Wenkai Zhong

Wide bandgap dithienobenzodithiophene-based π-conjugated polymers consisting of fluorinated benzotriazole and benzothiadiazole for polymer solar cells

Two wide bandgap donor–acceptor type π-conjugated polymers based on dithienobenzodithiophene as the donor unit and difluorobenzotriazole or difluorobenzothiadiazole as the acceptor unit were designed and synthesized. The copolymer based on difluorobenzothiadiazole exhibited more pronounced aggregations in chlorobenzene solutions than that of the copolymer based on difluorobenzotriazole. Both copolymers exhibited relatively wide bandgaps with deep highest occupied molecular orbitals, leading to high open circuit voltages of over 0.95 V for the fabricated polymer solar cells. These copolymers exhibited quite analogous hole mobility of about 0.1 cm2 V−1 s−1 as measured by organic field effect transistors. Bulk heterojunction polymer solar cells based on these copolymers as the electron-donating materials and PC71BM as the electron-accepting material exhibited relatively high performance, with the best power conversion efficiency of 7.45% attained for the copolymer based on the difluorobenzothiadiazole unit. These results demonstrated that the constructed wide bandgap π-conjugated polymers can be promising candidates for the fabrication of high performance solar cells with multi-junction architectures.

Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1,2,5]thiadiazole Unit for Polymer Solar Cells

We designed and synthesized two isomeric nonfullerene acceptors, IFBR-p and IFBR-d. These molecular semiconductors contain indacenodithiophene (IDT) as the central unit, adjacent asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole units, and are flanked with rhodanine as the peripheral units. The orientation of the two fluorine atoms (proximal, p, or distal, d), relative to IDT impacts most severely the film morphologies when blended with the electron-donating polymer PTzBI. Polymer solar cells based on PTzBI:IFBR-p give rise to a power conversion efficiency (7.3 ± 0.2%) that is higher than what is achieved with PTzBI:IFBR-d (5.2 ± 0.1%). This difference is attributed to the lower tendency for (over)crystallization by IFBR-p and the resulting more favorable morphology of the photoactive layer. These results highlight the subtle impact of substitution regiochemistry on the properties of nonfullerene acceptors through modulation of their self-assembly tendencies.

Side-chain modification of polyethylene glycol on conjugated polymers for ternary blend all-polymer solar cells with efficiency up to 9.27%

With the rapid progress achieved by all-polymer solar cells (all-PSCs), wide-bandgap copolymers have attracted intensive attention for their unique advantage of constructing complementary absorption profiles with conventional narrow-bandgap copolymers. In this work, we designed and synthesized a wide bandgap ternary copolymer PEG-2% which has the benzodithiophene-alt-difluorobenzotriazole as the backbone and the polyethylene glycol (PEG) modified side chain. The PBTA-PEG-2%:N2200 can be processed with a non-chlorinated solvent of 2-methyl-tetrahydrofuran (MeTHF) for the binary all-PSC, which exhibits a moderate photovoltaic performance. In particular, the ternary all-PSCs that consisting an additional narrow bandgap polymer donor PTB7-Th can also be processed with MeTHF, resulting in an unprecedented power conversion efficiency (PCE) of 9.27%, and a high PCE of 8.05% can be achieved with active layer thickness of 240 nm, both of which are the highest values so far reported from all-PSCs. Detailed investigations revealed that the dramatically improved device performances are attributable to the well-extended absorption band in the photoactive layer. Hence, developing novel copolymers with tailored side chains, and introducing additional polymeric components, can broaden the horizon for high-performance all-PSCs.

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.

Efficient saturated red light-emitting polyfluorenes containing iridium complexes in side chains

A series of electrophosphorescent copolymers were synthesized by Suzuki polymerization. A diketone-ended alkyl chain which is grafted in the N-position of the carbazole unit is used as a ligand to form a pendant cyclometalated Ir complex with 1-(9,9-dioctyl-9H-fluoren-2-yl)isoquinoline and 1-(N,N-diphenyl-benzenamine-4-yl)isoquinoline. The electroluminescence from the backbone of the copolymers is completely quenched by tethered iridium complexes even though with the content of iridium complexes being as low as 1 mol%. Saturated red light-emitting diodes were fabricated on the basis of these two series of copolymers. The best device performance with a maximum external quantum efficiency of 7.8% on the basis of copolymer PF-IrNiq1 was achieved, which was among the highest efficiencies of the reported electrophosphorescent red light-emitting polymers comprising iridium ligands.

