The research on the polymer-based solar cells (PSCs) has attracted an increasing amount of attention in recent years because PSCs pose potential advantages over mainstream inorganic-based solar cells, such as significantly reduced material/fabrication costs, flexible substrates, and light weight of finished solar cells. The research community has made great progress in the field of bulk heterojunction (BHJ) polymer solar cells since its inception in 1995. The power conversion efficiency (PCE), a key parameter to assess the performance of solar cells, has increased from 1% in the 1990s to over 8% just recently. These great advances are mainly fueled by the development of conjugated polymers used as the electron-donating materials in BHJ solar cells. In this Perspective, we first briefly review the progress on the design of conjugated polymers for polymer solar cells in the past 16 years. Since a conjugated polymer can be arbitrarily divided into three constituting components—the conjugated backbone, the si...

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Rational Design of High Performance Conjugated Polymers for Organic Solar Cells

A comprehensive database on physical properties of ionic liquids (ILs), which was collected from 109 kinds of literature sources in the period from 1984 through 2004, has been presented. There are 1680 pieces of data on the physical properties for 588 available ILs, from which 276 kinds of cations and 55 kinds of anions were extracted. In terms of the collected database, the structure-property relationship was evaluated. The correlation of melting points of two most common systems, disubstituted imidazolium tetrafluoroborate and disubstituted imidazolium hexafluorophosphate, was carried out using a quantitative structure-property relationship method.

Physical Properties of Ionic Liquids: Database and Evaluation

Ionic liquid crystals.

Polymer solar cells (PSCs) have attracted much attention because of their potential application in flexible, light-weight, and low-cost large-area devices through roll-to-roll printing. The bulk heterojunction PSCs showed advanced features in realizing high efficiencies and solution-processible devices. The active layer in this kind of device consists of an interpenetrating network formed by an electron-donor material blended with an electron-acceptor material. 3] Typically, conjugated polymers are used as electron donors and fullerene derivatives are used as the electron acceptors in the PSCs. Recently, power conversion efficiencies (PCEs) of 6–7% have been realized by using new conjugated polymer donors or new fullerene-derived acceptors. Short circuit current density (Jsc), open circuit voltage (Voc), and fill factors (FF) are key parameters for a PSC device, because the PCE of the device is proportional to the values of the three parameters. To broaden the response wavelength range of a PSC device by using conjugated side chains or narrowband-gap conjugated polymers is an effective way to realize high Jsc values. Conjugated polymers with lower HOMO levels are helpful in realizing high Voc and PCE values, as the Voc value of PSCs is directly proportional to the offset between the HOMO level of electron donor and the LUMO level of electron acceptor. PSiFDTBT, PFDTBT, and PCDTBT are three excellent examples for this concept. Consequently, by using conjugated polymers with lower HOMO levels and also narrow band gaps, high PCEs were realized in different families of conjugated polymers. Conjugated polymers based on benzo[1,2-b :4,5-b’]dithiophene (BDT) units have attracted interest as electron donors in the PSC field in recent years, since the report of Hou and Yang et al. on the synthesis and photovoltaic properties of a series of copolymers based on BDT. Many copolymers of BDT with different conjugated units, such as thieno[3,4b]thiophene (TT), 4,7-dithiophene-2-yl-2,1,3-benzothiadiazole (DTBT), N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD), and bithiazole, etc. were synthesized, and the copolymers showed promising photovoltaic properties. In these BDT-based polymers, the alternative copolymers of BDT and TT, namely PBDTTTs, are an important family of photovoltaic materials. For additional improvements in the photovoltaic performance of the PBDTTTs, structural modifications brought about by using different substituents on BDT, or the copolymerized moieties is of great importance. For example, Liang et al. introduced a fluorine atom into the TT unit of the PBDTTTs, and the HOMO level of the resulting polymer was successfully lowered by approximately 0.12 eV, and thus a higher Voc value was achieved, resulting in a great improvement of PCE. Hou et al. optimized PBDTTTs further by replacing the alkoxycarbonyl group on the TT unit with the alkylcarbonyl groups. The structural modification can also be carried out on the BDT units. In this work, we designed an 5-alkylthiophene-2-yl-substituted BDT monomer and synthesized two new PBDTTT-based polymers having either the thienylsubstituted BDT with alkoxycarbonyl-substituted thieno[3,4b]thiophene (TT-E) or the alkylcarbonyl-substituted thieno[3,4-b]thiophene (TT-C); that is PBDTTT-E-Tand PBDTTTC-T, respectively (Scheme 1). To fully investigate the effect of the thienyl-substituted BDTon the photovoltaic properties of the polymers, two corresponding PBDTTT polymers based on the alkoxy-substituted BDT (BDT-O), PBDTTT-E and PBDTTT-C (Scheme 1), were also prepared. The synthetic route of the thienyl-substituted BDT monomer (BDT-T) is shown in Scheme 1. The branched alkyl group 2-ethylhexyl was employed as the side chain on the thiophene to guarantee high solubility of the target polymers. The TT-E and TT-C monomers are commercially available. The polymers were prepared through a Stille coupling reaction between the bis(trimethyltin) BDT monomers (BDT-T and BDT-O) and the bromides (TT-E and TTC) as shown in Scheme 1. All the polymers are soluble in chloroform (CHCl3), chlorobenzene, and dichlorobenzene. Thermogravimetric analysis (TGA) measurements were employed to evaluate the thermal stability of the polymers. We found that the two-dimentional (2D) conjugated polymers based on alkylthienyl-substituted BDTs are much more stable than their analogues, the alkoxy-substituted BDTs. The TGA plots of these four polymers are shown in Figure 1. It can be seen that the decomposition temperatures [*] Dr. L. Huo, S. Zhang, F. Xu, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences Beijing 100190 (China) E-mail: hjhzlz@iccas.ac.cn

