Achieving the low interfacial tension by balancing crystallization and film-forming ability of the cathode interlayer for organic solar cells.

A series of molecules with imide units bridged by the core of thiophene-based groups, namely N-dimethylaminopropyl-4-thiophene-1,8- naphthalimide (NT), bis(N-dimethylaminopropyl)-4-thiophene-1,8-naphthalimide (NTN), and bis(N-dimethylaminopropyl)-4-bithiophene-1,8-naphthalimide (N2TN), have been reported as cathode interfacial materials (CIMs) to realize low interfacial tension with the blend in organic solar cells (OSCs). We evaluated the Ohmic contact between the active layer and these cathode interlayers basedon various characterizations, which is of great significance for further understanding these imide-based interlayers. It turned out that the homogeneous and continuous NTN interlayer as a CIM balanced the factors of crystallization and film-forming property, and broke through the limitation of poor conductivity and high aggregation in our previous work. Moreover, compared with NT and N2TN, the NTN interlayer achieve a combination of good solubility in methanol, efficient electron mobility, and aligned work function. These advantages of NTN are conducive to the realization of high-efficient interfacial electron collection and transfer, thus improving the short-circuit current density (JSC) and filling factor (FF) of devices. Therefore, the binary OSCs (PM6:Y6) based on NTN engineered aluminium-cathode with excellent stability demonstrate a maximum power conversion efficiency (PCE) of 16.56 %, which is higher than NT (PCE = 1.34 %) and N2TN (PCE = 13.90 %). The enhanced performance is ascribed to the improvement of JSC and FF, which is originated from the outstanding conductivity and high-quality interface of NTN. Surprisingly, the PM6:Y6-based semitransparent device with NTN obtain a PCE of 13.43 % with an average visible transmittance of 17.79 %, which is better than traditional PDINO. This study highlights a potential strategy for enhancing the performance of OSCs by the interface engineering via decreasing the interfacial intension.

Aminonaphthalimide-Based Molecular Cathode Interlayers for As-Cast Organic Solar Cells.

A series of imide-based small molecules, namely NA, NAA, and NEA with simple structures, were designed and synthesized by introducing different amine side-chains into the benzene unit of imide, which were used as cathode interfacial materials in organic solar cells (OSCs). The amine side-chain substitution positions were systematically investigated with these small-molecule imides. Compared with NA without amide chains-NAA, and NEA, with 3-dimethylaminopropylamine and ethylenediamine chains, respectively-show bathochromic shifts in absorption, decreased band gaps, and higher highest occupied molecular orbital (HOMO) energy levels. A power conversion efficiency (PCE) of 15.04 % was obtained with the NEA-based as-cast OSCs with a high open-circuit voltage and fill factor for PM6 : Y6 blend and the maximum PCE of 15.80 % was reached for as-cast PM6 : Y6 : IT-M ternary OSCs. NEA exhibits better conductivity, higher electron mobility, and stronger the capability of lower work function of cathode among three molecules, affording OSCs with better photovoltaic performance. Additionally, these three molecules show excellent thermal stability both in solution and in films at 150 °C. The results indicate that imide-based small molecules are promising cathode interfacial materials for commercial OSCs.

Enabling High-Performance Tandem Organic Photovoltaic Cells by Balancing the Front and Rear Subcells.

In tandem organic photovoltaics, the front subcell is based on large-bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow-bandgap acceptor for infrared absorption but a large-bandgap donor to realize a high open-circuit voltage. Unfortunately, most of the ultraviolet-visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large-bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole-transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared-absorbing polymer donor as a third component is introduced, which provides a second hole-generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short-circuit current density of the two-terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7 mA cm-2 (while retaining the open-circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.

Highly Conjugated, Fused-Ring, Quadrupolar Organic Chromophores with Large Two-Photon Absorption Cross-Sections in the Near-Infrared.

The two-photon absorption (2PA) properties are investigated for two series of organic, π-conjugated, fused-ring, quadrupolar A-π-D-π-A chromophores of the type originally developed as nonfullerene acceptors for organic photovoltaics. These molecules are found to exhibit large nondegenerate two-photon absorption (ND2PA) cross-sections (ca. 6-27 × 103 GM) in the near-infrared (NIR). In the first series, involving molecules of varying core size, ND2PA spectra and cross-sections characterized by femtosecond ND2PA spectroscopy in chloroform solutions reveal that increases in core size, and thus conjugation length, leads to substantially red-shifted and enhanced 2PA. In a second series, variation of the strength of the terminal acceptor (A) with constant core size (seven rings, indacene-based) led to less dramatic variation in the 2PA properties. Among the two core types studied, compounds in which the donor has a thieno[3,2-b]thiophene center demonstrate larger 2PA cross-sections than their indacene-centered counterparts, due to the greater electron-richness of their cores amplifying intramolecular charge transfer. Excited-state absorption (ESA) contributions to nonlinear absorption measured by open-aperture Z-scans are deduced for some of the compounds by analyzing the spectral overlap between 2PA bands and NIR ESA transitions obtained by ND2PA and transient absorption measurements, respectively. ESA cross-sections extracted from transient absorption and irradiance-dependent open-aperture Z-scans are in reasonable agreement, and their moderate magnitudes (ca. 10-21 m2) suggest that, although ESA contributions are non-negligible, the effective response is predominantly instantaneous 2PA.

