Kuan Liu

Fused Hexacyclic Nonfullerene Acceptor with Strong Near-Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency

A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 105 m-1 cm-1 , a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10-3 cm2 V-1 s-1 . The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.

Roll-coating fabrication of flexible organic solar cells: comparison of fullerene and fullerene-free systems

Flexible organic solar cells (OSCs) based on a blend of low-bandgap polymer donor PTB7-TH and non-fullerene small molecule acceptor IEIC were fabricated via a roll-coating process under ambient atmosphere. Both an indium tin oxide (ITO)-free substrate and a flexible ITO substrate were employed in these inverted OSCs. OSCs with flexible ITO and ITO-free substrates exhibited power conversion efficiencies (PCEs) up to 2.26% and 1.79%, respectively, which were comparable to those of the reference devices based on fullerene acceptors under the same conditions. This is the first example for all roll-coating fabrication procedures for flexible OSCs based on non-fullerene acceptors with the PCE exceeding 2%. The fullerene-free OSCs exhibited better dark storage stability than the fullerene-based control devices.

Enhancing the Performance of Polymer Solar Cells via Core Engineering of NIR‐Absorbing Electron Acceptors

In order to utilize the near-infrared (NIR) solar photons like silicon-based solar cells, extensive research efforts have been devoted to the development of organic donor and acceptor materials with strong NIR absorption. However, single-junction organic solar cells (OSCs) with photoresponse extending into >1000 nm and power conversion efficiency (PCE) >11% have rarely been reported. Herein, three fused-ring electron acceptors with varying core size are reported. These three molecules exhibit strong absorption from 600 to 1000 nm and high electron mobility (>1 × 10-3 cm2 V-1 s-1 ). It is proposed that core engineering is a promising approach to elevate energy levels, enhance absorption and electron mobility, and finally achieve high device performance. This approach can maximize both short-circuit current density (  JSC ) and open-circuit voltage (VOC ) at the same time, differing from the commonly used end group engineering that is generally unable to realize simultaneous enhancement in both VOC and JSC . Finally, the single-junction OSCs based on these acceptors in combination with the widely polymer donor PTB7-Th yield JSC as high as 26.00 mA cm-2 and PCE as high as 12.3%.

Spiro[fluorene-9,9′-xanthene]-based hole transporting materials for efficient perovskite solar cells with enhanced stability

Four spiro[fluorene-9,9′-xanthene] (SFX)-based hole transporting materials (HTMs) functionalized with four-armed arylamine moieties located at different positions are designed and synthesized. These compounds exhibit highest occupied molecular orbital (HOMO) energy levels of −4.9 to −5.1 eV and a hole mobility of 2.2 to 15 × 10−5 cm2 V−1 s−1 after doping. Perovskite solar cells (PSCs) based on a methylammonium lead iodide (MAPbI3) active layer using one of these HTMs (mp-SFX-2PA) exhibit power conversion efficiencies (PCEs) of up to 16.8%, which is higher than that of the control devices based on benchmark spiro-OMeTAD under the same conditions (15.5%). PSCs based on mp-SFX-2PA exhibit better stability (retain 90% of their initial PCEs after 2000 h storage in an ambient atmosphere) than the control devices based on spiro-OMeTAD (retain only 28% of their initial PCEs). mp-SFX-2PA based devices employing a mixed formamidinium lead iodide (FAPbI3)/methylammonium lead bromine (MAPbBr3) perovskite layer exhibit an improved PCE of 17.7%. The effects of arylamines and their location positions on device performance are discussed.

A perylene diimide based polymer: a dual function interfacial material for efficient perovskite solar cells

An n-type semiconducting copolymer of perylene diimide and dithienothiophene (PPDIDTT) is used as a dual function interfacial layer to modify the surface of perovskite films in inverted perovskite solar cells. The PPDIDTT layer can remarkably passivate the surface trap states of perovskite through the formation of a Lewis adduct between the under-coordinated Pb in perovskite and S in the dithienothiophene unit of PPDIDTT, and also shows efficient charge extraction and transfer properties. The PPDIDTT modified devices exhibit a maximum power conversion efficiency of 16.5%, superior to that of the control devices without PPDIDTT (15.3%). In addition, the device stability and hysteresis in J–V curves of the modified devices are also improved compared to those of the control devices.

