Shengjian Liu

High-Efficiency Polymer Solar Cells via the Incorporation of an Amino-Functionalized Conjugated Metallopolymer as a Cathode Interlayer

An amino-functionalized conjugated metallopolymer PFEN-Hg was developed as a cathode interlayer for inverted polymer solar cells. The resulting devices exhibited significantly improved performance with power conversion efficiencies exceeding 9%. Moreover, good device performance was achievable with the PFEN-Hg over a wider range of film thickness, likely due to the Hg-Hg interactions and improved π-π stacking.

Enhanced Photovoltaic Performance by Modulating Surface Composition in Bulk Heterojunction Polymer Solar Cells Based on PBDTTT-C-T/PC71BM

For the blend film of PBDTTT-C-T:PC71 BM, the use of 1,8-diiodooctane as the solvent additive enriches the polymer at the top surface, so that a power conversion efficiency of 9.13% is recorded in the inverted polymer solar cell based on the blend, which is much higher than that of the device with conventional structure.

A Series of New Medium‐Bandgap Conjugated Polymers Based on Naphtho[1,2‐c:5,6‐c]bis(2‐octyl‐[1,2,3]triazole) for High‐Performance Polymer Solar Cells

A series of novel conjugated copolymers based on naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) are synthesized. These copolymers exhibit medium bandgaps of ≈1.9 eV. One of them demonstrates a high performance of up to 6.10% power conversion efficiency in a bulk-heterojunction (BHJ) solar-cell device. The performance can be further enhanced to 7.11% when applied in an inverted device architecture, using PF3 N-OX as an interfacial modifier.

Donor–Acceptor Copolymers Based on Thermally Cleavable Indigo, Isoindigo, and DPP Units: Synthesis, Field Effect Transistors, and Polymer Solar Cells

A series of donor-acceptor type of π-conjugated copolymers based on tert-butoxycarbonyl (t-Boc) substituted indigo, isoindigo or diketopyrrolopyrrole as the acceptor unit and a benzodithiophene derivative as the donor unit was designed and synthesized. These copolymers can be readily dissolved in organic solvents and can produce uniform films by solution deposition. Thermal treatment of copolymer films at 200 °C for 10 min resulted in elimination of t-Boc side groups in nearly quantitative yield as suggested by thermogravimetric analysis and Fourier transform infrared spectroscopy. The elimination of the bulky t-Boc side groups resulted in the emergence of N-H···O═C hydrogen bonding interactions by virtue of the lactam structures of the indigo, isoindigo and diketopyrrolopyrrole units. Of particular interests is the distinctly increased field-effect mobility of these copolymers after thermal treatment, which may arise from the enhanced coplanarity and intermolecular ordering of the indigo, isoindigo or diketopyrrolopyrrole units after elimination of the bulky t-Boc side groups. These results demonstrate that the incorporation of latent side groups provides a viable strategy to construct conjugated polymers that can attain more ordered intermolecular stacking by simple thermal treatments. On the other hand, despite the thermal cleavage of t-Boc groups can also lead to increased ordering of polymer chains when blending with [6,6]-phenyl C71 butyric acid methyl ester, the photovoltaic performances of the resulting bulk heterojunction solar cells did not obviously increase due to the serious phase separation and coarsening of the film morphology.

Synthesis of two-dimensional π-conjugated polymers pendent with benzothiadiazole and naphtho[1,2- c :5,6- c ]bis[1,2,5]thiadiazole moieties for polymer solar cells

Four new 2D donor-acceptor conjugated polymers were designed and synthesized. These new polymers comprised fluorene-alt-triphenylamine or carbazole-alt-triphenylamine as the backbones, and pendants with 2,1,3-benzothiadiazole (BT) or naphtho[ 1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT) in a triphenylamine unit as the side groups. By changing the acceptor BT for a stronger electron-withdrawing unit of NT moiety in the side chain, the energy levels, absorption spectra, band gaps, and charge-transport abilities of the resultant polymers could be effectively tuned. Bulk heterojunction solar cells with these polymers as the electron donors and (6,6)-phenyl-C71-butyric acid methyl ester as the electron acceptor exhibited high open-circuit voltage (more than 0.8 eV). The power conversion efficiency can be improved from 1.37% to 3.52% by replacing the BT with an NT moiety, which indicates that introducing NT as the side-chain building block can be an effective strategy to construct efficient 2D conjugated polymers for PSCs.

An alcohol soluble amino-functionalized organoplatinum(II) complex as the cathode interlayer for highly efficient polymer solar cells

A small-molecule amino-functionalized organoplatinum(II) complex Pt–N has been developed. The unique properties of Pt–N, including a well-defined chemical structure, an excellent solubility in environmentally friendly polar solvents and good electron extraction to high work-function metals, make it a promising candidate for cathode interfacial modification of solution processed multilayer polymer solar cells (PSCs). The resultant PSCs with an ITO/PEDOT:PSS/PTB7:PC71BM/interlayer/Al device configuration exhibited significantly improved efficiencies from 3.62% for an unmodified device to 8.89% by using a Pt–N cathode interlayer, benefiting from the dramatic enhancement in the open circuit voltage (0.53 V for bare Al PSCs to 0.75 V for Pt–N/Al PSCs), the fill factor (43.58% for bare Al PSC to 72.49% for Pt–N/Al PSC), and a slight increase in short-circuit current density. These results indicate that Pt–N is a new promising candidate as the cathode interlayer for highly efficient PSCs.