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Replacing Alkoxy Groups with Alkylthienyl Groups: A Feasible Approach To Improve the Properties of Photovoltaic Polymers†

During the last decade the field of polymer photovoltaics has seen a tremendous improvement in both device efficiency and understanding of the underlying physical processes One has come to a point in which the prototypical large bandgap material system P3HT:PCBM is nearing optimal device performance In order to enhance efficiencies even further, research activities for new materials are needed with better aligned energy levels One interesting approach is by narrowing the donor bandgap to enhance light absorption Recent developments on small band gap (<20 eV) materials for photovoltaic applications are reviewed First, an introduction is given regarding the processes governing the exciton dissociation, charge transport requirements, energy level engineering of both donor and acceptor materials, and other parameters determining the photovoltaic performance The focus is on polymeric donor materials, which are subdivided by the type of monomeric units that constitute the backbone Finally, the synthetic

Small Bandgap Polymers for Organic Solar Cells (Polymer Material Development in the Last 5 Years)

The polymer light-emitting electrochemical cells (LECs) were fabricated with MEH−PPV as the luminescent polymer and the ionic liquid of imidazolium salts as the supporting electrolyte. The imidazolium salts utilized include various 1-methyl-3-alkylimidazolium salts with the alkyl substituents of butyl (bmim), dodecyl (dmim), tetradecyl (tmim), or hexadecyl (hmim) and the anions of PF6- or BF4-, which possess different melting points from room temperature for [bmim+][PF6-] to 83 °C for [hmim+][PF6-]. The electroluminescent (EL) properties and the electronic structure of the LECs were characterized by current−voltage (I−V), light intensity−voltage (L−V), and ac impedance measurements. It was found that the phase compatibility between the conjugated polymer and the ionic liquid determines the performance of the light-emitting devices, and the concentration of the ionic liquid and the ionic conductivity of the polymer blend films also play an important role. The imidazolium salts investigated in this work are...

Ionic liquid doped polymer light-emitting electrochemical cells

Synthesis and photovoltaic properties of the copolymers of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene and 2,5-thienylene-vinylene

Photovoltaic cells based on the blend of MEH-PPV and polymers with substituents containing C60 moieties

Three hyperbranched phenylene vinylenes (HPVs)—H-mn, H-es, and H-py—were used in fabricating polymer photovoltaic cells (PVCs). PVCs with photoactive layers composed of pure HPV, a blend of HPV and C60, and a blend of HPV, poly(2-methoxyl-5,2′-ethylhexyoxyl-1,4-phenylene vinylene) (MEH-PPV), and C60, were fabricated, and their current–voltage characteristics were measured and investigated. The PVC with photoactive layers composed of H-es and C60 showed promising photovoltaic characteristics, with an energy-conversion efficiency of 0.035%, when illuminated under 85 mW/cm2 white light. The short-circuit current and energy-conversion efficiency of the PVCs based on H-mn/C60 and H-py/C60 were greatly improved when they were blended with a small amount of MEH-PPV. The incident photon-to-collected-electron efficiency of the HPV/MEH-PPV/C60 PVCs indicated that both HPVs and MEH-PPV contributed to the photocurrent. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1459–1466, 2004

Hyperbranched conjugated polymers for photovoltaic applications

The invention relates to an photoconductive film for polymerized optical voltaic cell, and salt of 1-5 wt% is added to the composite film, wherein the salt may includes metal salt, organic ammonium and imidazole type ionic liquid.

Jing Qiao

Papers

Ionic liquid doped polymer light-emitting electrochemical cells

Synthesis and photovoltaic properties of the copolymers of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene and 2,5-thienylene-vinylene

Photovoltaic cells based on the blend of MEH-PPV and polymers with substituents containing C60 moieties

Hyperbranched conjugated polymers for photovoltaic applications

Photo sensitive thin film for polymer photovoltaic cells and method for making same