Junwu Chen

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Organization: South China University of Technology

Department: Institute of Polymer Optoelectronic Materials & Devices

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Polymer

Polymer Solar Cells

Organic Chemistry

Organic Solar Cells

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References

A Highly Crystalline Wide-Band-Gap Conjugated Polymer toward High-Performance As-Cast Nonfullerene Polymer Solar Cells

Co-reporter:Haiying Jiang, Zhen Wang, Lianjie Zhang, Anxing Zhong, Xuncheng Liu, Feilong Pan, Wanzhu Cai, Olle Inganäs, Yi Liu, Junwu Chen, and Yong Cao

ACS Applied Materials & Interfaces October 18, 2017 Volume 9(Issue 41) pp:36061-36061

Publication Date(Web):September 25, 2017

DOI:10.1021/acsami.7b10059

A new wide-band-gap conjugated polymer PBODT was successfully synthesized that showed high crystallinity and was utilized as the active material in nonfullerene bulk-heterojunction polymer solar cells (PSCs). The photovoltaic devices based on the as-cast blend films of PBODT with ITIC and IDIC acceptors showed notable power conversion efficiencies (PCEs) of 7.06% and 9.09%, with high open-circuit voltages of 1.00 and 0.93 V that correspond to low energy losses of 0.59 and 0.69 eV, respectively. In the case of PBODT:ITIC, lower exciton quenching efficiency and monomolecular recombination are found for devices with small driving force. On the other hand, the relatively higher driving force and suppressed monomolecular recombination for PBODT:IDIC devices are identified to be the reason for their higher short-circuit current density (Jsc) and higher PCEs. In addition, when processed with the nonchlorinated solvent 1,2,4-trimethylbenzene, a good PCE of 8.19% was still achieved for the IDIC-based device. Our work shows that such wide-band-gap polymers have great potential for the environmentally friendly fabrication of highly efficient PSCs.Keywords: bulk-heterojunction polymer solar cells; charge-transfer state; crystallinity; energy loss; nonfullerene acceptor; wide-band-gap polymer;

Siloxane-Terminated Side Chain Engineering of Acceptor Polymers Leading to Over 7% Power Conversion Efficiencies in All-Polymer Solar Cells

Co-reporter:Shizhen Feng, Chang Liu, Xiaofeng Xu, Xuncheng Liu, Lianjie Zhang, Yaowen Nian, Yong Cao, and Junwu Chen

ACS Macro Letters November 21, 2017 Volume 6(Issue 11) pp:1310-1310

Publication Date(Web):November 9, 2017

DOI:10.1021/acsmacrolett.7b00738

To investigate the influence of functional pendent groups on acceptor polymers and photovoltaic properties of all-polymer solar cells (all-PSCs), two novel acceptor polymers containing siloxane-terminated side chains are synthesized and characterized. Increasing the content of siloxane-terminated side chains can reduce π–π stacking distance and improve crystalline behavior, yet lead to poorer solubility of the acceptor polymers. By modulating the proper loadings of siloxane-terminated side chains on the acceptor polymers, the PBDB-T:PNDI-Si25 all-PSC attains a maximal power conversion efficiency (PCE) of 7.4% with an outstanding fill factor of 0.68. The results provide new insights for developing high-performance all-PSCs through functional group engineering on the acceptor polymers, to achieve good solubility, polymer miscibility, and blend morphology.

An extended π-conjugated area of electron-donating units in D–A structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells

Co-reporter:Ping Cai;Zhenhui Chen;Lianjie Zhang;Yong Cao

Journal of Materials Chemistry C 2017 vol. 5(Issue 11) pp:2786-2793

Publication Date(Web):2017/03/16

DOI:10.1039/C7TC00428A

Two D–A conjugated polymers, FBT-DThDT-1T and FBT-DThDT-TT, using 5,6-difluoro-2,1,3-thiadiazole (FBT) as the electron-accepting unit, and terthiophene or 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene as the electron-donating unit, respectively, were synthesized. Among them, the first batch of FBT-DThDT-TT with relatively low molecular weight (MW) can be denoted as FBT-DThDT-TT-L and the second batch of FBT-DThDT-TT with much higher MW can be denoted as FBT-DThDT-TT-H. FBT-DThDT-1T possesses a low FET hole mobility of 2.6 × 10−3 cm2 (V s)−1 and a poor power conversion efficiency (PCE) of 0.91% in inverted polymer solar cells (i-PSCs) under the illumination of AM1.5G, 100 mW cm−2 light. Compared with FBT-DThDT-1T, FBT-DThDT-TT with extended π-conjugation bears a TT replacing the middle thiophene of terthiophene on the backbone, which would increase the coplanarity of the polymer and thus facilitate both intermolecular packing and charge transport. FBT-DThDT-TT shows strong interchain aggregation in a room temperature solution, its absorption spectra in a room temperature solution and in a thin film were almost identical. The field-effect transistors based on FBT-DThDT-TT-L and FBT-DThDT-TT-H show improved hole mobilities of 0.38 and 0.20 cm2 (V s)−1, respectively. The i-PSCs based on FBT-DThDT-TT-L show a better PCE of 3.47%, and the i-PSCs based on FBT-DThDT-TT-H with a higher MW exhibit the best PCE up to 7.78%, with highly improved absorption capacity and miscibility with PC71BM. Moreover, with a 355 nm thick active layer, a PCE of 6.72% with a high FF of 67.8% is still obtained for FBT-DThDT-TT-H-based devices. The impressive results make FBT-DThDT-TT a promising candidate for applications of large-scale solution-processable PSCs.

An extended π-conjugated area of electron-donating units in D–A structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells

Co-reporter:Ping Cai;Zhenhui Chen;Lianjie Zhang;Yong Cao

Journal of Materials Chemistry C 2017 vol. 5(Issue 11) pp:2786-2793

Publication Date(Web):2017/03/16

DOI:10.1039/C7TC00428A

Low band gap conjugated polymers combining siloxane-terminated side chains and alkyl side chains: side-chain engineering achieving a large active layer processing window for PCE > 10% in polymer solar cells

Co-reporter:Xuncheng Liu;Li Nian;Ke Gao;Lianjie Zhang;Lechi Qing;Zhen Wang;Lei Ying;Zengqi Xie;Yuguang Ma;Yong Cao;Feng Liu

Journal of Materials Chemistry A 2017 vol. 5(Issue 33) pp:17619-17631

Publication Date(Web):2017/08/22

DOI:10.1039/C7TA05583H

Alternating and random conjugated copolymers with a siloxane-terminated side chain for a repeating unit based on 5,6-difluoro[2,1,3]benzothiadiazole (FBT) and quarterthiophene (4T) were synthesized, among which side-chain random copolymers PFBT4T-C5Si-50% and PFBT4T-C5Si-25% with low contents of 50% and 25% siloxane-terminated side chains, respectively, in conjunction with alkyl side chains were found to be more suitable for optoelectronic applications due to good film-forming in solution processing. Grazing incidence X-ray diffraction (GIXD) indicated that the siloxane-terminated side chain could induce PFBT4T-C5Si-50% and PFBT4T-C5Si-25% with face-on orientations, giving high 3-D hole transport in neat films as supported by a high hole mobility up to 2.46 cm2 V−1 s−1 in field-effect transistors and an SCLC hole mobility up to 5.9 × 10−2 cm2 V−1 s−1 in hole-only devices. Fast SCLC hole and electron transports were seen for their bulk-heterojunction (BHJ) blend films with PC71BM as the acceptor, due to the retention of a polymer face-on orientation. The BHJ blend film of PFBT4T-C5Si-25% showed lower film surface roughness, more balanced hole and electron transport, and relatively smaller phase separation when compared with PFBT4T-C5Si-50%, as evidenced by atomic force microscopy (AFM), transmission electron microscopy (TEM), SCLC, and resonant soft X-ray scattering (RSoXS) measurements. The PFBT4T-C5Si-25%-based PSCs with 270, 420, and 600 nm thick active layers exhibited outstanding power conversion efficiencies (PCEs) of 10.39%, 11.09%, and 10.15%, respectively, readily offering a high thickness tolerance to achieve an unprecedented wide active layer processing window for PCE > 10%. This is also the first PCE of more than 10% achieved by an active layer of a 600 nm thickness level in PSCs. Another notable feature is very high fill factors of more than 74% and 71% being achieved for very thick active layers of 420 and 600 nm, respectively. The results suggest that side-chain engineering through the incorporation of a partial siloxane-terminated side chain is a unique handle to afford new photovoltaic polymers with enhanced vertical carrier transport towards application in roll-to-roll processing of PSCs.

