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The rapid development of non-fullerene acceptors (NFAs) with strong near-infrared absorption has led to remarkably enhanced short-circuit current density (Jsc) values in organic solar cells (OSCs). NFAs based on the benzotriazole (Bz) fused-ring π-core have great potential in delivering both high Jsc and decent open-circuit voltage values due to their strong intramolecular charge transfer with reasonably low energy loss. In this work, we have designed and synthesized a series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F and EHN6SEH-4F, via regiospecific N-alkyl engineering based on the high-performance NFA mBzS-4F that was reported previously. The molecular packing of mBzS-4F, AN6SBO-4F, and EHN6SEH-4F single crystals was analyzed using X-ray crystallography in order to provide a comprehensive understanding of the correlation between the molecular structure, the charge-transporting properties, and the solar cell performance. Compared with the typical honeycomb single-crystal structure of Y6 derivatives, these NFAs exhibit distinctly different molecular packing patterns. The strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm−2.

All-polymer solar cells (all-PSCs) have achieved tremendous progress due to the recent advances of polymerized small molecule acceptors (PSMAs) and their power conversion efficiencies (PCEs) have reached over 14%. However, the practical applications of all-PSCs are still severely restricted by the lack of PSMAs with a broad absorption spectrum, high electron mobility, low energy loss, and good batch-to-batch reproducibility. Herein, a multi-selenophene-containing PSMA, namely PFY-3Se, based on a selenophene-fused SMA framework and a selenophene π-spacer was designed and developed. Compared to its thiophene analog PFY-0Se, PFY-3Se possesses a significantly red-shifted absorption (~30 nm), increased electron mobility, and improved intermolecular aggregation and packing. When matched with polymer donor PBDB-T, the PFY-3Se-based all-PSCs achieved an impressive PCE of 15.1% with both high short-circuit current density of 23.6 mA cm-2 and high fill factor of 0.737, and a low energy loss, which are among the best values in all-PSCs field reported so far and much better than those of thiophene analogs (PCE=13.0%). More importantly, PFY-3Se maintains similarly good batch-to-batch properties which is very helpful for realizing reproducible device performance. This is the first reported and also is very rare for the PSMAs so far which provides a potential candidate for efficient all-PSCs toward practical applications.

We have successfully demonstrated the effect of a thin organic bulk-heterojunction (BHJ) interlayer on improving the photovoltaic performance of lead sulfide (PbS) colloidal quantum dot (CQD) solar...

Organic nonlinear optical (NLO) materials are very important for high-speed information processing in addressing the challenges of reduced energy consumption and enhanced speed and bandwidth. In particular, organic second-order NLO materials are very promising for meeting the combined requirements of ultra-low energy and ultra-high bandwidth in electro-optic (EO) modulation, while organic third-order NLO materials have good potential for applications in ultra-speed all-optical signal processing (AOSP). This review highlights the recent significant progress made in organic second- and third-order NLO materials. For second-order NLO materials, the recent advances in the efficient and cost-effective synthesis of dipolar polyene chromophores and thin-film engineering for efficient electric field poling are summarized. The applications and prospects of these high-performance EO materials are also discussed. For third-order NLO materials, we discuss the molecular design strategies of cyanine dyes for AOSP applications, particularly focusing on anionic tricyanofuran (TCF)-based cyanines. We aim to provide a better understanding of the structure–property relationships for cyanine-based AOSP materials. Finally, a summary and outlook for advancing high-performance organic NLO materials are provided.

Both monoanionic dicarbene pincer chelate and dianionic azole-pyridine-carbazole cyclometalate were successfully employed in the preparation of respective bis-tridentate Ir(III) metal complexes (Cz6–9) in moderate yields. Tuning of emission to blue was achieved by the addition of dual methoxy substituents at the carbazole cyclometalate, as well as the introduction of either methoxy or dimethylamino group at the central pyridinyl fragment of the azole-pyridine-carbazole cyclometalate. Single-crystal X-ray diffraction study was conducted on complex Cz6 in an attempt to provide an unambiguous structural proof. Photophysical properties were next measured in degassed CH2Cl2 solution at RT, giving structureless emission with peak maximum spanning 460–508 nm and photoluminescence quantum yield of 22–87%. A TD-DFT calculation confirmed the dominance of azole-pyridine-carbazole cyclometalate in controlling the emission energy gap, while the carbene pincer was less influential on the detected emission peak wavelength, i.e. acted as an ancillary. Importantly, the experimentally detected radiative lifetime (e.g. 2.78–17.18 μs) showed a clear correlation to the calculated metal-to-ligand charge transfer percentage of the electronic transitions (10.8–7.5%). Organic light-emitting diode devices gave turn-on voltages of 4.3 and 4.3 V, maximum EQE = 16.3 and 12.2%, power efficiencies of 17.6 and 10.4 lm W−1 and with CIEx,y coordinates of (0.16, 0.26) and (0.16, 0.20) for Cz6 and 7 at 15 wt%, confirming their potential as blue dopants for phosphorescent organic light-emitting diodes.

