All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies

Organic Solar Cells Based on Non-Fullerene Low Molecular Weight Organic Semiconductor Molecules

The development of narrow bandgap A-D-A- and ADA'DA-type non-fullerene small molecule acceptors (NFSMAs) along with small molecule donors (SMDs) have led to significant progress in all-small molecule organic solar cells. Remarkable power conversion efficiencies, nearing the range of 17-18 %, have been realized. These efficiency values are on par with those achieved in OSCs based on polymeric donors. The commercial application of organic photovoltaic technology requires the design of more efficient organic conjugated small molecule donors and acceptors. In recent years the precise tuning of optoelectronic properties in small molecule donors and acceptors has attracted considerable attention and has contributed greatly to the advancement of all-SM-OSCs. Several reviews have been published in this field, but the focus of this review concerns the advances in research on OSCs using SMDs and NFSMAs from 2018 to the present. The review covers the progress made in binary and ternary OSCs, the effects of solid additives on the performance of all-SM-OSCs, and the recently developed layer-by-layer deposition method for these OSCs. Finally, we present our perspectives and a concise outlook on further advances in all-SM-OSCs for their commercial application.

Charge photogeneration and recombination dynamics in non-fullerene solar cells based on a polymer donor with a mono-fluorinated π-bridge

Non-symmetric fluorine substitution is an important route for designing π-conjugated polymers for high-performance solar cells. However, excited state dynamics in a non-symmetric fluorinated polymer and solar cells are poorly understood. In this work, excited state dynamics in a neat mono-fluorinated polymer (PTzBI-dF) film and blend films (PTzBI-dF:Y6) were studied using time-resolved spectroscopy. For the neat donor film, we found the activation energy of the PTzBI-dF film transiting from radiative states to non-radiative states to be ∼58 meV. Besides, we found that both exciton diffusion coefficient and exciton diffusion length in the PTzBI-dF film are higher than those in the PTzBI-Cl film, which has no fluorinated π-bridge. The high exciton diffusion length of the PTzBI-dF film benefits from its long exciton lifetime (∼327.8 ps), which is much longer than that of the polymer donors such as P3HT and PM6. For PTzBI-dF:Y6 blend films, we found that the energy level offsets of the HOMO (0.13 eV) and LUMO (0.23 eV) are sufficient to dissociate the acceptor and donor excitons, respectively. Besides, we found that carrier recombination in PTzBI-dF:Y6 and PTzBI-Cl:Y6 blend films are dominated by bimolecular recombination processes, and thermal annealing treatment has a weak influence on the charge photogeneration and recombination process of the PTzBI-dF:Y6 film at an ultrafast time scale. Thus, this work is helpful for understanding photoelectrical conversion processes in non-symmetric fluorinated polymer-based solar cells.

Research progress and application of high efficiency organic solar cells based on benzodithiophene donor materials

In recent decades, the demand for clean and renewable energy has grown increasingly urgent due to the irreversible alteration of the global climate change. As a result, organic solar cells (OSCs) have emerged as a promising alternative to address this issue. In this review, we summarize the recent progress in the molecular design strategies of benzodithiophene (BDT)-based polymer and small molecule donor materials since their birth, focusing on the development of main-chain engineering, side-chain engineering and other unique molecular design paths. Up to now, the state-of-the-art power conversion efficiency (PCE) of binary OSCs prepared by BDT-based donor materials has approached 20%. This work discusses the potential relationship between the molecular changes of donor materials and photoelectric performance in corresponding OSC devices in detail, thereby presenting a rational molecular design guidance for stable and efficient donor materials in future.

Remarkable enhancement of the nonlinear optical behavior towards asymmetric substituted D-π-A dithiophene-based compounds

