Chuanlang Zhan

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.

New advances in non-fullerene acceptor based organic solar cells

Non-fullerene organic solar cells (NF-OSCs), in which an n-type organic molecule instead of a fullerene derivative is utilized as the electron-acceptor material, have recently emerged as a new topic in the field of organic solar cells. Replacement of the traditional fullerene acceptor in the photoactive layer of a normal organic solar cell with the organic acceptor gives rise to several advantages, like light absorption and energy level tunability, diversity of donor-to-acceptor combination, and large-scale production of acceptor materials. Studies on NF-OSCs can be traced back to 1986, when the first bilayered organic solar cell was proposed. Unfortunately, they has been advancing very slowly and the power-conversion-efficiency (PCE) was only approaching or exceeding 2% up to 2012. Fast advances have been driven forward since 2013, when the PCE value first broke through 4%, and the reported PCE value has now reached about 8% after a short period of 3 years. If we turn to natural systems such as the photosynthesis systems I and II, in which Nature utilizes organic molecules to accomplish high-efficiency solar-to-chemical energy conversion through the cascade unidirectional electron–hole transfer paths, we can rationally expect an even higher PCE and a convincing future for NF-OSCs. In this review, we will address recent new progress in this sub-branch of organic solar cells.

Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications.

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic 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.

Switch from intra- to intermolecular H-bonds by ultrasound: induced gelation and distinct nanoscale morphologies.

During cooling of the ( R)-N-Fmoc-Octylglycine (Fmoc-OG)/cyclohexane solution, gelation is observed exclusively when ultrasound is used as an external stimulus, while deposit is obtained without sonication. The xerogel consists of entangled fibrous network made by interconnected nanofibers, while the deposit comprises large numbers of unbranched nanowires. It is found that the Fmoc-OG molecules form bilayer structures in both the deposit and the gel. However, the ratio ( R) between the Fmoc-OG molecules in a stable intramolecular H-bonding conformation and those in a metastable intermolecular H-bonding conformation can be tuned by the ultrasound, R (deposit) > R (gel). The increased population of the intermolecular H-bonding Fmoc-OG molecules induced by the ultrasonication facilitates to the interconnection of nanofibers for the formation of the fibrous network, and therefore gelation. The alteration in the morphologies and properties of the obtained nanomaterials induced by the ultrasound wave demonstrates a potential method for smart controlling of the functions of nanomaterials from the molecular level.

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.

Self-assembled hollow nanospheres strongly enhance photoluminescence.

We report that two molecular building blocks differ only by two protons, yet they form totally different nanostructures. The protonated one self-organized into hollow nanospheres (~200 nm), whereas the one without the protons self-assembled into rectangular plates. Consequently, the geometrically defined nanoassemblies exhibit radically different properties. As self-assembly directing units, protons impart ion-pairing and hydrogen-bonding probabilities. The plate-forming nanosystem fluoresces weakly, probably due to energy transfer among chromophores (Φ < 0.2), but the nanospheres emit strong yellow fluorescence (Φ ≈ 0.58-0.85).

Expanding the registry of aromatic amide foldamers: folding, photochemistry and assembly using diaza-anthracene units.

The synthesis of various 1,8-diaza-4,5-dialkoxy-2,7-anthracene dicarboxylic acid derivatives and their incorporation into cyclic and helically folded aromatic oligoamides are reported. The ability of the diaza-anthracene monomers to undergo photoaddition or head-to-tail photodimerization was investigated in the solid state and in solution. Quantitative conversion of a monomer diester to the corresponding head-to-tail photodimer could be achieved in the solid state without protection from oxygen. The formation of an emissive excimer between two diaza-anthracene units appended at the end of a helically folded oligomer was demonstrated. Intramolecular photodimerization was not observed in this compound, possibly due to the low thermal stability of the head-to-head photoadduct. A cyclic oligoamide composed of two diaza-anthracene and two pyridine units was shown to adopt a flat conformation and to form columnar stacks in the solid state. Longer, noncyclic oligoamides composed of one or two diaza-anthracene units were shown to adopt helical conformations that exist preferentially as double helical dimers.