Designs and understanding of small molecule-based non-fullerene acceptors for realizing commercially viable organic photovoltaics

Organic photovoltaics (OPVs) have emerged as a promising next-generation technology with great potential for portable, wearable, and transparent photovoltaic applications. Over the past few decades, remarkable advances have been made in non-fullerene acceptor (NFA)-based OPVs, with their power conversion efficiency exceeding 18%, which is close to the requirements for commercial realization. Novel molecular NFA designs have emerged and evolved in the progress of understanding the physical features of NFA-based OPVs in relation to their high performance, while there is room for further improvement. In this review, the molecular design of representative NFAs is described, and their blend characteristics are assessed via statistical comparisons. Meanwhile, the current understanding of photocurrent generation is reviewed along with the significant physical features observed in high-performance NFA-based OPVs, while the challenging issues and the strategic perspectives for the commercialization of OPV technology are also discussed.

This review describes the current understandings and the significant features observed in NFA-based OPVs, with a particular focus on photophysical, electrical, and morphological characteristics.     

High electron mobility fluorinated indacenodithiophene small molecule acceptors for organic solar cells

Indacenodithiophene (IDT) derivatives are kinds of the most representative and widely used cores of small molecule acceptors (SMAs) in organic solar cells (OSCs). Here we systematically investigate the influence of end-group fluorination density and position on the photovoltaic properties of the IDT-based SMAs IDIC-nF (n = 0, 2, 4). The absorption edge of IDIC-nF red-shifts with the π-π stacking and crystallinity improvement, and their electronic energy levels downshift with increasing n. Due to the advantages of J_sc and FF as well as acceptable V_oc, the difluorinated IDIC-2F acceptor based OSCs achieve the highest power conversion efficiency (PCE) of 13%, better than the OSC devices based on IDIC and IDIC-4F as acceptors. And the photovoltaic performance of the PTQ10: IDIC-2F OSCs is insensitive to the active layer thickness: PCE still keep high values of 12.00% and 11.46% for the devices with active layer thickness of 80 and 354 nm, respectively. This work verifies that fine and delicate modulation of the SMAs molecular structure could optimize photovoltaic performance of the corresponding OSCs. Meanwhile, the thickness-insensitivity property of the OSCs has potential for large-scale and printable fabrication technology.     

Ternary-organic photovoltaics with J71 as donor and two compatible nonfullerene acceptors

Recombining the advantages on photovoltaic parameters of two binary-organic photovoltaics (OPVs) into one ternary cell is an efficient strategy for selecting materials, in addition to the absorption spectra complementary among the used materials. The binary-OPVs with J71:BTP-4F-12 exhibit a power conversion efficiency (PCE) of 11.70%, along with a short-circuit-current-density (J_SC) of 23.61 mA cm⁻², an open-circuit-voltage (V_OC) of 0.841 V and a fill factor (FF) of 58.99%. Although the relatively low PCE of 10.92% and J_SC of 16.59 mA cm⁻² are achieved in J71:ITIC-based binary-OPVs, the V_OC of 0.935 V and FF of 70.40% are impressive compared with J71:BTP-4F-12-based OPVs. Optimal ternary-OPVs are achieved with J71:BTP-4F-12:ITIC as active layers by weight ratio of 1:0.48:0.72, delivering a markedly increased PCE of 13.05% with a V_OC of 0.903 V, a J_SC of 21.27 mA cm⁻² and a FF of 68.20%. An over 11.5% PCE improvement is obtained by recombining the advantages of binary-OPVs into ternary-OPVs with ITIC as photon harvesting reinforcing agent and morphology regulator. The good compatibility between BTP-4F-12 and ITIC provides large room to well optimize their relative content for achieving the well balanced three key photovoltaic parameters of ternary-OPVs.     

