Andong Zhang

An Electron Acceptor with Porphyrin and Perylene Bisimides for Efficient Non‐Fullerene Solar Cells

A star-shaped electron acceptor based on porphyrin as a core and perylene bisimide as end groups was constructed for application in non-fullerene organic solar cells. The new conjugated molecule exhibits aligned energy levels, good electron mobility, and complementary absorption with a donor polymer. These advantages facilitate a high power conversion efficiency of 7.4 % in non-fullerene solar cells, which represents the highest photovoltaic performance based on porphyrin derivatives as the acceptor.

Halogenated conjugated molecules for ambipolar field-effect transistors and non-fullerene organic solar cells

A series of halogenated conjugated molecules, containing F, Cl, Br and I, were easily prepared via Knoevenagel condensation and applied in field-effect transistors and organic solar cells. Halogenated conjugated materials were found to possess deep frontier energy levels and high crystallinity compared to their non-halogenated analogues, which is due to the strong electronegativity and heavy atom effect of halogens. As a result, halogenated semiconductors provide high electron mobilities up to 1.3 cm2 V−1 s−1 in transistors and high efficiencies over 9% in non-fullerene solar cells.

Pyridine-bridged diketopyrrolopyrrole conjugated polymers for field-effect transistors and polymer solar cells

Five wide or medium band gap diketopyrrolopyrrole (DPP)-based conjugated polymers with pyridine as bridges were developed for organic field-effect transistors (OFETs) and polymer solar cells (PSCs). By introducing copolymerized aromatic building blocks from strong electron-donating units to electron-deficient units into the conjugated backbone, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the DPP polymers were tailored to the low-lying position. Therefore, the polarity of charge transport in OFETs can be switched from p-type to n-type. The DPP polymer with a low-lying LUMO of −3.80 eV provides a hole-only mobility of 2.95 × 10−2 cm2 V−1 s−1, while an electron-only mobility of 1.24 × 10−3 cm2 V−1 s−1 is found in the DPP polymer with a LUMO of −4.22 eV. Further investigation of photovoltaic cells based on these DPP polymers shows a modest power conversion efficiency (PCE) of around 2%. Our results demonstrate that wide band gap pyridine-bridged DPP polymers have potential application in OFETs and OSCs by adjusting their energy level with alternated units on the conjugated backbone.

Bisperylene bisimide based conjugated polymer as electron acceptor for polymer-polymer solar cells

Perylene bisimide (PBI) unit has been widely used to design conjugated materials, which can be used as electron acceptor in organic solar cells due to its strong electron-deficient ability. In this work, a conjugated polymer based on PBI dimer as monomer was designed, synthesized, and compared to the conjugated polymer containing single PBI as repeating units. The two conjugated polymers were found to have similar molecular weight, absorption spectra and energy levels. Density functional theory calculation revealed that the PBI dimer-based polymer exhibited highly twisted conjugated backbone due to the large dihedral angle between the two PBI units. The PBI-based polymers as electron acceptor were applied into polymer-polymer solar cells, in which PBI dimer-based polymer solar cells were found to show a high short circuit current density (Jsc = 11.2 mA∙cm−2 and a high power conversion efficiency (PCE) of 4.5%. In comparison, the solar cells based on PBI-based polymer acceptor only provided a Jsc of 7.2 mA∙cm−2 and PCE of 2.5%. The significantly enhanced PCE in PBI dimer-based solar cells was attributed to the mixed phase in blended thin films, as revealed by atom force microscopy. This study demonstrates that PBI dimer can be used to design polymer acceptors for high performance polymerpolymer solar cells.

A perylene bisimide derivative with a LUMO level of −4.56 eV for non-fullerene solar cells

Conjugated polymers with LUMO levels of −4.00 eV and a perylene bisimide derivative with a LUMO level of −4.56 eV were used in non-fullerene solar cells in which power conversion efficiencies up to 1.4% were achieved.

Bis(perylene diimide) with DACH bridge as non-fullerene electron acceptor for organic solar cells

In this paper, we designed, synthesized, and characterized a set of non-fullerene small molecules based on bis(perylene diimide) with DACH bridge. Theoretical calculations make clear that the introduction of the DACH bridge into PPDI forms a U-shape framework, with pi–pi interactions between PDIs. We investigate the performances of non-fullerene solar cells comprising racemic and enantiomerically pure DACH-PPDIs as the electron acceptor and PTB7-Th as the electron donor. As a consequence, a power conversion efficiency (PCE) of 4.68% is achieved with inverted device architecture. Furthermore, the device behaviour, morphological feature and charge transport properties were also investigated. It is a potential way to design highly efficient non-fullerene organic solar cell by tuning the structure of PDI to reach highly efficient photovoltaic performances.

