Zhenhuan Lu

A Potential Perylene Diimide Dimer‐Based Acceptor Material for Highly Efficient Solution‐Processed Non‐Fullerene Organic Solar Cells with 4.03% Efficiency

A highly efficient acceptor material for organic solar cells (OSCs)--based on perylene diimide (PDI) dimers--shows significantly reduced aggregation compared to monomeric PDI. The dimeric PDI shows a best power conversion efficiency (PCE) approximately 300 times that of the monomeric PDI when blended with a conjugate polymer (BDTTTT-C-T) and with 1,8-diiodooctane as co-solvent (5%). This shows that non-fullerene materials also hold promise for efficient OSCs.

Solution-Processed DPP-Based Small Molecule that Gives High Photovoltaic Efficiency with Judicious Device Optimization

A solution-processed diketopyrrolopyrrole (DPP)-based small molecule, namely BDT-DPP, with broad absorption and suitable energy levels has been synthesized. The widely used solvents of chloroform (CF) and o-dichlorobenzene (o-DCB) were used as the spin-coating solvent, respectively, and 1,8-diiodooctane (DIO) was used as additive to fabricate efficient photovoltaic devices with BDT-DPP as the donor material and PC71BM as the acceptor material. Devices fabricated from CF exhibit poor fill factor (FF) of 43%, low short-circuit current density (Jsc) of 6.86 mA/cm(2), and moderate power conversion efficiency (PCE) of 2.4%, due to rapid evaporation of CF, leading to poor morphology of the active layer. When 0.3% DIO was added, the FF and Jsc were improved to 60% and 8.49 mA/cm(2), respectively, because of the better film morphology. Active layer spin-coated from the high-boiling-point solvent of o-DCB shows better phase separation than that from CF, because of the slow drying nature of o-DCB, offering sufficient time for the self-organization of active-layer. Finally, using o-DCB as the parent solvent and 0.7% DIO as the cosolvent, we obtained optimized devices with continuous interpenetrating network films, affording a Jsc of 11.86 mA/cm(2), an open-circuit voltage (Voc) of 0.72 V, an FF of 62%, and a PCE of 5.29%. This PCE is, to the best of our knowledge, the highest efficiency reported to date for devices prepared from the solution-processed DPP-based small molecules.

Significant improvement of photovoltaic performance by embedding thiophene in solution-processed star-shaped TPA-DPP backbone

Solution-processed star-shaped triphenylamine (TPA) derivatives and dialkylated diketopyrrolopyrrole (DPP)-based small molecules have been widely studied because they both yield promising photon-to-electron conversion. However, the power conversion efficiency (PCE) of covalent star-shaped TPA-DPP derivatives is still very low. To design star-shaped TPA-DPP derivatives with better photovoltaic performance, we embedded a thiophene ring in between the TPA and DPP units, namely TPA-T-DPP, and reported the comparative studies of the optoelectronic and photovoltaic properties of TPA-DPP and TPA-T-DPP. Benefiting from the covalent thiophene bridges, compared to the TPA-DPP solid film, the TPA-T-DPP film showed enhanced light-harvesting ability, for instance, an improved absorptivity (Abs. = 1.72/100 nm vs. 1.23/100 nm), a broader absorption band (131 nm vs. 107 nm) and a narrower band gap (1.86 eV vs. 1.91 eV), from cyclic voltammetry. Studies on the photovoltaic properties revealed that the best TPA-T-DPP:PC71BM based device showed a dramatically enhanced PCE of 2.95%, increased by 2.14 times with respect to the efficiency of the best TPA-DPP based device (1.38%). The improvement of PCE also was observed in the small molecule:PC61BM based devices (1.81% vs. 1.13%). Test of the hole mobilites of the active layer provided further insight into the impact of the embedded thiophene units. The hole mobility of the TPA-T-DPP:PC71BM blended films was higher by about one order of magnitude (1.16 × 10−2 cm2 V−1 s−1) than that of the TPA-DPP:PC71BM blended films (3.85 × 10−3 cm2 V−1 s−1). These results clearly indicated that embedding the thiophene ring enlarged the conjugation, thus enhanced the light-harvesting ability and hole mobility, while further significantly improving the device performance. Additionally, TPA-T-DPP was also used as the electron-acceptor material, and the best P3HT:TPA-T-DPP based device exhibited a very high open-circuit voltage (1.14 V), which was among the highest values reported for single-layered OSC devices.