Effects of including electron-withdrawing atoms on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene-based donor–acceptor polymers: towards an acceptor design for efficient polymer solar cells

Co-reporter:Ping Cai;Xiaofeng Xu;Jiangman Sun;Yong Cao

RSC Advances (2011-Present) 2017 vol. 7(Issue 33) pp:20440-20450

Publication Date(Web):2017/04/05

DOI:10.1039/C7RA01049D

Three new D–A polymers PIDTT-DTBO, PIDTT-DTBT and PIDTT-DTFBT, using indacenodithieno[3,2-b]thiophene (IDTT) as the electron-rich unit and benzoxadiazole (BO), benzodiathiazole (BT) or difluorobenzothiadiazole (FBT) as the electron-deficient unit, were synthesized via a Pd-catalyzed Stille polymerization. The included electron-withdrawing atoms of the acceptor portion were varied between O, S, and F for tailoring the optical and electrochemical properties and the geometry of structures. Their effects on the film topography, photovoltaic and hole-transporting properties of the polymers were thoroughly investigated via a range of techniques. As expected, the stronger electron-withdrawing BO unit affords red-shifted absorption, low-lying HOMO and LUMO levels for the polymer PIDTT-DTBO. However, it depicts lower hole mobility and a less efficient charge collection in the active layer compared to the polymer PIDTT-DTBT. In addition, degradation of the solubility is observed in the fluorinated polymer PIDTT-DTFBT. As a result, a BHJ PSC (ITO/PEDOT:PSS/polymer:PC71BM/interlayer/Al) fabricated with PIDTT-DTBT attains the best power conversion efficiency (PCE) of 4.91%. These results thus demonstrate the potential effects of electronegative atoms on IDTT-based polymers and the structure–function correlations of such electron-donor materials for efficient PSCs.

26 mA cm−2 Jsc from organic solar cells with a low-bandgap nonfullerene acceptor

Co-reporter:Zuo Xiao, Xue Jia, Dan Li, Shizhe Wang, ... Liming Ding

Science Bulletin 2017 Volume 62, Issue 22(Volume 62, Issue 22) pp:

Publication Date(Web):30 November 2017

DOI:10.1016/j.scib.2017.10.017

Alternating dithienobenzoxadiazole-based conjugated polymers for field-effect transistors and polymer solar cells

Co-reporter:Haitao Wang, Yongxiang Zhu, Zhulin Liu, Lianjie Zhang, Junwu Chen, Yong Cao

Organic Electronics 2016 Volume 31() pp:1-10

Publication Date(Web):April 2016

DOI:10.1016/j.orgel.2015.12.030

•Alternating conjugated polymers based on dithienobenzoxadiazole (DTfBO).•High hole mobility up to 0.54 cm2/(V s).•Good power conversion efficiency up to 5.63% for 250 nm thick active layer.Three new alternating copolymers derived from dithienobenzoxadizole (DTfBO) and different thiophene-based π-spacers, including terthiophene, quarterthiophene, and dithienyl flanked thienothiophene, were successfully synthesized. The DTfBO-based polymers possess optical band-gaps in the range of 1.84–1.89 eV and exhibit relatively deep HOMO levels between −5.36 eV and −5.50 eV. Due to strong interchain aggregation, DTfBO-based polymers could not be well dissolved in chlorobenzene at room temperature, but they could be processed with hot chlorobenzene solutions of ∼100 °C. Evolutions of UV absorption spectra of polymer solutions during heating process could differentiate their different aggregation ability, among which a repeating unit based on a DTfBO and a terthiophene could supply the strongest inter-chain interaction. Notably, the three DTfBO-based polymers displayed high field-effect hole mobilities between 0.21 and 0.54 cm2/(V s). In polymer solar cells (PSCs) with the three polymers as the donors, high open-circuit voltages between 0.87 and 0.93 V could be realized. For active layer thickness of 80 nm, the PSCs displayed power conversion efficiency (PCE) between 2.85% and 5.07%. A very high fill factor of 75.4% was achieved for the polymer comprising dithienyl flanked thienothiophene. With thicker ative layers of 250 nm, the three DTfBO-based polymers exhibited comparable PCEs of ∼5.61% due to obviously increased short-circuit currents. Our results suggest that DTfBO, a big coplanar heterocycle, is a promising building block to construct high mobility conjugated polymers for efficient thick-film PSCs.

A dual ternary system for highly efficient ITO-free inverted polymer solar cells

Co-reporter:Zuosheng Peng, Yuxin Xia, Feng Gao, Kang Xiong, Zhanhao Hu, David Ian James, Junwu Chen, Ergang Wang and Lintao Hou

Journal of Materials Chemistry A 2015 vol. 3(Issue 36) pp:18365-18371

Publication Date(Web):23 Jul 2015

DOI:10.1039/C5TA03831F

In this study, it has been found that a very fine nanostructure can be realized by mixing 1-chloronaphthalene (CN) – a high-boiling solvent – into a binary chlorobenzene (CB):1,8-diiodooctane (DIO) solvent mixture to form a ternary solvent system. An improvement in energy level alignment is also obtained by doping ICBA into a binary PTB7:PCBM[70] blend, whereby the ternary solute system provides a new pathway for charge transfer from PTB7 to the PCBM[70]:ICBA alloy. This is confirmed by imaging the surface morphology of the active layer using AFM and TEM, monitoring the transient film formation process and measuring the charge transfer states with Fourier transform photocurrent spectroscopy. An encouraging PCE of 7.65% is achieved from the dual ternary system, which is the highest value ever reported for an ITO-free inverted polymer solar cell with a PEDOT:PSS layer as the top semitransparent electrode – a system which is compatible with low-cost large-area roll-to-roll manufacturing.

Organic/Organic Cathode Bi-Interlayers Based on a Water-Soluble Nonconjugated Polymer and an Alcohol-Soluble Conjugated Polymer for High Efficiency Inverted Polymer Solar Cells

Co-reporter:Ping Cai, Hongfu Jia, Junwu Chen, and Yong Cao

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 50) pp:27871

Publication Date(Web):November 30, 2015

DOI:10.1021/acsami.5b09744

In this work, organic/organic cathode bi-interlayers based on a water-soluble nonconjugated polymer PDMC and an alcohol-soluble conjugated polymer PFN were introduced to modifythe ITO cathode for inverted polymer solar cells (PSCs). PDMC with ultrahigh molecular weight would facilitate to form strong adsorption on the ITO substrate, while PFN could provide both compatibly interfacial contacts with the bottom PDMC interlayer and the upper organic active layer. The PDMC/PFN cathode bi-interlayers could decrease work function of the ITO cathode to 3.8 eV, supplying the most efficient ohmic interfacial contacts for electron collection at the ITO cathode. With a PTB7:PC71BM blend as the active layer, inverted PSCs based on the PDMC/PFN cathode bi-interlayers showed the highest efficiency of 9.01% and the best air stability within 60 days if compared with devices based on a separate PDMC or PFN cathode interlayer. The results suggest that the PDMC/PFN cathode bi-interlayers would play an important role to achieve high efficiency and stable inverted PSCs.Keywords: alcohol-soluble conjugated polymer; cathode interlayer; inverted polymer solar cells; lifetime stability; water-soluble nonconjugated polymer

Hydrophilic Conjugated Polymers with Large Bandgaps and Deep-Lying HOMO Levels as an Efficient Cathode Interlayer in Inverted Polymer Solar Cells

Co-reporter:Yuanyuan Kan;Yongxiang Zhu;Zhulin Liu;Lianjie Zhang;Yong Cao

Macromolecular Rapid Communications 2015 Volume 36( Issue 15) pp:1393-1401

Publication Date(Web):

DOI:10.1002/marc.201500163

Using d-limonene as the non-aromatic and non-chlorinated solvent for the fabrications of high performance polymer light-emitting diodes and field-effect transistors

Co-reporter:Yongxiang Zhu, Zhenhui Chen, Yong Yang, Ping Cai, Junwu Chen, Yuanyuan Li, Wei Yang, Junbiao Peng, Yong Cao