Perfluoroalkyl-substituted conjugated polymers as electron acceptors for all-polymer solar cells: the effect of diiodoperfluoroalkane additives

A series of six diketopyrrolopyrrole (DPP) based conjugated polymers with a varying content of solubilizing perfluoroalkyl chains were synthesized. Based on a systematic investigation of the influence of the solvent on the photovoltaic performance, it is found that 1,6-diiodoperfluorohexane (IC6F12I) is an effective solvent additive to enhance the power conversion efficiency (PCE) of DPP polymers with perfluoroalkyl side chains. The polymers consist of thiazole-flanked DPP units that alternate along the main chain with varying ratios of thiophene (T) and perfluoroalkyl benzodithiophene (FBDT) units. The polymers possess high molecular weights, narrow band gaps and good crystalline properties. The DPP polymers were used as electron acceptors in bulk heterojunction solar cells with another DPP polymer as the electron donor. A solvent mixture of CHCl3 : 1-chloronaphthalene (1-CN) is found to provide the best PCE of 2.9% in non-fluorine based DPP polymer solar cells, but yields a low PCE of 0.52% for perfluoroalkyl-containing polymer solar cells. Perfluoroalkyl-containing polymer solar cells fabricated from CHCl3 with IC6F12I as the processing additive show a significantly improved PCE of 2.1%. The morphology analysis of the blend films reveals that IC6F12I as an additive improves the micro-phase separation between the polymer donor and acceptor, which results in enhanced charge generation.

Conjugated polymer acceptors based on fused perylene bisimides with a twisted backbone for non-fullerene solar cells

In this work, we present a new strategy to use fused and twisted conjugated backbones to construct conjugated small molecules and polymers as electron acceptors for non-fullerene solar cells. The new conjugated materials containing binary perylene bisimide (PBI) units linked with coplanar thieno[3,2-b]thiophene (as trans-PBI) or twisted thieno[2,3-b]thiophene (as cis-PBI) and the corresponding conjugated polymer cis-polyPBI were developed. A fused conjugated backbone ensures good charge transport, in which the twisted polymer cis-polyPBI was found to show the highest electron mobility of 1.2 × 10−2 cm2 V−1 s−1 in field-effect transistors. Meanwhile, a twisted conjugated backbone effectively prevents the aggregation and crystallization of PBI units, resulting in isotropic charge transport and finely tuned micro-phase separation in bulk-heterojunction thin films. The high electron mobility and isotropic crystallinity make the fused and twisted electron acceptors achieve high power conversion efficiencies above 6% in non-fullerene solar cells, while the coplanar molecule trans-PBI as the electron acceptor shows a very low efficiency of 0.13%.

All polymer solar cells with diketopyrrolopyrrole-polymers as electron donor and a naphthalenediimide-polymer as electron acceptor

Four typical diketopyrrolopyrrole (DPP)-based conjugated polymers were used as electron donors in all-polymer solar cells (PSCs) with a naphthalenediimide-based polymer N2200 as the electron acceptor. The four DPP polymers have near-infrared absorption spectra up to 1000 nm and suitable energy levels for charge separation from donor to acceptor. DPP polymer : N2200 cells were found to have high open circuit voltages in comparison to fullerene-based solar cells but with low short circuit current densities and fill factors, so that the power conversion efficiencies of these cells were relatively low (0.45–1.7%). These blends relatively had balanced but low hole and electron mobilities from space charge limit current measurements, small surface roughness, and highly quenched photoluminescence (PL) from steady-state PL. These studies show that the low photocurrent and performance arise from the miscibility of the DPP and N2200 polymers, which enhances the charge recombination. The finding was further confirmed by grazing incidence X-ray diffraction and resonant soft X-ray scattering. All the PSCs based on DPP polymers were investigated, opening further studies based on these systems due to the broad absorption, high carrier mobilities and good crystalline properties of DPP polymers.

Conjugated polymers with deep LUMO levels for field-effect transistors and polymer–polymer solar cells

Three thiazole-bridged DPP polymers with deep lowest unoccupied molecular orbital (LUMO) levels were designed for field-effect transistors (FETs) and polymer–polymer solar cells. By introducing thiazole–thiazole coupled segments, perfluoroalkyl side chains or strong electron-deficient naphthalenediimide units into the conjugated backbone the three thiazole-bridged DPP polymers have LUMO levels of −4.0 to −4.4 eV. The three DPP polymers exhibit optical absorption in the near-infrared region, crystallinity and an electron mobility of around 0.01 cm2 V−1 s−1 in bottom contact FETs. The polymers were applied as electron acceptors in polymer–polymer solar cells to provide PCEs of around 0.4%. The low PCEs are mainly due to low short-circuit currents (Jsc) and attributed to large phase separation. Our results demonstrate several efficient strategies to lower the energy levels of conjugated polymers in order to be used as universal acceptors for photovoltaic cells.