Xinhui Lu

A Facile Planar Fused-Ring Electron Acceptor for As-Cast Polymer Solar Cells with 8.71% Efficiency

A planar fused-ring electron acceptor (IC-C6IDT-IC) based on indacenodithiophene is designed and synthesized. IC-C6IDT-IC shows strong absorption in 500-800 nm with extinction coefficient of up to 2.4 × 10(5) M(-1) cm(-1) and high electron mobility of 1.1 × 10(-3) cm(2) V(-1) s(-1). The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibit power conversion efficiencies of up to 8.71%.

A spirobifluorene and diketopyrrolopyrrole moieties based non-fullerene acceptor for efficient and thermally stable polymer solar cells with high open-circuit voltage

In this study, we design and synthesize a new non-fullerene electron acceptor, SF(DPPB)4, in which a spirobifluorene (SF) core is installed with four benzene endcapped diketopyrrolopyrrole (DPP) arms. SF(DPPB)4 exhibits energy levels matching perfectly with those of the commonly used poly(3-hexyl thiophene) (P3HT) donor in polymer solar cells (PSCs). Furthermore, a designed cross-shaped molecular geometry helps in suppressing strong intermolecular aggregation in the P3HT : SF(DPPB)4 blend, leading to efficient non-fullerene PSCs. The resultant devices give a maximum power conversion efficiency (PCE) of 5.16% with an extremely high open-circuit voltage (Voc) of 1.14 V. In contrast, the devices based on P3HT : PC61BM blends provide a PCE of 3.18% with a Voc of 0.62 V. Finally, we observe that the P3HT : SF(DPPB)4 devices exhibit significantly improved thermal stability from that of the P3HT : PC61BM devices; upon thermal treatment at 150 °C for 3 h, the PCEs of P3HT : SF(DPPB)4 devices remain unchanged, whereas those of the P3HT : PC61BM devices drop drastically to below 1%. The abovementioned results demonstrate that the new design strategy of employing a high-performance non-fullerene acceptor, SF(DPPB)4, is promising for the future practical application of PSCs.

Realizing Small Energy Loss of 0.55 eV, High Open‐Circuit Voltage >1 V and High Efficiency >10% in Fullerene‐Free Polymer Solar Cells via Energy Driver

A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.

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.

Nanoimprint-Induced Molecular Orientation in Semiconducting Polymer Nanostructures

The morphology and orientation of thin films of the polymer poly-3(hexylthiophene)-important parameters influencing electronic and photovoltaic device performance-have been significantly altered through nanoimprinting with 100 nm spaced grooves. Grazing-incidence small-angle X-ray scattering studies demonstrate the excellent fidelity of the pattern transfer, while wide-angle scattering convincingly shows an imprinting-induced π-π reorientation and polymer backbone alignment along the imprinted grooves. Surprisingly, temperature-dependent scattering measurements indicate that the imprinted induced orientation and alignment remain intact even at temperatures where the imprinted topographical features nearly vanish.

Bilayer order in a polycarbazole-conjugated polymer

One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymers molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

A non-fullerene acceptor with a fully fused backbone for efficient polymer solar cells with a high open-circuit voltage

Herein, we design and synthesize a perylene diimide derivative with a fully fused backbone, FITP, which possesses an elevated lowest unoccupied molecular orbital level and high electron mobility. Consequently, polymer solar cells with FITP as the acceptor can provide the best efficiency of 7.33% with a high voltage of 0.99 V.

Improved photon-to-electron response of ternary blend organic solar cells with a low band gap polymer sensitizer and interfacial modification

Incorporating two polymer donors with different bandgaps to compose a ternary blend bulk heterojunction (BHJ) is proved to be an effective approach to improve the device performance of BHJ polymer solar cells (PSCs). Here, we demonstrate an efficient ternary PSC consisting of a polythieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host blend sensitized by a low band gap (LBG) polymer poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:20,30-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothiadiazole)] (PDTP-DFBT). The addition of the PDTP-DFBT sensitizer remarkably extended the PSC photon to electron response from 750 to 900 nm, which increased the Jsc from 15.12 to 16.27 mA cm−2, and the device performance from 8.08% to 8.63%. A study on the morphology involving the atomic force microscopy mapping and grazing incident X-ray diffraction showed that the incorporation of PDTP-DFBT improved the crystallinity of the PTB7 film with most of the sensitizers associated with the PTB7 domains when blending with a PC71BM film. This observation, together with the unchanged Voc of the ternary PSC, implies a sensitizing mechanism with addition of PDTP-DFBT. After further interfacial modification with a capronic acid self-assembling monolayer (C3-SAM), a higher PCE of 9.06% was achieved, which is among the highest values of efficient ternary PSCs. Our work suggests that a sensitizing mechanism of ternary blends compensates for the light absorbing limitation of binary blend PSCs for high device performance.

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%.

Nanostructured surfaces frustrate polymer semiconductor molecular orientation.

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C61-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction.