Peng Wang

With respect to the electron-withdrawing acceptors of D–A−π–A organic dyes, reports on the second electron-donating donors for D–D−π–A organic dyes are very limited. Both of the dyes have attracted significant attention in the field of dye-sensitized solar cells (DSCs). In this work, four new D–D−π–A organic dyes with dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or bis(amine) donor have been designed and synthesized for a investigation of the influence of the terminal electron donor in D–D−π–A organic dye-sensitized solar cells. It is found that DTP is a promising building block as the terminal electron donor when introduced in the dithiophenepyrrole direction, but not just a good bridge, which exhibits several characteristics: (i) efficiently increasing the maximum molar absorption coefficient and extending the absorption bands; (ii) showing stronger charge transfer interaction as compared with the pyrrole direction; (iii) beneficial to photocurrent generation of DSCs employing cobalt electrolytes. DSCs based on M45 with the Co-phen electrolyte exhibit good light-to-electric energy conversion efficiencies as high as 9.02%, with a short circuit current density (JSC) of 15.3 mA cm–2, open circuit voltage (VOC) of 867 mV and fill factor (FF) of 0.68 under AM 1.5 illumination (100 mW cm–2). The results demonstrate that N,S-heterocycles such as DTP unit could be promising candidates for application in highly efficient DSCs employing cobalt electrolyte.Keywords: dithieno[3,2-b:2′,3′-d]pyrrole; dye-sensitized solar cells; D−D−π−A organic dyes; electron donor; photovoltaic performance

Push–pull type porphyrin based sensitizers: The effect of donor structure on the light-harvesting ability and photovoltaic performance

Co-reporter:Qingbiao Qi, Renzhi Li, Jie Luo, Bin Zheng, Kuo-Wei Huang, Peng Wang, Jishan Wu

Dyes and Pigments 2015 Volume 122() pp:199-205

Publication Date(Web):November 2015

DOI:10.1016/j.dyepig.2015.06.019

•Two new push–pull type porphyrin dyes with different donors were synthesized.•Dye with diphenyl amino-anthryl as donor showed improved light harvesting.•Dye with diphenyl amino-anthryl as donor showed highest performance.Push–pull type porphyrin-based sensitizers have become promising candidates for high-efficiency dye sensitized solar cells (DSCs). It is of importance to understand the fundamental structure-physical property-photovoltaic performance relationships by varying the donor and acceptor moieties. In this work, two new porphyrin-based sensitizers, WW-7 and WW-8, were synthesized and compared with the known sensitizer YD20. All the three dyes have the same porphyrin core and acceptor group (ethynylbenzoic acid) but their donor groups vary from the triphenylamine in YD20 to meso-diphenylaminoanthracene in WW-7 to N-phenyl carbazole in WW-8. Co(II/III)-based DSC device characterizations revealed that WW-7 showed enhanced light harvesting ability in comparison to YD20 with improved incident photon-to-collected electron conversion efficiencies (IPCEs). As a result, WW-7 displayed much higher short circuit current (Jsc: 13.54 mA cm−2) and open-circuit voltage (Voc: 0.829 V), with a power conversion efficiency (η) as high as 7.7%. Under the same conditions, YD20 cell exhibited a power conversion efficiency of 6.6% and the dye WW-8 showed even lower efficiency (η = 4.6%). Detailed physical measurements and theoretic calculations were conducted to understand the difference and reveal how three different donor structures affect their molecular orbital profile, light-harvesting ability, energy level alignment, and eventually the photovoltaic performance.Replacing one phenyl ring in the triphenylamine donor moiety by an anthryl unit in the push–pull porphyrin based sensitizers significantly improved the light harvesting capability and power conversion efficiency.

Titanium Dioxide Nanomaterials for Photovoltaic Applications

Co-reporter:Yu Bai, Iván Mora-Seró, Filippo De Angelis, Juan Bisquert, and Peng Wang

Chemical Reviews 2014 Volume 114(Issue 19) pp:10095

Publication Date(Web):March 25, 2014

DOI:10.1021/cr400606n

Stepwise assembled nickel–cobalt-hydroxide hetero-accumulated nanocrystalline walls on reduced graphene oxide/nickel foams: an adjustable interface design for capacitive charge storage

Co-reporter:Zhongyu Qian, Tao Peng, Liangti Qu, Jun Wang and Peng Wang

Journal of Materials Chemistry A 2014 vol. 2(Issue 14) pp:4894-4898

Publication Date(Web):20 Jan 2014

DOI:10.1039/C3TA15072K

To achieve a balance of a high specific capacitance between the electroactive materials and electrodes with adjustable specific energy and power, evaporation-induced nickel–cobalt-hydroxide hetero-accumulated nanocrystalline walls on reduced graphene oxide/nickel foams are presented by different Ni-doping and dipping numbers. The flexible solution-based strategy promotes a new interface design for capacitive charge storage.

Joint Electrical, Photophysical, and Photovoltaic Studies on Truxene Dye-Sensitized Solar Cells: Impact of Arylamine Electron Donors

Co-reporter:Zhihui Wang;Dr. Mao Liang;He Wang; Peng Wang;Dr. Fangyi Cheng;Dr. Zhe Sun; Xue Song

ChemSusChem 2014 Volume 7( Issue 3) pp:795-803

Publication Date(Web):

DOI:10.1002/cssc.201301155

Abstract

The judicious design of electron donors is one of the viable tactics to improve the efficiency of organic dyes for dye-sensitized solar cells (DSCs) employing outer-sphere redox couples. Herein, a hexahexyltruxene-substituted 4-(hexyloxy)-N-phenylaniline (HT-HPA) segment is constructed and employed as the electron donor in two organic push–pull dyes (M28 and M29) with high molar absorption coefficient values. Relative to its congener (C241) possessing the dihexyloxy-substituted triphenylamine electron donor, M29 exhibits red-shifted absorption as well as enhanced maximum molar absorption coefficient values. A thorough comparison with M29 and C241 demonstrates that the HT-HPA segment adequately insulates the TiO₂ surface from the electrolyte, which prevents back-recombination and prolongs electron lifetime in the semiconductor. The diminishment of charge recombination not only enables attainment of strikingly high photovoltages (approaching 1 V), but also overcompensates the disadvantageous impact of lower dye-load amounts. As a result, the dye transformation from C241 to M29 brings forth an efficiency improvement from 7.3 % to 8.5 % at the 100 mW cm⁻² simulated AM1.5 conditions. Our work should shed light on the future design of more powerful push–pull organic photosensitizers for iodine-free DSCs.

Organic dye-sensitized solar cells with a cobalt redox couple: influences of π-linker rigidification and dye–bath solvent selection

Co-reporter:Ning Cai, Renzhi Li, Yinglin Wang, Min Zhang and Peng Wang

Energy & Environmental Science 2013 vol. 6(Issue 1) pp:139-147

Publication Date(Web):01 Nov 2012

DOI:10.1039/C2EE23592G

The rigidification of π-conjugated linkers represents a viable strategy towards the energy-level engineering of organic push–pull photosensitizers in dye-sensitized solar cells. In this paper we deploy 4-hexylphenyl substituted cyclopenta[1,2-b:5,4-b′]dithiophene[2′,1′:4,5]thieno[2,3-d]thiophene as the π-linker of a D-π-A dye, which displays an improved molar absorption coefficient and a red-shifted absorption peak in contrast to its model dye with the 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene segment. The energy-gap reduction is concomitant with negative and positive shifts of ground-state and excited-state redox potentials, which however do not exert an adverse impact on the net charge separation yield at the titania/dye/electrolyte interface, probably due to the formation of a favored microstructured dye assembly. Furthermore, the dye load amount can be tuned by changing the bath solvent and has a significant influence on some key photovoltaic features such as the photocurrent and photovoltage, the latter of which is dissected via the joint charge extraction and photovoltage decay experiments. The charge recombination lifetime could be roughly rationalized by analyzing the damping of signals on emitted electrons from titania in the X-ray photoelectron spectroscopy measurements.