Bis-tridentate Ir(III) metal complexes bring forth interesting photophysical properties, among which the orthogonal arranged, planar tridentate chelates could increase the emission efficiency due to the greater rigidity and, in the meantime, allow strong interligand stacking that could deteriorate the emission efficiency. We bypassed this hurdle by design of five bis-tridentate Ir(III) complexes (-), to which both of their monoanionic ancillary and dianionic chromophoric chelate were functionalized derivative of 2-pyrazolyl-6-phenylpyridine, i.e. pzpyphH parent chelate. Hence, addition of phenyl substituent to the pyrazolyl fragment of pzpyphH gave rise to the precursors of monoanionic chelate (A1H-A3H), on which the additional -butyl and/or methoxy groups were introduced at the selected positions for tuning their steric and electronic properties, while precursors of dianionic chelates was judiciously prepared with an isoquniolinyl central unit on pziqphH in giving the red-shifted emission (cf. L1H and L2H). Factors affected their photophysical properties were discussed by theoretical methods based on DFT and TD-DFT calculation, confirming that the T excited state of all investigated Ir(III) complexes shows a mixed metal-to-ligand charge transfer (MLCT), intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and ligand-centered (LC) transition character. In contrast, the poor quantum yield of is due to the facilitation of the nonradiative decay in comparison to the radiative process. As for potential OLED applications, Ir(III) complex gives superior performance with max. efficiencies of 28.17%, 41.25 cd·A and 37.03 lm·W, CIE = 0.63, 0.37 at 50 mA cm, and small efficiency roll-off.

Summary Among all the solutions to improve long-term stability of perovskite solar cells, p-i-n heterojunction with all-metal-oxide charge transporting layers show their attractive features. However, these devices require an efficient electron-transporting layer (ETL) fabricated on top of the perovskites in which the commonly used oxide nanocrystals are limited by their imperfect surface. Here, a novel surface-passivation strategy was adopted by anchoring ZnO nanoparticles with fullerene nano-shells (Fa-ZnO) to mitigate trap states and passivate surface hydroxyl groups. We evidence the Fa-ZnO can be easily processed on top of perovskite as a high-quality ETL that improves electron extraction efficiency and suppresses ion diffusion/moisture penetration. In addition, the presence of fullerene shells allows Fa-ZnO nanoparticles to penetrate into perovskite precursors before crystallization, resulting in n-type sensitized configuration. With mesoscopic NiOx as hole-collecting contact, the p-n dual sensitization configuration has enabled all-metal-oxide devices to achieve state-of-the-art efficiencies of 21.1% with greatly improved performance longevity.

Organic semiconductors combining high mobility and efficient solid fluorescence are in high demand for developing organic light-emitting transistors and electrically driven organic lasers. But, achieving such dual functional molecules is extremely challenging. In this manuscript, we report 2-phenylbenzo[g]furo[2,3-b]quinoxaline derivatives (3a–3c) and develop a new synthesis method for the furo[2,3-b]quinoxaline core. It was found that 3a exhibited favorable aggregation induced emissive enhancement behavior and reasonable hole mobility. The absolute photoluminescence quantum yield of 3a is determined to be 22.1% in solid powders and can reach a maximum of 19.7% in 50% water in THF, twice that in CH2Cl2 solution (9.2%) and dispersed in PS film (9.3%). The single crystal and thin film organic field-effect transistor of 3a show a hole mobility of 2.58 × 10−2 cm2 V−1 s−1 and 5.7 × 10−3 cm2 V−1 s−1, respectively. Our results demonstrated that the 2-phenyl-benzo[g]furo[2,3-b]quinoxaline skeleton is a promising candidate for building multifunctional organic optoelectronics.

We explore the effects of nonradiative recombination at the extracting contacts on the achievable performance of halide perovskite photovoltaic cells. First, we perform device simulations using standard drift-diffusion models with experimental semiconductor parameters matching those of methylammonium lead triiodide (MAPbI3). We quantify the range of surface recombination velocities (SRVs) that would allow this archetypal perovskite to reach power conversion efficiencies of 27%. In particular, for contacts with well-aligned energy levels, SRVs of ∼1–10 cm/s should enable open-circuit voltages of 1.30 V, within 96% of the Shockley–Queisser limit. Next, we use time-resolved photoluminescence to experimentally determine the SRVs on 14 different common electron- and hole-extracting contacts, including TiO2, SnO2, ZnO, PCBM, ITIC, ICBA, TPBi, PEDOT:PSS, PTAA, PVK, NiO, MoO3, WO3, and spiro-OMeTAD. These results point the way to the selection and rational engineering of better contacts as a means to achieve high...

Delicately designed dopant-free hole-transporting materials (HTMs) with ordered structure have become one of the major strategies to achieve high-performance perovskite solar cells (PSCs). In this work, we report two donor-π linker-donor (D-π-D) HTMs, N01 and N02, which consist of facilely synthesized 4,8-di(n-hexyloxy)-benzo[1,2-b:4,5-b']dithiophene as a π linker, with 10-bromohexyl-10H-phenoxazine and 10-hexyl-10H-phenoxazine as donors, respectively. The N01 molecules form a two-dimensional conjugated network governed by C-H⋅⋅⋅O and C-H⋅⋅⋅Br interaction between phenoxazine donors, and synchronously construct a three-dimension lamellar structure with the aid of interlaminar π-π interaction. Consequently, N01 as a dopant-free small-molecule HTM exhibits a higher intrinsic hole mobility and more favorable interfacial properties for hole transport, hole extraction and perovskite growth, enabling an inverted PSC to achieve a very impressive power conversion efficiency of 21.85 %.