Suppressing trap states and energy loss by optimizing vertical phase distribution through ternary strategy in organic solar cells

Suppressing the trap-state density and the energy loss via ternary strategy was demonstrated.Favorable vertical phase distribution with donors(acceptors)accumulated(depleted)at the interface of active layer and charge extraction layer can be obtained by introducing appropriate amount of polymer acceptor N2200 into the systems of PBDB-T:IT-M and PBDB-TF:Y6.In addition,N2200 is gradiently distributed in the vertical direction in the ternary blend film.Various measurements were carried out to study the effects of N2200 on the binary systems.It was found that the optimized morphology especially in vertical direction can significantly decrease the trap state density of the binary blend films,which is beneficial for the charge transport and collection.All these features enable an obvious decrease in charge recombination in both PBDB-T:IT-M and PBDB-TF:Y6 based organic solar cells(OSCs),and power conversion efficiencies(PCEs)of 12.5%and 16.42%were obtained for the ternary OSCs,respectively.This work indicates that it is an effective method to suppress the trap state density and thus improve the device performance through ternary strategy.     

High efficiency ternary organic solar cells enabled by compatible dual-donor strategy with planar conjugated structures

Ternary organic solar cells(OSCs) have received extensive attention for improving the power conversion efficiency(PCE) of organic photovoltaics(OPVs). In this work, a novel donor material(ECTBD) consisting of benzodithiophene(BDT) central electron donor unit was developed and synthesized. The small molecular donor has the same central unit as PM6. The addition of ECTBD into PM6:Y6 system could improve the morphology of active blend layer. In addition, ECTBD showed good morphologically compatibility when blending with PM6:Y6 host, resulting in the improvement of fill factor and current density. As a result, the ternary devices based on PM6:ECTBD:Y6 ternary system achieved a highest PCE of 16.51% with fill factor of 76.24%, which was much higher than that of the binary devices(15.7%). Overall, this work provided an effective strategy to fabricate highly efficient ternary organic solar cells through design of the novel small molecular donor as the third component.     

Over 18% binary organic solar cells enabled by isomerization of non-fullerene acceptors with alkylthiophene side chains

Side-chain engineering has been demonstrated as an effective method for fine-tuning the optical, electrical, and morphological properties of organic semiconductors toward efficient organic solar cells (OSCs). In this work, three isomeric non-fullerene small molecule acceptors (SMAs), named BTP-4F-T2C8, BTP-4F-T2EH and BTP-4F-T3EH, with linear and branched alkyl chains substituted on the α or β positions of thiophene as the side chains, were synthesized and systematically investigated. The results demonstrate that the size and substitution position of alkyl side chains can greatly affect the electronic properties, molecular packing as well as crystallinity of the SMAs. After blending with donor polymer D18-Cl, the prominent device performance of 18.25% was achieved by the BTP-4F-T3EH-based solar cells, which is higher than those of the BTP-4F-T2EH-based (17.41%) and BTP-4F-T2C8-based (15.92%) ones. The enhanced performance of the BTP-4F-T3EH-based devices is attributed to its stronger crystallinity, higher electron mobility, suppressed biomolecular recombination, and the appropriate intermolecular interaction with the donor polymer. This work reveals that the side chain isomerization strategy can be a practical way in tuning the molecular packing and blend morphology for improving the performance of organic solar cells.

     

Over 16.7% efficiency of ternary organic photovoltaics by employing extra PC71BM as morphology regulator