CONTEXT

Nonlinear optics (NLO) is an interesting field that discloses the interaction between intense light and matter, leading to a deeper understanding of NLO phenomena. Organic chromophores are considered as promising materials for NLO due to their exceptional structural versatility, ease of processing, and rapid response to NLO effects. Functional materials based on thiophene have been indispensable in advancing organic optoelectronics. Specifically, dithiophene-based compounds display weaker aromaticity, reduced steric hindrance, and additional sulfur-sulfur interactions. Hence, by utilizing dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) as the core structure, designing of a set of organic compounds with D1-π-D2-π-A-type framework, namely ZR1D1-ZR1D8, was carried out in this study. The analysis of frontier molecular orbitals (FMOs) revealed that compound ZR1D2 has the lowest band gap of 1.922 eV among all the investigated chromophores. The correlation of global reactivity parameters (GRPs) with the band gap values indicates that ZR1D2 displays a hardness of 0.961 eV and a softness of 0.520 eV-1. Among the studied compounds, ZR1D2 demonstrated a broad absorption spectrum that extended across the visible region. The maximum absorption wavelengths were observed at 766.470 nm for ZR1D2 and 749.783 nm for ZR1D5. These DTBDT-based dyes exhibit a remarkable NLO response with exceptionally high first hyperpolarizability (βtot) values. Among them, compound ZR1D2 stands out with the highest average linear polarizability (⟨α⟩ = 3.0 × 10-22 esu), first hyperpolarizability (βtot = 4.1 × 10-27 esu), and second hyperpolarizability (γtot = 7.5 × 10-32 esu) values. In summary, this investigation offers valuable insights into the potential use of DTBDT-based organic chromophores, particularly ZR1D2, for advanced applications in NLO. These findings suggest promising opportunities for researchers to synthesize these molecules and utilize these compounds in hi-tech NLO-based applications.

METHODOLOGY

The density functional theory computations were performed at the M06/6-311G(d,p) functional to explore their structural effects on electronic and NLO findings. Various analyses like highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, absorption maxima, density of states, open circuit voltage, binding energies of electrons and holes, and transition density matrix are employed to investigate photovoltaic efficiencies of the derivatives. Different software packages like Avogadro, Multiwfn, Origin, GaussSum, PyMOlyze, and Chemcraft were used to deduce conclusions from the output files.

Using an external electric field to tune active layer morphology enabling high-efficiency organic solar cells via ambient blade coating

The nanoscale morphology of the photoactive layer notably impacts the performance of organic solar cells (OSCs). Conventional methods to tune the morphology are typically chemical approaches that adjust the properties (such as solubility and miscibility) of the active components including donor, acceptor, and/or additive. Here, we demonstrate a completely different approach by applying an external electric field (EEF) on the active layer during the wet coating. The EEF-coating method is perfectly compatible with an ambient blade coating using environmentally friendly solvents, which are essential requirements for industrial production of OSCs. A record 18.6% efficiency is achieved using the EEF coating, which is the best value for open-air, blade-coated OSCs to date. Our findings suggest broad material applicability and attribute-enhanced performance to EEF-induced fiber formation and long-range ordering of microstructures of acceptor domains. This technique offers an effective method for producing high-performance OSCs, especially suited for industry OSC production based on open-air printing.

Optimization of Charge Management and Energy Loss in All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) have received tremendous attention in recent decades because of their advantages over their polymer counterparts. These advantages include well-defined chemical structures, easy purification, and negligible batch-to-batch variation. Remarkable progress with a power conversion efficiency (PCE) of over 17% has recently been achieved with improved charge management (FF × JSC) and reduced energy loss (Eloss). Morphology control is the key factor in the progress of ASM-OSCs, which remains a significant challenge because of the similarities in the molecular structures of the donors and acceptors. In this review, the effective strategies for charge management and/or Eloss reduction from the perspective of effective morphology control are summarized. The aim is to provide practical insights and guidance for material design and device optimization to promote further development of ASM-OSCs to a level where they can compete with or even surpass the efficiency of polymer solar cells.

High-Precision Tailored Polymer Molecular Weights for Specific Photovoltaic Applications through Ultrasound-Induced Simultaneous Physical and Chemical Events

It is highly challenging to reproducibly prepare semiconducting polymers with targeted molecular weight tailored for next-generation photovoltaic applications. Once such an easily accessible methodology is established, which can not only contribute to overcome the current limitation of the statistically determined nature of semiconducting polymers, but also facilitate rapid incorporation into the broad synthetic chemists' toolbox. Here, we describe a simple yet robust ultrasonication-assisted Stille polymerization for accessing semiconducting polymers with high-precision tailored molecular weights (from low to ultrahigh molecular weight ranges) while mitigating their interbatch variations. We propose that ultrasound-induced simultaneous physical and chemical events enable precise control of the semiconducting polymers' molecular weights with high reproducibility to satisfy all the optical/electrical and morphological demands of diverse types of high-performance semiconducting polymer-based devices; as demonstrated in in-depth experimental screenings in applications of both organic and perovskite photovoltaics. We believe that this methodology provides a fast development of new and existing semiconducting polymers with the highest-level performances possible on various photovoltaic devices.

Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet-Visible Absorption Spectroscopy

In addition to the donor-acceptor nano phases, the intermixed phase within the organic blends is crucial for the photovoltaic performance and stability of the bulk-heterojunction organic solar cells (OSCs). Here, the intermixed phase of a representative M-PhS:BTP-eC9 all-small-molecule organic solar cell was investigated by a concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy method, where a shift of the absorption maximum wavelength was measured for the acceptor component with the increase of the acceptor concentration. The blend ratios of the acceptor to the donor in the intermixed phase, corresponding to the critical concentration for the formation of the acceptor nanophase (CAP), were determined to be 0.35, 0.20, and 0.15 for the as-cast, thermal annealing (TA), and the combined TA and solvent vapor annealing films. These results indicated that M-PhS and BTP-eC9 are kinetically well intermixed during spin coating, whereas TA and the following solvent annealing promote the crystallization of BTP-eC9 molecules out of the intermixed phase. The photovoltaic performance of the M-PhS:BTP-eC9 cells with different blend ratios was investigated. The formation of the BTP-eC9 nano phase in the blend film leads to stable VOC and fast increased JSC, which can be understood by the reduction of bimolecular charge recombination and the formation of electron transporting pathways within the photoactive layer. Similarly, the critical concentration for the formation of the donor phase was estimated to be 0.15 by measuring the stabilized VOC and increased JSC values of the cells with different donor blending ratios. More importantly, after a fast "burn-in" thermal degradation, the M-PhS:BTP-eC9 cell showed excellent thermal stability aging at 85 °C for over 1128 h, which is in good accordance with the unchanged intermixed phases measured by the UV-vis spectra of the annealed films. The current work demonstrates the feasibility of the spectroscopy method to investigate the intermixed phases for organic bulk-heterojunction solar cells and proves that all-small-molecule solar cells can be intrinsically very stable.

Efficient all-small-molecule organic solar cells processed with non-halogen solvent

All-small-molecule organic solar cells with good batch-to-batch reproducibility combined with non-halogen solvent processing show great potential for commercialization. However, non-halogen solvent processing of all-small-molecule organic solar cells are rarely reported and its power conversion efficiencies are very difficult to improve. Herein, we designed and synthesized a small molecule donor BM-ClEH that can take advantage of strong aggregation property induced by intramolecular chlorine-sulfur non-covalent interaction to improve molecular pre-aggregation in tetrahydrofuran and corresponding micromorphology after film formation. Tetrahydrofuran-fabricated all-small-molecule organic solar cells based on BM-ClEH:BO-4Cl achieved high power conversion efficiencies of 15.0% in binary device and 16.1% in ternary device under thermal annealing treatment. In contrast, weakly aggregated BM-HEH without chlorine-sulfur non-covalent bond is almost inefficient under same processing conditions due to poor pre-aggregation induced disordered π-π stacking, indistinct phase separation and exciton dissociation. This work promotes the development of non-halogen solvent processing of all-small-molecule organic solar cells and provides further guidance.

Toward the search for new photosensitizers for DSSCs: theoretical study of both substituted Zn(II) and Si(IV) phthalocyanines

We report a density functional theory (DFT) study performed for a set of 66 compounds based on zinc(II) and silicon(IV) phthalocyanines (Pcs) with potential applications in dye-sensitized solar cells (DSSCs). The effect of the metal center (Zn, Si), periplanar and axial substituents, and anchor groups like anhydrous, carboxyl, and catechol on the electronic, optical, photovoltaics, and adsorption properties is investigated. Using the TD-DFT methodology and M06 and CAM-B3LYP functionals, we calculated the absorption spectra on optimized structures and in the solution phase but not on structures relaxed in the solvent. We obtained a strong Q band and a weak Soret band in the UV-Vis region, which are attributed to the transitions of type π-π* as described by the Gouterman orbitals. Q bands calculated show absorption up to 667 nm for ZnPcs and up to 769 nm for SiPcs, suggesting an essential role of the metal atom. The systems have a bathochromic effect in the order of secondary amine > primary amine > hydroxyl > amide > ester. We also found that the anhydrous and carboxyl groups favor absorption at longer wavelengths than the catechol group. The ZnPc systems show a slightly larger electron injection ΔGinj (∼1.1 eV) than SiPcs (∼0.9 eV), with similar values for the three anchor groups. The interaction energies (Eint) between ZnPcs/SiPcs and TiO2 in molecular and periodic configuration and corrected by the counterpoise method indicate that SiPcs predict more negative values than ZnPcs. The anchor group effect is relevant; the carboxyl moiety leads to stronger interactions than the anhydrous moiety. The strategies used could help to identify new photosensitizers for DSSCs.