Passivating surface and bulk defects of perovskite films has been proven to be an effective way to minimize nonradiative recombination losses in perovskite solar cells (PVSCs). The lattice interference and perturbation of atomic periodicity at the perovskite surfaces often significantly affect the material properties and device efficiencies. By tailoring the terminal groups on the perovskite surface and modifying the surface chemical environment, the defects can be reduced to enhance the photovoltaic performance and stability of derived PVSCs. Here, we report a rationally designed bifunctional molecule, piperazinium iodide (PI), containing both R2NH and R2NH2+ groups on the same six-membered ring, behaving both as an electron donor and an electron acceptor to react with different surface-terminating ends on perovskite films. The resulting perovskite films after defect passivation show released surface residual stress, suppressed nonradiative recombination loss, and more n-type characteristics for sufficient energy transfer. Consequently, charge recombination is significantly suppressed to result in a high open-circuit voltage (VOC) of 1.17 V and a reduced VOC loss of 0.33 V. A very high power conversion efficiency (PCE) of 23.37% (with 22.75% certified) could be achieved, which is the highest value reported for inverted PVSCs. Our work reveals a very effective way of using rationally designed bifunctional molecules to simultaneously enhance the device performance and stability.

We study how tailoring noncovalent interactions through the regiochemistry of common donor–acceptor polymers can alter their excited-state electronic properties. Specifically, we compare two regioregular analogues (referred to as P1 and P2) of the widely used donor–acceptor copolymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate], which differ only in the orientation of a fluorine atom on the thieno[3,4-b]thiophene acceptor units with respect to the benzodithiophene unit. We show that in thin films, this subtle change along the polymer backbone results in a transformation from typical H-like to an unusual HJ-like aggregate behavior, indicating significant differences in intrachain exciton coupling along the polymer backbone and interchain coupling between polymer chains. We also use electroabsorption spectroscopy to relate the dominant coupling mechanisms to differences in the excess polarization volume of the excitons in P1 and P...

Two novel non-fullerene acceptors, SN6IC and SN6IC-4F, based on an S,N-heteroacene backbone were designed and synthesized. The cyclopentadiene fragments of commonly used acceptors were replaced with pyrrole rings to improve the electron-donating ability to increase the energy levels of the molecules. Both acceptors match well with the absorption and energy levels of the polymer donor PBDB-T, and PBDB-T:SN6IC-4F-based solar cells showed an excellent power conversion efficiency of 13.2%, with a relatively small VOC loss of 0.54 eV. This study proves that the introduction of a nitrogen atom to replace the sp3-hybridized carbon in the fused ring is very effective for making highly efficient NFAs to further improve the performance of organic solar cells.

Photochemical changes in the optical characteristics of solid polymers form the basis for many important technologies, but few examples are demonstrated in diamine-cured epoxy, one of the most important structural polymer systems. We have observed that diamine-cured epoxies containing the 4,4′-diaminodiphenyl methane (DDM) framework display changes in both absorption and fluorescence in response to UV light. The change from original “blue” to photo-activated “red” emission can be accomplished by illuminating samples of DDM-containing epoxy with photons of 254 nm or 370 nm wavelengths, followed by excitation of the newly-generated red-emitting fluorophore with 350–400 nm light. Studies of the monomer constituents and of many formulations of diamine-cured epoxy identify the DDM structure as the responsive moiety, both from the epoxy or diamine monomer. The fluorescence change is accompanied by the formation of orange and green chromophores; the orange chromophore is the fluorophore, with a broad 410 nm excitation and 607 nm emission. Our work shows the “blue-to-red” transition to be irreversible and independent of atmospheric oxygen; the fluorophore’s impermanence with time and its radical spectral signature identify it as a reactive intermediate rather than a photo-oxidation product. The central methylene radical of the benzoidal DDM structure is proposed as a red-emitting fluorophore/orange chromophore, with the green chromophore being a quinoidal methine resulting from the DDM radical.

An innovative strategy using strontium chloride (SrCl 2 ) as an additive to modulate the crystallization of formamidinium lead triiodide perovskite was developed in a sequential two-step deposition. The results from structural, elemental, and morphological analyses reveal that the SrCl 2 additive is capable of improving the quality and morphology of the pre-deposited PbI 2 layer by forming a PbI 2 -SrCl 2 interphase complex through plausible solid-solid interactions. This modulates the subsequent reaction between PbI 2 and formamidinium iodide (FAI) to enhance the crystallinity of the prepared perovskite film with denser and larger grain domains. The concomitantly enhanced optoelectronic properties of the SrCl 2 -derived perovskite film enables an improved power conversion efficiency (PCE) of 18.44% with good reproducibility, which surpasses the performance (PCE: 15.94%) of the control device without SrCl 2 . This work demonstrates a useful methodology to modulate the morphology and quality of perovskites through solid-solid interactions for achieving high-performance solar cells.

