Shuixing Dai

Single‐Junction Binary‐Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push-pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC ). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71 BM (PCE = 5.22%).

Fused Tris(thienothiophene)‐Based Electron Acceptor with Strong Near‐Infrared Absorption for High‐Performance As‐Cast Solar Cells

A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600-950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 105 m-1 cm-1 , low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10-3 cm2 V-1 s-1 . Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.

Perylene and naphthalene diimide polymers for all-polymer solar cells: a comparative study of chemical copolymerization and physical blend

Five copolymers, having 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b′]-dithiophene as a donor unit, and perylene diimide (PDI) and/or naphthalene diimide (NDI) as acceptor moieties, were synthesized by Stille coupling copolymerization, and used as electron acceptors in solution-processed polymer solar cells (PSCs). All five copolymers exhibited broad absorption in the region of 300–800 nm. The LUMO energy level of the resulting copolymers was from −3.90 to −3.77 eV and the HOMO energy level had little variation from −5.65 to −5.57 eV. Among binary blend PSCs using P3HT as a donor and these polymers as acceptors, PPDI25-co-NDI75-based devices (P3HT : PPDI25-co-NDI75 = 3 : 1, w/w) yielded the best power conversion efficiency (PCE) of up to 1.54%. Among ternary blend PSCs using P3HT as a donor and PDI polymer PPDI100 and NDI polymer PNDI100 as coacceptors, the P3HT : PPDI100 : PNDI100 (3 : 0.25 : 0.75, w/w) ternary blend afforded the best PCE of 0.83%. All ternary blends based on P3HT : PPDI100 : PNDI100 showed decreased VOC, JSC, FF and PCE compared to the corresponding binary blends based on P3HT : PPDI-co-NDI.

Oligothiophene-bridged perylene diimide dimers for fullerene-free polymer solar cells: effect of bridge length

A series of perylene diimide (PDI) dimers (PnTP, n = 0–3) with oligothiophenes as bridges were designed, theoretically calculated, synthesized, and developed as electron acceptors for polymer solar cells (PSCs). The effects of oligothiophene bridge length on the absorption, energy level, charge transport, morphology and photovoltaic properties of the molecules were investigated. These molecules exhibited good thermal stability with decomposition temperatures of 367–413 °C, broad and strong absorption in the visible region (400–700 nm), and appropriate HOMO (−5.74 to −5.61 eV) and LUMO (−3.84 to −3.72 eV) energy levels. When blending these PDI acceptors with a low bandgap polymer donor PBDTTT-C-T, the PSCs exhibited power conversion efficiencies of 0.76–3.61%.

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%.

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.

Efficient and stable organic solar cells via a sequential process

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted considerable attention in the last two decades. Sequentially solution processed BHJ (s-BHJ) have been developed in recent years. s-BHJ not only maintain some advantages of mixed BHJ (m-BHJ), but also exhibit other advantages over m-BHJ. However, to date, s-BHJ OSCs exhibit relatively lower efficiency and have received much less attention compared with m-BHJ OSCs. Moreover, there have been rare systematic comparisons between m-BHJ and s-BHJ OSCs. In this work, we systematically compare the m-BHJ and s-BHJ OSCs based on a classical system PTB7-TH/PC71BM in terms of film morphology, domain size and purity, molecular orientation and aggregation, vertical phase separation, charge transport, efficiency and stability. The s-BHJ OSCs without additives exhibit efficiencies as high as 8.6%, which is similar to that of m-BHJ OSCs with additives (8.5%) and is the highest reported for s-BHJ OSCs. More importantly, the s-BHJ OSCs show much better device stability than the m-BHJ OSCs. This study demonstrates that employing s-BHJ is a promising strategy towards efficient and stable OSCs.

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.

Rylene diimide and dithienocyanovinylene copolymers for polymer solar cells

Two polymers containing (E-2,3-bis(thiophen-2-yl)acrylonitrile (CNTVT) as a donor unit, perylene diimide (PDI) or naphthalene diimide (NDI) as an acceptor unit, are synthesized by the Stille coupling copolymerization, and used as the electron acceptors in the solution-processed organic solar cells (OSCs). Both polymers exhibit broad absorption in the region of 300–850 nm. The LUMO energy levels of the resulted polymers are ca.–3.93 eV and the HOMO energy levels are–5.97 and–5.83 eV. In the binary blend OSCs with PTB7-Th as a donor, PDI polymer yields the power conversion efficiency (PCE) of up to 1.74%, while NDI polymer yields PCE of up to 3.80%.

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%.