Organic Electronics 2015 Volume 23() pp:193-198

Publication Date(Web):August 2015

DOI:10.1016/j.orgel.2015.05.004

•d-Limonene, a natural product, as the non-aromatic and non-chlorinated solvent.•Good salvation ability to conjugated polymers.•d-Limonene processed blue PLEDs with EQEmax = 3.57%.•High performance OFETs with μh = 1.06 cm2 (V s)−1.Aiming to environment protection, green solvents are crucial for commercialization of solution-processed optoelectronic devices. In this work, d-limonene, a natural product, was introduced as the non-aromatic and non-chlorinated solvent for processing of polymer light-emitting diodes (PLEDs) and organic field effect transistors (OFETs). It was found that d-limonene could be a good solvent for a blue-emitting polyfluorene-based random copolymer for PLEDs and an alternating copolymer FBT-Th4(1,4) with high hole mobility (μh) for OFETs. In comparisons to routine solvent-casted films of the two conjugated polymers, the resulting d-limonene-deposited films could show comparable film qualities, based on UV–vis absorption spectra and observations by atomic force microscopy (AFM). With d-limonene as the processing solvent, efficient blue PLEDs with CIE coordinates of (0.16, 0.16), maximum external quantum efficiency of 3.57%, and luminous efficiency of 3.66 cd/A, and OFETs with outstanding μh of 1.06 cm2 (V s)−1 were demonstrated. Our results suggest that d-limonene would be a promising non-aromatic and non-chlorinated solvent for solution processing of conjugated polymers and molecules for optoelectronic device applications.Graphical abstract

Low band-gap benzodithiophene-thienothiophenecopolymers: the effect of dual two-dimensional substitutions on optoelectronic properties

Co-reporter:Zhulin Liu;Jiangman Sun;Yongxiang Zhu;Peng Liu;Lianjie Zhang

Science China Chemistry 2015 Volume 58( Issue 2) pp:267-275

Publication Date(Web):2015 February

DOI:10.1007/s11426-014-5223-7

Two new conjugated copolymers, PBDT-T6-TTF and PBDT-T12-TTF, were derived from a novel 4-fluorobenzoyl thienothiophene (TTF). In addition, two types of benzodithiophene (BDT) units with 2,3-dihexylthienyl (T6) and 2,3-didodecylthienyl (T12) substituents, respectively, were successfully synthesized. The effect of the dual two-dimensional (2D) substitutions of the building blocks upon the optoelectronic properties of the polymers was investigated. Generally, the two polymers exhibited good solubility and broad absorption, showing similar optical band gaps of ∼1.53 eV. However, PBDT-T6-TTF with its shorter alkyl chain length possessed a larger extinction coefficient in thin solid film. The highest occupied molecular orbital (HOMO) level of PBDT-T6-TTF was located at −5.38 eV while that of PBDT-T12-TTF was at −5.51 eV. In space charge-limited-current (SCLC) measurement, PBDT-T6-TTF and PBDT-T12-TTF displayed respective hole mobilities of 3.0×10−4 and 1.6×10−5 cm2 V−1s−1. In polymer solar cells, PBDT-T6-TTF and PBDT-T12-TTF showed respective power conversion efficiencies (PCEs) of 2.86% and 1.67%. When 1,8-diiodooctane (DIO) was used as the solvent additive, the PCE of PBDT-T6-TTF was remarkably elevated to 4.85%, but the use of DIO for the PBDT-T12-TTF-blend film resulted in a lower PCE of 0.91%. Atomic force microscopy (AFM) indicated that the superior efficiency of PBDT-T6-TTF with 3% DIO (v/v) should be related to the better continuous phase separation of the blend film. Nevertheless, the morphology of the PBDT-T12-TTF deteriorated when the 3% DIO (v/v) was added. Our results suggest that the alkyl-chain length on the 2D BDT units play an important role in determining the optoelectronic properties of dual 2D BDT-TT-based polymers.

Efficient Blue Light-Emitting Diodes Based on Conjugated Polymers with Fluorinated Silole Chemically Doped in Fluorene–Carbazole Main Chain

Co-reporter:Lianjie Zhang;Zhulin Liu;Xiaofei Zhang

Journal of Inorganic and Organometallic Polymers and Materials 2015 Volume 25( Issue 1) pp:64-72

Publication Date(Web):2015 January

DOI:10.1007/s10904-014-0078-0

A series of random conjugated copolymers PFCFS1-20 with 2,5-diphenyl-3,4-bis(3-fluorophenyl)-silole (fluorinated TPS) chemically doped in 2,7-fluorene-2,7-carbazole main chain were successfully synthesized and characterized. Polymers PFCFS1-20 with fluorinated TPS contents from 1 to 20 % possess high weight-average molecular weights between 63.5 and 114.1 kg mol−1. The absorption spectra of THF solutions and films of PFCFS1-20 are very similar, showing peaks around 385 nm and optical band gaps around 2.87 eV. Almost no red-shifts of the absorption peaks from a solution to a solid state film for the four copolymers indicate that their interchain interactions are extremely weak. PFCFS1-20 solutions show blue emissions with very limited intrachain excitation energy transfers. The films of PFCFS1-20 also display blue emissions with main peaks at 449 nm for PFCFS1 and 475 nm for PFCFS20, suggesting that the fluorinated TPS may comprise more twisted phenyl peripherals. Two types of electroluminescence (EL) devices with configurations of ITO/PEDOT:PSS/PFCFS/CsF/Al and ITO/PEDOT:PSS/PFCFS/TPBI/CsF/Al were investigated. It was found that PFCFS1-20 in the two types of EL devices could show blue to sky-blue emissions. Among PFCFS1-20, for EL devices without TPBI layer, PFCFS5 possesses the best device performances with maximum luminous efficiency (LEmax) of 1.46 cd A−1 and maximum external quantum efficiency (EQEmax) of 0.93 %. The insertion of TPBI as the electron transport layer remarkably improves the EL performances. Also PFCFS5 displays the best LEmax of 4.59 cd A−1 and EQEmax of 3.21 %. The latter one is among the best efficiency so far reported for silole-containing polymers. Our results suggest that the fluorinated TPS is an efficient chromophore for constructions of blue-emissive conjugated polymers.

Low Band-Gap Conjugated Polymers with Strong Interchain Aggregation and Very High Hole Mobility Towards Highly Efficient Thick-Film Polymer Solar Cells

Co-reporter:Zhenhui Chen;Ping Cai;Xuncheng Liu;Lianjie Zhang;Linfeng Lan;Junbiao Peng;Yuguang Ma ;Yong Cao

Advanced Materials 2014 Volume 26( Issue 16) pp:2586-2591

Publication Date(Web):

DOI:10.1002/adma.201305092

Dithienobenzothiadiazole-Based Conjugated Polymer: Processing Solvent-Relied Interchain Aggregation and Device Performances in Field-Effect Transistors and Polymer Solar Cells

Co-reporter:Jun Huang;Yongxiang Zhu;Lianjie Zhang;Junbiao Peng ;Yong Cao

Macromolecular Rapid Communications 2014 Volume 35( Issue 22) pp:1960-1967

Publication Date(Web):

DOI:10.1002/marc.201400461

Using ultra-high molecular weight hydrophilic polymer as cathode interlayer for inverted polymer solar cells: Enhanced efficiency and excellent air-stability

Co-reporter:Ping Cai, Shu Zhong, Xiaofeng Xu, Junwu Chen, Wei Chen, Fei Huang, Yuguang Ma, Yong Cao

Solar Energy Materials and Solar Cells 2014 Volume 123() pp:104-111

Publication Date(Web):April 2014

DOI:10.1016/j.solmat.2014.01.009

•Water soluble and ultra-high molecular weight hydrophilic polymer as cathode interlayer on ITO.•Interlayer polymer shifts ITO work function to ~3.7 eV.•Inverted solar cells show higher efficiency and better air-stability when compared with well-known ZnO cathode interlayer.•Degradation mechanism analysis constructively focused on cathode interfaces.New cathode interlayers based on a water soluble cationic polyacrylamide (C-PAM) with a viscosity-averaged molecular weight of 3 million were introduced as cathode interlayers in inverted solar cells. The neat C-PAM and 5% CsF doped C-PAM thin layers on ITO substrate could decrease the work-function of ITO and hence lead to suitable energy level alignments for efficient electron collection. Obvious enhancements of energy conversion efficiency as well as air-stability were found, in comparison to well-known ZnO interlayer based device. The possible factors involving deteriorations at two interfaces of ITO/interlayer and interlayer/active layer during more than 1 year storage under an ambient condition were analyzed. Our results suggest that the interlayer polymer with giant molecular size and numerous side groups of hydrophilic amide and quaternary ammonium would facilitate the formations of large and intimate single-molecular binding area with the ITO substrate and the upper active layer, and might afford more reliable interfacial contacts to resist possible moisture- and/or stress-induced delamination. Therefore, hydrophilic polymer with an ultra-high molecular weight would be an ideal cathode interlayer for efficient and air-stable inverted solar cells.

