Guitao Feng

All-small-molecule organic solar cells based on an electron donor incorporating binary electron-deficient units

In this work, solution-processed organic solar cells with conjugated small molecules both as electron donors and electron acceptors were studied, where the influence of the chemical structures of the donor and acceptor on the device performance was systematically investigated. A small molecular donor incorporating binary electron-deficient units, diketopyrrolopyrrole and pentacyclic aromatic bislactam, was synthesized to provide a low band gap of 1.65 eV and low-lying energy levels. Three molecules, from a fullerene derivative to non-fullerene perylene bisimide-based acceptors, were selected as electron acceptors to construct organic solar cells. The results showed that fullerene-based solar cells provided power conversion efficiencies (PCEs) of up to 4.8%, while the non-fullerene solar cells also exhibited promising PCEs of 2.4% and 3.5%, with a photoresponse of up to 750 nm. Further analysis of the bulk-heterojunction systems between donors and acceptors revealed that the relatively low carrier mobilities of the non-fullerene acceptors and the large phase separations are mainly responsible for the less efficient solar cells. Our results demonstrate that molecules containing several electron-deficient units can effectively reduce the band gap of small molecules, and thus offer great potential for realizing high performance fullerene and non-fullerene solar cells.

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.

Poly(pentacyclic lactam-alt-diketopyrrolopyrrole) for field-effect transistors and polymer solar cells processed from non-chlorinated solvents

A semi-crystalline conjugated polymer based on two electron-deficient units, pentacyclic lactam (PCL) and diketopyrrolopyrrole (DPP), was designed and synthesized for application in field-effect transistors (FETs) and polymer solar cells (PSCs). The polymer has a high molecular weight, near-infrared absorption up to 900 nm and good solubility in toluene. When the polymer thin films were solution-processed from toluene with diphenyl ether as an additive, the FET devices achieved a high hole mobility of 0.81 cm(2) V-1 s(-1). With the same solution-processing solvents, bulk-heterojunction solar cells based on this polymer as an electron donor provided a power conversion efficiency of 4.7% with an optimal energy loss of 0.65 eV due to its deep lowest unoccupied molecular orbital level. Further study on the morphology of the pure polymer or blend thin films by atomic force microscopy, transmission electron microscopy and 2D grazing-incidence wide-angle X-ray scattering reveals that the new polymer has good crystalline property, which is mainly due to the coplanar nature of the conjugated backbone. This work demonstrates that conjugated polymers incorporating several electron-deficient units can be potentially used in high performance FETs and PSCs.

Enhanced electrochemical lithium storage activity of LiCrO2 by size effect

Cr8O21 was chemically lithiated using a lithium–biphenyl–dimethoxyethane solution. Lithiated Cr8O21 shows a structure in which as-formed LiCrO2 units are sandwiched between Cr2O3 superlattice layers. Chemically lithiated Cr8O21 shows a delithiation capacity of 200 mAh g−1. It means that LiCrO2 units in lithiated Cr8O21 are electrochemically active. This finding is opposite to previous reports that LiCrO2 materials have very poor Li-storage capacities. Our new result implies that LiCrO2 with extremely small domain size could show enhanced reactivity. This proposal is proved unambiguously by the fact that LiCrO2 powder materials with smaller grain size ( 50 nm). In addition, it is found that the cation mixing is more significantly in LiCrO2 materials with smaller grain size, which seems a key factor for the storage and transport of lithium in layered Cr-based materials. The cation mixing may also explain the result that the lattice parameters of LiCrO2 do not change significantly upon lithium extraction and insertion, investigated by in situ and ex situXRD techniques.

A new strategy for designing polymer electron acceptors: electronrich conjugated backbone with electron-deficient side units

Three “double-cable” conjugated polymers with thienopyrroledione-based backbone and perylene bisimide as side units were designed as electron acceptor for polymer-polymer solar cells. The polymers show broad absorption spectra and low-lying frontier energy levels due to both aromatic backbone and side units. The new double-cable polymers can be used as electron acceptor to combine with several electron donors, in which power conversion efficiencies above 3% could be achieved.

Multifunctional Diketopyrrolopyrrole-Based Conjugated Polymers with Perylene Bisimide Side Chains

Two conjugated polymers based on diketopyrrolopyrrole (DPP) in the main chain with different content of perylene bisimide (PBI) side chains are developed. The influence of PBI side chain on the photovoltaic performance of these DPP-based conjugated polymers is systematically investigated. This study suggests that the PBI side chains can not only alter the absorption spectrum and energy level but also enhance the crystallinity of conjugated polymers. As a result, such polymers can act as electron donor, electron acceptor, and single-component active layer in organic solar cells. These findings provide a new guideline for the future molecular design of multifunctional conjugated polymers.

Enhancing the photovoltaic performance of binary acceptor-based conjugated polymers incorporating methyl units

Three conjugated polymers incorporating pentacyclic lactam (PCL) and diketopyrrolopyrrole (DPP) units into conjugated polymers were designed and synthesized, in which the aromatic linkers between PCL and DPP varied from thiophene to methylthiophene. Methylated polymers were found to show slightly blue-shift absorption and high-lying energy levels compared to non-methylated polymers. The three polymers also exhibit good crystalline properties and hole mobilities up to 0.57 cm2 V−1 s−1 in field-effect transistors. Non-methylated polymers as electron donors in solar cells show an efficiency of 4.2% with a relatively low short circuit current density (Jsc) of 8.3 mA cm−2, while methylated polymers exhibit dramastically enhanced Jsc of 12.8 mA cm−2 and PCEs up to 6.1%. Micro-phase separation in bulk-heterojunction thin films were systematically investigated, in which methylated polymers in blended thin films were found to provide better micro-phase separation with small crystal domain. The observation can explain their improved photocurrent in solar cells. Our studies demonstrate that by intentionally structural modification, conjugated polymers containing several electron-deficient units can have the great potential application in high performance solar cells.