Conjugated linker correlated energetics and kinetics in dithienopyrrole dye-sensitized solar cells

Co-reporter:Jing Zhang, Zhaoyang Yao, Yanchun Cai, Lin Yang, Mingfei Xu, Renzhi Li, Min Zhang, Xiandui Dong and Peng Wang

Energy & Environmental Science 2013 vol. 6(Issue 5) pp:1604-1614

Publication Date(Web):15 Mar 2013

DOI:10.1039/C3EE40375K

We report two triarylamine-cyanoacrylic acid based push–pull dyes C252 and C253 featuring the π-conjugated linkers of 2,6-di(thiophen-2-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole and 4H,4′H-2,2′-bidithieno[3,2-b:2′,3′-d]pyrrole, respectively. Benefitting from an improved coplanarity of the conjugated units, the C253 dye displays a red-shifted absorption peak and an enhanced maximum molar absorption coefficient in comparison with C252. However, this pattern of conjugated linker alternation is associated with an 80 mV negative shift of the ground-state oxidation potential, which dominates an almost 5 times reduced rate of hole injection from the oxidized state of C253 to the divalent tris(2,2′-bipyridine)cobalt (Co-bpy) cation in the redox electrolyte, resulting in a considerably poor net charge separation yield. On the other side, a dye-sensitized solar cell employing the C252 photosensitizer and the Co-bpy electrolyte exhibits a good power conversion efficiency of 9.5% measured under the 100 mW cm−2 simulated AM1.5 sunlight. The dissimilarity of cell photovoltage is scrutinized by evaluating the shift of the titania conduction band edge and the variation of interfacial charge recombination kinetics, the latter of which presents a clear correlation with dye coating thickness on titania derived from X-ray photoelectron spectroscopy measurements. Our work has underlined the important energetic and kinetic interplays which should be seriously considered in the further optimization of active components in dye-sensitized solar cells.

N-Annulated Perylene as An Efficient Electron Donor for Porphyrin-Based Dyes: Enhanced Light-Harvesting Ability and High-Efficiency Co(II/III)-Based Dye-Sensitized Solar Cells

Co-reporter:Jie Luo ; Mingfei Xu ; Renzhi Li ; Kuo-Wei Huang ; Changyun Jiang ; Qingbiao Qi ; Wangdong Zeng ; Jie Zhang ; Chunyan Chi ; Peng Wang ;Jishan Wu

Journal of the American Chemical Society 2013 Volume 136(Issue 1) pp:265-272

Publication Date(Web):December 17, 2013

DOI:10.1021/ja409291g

Porphyrin-based dyes recently have become good candidates for dye-sensitized solar cells (DSCs). However, the bottleneck is how to further improve their light-harvesting ability. In this work, N-annulated perylene (NP) was used to functionalize the Zn-porphyrin, and four “push–pull”-type NP-substituted and fused porphyrin dyes with intense absorption in the visible and even in the near-infrared (NIR) region were synthesized. Co(II/III)-based DSC device characterizations revealed that dyes WW-5 and WW-6, in which an ethynylene spacer is incorporated between the NP and porphyrin core, showed pantochromatic photon-to-current conversion efficiency action spectra in the visible and NIR region, with a further red-shift of about 90 and 60 nm, respectively, compared to the benchmark molecule YD2-o-C8. As a result, the short-circuit current density was largely increased, and the devices displayed power conversion efficiencies as high as 10.3% and 10.5%, respectively, which is comparable to that of the YD2-o-C8 cell (η = 10.5%) under the same conditions. On the other hand, the dye WW-3 in which the NP unit is directly attached to the porphyrin core showed a moderate power conversion efficiency (η = 5.6%) due to the inefficient π-conjugation, and the NP-fused dye WW-4 exhibited even poorer performance due to its low-lying LUMO energy level and nondisjointed HOMO/LUMO profile. Our detailed physical measurements (optical and electrochemical), density functional theory calculations, and photovoltaic characterizations disclosed that the energy level alignment, the molecular orbital profile, and dye aggregation all played very important roles on the interface electron transfer and charge recombination kinetics.

Improving the Photovoltage of Dithienopyrrole Dye-Sensitized Solar Cells via Attaching the Bulky Bis(octyloxy)biphenyl Moiety to the Conjugated -Linker

Co-reporter:Ning Cai;Jing Zhang;Mingfei Xu;Min Zhang

Advanced Functional Materials 2013 Volume 23( Issue 28) pp:3539-3547

Publication Date(Web):

DOI:10.1002/adfm.201203348

Abstract

The judicious design of 3D giant organic dye molecules to enable the formation of a porous photoactive layer on the surface of titania is one of the viable tactics to abate the adverse interfacial charge recombination in dye-sensitized solar cells (DSCs) employing outer-sphere redox couples. Here 2′,6′-bis(octyloxy)-biphenyl substituted dithieno[3,2-b:2′,3′-d]pyrrole segment is constructed and employed as the π-linker of a high molar absorption coefficient organic push-pull dye. With respect to its congener possessing the hexyl substituted dithieno[3,2-b:2′,3′-d]pyrrole linker, the new dye can self-assemble on the surface of titania to afford a porous organic coating, which effectively slow down the kinetics of charge recombination of titania electrons with both outer-sphere tris(1,10-phenanthroline)cobalt(III) ions and photooxidized dye molecules, improving the cell photovoltage. In addition, the diminishments of charge recombination via modulating the microstructure of interfacial functional zone can also overcompensate the disadvantageous impact of reduced light-harvesting and evoke an enhanced photocurrent output, bringing forth an efficiency improvement from 7.5% to 9.3% at the 100 mW cm⁻², simulated AM1.5 conditions.

Engineering of Push-Pull Thiophene Dyes to Enhance Light Absorption and Modulate Charge Recombination in Mesoscopic Solar Cells

Co-reporter:Ning Cai;Yinglin Wang;Mingfei Xu;Ye Fan;Renzhi Li;Min Zhang

Advanced Functional Materials 2013 Volume 23( Issue 14) pp:1846-1854

Publication Date(Web):

DOI:10.1002/adfm.201202562

Abstract

The elaborate selection of diverse π-conjugated segments which bridge the electron donors and acceptors in organic push-pull dyes can not only tune the molecular energy-levels but also impact the interfacial energetics and kinetics of dye-sensitized solar cells (DSCs). In this paper, a series of triphenylamine-cyanoacrylic acid photosensitizers is reported with TT, EDOT-BT, EDOT-CPDT, and CPDT-EDOT (herein TT, EDOT, BT, and CPDT denote terthiophene, ethylenedioxythiophene, bithiophene, and cyclopentadithiophene, respectively) as the π-linkers, and the dye-structure correlated photocurrent and photovoltage features of DSCs based on a cobalt electrolyte are scrutinized via analyzing light absorption and multichannel charge transfer kinetics. Both stepwise incorporation of more electron-rich blocks and rational modulation of connection order of dissimilar segments can result in a negative movement of ground-state redox potential and a red-shift of the absorption peak. While these styles of reducing energy-gap do not exert too much influence on the electron injection from photoexcited dye molecules to titania, the dyestuff employing the EDOT-BT linker presents a faster interfacial charge recombination and a slower dye regeneration, accounting for its inferior cell efficiency of 5.3% compared to that of 9.4% at the AM1.5G conditions achieved by the CPDT-EDOT dye.