Additive-Assisted Control over Phase-Separated Nanostructures by Manipulating Alkylthienyl Position at Donor Backbone for Solution-Processed, Non-Fullerene, All-Small-Molecule Solar Cells

A non-fullerene, all-small-molecule solar cell (NF-SMSC) device uses the blend of a small molecule donor and a small molecule acceptor as the active layer. Aggregation ability is a key factor for this type of solar cell. Herein, we used the alkylthienyl unit to tune the aggregation ability of the diketopyrrolopyrrole (DPP)-based small molecule donors. Replacing two alkoxyl units in BDT-O-DPP with two alkylthienyl units yields BDT-T-DPP, and further introducing another two alkylthienyl units into the backbone produces BDT-T-2T-DPP. With the introduction of alkylthienyl, the backbone becomes twisted. As a result, the ππ-stacking strength, aggregation ability, and crystallite size all obey the sequence of BDT-O-DPP > BDT-T-DPP > BDT-T-2T-DPP. When selected a reported perylene diimide dimer of bis-PDI-T-EG as acceptor, the best NF-SMSC device exhibits a power conversion efficiency of 1.34, 2.01, and 1.62%, respectively, for the BDT-O-DPP, BDT-T-DPP, and BDT-T-2T-DPP based system. The BDT-T-DPP/bis-PDI-T-EG system yields the best efficiency of 2.01% among the three combinations. This is due to the moderate aggregation ability of BDT-T-DPP yields moderate phase size of 30-50 nm, whereas the strong aggregation ability of BDT-O-DPP gives a bigger size of 50-80 nm, and the weak aggregation ability of BDT-T-2T-DPP produces a smaller size of 10-30 nm. The BDT-T-DPP/bis-PDI-T-EG combination exhibits balanced hole/electron mobility of 0.022/0.016 cm(2)/(V s), whereas the BDT-O-DPP/bis-PDI-T-EG and the BDT-T-2T-DPP/bis-PDI-T-EG blend show a hole/electron mobility of 0.0011/0.0057 cm(2)/(V s) and 0.0016/0.11 cm(2)/(V s), respectively.

Benzodithiophene bridged dimeric perylene diimide amphiphiles as efficient solution-processed non-fullerene small molecules

Two amphiphilic and highly twisting perylene diimide (PDI) dimers, Bis-PDI-BDT-EG, were synthesized by using 4,8-bis(2-(2-ethylhexylthienyl) benzo[1,2-b:4,5-b′]dithiophene (BDT-T) and 4,8-bis(2-ethylhexyloxy) BDT (BDT-O) as covalent bridges at the 7,7′-positions, while at the 1,1′-positions, they were functionalized with weakly solvophobic 2-methoxylethoxyl (EG) units. The subtle structural differences between BDT-O and BDT-T lead to distinct aggregation abilities: with respect to the over-strong aggregation ability of the BDT-O bridged dimer 2, the BDT-T bridged dimer 1 shows largely reduced aggregation ability and is solution-processable in the commonly used organic solvent. The highly twisted conformation between the PDI–BDT–PDI planes produced steric-pairing effects, which directed ordered packing of dimer 1. When dimer 1 was blended with P3HT in a weight D/A ratio of 1 : 2.5, the electron mobility (μe) was 3.4 × 10−5 cm2 V−1 s−1 and the best PCE was 1.72%. Slowing the solvent evaporation speed benefited the packing order of the PDI dimer, and the μe value was slightly increased to 6.0 × 10−5 cm2 V−1 s−1. The best PCE was improved up to 1.87%. The μe was further increased up to 3.4 × 10−4 cm2 V−1 s−1 when the D/A ratio was decreased down to 1 : 2.2 and the best PCE of 1.95% was achieved. Solid absorption spectra and XRD data of the blended films supported the improvement of the packing order of the PDI dimer by slowing the solvent annealing speed. AFM images supported the largely reduced aggregation ability of dimer 1 when blended with P3HT. The observed phase size of 35 nm is formed under the slow solvent annealing speed and a D/A ratio of 1 : 2.2. Our results revealed that the amphiphilic nature of the bridged aromatic unit reduces the aggregation ability and facilitates the ordered packing of the PDI units, contributing to the improvement of efficiency.

