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...

The conventional anode for organic photovoltaics (OPVs), indium tin oxide (ITO), is expensive and brittle, and thus is not suitable for use in roll-to-roll manufacturing of OPVs. In this study, fully solution-processed polymer bulk heterojunction (BHJ) solar cells with anodes made from silver nanowires (Ag NWs) have been successfully fabricated with a configuration of Ag NWs/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer:phenyl-C(61)-butyric acid methyl ester (PCBM)/Ca/Al. Efficiencies of 2.8 and 2.5% are obtained for devices with Ag NW network on glass and on poly(ethylene terephthalate) (PET), respectively. The efficiency of the devices is limited by the low work function of the Ag NWs/PEDOT:PSS film and the non-ideal ohmic contact between the Ag NW anode and the active layer. Compared with devices based on the ITO anode, the open-circuit voltage (V(oc)) of solar cells based on the Ag NW anode is lower by ~0.3 V. More importantly, highly flexible BHJ solar cells have been firstly fabricated on Ag NWs/PET anode with recoverable efficiency of 2.5% under large deformation up to 120°. This study indicates that, with improved engineering of the nanowires/polymer interface, Ag NW electrodes can serve as a low-cost, flexible alternative to ITO, and thereby improve the economic viability and mechanical stability of OPVs.

Here we demonstrate a conceptually new approach, the parallel-like bulk heterojunction (PBHJ), which maintains the simple device configuration and low-cost processing of single-junction BHJ cells while inheriting the major benefit of incorporating multiple polymers in tandem cells. In this PBHJ, free charge carriers travel through their corresponding donor-polymer-linked channels and fullerene-enriched domain to the electrodes, equivalent to a parallel-like connection. The short-circuit current (J(sc)) of the PBHJ solar cell is nearly identical to the sum of those of the individual "subcells", while the open-circuit voltage (V(oc)) is between those of the "subcells". Preliminary optimization of the PBHJ devices gives improvements of up to 40% in J(sc) and 30% in overall efficiency (η) in comparison with single-junction BHJ devices.

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...