Effects of pyridyl group orientations on the optoelectronic properties of regio-isomeric diketopyrrolopyrrole based π-conjugated polymers

Two novel regio-isomeric π-conjugated polymers consisting of the pyridyl unit flanked by diketopyrrolopyrrole as the electron-accepting unit and 2,5-bis(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene as the electron-donating unit were designed and synthesized. The comparison of the optical and electrochemical properties indicated that the copolymer based on the nitrogen atom proximal to the central diketopyrrolopyrrole unit (p-PDBPy) exhibited bathochromic shifted absorption spectra and narrower bandgaps than the counterpart copolymer (d-PDBPy) with the distally oriented nitrogen atom, which can be correlated with the stronger intermolecular aggregation of the former as a result of the different intrinsic molecular geometry of the polymer backbone. Of particular interest is that the copolymer p-PDBPy exhibited a moderate hole mobility of 0.35 cm2 V−1 s−1, which is about three orders of magnitude higher than the hole mobility of 3.2 × 10−4 cm2 V−1 s−1 obtained based on the counterpart copolymer d-PDBPy, as measured using organic field-effect transistors. These results demonstrated that the delicate control of the pyridyl orientations along the polymer backbone is of vital importance for the molecular design of π-conjugated polymers for high-performance organic electronic devices.

Highly efficient single-layer blue polymer light-emitting diodes based on hole-transporting group substituted poly(fluorene-co-dibenzothiophene-S,S-dioxide)

We developed a series of high-performance blue light-emitting polymers that contain hole-transport moieties comprising carbazole or triphenylamine substituents in the side chains of random copolymer poly(fluorene-co-dibenzothiophene-S,S-dioxide) (PFSO). The incorporation of these hole-transport species had negligible effects on the optical properties of the polymers but obviously facilitated hole injection from the anode, leading to significant improvements in hole/electron flux in the emissive layer. Interestingly, the copolymers that comprised either carbazole or triphenylamine substituents exhibited dramatically enhanced luminous efficiencies compared with the original copolymer. A PFSO-T5-based device yielded an excellent luminous efficiency of 7.04 cd A−1 at a brightness of 1000 cd m−2 with CIE coordinates of (0.16, 0.18), the highest efficiency reported so far for blue light-emitting polymers based on single-layer devices. The electroluminescent spectra of the PFSO-T5-based device exhibited excellent stability, as evidenced by a lack of change as the driving voltage increased from 4 to 8 V. These results indicate that the introduction of a hole-transporting group in the side chain is a very promising strategy for the development of highly efficient blue polymers for single-layer devices.

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.

Asymmetric Alkyl Side-Chain Engineering of Naphthalene Diimide-Based n-Type Polymers for Efficient All-Polymer Solar Cells

The design and synthesis of three n-type conjugated polymers based on a naphthalene diimide-thiophene skeleton are presented. The control polymer, PNDI-2HD, has two identical 2-hexyldecyl side chains, and the other polymers have different alkyl side chains; PNDI-EHDT has a 2-ethylhexyl and a 2-decyltetradecyl side chain, and PNDI-BOOD has a 2-butyloctyl and a 2-octyldodecyl side chain. These copolymers with different alkyl side chains exhibit higher melting and crystallization temperatures, and stronger aggregation in solution, than the control copolymer PNDI-2HD that has the same side chain. Polymer solar cells based on the electron-donating copolymer PTB7-Th and these novel copolymers exhibit nearly the same open-circuit voltage of 0.77 V. Devices based on the copolymer PNDI-BOOD with different side chains have a power-conversion efficiency of up to 6.89%, which is much higher than the 4.30% obtained with the symmetric PNDI-2HD. This improvement can be attributed to the improved charge-carrier mobility and the formation of favorable film morphology. These observations suggest that the molecular design strategy of incorporating different side chains can provide a new and promising approach to developing n-type conjugated polymers.