Zhenfeng Wang

Optimisation of processing solvent and molecular weight for the production of green-solvent-processed all-polymer solar cells with a power conversion efficiency over 9%

Increasing interest has been devoted to developing high-performance all-polymer solar cells (all-PSCs) owing to their specific advantages in light absorption and long-term stability. In this work, we systematically investigated the synergistic effects of processing solvents and molecular weight on the photovoltaic performance of all-PSCs, which consist of an n-type polymer N2200 and a p-type wide bandgap polymer PTzBI that are made up of benzodithiophene and imide-functionalized benzotriazole units. It is noted that increasing the molecular weight of N2200 can simultaneously enhance exciton generation and dissociation, reduce bimolecular recombination, and facilitate charge extraction. The films processed with the environmentally-friendly solvent 2-methyl-tetrahydrofuran (MeTHF) exhibit a more favourable film morphology than those processed with commonly used halogenated solvents. The all-PSC consisting of the high molecular weight N2200 and PTzBI processed with the environmentally friendly solvent MeTHF presents a remarkable power conversion efficiency of 9.16%, which is the highest value so far observed for all-PSCs. Of particular interest is that the PCE remains 6.37% with the active layer thickness of 230 nm. These observations imply the great promise of the developed all-PSCs for practical applications toward high-throughput roll-to-roll technology.

Non-fullerene acceptors based on fused-ring oligomers for efficient polymer solar cells via complementary light-absorption

We designed and synthesized two novel non-fullerene small molecule acceptors (IDT-N and IDT-T-N) that consist of indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (N) as a novel electron-withdrawing end group. IDT-N and IDT-T-N consisting of the naphthyl-based N group exhibited an expanded plane compared to phenyl-based indanone (INCN), which strengthened the intramolecular push–pull effect between the core donor unit and the terminal acceptor units. This strengthened effect resulted in a reduced bandgap that was beneficial for solar photon collection and increased short-circuit current density of the resulting devices. IDT-N and IDT-T-N exhibited red-shifted absorptions and smaller optical bandgaps than the corresponding phenyl-fused indanone end-capped chromophores. Both acceptors exhibited broad absorptions and energy levels that were well-matched with the donor materials. Polymer solar cells based on IDT-N and IDT-T-N and two representative polymer donors (PTB7-Th and PBDB-T) exhibited impressive photovoltaic performances. The devices based on the PBDB-T:IDT-N system exhibited a power conversion efficiency of up to 9.0%, with a short-circuit current density of 15.88 mA cm−2 and a fill factor of 71.91%. These results demonstrate that IDT-N and IDT-T-N are promising electron acceptors for use in polymer solar cells.

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.

Electron acceptors with a truxene core and perylene diimide branches for organic solar cells: the effect of ring-fusion

In this work, a star-shaped planar acceptor named FTr-3PDI was synthesized via ring-fusion between truxene core and three bay-linked perylene diimide (PDI) branches. Compared to the unfused non-planar acceptor Tr-3PDI, FTr-3PDI exhibits better structural rigidity and planarity, as well as more effective conjugation between truxene core and PDI branches. As a result, FTr-3PDI shows up-shifted energy levels, enhanced light absorption coefficient, increased electron mobility, and more favorable phase separation morphology in bulk-heterojunction (BHJ) blend films as compared to Tr-3PDI. Consequently, FTr-3PDI afforded higher power conversion efficiency (PCE) in BHJ solar cells when blended with a polymer donor PTB7-Th. This work demonstrates that ring-fusion is a promising molecular design strategy to combine the merits of truxene and PDI for non-fullerene acceptors used in organic solar cells.

Perylene Diimide Based Isomeric Conjugated Polymers as Efficient Electron Acceptors for All-polymer Solar Cells

We present here a series of perylene diimide (PDI) based isomeric conjugated polymers for the application as efficient electron acceptors in all-polymer solar cells (all-PSCs). By copolymerizing PDI monomers with 1,4-diethynylbenzene (para-linkage) and 1,3- diethynylbenzene (meta-linkage), isomeric PDI based conjugated polymers with parallel and non-parallel PDI units inside backbones were obtained. It was found that para-linked conjugated polymer (PA) showed better solubility, stronger π-π stacking, more favorable blend morphology, and better photovoltaic performance than those of meta-linked conjugated polymers (PM) did. Device based on PTB7-Th:PA (PTB7-Th:poly{4,8-bis[5-(2-ethylhexyl)-thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)- carbonyl]thieno[3,4-b]thiophene-4,6-diyl}) showed significantly enhanced photovoltaic performance than that of PTB7-Th:MA (3.29% versus 0.92%). Moreover, the photovoltaic performance of these polymeric acceptors could be further improved via a terpolymeric strategy. By copolymerizing a small amount of meta-linkages into PA, the optimized terpolymeric acceptors enabled to enhance photovoltaic performance with improved the short-circuit current density (Jsc) and fill factor (FF), resulting in an improved power conversion efficiency (PCE) of 4.03%.

Adjusting Aggregation Modes, Photophysical and Photovoltaic Properties of Diketopyrrolopyrrole-Based Small Molecules by Introducing B←N Bonds

Abstract The packing mode of small‐molecular semiconductors in thin films is an important factor that controls the performance of their optoelectronic devices. Designing and changing the packing mode by molecular engineering is challenging. Three structurally related diketopyrrolopyrrole (DPP)‐based compounds were synthesized to study the effect of replacing C−C bonds by isoelectronic dipolar B←N bonds. By replacing one of the bridging C−C bonds on the peripheral fluorene units of the DPP molecules by a coordinative B←N bond and changing the B←N bond orientation, the optical absorption, fluorescence, and excited‐state lifetime of the compounds can be tuned. The substitution alters the preferential aggregation of the molecules in the solid state from H‐type (for C−C) to J‐type (for B←N). Introducing B←N bonds thus provides a subtle way of controlling the packing mode. The photovoltaic properties of the compounds were evaluated in bulk heterojunctions with a fullerene acceptor and showed moderate performance as a consequence of suboptimal morphologies, bimolecular recombination, and triplet‐state formation.

11.2% All-Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

All-polymer solar cells (all-PSCs) that contain both p-type and n-type polymeric materials blended together as light-absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of small molecular-based organic solar cells. In this work, the interfacial stability is studied by using highly stable all-polymer solar cell as a platform. It is found that the thermally deposited metal electrode atoms can diffuse into the active layer during device storage, which consequently greatly decreases the power conversion efficiency. Fortunately, the diffusion of metal atoms can be slowed down and even blocked by using thicker interlayer materials, high-glass-transition-temperature interlayer materials, or a tandem device structure. Learning from this, homojunction tandem all-PSCs are successfully developed that simultaneously exhibit a record power conversion efficiency over 11% and remarkable stability with efficiency retaining 93% of the initial value after thermally aging at 80 °C for 1000 h.