Xiangyi Meng

Three-Bladed Rylene Propellers with Three-Dimensional Network Assembly for Organic Electronics

Two kinds of conjugated C3-symmetric perylene dyes, namely, triperylene hexaimides (TPH) and selenium-annulated triperylene hexaimides (TPH-Se), are efficiently synthesized. Both TPH and TPH-Se have broad and strong absorption in the region 300-600 nm together with suitable LUMO levels of about -3.8 eV. Single-crystal X-ray diffraction studies show that TPH displays an extremely twisted three-bladed propeller configuration and a unique 3D network assembly in which three PBI subunits in one TPH molecule have strong π-π intermolecular interactions with PBI subunits in neighboring molecules. The integration of selenophene to TPH endows TPH-Se with a more distorted propeller configuration and a more compact 3D network assembly due to the Se···O interactions. A single-crystal transistor confirms that both TPH and TPH-Se possess good electron-transport ability. TPH and TPH-Se acceptor-based solar cells show high power conversion efficiency of 8.28% and 9.28%, respectively, which mainly results from the combined properties of broad and strong absorption ability, appropriate LUMO level, desirable aggregation, high electron mobility, and good film morphology with the polymer donor.

High performance all-polymer solar cells by synergistic effects of fine-tuned crystallinity and solvent annealing

Growing interests have been devoted to the design of polymer acceptors as potential replacement for fullerene derivatives for high-performance all polymer solar cells (all-PSCs). One key factor that is limiting the efficiency of all-PSCs is the low fill factor (FF) (normally <0.65), which is strongly correlated with the mobility and film morphology of polymer:polymer blends. In this work, we find a facile method to modulate the crystallinity of the well-known naphthalene diimide (NDI) based polymer N2200, by replacing a certain amount of bithiophene (2T) units in the N2200 backbone by single thiophene (T) units and synthesizing a series of random polymers PNDI-Tx, where x is the percentage of the single T. The acceptor PNDI-T10 is properly miscible with the low band gap donor polymer PTB7-Th, and the nanostructured blend promotes efficient exciton dissociation and charge transport. Solvent annealing (SA) enables higher hole and electron mobilities, and further suppresses the bimolecular recombination. As expected, the PTB7-Th:PNDI-T10 solar cells attain a high PCE of 7.6%, which is a 2-fold increase compared to that of PTB7-Th:N2200 solar cells. The FF of 0.71 reaches the highest value among all-PSCs to date. Our work demonstrates a rational design for fine-tuned crystallinity of polymer acceptors, and reveals the high potential of all-PSCs through structure and morphology engineering of semicrystalline polymer:polymer blends.

Alkyl Side-Chain Engineering in Wide-Bandgap Copolymers Leading to Power Conversion Efficiencies over 10%

A series of wide-bandgap (WBG) copolymers with different alkyl side chains are synthesized. Among them, copolymer PBT1-EH with moderatly bulky side chains on the acceptor unit shows the best photovoltaic performance with power conversion efficiency over 10%. The results suggest that the alkyl side-chain engineering is an effective strategy to further tuning the optoelectronic properties of WBG copolymers.

High Efficiency Nonfullerene Polymer Solar Cells with Thick Active Layer and Large Area

In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (Voc ) of 0.92 V, a short-circuit current density (Jsc ) of 16.4 mA cm-2 , and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm2 . These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.

Optimized Fibril Network Morphology by Precise Side-Chain Engineering to Achieve High-Performance Bulk-Heterojunction Organic Solar Cells

A polymer fibril assembly can dictate the morphology framework, in forming a network structure, which is highly advantageous in bulk heterojunction (BHJ) organic solar cells (OSCs). A fundamental understanding of how to manipulate such a fibril assembly and its influence on the BHJ morphology and device performance is crucially important. Here, a series of donor-acceptor polymers, PBT1-O, PBT1-S, and PBT1-C, is used to systematically investigate the relationship between molecular structure, morphology, and photovoltaic performance. The subtle atom change in side chains is found to have profound effect on regulating electronic structure and self-assembly of conjugated polymers. Compared with PBT1-O and PBT1-S, PBT1-C-based OSCs show much higher photovoltaic performance with a record fill factor (FF) of 80.5%, due to the formation of optimal interpenetrating network morphology. Such a fibril network strategy is further extended to nonfullerene OSCs using a small-molecular acceptor, which shows a high efficiency of 12.7% and an FF of 78.5%. The results indicate the formation of well-defined fibrillar structure is a promising approach to achieving a favorable morphology in BHJ OSCs.