4,5-Ethylene-2,7-Carbazole-Based Medium-Bandgap Conjugated Polymers with Low-Lying HOMO Levels Toward Efficient Polymer Solar Cells with High Open-Circuit Voltage

Co-reporter:Jiangman Sun;Yongxiang Zhu;Xiaofeng Xu;Chen Zhang;Hongzheng Chen;Yong Cao

Macromolecular Chemistry and Physics 2014 Volume 215( Issue 11) pp:1052-1059

Publication Date(Web):

DOI:10.1002/macp.201400001

D-A copolymers based on 5,6-difluorobenzotriazole and oligothiophenes: Synthesis, field effect transistors, and polymer solar cells

Co-reporter:Xuncheng Liu, Ping Cai, Zhenhui Chen, Lianjie Zhang, Xiaofei Zhang, Jiangman Sun, Haitao Wang, Junwu Chen, Junbiao Peng, Hongzheng Chen, Yong Cao

Polymer 2014 Volume 55(Issue 7) pp:1707-1715

Publication Date(Web):1 April 2014

DOI:10.1016/j.polymer.2014.02.046

Two new 5,6-difluorobenzotriazole (FBTA)-oligothiophene copolymers PFBTA-3T and PFBTA-4T, comprising terthiophene (3T) and quaterthiophene (4T) on the backbone, respectively, were successfully synthesized. A new route to synthesize FBTA monomer was established. Polymers PFBTA-3T and PFBTA-4T exhibited good solubility in common organic solvents and good thermal stability. In comparison to poly (3-hexylthiophene), the incorporations of the FBTA as in PFBTA-3T and PFBTA-4T could result in smaller band gaps around 1.83 eV for the two copolymers. The HOMO levels of PFBTA-3T and PFBTA-4T were −5.49 and −5.31 eV, respectively, while their LUMO levels were −3.65 and −3.90 eV, respectively. In field-effect transistors fabricated without high temperature thermal annealing, PFBTA-3T and PFBTA-4T could display hole mobilities of 1.68 × 10−3 and 1.31 × 10−2 cm2 V−1 s−1, respectively. The mobility for PFBTA-4T is the highest among the reported FBTA-based polymers, suggesting that FBTA is a promising heterocycle to construct polymers with high mobility. Polymer solar cells were also fabricated with PFBTA-3T and PFBTA-4T as the donor and PC61BM as the acceptor. With copolymer: PC61BM = 1:1.5 as the active layers, polymer solar cells showed power conversion efficiencies of 3.0% and 2.51% for PFBTA-3T and PFBTA-4T, respectively.

Low band-gap DA conjugated copolymers based on anthradithiophene and diketopyrrolopyrrole for polymer solar cells and field-effect transistors

Co-reporter:Jun Huang;Yongxiang Zhu;Lianjie Zhang;Ping Cai;Xiaofeng Xu;Yong Cao

Journal of Polymer Science Part A: Polymer Chemistry 2014 Volume 52( Issue 12) pp:1652-1661

Publication Date(Web):

DOI:10.1002/pola.27163

ABSTRACT

Two conjugated copolymers PADT-DPP and PADT-FDPP based on anthradithiophene and diketopyrrolopyrrole, with thiophene and furan as the π-conjugated bridge, respectively, were successfully synthesized and characterized. The number-averaged molecular weights of the two polymers are 38.7 and 30.2 kg/mol, respectively. Polymers PADT-DPP and PADT-FDPP exhibit broad absorption bands and their optical band gaps are 1.44 and 1.50 eV, respectively. The highest occupied molecular orbital energy level of PADT-DPP is located at −5.03 eV while that of PADT-FDPP is at −5.16 eV. In field-effect transistors, PADT-DPP and PADT-FDPP displayed hole mobilities of 4.7 × 10⁻³ and 2.7 × 10⁻³ cm²/(V s), respectively. In polymer solar cells, PADT-DPP and PADT-FDPP showed power conversion efficiency (PCE) of 3.44% and 0.29%, respectively. Atomic force microscopy revealed that the poor efficiency of PADT-FDPP should be related to the large two-phase separation in its active layer. If 1,8-diiodooctane (DIO) was used as the solvent additive, the PCE of PADT-DPP remained almost unchanged due to very limited morphology variation. However, the addition of DIO could remarkably elevate the PCE of PADT-FDPP to 2.62% because of the greatly improved morphology. Our results suggest that the anthradithiophene as an electron-donating polycyclic system is useful to construct new D–A alternating copolymers for efficient polymer solar cells. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1652–1661

Electroluminescence performances of 1,1-bis(4-(N,N-dimethylamino)phenyl)-2,3,4,5-tetraphenylsilole based polymers in three cathode architectures

Co-reporter:ZhiTian Liu;SuJun Hu;LinHua Zhang;JunBiao Peng

Science China Chemistry 2013 Volume 56( Issue 8) pp:1129-1136

Publication Date(Web):2013 August

DOI:10.1007/s11426-013-4875-z

A new silole monomer with two 4-(N,N-dimethylamino)phenyl substitutions on silicon atom as designed and synthesized. Three copolymers PF-N-HPS1, PF-N-HPS10 and PF-N-HPS20 were then obtained by copolymerizations of 2,7-fluorene derivatives with the silole monomer at feed ratios of 1%, 10%, and 20%. Their UV-vis absorption, electrochemical, photoluminescent, and electroluminescent (EL) properties were investigated. PF-N-HPS possessed HOMO levels of −5.25–−5.58 eV, and showed green emissions. Using PF-N-HPS as the emissive layer, three different polymer light-emitting diodes were fabricated as device A with ITO/PEDOT/PF-N-HPS/Al, device B with ITO/PEDOT/PF-N-HPS/Ba/Al, and device C with ITO/PEDOT/PF-N-HPS/TPBI/Ba/Al. For the device A, PF-N-HPS only showed very low EL efficiency of 0.06–0.33 cd/A, indicating that the Al cathode could not inject electron efficiently to the emissive polymers containing the 4-(N,N-dimethylamino)phenyl groups. For the device B, low work function Ba supplied better electron injections, and the EL efficiency could be improved to 0.85–1.44 cd/A. TPBI with a deep HOMO level of −6.2 eV could enhance electron transport and hole blocking. Thus modified recombinations and largely elevated EL efficiency of 4.56–7.96 cd/A were achieved for the device C. The separation of the emissive layer and metal cathode with the TPBI layer may also suppress exciton quenching at the cathode interface.

Donoracceptor copolymers based on phenanthrene as electron-donating unit: Synthesis and photovoltaic performances

Co-reporter:Zhenhui Chen;Ping Cai;Lianjie Zhang;Yongxiang Zhu;Xiaofeng Xu;Jiangman Sun;Jun Huang;Xuncheng Liu;Hongzheng Chen;Yong Cao

Journal of Polymer Science Part A: Polymer Chemistry 2013 Volume 51( Issue 23) pp:4966-4974

Publication Date(Web):

DOI:10.1002/pola.26948

ABSTRACT

Using 9,10-bis(dodecyloxy)phenanthrene as electron-donating unit and 4,7-dithienyl-5,6-bis(dodecyloxy)benzothiadiazole, 4,7-dithienyl-5,6-bis(octyloxy)benzoxadiazole, 5,8-dithienyl-2,3-bis(para-octyloxyphenyl)quinoxaline, and 5,8-dithienyl-2,3-bis(meta-octyloxyphenyl)quinoxaline as electron-accepting unit, four D–A copolymers PPA-DTBT, PPA-DTBX, PPA-p-DTQ, and PPA-m-DTQ, respectively, were successfully synthesized as new polymeric donors for photovoltaic cells. All the alternating copolymers can show two absorption bands, both in solutions and thin films. The optical bandgaps of the polymers are quite close, which are between 1.93 and 2.00 eV. The HOMO and LUMO levels of the polymers are also comparable of −5.52 ± 0.03 eV and −3.57 ± 0.03 eV, respectively. Thus, using the dialkoxyphenanthrene as the D unit could afford D–A copolymers with deep-lying HOMO levels, which would be an important factor to achieve high open-circuit voltages (V_oc) in bulk-heterojunction solar cells. With the copolymers as the donor and PC₇₁BM as the acceptor, the resulting solar cells could display good V_oc between 0.86 and 0.88 V. Among the four copolymers, PPA-DTBT containing the dialkoxybenzothiadiazole unit showed the best power conversion efficiency of 3.03% because of its relatively higher hole mobility and better phase separation. The results suggest that dialkoxyphenanthrene is a valuable electron-donating unit in the constructions of D–A copolymers for efficient solar cells with high V_oc. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4966–4974