A series of six Ir(III)-based blue-emitting bis-tridentate phosphors, for which a 6-pyrazoly-2-phenoxypyridine (pzyPx) chelate is functionalized with either a m- or p-CF3 substituent at the terminal phenoxy site, and a tert-butyl substituent at the 4-position of the central pyridyl group, while the substituent of a carbene pincer ancillary is varied from methyl, isopropyl and to benzyl, are strategically designed and synthesized. A majority of these blue phosphors exhibit a nearly unitary emission quantum yield (Φ) in degassed solution and enhanced stability against photo-degradation in degassed toluene solution comparable to that of green, sky-blue and true-blue emitters, i.e. fac-[Ir(ppy)3], FIrpic and mer-[Ir(pmp)3], under irradiation with a xenon lamp at a power density of 620 W m−2 and at 35 °C. Corresponding OLED devices are fabricated using these pzyPx functionalized Ir(III) phosphors co-doped in a 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN) host material, giving a max. EQE in the range of 8.0–12.2% and stable CIEx,y coordinates between (0.16, 0.22) and (0.17, 0.27). Importantly, the max. EQE of these OLEDs is qualitatively proportional to the phosphorescence radiative lifetime recorded in either the solution state or the doped host matrix, showing a critical factor for the development of successive blue emitters.

Free-standing electrolytes based on Li(G4)TFSI and chemically-crosslinked poly(ethylene glycol), as well as functional co-monomers and/or small amounts of volatile diluent, are demonstrated to reach very high room-temperature ionic conductivities (>2 × 10−3 S cm−1) and electrochemically-measured lithium transference numbers (>0.5). An unexpected dependence of properties on polymer length/crosslinking is observed and explained in terms of competing Li+ transport processes. These electrolytes enable lithium symmetric cells to cycle >600 h at low overpotential and without dendrite growth. Future optimization routes are also discussed.

Although the inverted perovskite solar cells (PeSCs) have many advantages such as simple device fabrication, high stability and small hysteresis, the efficiency of inverted mesoporous PeSCs are still lower than those of normal-structure. For developing inverted mesoporous perovskite solar cells and the future design of tandem devices, a p-type metal oxide with high surface area and good charge carrier mobility is of paramount importance. Here, we develop a bilayer structure of p-type Cu:NiOx nanoparticle-based mesoporous and Cu-doped NiOx blocking layers to achieve efficient charge collection at the NiOx/perovskite interface with minimized recombination loss. Our strategy enables the fabrication of centimeter-sized perovskite solar cells with a decent efficiency of 18.1%, significantly improved stability, and negligible hysteresis. The rational design of the p-type Cu-doped metal oxide bilayer provides an effective strategy for future development of inverted architecture based mesoporous solar cells.

The power-conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have increased rapidly from about 4% to 22% during the past few years. One of the major challenges for further improvement ...

The high chemo-selectivity of bis-aldehydes for condensation with both basic and acidic active methylene compounds facilitates a one-pot, three-component reaction to produce dipolar polyene chromophores as the major products for electro-optics.

There is growing interest in the bis-tridentate Ir(III) emitters as they are expected to display both improved emission efficiency and improved photostability. Herein, we turned to the new emitters m2h-1–3 and m6h-1–3, bearing a pincer carbene ancillary and a chromophoric chelate derived from judiciously selected phenyl-pyrimidine-pyrazole entities (pzm2hF)H2 and (pzm6hF)H2, which differ in terms of the location of phenyl and pyrazole substituents on the central pyrimidine. Density functional theory calculations revealed a notable change in the spin density distribution from the pyrimidine-pyrazolate entity in m2h to the pyrimidine-phenyl fragment in m6h. As a consequence, the m6h emitters exhibited both shortened emission lifetimes and improved stabilities during extensive photolysis in solution, while corresponding organic light-emitting diodes (OLEDs) doped with green-emitting m6h-1 and sky-blue-emitting m6h-2 and m6h-3 exhibited external quantum efficiencies of 17.6, 15.9, and 17.6%, respectively, sup...

A novel near-infrared-light-absorbing fused-ring electron acceptor, STIC, was developed for organic solar cells. STIC exhibited a narrow bandgap with an absorption edge reaching 940 nm, which was ascribed to the strong electron-rich selenophene-thieno[3,2-b]thiophene-selenophene (ST) unit and strong intramolecular charge transfer of STIC. Also, STIC-based devices showed low open-circuit voltage (Voc) loss values, attributed to the rigid ST core providing low reorganization energy. The device based on the PBDB-T:STIC blend exhibited a power conversion efficiency (PCE) of 9.68% with a high short-circuit current density (Jsc) of 19.96 mA cm-2 and a low Voc loss of 0.55 eV.

