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Journal ArticleDOI

2-Alkyl-5-thienyl-Substituted Benzo[1,2-b:4,5-b′]dithiophene-Based Donor Molecules for Solution-Processed Organic Solar Cells

TL;DR: Two novel, symmetrical, and linear A-D-A-type π-conjugated donor molecules each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core are synthesized and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units.
Abstract: In this study, we have strategically designed and convergently synthesized two novel, symmetrical, and linear A–D–A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. We thoroughly characterized both of these materials and investigated the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties. We then fabricated solution-processed bulk heterojunction organic solar cells incorporating TBDTCNR and TBDTCN. Among our tested devices, the one containing TBDTCNR and [6,6]-phenyl-C61-butyric acid methyl ester in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (Jsc) of 9.08 mA cm–2, an open circuit voltage (Voc) of 0.90 V, and an impressive fill factor (FF) of ...

Summary (1 min read)

1. INTRODUCTION

  • Solution-processed polymer solar cells (PSCs) are emerging as promising alternatives to silicon-based solar cells; they provide enormous benefits as a result of their low cost, amenability, high-throughput roll-to-roll processing, and ready structural modifications.
  • 6,8,9 Nevertheless, research into SMOSCs requires much further attention if the authors are to solve their shortcomings.

2. EXPERIMENTAL SECTION

  • 1. Fabrication and Characterization of Organic Solar Cells (OSCs).
  • After routine solvent cleaning, the ITO substrates were treated with UV ozone for 15 min and then spin-coated with the PEDOT:PSS layer (∼30 nm) at 4000 rpm.
  • The photovoltaic performance was characterized within a N2-filled glove box under an AM 1.5G filter (100 mW cm−2) using a Newport Oriel Solar Simulator (Thermal Oriel 1000W), as calibrated using a Si photodiode.
  • The configurations of the hole- and electron-only devices were ITO/ PEDOT:PSS/small molecule:PC61BM/V2O5/Al and ITO/Cs2CO3/ small molecule:PC61BM/Ca/Al, respectively.

3. RESULTS AND DISCUSSION

  • Scheme 1 presents the synthetic route the authors followed to prepare TBDTCNR and TBDTCN.
  • TBDTCN possessed a relatively low HOMO energy level relative to that of TBDTCNR, due to its high onset oxidation potential resulting from the presence of its more highly electron deficient CN units.
  • The corresponding devices incorporating TBDTCNR:PC61BM at weight ratios of 1:0.25, 1:0.50, and 1:0.75 provided lower EQEs obtained at lower values of Jsc, because of imbalances in the hole and electron mobilities and unfavorable morphologies , as the authors discuss below.
  • Transmission electron microscopy (TEM) images were recorded both pristine films of TBDTCNR and the blend film TBDTCNR/PC61BM (wt. ratio 1:0.4) and presented in Supporting Information Figure S10.

4. CONCLUSIONS

  • The authors have synthesized TBDTCNR and TBDTCN two πconjugated small molecules having A−D−A architectures based on TBDT as the central donating core and modified with different end groups and correlated their photophysical and photovoltaic properties.
  • These TBDT-based species possessed deep HOMO energy levels and provided devices exhibiting good values of Voc. TBDTCNR showed not only good packing in the solid state but also superior charge transport properties and favorable nanoscale morphology relative to TBDTCN and thus pronounced to the higher PCE values.
  • Further engineering of the molecular structure and optimization of the morphology, and variation of the acceptor moieties, are currently underway in their laboratory in the quest to boost device efficiencies.
  • ■ ASSOCIATED CONTENT *S Supporting Information 1HNMR spectra, 13CNMR spectra, TGA and DSC curves, AFM height images, synthesis details, characterization, DFT calculations AFM, TEM images of the blends and transfer curves.
  • This materials are available free of charge via the Internet at http://pubs.acs.org.

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Journal ArticleDOI
TL;DR: This review offered an overview of the organic photovoltaic materials based on BDT from the aspects of backbones, functional groups, alkyl chains, and device performance, trying to provide a guideline about the structure-performance relationship.
Abstract: Advances in the design and application of highly efficient conjugated polymers and small molecules over the past years have enabled the rapid progress in the development of organic photovoltaic (OPV) technology as a promising alternative to conventional solar cells. Among the numerous OPV materials, benzodithiophene (BDT)-based polymers and small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking 10% efficiency barrier in the single junction OPV devices. Remarkably, the OPV device featured by BDT-based polymer has recently demonstrated an impressive PCE of 11.21%, indicating the great potential of this class of materials in commercial photovoltaic applications. In this review, we offered an overview of the organic photovoltaic materials based on BDT from the aspects of backbones, functional groups, alkyl chains, and device performance, trying to provide a guideline about the structure-performance relationship. We believe more exciting BDT-based photovol...

