Huaxing Zhou

Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer−Fullerene Solar Cells

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.

Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7 % Efficiency

a) fluorine is the mostelectronegative element, with a Pauling electronegativity of4.0, which is much larger than that of hydrogen (2.2);b) fluorine is the smallest electron-withdrawing group (vander Waals radius, r=1.35 , only slightly larger than hydro-gen, r=1.2 ). Furthermore, these fluorine atoms often havea great influence on inter- and intramolecular interactionsthrough C-F···H, F···S, and C-F···p

Solution-Processed Flexible Polymer Solar Cells with Silver Nanowire Electrodes

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.

Enhanced Photovoltaic Performance of Low‐Bandgap Polymers with Deep LUMO Levels

As a potential low-cost alternative to mainstream silicon solar cells, bulk heterojunction (BHJ) polymer solar cells have attracted a significant amount of attention in the research community. Fullerene derivatives (such as [6,6]-phenyl-C61butyric acid methyl ester, PC61BM) have been extensively used as the n-type semiconductor in BHJ solar cells because of their superior electron-accepting and transport behavior. However, these fullerene derivatives are usually poor light absorbers, thereby leaving the task of light absorbing to the conjugated polymers. Moreover, fullerene derivatives usually have fixed energy levels (e.g., a lowest unoccupied molecular orbital (LUMO) of 4.3 eV), which dictate that the proposed “ideal” conjugated polymer should exhibit a low highest occupied molecular orbital (HOMO) energy level of 5.4 eV and a small bandgap of 1.5 eV. Therefore, a significant amount of effort has been devoted to engineering the bandgap and energy levels of conjugated polymers. As a result, a few highly efficient polymers have been reported with the record high efficiency surpassing 7%. To simultaneously lower the HOMO energy level and the bandgap as required by the ideal polymer, a “weak donor– strong acceptor” strategy was proposed. A few such materials, by incorporating weak donor moieties based on fused aromatic systems and a strong acceptor based on 4,7dithien-2-yl-2,1,3-benzothiadiazole (DTBT), have been successfully demonstrated with high efficiency in typical BHJ devices. In these conjugated polymers, close to ideal HOMO energy levels were achieved (e.g., 5.33 eV), which led to an observed open circuit voltage (Voc) as high as 0.83 V. [4a] However, the bandgaps of these materials were still larger than the proposed 1.5 eV of ideal polymers, which explains why mediocre short-circuit currents (Jsc) were obtained. Logically, to further improve the efficiency, a smaller bandgap is needed to achieve a higher short-circuit current (Jsc), while the low HOMO energy level should still be maintained. Fortunately, our previous study indicated that the LUMO of donor–acceptor copolymers largely resides on the acceptor moiety. Therefore, we envisioned that incorporating a more electron deficient acceptor to lower the LUMO would lead to a smaller bandgap and maintain the low HOMO energy level in the newly designed materials. Compared with benzene, pyridine is p-electron deficient. Therefore, if we replaced the benzene in the 2,1,3-benzothiadiazole (BT) unit with pyridine, the new acceptor, thiadiazolo[3,4-c]pyridine (PyT), would be one such stronger acceptor. A similar strategy has been demonstrated recently by Leclerc et al. The copolymer of a carbazole unit with a thienyl-flanked PyT unit (PCDTPT) did show a much lower LUMO level compared with that of the copolymer with a BT unit. However, a low efficiency was obtained, presumably because of the low molecular weight and low solubility of PCDTPT. To solve these issues, we employed the strategy of a “soluble” acceptor 5a] by flanking the PyT moiety with two alkylated thienyl units, which converted the PyT into the new, soluble, stronger acceptor DTPyT. As demonstrated in our previous study, anchoring of alkyl chains to the 4-position of the thienyl units of DTPyT would only significantly improve the molecular weight and solubility of the resulting polymers without introducing much steric hindrance. Herein, we report the synthesis of a series of weak donor– strong acceptor polymers, PNDT–DTPyT, PQDT–DTPyT, and PBnDT–DTPyT, by copolymerizing various donor moieties, namely naphtho[2,1-b :3,4-b’]dithiophene (NDT), dithieno[3,2-f :2’,3’-h]quinoxaline (QDT), and benzo[1,2b :4,5-b’]dithiophene (BnDT), with the newly conceived soluble DTPyT acceptor moiety (Scheme 1). Our preliminary investigation on the photovoltaic properties of these polymers in typical BHJ devices using PC61BM as the electron acceptor showed highly respectable power conversion efficiencies (PCEs) of over 5.5% for PQDT–DTPyT, and over 6% for PBnDT–DTPyT and PNDT–DTPyT. The synthesis of the alkylated DTPyT is modified from the reported procedure (see the Supporting Information for experimental details). The other comonomers—alkylated NDT, QDT, and BnDT—were prepared by established literature procedures. 7] Three polymers, PNDT–DTPyT, PQDT–DTPyT, and PBnDT–DTPyT, were synthesized by the microwave-assisted Stille polycondensation between alkylated dibrominated DTPyT and the corresponding distannylated monomers. Crude polymers were purified by [*] H. Zhou, S. C. Price, K. J. Knight, Prof. Dr. W. You Department of Chemistry University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3290 (USA) Fax: (+ 1)919-962-2388 E-mail: wyou@email.unc.edu Homepage: http://www.chem.unc.edu/people/faculty/you/group/ index.html

Disentangling the impact of side chains and fluorine substituents of conjugated donor polymers on the performance of photovoltaic blends

Side chains and fluorine (F) substituents on conjugated polymers have shown significant impact on the photovoltaic properties of polymer-based bulk heterojunction (BHJ) solar cells, but their respective impact is largely studied independently. In order to disentangle the effect of side chains and F substituents, we comprehensively investigate a series of conjugated polymers with an identical backbone (PNDT–DTBT) but different combinations of side chains and F substituents. Surprisingly, these seemingly marginal changes to the polymer backbone strongly influence the morphology and structure in BHJ thin films (e.g., domain size/purity and the relative orientation of polymer crystallites), as manifested by resonant soft X-ray scattering (R-SoXS) and grazing-incidence wide-angle X-ray scattering (GI-WAXS), thereby exerting significant impact on the photovoltaic properties of these conjugated polymer-based BHJ cells. Devices based on the polymer with long bulky side chains and F substituents (C8,4-C6,2F) simultaneously exhibit large open circuit voltage (Voc), high short circuit current (Jsc) and good fill factor (FF), with an efficiency as high as 5.6% for this series of PNDT–DTBT polymers.