In this work, an efficient inverted perovskite solar cell with decent ambient stability is successfully demonstrated by employing an n-type polymer, poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200), as the electron-transporting layer (ETL). The device performance can be further enhanced from a power conversion efficiency (PCE) of 15 to 16.8 % by tailoring the electronic properties of N2200 with a polymeric additive, poly[9,9-bis(6'-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)-hexyl) fluorene] (PFN-Ox). More importantly, the device derived from this hybrid ETL can maintain good ambient stability inherent from the pristine N2200 ETL, for which 60-70 % of initial PCE can be retained after being stored in air with 10-20 % humidity for 45 days.

Cyanines with long conjugation length such as heptamethines are well known to have strong intramolecular and/or intermolecular interactions (i.e., ion paring and aggregation), which can affect their structures (i.e., symmetry breaking) and optical properties remarkably. In this paper, we report a covalently linked complementary cyanine complex of cationic and anionic heptamethines forming a highly polarizable zwitterionic cyanine–cyanine salt. The effect of the modification was studied in detail on both its electronic structure and its optical properties. This novel zwitterionic salt was found to exhibit a decreased ion-pairing-induced charge localization but with an increased electronic coupling between the excited states of the cyanine cation and the anion, resulting in unusual optical properties compared to closely related noncovalent complementary cyanine salts. The dual-arm Z-scan technique was used to study their third-order NLO properties, suggesting the electronic coupling in such preorganized zwi...

By creating an effective π-orbital hybridization between the fullerene cage and the aromatic anchor (addend), the azafulleroid interfacial modifiers exhibit enhanced electronic coupling to the underneath metal oxides. High power conversion efficiency of 10.3% can be achieved in organic solar cells using open-cage phenyl C61 butyric acid methyl ester (PCBM)-modified zinc oxide layer.

A new class of rationally designed mechanophores is developed for highly sensitive built-in strain sensors in polymer composites. These mechanophores are designed to regenerate the π-conjugation pathway between the electron donor and electron acceptor by force-induced cleavage of the covalent bond to form a fluorescent dipolar dye.

While organo-metal halide perovskite photovoltaics have seen rapid development, growth of high quality material remains a challenge. Herein, we report a facile post deposition treatment utilizing coordination between methylamine (CH3NH2) vapor and CH3NH3PbI3 perovskite that rapidly improves film quality, enhancing power conversion efficiency (PCE) by ∼9%. We further comprehensively analyze the physical impact of this process with regard to the material's optoelectronic properties and its detailed microstructural changes. Connecting this with an analysis of the source of organo-metal halide perovskite reactivity toward the vapor as well as phase behavior as a function of CH3NH2 vapor pressure and time, we provide design rules for the broad, rational extension of this process to new systems and scales.

With developments in materials, thin-film processing, fine-tuning of morphology, and optimization of device fabrication, the performance of organic solar cells (OSCs) has improved markedly in recent years. Designing low-bandgap materials has been a focus in order to maximize solar energy conversion. However, there are only a few successful low-bandgap donor materials developed with near-infrared (NIR) absorption that are well matched to the existing efficient acceptors. Porphyrin has shown great potential as a useful building block for constructing low-bandgap donor materials due to its large conjugated plane and strong absorption. Porphyrin-based donor materials have been shown to contribute to many record-high device efficiencies in small molecule, tandem, ternary, flexible, and OSC/perovskite hybrid solar cells. Specifically, non-fullerene small-molecule solar cells have recently shown a high power conversion efficiency of 12% using low-bandgap porphyrin. All these have validated the great potential of porphyrin derivatives as effective donor materials and made DPPEZnP-TRs a family of best low-bandgap donor materials in the OSC field so far. Here, recent progress in the rational design, morphology, dynamics, and multi-functional applications starting from 2015 will be highlighted to deepen understanding of the structure-property relationship. Finally, some future directions of porphyrin-based OSCs are presented.

Functional 2-pyrazolyl-6-phenylpyridine chelates-namely, (pzpyphBu)H2 and (pzpyphCF3)H2 and phosphines-are successfully employed in the preparation of emissive Ir(III) metal complexes, for which the reaction with phosphine such as PPh3, PPh2Me, and PPh2(CH2Ph) afford corresponding Ir(III) complexes [Ir(pzpyphBu)(PPh3)2H] (1a), [Ir(pzpyphCF3)(PPh2R)2H] (2a-2c), R = Ph, Me, CH2Ph, which also show an equatorial coordinated hydride. In contrast, treatment with 1,2-bis(diphenylphosphino)benzene (dppb) and 1,2-bis(diphenylphosphino)ethane (dppe) yields the isomeric products [Ir(pzpyphBu)(dppb)H] (3a) and [Ir(pzpyphBu)(dppe)H] (3b), for which the distinctive, axial hydride undergoes rapid chlorination, forming chlorinated complexes [Ir(pzpyphBu)(dppb)Cl] (4a) and [Ir(pzpyphBu)(dppe)Cl] (4b), respectively. On the other hand, upon extensive heating of 2c, one of its coordinated PPh2(CH2Ph) exhibits benzyl cyclometalation and hydride elimination to afford [Ir(pzpyphCF3)(PPh2R)(PPh2R')] (5c and 6c) R = CH2Ph and R' = CH2( o-C6H4) as the kinetic and thermodynamic products, respectively. Their structural, photophysical, and electrochemical properties are examined and further affirmed by the computational approaches.