Fluorinated fused nonacyclic interfacial materials for efficient and stable perovskite solar cells

Three fused-ring n-type semiconductors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2,3-d]thiophene) end-capped with 1,1-dicyanomethylene-3-indanone substituted by different numbers of fluorine atoms (INIC series) are employed as interfacial materials to modify the surface of the perovskite film in inverted planar perovskite solar cells (PSCs). Due to fast interfacial charge extraction and efficient trap passivation, PSCs based on INIC series exhibit a maximum power conversion efficiency of 19.3% without any hysteresis, which is superior to control devices without INIC series (16.6%). Moreover, the strong water-resistance ability of fluorinated INIC significantly enhances the ambient stability of the PSCs. The effects of fluorine atom number on the device performance are discussed.

High-Mobility p-Type Organic Semiconducting Interlayer Enhancing Efficiency and Stability of Perovskite Solar Cells

A high‐mobility p‐type organic semiconductor based on benzodithiophene and diketopyrrolopyrrole with linear alkylthio substituents (BDTS‐2DPP) is used as a dual function interfacial layer to modify the interface of perovskite/2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene in planar perovskite solar cells. The BDTS‐2DPP layer can remarkably passivate the surface defects of perovskite through the formation of Lewis adduct between the under‐coordinated Pb atoms in perovskite and S atoms in BDTS‐2DPP, and also shows efficient hole extraction and transfer properties. The devices with BDTS‐2DPP interlayer show a peak power conversion efficiency of 18.2%, which is higher than that of reference devices without the BDTS‐2DPP interlayer (16.9%). Moreover, the hydrophobic BDTS‐2DPP interlayer effectively protects the perovskite against moisture, leading to enhanced device stability.

Enhancing the performance of a fused-ring electron acceptor via extending benzene to naphthalene

We compared an indacenodithiophene(IDT)-based fused-ring electron acceptor IDIC1 with its counterpart IHIC1 in which the central benzene unit is replaced by a naphthalene unit, and investigated the effects of the benzene/naphthalene core on the optical and electronic properties as well as on the performance of organic solar cells (OSCs). Compared with benzene-cored IDIC1, naphthalene-cored IHIC1 shows a larger π-conjugation with stronger intermolecular π–π stacking. Relative to benzene-cored IDIC1, naphthalene-cored IHIC1 shows a higher lowest unoccupied molecular orbital energy level (IHIC1: −3.75 eV, IDIC1: −3.81 eV) and a higher electron mobility (IHIC1: 3.0 × 10−4 cm2 V−1 s−1, IDIC1: 1.5 × 10−4 cm2 V−1 s−1). When paired with the polymer donor FTAZ that has matched energy levels and a complementary absorption spectrum, IHIC1-based OSCs show higher values of open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency relative to those of the IDIC1-based control devices. These results demonstrate that extending benzene in IDT to naphthalene is a promising approach to upshift energy levels, enhance electron mobility, and finally achieve higher efficiency in nonfullerene acceptor-based OSCs.

A low temperature processed fused-ring electron transport material for efficient planar perovskite solar cells

A fused-ring electron acceptor based on indacenodithiophene (IDIC) was used to replace TiO2 and work as an electron transport layer in planar n–i–p perovskite solar cells. IDIC improves perovskite crystallinity and film quality due to its hydrophobicity and incompatible wetting surface. IDIC facilitates electron extraction and transport due to its high mobility and suitable energy levels matched with the perovskite. IDIC reduces charge recombination in the devices due to trap passivation at the perovskite surface. The IDIC-based devices exhibit a champion power conversion efficiency of 19.1%, which is higher than that of TiO2-based devices (17.4%). Moreover, the device stability is significantly improved by IDIC.

Enhancing the performance of the electron acceptor ITIC-Th via tailoring its end groups

We choose the high-performance nonfullerene acceptor ITIC-Th as an example, and incorporate electron-donating methoxy and electron-withdrawing F groups onto the terminal group 1,1-dicyanomethylene-3-indanone (IC) to construct a small library of four fused-ring electron acceptors. With this series, we systematically investigate the effects of the substituents on the end-groups on the electronic properties, charge transport, film morphology, and photovoltaic properties of the ITIC-Th series. The electron-withdrawing ability increases from methoxylated to unsubstituted, fluorinated, and difluorinated IC, leading to a downshift of energy levels and a redshift of absorption spectra. Optimized organic solar cells based on the ITIC-Th series show power conversion efficiencies ranging from 8.88% to 12.1%.