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Wang Ni1, Xiangjian Wan1, Miaomiao Li1, Yunchuang Wang1, Yongsheng Chen1 
TL;DR: A-D-A small molecules have drawn more and more attention in solution-processed organic solar cells due to the advantages of a diversity of structures, easy control of energy levels, etc.

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TL;DR: In this paper, the progress made in the field of small molecules containing BDT units for solution-processed organic photovoltaic cells is reviewed, and insights into several important aspects regarding the design and synthesis of BDT based small molecules are also included.
Abstract: Solution processed small molecule based solar cells have become a competitive alternative to their polymer counterparts due to the advantages of their defined structure and thus less batch to batch variation. With a large and rigid planar conjugated structure, the benzo[1,2-b:4,5-b′]dithiophene (BDT) unit has become one of the most widely used and studied building blocks for high performance small molecule based photovoltaic devices. In this review article, we review the progress made in the field of small molecules containing BDT units for solution-processed organic photovoltaic cells. Insights into several important aspects regarding the design and synthesis of BDT based small molecules are also included.

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TL;DR: In this paper, a two-dimensional organic small molecule, DCA3T(T-BDT), was designed and synthesized for solution-processed organic solar cells (OSCs).
Abstract: A new two-dimensional (2D) organic small molecule, DCA3T(T-BDT), was designed and synthesized for solution-processed organic solar cells (OSCs). DCA3T(T-BDT) exhibited a deep HOMO energy level (−5.37 eV) and good thermal stability. The morphologies of the DCA3T(T-BDT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blends were investigated by atomic force microscopy and the crystallinity was explored by X-ray diffraction (XRD) and 2D grazing incidence wide-angle X-ray scattering (GIWAXS), respectively. The morphologies of the blends were strongly influenced by the blend ratio of DCA3T(T-BDT):PC61BM and annealing temperature. The effect of thermal annealing on the photovoltaic performance of DCA3T(T-BDT)-based small molecule organic solar cells (SMOSCs) was studied in detail. When DCA3T(T-BDT) was used as a donor with PC61BM as an acceptor, high efficiency SMOSCs with a power conversion efficiency of 7.93%, a high Voc of 0.95 V, Jsc of 11.86 mA cm−2 and FF of 0.70 were obtained by a thermal annealing process at only 60 °C, which offers obvious advantages for large scale production compared with solvent additive or interfacial modification treatment.

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TL;DR: In this paper, a mini review of small molecules with impressive photovoltaic performance especially reported in the last two years was highlighted, and the relationship between molecular structure and device performance was analyzed, which drew some rules for rational molecular design.
Abstract: With the rapid development in recent years, small-molecule organic solar cell is challenging the dominance of its counterpart, polymer solar cell. The top power conversion efficiencies of both single and tandem solar cells based on small molecules have surpassed 9%. In this mini review, achievements of small molecules with impressive photovoltaic performance especially reported in the last two years were highlighted. The relationship between molecular structure and device performance was analyzed, which draws some rules for rational molecular design. Five series of p- and n-type small molecules were selected based on the consideration of their competitiveness of power conversion efficiencies.

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References
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TL;DR: In this article, the authors measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly[2methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), with Au anodes and Ca, Al, and Au cathodes.
Abstract: We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current.

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TL;DR: In inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% are demonstrated using ZnO and PEIE as an interfacial layer and are relatively stable in air compared to conventional cells.
Abstract: We successfully demonstrate inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% using ZnO and PEIE as an interfacial layer. Modification of ZnO with a cost-effective PEIE thin layer increases the efficiency of the inverted cell as a result of reducing the work function of the cathode and suppressing the trap-assisted recombination. In addition to the high efficiency, the inverted SM solar cells are relatively stable in air compared to conventional cells.

