Liuliu Feng

Achieving over 10% efficiency in a new acceptor ITTC and its blends with hexafluoroquinoxaline based polymers

A new small molecule, ITTC, bearing an indacenodithieno[3,2-b]thiophene core and a 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile end group, was designed, synthesized and characterized as a non-fullerene electron acceptor. The ITTC possesses strong and broad light absorption, high and balanced charge mobility and a nanoscale interpenetrating morphology when blended with a recently synthesized hexafluoroquinoxaline based polymer donor-HFQx-T. HFQx-T was obtained from a Stille coupling copolymerization of a 2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene electron donating unit and a hexafluoroquinoxaline based electron accepting unit. The device employing HFQx-T as the donor and ITTC as the acceptor delivered a power conversion efficiency (PCE) of 8.19% without any post-treatment. After thermal annealing, an impressive PCE of 10.4% was obtained. This performance is among the highest PCEs reported for fullerene-free polymer solar cells up to date. This study demonstrates the great potential of ITTC as n-type materials for organic electronics.

Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency

Through introducing six fluorine atoms onto quinoxaline (Qx), a new electron acceptor unit-hexafluoroquinoxaline (HFQx) is first synthesized. On the basis of this unit, we synthesize a new donor-acceptor (D-A) copolymer (HFQx-T), which is composed of a benzodithiophene (BDT) derivative donor block and an HFQx accepting block. The strong electron-withdrawing properties of fluorine atoms increase significantly the open-circuit voltage (Voc) by tuning the highest occupied molecular orbital (HOMO) energy level. In addition, fluorine atoms enhance the absorption coefficient of the conjugated copolymer and change the film morphology, which implies an increase of the short-circuit current density (Jsc) and fill factor (FF). Indeed, the HFQx-T:ITIC blended film achieves an impressive power conversion efficiency (PCE) of 9.4% with large short-current density (Jsc) of 15.60 mA/cm2, high Voc of 0.92 V, and FF of 65% via two step annealing (thermal annealing (TA) and solvent vapor annealing (SVA) treatments). The excellent results obtained show that the new copolymer HFQx-T synthesized could be a promising candidate for organic photovoltaics.

Optimizing the conjugated side chains of quinoxaline based polymers for nonfullerene solar cells with 10.5% efficiency

Quinoxaline (Qx) has an easily modifiable structure, which allows for fine-tuning its properties through optimizing the length of side chains and the kinds of aromatic rings in conjugated side chains. Qx based polymer PBDTT-ffQx with alkoxy substituted fluorobenzene side chains shows a power conversion efficiency (PCE) of 8.47% in nonfullerene solar cells. In this work, we change the alkoxy substituted fluorobenzene side chains to alkyl substituted fluorothiophene in the Qx unit and employ the benzodithiophene (BDT) unit with different lengths of alkyl thiophene side chains to obtain two new Qx based polymers, named TTFQx-T1 and TTFQx-T2, respectively. The TTFQx-T1:ITIC blend film has clear nanoscale phase separation and a suitable domain size, and the device with the TTFQx-T1:ITIC blend film shows lower geminate recombination and higher charge mobility than those with TTFQx-T2:ITIC. Therefore, the TTFQx-T1:ITIC based device exhibits a higher PCE of 10.52% than that based on the TTFQx-T2:ITIC blend with a moderate PCE of 7.22%. The inverted devices from TTFQx-T1:ITIC blends with a larger active area of 16 mm2 than the conventional device (4.5 mm2) show a good PCE of 9.21%. The results highlight that the alkyl lengths and the kinds of aromatic rings in side chains are significant for the development of high performance photovoltaic polymers. Optimizing the conjugated side chains of Qx based polymers is an efficient way to improve photovoltaic properties, and Qx based polymers are potential candidates for fabricating highly efficient polymer solar cells.

A New Electron Acceptor with meta‐Alkoxyphenyl Side Chain for Fullerene‐Free Polymer Solar Cells with 9.3% Efficiency

Abstract A new small molecule acceptor, m‐ITIC‐OR, based on indacenodithieno[3,2‐b]thiophene core with meta‐alkoxyphenyl side chains, is designed and synthesized. The m‐ITIC‐OR film shows broader and redshift absorption compared to its solution and matched energy levels with a hexafluoroquinoxaline‐based polymer donor‐HFQx‐T. Here, polymer solar cells (PSCs) by blending an HFQx‐T donor and an m‐ITIC‐OR acceptor as an active layer deliver the power conversion efficiency (PCE) of 6.36% without any posttreatment. The investigations demonstrate that the HFQx‐T:m‐ITIC‐OR blend films possess higher and more balanced charge mobility, negligible bimolecular recombination, and nanoscale interpenetrating morphology after thermal annealing (TA) treatment. Through a simple TA treatment at 150 °C for 5 min, an impressive PCE of 9.3% is obtained. This efficiency is among one of the highest PCEs for additive free PSCs. This is the first time alkoxyphenyl side chain is introduced into nonfullerene electron acceptor; more interestingly, the new electron acceptor (m‐ITIC‐OR) in this work shows a great potential for highly efficient photovoltaic properties.

