Shanlin Zhang

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.

A Self‐Assembly Phase Diagram from Amphiphilic Perylene Diimides

Supramolecular forces govern self-assembly and further determine the final morphologies of self-assemblies. However, how they control the morphology remains hitherto largely unknown. In this paper, we have discovered that the self-assembled nanostructures of rigid organic semiconductor chromophores can be finely controlled by the secondary forces by fine-tuning the surrounding environments. In particular, we used water/methanol/hydrochloric acid to tune the environment and observed five different phases that resulted from versatile molecular self-assemblies. The representative self-assembled nanostructures were nanotapes, nanoparticles and their 1D assemblies, rigid microplates, soft nanoplates, and hollow nanospheres and their 1D assemblies, respectively. The specific nanostructure formation is governed by the water fraction, R(w), and the concentration of hydrochloric acid, [HCl]. For instance, nanotapes formed at low [HCl] and R(w) values, whereas hollow nanospheres formed when either the HCl concentration is high, or the water fraction is low, or both. The significance of this paper is that it provides a useful phase diagram by using R(w) and [HCl] as two variables. Such a self-assembly phase diagram maps out the fine control that the secondary forces have on the self-assembled morphology, and thus allows one to guide the formation toward a desired nanostructure self-assembled from rigid organic semiconductor chromophores by simply adjusting the two key parameters of R(w) and [HCl].

Phenyl-1,3,5-trithienyl-diketopyrrolopyrrole: a molecular backbone potentially affording high efficiency for solution-processed small-molecule organic solar cells through judicious molecular design.

Finding new molecular backbones is necessary for further advances in solution-processed small-molecule organic solar cells (SM-OSCs). Increasing molecular π conjugation generally enhances the light-harvesting ability, and the resulting strong π-π-stacking interactions improve the charge-carrier transport ability; both increase the efficiency. In this study, we focus on the phenyl-1,3,5-trithienyl (3T-P) backbone because of its C3 symmetry, planarity, and particularly high conjugation between the three arms through the core phenyl unit. When the three arms were functionalized with diketopyrrolopyrrole (DPP) units to afford 3D-T-P, only modest efficiency was achieved (1.16%). Introduction of 4,8-bis(2-(2-ethylhexylthienyl)) benzodithiophene (BDT) between the 3T-P and DPP units to give 3D-B-T-P enhanced the light-harvesting ability, and particularly improved the hole mobility by 1.5 orders of magnitude (5.91×10(-2) versus 1.05×10(-3) cm(2) V(-1) s(-1)). When using PC71BM as the acceptor material, 3D-B-T-P gave the best power conversion efficiency (PCE) of 2.27%, which is about 1.9 times higher than the best efficiency from 3D-T-P (≈1.16%). The efficiency can be improved up to 3.60% with 3% (v/v) of 1,8-diiodooctane (DIO) as the cosolvent and thermal annealing at 100 °C for 10 min. This PCE is, to the best of our knowledge, the highest efficiency reported to date among the phenyl-1,3,5-based C3-symmetric molecules. Removing one DPP unit from 3D-T-P to form 2D-T-P, or from 3D-B-T-P to form 2D-B-T-P both decreased the light-harvesting ability and the hole mobility, thereby affording lower efficiency. Taken together, our results demonstrate that the planar phenyl-1,3,5-trithienyl-based C3 -symmetric structure can be a promising backbone, and enhancing the conjugation of the 3D-T-P backbone can effectively improve the device performance.

Terminal moiety-driven electrical performance of asymmetric small-molecule-based organic solar cells

With respect to the successes from symmetric small molecules, asymmetric ones have recently emerged as an alternative choice. In this paper, we present the synthesis and photovoltaic properties of four asymmetric small molecule donors. The benzo[1,2-b:4,5-b′]dithiophene (BDT) end in the asymmetric push–pull CNR–DPP–BDT (CNR = octyl-2-cyano-3-(thiophen-2-yl)acrylate, DPP = diketopyrrolopyrrole) was tailored from hydrogen (H) to thiophene (T), 2-hexylbithiophene (HTT), and CNR, respectively, obtaining M1, M2, M3, and M4. In this order, the donor–donor interactions are enhanced with an increase of intermolecular forces, such as π–π-stacking and van de Waals forces, which enhances aggregation of the donor molecules. In parallel, the molecular dipolarity (|μ|) increases in this order from 5.39 D to 6.26 D, 6.34 D, and 6.92 D, respectively, gradually deviating from the value of PC71BM (5.01 D). The increase in the donor-to-PC71BM doplarity difference acts as another factor for enhancing the donor-to-PC71BM phase-separation. The donor/acceptor domain sizes increase from M1 (10 nm) to M2 (20 nm) and M3 (50 nm), and even formation of island-like mesostructured PC71BM aggregates (200 nm) for M4, corresponding to declined short-circuit current density (Jsc) and fill factor (FF) as well as hole mobility (μh) from M1 to M4. This work reveals that control over the terminal moieties of asymmetric small molecules can be an important factor in tailoring photovoltaic performance.