ying
;
Hsu, Jhih-Yang
;
Huang, Po-Kai
;
Cheng, Yen-Ju
A ladder-type dithieno[3,2-b: 6,7-b'] fluorene (DTF), where the central fluorene is fused with two outer thiophene rings at its 2,3- and 6,7-junctions, is developed. The pentacyclic DTF monomers were polymerized with dithienodiketopyrrolopyrrole (DPP) acceptors to afford three alternating donor-acceptor copolymers PDTFDPP16, PDTFDPP20, and PDTFDPP32 incorporating different aliphatic side chains (R-1 group at DTF; R-2 group at the DPP moieties). The side-chain variations in the polymers play a significant role in determining not only the intrinsic molecular properties but also the intermolecular packing. As evidenced by the 2-dimensional GIXS measurements, PDTFDPP16 with octyl (R-1) and 2-ethylhexyl (R-2) side chains tends to align in an edge-on pi-stacking orientation, whereas PDTFDPP20 using 2-butyloctyl (R-1) and 2-ethylhexyl (R-2) adopts a predominately face-on orientation. PDTFDPP32 with the bulkiest 2-butyloctyl (R-1) and 2-octyldodecyl (R-2) side chains shows a less ordered amorphous character. The OFET device using PDTFDPP20 with a face-on orientation determined by GIXS measurements achieved a high hole-mobility of up to 5 cm(2) V-1 s(-1). The high rigidity and coplanarity of the DTF motifs play an important role in facilitating intramolecular 1-dimensional charge transport within the polymer backbones. The implementation of main-chain coplanarity and side-chain engineering strategies in this research provides in-depth insights into structure-property relationships for guiding development of high-mobility OFET polymers.