481 citations

Journal ArticleDOI
TL;DR: This work has shown that attaching C2-pyrene as the small molecule end-group to a diketopyrrolopyrrole core leads to materials with a tight, aligned crystal packing and favorable morphology dictated by π-π interactions, resulting in high power conversion efficiencies and high fill factors.
Abstract: Efficient organic photovoltaic (OPV) materials are constructed by attaching completely planar, symmetric end-groups to donor-acceptor electroactive small molecules. Appending C2-pyrene as the small molecule end-group to a diketopyrrolopyrrole core leads to materials with a tight, aligned crystal packing and favorable morphology dictated by π-π interactions, resulting in high power conversion efficiencies and high fill factors. The use of end-groups to direct molecular self-assembly is an effective strategy for designing high-performance small molecule OPV devices.

361 citations

Journal ArticleDOI
TL;DR: A tailor-made medium-band gap fluorinated quinoxaline-based conjugated polymer of PBDT-TFQ was designed and synthesized as a donor material for bulk-heterojunction (BHJ) solar cells.
Abstract: A tailor-made medium-band gap fluorinated quinoxaline-based conjugated polymer of PBDT-TFQ was designed and synthesized as a donor material for bulk-heterojunction (BHJ) solar cells. This polymer is possessed of an intrachain donor–acceptor architecture and exhibits a broad and strong absorption spectrum across the entire UV–vis region. The introduction of F atoms with high electron affinity to the quinoxaline moiety is effective in further lowering both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of PBDT-TFQ to attain higher open-circuit voltage (Voc). With an optimized blend ratio of PBDT-TFQ:PC71BM (1:1, w/w), a high power conversion efficiency (PCE) of 8.0% was obtained, with a Voc of 0.76 V, a short-circuit current density (Jsc) of 18.2 mA cm–2, and a fill factor (FF) of 58.1% under AM 1.5G irradiation. The resulting copolymer reveals an outstanding Jsc value, arising from the higher hole mobility of PBDT-TFQ, together with the better...

323 citations

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TL;DR: In this article, the authors investigated the power conversion efficiency of small-molecule-based organic photovoltaic (OPV) cells for an alternate of silicon semiconductor-based solar cells.
Abstract: In the past few years, great progress has been made in organic photovoltaic (OPV) cells for an alternate of silicon semiconductorbased solar cells. OPV has the advantages of clean, low-cost, flexibility, and the possibility of roll-to-roll production.[1–4] Currently, most of the works have been focused on polymer donor molecules using bulk heterojunction (BHJ) architecture and [6,6]-phenyl-C61–butyric acid methyl ester (PC61BM) as the acceptor.[5,6] Indeed, in addition to the currently better OPV performance than small molecules, polymers have the advantages for such as better film forming quality and so on.[7] However, it cannot be denied that there are disadvantages for polymer-based OPV, such as batch to batch reproducibility, difficulty of purification, and so on. In contrast, small molecules intrinsically do not have such flaws;[8] additionally, their band structures could be tuned easily with much more choices of chemical modification. Furthermore, small molecules generally have higher charge mobility and open voltages.[9,10] However, even with these advantages, small-molecule-based OPV cells have not been investigated as intensively as that of their polymer counterparts because one of the major problems for small molecules is their generally poor film quality when using the simple solution spinning process.[11] This has been hampering their performance, and indeed their power conversion efficiencies (PCEs) (4%–5%)[12–18] are still significantly lower compared with that (>7%)[19–25] from polymers. It is thus expected that better PCE could be achieved when their intrinsic bad film quality and morphology in BHJ architecture could be improved combining with their other advantages. But to achieve this, careful molecule design has to be carried out to address many factors collectively, including their molar absorption, morphology compatibility with the acceptors for a better film quality, and so on.

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Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "2‐alkyl-5-thienyl-substituted benzo[1,2‐b:4,5‐b′]dithiophene-based donor molecules for solution-processed organic solar cells" ?

In this study, the authors have strategically designed and convergently synthesized two novel, symmetrical, and linear A− D−A-type π-conjugated donor molecules ( TBDTCNR, TBDTCN ), each containing a planar electron-rich 2-octylthiene-5-ylsubstituted benzodithiophene ( TBDT ) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate ( CNR ) or dicyanovinyl ( CN ) units. The authors thoroughly characterized both of these materials and investigated the effects of the end groups ( CNR, CN ) on their optical, electrochemical, morphological, and photovoltaic properties. The FFs of these solutionprocessed small-molecule organic solar cells ( SMOSCs ) are outstanding when compared with those recently reported for benzodithiophene ( BDT ) -based SMOSCs, because of the high crystallinity and excellent stacking properties of the TBDT-based compounds.