Joint electrical, photophysical and computational studies on D-π-A dye sensitized solar cells: the impacts of dithiophene rigidification

Co-reporter:Mingfei Xu, Min Zhang, Mariachiara Pastore, Renzhi Li, Filippo De Angelis and Peng Wang

Chemical Science 2012 vol. 3(Issue 4) pp:976-983

Publication Date(Web):20 Dec 2011

DOI:10.1039/C2SC00973K

The rigidification of π-conjugated segments represents a feasible tactic towards energy-level engineering of organic D-π-A dyes in mesoscopic titania solar cells. In this work, comparions of four dyes with the di(3-hexylthiophene), dihexyldithienosilole, dihexylcyclopentadithiophene and N-hexyldithienopyrrole linkers have revealed some general influences of π-linker rigidification on the optoelectronic features of titania solar cells employing a cobalt(II/III) redox electrolyte, in terms of energetic and kinetic viewpoints. Compared to a dye with the di(3-hexylthiophene) linker, its three counterparts with rigidified dithiophene blocks present bathochromic and hyperchromic absorptions of solar photons. Transient absorption measurements have shown that the incorporation of Si-, C- and N-bridged dithiophene segments decelerates the dye regeneration kinetics. The rigidification of π-conjugated dithiophene linkers brings forth a general open-circuit photovoltage diminishment, in the range from 60 to 190 mV. Further insightful impedance analyses have disclosed that the open-circuit photovoltage reduction, due to the π-linker alternation from di(3-hexylthiophene) to N-hexyldithienopyrrole, is predominantly caused by an adverse downward displacement of the titania conduction band edge, despite a positive contribution from attenuated charge recombination at the titania/electrolyte interface.

Modulating the assembly of organic dye molecules on titania nanocrystals via alkyl chain elongation for efficient mesoscopic cobalt solar cells

Co-reporter:Yiming Cao, Ning Cai, Yinglin Wang, Renzhi Li, Yi Yuan and Peng Wang

Physical Chemistry Chemical Physics 2012 vol. 14(Issue 23) pp:8282-8286

Publication Date(Web):03 May 2012

DOI:10.1039/C2CP41314K

Through elongating the end or side alkyl chains of dye molecules, we decorate anatase nanocrystals with a thicker organic assembly featuring a smaller tilt angle of the D–π-A backbone with respect to the surface normal, which retards the interfacial charge recombination and confers a higher photovoltage output on mesoscopic cobalt solar cells displaying an over 10% power conversion efficiency at the AM1.5G conditions.

Electrical and photophysical analyses on the impacts of arylamine electron donors in cyclopentadithiophene dye-sensitized solar cells

Co-reporter:Mingfei Xu, Difei Zhou, Ning Cai, Jingyuan Liu, Renzhi Li and Peng Wang

Energy & Environmental Science 2011 vol. 4(Issue 11) pp:4735-4742

Publication Date(Web):06 Oct 2011

DOI:10.1039/C1EE02432A

The electron donor of a D-π-A dye is known for its capability to tune both the electronic trait and packing mode of dye molecules chemisorbed on titania nanocrystals of dye-sensitized solar cells (DSCs), bringing on the opportunity to impact cell performance by modulating the physicochemical characteristics at the titania/dye/electrolyte interface. In this paper, we scrutinize the influences of arylamine electron donors on the optoelectronic features of thin-film DSCs employing a tris(1,10-phenanthroline)cobalt(II/III) redox electrolyte, by use of four cyclopentadithiophene dyes (C218, C244, C245 and C246) with the respective dihexyloxy-, diphenothiazinyl- or di-tert-butylphenyl-substituted triphenylamine and N-hexyl-carbazole electron donors. Amongst these electron donors, dihexyloxy-substituted triphenylamine is found to present the strongest electron-donating capacity, endowing the corresponding C218 dye with evidently red-shifted light absorption in comparison with the other three congeners. Transient absorption measurements show that all DSCs exhibit expeditious dye regeneration, guaranteeing efficient long-distance charge separation at the titania/dye/redox couple interface. Furthermore, it is worthwhile noting that the C218 dye prompts the highest open-circuit photovoltage amongst these chromophores, which is primarily attributed to the positive effect of slow cobalt(III) interception of titania electrons, highlighting the superiority of applying dihexyloxy-substituted triphenylamine as the electron donor for a cyclopentadithiophene dye.

The structure–property relationship of organic dyes in mesoscopic titania solar cells: only one double-bond difference

Co-reporter:Jingyuan Liu, Difei Zhou, Mingfei Xu, Xiaoyan Jing and Peng Wang

Energy & Environmental Science 2011 vol. 4(Issue 9) pp:3545-3551

Publication Date(Web):21 Jul 2011

DOI:10.1039/C1EE01849C

A photosensitizer plays a crucial role in the conversion of solar light to electricity in dye-sensitized solar cells (DSCs). Thereby an all-sided apprehension on its structure–property relationship should to a large extent expedite rational material design. In this paper, we investigate some general impacts of one double-bond variation upon the optoelectronic features of three pairs of cyanoacrylic acid dyes. One double-bond insertion between a triphenylamine electron donor and π-conjugated heterocycles such as thiophene, 3,4-ethylenedioxythiophene and 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b]dithiophene endows a bathochromic effect in terms of electronic absorption of a dye-coated titania film, but leads to a relatively lower electron injection yield as indicated by transient emission measurements. Moreover, this style of one more double-bond incorporation brings forth a lower open-circuit photovoltage, owing to the occurrences of a downwards shifted titania conduction band edge and an augmented rate constant of charge recombination at the titania/electrolyte interface as revealed by electrical impedance analyses.

Mesoscopic titania solar cells with the tris(1,10-phenanthroline)cobalt redox shuttle: uniped versus biped organic dyes

Co-reporter:Jingyuan Liu, Jing Zhang, Mingfei Xu, Difei Zhou, Xiaoyan Jing and Peng Wang

Energy & Environmental Science 2011 vol. 4(Issue 8) pp:3021-3029

Publication Date(Web):14 Jun 2011

DOI:10.1039/C1EE01633D

The combination of some non-iodine one-electron outer-sphere redox shuttles with high-absorption-coefficient organic D–π–A dyes has very recently brought forth a new opportunity towards efficiency enhancement of dye-sensitized solar cells (DSCs). Thereby a comprehensive understanding on the structure–property relationship of metal-free organic photosensitizers will play a pivotal role in further development of high-efficiency devices iodine-free. Herein, we scrutinize some general influences of the cyanoacrylic-acid-anchoring-group number of organic dyes upon the optoelectronic features of mesoscopic titania solar cells based on the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle, by employing three pairs of uniped and biped dyes with thiophene, 3,4-ethylenedioxythiophene and 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene as the π-conjugated spacers. It is found that regardless of the spacer selection, a biped dye confers an improved light absorption coefficient of a stained titania film in comparison with its uniped analogue, which is highly desirable for cells with a one-electron outer-sphere redox shuttle, because in this type of cells a very thin titania film is always needed to satisfy the mass transport and electron collection requirements. However, our electrical impedance analyses have revealed that the alteration from a uniped chromophore to its biped congener evokes not only a downward displacement of the titania conduction band edge but also an acceleration of interfacial charge recombination of the titania electrons with cobalt(III) ions, leading to an attenuated open-circuit photovoltage, highlighting the future necessity to design a sterically bulky organic dye featuring superior light-harvesting capacity.

Efficient organic dye-sensitized thin-film solar cells based on the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle

Co-reporter:Difei Zhou, Qingjiang Yu, Ning Cai, Yu Bai, Yinghui Wang and Peng Wang

Energy & Environmental Science 2011 vol. 4(Issue 6) pp:2030-2034

Publication Date(Web):20 Apr 2011

DOI:10.1039/C0EE00735H

We deploy the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle in conjunction with the high-absorption-coefficient organic photosensitizer C218 to fabricate an iodine-free dye-sensitized solar cell displaying an unprecedented 8.3% efficiency at an irradiation of AM1.5 full sunlight.

An Organic D-π-A Dye for Record Efficiency Solid-State Sensitized Heterojunction Solar Cells

Co-reporter:Ning Cai, Soo-Jin Moon, Lê Cevey-Ha, Thomas Moehl, Robin Humphry-Baker, Peng Wang, Shaik M. Zakeeruddin, and Michael Grätzel

Nano Letters 2011 Volume 11(Issue 4) pp:1452-1456

Publication Date(Web):March 4, 2011

DOI:10.1021/nl104034e

The high molar absorption coefficient organic D-π-A dye C220 exhibits more than 6% certified electric power conversion efficiency at AM 1.5G solar irradiation (100 mW cm−2) in a solid-state dye-sensitized solar cell using 2,2′,7,7′-tetrakis(N,N-dimethoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. This contributes to a new record (6.08% by NREL) for this type of sensitized heterojunction photovoltaic device. Efficient charge generation is proved by incident photon-to-current conversion efficiency spectra. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.