Tethered Small-Molecule Acceptor Refines Hierarchical Morphology in Ternary Polymer Solar Cells: Enhanced Stability and 19% Efficiency

Polymer solar cells (PSCs) are promising for efficient solar energy conversion, but achieving high efficiency and device longevity within a bulk-heterojunction (BHJ) structure remains a challenge. Traditional small-molecule acceptors (SMAs) in the BHJ blend show thermodynamic instability affecting the morphology. In contrast, tethered SMAs exhibit higher glass transition temperatures, mitigating these concerns. Yet, they might not integrate well with polymer donors, causing pronounced phase separation and overpurification of mixed domains. Herein, a novel ternary device is introduced that uses DY-P2EH, a tethered dimeric SMA with conjugated side-chains as host acceptor, and BTP-ec9, a monomeric SMA as secondary acceptor, which respectively possess hypomiscibility and hypermiscibility with the polymer donor PM6. This unique combination affords a parallel-connected ternary BHJ blend, leading to a hierarchical and stable morphology. The ternary device achieves a remarkable fill factor of 80.61% and an impressive power conversion efficiency of 19.09%. Furthermore, the ternary device exhibits exceptional stability, retaining over 85% of its initial efficiency even after enduring 1100 h of thermal stress at 85 °C. These findings highlight the potential advantage of tethered SMAs in the design of ternary devices with a refined hierarchical structure for more efficient and durable solar energy conversion technologies.

Remarkable Enhancement of Hole Mobility of Novel DA-D'-AD Small Molecules by Thermal Annealing: Effect of the D'-Bridge Block

Conjugated small molecules are advanced semiconductor materials with attractive physicochemical and optoelectronic properties enabling the development of next-generation electronic devices. The charge carrier mobility of small molecules strongly influences the efficiency of organic and hybrid electronics based on them. Herein, we report the synthesis of four novel small molecules and their investigation with regard to the impact of molecular structure and thermal treatment of films on charge carriers' mobility. The benzodithiophene-containing compounds (BDT) were shown to be more promising in terms of tuning the morphology upon thermal treatment. Impressive enhancement of hole mobilities by more than 50 times was found for annealed films based on a compound M4 comprising triisopropylsilyl-functionalized BDT core. The results provide a favorable experience and strategy for the rational design of state-of-the-art organic semiconductor materials (OSMs) and for improving their charge-transport characteristics.

Efficient Standalone Flexible Small Molecule Organic Solar Cell Devices: Structure-Performance Relation Among Tetracyanoquinodimethane Derivatives

Currently, very few dicyano and tetracyanoquinodimethane (TCNQ) based molecules are utilized as active layers, sandwiched between the electron and hole transport layer in organic solar cell (OSC) devices. Nevertheless, simple mono- and disubstituted TCNQ derivatives as exclusively active layers are yet unexplored and provide scope for further investigation. In this study, TCNQ derivatives with varying amine substituents, namely, AEPYDQ (1), BMEDDQ (2), MATBTCNQ (3), and MITATCNQ (4), were explored as efficient standalone, flexible, all small molecule OSC devices. Particularly, 1 resulted in the highest device efficiency of 11.75% with an aromatic amine, while 2 possessing an aliphatic amine showed the lowest power conversion efficiency (PCE; 2.12%). Notably, the short circuit current density (JSC) of device 1 increased from 2 mA/cm2 in the dark to 9.12 mA/cm2 under light, indicating a significant boost in the current generation. Further, 1 manifested more crystallinity than others. Interestingly, 4 exhibited a higher PCE (5.90%) than 3 (PCE is 2.58%), though 3 is disubstituted with an aromatic amine, probably attributed to the electron-withdrawing effects of the -CF3 and -CN groups in 3 reducing the available π-electron density for stacking. Therefore, this study emphasizes crystallinity, significantly on the PCE, offering insights into the design of many such efficient OSCs.

