Ruihao Xie

Synthesis and characterization of π-conjugated copolymers based on alkyltriazolyl substituted benzodithiophene

A series of novel electron-donating building blocks of alkyltriazolyl substituted benzodithiophene were synthesized on the basis of Cu(I)-catalyzed azide–alkyne cycloaddition. The alternating donor–acceptor type of π-conjugated copolymers by using such alkyltriazolyl substituted benzodithiophene as donor units and diketopyrrolopyrrole as acceptor units were synthesized via Suzuki polymerization. The resultant copolymers exhibited good thermal properties and can be easily dissolved in various organic solvents. All copolymers show quite comparable absorption profiles in the range of 300–850 nm with the absorption onset at about 1.4 eV, where the copolymer with longer side chains exhibited a slightly enhanced absorption coefficient. The cyclic voltammetry measurements indicated the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals located at about −5.40 and −3.40 eV, respectively, which were nearly independent of the size of alkyl side chains. Polymer solar cells based on the resultant copolymers as electron-donating materials and PC71BM as the electron-accepting material exhibited moderate photovoltaic performances.

8.0% Efficient all-polymer solar cells based on novel starburst polymer acceptors

The polymer N2200, with its π-conjugated backbone composed of alternating naphthalene diimide (NDI) and bithiophene (DT) units, has been widely used as an acceptor for all-polymer solar cells (all-PSCs) owing to its high electron mobility and suitable ionization potential and electron affinity. Here, we developed two naphthalene diimide derivatives by modifying the molecular geometry of N2200 through the incorporation of a truxene unit as the core and NDI-DT as the branches. These starburst polymers exhibited absorption spectra and molecular orbital energy levels that were comparable to N2200. These copolymers were paired with the wide-bandgap polymer donor PTzBI-O to fabricate all-polymer solar cells (all-PSCs), which displayed impressive power conversion efficiencies up to 8.00%. The improved photovoltaic performances of all-PSCs based on these newly developed starburst acceptors can be ascribed to the combination of increased charge carrier mobilities, reduced bimolecular recombination, and formation of more favorable film morphology. These findings demonstrate that the construction of starburst polymer acceptors is a feasible strategy for the fabrication of high-performance all-PSCs.

Introducing cyclic alkyl chains into small-molecule acceptors for efficient polymer solar cells

A new acceptor–donor–acceptor type small molecule acceptor, namely IDT-HN, has been developed, which consists of a newly developed 2-(3-oxo-2,3,5,6,7,8-hexahydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile as the peripheral electron-withdrawing group and indaceno[1,2-b:5,6-b′]dithiophene as the electron-donating core. Compared with the reference molecule (IDT-IC) that bears phenyl-fused indanone as the end groups, IDT-HN exhibited an elevated lowest unoccupied molecular orbital level. By utilizing IDT-HN as the electron acceptor and a wide bandgap conjugated polymer, PBDB-T, as the electron donor, optimized devices exhibited an impressively high power conversion efficiency of up to 10.22% with simultaneously improved open-circuit voltage, short-circuit current and fill factor. The improved photovoltaic performance can be attributed to the widened and intensified absorption spectra, improved electron mobility, more ordered π–π packing structure, and the symmetric carrier transport mobility of the IDT-HN-based blend in comparison to those obtained from devices based on the reference molecule IDT-IC. These results indicate that the cyclic alkyl moiety incorporated into the peripheral groups plays a critical role in improving the performance of the corresponding solar cell devices. In addition, the power conversion efficiency of the devices remains at 91% of its optimum performance with a film thickness of 250 nm, indicating its great potential for future practical application.

Efficient Non-fullerene Organic Solar Cells Enabled by Sequential Fluorination of Small-Molecule Electron Acceptors

Three small-molecule non-fullerene electron acceptors containing different numbers of fluorine atoms in their end groups were designed and synthesized. All three acceptors were found to exhibit relatively narrow band gaps with absorption profiles extending into the near-infrared region. The fluorinated analog exhibited enhanced light-harvesting capabilities, which led to improved short-circuit current densities. Moreover, fluorination improved the blend film morphology and led to desirable phase separation that facilitated exciton dissociation and charge transport. As a result of these advantages, organic solar cells based on the non-fullerene acceptors exhibited clearly improved short-circuit current densities and power conversion efficiencies compared with the device based on the non-fluorinated acceptor. These results suggest that fluorination can be an effective approach for the molecular design of non-fullerene acceptors with near-infrared absorption for organic solar cells.