Engineering Organic Sensitizers for Iodine-Free Dye-Sensitized Solar Cells: Red-Shifted Current Response Concomitant with Attenuated Charge Recombination

Co-reporter:Yu Bai ; Jing Zhang ; Difei Zhou ; Yinghui Wang ; Min Zhang

Journal of the American Chemical Society 2011 Volume 133(Issue 30) pp:11442-11445

Publication Date(Web):July 7, 2011

DOI:10.1021/ja203708k

With a new metal-free donor–acceptor photosensitizer featuring the 2,6-bis(thiophen-2-yl)-4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-conjugated spacer and the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle, we present a highly efficient iodine-free dye-sensitized solar cell displaying a power conversion efficiency of 9.4% measured at 100 mW cm–2 simulated AM1.5 conditions.

Co-reporter:Min Zhang, Jingyuan Liu, Yinghui Wang, Difei Zhou and Peng Wang

Chemical Science 2011 vol. 2(Issue 7) pp:1401-1406

Publication Date(Web):17 May 2011

DOI:10.1039/C1SC00199J

The iodide/triiodide redox shuttle is an unparalleled option in making efficient dye-sensitized solar cells (DSCs) but also presents some prominent constraints on the development of new photosensitizers, semiconductors and counter electrodes. In this paper, a cobalt electrolyte has been identified to offer an advantageous impact of π-conjugation extension in push–pull organic dyes, upon the interfacial charge recombination kinetics and thus the open-circuit photovoltage of DSCs, in sharp contrast with the conventional iodine congener. The usage of a solely visible-light-absorption dye has generated a DSC free of corrosive iodine, exhibiting an impressive overall power conversion efficiency of 8.0% at the 100 mW cm−2 AM1.5G conditions.

Perylene Anhydride Fused Porphyrins as Near-Infrared Sensitizers for Dye-Sensitized Solar Cells

Co-reporter:Chongjun Jiao, Ningning Zu, Kuo-Wei Huang, Peng Wang, and Jishan Wu

Organic Letters 2011 Volume 13(Issue 14) pp:3652-3655

Publication Date(Web):June 23, 2011

DOI:10.1021/ol201303h

Two perylene anhydride fused porphyrins 1 and 2 have been synthesized and employed successfully in dye-sensitized solar cells (DSCs). Both compounds showed broad incident monochromatic photon-to-current conversion efficiency spectra covering the entire visible spectral region and even extending into the near-infrared (NIR) region up to 1000 nm, which is impressive for ruthenium-free dyes in DSCs.

High-efficiency organic dye-sensitized mesoscopic solar cells with a copper redox shuttle

Co-reporter:Yu Bai, Qingjiang Yu, Ning Cai, Yinghui Wang, Min Zhang and Peng Wang

Chemical Communications 2011 vol. 47(Issue 15) pp:4376-4378

Publication Date(Web):14 Mar 2011

DOI:10.1039/C1CC10454C

Based upon the bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) electron mediator, we present an iodine-free dye-sensitized solar cell exhibiting an impressive power conversion efficiency of 7.0% at 100 mW cm−2 air mass global (AM1.5G) conditions, which rivals that of a control cell with a conventional iodine redox couple.

Co-reporter:Min Zhang, Jing Zhang, Yu Bai, Yinghui Wang, Mei Su and Peng Wang

Physical Chemistry Chemical Physics 2011 vol. 13(Issue 9) pp:3788-3794

Publication Date(Web):24 Dec 2010

DOI:10.1039/C0CP02728F

In solvent-free ionic liquid electrolytes for efficient dye-sensitized solar cells, iodide and non-iodide melts are ordinarily blended in order to attain a high dynamic fluidity and thereby meet a fast mass transport of electroactive species. This common tactic could bring forth a prominent impact of various anions on cell efficiency by altering photocurrent and/or photovoltage. Herein we report evident effects of the dicyanamideversustetracyanoborate anion on the energetics of titania conduction band edge and the kinetics of multi-channel charge-transfer reactions in cells employing a high absorption coefficient ruthenium sensitizer C106. A slightly shorter photoluminescence lifetime of C106 grafted on alumina is probed for the tetracyanoborate-based reference cell with respect to the dicyanamide counterpart. However, owing to a more favourable thermodynamic driving force of ∼90 meV, the tetracyanoborate anion prompts an almost 3-fold faster electron injection from the excited-state dye to titania than dicyanamide, leading to a higher charge separation yield, which is in good agreement with an almost indistinguishable ratio of external quantum efficiency enhancement in the whole spectral response region. Compared to tetracyanoborate, the presence of dicyanamide at the titania/electrolyte interface evokes a 27-fold smaller interfacial electron exchange rate (K) with triiodide, accounting for the open-circuit photovoltage variation observed in current–voltage measurements.

Lithium-Modulated Conduction Band Edge Shifts and Charge-Transfer Dynamics in Dye-Sensitized Solar Cells Based on a Dicyanamide Ionic Liquid

Co-reporter:Yu Bai, Jing Zhang, Yinghui Wang, Min Zhang, and Peng Wang

Langmuir 2011 Volume 27(Issue 8) pp:4749-4755

Publication Date(Web):March 25, 2011

DOI:10.1021/la200156m

Lithium ions are known for their potent function in modulating the energy alignment at the oxide semiconductor/dye/electrolyte interface in dye-sensitized solar cells (DSCs), offering the opportunity to control the associated multichannel charge-transfer dynamics. Herein, by optimizing the lithium iodide content in 1-ethyl-3-methylimidazolium dicyanamide-based ionic liquid electrolytes, we present a solvent-free DSC displaying an impressive 8.4% efficiency at 100 mW cm−2 AM1.5G conditions. We further scrutinize the origins of evident impacts of lithium ions upon current density−voltage characteristics as well as photocurrent action spectra of DSCs based thereon. It is found that, along with a gradual increase of the lithium content in ionic liquid electrolytes, a consecutive diminishment of the open-circuit photovoltage arises, primarily owing to a noticeable downward movement of the titania conduction band edge. The conduction band edge displacement away from vacuum also assists the formation of a more favorable energy offset at the titania/dye interface, and thereby leads to a faster electron injection rate and a higher exciton dissociation yield as implied by transient emission measurements. We also notice that the adverse influence of the titania conduction band edge downward shift arising from lithium addition upon photovoltage is partly compensated by a concomitant suppression of the triiodide involving interfacial charge recombination.

Anion Effects in Organic Dye-Sensitized Mesoscopic Solar Cells with Ionic Liquid Electrolytes: Tetracyanoborate vs Dicyanamide

Co-reporter:Difei Zhou ; Yu Bai ; Jing Zhang ; Ning Cai ; Mei Su ; Yinghui Wang ; Min Zhang

The Journal of Physical Chemistry C 2011 Volume 115(Issue 3) pp:816-822

Publication Date(Web):December 15, 2010

DOI:10.1021/jp109803n

We compare the impacts of tetracyanoborate and dicyanamide anions in solvent-free ionic liquid electrolytes upon the optoelectronic features of dye-sensitized solar cells based on an organic push−pull chromophore. With respect to dicyanamide, tetracyanoborate confers a bathochromic and higher photocurrent response upon a dye-coated mesoporous titania film. Numerical simulations on electrical impedance spectroscopies disclose that tetracyanoborate anions evoke a downward displacement of the conduction-band edge of an electrolyte-sinking nanocrystalline film in comparison with dicyanamide, generating a more favorable energy-offset at the titania/dye interface and therefore a better exciton dissociation yield, as proved via the transient emission measurements. An effective recombination reaction rate constant U0k is used to depict the charge-transfer behavior at the titania/electrolyte interface in dye-sensitized solar cells. The negative impact of an about 4 times larger U0k of the dicyanamide cell is overwhelmed by the noticeably higher conduction band edge, leading to the experimentally observed anion-dependent open-circuit photovoltage.