A Theoretical Study on the Underlying Factors of the Difference in Performance of Organic Solar Cells Based on ITIC and Its Isomers

Recently, non-fullerene-based organic solar cells (OSCs) have made great breakthroughs, and small structural differences can have dramatic impacts on the power conversion efficiency (PCE). We take ITIC and its isomers as examples to study their effects on the performance of OSCs. ITIC and NFBDT only differed in the side chain position, and they were used as models with the same donor molecule, PBDB-T, to investigate the main reasons for the difference in their performance in terms of theoretical methods. In this work, a detailed comparative analysis of the electronic structure, absorption spectra, open circuit voltage and interfacial parameters of the ITIC and NFBDT systems was performed mainly by combining the density functional theory/time-dependent density functional theory and molecular dynamics simulations. The results showed that the lowest excited state of the ITIC molecule possessed a larger ∆q and more hybrid FE/CT states, and PBDB-T/ITIC had more charge separation paths as well as a larger kCS and smaller kCR. The reason for the performance difference between PBDB-T/ITIC and PBDB-T/NFBDT was elucidated, suggesting that ITIC is a superior acceptor based on a slight modulation of the side chain and providing a guiding direction for the design of superior-performing small molecule acceptor materials.

Synthesis and Characterization of the Donor-Acceptor Conjugated Polymer PBDB-T Implementing Group IV Element Germanium

Here, we synthesized and characterized a novel two-dimensional (2D) conjugated electron donor-acceptor (D-A) copolymer (PBDB-T-Ge), wherein the substituent of triethyl germanium was added to the electron donor unit of the polymer. The Turbo-Grignard reaction was used to implement the group IV element into the polymer, resulting in a yield of 86%. This corresponding polymer, PBDB-T-Ge, exhibited a down-shift in the highest occupied molecular orbital (HOMO) level to -5.45 eV while the lowest unoccupied molecular orbital (LUMO) level was -3.64 eV. The peaks in UV-Vis absorption and the PL emission of PBDB-T-Ge were observed at 484 nm and 615 nm, respectively.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Modulation of Molecular Stacking via Tuning 2-Ethylhexyl Alkyl Chain Enables Improved Efficiency for All-Small-Molecule Organic Solar Cells

There is always a dilemma between strong π-π stacking/crystallinity and suitable domain size for all-small-molecule organic solar cells (ASM-OSCs), which puts forward higher requirements for the design of molecular donors. In this work, a series of novel molecular donors with different positional 2-ethylhexy (EH) attachments are designed and synthesized, named SM-R, SM-REH, SM-EH-R, and SM-EH-REH. It is found that EH-substitution on end groups (SM-REH) enables improved π-π interaction and crystallinity but with decreased solubility and phase size, leading to the improved efficiency of 15.6% as compared to 14.0% of SM-R. In contrast, EH-substitution on the π-bridge (SM-EH-R) significantly suppresses π-π stacking and increases the solubility, resulting in the lower efficiency of 11.9%. The further EH-substitution on end-groups of SM-EH-R, namely, SM-EH-REH, recovers the π-π stacking strength and obtains a moderate efficiency of 14.4%. Despite the higher crystallinity and increased π-π stacking in some molecules, the blend films show the gradually decreased domain size in the sequence of SM-R, SM-REH, SM-EH-R, and SM-EH-REH owing to the steric hindrance of the EH-chain. Overall, this work indicates that obtaining the higher π-π stacking/crystallinity and decreased domain size is achievable by tuning the EH-chain substitution, which paves the way to further improve the photovoltaic performance of ASM-OSCs.

End Group Effect of Asymmetric Benzodithiophene-Based Donor with Liquid-Crystal State for Small-Molecule Binary Solar Cell

Liquid-crystal small molecule donor (LC-SMD) is a new type organic semiconductor, which is attractive not only for the easy synthesis and purification, well-defined chemical structures, etc., but also for the LC state that makes the crystallinity and aggregation state of molecules adjustable. Here, one new LC-SMD (a-BTR-H4) is synthesized with 1D alkoxyl and 2D thiophene-alkylthiol side-chained benzo[1,2-b:4,5-b']dithiophene core, trithiophene π-bridge, and 3-(2-ethylhexyl) rhodanine end group. a-BTR-H4 shows low LC transition temperature, 117 °C, however, counterpart material (a-BTR-H5) with the same main structure but 3-ethyl rhodanine terminal group does not show LC properties. Although a-BTR-H4/H5 show similar Ultraviolet-visible absorption spectrum and energy levels, a-BTR-H4 affords relatively high photovoltaic performances due to favorable blend morphology produced by the consistent annealing temperature of Y6-based accepters and liquid crystal temperature of donors. Preliminary results indicate that a-BTR-H4 gains a power conversion efficiency (PCE) of 11.36% for Y6-based devices, which is ascribed to better light harvest as well as balanced carrier generation and transport, while a-BTR-H5 obtains 7.57% PCE. Therefore, some materials with unique nematic LC phase have great application potential in organic electronics, and further work to utilize a-BTR-H4 for high-performance device is underway.