Effects of structure-manipulated molecular stacking on solid-state optical properties and device performances

Four conjugated copolymers with phthalimide (PhI) or thieno[3,4-c]pyrrole-4,6-dione (TPD) as the acceptor, thiophene (T) or selenophene (Se) as the spacer and 3,3′-didodecyl-2,2′-bithiophene (BT) as the common donor, namely, PPhI-T, PPhI-Se, PTPD-T and PTPD-Se, have been synthesized and the effects of intra- and intermolecular interactions on the optical properties, molecular stacking, and organic electronic device performances were investigated. The intramolecular S(Se)⋯O (carbonyl) interactions between the spacer and the PhIs or TPDs carbonyl and the intermolecular reciprocity between the polymeric backbones differ from each other as the spacer and the acceptor were varied. Among the four polymers, PPhI-T with the weakest intramolecular S⋯O interaction and intermolecular backbone reciprocity exhibited the poorest photovoltaic performance with a PCE of 0.31%. When the T spacer was replaced by the more polarized Se spacer, the resultant copolymer PPhI-Se exhibited stronger intra- and intermolecular interactions, resulting in better optical properties with a PCE of 0.94% when blended with PC71BM. When PhI is replaced with the more polarized TPD unit, the TPD-based polymers, PTPD-T and PTPD-Se, showed even better coplanarity compared to that of the PhI-based polymers, with a PCE of 2.04% for PTPD-T and 1.52% for PTPD-Se blended with PC71BM. To the best of our knowledge, this is the first systematic study on the influences of structure-manipulated molecular stacking on solid-state optical properties and electronic device performance through modulations of the intramolecular and intermolecular interactions.

In situ ion-exchange synthesis of SnS2/g-C3N4 nanosheets heterojunction for enhancing photocatalytic activity

In this paper, free standing graphitic carbon nitride (g-C3N4) nanosheets have been synthesized by mixed solvents (water/IPA = 2/1) liquid phase exfoliation, and a series of SnS2/g-C3N4 heterojunctions with different contents of SnS2 have been prepared via a simple ion-exchange process. Exfoliated g-C3N4 presents a two-dimension sheet-like structure with the thickness of 2.8 nm and small SnS2 nanoparticles with diameter of 5–10 nm are well anchored on the surface of g-C3N4 nanosheets, which was proved by transmission electron microscopy (TEM) and atomic force microscope (AFM). The 4.0-SnS2/g-C3N4 sample shows the highest photocurrent density of 13.66 μA cm−2 at 0.8 V, which is about 1.5 time of the g-C3N4 nanosheets and 2 times of the bulk g-C3N4, respectively. Photocatalytic measurement also demonstrated that constructing heterojunction of SnS2/g-C3N4 can improve the photocatalytic efficiency as compared to pure g-C3N4 and g-C3N4 nanosheets. The highly effective photoelectrochemical and photocatalytic activities of SnS2/g-C3N4 heterojunctions are attributed to the efficient separation of photogenerated hole–electron pairs. This work may provide a novel concept for the rational design of high performance g-C3N4-based photocatalysts.

A new solution-processed diketopyrrolopyrrole donor for non-fullerene small-molecule solar cells

In solution-processed non-fullerene small-molecule solar cells (NF-SMSCs), the bulk-heterojunction active layer is blended by a small molecule donor and a non-fullerene small molecule acceptor. Synthesis of solution-processed small molecule donors is of the same importance as designing non-fullerene small molecule acceptors. In this paper, a new solution-processed diketopyrrolopyrrole (DPP)-based small molecule donor, namely DPP-BDT-T, was synthesized. The pure DPP-BDT-T film covers a broad spectrum from 500 nm to 700 nm with a low band gap of 1.72 eV. By choosing our newly reported perylene diimide (PDI) dimer, bis-PDI-T-EG, as the non-fullerene small molecule acceptor, the best NF-SMSC device showed a low efficiency of 0.12%. When using 2% 1,8-diiodooctane (DIO) as the additive, more acceptor molecules formed into π–π-stacks, accompanied by the increase of the phase size from 15 nm to 50 nm and the formation of continuous interpenetrating networks. This in turn enhanced the hole and electron mobilities (μh = 1.6 × 10−2vs. 5.8 × 10−4 cm2 V−1 s−1 and μe = 2.3 × 10−5vs. 6.1 × 10−7 cm2 V−1 s−1) and the efficiency was enhanced to 1.6%. In another respect, the fluorescent emission from the blend films was enhanced by 10 times after using 2% DIO as the additive, suggesting less efficient photon-induced exciton separation at the interfaces of the donor and acceptor nanostructures. Accordingly, our case suggests that efficient sweepout of the separated electrons and holes from the nanostructural interfaces plays a role for efficient NF-SMSCs.