An Energetic and Kinetic View on Cyclopentadithiophene Dye-Sensitized Solar Cells: The Influence of Fluorine vs Ethyl Substituent

Co-reporter:Difei Zhou ; Ning Cai ; Huijin Long ; Min Zhang ; Yinghui Wang

The Journal of Physical Chemistry C 2011 Volume 115(Issue 7) pp:3163-3171

Publication Date(Web):February 3, 2011

DOI:10.1021/jp110384n

A comprehensive understanding on the structure−property relationship of metal-free organic chromophores in dye-sensitized solar cells will form the basis of further design of advanced materials and realization of considerable enhancement of device performance, boosting the widespread utilization of solar energy at an affordable cost. In this paper we report profound influences of the fluorine versus ethyl substituent of the cyclopentadithiophene conjugated spacer of push−pull photosensitizers, upon the energetic and kinetic characteristics of dye-sensitized solar cells. Joint electrochemical and spectral measurements reveal that with respect to the fluorine-containing dye, the ethyl counterpart exhibits a 0.18 V more negative excited-state redox potential upon anchoring on titania, which overwhelms the 0.01 eV upward conduction band edge shift of the dye-coated titania film. The ethyl-correlated more favorable dye/titania interface energetics benefits a remarkably higher electron injection yield, which is verified with the transient emission and photocurrent action spectrum measurements. Furthermore, analysis on the electrical impedance data affords a notably smaller recombination reaction order for the cell made from the ethyl-substituting chromophore than that of the fluorine counterpart, accounting for a remarkably suppressed interfacial charge recombination with triiodide and thus explaining the observed higher open-circuit photovoltage.

Joint Photophysical and Electrical Analyses on the Influence of Conjugation Order in D-π-A Photosensitizers of Mesoscopic Titania Solar Cells

Co-reporter:Jingyuan Liu ; Difei Zhou ; Fangfang Wang ; Francisco Fabregat-Santiago ; Sara G. Miralles ; Xiaoyan Jing ; Juan Bisquert

The Journal of Physical Chemistry C 2011 Volume 115(Issue 29) pp:14425-14430

Publication Date(Web):July 6, 2011

DOI:10.1021/jp203313b

Diverse thiophene-containing blocks have been employed as the π-conjugated spacers of organic D-π-A dyes. In the case that multiple segments with distinguishable electronic features are applied, their conjugation sequence could potently affect optoelectric behaviors of photosensitizers in mesoscopic titania solar cells. In this work, we address this issue by designing three organic dyes (C225, C226, and C227), wherein the dihexyl-substituted cyclopentadithiophene moiety is stepwise shifted from the electron acceptor side to the donor one, along with the additional use of two 3-hexylthiophene rings as the conjugated spacing unit. With respect to C225 and C226, C227 presents a relatively inefficient photoinduced electron injection as indicated by photoluminescence measurements, which accounts for its lower efficiencies of converting incident monochromatic photons to collected electrons. Transient absorption measurements suggest that the charge recombination between oxidized dye molecules and titania electrons gradually decelerates from C225 to C227, while the interception of oxidized dye molecules by iodide ions exhibits an apparent driving force dependent, Marcus normal region behavior.

Probing Dye-Correlated Interplay of Energetics and Kinetics in Mesoscopic Titania Solar Cells with 4-tert-Butylpyridine

Co-reporter:Huijin Long ; Difei Zhou ; Min Zhang ; Chuyao Peng ; Satoshi Uchida

The Journal of Physical Chemistry C 2011 Volume 115(Issue 29) pp:14408-14414

Publication Date(Web):June 21, 2011

DOI:10.1021/jp202826m

The adsorption of 4-tert-butylpyridine (TBP) on the surface of oxide semiconductors can tune their interband state distributions as well as the conduction band edges, offering a straightforward tactic to modulate the energy alignments at the titania/dye/electrolyte interface in dye-sensitized solar cells. In this work, by stepwise augmenting the TBP concentration in electrolytes, we investigate the energetic and kinetic interplays in mesoscopic titania solar cells based upon two push–pull organic dyes C218 and D205. TBP at a higher concentration, while better retarding triiodide interception of photoinjected electrons in C218 cells, has strikingly induced an acceleration of interfacial charge recombination with triiodide in devices employing the D205 dye, accounting for a much smaller photovoltage enhancement with respect to the C218 counterpart. Transient emission measurements disclose that the exciton dissociation at the titania/C218 dye interface does not noticeably vary despite a TBP-correlated conduction band edge elevation, which has otherwise prompted an evident Gibbs free-energy-dependent rate of electron injection in cells based on the D205 photosensitizer.

Stable and efficient dye-sensitized solar cells: photophysical and electrical characterizations

Co-reporter:Qingjiang Yu, Difei Zhou, Yushuai Shi, Xiaoying Si, Yinghui Wang and Peng Wang

Energy & Environmental Science 2010 vol. 3(Issue 11) pp:1722-1725

Publication Date(Web):09 Sep 2010

DOI:10.1039/C0EE00264J

We investigate the lithium cation effects in high-performance dye-sensitized solar cells, which exhibit ∼10% efficiency and excellent thermal and light-soaking stability during 1000-h accelerated aging tests.

Oligothiophene dye-sensitized solar cells

Co-reporter:Jingyuan Liu, Renzhi Li, Xiaoying Si, Difei Zhou, Yushuai Shi, Yinghui Wang, Xiaoyan Jing and Peng Wang

Energy & Environmental Science 2010 vol. 3(Issue 12) pp:1924-1928

Publication Date(Web):20 Oct 2010

DOI:10.1039/C0EE00304B

We scrutinize the energetic and kinetic interplays in oligothiophene dye-sensitized solar cells via analyzing electrical impedance as well as transient emission and absorption spectroscopies.

Influence of the electrolyte cation in organic dye-sensitized solar cells: lithium versus dimethylimidazolium

Co-reporter:Renzhi Li, Daxing Liu, Difei Zhou, Yushuai Shi, Yinghui Wang and Peng Wang

Energy & Environmental Science 2010 vol. 3(Issue 11) pp:1765-1772

Publication Date(Web):02 Sep 2010

DOI:10.1039/C0EE00223B

We investigate the influence of electrolyte cations such as lithium and dimethylimidazolium on the spectroscopic and electrical characteristics of dye-sensitized solar cells based on a metal-free chromophore C218. An evident bathochromic effect of lithium with respect to dimethylimidazolium is noticed for the C218 dye-coated nanocrystalline titania film via measuring electronic absorption and photocurrent action spectra. In comparison with dimethylimidazolium, the use of lithium as the electrolyte cation evokes a downward shift of the excited-state redox potential of the C218 sensitizer by 120 mV, and that of the conduction band edge of the nanocrystalline TiO2 film by 390 meV. The resultant remarkable variation in the energy alignment at the titania/dye interface brings on dissimilar electron injection yields as revealed by transient emission measurements. Modulating the titania surface states distribution with lithium relative to dimethylimidazolium induces a slightly declining electron diffusion coefficient in the mesoporous titania film. However, the adsorption of lithium cations on titania strongly retards the interfacial charge recombination compared to dimethylimidazolium, contributing to an over one order of magnitude of enhancement of electron diffusion length. The improvement of electron diffusion length has a noticeable effect on the charge collection yield, which can also be observed by measuring monochromatic incident photon-to-electron conversion efficiencies. Further interface engineering is needed to enhance both the photovoltage and photocurrent, making use of the full power of metal-free organic dyes.