Hydrophilic poly(triphenylamines) with phosphonate groups on the side chains: synthesis and photovoltaic applications

Co-reporter:Xiaofeng Xu, Yongxiang Zhu, Lianjie Zhang, Jiangman Sun, Jun Huang, Junwu Chen and Yong Cao

Journal of Materials Chemistry A 2012 vol. 22(Issue 10) pp:4329-4336

Publication Date(Web):06 Jan 2012

DOI:10.1039/C1JM14970A

Two triphenylamine-based homopolymers PTPA-EP and PTPA-PO3Na2, comprising diethyl phosphonate and sodium phosphonate end groups on side chains, respectively, were synthesized. The UV-vis absorption and photoluminescence (PL) properties of the PTPA-EP and PTPA-PO3Na2 are mainly determined by the conjugated poly(triphenylamine) main chain. The PTPA-EP and PTPA-PO3Na2 possess comparable HOMO levels of around −5.03 eV. The PTPA-EP, with better solubility than PTPA-PO3Na2 in hydrophilic solvents, was utilized as cathode interlayer to construct efficient bulk-heterojunction photovoltaic cells with a low bandgap poly(2,7-carbazole) (PCDTBT) as the polymer donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor. The work function of ITO was shifted to −4.3 eV by PTPA-EP, which matches well with the LUMO level of PC71BM for good electron extraction. Inverted solar cells with a device configuration of ITO/PTPA-EP/active layer/MoO3/Al exhibited a power conversion efficiency (PCE) of 4.59%, which is a good efficiency among inverted solar cells with an organic interlayer on an ITO cathode. The PCE shows a 79% increase in comparison to that of a bare ITO cathode, though the efficiency is lower than 5.13% for an inverted solar cell with an inorganic ZnO interlayer on ITO. Moreover, a conventional solar cell with a device configuration of ITO/PEDOT:PSS/active layer/PTPA-EP/Al could show a better PCE of 5.27%. The results indicate that PTPA-EP is a promising new cathode interlayer for high efficiency inverted and conventional solar cells.

Controlled synthesis of CdTe nanocrystals for high performanced Schottky thin film solar cells

Co-reporter:Shi Sun, Hongmei Liu, Yuping Gao, Donghuan Qin and Junwu Chen

Journal of Materials Chemistry A 2012 vol. 22(Issue 36) pp:19207-19212

Publication Date(Web):06 Aug 2012

DOI:10.1039/C2JM34280D

We report the synthesis of CdTe semiconductor nanocrystals (NCs) with multi-armed, rod and tetrapod shapes, developed by a facile solution process using a cadmium carboxylate as the precursor at moderate temperatures of 220–300 °C. It was found that the size and morphology of the CdTe NCs were related to synthesis parameters such as the reaction temperature, carboxyl chain length and carboxylic acid. Based on these CdTe NCs, photovoltaic cells (PVC) in simple Schottky diode configuration of ITO/CdTe/Al were fabricated. The performances of the CdTe NCs solar cells were found to strongly rely on the morphology and surface ligand of the CdTe NCs. The best device could show a power conversion efficiency (PCE) of 5.15% under AM 1.5G illumination at 100 mW cm−2, which is the highest efficiency of a CdTe NC-based Schottky solar cell reported to date. Notably, in comparison to the ITO/CdTe/LiF/Al device, the device configuration of ITO/CdTe/Al showed higher efficiency and better air-stability.

High Efficiency and High Voc Inverted Polymer Solar Cells Based on a Low-Lying HOMO Polycarbazole Donor and a Hydrophilic Polycarbazole Interlayer on ITO Cathode

Co-reporter:Jiangman Sun, Yongxiang Zhu, Xiaofeng Xu, Linfeng Lan, Lianjie Zhang, Ping Cai, Junwu Chen, Junbiao Peng, and Yong Cao

The Journal of Physical Chemistry C 2012 Volume 116(Issue 27) pp:14188-14198

Publication Date(Web):June 12, 2012

DOI:10.1021/jp3009546

In this work, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-5,6-bis(dodecyloxy)-2,1,3-benzothiadiazole)] (PCDTBT12) was synthesized as the polymer donor for photovoltaic application. PCDTBT12 possesses a band gap of 1.99 eV, a low-lying HOMO of −5.6 eV, and good hole mobility up to 4.1 × 10–3 cm2 V–1 s–1. With ZnO as the interlayer on an ITO cathode, a PCDTBT12-based inverted solar cell showed a high open-circuit voltage of 0.98 V and a good power conversion efficiency (PCE) of 5.53%, suggesting that PCDTBT12 would be a promising donor material in the fabrication of a subcell for shorter wavelength absorption in a tandem solar cell. Using PC-P, a homopolymer of 2,7-carbazole with hydrophilic phosphonate side chains, as an interlayer polymer on the ITO cathode could further elevate the efficiency to 6.04% because of increased current (higher efficiency of 6.2% was achieved for a smaller cell area of 0.045 cm2). The efficiencies are the highest ones so far reported for an inverted solar cell with an organic cathode interlayer. It was proposed that the hydrophilic side chains of PC-P supplied a subgap state for electron transport. The two devices showed comparable air stability, and retained over 96% of their initial PCEs after storage in air for more than 1 month. Therefore, a hydrophilic conjugated polymer as the cathode interlayer, already shown in outstanding cathode modifications in conventional polymer solar cells, will play an important role in the future development of high efficiency and air-stable inverted solar cells.

High efficiency inverted polymeric bulk-heterojunction solar cells with hydrophilic conjugated polymers as cathode interlayer on ITO

Co-reporter:Yongxiang Zhu, Xiaofeng Xu, Lianjie Zhang, Junwu Chen, Yong Cao

Solar Energy Materials and Solar Cells 2012 97() pp: 83-88

Publication Date(Web):

DOI:10.1016/j.solmat.2011.09.030

Largely Enhanced Efficiency with a PFN/Al Bilayer Cathode in High Efficiency Bulk Heterojunction Photovoltaic Cells with a Low Bandgap Polycarbazole Donor

Co-reporter:Zhicai He;Chen Zhang;Xiaofeng Xu;Lianjie Zhang;Liang Huang;Hongbin Wu;Yong Cao

Advanced Materials 2011 Volume 23( Issue 27) pp:3086-3089

Publication Date(Web):

DOI:10.1002/adma.201101319

A Series of Energy-Transfer Copolymers Derived from Fluorene and 4,7-Dithienylbenzotriazole for High Efficiency Yellow, Orange, and White Light-Emitting Diodes

Co-reporter:Lianjie Zhang;Sujun Hu;Zhenhui Chen;Hongbin Wu;Junbiao Peng ;Yong Cao

Advanced Functional Materials 2011 Volume 21( Issue 19) pp:3760-3769

Publication Date(Web):

DOI:10.1002/adfm.201100844

Abstract

Eight random and alternating copolymers PF-DTBTA derived from 2,7-fluorene and 4,7-dithienylbenzotriazole (DTBTA) were synthesized. Thin solid films of the energy-transfer copolymers possess high absolute photoluminescence (PL) quantum yields (Φ_PL) between 60−72%. Inserting PVK layer between anode and emissive layer could show higher electroluminescence (EL) performances due to PVK-enhanced hole injection. Random copolymers PF-DTBTA1−15, with DTBTA molar contents from 1% to 15%, displayed yellow EL spectra with high external quantum efficiency (EQE_max) up to 5.78%. PF-DTBTA50, the alternating copolymer, showed an orange EL with EQE_max of 3.3%. The good Φ_PL and EQE_max of the PF-DTBTA50 with very high DTBTA content indicate that DTBTA is a high efficiency chromophore with very low concentration quenching effects in the solid state PL and EL processes. PF-DTBTA0.03−0.1 could emit white EL due to partial energy transfer from fluorene segments to DTBTA units. Moreover, white EL devices, with forward-viewing maximum luminous efficiency up to 11 cd/A and stable white EL spectra (CIE coordinates of (0.33, 0.43)) in high current range from 5 mA to 60 mA, could be realized from the non-doped polymer with simple binary structure. Our results suggest that DTBTA has big potential to construct high performanced EL polymers or oligomers.