Optical interference on the measurement of film-depth-dependent light absorption spectroscopy and a correction approach

Organic thin films usually feature vertical phase segregation, and film-depth-dependent light absorption spectroscopy is an emerging characterization method to study the vertical phase separation of active layer films in organic electronics field. However, the interference effects on thin films can lead to optical errors in their characterization results. In this work, the interference effects on fluctuations of peak intensity and peak position of film-depth-dependent light absorption spectroscopy are investigated. Subsequently, a numerical method based on inverse transfer matrix is proposed to obtain the optical constants of the active layer through the film-depth-dependent light absorption spectroscopy. The extinction coefficient error in the non-absorbing wavelength range caused by interference effect is reduced by ∼95% compared with the traditional film-depth-dependent light absorption spectroscopy measurement. Thus, the optical properties of the thin film and quantitative spectrographic analysis based on these optical constants largely avoid the effects of interference including fluctuations of peak intensity and peak position. It is concluded that for many morphologically homogenously films, the spatial (film-depth) resolution of this film-depth-dependent light absorption spectroscopy can be optimized to be <1 nm. Subsequently, this modified film-depth-dependent light absorption spectroscopy approach is employed to simulate the local optical properties within devices with a multilayer architecture.

Bifacial Dye Membranes: Ultrathin and Free-Standing although not Being Covalently Bound

Layers of aligned dyes are key to photo-driven charge separation in dye sensitized solar cells, but cannot be exploited as rectifying membranes in photocatalysis to separate half-cells because they are not sufficiently stable. While impressive work on the fabrication of stable noncovalent membranes has been recently demonstrated, these membranes are inherently suffering from non-uniform orientation of the constituting dyes. To stabilize layers made from uniformly assembled and aligned dyes, they can be covalently cross-linked via functional groups or via chromophores at the expense of their optical properties. Here stable membranes from established dyes are reported that do not need to be elaborately functionalized nor do their chromophores need to be destroyed. These membranes are free-standing, although being only non-covalently linked. To enable uniform dye-alignment, Langmuir layers made from linear, water-insoluble dyes are used. That water-soluble charge transfer dyes adsorb onto and intercalate into the Langmuir layer from the aqueous subphase, thus yielding free-standing, molecularly thin membranes are demonstrated. The developed bifacial layers consist almost entirely of π-conjugated units and thus can conduct charges and can be further engineered for optoelectronic and photocatalytic applications.

Recent Progress in All-Small-Molecule Organic Solar Cells

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed.

Impact of end-group modifications and planarity on BDP-based non-fullerene acceptors for high-performance organic solar cells by using DFT approach

With the aim to enhance the photovoltaic properties of organic solar cells (OSCs), seven new non-fullerene acceptors (K1-K7) have been designed by end-group modifications of benzo[2,1-b:3,4-b']bis(4H-dithieno[3,2-b:2',3'-d]pyrrole) (BDP)-based small molecule "MH" (which is taken as our reference R) using computational techniques. To investigate their various optoelectronic parameters, DFT studies were applied using the B3LYP functional at 6-31G (d, p) basis set. The measurement of molecular planarity parameter (MPP) and span of deviation from plane (SDP) confirmed the planar geometries of these structures resulting in enhanced conjugation. Frontier molecular orbital (FMO) and density of states (DOS) analyses confirmed shorter band gaps of K1-K7 as compared to R, which promotes charge transfer in them. Optical properties demonstrated that these compounds have absorption range from 692 to 711 nm, quite better than the 684 nm of reference R. Molecular electrostatic potential (MEP) and Mulliken' charge distribution analysis also revealed the presence of epic charge separation in these structures. K1-K7 showed enhanced LHE values as compared to R putting emphasis on their better abilities to produce charge carrier by absorption of light. Reorganization energies showed that all newly designed compound could have better rate of charge carrier mobility (except K4) than R. Calculations of open-circuit voltage (V_oc) and fill factor (FF) revealed its highest values for K3 and K4. Among newly designed molecules, K3 showed betterment in all its investigated parameters, making it a strong candidate to get enhanced power conversion efficiencies of OSCs.