Enhanced-Light-Harvesting Amphiphilic Ruthenium Dye for Efficient Solid-State Dye-Sensitized Solar Cells

Co-reporter:Mingkui Wang;Soo-Jin Moon;Difei Zhou;Florian Le Formal;Ngoc-Lê Cevey-Ha;Robin Humphry-Baker;Carole Grätzel;Shaik M. Zakeeruddin;Michael Grätzel

Advanced Functional Materials 2010 Volume 20( Issue 11) pp:1821-1826

Publication Date(Web):

DOI:10.1002/adfm.200902396

Abstract

A ruthenium sensitizer (coded C101, NaRu (4,4′-bis(5-hexylthiophen-2-yl)-2,2′-bipyridine) (4-carboxylic acid-4′-caboxylate-2,2′-bipyridine) (NCS)₂) containing a hexylthiophene-conjugated bipyridyl group as an ancillary ligand is presented for use in solid-state dye-sensitized solar cells (SSDSCs). The high molar-extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4-carboxylic acid-4′-carboxylate) (4,4′-dinonyl-2,2′-bipyridine) (NCS)₂). In combination with an organic hole-transporting material (spiro-MeOTAD, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine) 9, 9′-spirobifluorene), the C101 sensitizer exhibits an excellent power-conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm⁻²) irradiation in a SSDSC. From electronic-absorption, transient-photovoltage-decay, and impedance measurements it is inferred that extending the π-conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.

Efficient Dye-Sensitized Solar Cells with an Organic Photosensitizer Featuring Orderly Conjugated Ethylenedioxythiophene and Dithienosilole Blocks

Co-reporter:Wangdong Zeng, Yiming Cao, Yu Bai, Yinghui Wang, Yushuai Shi, Min Zhang, Fangfang Wang, Chunyue Pan and Peng Wang

Chemistry of Materials 2010 Volume 22(Issue 5) pp:1915

Publication Date(Web):January 22, 2010

DOI:10.1021/cm9036988

In view of the limited ruthenium resource, metal-free organic dyes may play a prominent role in the coming large-scale application of cost-effective dye-sensitized solar cells, if their efficiency and stability can be considerably improved. In this paper we utilized a binary π-conjugated spacer of ethylenedioxythiophene and dithienosilole to construct a high molar absorption coefficient push−pull dye, characteristic of an intramolecular charge-transfer band peaking at 584 nm measured in chloroform. In comparison with the standard ruthenium sensitizer Z907, this metal-free chromophore C219 endowed a nanocrystalline titania film with an evident light-harvesting enhancement, leading to an unprecedented 10.0−10.3% efficiency at the AM1.5G conditions for dye-sensitized solar cells with nonruthenium dyestuffs, although a highly volatile electrolyte was used. Transient absorption measurements have revealed that even if the kinetics of back-electron transfer and dye regeneration are considerably different for Z907 and C219, the branching ratios of these two charge-transfer channels are over 35 for both dyes, ensuring a high yield of net charge separation at the titania/dye/electrolyte interface. A solvent-free ionic liquid cell with C219 as the sensitizer exhibited an impressive efficiency of 8.9% under a low light intensity of 14.39 mW cm−2, making it very favorable for the indoor application of flexible dye-sensitized solar cells.

Synchronously Reduced Surface States, Charge Recombination, and Light Absorption Length for High-Performance Organic Dye-Sensitized Solar Cells

Co-reporter:Renzhi Li, Jingyuan Liu, Ning Cai, Min Zhang and Peng Wang

The Journal of Physical Chemistry B 2010 Volume 114(Issue 13) pp:4461-4464

Publication Date(Web):March 17, 2010

DOI:10.1021/jp101222s

We employ the 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) segment as a conjugated spacer to construct an extremely high-molar-absorption-coefficient organic chromophore for dye-sensitized solar cells, exhibiting a high power conversion efficiency of 8.95% measured under irradiation of 100 mW cm−2 AM1.5G sunlight. Our comparative experiments have proved the prominent merit of employing CPDT instead of the prevailing 2,2′-dithiophene (DT) as the building block for the further dye design. We also have demonstrated that a controllable coassembling of dye molecules and electrolyte components on semiconducting nanocrystals can reduce surface states and inhibit charge recombination synchronously.

High efficiency solid-state sensitized heterojunction photovoltaic device

Co-reporter:Mingkui Wang, Jingyuan Liu, Ngoc-Le Cevey-Ha, Soo-Jin Moon, Paul Liska, Robin Humphry-Baker, Jacques-E. Moser, Carole Grätzel, Peng Wang, Shaik M. Zakeeruddin, Michael Grätzel

Nano Today 2010 Volume 5(Issue 3) pp:169-174

Publication Date(Web):June 2010

DOI:10.1016/j.nantod.2010.04.001

The high molar extinction coefficient heteroleptic ruthenium dye, NaRu(4,4′-bis(5-(hexylthio)thiophen-2-yl)-2,2′-bipyridine) (4-carboxylic acid-4′-carboxylate-2,2′-bipyridine) (NCS)2, exhibits certified 5% electric power conversion efficiency at AM 1.5 solar irradiation (100 mW cm−2) in a solid-state dye-sensitized solar cell using 2,2′,7,7′-tetrakis-(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. This demonstration elucidates a class of photovoltaic devices with potential for low-cost power generation.

High-Efficiency Dye-Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States

Co-reporter:Qingjiang Yu, Yinghui Wang, Zhihui Yi, Ningning Zu, Jing Zhang, Min Zhang, and Peng Wang

ACS Nano 2010 Volume 4(Issue 10) pp:6032

Publication Date(Web):October 5, 2010

DOI:10.1021/nn101384e

We present a time-saving staining protocol based upon the low-toxicity and nonvolatile solvent dimethyl sulfoxide for a high-efficiency dye-sensitized solar cell, which is very critical for the roll-to-roll flexible cell production. Power conversion efficiencies of 11.7−12.1% were achieved under AM1.5G simulated sunlights. The intrinsic roles of lithium cations on efficiency enhancement were scrutinized by measuring transient absorption and electrical impedance spectroscopies. Our studies have revealed that lithium ions can enhance exciton dissociation at the energy-offset dye/titania interface effectively. Charge collection is not found to be a crucial current loss channel in our high-efficiency dye-sensitized solar cell.Keywords: charge recombination; exciton dissociation; impedance spectroscopy; solar cells; surface states; transient absorption spectroscopy

Employ a bisthienothiophene linker to construct an organic chromophore for efficient and stable dye-sensitized solar cells

Co-reporter:Guangliang Zhang, Yu Bai, Renzhi Li, Dong Shi, Sophie Wenger, Shaik M. Zakeeruddin, Michael Grätzel and Peng Wang

Energy & Environmental Science 2009 vol. 2(Issue 1) pp:92-95

Publication Date(Web):27 Nov 2008

DOI:10.1039/B817990E

We employed the bisthienothiophene conjugated linker along with a hydrophobic triphenylamine electron-donor and a hydrophilic cyanoacrylic acid electron-acceptor to construct a high molar extinction coefficient organic photosensitizer, exhibiting a power conversion efficiency of 8.0% measured under irradiation of air mass 1.5 global (AM 1.5G) full sunlight.

High efficiency and stable dye-sensitized solar cells with an organic chromophore featuring a binary π-conjugated spacer

Co-reporter:Guangliang Zhang, Hari Bala, Yueming Cheng, Dong Shi, Xueju Lv, Qingjiang Yu and Peng Wang

Chemical Communications 2009 (Issue 16) pp:2198-2200

Publication Date(Web):02 Mar 2009

DOI:10.1039/B822325D

We employed a binary spacer of orderly conjugated 3,4-ethyldioxythiophene and thienothiophene to construct a wide-spectral response organic chromophore for dye-sensitized solar cells, exhibiting a high power conversion efficiency of 9.8% measured under irradiation of 100 mW cm−2 air mass 1.5 global (AM1.5G) sunlight and an excellent stability.