Sky-blue and blue-emitting, carbazolyl functionalized, bis-tridentate Ir(III) phosphors Cz-1-Cz-3 with bright emission and short radiative lifetime are successfully synthesized in a one-pot manner. They exhibit very high photostability against UV-vis irradiation in degassed toluene, versus both green and true-blue-emitting reference compounds, i.e., fac-[Ir(ppy)3] and mer-[Ir(pmp)3]. Organic light-emitting diodes (OLEDs) based on Cz-2 exhibit maximum external quantum efficiency (EQE) of 21.6%, EQE of 15.1% at 100 cd m-2, and with CIE x,y coordinates of (0.17, 0.25). This study provides a conceptual solution to the exceedingly stable and efficient blue phosphor. It is promising that long lifespan blue OLED based on these emitters can be attained with further engineering of devices suitable for commercial application.

Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction organic solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochemical stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused aromatic cores with strong electron-accepting end groups. The key structure–property relationships, donor–acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field. Non-fullerene acceptors have been widely used in organic solar cells over the past 3 years. This Review focuses on the two most promising classes of non-fullerene acceptors — rylene diimide-based materials and fused-ring electron acceptors — and discusses structure–property relationships, donor– acceptor matching criteria and device physics, as well as future research directions for the field.

Using an analysis based on Marcus theory, we characterize losses in open-circuit voltage (VOC) due to changes in charge-transfer state energy, electronic coupling, and spatial density of charge-transfer states in a series of polymer/fullerene solar cells. We use a series of indacenodithiophene polymers and their selenium-substituted analogs as electron donor materials and fullerenes as the acceptors. By combining device measurements and spectroscopic studies (including subgap photocurrent, electroluminescence, and, importantly, time-resolved photoluminescence of the charge-transfer state) we are able to isolate the values for electronic coupling and the density of charge-transfer states (NCT), rather than the more commonly measured product of these values. We find values for NCT that are surprisingly large (∼4.5 × 1021–6.2 × 1022 cm–3), and we find that a significant increase in NCT upon selenium substitution in donor polymers correlates with lower VOC for bulk heterojunction photovoltaic devices. The inc...

An understanding of structure-property relationships in cyanine dyes is critical for their design and application. Anionic and cationic cyanines can be organized into complementary cyanine salts, offering potential building blocks to modulate their intra/intermolecular interactions in the solid state. Here, we demonstrate how the structures of these complementary salts can be tuned to achieve highly ordered J-type supramolecular aggregate structures of heptamethine dyes in crystalline solids.

In this study, we investigate the influence of molecular geometry of the donor polymers and the perylene diimide dimers (di-PDIs) on the bulk heterojunction (BHJ) morphology in the nonfullerene polymer solar cells (PSCs). The results reveal that the pseudo 2D conjugated poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) has better miscibility with both bay-linked di-PDI (B-di-PDI) and hydrazine-linked di-PDI (H-di-PDI) compared to its 1D analog, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), to facilitate more efficient exciton dissociation in the BHJ films. However, the face-on oriented - stacking of PTB7-Th is severely disrupted by the B-di-PDI due to its more flexible structure. On the contrary, the face-on oriented - stacking is only slightly disrupted by the H-di-PDI, which has a more rigid structure to provide suitable percolation pathways for charge transport. As a result, a very high power conversion efficiency (PCE) of 6.41% is achieved in the PTB7-Th:H-di-PDI derived device. This study shows that it is critical to pair suitable polymer donor and di-PDI-based acceptor to obtain proper BHJ morphology for achieving high PCE in the nonfullerene PSCs.

By studying the regio- and chemoselectivity of fluoro-substituted thienothiophene and benzodithiophene copolymers, we found polymers made from conventional one-pot polycondensation reaction consist of two distinctly different segments with a ratio of 0.36/0.64. Through further comparative studies of neat regioregular polymers based on each individual segment, we have identified the specific segment that contributes to the superior absorption, packing order, and charge mobility in the corresponding polymers. The unique structure-property relationships are the result of cooperative molecular arrangements of the key segment and noncovalent interaction between the fluoro group and the aromatic proton on the thiophene side-chain of the polymers.

The panchromatic operation of quasi-solid state dye sensitized solar cells is one of the most important criteria for enhancing the performance of solar cells. This paper presents a novel strategy for designing efficient panchromatic quasi-solid state dye sensitized solar cells that are enhanced by Forster resonance energy transfer. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran is excited by the irradiation of visible light with a short wavelength, and the excited energy is then transferred to a blue organic photosentizer, symmetrically structured squaraine. The addition of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran to the quasi-solid electrolyte improves the photovoltaic performance of squaraine-sensitized solar cells. The optimized quasi-solid state dye sensitized solar cell exhibits approximately 1.8 times higher overall conversion efficiency than the solar cell without any fluorescence materials. In particular, the photocurrent is enhanced significantly at the absorption wavelength range of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, which means that the panchromatic quasi-solid state dye sensitized solar cell has been designed successfully.