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length through photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light, the azobenzene undergoes trans-to-cis isomerization, thus mechanically inducing the oligothiophene to assume a planar conformation (extended π-conjugation). Exposure to 254 nm wavelength promotes azobenzene cis-to-trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis-to-trans thermal relaxation at a rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized by using steady-state UV-vis spectroscopy, X-ray crystallography and quantum-chemical modeling. Finally, we tested the proposed light-responsive oligothiophenes in field-effect transistors to probe the photo-induced tuning of their electronic properties.

Ternary Blend Organic Solar Cells: Understanding the Morphology from Recent Progress

Ternary blend organic solar cells (TB-OSCs) incorporating multiple donor and/or acceptor materials into the active layer have emerged as a promising strategy to simultaneously improve the overall device parameters for realizing higher performances than binary devices. Whereas introducing multiple materials also results in a more complicated morphology than their binary blend counterparts. Understanding the morphology is crucially important for further improving the device performance of TB-OSC. This review introduces the solubility and miscibility parameters that affect the morphology of ternary blends. Then, this review summarizes the recent processes of morphology study on ternary blends from the aspects of molecular crystallinity, molecular packing orientation, domain size and purity, directly observation of morphology, vertical phase separation as well as morphological stability. Finally, summary and prospects of TB-OSCs are concluded.

Molecular Engineering of Peripheral Substitutions to Construct Efficient Acridine Core-Based Hole Transport Materials for Perovskite Solar Cells

The development of highly efficient hole transport materials (HTMs) for perovskite solar cells (PSCs) has been a hot research topic. Acridine and its derivatives are gradually utilized as new blocks for optoelectronic applications, which stems from its rigid conjugated structure, shedding a new light on this old molecule. Meanwhile, its application in PSCs as a HTM has not been well explored, and the efficiency of 9,10-dihydroacridine (ACR)-based HTMs is relatively low. In this work, we conduct a systematic modulation of the peripheral substituents for ACR core building block-based HTMs and investigate the effects of the electron-donating ability and π-conjugation of peripheral groups on the photovoltaic performance of the corresponding HTMs. It is found that the peripheral groups with a weaker electron-donating ability and stronger π-conjugation are more suitable for the acridine core, which itself has a stronger electron-donating ability. Through molecular engineering, the newly developed HTM ACR-PhDM achieves an impressive power conversion efficiency of 23.5%. Our work lays the foundation for the design and development of efficient HTMs in the future.

Achieving and Understanding of Highly Efficient Ternary Organic Photovoltaics: From Morphology and Energy Loss to Working Mechanism

Ternary strategy, adding an additional donor (D) or acceptor (A) into conventional binary D:A blend, has shown great potential in improving photovoltaic performances of organic photovoltaics (OPVs) for practical applications. Herein, this review is presented on how efficient ternary OPVs are realized from the aspects of morphology, energy loss, and working mechanism. As to morphology, the role of third component on the formation of preferred alloy-like-phase and vertical-phase, which are driven by the miscibility tuning, is discussed. For energy loss, the effect of the third component on the luminescence enhancement and energetic disordering suppression, which lead to favorable increase of voltage, is presented. Regarding working mechanism, dilution effect and relationships between two acceptors or donor/acceptor, which explain the observed device parameters variations, are analyzed. Finally, some future directions concerning ternary OPVs are pointed out. Therefore, this review can provide a comprehensive understanding of working principles and effective routes for high-efficiency ternary systems, advancing the commercialization of OPVs.

Perylene Diimide-Fused Dithiophenepyrroles with Different End Groups as Acceptors for Organic Photovoltaics