Efficient three-color white light-emitting diodes from a single polymer with PFN/Al bilayer cathode

Co-reporter:Jie Luo, Xianzhen Li, Junwu Chen, Fei Huang, Yong Cao

Synthetic Metals 2011 Volume 161(17–18) pp:1982-1986

Publication Date(Web):September 2011

DOI:10.1016/j.synthmet.2011.07.006

In this work, a bilayer cathode, composed by alcohol-soluble poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) and high work-function Al, was utilized for efficient electron injection in non-doped white light-emitting diodes (WPLEDs) based on an electroluminescent single conjugated polymer PFO-R005-G010 that could show red, green, and blue (RGB) emissions simultaneously. The non-doped WPLEDs with the bilayer cathode could display higher luminous efficiency (LE) and better white light CIE coordinates, than those of well-known Ba/Al cathode. A pure white light (CIE coordinates of (0.33, 0.33)) containing spectral ranges of both deep blue and deep red as well as a good forward-viewing maximum LE of 6.62 cd/A at luminance of 200 cd/m2 could be achieved for a device with a PFN thickness of 10 nm and a thermal annealing process before Al deposition. The WPLED could show a maximum luminance of 9500 cd/m2 and good spectral stability at high luminance range. The results suggest that material systems for high efficiency three color WPLEDs, already realized by Ca/Al or Ba/Al cathode, could be the promising candidates for WPLEDs with a bilayer cathode.Graphical abstractHighlights► Non-doped white light-emitting diodes based on a single conjugated polymer. ► Showing red, green, and blue (RGB) emissions simultaneously. ► Higher efficiency with PFN/Al cathode than Ba/Al cathode. ► Efficiency of 6.62 cd/A and luminance up to 9500 cd/m2.

Conjugated polyelectrolytes and neutral polymers with poly(2,7-carbazole) backbone: Synthesis, characterization, and photovoltaic application

Co-reporter:Xiaofeng Xu;Wanzhu Cai;Yong Cao

Journal of Polymer Science Part A: Polymer Chemistry 2011 Volume 49( Issue 5) pp:1263-1272

Publication Date(Web):

DOI:10.1002/pola.24547

Abstract

Poly(2,7-carbazole) neutral polymers (PC-N, PC-NOH, and PC-P) and polyelectrolytes (PC-NBr and PC-SO₃Na) with hydrophilic pendant groups of ammonium, phosphonate, and sulfonate were synthesized as interlayers for cathode modifications in bulk-heterojunction photovoltaic cells (BHJ PVCs). The absorptions of the polymers were determined by the poly(2,7-carbazole) backbone, showing absorption peaks at ∼390 nm for their solutions and films. Because of large intermolecular interactions, excimer emissions with wavelengths higher than 500 nm were found in the photoluminescence spectra of the films of the polymers, which weakened the light emissions of the polymers. PC-N, PC-NBr, PC-NOH, and PC-P possessed comparable HOMO levels of −5.23 eV and LUMO levels of −2.4 eV, but HOMO and LUMO levels of PC-SO₃Na were up-lying to −4.91 and −2.12 eV, respectively. PC-N, PC-NBr, PC-NOH, and PC-P were selected to construct thin interlayers in BHJ PVCs with PFO-DBT35:PCBM = 1:4 as the active layer. Compared with traditional Al cathode, bilayer cathodes with the interlayers showed improvements of open-circuit voltages and short-circuit currents of the PVCs. PC-NOH was the best for the photovoltaic performances and over 20% increase of power conversion efficiency (PCE) was achieved. The bilayer cathodes would have great potential to further elevate PCE of BHJ PVCs with other active layer materials. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Bulk-Heterojunction Solar Cells with Benzotriazole-Based Copolymers as Electron Donors: Largely Improved Photovoltaic Parameters by Using PFN/Al Bilayer Cathode

Co-reporter:Lianjie Zhang, Chao He, Junwu Chen, Pan Yuan, Liang Huang, Chen Zhang, Wanzhu Cai, Zhitian Liu, and Yong Cao

Macromolecules 2010 Volume 43(Issue 23) pp:9771-9778

Publication Date(Web):November 3, 2010

DOI:10.1021/ma102080c

Three benzotriazole (BTA)-based conjugated polymers PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, with electron-donating segments of fluorene, carbazole, and dialkoxybenzene, respectively, were successfully synthesized as new polymeric donors in bulk-heterojunction (BHJ) photovoltaic cells (PVCs). All the copolymers exhibited good solubility in common organic solvents and good thermal stability. The optical band gaps for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are 2.24, 2.18, and 1.87 eV, respectively. The HOMO levels for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are −5.67, −5.54, and −5.20 eV, respectively, which were determined by the electron-donating segments. The LUMO levels for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are −3.43, −3.36, and −3.33 eV, respectively, which were mainly dominated by the BTA unit. For a blend ratio of polymer:PCBM = 1:2, BHJ PVCs with Al cathode displayed power conversion efficiencies (PCE) of 0.9%, 1.51%, and 1.16% for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, respectively. Alcohol-soluble poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) was selected for cathode modification. For BHJ PVCs with PF-DTBTA and PCz-DTBTA as the donors, open-circuit voltages (Voc) of the PVCs could all be elevated with the PFN/Al bilayer cathode, whereas no such improvements were found for PVCs with PPh-DTBTA as the donor. The behaviors could be attributed to the different Voc losses when using Al cathode. Using PFN/Al bilayer cathode could also improve short-curcuit current and fill factor for the three BTA-based copolymers. The N−N interactions between the BTA-based polymers and the PFN may modify interfacial contact, resulting in enhancement of the electron extraction from the acceptor phase to the cathode and decreasing hole−electron recombination in the active layer. Consequently, PFN/Al bilayer cathode elevated PCE values to 1.3%, 2.75%, and 1.39% for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, respectively. The most significant increasing of PCE with a calculated value of 80% was found for PCz-DTBTA as the donor, and this might be related to the additional N−N interactions between the carbazole segments and the PFN. The results would supply useful information to understand the contribution of an interfacial layer on the photovoltaic performance. Our results also suggest that the bilayer cathode would have great potential to elevate PCE of BHJ PVCs.

Development of Novel Conjugated Donor Polymers for High-Efficiency Bulk-Heterojunction Photovoltaic Devices

Co-reporter:Junwu Chen and Yong Cao

Accounts of Chemical Research 2009 Volume 42(Issue 11) pp:1709

Publication Date(Web):July 2, 2009

DOI:10.1021/ar900061z

Solar cells are one attractive method for harnessing inexhaustible clean energy from the sun. Organic photovoltaic technology is emerging as a potential competitor to silicon-based photovoltaic cells (PVCs), and their power conversion efficiencies (PCE) can now exceed 6%. Polymeric bulk-heterojunction (BHJ) PVCs, whose photoactive layer is composed of a blend of bicontinuous and interpenetrating donors and acceptors, can maximize interfacial area between the donor and the acceptor. Classic polymer donors, such as dialkoxy-substituted poly(para-phenylene vinylene)s (PPVs) and poly(3-hexylthiophene) (P3HT), have been widely investigated. However, advances in synthetic methodology provide new avenues for the development of novel conjugated polymer donors with improved power conversion efficiencies. Recently, researchers have achieved great advances in this area. This Account primarily focuses on novel donor polymers that have shown power conversion efficiencies greater than 1%. 2,1,3-Benzothiadiazole, thiophene, thieno[3,4-b]pyrazine, quinoxaline, and silole have emerged as useful heterocycles for constructing a variety of conjugated polymers for photovoltaic applications. We summarize useful information, such as molecular weights, absorption, bandgap, energy levels, and their photovoltaic performances with detailed device parameters (see comparison tables), about these novel donor polymers. We use statistical summaries to evaluate several important parameter relationships among these polymer donors including open-circuit voltage versus HOMO, power conversion efficiency versus bandgap, and power conversion efficiency versus hole mobility. Further statistical analysis of the data listed in these tables may guide further structural design and evaluation of polymer donor materials.

Silole-Containing Polymers: Chemistry and Optoelectronic Properties

Co-reporter:Junwu Chen;Yong Cao

Macromolecular Rapid Communications 2007 Volume 28(Issue 17) pp:1714-1742

Publication Date(Web):8 AUG 2007

DOI:10.1002/marc.200700326

In this review, the synthesis and optoelectronic properties of various π-conjugated-, σ-conjugated-, pendanted-, and hyperbranched or dendritic silole-containing polymers (SCPs) are described. So far, substituted silole, dibenzosilole, dithienosilole, and bis-silicon-bridged stilbene have been incorporated into SCPs. The tunable bandgaps from 4.0–1.55 eV, variable fluorescent colors from UV to blue, green, and red (RGB) light, fluorescent chemo-sensors for 2,4,6-trinitrotoluene (TNT)-type explosives, aggregation-induced emission, efficient electroluminescence emissions for RGB lights, phosphorescent hosts with high triplet energy level, efficient solar cells, stable field-effect transistors with high hole mobility in air, and attenuation of strong laser power, are the important features of SCPs.