Ternary organic photovoltaics(OPVs) are fabricated with PBDB-T-2 Cl:Y6(1:1.2, wt/wt) as the host system and extra PC_(71)BM as the third component. The PBDB-T-2 Cl:Y6 based binary OPVs exhibit a power conversion efficiency(PCE) of 15.49% with a short circuit current(J_(SC)) of 24.98 m A cm~(-2), an open circuit voltage(V_(OC)) of 0.868 V and a fill factor(FF) of 71.42%. A 16.71%PCE is obtained in the optimized ternary OPVs with PBDB-T-2 Cl:Y6:PC_(71)BM(1:1.2:0.2, wt/wt) active layer, resulting from the synchronously improved J_(SC) of 25.44 m A cm~(-2), FF of 75.66% and the constant V_(OC)of 0.868 V. The incorporated PC_(71)BM may prefer to mix with Y6 to finely adjust phase separation, domain size and molecular arrangement in ternary active layers, which can be confirmed from the characterization on morphology, 2 D grazing incidence small and wide-angle X-ray scattering, as well as Raman mapping. In addition, PC_(71)BM may prefer to mix with Y6 to form efficient electron transport channels, which should be conducive to charge transport and collection in the optimized ternary OPVs. This work provides more insight into the underlying reasons of the third component on performance improvement of ternary OPVs, indicating ternary strategy should be an efficient method to optimize active layers for synchronously improving photon harvesting, exciton dissociation and charge transport, while keeping the simple cell fabrication technology.     

Easy preparation of multifunctional ternary PdNiP/C catalysts toward enhanced small organic molecule electro-oxidation and hydrogen evolution reactions

The small organic molecule electro-oxidation (OMEO) and the hydrogen evolution (HER) are two important half-reactions in direct liquid fuel cells (DLFCs) and wa...     

Design, microwave-assisted synthesis, and photophysical properties of small molecule organic antennas for luminescence resonance energy transfer

A highly efficient microwave-assisted method was successfully developed for the synthesis of a library of carbostyril analogues. The reaction time for synthesis of carbostyril analogues was drastically reduced from a reported 18-58 h to only 80 min. Compounds obtained directly from each synthesis were more than 90% pure and did not require any further purification. On the basis of the fluorescence spectra of the compounds in the initial library, four carbostyril analogues were designed. Two of these analogues showed very favorable fluorescence profiles and have the potential to be used as small molecule organic antennas for LRET studies.     

Functional 2-benzyl-1,2-dihydro[60]fullerenes as acceptors for organic photovoltaics: facile synthesis and high photovoltaic performances

A new series of functional 2-benzyl-1,2-dihydro[60]fullerenes, BnHCs, were synthesized efficiently via co-catalyzed selective monofunctionalization of C₆₀ with functional benzyl bromides. Photophysical and electrochemical properties of the new BnHCs were investigated. PSCs based on 2-MeO–4-CO₂Me–BnHC as new acceptor and P3HT as donor showed a power conversion efficiency of 3.75%, which is comparable to that of PC₆₁BM under the same device conditions.     

Realizing high-performance ternary organic solar cell with thick active layer via incorporating a liquid crystalline small molecule as a third component

正In the past two decades,organic solar cells(OSCs)have attracted significant attention owing to their merits of achieving light-weight,flexible,low-cost devices[1,2].Ternary OSC as an efficient way to improve the power conversion efficiencies(PCEs)of OSCs has inspired great interest from researchers     

Design strategy and bioimaging of small organic molecule multicolor fluorescent probes

Science China Chemistry - Multicolor fluorescent probes based on small organic molecules have the advantages of low cost, good biocompatibility, easily modifiable molecular structures and...     

Dimeric naphthalene diimide based small molecule acceptors: synthesis,characterization, and photovoltaic properties

Herein, we present two naphthalene diimide (NDI) dimer based small molecule acceptors, Bis-NDI-T-EG and Bis-NDI-BDT-EG, in which two NDI units are bridged, respectively, by a thiophene (T) or a benzodithiophene (BDT), and further symmetrically substituted by 2-methoxylethoxyl (EG), both through the bay-region. These two NDI dimers exhibit broad absorption in the visible region of 300–650 (800) nm and display a HOMO/LUMO energy level of −5.88 eV/−3.80 eV and −5.46 eV/−3.78 eV, respectively. When PBDTTT-C-T was chosen as the polymer donor, Bis-NDI-T-EG and Bis-NDI-BDT-EG exhibited efficiencies of 1.31% and 1.24%, respectively.     