Conjugation of Selenophene with Bipyridine for a High Molar Extinction Coefficient Sensitizer in Dye-Sensitized Solar Cells

Co-reporter:Feifei Gao, Yueming Cheng, Qingjiang Yu, Shi Liu, Dong Shi, Yunhui Li and Peng Wang

Inorganic Chemistry 2009 Volume 48(Issue 6) pp:2664-2669

Publication Date(Web):January 30, 2009

DOI:10.1021/ic802289e

A high molar extinction coefficient heteroleptic polypyridyl ruthenium sensitizer, featuring a conjugated electron-rich selenophene unit in its ancillary ligand, has been synthesized and demonstrated as an efficient sensitizer in dye-sensitized solar cells. A nanocrystalline titania film stained with this sensitizer shows improved optical absorptivity, which is highly desirable for dye-sensitized solar cells with a thin photoactive layer. With preliminary testing, this sensitizer has already achieved a high efficiency of 10.6% measured under the air mass 1.5 global conditions.

Dye-Sensitized Solar Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2-(Hexylthio)thiophene Conjugated Bipyridine

Co-reporter:Yiming Cao, Yu Bai, Qingjiang Yu, Yueming Cheng, Shi Liu, Dong Shi, Feifei Gao and Peng Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 15) pp:6290-6297

Publication Date(Web):2017-2-22

DOI:10.1021/jp9006872

We conjugated 2-(hexylthio)thiophene with bipyridine to construct a new heteroleptic polypyridyl ruthenium sensitizer exhibiting a charge-transfer band at 550 nm with a molar extinction coefficient of 18.7 × 103 M−1 cm−1. In contrast to its analogues Z907 and C101, a mesoporous titania film stained with this new sensitizer featured a short light absorption length, allowing for the use of a thin photoactive layer for efficient light-harvesting and conversion of solar energy to electricity. With a preliminary testing, we have reached 11.4% overall power conversion efficiency measured at the air mass 1.5 global conditions. Transient photoelectrical decays and electrical impedance spectra were analyzed to picture the intrinsic physics of temperature-dependent photovoltage and photocurrent.

Dye-Sensitized Solar Cells Based on Organic Sensitizers with Different Conjugated Linkers: Furan, Bifuran, Thiophene, Bithiophene, Selenophene, and Biselenophene

Co-reporter:Renzhi Li, Xueju Lv, Dong Shi, Difei Zhou, Yueming Cheng, Guangliang Zhang and Peng Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 17) pp:7469-7479

Publication Date(Web):2017-2-22

DOI:10.1021/jp900972v

Six organic dyes with different conjugated linkers such as furan, bifuran, thiophene, bithiophene, selenophene, and biselenophene have been prepared in combination with the dihexyloxy-substituted triphenylamine donor and the cyanoacrylic acid acceptor. In conjunction with an acetonitrile-based electrolyte and a solvent-free ionic liquid electrolyte, these dyes exhibit 6.88−7.77% and 6.39−7.00% efficiencies, respectively. We have demonstrated that furan and selenophene can be employed as building blocks of sensitizers in stable solar cells for the first time. We have also studied the influence of heteroatoms on photocurrents and photovoltages with the aid of quantum calculations and transient photoelectrical decay measurements. Temperature-dependent electrical impedance experiments have shown that a relatively low external quantum efficiency of the dye with biselenophene linker is not related to the charge collection yield in the case of an acetonitrile electrolyte.

An Extremely High Molar Extinction Coefficient Ruthenium Sensitizer in Dye-Sensitized Solar Cells: The Effects of π-Conjugation Extension

Co-reporter:Qingjiang Yu, Shi Liu, Min Zhang, Ning Cai, Yuan Wang and Peng Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 32) pp:14559-14566

Publication Date(Web):July 8, 2009

DOI:10.1021/jp904096g

We report a heteroleptic ruthenium complex (C107) featuring the electron-rich 5-octyl-2,2′-bis(3,4-ethylenedioxythiophene) moiety conjugated with 2,2′-bipyridine and exhibiting 10.7% power conversion efficiency measured at the AM1.5G conditions, thanks to the enhanced light-harvesting that is closely related to photocurrent. This C107 sensitizer has an extremely high molar extinction coefficient of 27.4 × 103 M−1 cm−1 at 559 nm in comparison to its analogue C103 (20.5 × 103 M−1 cm−1 at 550 nm) or Z907 (12.2 × 103 M−1 cm−1 at 521 nm) with the corresponding 5-hexyl-3,4-ethylenedioxythiophene- or nonyl-substituted bipyridyl unit. The augmentation of molar extinction coefficients and the bathochromic shift of low-energy absorption peaks along with the π-conjugation extension are detailed by TD-DFT calculations. The absorptivity of mesoporous titania films grafted with Z907, C103, or C107 sublinearly increases with the molar extinction coefficient of sensitizers, which is consistent with the finding derived from the surface coverage measurements that the packing density of those sensitizers decreases with the geometric enlargement of ancillary ligands. When the dye-coated titania film is immersed in a high-efficiency redox electrolyte, a lower density molecule grafting on titania nanocrystals leads to more deep electronic states and a faster charge recombination at the titania/dye/electrolyte interface at a given electron Fermi level, explaining the observation of a larger dark current and a lower open-circuit photovoltage. Electrical impedance analysis further reveals that the electron diffusion length in nanocrystalline titania films is shortened along with the π-conjugation extension of ancillary ligands.

N-Methyl-N-Allylpyrrolidinium Based Ionic Liquids for Solvent-Free Dye-Sensitized Solar Cells

Co-reporter:Ning Cai, Jing Zhang, Difei Zhou, Zhihui Yi, Jin Guo and Peng Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 10) pp:4215-4221

Publication Date(Web):2017-2-22

DOI:10.1021/jp810973q

We prepared four new ionic liquids consisting of N-methyl-N-allylpyrrolidinium cation in conjunction with anions including iodide, nitrate, thiocyanate, and dicyanamide, respectively, and measured their physical properties of density, viscosity, and conductivity. Owing to the relatively lower melting point of electroactive N-methyl-N-allylpyrrolidinium iodide, in combination with three other nonelectroactive ionic liquids, we could construct solvent-free electrolytes possessing high iodide concentrations for dye-sensitized solar cells. We correlated temperature-dependent electrolyte viscosity with molar conductivity and triiodide mobility through applying an empirical Walden’s rule and a modified Stokes−Einstein equation, respectively. We have further found that these anions (nitrate, thiocyanate, and dicyanamide) have different influences on surface states and electron transport in the mesoporous titania film, resulting in different photovoltages and photocurrents of dye-sensitized solar cells.

Tuning the Energy Level of Organic Sensitizers for High-Performance Dye-Sensitized Solar Cells

Co-reporter:Mingfei Xu, Sophie Wenger, Hari Bala, Dong Shi, Renzhi Li, Yanzhou Zhou, Shaik M. Zakeeruddin, Michael Grätzel and Peng Wang

The Journal of Physical Chemistry C 2009 Volume 113(Issue 7) pp:2966-2973

Publication Date(Web):2017-2-22

DOI:10.1021/jp809319x

Cost-effective organic sensitizers will play a pivotal role in the future large-scale production and application of dye-sensitized solar cells. Here we report two new organic D-π-A dyes featuring electron-rich 3,4-ethylenedioxythiophene- and 2,2′-bis(3,4-ethylenedioxythiophene)-conjugated linkers, showing a remarkable red-shifting of photocurrent action spectra compared with their thiophene and bithiophene counterparts. On the basis of the 3-{5′-[N,N-bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2′-bis(3,4-ethylenedioxythiophene)-5-yl}-2-cyanoacrylic acid dye, we have set a new efficiency record of 7.6% for solvent-free dye-sensitized solar cells based on metal-free organic sensitizers. Importantly, the cell exhibits an excellent stability, keeping over 92% of its initial efficiency after 1000 h accelerated tests under full sunlight soaking at 60 °C. This achievement will considerably encourage further design and exploration of metal-free organic dyes for higher performance dye-sensitized solar cells. We have also scrutinized the physical origins of the relatively low photocurrent and photovoltage obtained with an ionic liquid electrolyte compared to a volatile acetonitrile-based electrolyte through transient and modulated photoelectrical measurements.