A wide-bandgap conjugated polymer for highly efficient inverted single and tandem polymer solar cells

We synthesized a wide-bandgap conjugated polymer (named PTZ1) based on thienyl-substituted benzodithiophene as the donor unit and thiazolothiazole as the acceptor unit for photovoltaic applications. The polymer exhibits a desirable broad bandgap of 1.97 eV with the maximum absorption edge of 630 nm, a deep highest occupied molecular orbital (HOMO) energy level of −5.31 eV and a relatively high hole mobility of 3.86 × 10−3 cm2 V−1 s−1. Consequently, single junction PSCs based on PTZ1 exhibit outstanding performance with a PCE of 7.7% and a high Voc of 0.94 V, which are among the highest values for PSCs based on conjugated polymers with a broad bandgap near to 2.0 eV. The excellent performance is attributed to the high polymer crystallinity, favorable backbone orientation and continuous interpenetrating network in blend films. The tandem PSCs based on PTZ1 as the donor material in the front cell exhibited a high PCE of 10.3% with a Voc of 1.65 V.

High-performance conjugated terpolymer-based organic bulk heterojunction solar cells

Recently, conjugated terpolymers comprising three components have attracted tremendous attention. However, quite a few examples of high-performance terpolymers have been reported. We present here two novel terpolymers named PtDDA and PtDAA, in which bithiophene (BT) and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (T1) were chosen as the donor and acceptor units, respectively. Thieno[3,2-b]thiophene (TT) and thiazolo[5,4-d]thiazole (TTz) were used as the third component. It is interesting to find that the PtDDA terpolymer shows a typical D1–D2–D1–A1 structure while PtDAA shows a D1–A1–D1–A2 structure. Without using additives or post-annealing processes, PtDAA-based solar cells show a high PCE of 8.1%, with an unprecedented fill factor (FF) of 0.74, which is much higher than those of PtDDA-based devices (PCE = 3.4%, FF = 0.55). The high efficiency of 8.1% is one of the highest values reported so far for organic solar cells based on conjugated terpolymers. The high performance is mainly ascribed to the efficient carrier transport in the PtDAA:PC71BM active layer, high crystallinity of PtDAA, and high domain purity. The results suggest that constructing conjugated terpolymers with one donor and two acceptor units is an effective strategy for designing high-performance solar cell materials.

Influence of alkyl chains on photovoltaic properties of 3D rylene propeller electron acceptors

A series of propeller-shaped triperylene hexaimide (TPH) non-fullerene acceptors, featuring branched alkyl side chains with different lengths (TPH-4, TPH-5, TPH-6, and TPH-7), have been designed and synthesized. The effects of the branched alkyl chain length on the physical properties, thin-film microstructure, molecular packing, charge transport and the resulting photovoltaic properties of these materials have been systematically investigated. It was found that TPH-7 with the longest alkyl side chain showed the best photovoltaic performance compared with three other TPH molecules, and an outstanding power conversion efficiency (PCE) of 8.6% under AM 1.5G irradiation (100 mW cm−2) has been obtained using a wide-band-gap polymer PDBT-T1 as the electron donor. These results demonstrate that finely tailoring alkyl substituents on TPH-based small molecular acceptors critically impacts the structural order of thin films and molecular orientation, and thus the photovoltaic performance.

Enhanced open-circuit voltage in methoxyl substituted benzodithiophene-based polymer solar cells

The open-circuit voltage (Voc) is one of the important parameters that influence the power conversion efficiency (PCE) of polymer solar cells. Its value is mainly determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Therefore, decreasing the HOMO value of the polymer could lead to a high Voc and thus increasing the cell efficiency. Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene (BDT) unit. The polymer with the methoxyl functionl group (POBDT(S)-T1) exhibited a HOMO value of–5.65 eV, which is deeper than that (–5.52 eV) of polymer without methoxyl unit (PBDT(S)-T1). As a result, POBDT(S)-T1-based solar cells show a high Voc of 0.98 V and PCE of 9.2%. In contrast, PBDT(S)-T1-based devices show a relatively lower Voc of 0.89 V and a moderate PCE of 7.4%. The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.