Hybridization detection of DNA by measuring organic small molecule amplified resonance light scattering signals

The interaction of organic small molecules (OSMs) with a biological molecule is very important. In this contribution, quinone-imine dyes including Acridine Yellow (AY), Neutral Red (NR), Acridine Orange (AO), Brilliant Cresyl Blue (BCB), Thionin (TN), Azur A (AA), Azur B (AB), and Methylene Blue (MB) respectively with double strand DNA (dsDNA) and single strand DNA (ssDNA) were investigated based on the measurements of enhanced resonance light scattering (RLS) and TEM. Mechanism investigations have shown that groove binding occurs between dsDNA and these OSMs, which depends on G-C sequences of dsDNA and the volumes of OSMs. With the amplified RLS signals resulting from the interactions of OSMs with DNA, a new technique has been proposed to detect the hybridization and mismatch of DNA labeling neither the target nor the probe DNA. The results have suggested that the extent of the amplified RLS signals of dsDNA by AY is the maximum among these eight OSMs, and therefore, it has been selected as a typical model system for further discussions.     

Positronium inhibition and quenching by organic electronic acceptors and charge transfer complexes

Positron lifetime measurements were performed on a series of organic electron acceptors and charge-transfer complexes in solution. The acceptors cause both positronium (Ps) inhibition (with maybe one exception) and quenching, but when an acceptor takes part in a charge-transfer complex the inhibition intensifies and the quenching almost vanishes. The reaction constants between ortho-Ps and the acceptors were determinded to be: 1.5 × 10¹⁰ M⁻¹ s⁻¹ for SO₂ in dioxane 3.7 × 10¹⁰ M⁻¹ s⁻¹ for SO₂ in n-heptane, 3.4 × 10¹⁰ M⁻¹ s⁻¹ for tetracyanoquinodimethane in tetrahydrofurane and 1.6 × 10¹⁰ M⁻¹ s⁻¹ for tetracyanoethylene in dioxane. From the ortho-Ps lifetimes in solutions containing charge-transfer complexes complexity constants were determined that were in reasonable agreement with constants obtained from optical data. The influence of acceptors and charge-transfers complexes on the Ps yield was interpreted in terms of the spur reaction model of Ps formation. Correlation was also made to gas phase reaction between electron acceptors and free electron, as well as to pulse radiolysis data.     

中变质煤小分子相对煤的吸附及渗流特性影响

为研究煤中小分子对煤的瓦斯吸附及流动特性的影响,采用四氢呋喃溶剂萃取煤中可溶有机小分子,得到萃取后煤样(残煤)。对原煤和残煤分别进行甲烷等温吸附、解吸实验和径向渗流实验;同时,采用氮气吸附法测试了它们的孔隙结构参数。结果表明,残煤的饱和吸附量a值低于原煤,吸附常数b值增大,萃取小分子降低了煤对甲烷的吸附能力,提高了低压段(<4 MPa)煤对甲烷的解吸速率;原煤的孔体积和平均孔径高于残煤,比表面积和微孔孔体积却减少;萃取后,煤粒表面传质阻力和扩散阻力均减小,残煤甲烷解吸速率和解吸量均高于原煤;同一渗流条件下,残煤的渗透率明显高于原煤。分析认为,煤中部分小分子被溶解后,煤孔隙结构的改变,降低了煤对瓦斯的吸附能力,减小了甲烷在煤粒中的内外扩散阻力,扩大了瓦斯在煤中的流动通道,改变了煤层储运特性,为煤储层的化学增透提供依据。     

Viscoelastic properties of an organic hybrid of chlorinated polyethylene and a small molecule