Thieno[3,2-b]pyrrolo-Fused Pentacyclic Benzotriazole-Based Acceptor for Efficient Organic Photovoltaics

A novel nonfullerene small molecular acceptor (BZIC) based on a ladder-type thieno[3,2-b]pyrrolo-fused pentacyclic benzotriazole core (dithieno[3,2-b]pyrrolobenzotriazole, BZTP) and end-capped with 1,1-dicyanomethylene-3-indanone (INCN) has been first reported in this work. Through introducing multifused benzotriazole and INCN, BZIC could maintain a high-lying lowest unoccupied molecular orbital (LUMO) energy level of -3.88 eV. Moreover, BZIC shows a low optical bandgap of 1.45 eV with broad and efficient absorption band from 600 to 850 nm due to increased π-π interactions by the covalently locking thiophene and benzotriazole units. A power conversion efficiency of 6.30% is delivered using BZIC as nonfullerene acceptor and our recently synthesized hexafluoroquinoxaline-based polymer HFQx-T as donor. This is the first time to synthesize mutifused benzotriazole-based molecules as nonfullerene electron acceptor up to date. The preliminary results demonstrate that the mutifused benzotriazole derivatives hold great potential for efficient photovoltaics.

Side-chain fluorination on the pyrido[3,4-b]pyrazine unit towards efficient photovoltaic polymers

A series of new polymer donors (PT-PP, PT-2fPP and PT-4fPP) were synthesized based on alkylthiophene substituted benzodithiophene (BDT-T) and pyrido[3,4-b]pyrazine (PP) building blocks and the effects of fluorination on the polymer properties were explored. Photophysical properties, charge mobilities and morphologies of the three polymers have been intensively investigated. The results indicated that the introduction of the fluorine atom at meta-positions of phenyl substituted PP unit hardly affected their highest occupied molecular orbital (HOMO) level. More importantly, controlling the degree of side-chain fluorination in the polymers is crucial for optimizing the blend morphology. Three polymers showed different photovoltaic properties. The polymer solar cell (PSC) based on the single layer device structure of ITO/PEDOT:PSS/PT-4fPP:PC71BM (1:1, w:w)/ZrAcac/Al demonstrates a high power conversion efficiency (PCE) of 7.61% under the illumination of AM 1.5G, 100 mW cm−2, which is the highest value for PP-based PSCs.

A new fluoropyrido[3,4-b]pyrazine based polymer for efficient photovoltaics

In order to improve the photovoltaic performance of pyrido[3,4-b]pyrazine (PP)-based polymers, a new fluoropyrido[3,4-b]pyrazine based D–A type polymer (BDT-S-fPP) was synthesized. The optical, electrochemical and photovoltaic properties were well investigated. A power conversion efficiency (PCE) over 6.0% with a single junction device was obtained, which is the highest PCE among the PP-based polymers reported to date.

Low temperature, fast synthesis and ionic conductivity of Li6MLa2Nb2O12 (M = Ca, Sr, Ba) garnets

In this paper, we report low temperature, fast synthesis of Li6MLa2Nb2O12 (M = Ca, Sr, Ba) with the cubic garnet structure by sol–gel process. The optimized synthesis condition is 775 °C for 6 h with 10% excess lithium salt. The calcination temperature is nearly 125 °C lower than that in the solid state reaction, and the calcination time(~6 h) is shorter than in the solid state reaction(~24 h). Qualitative phase analysis by X-ray powder diffraction patterns combined with the Rietveld method reveals garnet type compounds as major phases. The cubic lattice parameter is found to increase with increasing size of the alkaline earth ions under the same preparation conditions. The density was found to be increasing with increasing ionic radius of the alkaline earth elements. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the compounds, the Li6BaLa2Nb2O12 exhibits the highest ionic conductivity of 1.2 × 10−5 S cm−1 at room temperature.Graphical Abstract