Simple Silole-Containing Polyfluorene for White Electroluminescence with Simultaneous Blue, Green, and Red Emission

Co-reporter:Feng Wang;Lei Wang;Yong Cao

Macromolecular Rapid Communications 2007 Volume 28(Issue 20) pp:2012-2018

Publication Date(Web):3 SEP 2007

DOI:10.1002/marc.200700423

Novel conjugated silole-containing polyfluorenes, with green- and red-emissive siloles on the backbone of the blue-emissive polyfluorene are synthesized for white light electroluminescence (EL) from a single polymer with simultaneous red, green, and blue (RGB) emission. The CIE coordinates (0.33, 0.36) of the white light EL spectra are very close to that for pure white light (0.33, 0.33). The EL spectra are also quite stable at different applied voltages or brightness. The relative intensities for the three RGB peaks, at 450, 505, and 574 nm, were 0.94, 1, and 0.97, respectively, which demonstrates a balanced simultaneous RGB emission. A maximum luminous efficiency of 2.03 cd · A⁻¹ for a brightness of 344 cd · m⁻², and a luminous efficiency of 1.86 cd · A⁻¹ for a more practical brightness of 2 703 cd · m⁻², were achieved.

Synthesis and optoelectronic properties of silole-containing polyfluorenes with binary structures

Co-reporter:Zhitian Liu;Lei Wang;Xiaoying Ouyang;Yong Cao;Feng Wang

Journal of Polymer Science Part A: Polymer Chemistry 2007 Volume 45(Issue 5) pp:756-767

Publication Date(Web):5 JAN 2007

DOI:10.1002/pola.21829

2,5-Bis(2-bromofluorene-7-yl)silole was prepared by a modified one-pot synthesis with a reverse addition procedure, from which novel silole-containing polyfluorenes with binary random and alternating structures (silole contents between 4.5 and 25% and high M_w up to 509 kDa were successfully synthesized. The well-defined repeating unit of the alternating copolymer comprises a terfluorene and a silole ring. Optoelectronic properties including UV absorption, electrochemistry, photoluminescence (PL), and electroluminescence (EL) of the copolymers were examined. The different excitation energy transfers from fluorene to silole of the copolymers in solution and in the solid state were compared. The films of the copolymers showed silole-dominant green emissions with high absolute PL quantum yields up to 83%. EL devices of the copolymers with a configuration of ITO/PEDOT/copolymer/Ba/Al displayed exclusive silole emissions peaked at around 543 nm and the highest EL efficiency was achieved with the alternating copolymer. Using the alternating copolymer and poly(9,9-dioctylfluorene) as the blend-type emissive layer, a maximum external quantum efficiency of 1.99% (four times to that of the neat film) was realized, which was a high efficiency so far reported for silole-containing polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 756–767, 2007

Synthesis and Optoelectronic Properties of Random Copolymers Derived from Fluorene and 2,5-Bis(2,1,3-benzothiadiazolyl)silole

Co-reporter:Yongqiang Liu;Zhao Chen;Feng Wang;Yong Cao

Polymer Bulletin 2007 Volume 59( Issue 1) pp:31-44

Publication Date(Web):2007 June

DOI:10.1007/s00289-007-0736-2

A novel series of soluble conjugated random copolymers (PFO-BTS) derived from 9,9-dioctylfluorene (FO) and bis(2,1,3-benzothiadiazolyl)silole (BTS) were synthesized by Suzuki coupling reactions. The feed ratios of FO to BTS were 99:1, 95:5, 90:10, and 85:15. Chemical structures and optoelectronic properties of the copolymers were characterized by elemental analysis, NMR, UV absorption, cyclic voltammetry, photoluminescence (PL), and electroluminescence (EL). The elemental analyses of the copolymers indicated that FO and BTS contents in the copolymers were very close to that of the feed compositions. Compared with the solution PL, complete PL excitation energy transfer from the PFO segments to the BTS units could be achieved by film PL at lower BTS content. The films of the copolymers exhibited PL quantum yields between 22 and 34%. EL devices with a configuration of ITO/PEDOT/PFO-BTS/Ba/Al demonstrated that the BTS units could serve as powerful exciton traps, giving orange-red EL emissions. The PFO-BTS15 was utilized to fabricate blend-type PLEDs with the PFO, the EL efficiency was improved to 1.37% with a weight ratio of PFO-BTS15 : PFO = 1 : 4.

Luminescence and photovoltaic cells of benzoselenadiazole-containing polyfluorenes

Co-reporter:Jie luo, Qiong Hou, Junwu Chen, Yong Cao

Synthetic Metals 2006 Volume 156(5–6) pp:470-475

Publication Date(Web):1 March 2006

DOI:10.1016/j.synthmet.2006.01.009

In this work, light-emitting and photovoltaic properties of new low band-gap copolymers (PFO-DBSe) based on 9,9-dioctylfluorene and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole (DBSe) were investigated. The peak positions of photoluminescence and electroluminescence (EL) of the copolymers reach 758 and 727 nm, respectively, and locate in near-infrared light region. At a current of ∼5 mA, EL emission with peak position of 701 nm and a maximum external quantum efficiency of 1.62% was achieved from EL device with poly(9,9-dioctylfluorene) and the copolymer with DBSe content of 5% as the blend-type emitter, by utilizing of a configuration of ITO/PEDOT/PVK/emitter/CsF/Al. This is a high efficiency for deep red EL emission. The copolymer with DBSe content of 35% possesses good solar light spectral coverage according to its absorption spectrum, and a bulk-heterojunction photovoltaic cell with the copolymer as the electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor displayed an energy conversion efficiency of 0.91% under an AM1.5 solar simulator at 100 mW/cm2.

Fluorescence responses of 1-methyl-1,2,3,4,5-pentaphenylsilole thin layer to vapors of common solvents

Co-reporter:Junwu Chen, Yong Cao

Sensors and Actuators B: Chemical 2006 Volume 114(Issue 1) pp:65-70

Publication Date(Web):30 March 2006

DOI:10.1016/j.snb.2005.04.011

The fluorescence responses of 1-methyl-1,2,3,4,5-pentaphenylsilole (MPS) thin layers on thin-layer chromatography silica plates in the sealed atmospheres of toxic, inflammable, and explosive solvents at room temperature were investigated. The MPS thin layer showed large fluorescence responses to approximately saturated vapors of solvents such as dichloromethane, chloroform, tetrahydrofuran, diethyl ether, benzene, n-hexane, acetone, toluene, and ethyl acetate with fluorescence quenching close to 100% within 180 s. Vapors of methanol and ethanol quenched the MPS fluorescence less than 40% within 180 s and n-butanol and DMF caused very limited response with fluorescence quenching less than 10% within 180 s. The solvent vapor molecules activated the rotational relaxations of the peripheries of MPS, causing the fluorescence quenching. The quenching ability or speed of a solvent was mainly determined by its affinity for the periphery rotations and its vapor pressure. Higher affinity for the periphery rotations or higher vapor pressure would enhance the quenching. The different fluorescence responses showed the possibility to differentiate different solvents.

Conjugated Silole and Carbazole Copolymers: Synthesis, Characterization, Single-Layer Light-Emitting Diode, and Field Effect Carrier Mobility

Co-reporter:Junwu Chen;Kaixia Yang;Lintao Hou;Feng Wang;Yong Cao;Yan Wang

Macromolecular Chemistry and Physics 2005 Volume 206(Issue 21) pp:2190-2198

Publication Date(Web):21 OCT 2005

DOI:10.1002/macp.200500281

Summary: Soluble conjugated random and alternating copolymers (PCz-PSP) derived from N-hexyl-3,6-carbazole (Cz) and 1,1-dimethyl-2,3,4,5-tetraphenylsilole (PSP) were synthesized by palladium(0)-catalyzed Suzuki coupling reactions. The feed ratios of Cz to PSP were 95:5, 90:10, 80:20, 70:30, and 50:50. Chemical structures and optoelectronic properties of the copolymers were characterized by ¹H NMR, ¹³C NMR, UV absorption, cyclic voltammetry, photoluminescence, electroluminescence, and field effect transistor. HOMO levels of the copolymers are between −5.15 and −5.34 eV. Single-layer devices with a configuration of ITO/copolymer/Ba/Al were fabricated and the copolymer with PSP content of 20% displayed the highest external quantum efficiency of 0.77%. Field effect transistors with tantalum pentoxide-polyacrylonitrile double insulators demonstrated that hole mobilities of the copolymers decreased with their PSP contents, and the hole mobility up to 9.3 × 10⁻⁶ cm² · (V · s)⁻¹ could be achieved.