The dynamic mechanical properties of an organic hybrid consisting of chlorinated polyethylene (CPE) and N,N‐dicyclohexyl‐2‐benzothiazolyl sulfenamide (DZ) were investigated. All the CPE/DZ hybrids showed a single loss tangent (tan δ) peak in the mechanical spectra. The peak area under the tan δ/temperature curves around the mechanical loss peak was examined to characterize the damping properties of the CPE/DZ hybrids. We found that there exists a bending point in the relation between the glass‐transition temperature (T_g) and DZ content and that the value of T_g is saturated in the higher DZ contents, suggesting that excess DZ molecules show self‐aggregation and are reorganized. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1341–1347, 2000     

New strong organic acceptors by cycloaddition of TCNE and TCNQ to donor-substituted cyanoalkynes

Donor-substituted 1,1,2,4,4-pentacyanobuta-1,3-dienes and a cyclohexa-2,5-diene-1,4-diylidene-expanded derivative were prepared by a [2 + 2] cycloaddition of tetracyanoethene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) to anilino-substituted cyanoalkynes, followed by retro-electrocyclisation; they feature intense bathochromically-shifted intramolecular charge-transfer bands and undergo their first one-electron reductions at potentials similar to those reported for TCNE and TCNQ.     

Influence of the replacement of alkoxyl with alkylthienyl on photovoltaic properties of two small molecule donors for organic solar cells

Two benzo[1,2-b:4,5-b￠]dithiophene(BDT)-based small molecule(SM) donor materials with identical conjugated backbones but different substitution groups, named as DRTB-O and DRTB-T, were well explored to demonstrate the influence of the replacement of alkoxy with alkylthienyl on their photovoltaic properties in fullerene-based and fullerene-free organic solar cells(OSCs). The study shows that the two SM donors possess similar absorption spectra and energy levels but different crystalline structures in solid films. The carrier transport property and phase separation morphologies of the blend films have also been fully investigated.By employing PC71 BM as the acceptor, the power conversion efficiency(PCE) of DRTB-O:PC71BM and DRTB-T:PC71BM based devices were 4.91% and 7.08%, respectively. However, by blending with IDIC, the two SM donors exhibited distinctly different photovoltaic properties in fullerene-free OSCs, and the PCE of DRTB-O:IDIC and DRTB-T:IDIC based devices were 0.15% and9.06%, respectively. These results indicate that the replacement of alkoxyl with alkylthienyl in designing SM donor materials plays an important role in the application of fullerene-free OSCs.     

Deposition of small organic molecules by the displacement of two immiscible supercritical phases

A new coating process is described (deposition from two immiscible supercritical phases, or DISP) in which a solution of supercritical carbon dioxide (scCO2) with a desired solute is displaced by supercritical helium (scHe). After depressurization, the solute is deposited on substrates initially submerged in the coating solvent. Micron-sized particles and thin films of sucrose octaacetate (SOA) were formed on silicon wafer substrate coupons from DISP at relatively low temperatures and pressures (< or = 6500 psi and < or = 60 degrees C). The particle size, film thickness, and morphology of SOA were characterized as a function of coating conditions-solution concentrations, withdrawal velocities, and pressures. Particles in the range of 1-14 microm in diameter were deposited at low solute concentrations (< or = 0.2 wt % at 4500 psi), whereas films in the range of 0.1-0.5 microm in thickness were deposited at higher solute concentrations (> or = 1.5 wt % at 4500 psi). Particle sizes decreased with increasing displacement velocity and increasing pressure. Estimates of characteristic times for diffusion and nucleation indicate that DISP is a diffusion-limited process. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology, including defect formations and film roughness. Highly uniform films with low root-mean-square (RMS) roughness (approximately 10 angstroms) were obtained at a low displacement velocity of 0.0035 cm/s, while ring-like defect structures were observed in films deposited at a higher displacement velocity of 0.035 cm/s. The film thickness and morphology of the films deposited from DISP were compared with films from normal dip coating with typical organic solvents (acetone and toluene). Films deposited from scCO2 by DISP were much thicker, more uniform, and exhibited much fewer drying defects and lower RMS roughness compared with films from the organic solvents.