High-Performance Liquid and Solid Dye-Sensitized Solar Cells Based on a Novel Metal-Free Organic Sensitizer

Co-reporter:Mingkui Wang;Mingfei Xu;Dong Shi;Renzhi Li;Feifei Gao;Guangliang Zhang;Zhihui Yi;Robin Humphry-Baker;Shaik M. Zakeeruddin;Michael Grätzel

Advanced Materials 2008 Volume 20( Issue 23) pp:4460-4463

Publication Date(Web):

DOI:10.1002/adma.200801178

A new heteroleptic ruthenium sensitizer enhances the absorptivity of mesoporous titania film for a high efficiency dye-sensitized solar cell

Co-reporter:Feifei Gao, Yuan Wang, Jing Zhang, Dong Shi, Mingkui Wang, Robin Humphry-Baker, Peng Wang, Shaik M. Zakeeruddin and Michael Grätzel

Chemical Communications 2008 (Issue 23) pp:2635-2637

Publication Date(Web):04 Apr 2008

DOI:10.1039/B802909A

A heteroleptic polypyridyl ruthenium complex, cis-Ru(4,4′-bis(5-octylthieno[3,2-b]thiophen-2-yl)-2,2′-bipyridine)(4,4′-dicarboxyl-2,2′-bipyridine)(NCS)2, with a high molar extinction coefficient of 20.5 × 103 M−1 cm−1 at 553 nm has been synthesized and demonstrated as a highly efficient sensitizer for a dye-sensitized solar cell, giving a power conversion efficiency of 10.53% measured under an irradiation of air mass 1.5 global (AM 1.5G) full sunlight.

Energy-Level and Molecular Engineering of Organic D-π-A Sensitizers in Dye-Sensitized Solar Cells

Co-reporter:Mingfei Xu, Renzhi Li, Nuttapol Pootrakulchote, Dong Shi, Jin Guo, Zhihui Yi, Shaik M. Zakeeruddin, Michael Grätzel and Peng Wang

The Journal of Physical Chemistry C 2008 Volume 112(Issue 49) pp:19770-19776

Publication Date(Web):November 12, 2008

DOI:10.1021/jp808275z

A series of organic D-π-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length.

Tetrahydrothiophenium-Based Ionic Liquids for High Efficiency Dye-Sensitized Solar Cells

Co-reporter:Chengcheng Xi ; Yiming Cao ; Yueming Cheng ; Mingkui Wang ; Xiaoyan Jing ; Shaik M. Zakeeruddin ; Michael Grätzel

The Journal of Physical Chemistry C 2008 Volume 112(Issue 29) pp:11063-11067

Publication Date(Web):June 26, 2008

DOI:10.1021/jp802798k

Binary melts of S-ethyltetrahydrothiophenium iodide and dicyanoamide (or tricyanomethide) have been employed for dye-sensitized solar cells with high power conversion efficiencies up to 6.9% under the illumination of AM 1.5G full sunlight. We have further shown that the transport of triiodide in ionic liquids with high iodide concentration is viscosity-dependent in terms of a physical diffusion coupled bond exchange mechanism apart from the simple physical diffusion. In addition, we have found that some anions of ionic liquid electrolytes such as dicyanoamide have a significant influence on surface states and electron transport in the mesoporous semiconducting film.

Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes

Co-reporter:Yiming Cao, Jing Zhang, Yu Bai, Renzhi Li, Shaik M. Zakeeruddin, Michael Grätzel and Peng Wang

The Journal of Physical Chemistry C 2008 Volume 112(Issue 35) pp:13775-13781

Publication Date(Web):2017-2-22

DOI:10.1021/jp805027v

We systematically studied the temperature-dependent physicochemical properties, such as density, conductivity, and fluidity, of 1,3-dialkylimidazolium iodides. In combination with the amphiphilic Z907Na sensitizer, we have found that it is important to use low-viscosity iodide melts with small cations to achieve high-efficiency dye-sensitized solar cells. By employing high-fluidity eutectic-based melts the device efficiencies considerably increased compared to those for cells with the corresponding state of the art ionic liquid electrolytes. We propose a modified Stokes−Einstein equation by correlating ion mobility and fluidity to quantitatively depict the triiodide transport in ionic liquid electrolytes. These studies reveal that the viscosity-dependent transport of triiodide in ionic liquid electrolytes with high iodide concentration can be explained by two parallel processes. Apart from the normal physical diffusion, the coupling process of physical diffusion and bond exchange is responsible for the observed abnormally high diffusion coefficients. This work has provided useful insight for further improvement of solvent-free electrolytes based on rational design of their constituents, facilitating the large-scale practical application of lightweight, flexible dye-sensitized solar cells.

New Organic Sensitizer for Stable Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes

Co-reporter:Dong Shi, Yiming Cao, Nuttapol Pootrakulchote, Zhihui Yi, Mingfei Xu, Shaik M. Zakeeruddin, Michael Grätzel and Peng Wang

The Journal of Physical Chemistry C 2008 Volume 112(Issue 44) pp:17478-17485

Publication Date(Web):2017-2-22

DOI:10.1021/jp807191w

We report a high molar extinction coefficient metal-free sensitizer composed of a triarylamine donor in combination with the 2-(2,2′-bithiophen-5-yl)acrylonitrile conjugation unit and cyanoacrylic acid as an acceptor. In conjugation with a volatile acetonitrile-based electrolyte or a solvent-free ionic liquid electrolyte, we have fabricated efficient dye-sensitized solar cells showing a corresponding 7.5% or 6.1% efficiency measured under the air mass 1.5 global sunlight. The ionic liquid cell exhibits excellent stability during a 1000 h accelerated test under the light-soaking and thermal dual stress. Intensity-modulated photocurrent and photovolatge spectroscopies were employed along with the transient photoelectrical decay measurements to detail the electron transport in the mesoporous titania films filled with these two electrolytes.

New Efficiency Records for Stable Dye-Sensitized Solar Cells with Low-Volatility and Ionic Liquid Electrolytes

Co-reporter:Dong Shi, Nuttapol Pootrakulchote, Renzhi Li, Jin Guo, Yuan Wang, Shaik M. Zakeeruddin, Michael Grätzel and Peng Wang

The Journal of Physical Chemistry C 2008 Volume 112(Issue 44) pp:17046-17050

Publication Date(Web):2017-2-22

DOI:10.1021/jp808018h

We report a high molar extinction coefficient heteroleptic polypyridyl ruthenium sensitizer, featuring an electron-rich 3,4-ethylenedioxythiophene unit in its ancillary ligand. A nanocrystalline titania film stained with this sensitizer shows an improved optical absorption, which is highly desirable for practical dye-sensitized solar cells with a thin photoactive layer, facilitating the efficient charge collection. In conjunction with low-volatility and solvent-free electrolytes, we achieved 9.6−10.0% and 8.5−9.1% efficiencies under the air-mass 1.5 global solar illumination. These dye-sensitized solar cells retain over 90% of the initial performance after 1000 h full sunlight soaking at 60 °C.

High-efficiency organic dye-sensitized mesoscopic solar cells with a copper redox shuttle

Co-reporter:Yu Bai, Qingjiang Yu, Ning Cai, Yinghui Wang, Min Zhang and Peng Wang

Chemical Communications 2011 - vol. 47(Issue 15) pp:NaN4378-4378

Publication Date(Web):2011/03/14

DOI:10.1039/C1CC10454C

High efficiency and stable dye-sensitized solar cells with an organic chromophore featuring a binary π-conjugated spacer

Co-reporter:Guangliang Zhang, Hari Bala, Yueming Cheng, Dong Shi, Xueju Lv, Qingjiang Yu and Peng Wang

Chemical Communications 2009(Issue 16) pp:NaN2200-2200

Publication Date(Web):2009/03/02

DOI:10.1039/B822325D

Joint electrical, photophysical and computational studies on D-π-A dye sensitized solar cells: the impacts of dithiophene rigidification

Co-reporter:Mingfei Xu, Min Zhang, Mariachiara Pastore, Renzhi Li, Filippo De Angelis and Peng Wang