Two new ladder-type conjugated polymers, PIDTT-DFBT-EH and PIDTT-F-PhanQ-EH, are prepared through the copolymerization of heptacyclic IDTT with F-PhanQ and DFBT electron deficient moieties. The introduction of 4-(2-ethylhexyloxy)-phenyl (EHOPh) side-chains onto the polymer is beneficial for achieving high molecular weight and good solution-processability of materials. The derived polymer solar cells yielded PCEs of 5.48% (PIDTT-DFBT-EH) and 5.14% (PIDTT-F-PhanQ-EH), without engaging post-solvent or solvent additive treatments.

In this study we synthesized a series of solution-processable small molecules comprising 2,2′-bithiophene (BTh), terthiophene (TTh), and thiobarbituric acid (TB) units as the central core, π-conjugated spacer, and acceptor end-capping moieties, respectively, but with alkyl side-chains of different lengths presented from their central BTh units (TBTThBTh-H, TBTThBTh-C4, TBTThBTh-C8, TBTThBTh-C12). We then investigated the structure–property relationships of these compounds in terms of their packing behaviors and bulk heterojunction (BHJ) photovoltaic properties. And we found that the packing of these molecules in neat films is critically dependent of their side-chain lengths, as evidenced by the variations in their lamellar structures determined with grazing-incidence wide-angle X-ray scattering (GIWAXS). The power conversion efficiencies (PCEs) of the photovoltaic BHJ devices comprising these small molecules and PC61BM exhibited zigzag-shaped variations with respect to the alkyl side-chain lengths, with the PCE of devices incorporating TBTThBTh-H and TBTThBTh-C8 being higher than those of devices incorporating TBTThBTh-C4 and TBTThBTh-C12. Using GIWAXS to probe the molecular packing in the BHJ active layers, we found that the alkyl chain lengths of the small molecules had a large impact on the formation of crystallites in the BHJ films; the molecules with more uniform and shorter alkyl side-chain lengths provide stronger intermolecular interactions, being more favorable for the crystallization of these molecules.

Recently, the evolved intermediate phase based on iodoplumbate anions that mediates perovskite crystallization has been embodied as the Lewis acid-base adduct formed by metal halides (serve as Lewis acid) and polar aprotic solvents (serve as Lewis base). Based on this principle, it is proposed to constitute efficient Lewis acid-base adduct in the SnI2 deposition step to modulate its volume expansion and fast reaction with methylammonium iodide (MAI)/formamidinium iodide (FAI) (FAI is studied hereafter). Herein, trimethylamine (TMA) is employed as the additional Lewis base in the tin halide solution to form SnY2 -TMA complexes (Y = I- , F- ) in the first-step deposition, followed by intercalating with FAI to convert into FASnI. It is shown that TMA can facilitate homogeneous film formation of a SnI2 (+SnF2 ) layer by effectively forming intermediate SnY2 -TMA complexes. Meanwhile, its relatively larger size and weaker affinity with SnI2 than FA+ ions will facilitate the intramolecular exchange with FA+ ions, thereby enabling the formation of dense and compact FASnI3 film with large crystalline domain (>1 µm). As a result, high power conversion efficiencies of 4.34% and 7.09% with decent stability are successfully accomplished in both conventional and inverted perovskite solar cells, respectively.

Two new donor–acceptor alternating wide band gap copolymers, PTTDBC-PhQC8 and PTTDBC-BTC12, based on an electron-rich tetrathienodibenzocarbazole (TTDBC) donor have been designed and synthesized, wherein two additional thienyl rings are fused into a 2,7-dithienylcarbazole skeleton to reinforce the structural coplanarity and rigidity of polymers. The quinoid thiophene in TTDBC can endow not only a wide optical band gap (∼1.9 eV) but also a deep HOMO (∼5.3 eV) for the resulting polymers. The conventional configuration solar cells based on PTTDBC-BTC12 exhibit a large Voc of 0.91 V and a power conversion efficiency of 4.30% (with a Jsc of 9.27 mA cm–2 and a FF of 0.51), while a slightly higher PCE of 4.50% can be achieved for the inverted structure devices. We believe that these wide band gap polymers have potential to be used for tandem cell applications.

We demonstrate the significance of ions with an ordered arrangement for enhancing the electron injection in polymer light-emitting diodes (PLEDs) and the electron extraction in polymer solar cells (PSCs) by using tetraoctylammonium bromide (TOAB) as a model compound. The spin-coated films of TOAB are prepared on the active layers of both PLEDs and PSCs without any annealing (TOAB-No) as well as with annealing at 60 °C (TOAB-60) and 100 °C (TOAB-100). Subsequently, they are investigated by atomic force microscopy (AFM) and synchrotron X-ray diffraction (XRD) as well as simultaneously employed as the interfacial layers in PLEDs and PSCs. The AFM results show that the active layers are well covered with TOAB-No and TOAB-100, but poorly covered with TOAB-60 due to the formation of large islands. The XRD results show that TOAB-No and TOAB-60 are polycrystalline with the (002) lattice planes parallel to the surface of the active layers while TOAB-100 is amorphous. Of note, the electron injection in PLEDs and the electron extraction in PSCs are significantly enhanced using TOAB-No and TOAB-60, but both are worsened using TOAB-100. Evidently the enhanced electron injection/extraction is mainly attributed to the ordered arrangement of ions in the TOAB crystals. This study indicates that with an ordered arrangement of ions, all ionic compounds should be excellent for enhancing the electron injection/extraction in polymer optoelectronic devices.