Low temperature, solution-processed ambipolar field-effect transistors based on polymer/self-assembled monolayer modified InOx hybrid structures for balanced hole and electron mobilities exceeding 1 cm2 V−1 s−1

Co-reporter:Sheng Sun, Yuzhi Li, Linfeng Lan, Peng Xiao, Zhenhui Chen, Zhenguo Lin, Junwu Chen, Junbiao Peng, Yong Cao

Organic Electronics (April 2017) Volume 43() pp:

Publication Date(Web):1 April 2017

DOI:10.1016/j.orgel.2017.01.029

•Ambipolar field-effect transistors (FETs) based on solution-processed organic-inorganic bilayer structures are investigated.•A self-assembled monolayer (SAM) of octadecyl-phosphonic acid is introduced to modify the organic-inorganic interface.•The complementary metal oxide semiconductor (CMOS)-like inverters based on ambipolar FETs are fabricated.Ambipolar field-effect transistors (FETs) based on solution-processed organic-inorganic bilayer structures were investigated. An amorphous indium oxide (InOx) film, as the n-type semiconducting layer, was prepared with an environmentally friendly method and annealed at a low temperature; and a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer, FBT-Th4(1,4), was spin-coated on the InOx film as the p-type semiconducting layer. To improve the p-type mobility, a self-assembled monolayer (SAM) of octadecyl-phosphonic acid was introduced to modify the surface of InOx. The ambipolar FETs showed high and well-balanced hole and electron mobilities of 1.1 and 1.5 cm2 V−1 s−1, respectively. Furthermore we found that ambipolar FETs could be integrated into functional complementary metal oxide semiconductor (CMOS)-like inverters.Download high-res image (233KB)Download full-size image

Hydrophilic poly(triphenylamines) with phosphonate groups on the side chains: synthesis and photovoltaic applications

Co-reporter:Xiaofeng Xu, Yongxiang Zhu, Lianjie Zhang, Jiangman Sun, Jun Huang, Junwu Chen and Yong Cao

Journal of Materials Chemistry A 2012 - vol. 22(Issue 10) pp:NaN4336-4336

Publication Date(Web):2012/01/06

DOI:10.1039/C1JM14970A

Controlled synthesis of CdTe nanocrystals for high performanced Schottky thin film solar cells

Co-reporter:Shi Sun, Hongmei Liu, Yuping Gao, Donghuan Qin and Junwu Chen

Journal of Materials Chemistry A 2012 - vol. 22(Issue 36) pp:NaN19212-19212

Publication Date(Web):2012/08/06

DOI:10.1039/C2JM34280D

A dual ternary system for highly efficient ITO-free inverted polymer solar cells

Co-reporter:Zuosheng Peng, Yuxin Xia, Feng Gao, Kang Xiong, Zhanhao Hu, David Ian James, Junwu Chen, Ergang Wang and Lintao Hou

Journal of Materials Chemistry A 2015 - vol. 3(Issue 36) pp:NaN18371-18371

Publication Date(Web):2015/07/23

DOI:10.1039/C5TA03831F

An extended π-conjugated area of electron-donating units in D–A structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells

Co-reporter:Ping Cai, Zhenhui Chen, Lianjie Zhang, Junwu Chen and Yong Cao

Journal of Materials Chemistry A 2017 - vol. 5(Issue 11) pp:NaN2793-2793

Publication Date(Web):2017/02/21

DOI:10.1039/C7TC00428A

An extended π-conjugated area of electron-donating units in D–A structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells

Co-reporter:Ping Cai, Zhenhui Chen, Lianjie Zhang, Junwu Chen and Yong Cao

Journal of Materials Chemistry A 2017 - vol. 5(Issue 11) pp:NaN2793-2793

Publication Date(Web):2017/02/21

DOI:10.1039/C7TC00428A

Solution-processed small molecules with ethynylene bridges for highly efficient organic solar cells

Co-reporter:Yuanyuan Kan, Chang Liu, Lianjie Zhang, Ke Gao, Feng Liu, Junwu Chen and Yong Cao

Journal of Materials Chemistry A 2016 - vol. 4(Issue 38) pp:NaN14728-14728

Publication Date(Web):2016/08/23

DOI:10.1039/C6TA04442E

Two acceptor–donor–acceptor (A–D–A) conjugated molecules DPP-E-BDT and DPP-E-BDT-T, using diketopyrrolopyrrole (DPP) as the A-unit, ethynylene bridge flanked benzo-[1,2-b:4,5-b′]dithiophene (E-BDT) as the central D-unit, and different 4,8-substitutions on the BDT were synthesized by Sonogashira coupling reactions for solution-processed organic solar cells (OSCs). The insertion of electron-withdrawing ethynylene bridges (sp hybridization) in the DPP-E-BDT and DPP-E-BDT-T molecules leads to planar and enlarged aromatic skeletons, larger band gaps, and deeper HOMO levels. 4,8-Dithienyl substitution on BDT in DPP-E-BDT-T results in additional conjugation extension to give a slightly smaller band gap compared to the 4,8-dialkoxy substitution in target molecules. Bulk heterojunction solar cells using DPP-E-BDT and DPP-E-BDT-T as the donor materials and fullerene acceptor showed a high open-circuit voltage of 0.89 V and moderate current densities of 10.9 mA cm−2. Besides, quite high fill factors (73.6%) could be obtained. Power conversion efficiencies (PCE) of 7.12% were obtained for DPP-E-BDT-T blends, which is the highest efficiency among small molecules based on DPP and BDT units. In active layer fabrication, 1,8-diiodooctane (DIO) was used as a solvent additive and subsequent thermal annealing treatment was also employed. We saw that these combined treatments led to balanced hole and electron transports, with values around 1.2 × 10−4 cm2 V−1 s−1 for the active layers. These results demonstrated that ethynylene bridges in small molecule donors are quite useful, both in tuning the electronic structure and in defining the thin film morphology, thus would be a promising method to enhance photovoltaic performances of the resulting materials.

2,7-Carbazole-1,4-phenylene Copolymers with Polar Side Chains for Cathode Modifications in Polymer Light-Emitting Diodes

Co-reporter:Xiaofeng Xu ; Bing Han ; Junwu Chen ; Junbiao Peng ; Hongbin Wu ;Yong Cao

Macromolecules () pp:

Publication Date(Web):May 10, 2011

DOI:10.1021/ma200191p

Alcohol-soluble 2,7-carbazole-1,4-phenylene copolymers PCP-NOH and PCP-EP, comprising surfactant-like diethanolamino and phosphonate end groups on the side chains, respectively, were synthesized and utilized as electron injection layer (EIL) in combination with high work function Al electrode in polymer light-emitting diodes (PLEDs). The UV–vis absorption and photoluminescence properties of the PCP-NOH and PCP-EP are mainly determined by the conjugated 2,7-carbazole-1,4-phenylene main chain. The PCP-NOH and PCP-EP possess comparable HOMO levels of −5.20 eV and LUMO levels arround −2.35 eV. Multilayer PLEDs with a device configuration of ITO/PEDOT:PSS (40 nm)/emissive layer (70 nm)/EIL (15 nm)/Al (100 nm) were successfully fabricated. With fluorescent PFO-DBT15 as the emissive layer, the PLEDs using the PCP-NOH and PCP-EP as the EIL displayed luminous efficiency (LE) of 1.01 and 0.88 cd/A, respectively, all obviously higher than 0.015 cd/A for sole Al cathode and 0.58 cd/A for the well-known Ba/Al cathode. With a phosphorescent Ir(mppy)3-doped polymer blend (PVK:PBD:Ir(mppy)3 = 100:30:1) as the emissive layer, the PLEDs with the EIL showed high maximum LE of 39.3 cd/A for PCP-NOH and 42.5 cd/A for PCP-EP, in comparison to 0.35 cd/A for sole Al cathode and 32.2 cd/A for the Ba/Al cathode. Particularly, the PLED with PCP-EP as EIL exhibited an excellent LE of 33.3 cd/A at a current density of 61.1 mA/cm2 (luminance = 20 300 cd/cm2), showing weak roll-off of efficiency at high current density. Inserting the polar alcohol-soluble polymers as interlayer can significantly increase built-in potentials in the PLEDs, from which electron injection barrier from the Al electrode is decreased. The results indicate that the PCP-NOH and PCP-EP are excellent electron injection polymers for high-performance PLEDs with high work function Al electrode.