Large-scale, ultra-dense and vertically standing zinc phthalocyanine π–π stacks as a hole-transporting layer on an ITO electrode

Vertically standing π–π stacks play a key role in advancing the charge transporting properties in the fields of some organic materials and devices such as organic solar cells, organic light-emitting diodes and photodetector. However, realization of large-scale, ultra-dense and vertically standing π–π stacks of organic semiconductors is still a big challenge. By using an amide armed ZnPc–COOH molecule, we show herein a facile solution deposition method to prepare large-scale (>2 × 3 mm2), ultra-dense (completely covering the ITO surface) and vertically standing π–π stacks through supramolecular self-assembly. These vertically standing π–π stacks show a high conductivity and hole mobility, of the order of 10−3 S cm−1 and 10−3 cm2 V−1 s−1, respectively, and act as the hole-transporting layer on the ITO electrode in organic solar cells.

The leverage effect of the relative strength of molecular solvophobicity vs. solvophilicity on fine-tuning nanomorphologies of perylene diimide bolaamphiphiles

Previously, we have found that full protonation of the two pyridyloxyl groups of 1,7-bispyridyloxyl-N,N′-bis(2-ethylhexyl)perylene diimide (PDI) (molecule 1) leads to formation of highly fluorescent nanospheres, due to formation of 1,7-bis(4-oxylpyridinium chloride) dramatically enhancing the inter-chromophore interactions in the bay-region (J. Am. Chem. Soc., 2011, 133, 11022–11025; Chem.–Eur. J., 2012, 18, 12305–12313). Molecular modeling revealed that the two pyridyloxyl groups in molecule 1 pointed outside the same facet of the PDI plane, forming a rigid PDI-based bolaamphiphile. In order to more fully investigate the effects of the molecular solvophobicity on the bay-region vs. the molecular solvophilicity including that from the imide-direction and from the solvophilic PDI unit, Fsolvophob/solvophil, on fine-tuning nanomorphologies and properties, we reduced the molecular solvophilicity by replacing the two 2-ethylhexyl (EH) tails in molecule 1 with two shorter cyclohexyl (CH) tails, while maintaining the two 1,7-bispyridyloxyl units, forming molecule 2. Furthermore, we replaced one pyridyloxyl group in molecule 2 with another weaker solvophobic 2-methoxyethoxyl unit, forming molecule 3 to tune the molecular solvophobicity in the bay-region. Morphological studies demonstrated that molecule 2 formed 70–400 nm sized hollow nanospheres in a polar solvent mixture of dichloromethane (DCM)–ethanol (EtOH) and ∼100 nm sized hollow nanoparticles in a weak apolar environment of DCM–methylcyclohexane (MCH) mixture with RMCH = 10–40% (v/v). Upon a further increase of the surrounding apolarity by increasing the RMCH, plate morphologies of nanorods and microplates formed, accompanying with the π–π-stacking changing from the co-facial mode to slippage mode. Differently, molecule 3 always formed platelike nanostructures such as nanotapes in DCM–EtOH mixtures and nano-rhombuses in DCM–MCH mixtures with the molecules adopting co-facial π–π-stacking in both nanostructures. Taken together, the self-assembly and the final nanomorphologies of the PDI-based bolaamphiphiles are both significantly controlled by a small change of Fsolvophob/solvophil and such a leverage effect of the control from Fsolvophob/solvophil is amplified by changing the solvent polarity, for example, fine-tuning REtOH and RMCH.