This study investigates the role of self-assembled tetraoctylammonium bromide (TOAB) on various conjugated polymers, namely green-emissive poly(9,9-dialkylfluorene) derivative (G-PF), super yellow poly(phenylenevinylene) (SY-PPV), and poly(3-hexylthiophene) (P3HT), in light-emitting devices based on these polymers. Atomic force microscopy images show that TOAB forms a homogeneous thin film with small grains on G-PF and SY-PPV, but forms an inhomogeneous film with large grains on P3HT. With TOAB deposited on these polymers, the emitting devices show better luminescence efficiency but lower current density compared to those of a Ca/Al cathode device. At a bias of 7 V, TOAB/Al-based devices have 1.67, 1.15, and 0.75 times the current density of the corresponding G-PF/Al-, SY-PPV/Al-, and P3HT/Al-based devices (159, 48, and 1233 mA cm−2), for which the hole injection barriers are 0.4, 0.1, and 0.0 eV, respectively. The increase in current density is attributed to the reduction of the electron injection barrier, which is due to the self-assembly of TOAB on the hydrophobic polymer surfaces for elevating the vacuum level by the interfacial dipole, as evidenced by synchrotron X-ray diffraction results. Of note, the ratio of the current density obtained for devices with and without TOAB decreases with decreasing hole injection barrier. The P3HT-based device shows a current density ratio of less than one (0.75), and thus has the highest light intensity (401 cd m−2) due to the hole blocking capability of TOAB.

A synergistic approach combining new material design and interfacial engineering of devices is adopted to produce high efficiency inverted solar cells. Two new polymers, based on an indacenodithieno[3,2-b]thiophene-difluorobenzothiadiazole (PIDTT-DFBT) donor–acceptor (D–A) polymer, are produced by incorporating either an alkyl thiophene (PIDTT-DFBT-T) or alkyl thieno[3,2-b]thiophene (PIDTT-DFBT-TT) π-bridge as spacer. Although the PIDTT-DFBT-TT polymer exhibits decreased absorption at longer wavelengths and increased absorption at higher energy wavelengths, it shows higher power conversion efficiencies in devices. In contrast, the thiophene bridged PIDTT-DFBT-T shows a similar change in its absorption spectrum, but its low molecular weight leads to reduced hole mobilities and performance in photovoltaic cells. Inverted solar cells based on PIDTT-DFBT-TT are explored by modifying the electron-transporting ZnO layer with a fullerene self-assembled monolayer and the MoO3 hole-transporting layer with graphene oxide. This leads to power conversion efficiencies as high as 7.3% in inverted cells. PIDTT-DFBT-TT's characteristic strong short wavelength absorption and high efficiency suggests it is a good candidate as a wide band gap material for tandem solar cells.

A series of cyclopentadithiophene-based low band gap conjugated polymers with varied side-chain patterns and F-substituents were synthesized. By replacing the shorter 2-ethylhexyl (EH) side-chain with the longer 3,7-dimethyloctyl (DMO) side-chain, it resulted in significant changes to the optical, electrochemical, and morphological properties of the polymers, as well as the subsequent performance of devices made from these materials. Solar cells fabricated from polymer with 2-ethylhexyl (EH) side-chain and monofluoro substituent exhibits increased open circuit voltage, short circuit current and fill factor, resulting in the highest power conversion efficiency (5.5%) in this series of polymers. This finding provides valuable insight for making more efficient low band gap polymers.

A new approach to polymer cross-linking is investigated using a cascading cycloreversion of a maleimide-furan adduct and a double 1,3-dipolar cycloaddition between a sydnone and maleimide. The cross-linking proceeds quantitatively above 63 °C, despite the polymer possessing no observable glass transition temperature. The resulting polymer network possesses a high thermal stability (>300 °C) due to the irreversibility of the sydnone-maleimide cycloaddition, which releases CO2 during the cross-linking, driving the reaction. The rigid three-dimensional structure of the bis-maleimide-sydnone cycloadduct produced local free volumes in films, decreasing the dielectric constant of the material.

A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ~(CH2)12 PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm2 V−1 s−1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm2 V−1 s−1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.

A new, all room-temperature solution process is developed to fabricate efficient, low-cost, and stable perovskite solar cells (PVSCs). The PVSCs show high efficiency of 17.10% and 14.19%, with no hysteresis on rigid and flexible substrates, respectively, which are the best efficiencies reported to date for PVSCs fabricated by room-temperature solution-processed techniques. The flexible PVSCs show a remarkable power-per-weight of 23.26 W g-1 .

In this paper, an electron donor–acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michler’s base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.