High-detectivity inverted near-infrared polymer photodetectors using cross-linkable conjugated polyfluorene as an electron extraction layer

We report an alcohol- and water-soluble and cross-linkable conjugated polymer used as an electron extraction layer (EEL) in near-infrared (NIR) polymer photodetectors (PDs) with an inverted device structure. Effectively suppressed dark current with significantly improved photocurrent result in enhanced detectivities for the inverted NIR polymer PDs incorporating the polymer EEL. Operating at room temperature, the inverted polymer PDs possess a spectral response from 350 nm to 1100 nm, responsivity of 116 mA W−1 and detectivity of 1.02 × 1013 cm Hz1/2 W−1. Our results provide a promising pathway for fabricating high-sensitivity inverted NIR polymer PDs.

Three pyrido[2,3,4,5-lmn]phenanthridine derivatives and their large band gap copolymers for organic solar cells

Three novel pyrido[2,3,4,5-lmn]phenanthridine derivatives of 4,9-dialkylpyrido[2,3,4,5-lmn]phenanthridine-5,10-dione, 10-alkoxy-4-alkylpyrido[2,3,4,5-lmn]phenanthridin-5-one, and 5,10-dialkoxypyrido[2,3,4,5-lmn]phenanthridine were synthesized as building blocks of copolymers Pa, Pb and Pc, giving relatively large band gaps of 2.13, 2.19 and 2.21 eV, respectively. Bulk-heterojunction (BHJ) solar cells were fabricated in a typical device configuration by utilizing the resulting polymers as the donor of the active layer. A power conversion efficiency (PCE) of 3.47% was achieved from the Pa-based device without any additives or thermal annealing. A further improved PCE of 4.54% was achieved in inverted device architecture, and it is hitherto the highest efficiency of BHJ polymer solar cells using a donor polymer with a band gap above 2.1 eV.

Dithienosilole-benzothiadiazole-based ternary copolymers with a D1–A–D2–A structure for polymer solar cells

A series of narrow band gap conjugated copolymers comprising two electron-rich (donor, D) and one electron-deficient (acceptor, A) moieties regularly alternating along the polymer backbone were designed and synthesized. The ternary copolymers with the repeating unit in a D1–A–D2–A manner were constructed by copolymerizing a bisstannyled-D1 (D1 = cyclopentadithiophene or benzodithiophene-derivatives) and a dibromo-monomer (Br–A–D2–A–Br, D2 = dithienosilole, A = benzothiadiazole) through a palladium-catalyzed Stille polymerization. It was recognized that the optical properties, frontier molecular orbital energy levels and the photovoltaic performance of the resulting copolymers can be influenced by modifying the copolymerized D1 moiety. By carefully optimizing the electron-donating behaviours and substitutions of the D1 unit, bulk-heterojunction solar cells with a power conversion efficiency of 4.11% were achieved based on an inverted device configuration of ITO/PFN-OX/PBDT-O-ADA:PC71BM/MoO3/Al. These results demonstrated that the construction of regularly alternating narrow band gap conjugated ternary copolymers can be an effective strategy for the development of electron-donating materials for polymer solar cells.

Novel donor–acceptor type conjugated polymers based on quinoxalino[6,5-f]quinoxaline for photovoltaic applications

The field of polymer solar cells has undergone tremendous advancement in terms of power conversion efficiency in the past decade. However, there is still an urgent requirement to further enhance device performance for achieving large-scale commercialization. Here, we report the design and synthesis of three novel conjugated polymers alternatively copolymerized by a newly developed 2,3,8,9-tetrakis(3-(alkoxy)phenyl)-6,12-di(thiophen-2-yl)-2,3,8,9-tetrahydroquinoxalino[6,5-f]quinoxaline (DTNQx) acceptor unit and a conventional two-dimensional alkylthienyl-substituted benzodithiophene (BDT) unit. These polymers possess an identical main chain but different side chains. The structure–property relationship is systematically studied. All polymers exhibit an optical bandgap around 1.7 eV and a HOMO energy level around −5.16 eV. Compared with the quinoxaline unit, fused quinoxaline DTNQx leads to a downshifted HOMO energy level in the resulting polymers by 0.04 eV while keeping the bandgap unchanged. By changing the side chain length and by introducing branched side chains, polymer properties including absorption spectra and hole mobilities could be finely tuned. Under optimal conditions, polymer P2 shows the highest hole mobility of 2.7 × 10−4 cm2 V−1 s−1 and the best photovoltaic performance, with Jsc = 9.1 mA cm−2, Voc = 0.88 V, FF = 48%, and PCE = 3.8%.