High-Performance Fluorinated Fused-Ring Electron Acceptor with 3D Stacking and Exciton/Charge Transport.

A new fluorinated electron acceptor (FINIC) based on 6,6,12,12-tetrakis(3-fluoro-4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-donating central core and 5,6-difluoro-3-(1,1-dicyanomethylene)-1-indanone as the electron-deficient end groups is rationally designed and synthesized. FINIC shows similar absorption profile in dilute solution to the nonfluorinated analogue INIC. However, compared with INIC, FINIC film shows red-shifted absorption, down-shifted frontier molecular orbital energy levels, enhanced crystallinity, and more ordered molecular packing. Single-crystal structure data show that FINIC molecules pack into closer 3D "network" motif through H-bonding and π-π interaction, while INIC molecules pack into incompact "honeycomb" motif through only π-π stacking. Theoretical calculations reveal that FINIC has stronger electronic coupling and more molecular interactions than INIC. FINIC has higher electron mobilities in both horizontal and vertical directions than INIC. Moreover, FINIC and INIC support efficient 3D exciton transport. PBD-SF/FINIC blend has a larger driving force for exciton splitting, more efficient charge transfer and photoinduced charge generation. Finally, the organic solar cells based on PBD-SF/FINIC blend yield power conversion efficiency of 14.0%, far exceeding that of the PBD-SF/INIC-based devices (5.1%).

High-Performance Fused Ring Electron Acceptor-Perovskite Hybrid.

We report the fused ring electron acceptor (FREA)-perovskite hybrid as a promising platform to fabricate organic-inorganic hybrid solar cells with simple preparation, high efficiency, and good stability. The FREA-perovskite hybrid films exhibit larger grain sizes and stronger crystallinity than the pristine perovskite films. Moreover, the FREA molecules can form coordination bonding with undercoordinated Pb atoms and passivate the trap states in the perovskite films. Time-resolved photoluminescence and transient absorption measurements reveal that FREA facilitates efficient electron extraction and collection. Transient photocurrent and photovoltage measurements suggest faster charge transfer and reduced charge recombination in solar cells based on FREA-perovskite hybrid films. Consequently, solar cells based on FREA-perovskite hybrid films yield a champion efficiency of 21.7% with enhanced stability, which is higher than that of the control devices based on pristine perovskite films (19.6%).

Convenient fabrication of conjugated polymer semiconductor nanotubes and their application in organic electronics.

Organic heterojunction is indispensable in organic electronic devices, such as organic solar cells, organic light-emitting diodes and so on. Fabrication of core-shell nanostructure provides a feasible and novel way to prepare organic heterojunction, which is beneficial for miniaturization and integration of organic electronic devices. Fabrication of nanotubes which constitute the core-shell structure in large quantity is the key for the realization of application. In this work, a simple and convenient method to prepare nanotubes using conjugated copolymer of perylene diimide and dithienothiophene (P(PDI-DTT)) was demonstrated. The relationship between preparation conditions (solvent atmosphere, solution concentration and pore diameter of templates) and morphology of nanostructure was studied systematically. P(PDI-DTT) nanotubes could be fabricated in regular shape and large quantity by preparing the solution with appropriate concentration and placing anodic aluminium oxide template with nanopore diameter of 200 nm in the solvent atmosphere. The tubular structure was confirmed by scanning electron microscopy. P(PDI-DTT) nanotubes exhibited electron mobility of 0.02 cm2 V-1 s-1 in field-effect transistors under ambient condition. Light-emitting nanostructures were successfully fabricated by incorporating tetraphenylethylene into polymer nanotubes.

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%.

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%.

Naphthodithiophene-Based Nonfullerene Acceptor for High-Performance Organic Photovoltaics: Effect of Extended Conjugation.

Naphtho[1,2-b:5,6-b']dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: -3.78 eV vs IHIC2: -3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10-3 cm2 V-1 s-1 vs IHIC2: 5.0 × 10-4 cm2 V-1 s-1 ). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.

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 Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells.

We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0-2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550-850 nm) and strong absorption with high extinction coefficients of (2.1-2.5) × 105 M-1 cm-1. Fluorine substitution downshifts the LUMO energy level, red-shifts the absorption spectrum, and enhances electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.