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 side chains, and the substituentswe then focus on the rational design of conjugated polymers by separately discussing the influence of each component on the physical and photovoltaic (PV) properties of these polymers. Special attention is paid to the design of donor− acceptor type low-band-gap polymers because this approach is prevailing in the literature with its unique features. In doing so, we strive to extract useful rules for the rational design of conjugated polymers with predictable properties. We conclude by proposing future research opportunities to achieve even higher PCEs for PSCs.

Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.

Fluorinated organic molecules exhibit a series of unique features such as great thermal and oxidative stability, [1] elevated resistance to degradation, [2] enhanced hydrophobicity, high lipophobicity of perfluorinated substances, [3] and inverted charge density distribution in fluorinated aromatic compounds.[4] These special features are related to the unique properties of the fluorine atom:[5] a) fluorine is the most electronegative element, with a Pauling electronegativity of 4.0, which is much larger than that of hydrogen (2.2); b) fluorine is the smallest electron-withdrawing group (van der Waals radius, r = 1.35 , only slightly larger than hydrogen, r = 1.2 ). Furthermore, these fluorine atoms often have a great influence on inter-and intramolecular interactions through C-F···H, F···S, and C-F···p F interactions. [2,6] As a result, fluorinated conjugated materials have been explored for their applications in organic field-effect transistors (OFET) [7] and organic light-emitting diodes (OLED). [4,8] However, there are only a few examples of applying fluorinated compounds in organic photovoltaics, [9] especially as p-type semiconductors in bulk heterojunction (BHJ) polymer solar cells.Since the fluorine atom is a strong electron-withdrawing substituent, the introduction of F into the conjugated backbone would lower both the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the conjugated polymers, as demonstrated by Heeger and BrØdas in a theoretical study of poly(phenylene vinylene) having various substituents. [10] Experimentally, Yu et al. confirmed the electronic effect of the fluorine substituent in their study of a series of benzodithiophene thieno[3,4-b]thiophene copolymers.[9b] After one fluorine atom was substituted onto the thieno [3,4-b]thiophene unit, the copolymer exhibited decreased LUMO and HOMO energy levels, but with a similar band gap, as compared with those of the nonfluorinated analogue. A larger open-circuit voltage (V oc ) was observed from the BHJ device based on the F-substituted polymer, and this difference is largely because of the lower HOMO energy level. Moreover, the short-circuit current (J sc ) and the fill factor (FF) were noticeably increased by judicious selection of solvent and additives, [11] possibly because of an optimized film morphology facilitated by these F atoms. A similar enhancement on the morphology by employing F atoms was observed by Kim et al. in their study of poly(3-hexylthiophene) (P3HT) having various end-groups.[9a] The CF 3 end-group-modified P3HT showed significant improvement in both the J sc and FF values for its BHJ devices, thus leading to a 40 % increase in the efficiency (h). The much improved morphology of the polymer/PC 61 BM blend was attributed to the decreased surface energy of the fluorine-containing polymer. However, there has been no precedent study on the photovoltaic properties of F-containing low-band-gap polymers constructed using the donor-acceptor strategy, [12] which is a comm...

OS with sunitinib was not superior or equivalent but was significantly inferior to sorafenib. OS was comparable in Asian and hepatitis B-infected patients. OS was superior in hepatitis C-infected patients who received sorafenib. Sunitinib-treated patients reported more frequent and severe toxicity.