In this study, we synthesized four new A-DA'D-A acceptors (where A and D represent acceptor and donor chemical units) incorporating perylene diimide units (A') as their core structures and presenting various modes of halogenation and substitution of the functional groups at their end groups (A). In these acceptors, by fusing dithiophenepyrrole (DTP) moieties (D) to the helical perylene diimide dimer (hPDI) to form fused-hPDI (FhPDI) cores, we could increase the D/A' oscillator strength in the cores and, thus, the intensity of intramolecular charge transfer (ICT), thereby enhancing the intensity of the absorption bands. With four different end group units─IC2F, IC2Cl, IO2F, and IO2Cl─tested, each of these acceptor molecules exhibited different optical characteristics. Among all of these systems, the organic photovoltaic device incorporating the polymer PCE10 blended with the acceptor FhPDI-IC2F (1:1.1 wt %) had the highest power conversion efficiency (PCE) of 9.0%; the optimal PCEs of PCE10:FhPDI-IO2F, PCE10:FhPDI-IO2Cl, and PCE10:FhPDI-IC2Cl (1:1.1 wt %) devices were 5.2, 4.7, and 7.7%, respectively. The relatively high PCE of the PCE10:FhPDI-IC2F device resulted primarily from the higher absorption coefficients of the FhPDI-IC2F acceptor, lower energy loss, and more efficient charge transfer; the FhPDI-IC2F system experienced a lower degree of geminate recombination─as a result of improved delocalization of π-electrons along the acceptor unit─relative to that of the other three acceptors systems. Thus, altering the end groups of multichromophoric PDI units can increase the PCEs of devices incorporating PDI-derived materials and might also be a new pathway for the creation of other valuable fused-ring derivatives.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Modulation of the Fluorination Site on Side-Chain Thiophene Improved Efficiency in All-Small-Molecule Organic Solar Cells

Fine-tuning the phase-separated morphology is of great importance to achieve efficient all-small-molecule organic solar cells (ASM-OSCs). In this work, a pair of isomers are designed and synthesized, namely, BDT-UF and BDT-DF, in which the fluorine atom in BDT-UF is close to the alkyl chain of side-chain thiophene, while that in BDT-DF is close to the center core. Owing to the noncovalent interaction between fluorine and hydrogen, BDT-DF shows a smaller dihedral angle between the thiophene side chain and the BDT core, which causes better molecular planarity. When mixed with N3, BDT-UF shows better miscibility, higher crystallinity, and more ordered molecule stacking in the blend film. Finally, the device of BDT-DF:N3 gains a power conversion efficiency (PCE) of 14.5%, while that of BDT-UF:N3 shows an increase in Voc, Jsc, and FF and gains a PCE of 15.1%. Our work exhibits a way of adjusting the substitution site of fluorine atoms to improve the PCE of ASM-OSCs.

Small reorganization energy acceptors enable low energy losses in non-fullerene organic solar cells

Minimizing energy loss is of critical importance in the pursuit of attaining high-performance organic solar cells. Interestingly, reorganization energy plays a crucial role in photoelectric conversion processes. However, the understanding of the relationship between reorganization energy and energy losses has rarely been studied. Here, two acceptors, Qx-1 and Qx-2, were developed. The reorganization energies of these two acceptors during photoelectric conversion processes are substantially smaller than the conventional Y6 acceptor, which is beneficial for improving the exciton lifetime and diffusion length, promoting charge transport, and reducing the energy loss originating from exciton dissociation and non-radiative recombination. So, a high efficiency of 18.2% with high open circuit voltage above 0.93 V in the PM6:Qx-2 blend, accompanies a significantly reduced energy loss of 0.48 eV. This work underlines the importance of the reorganization energy in achieving small energy losses and paves a way to obtain high-performance organic solar cells.

All-Small-Molecule Organic Solar Cells with Efficiency Approaching 16% and FF over 80

Molecule engineering has been demonstrated as a valid strategy to adjust the active layer morphology in all-small-molecule organic solar cells (ASM-OSCs). In this work, two non-fullerene acceptors (NFAs), FO-2Cl and FO-EH-2Cl, with different alkyl side chains are reported and applied in ASC-OSCs. Compared with FO-2Cl, FO-EH-2Cl is designed by replacing the octyl alkyl chains with branched iso-octyl alkyl chains, leading to an enhanced molecular packing, crystallinity, and redshifted absorption. With a small molecule BSFTR as donor, the device of BSFTR:FO-EH-2Cl obtains a better morphology and achieves a higher power conversion efficiency (PCE) of 15.78% with a notable fill factor (FF) of 80.44% than that of the FO-2Cl-based device with a PCE of 15.27% and FF of 78.41%. To the authors' knowledge, the FF of 80.44% is the highest value in ASM-OSCs. These results demonstrate a good example of fine-tuning the molecular structure to achieve suitable active layer morphology with promising performance for ASM-OSCs, which can provide valuable insight into material design for high-efficiency ASM-OSCs.

Influence of Large Steric Hinderance Substituent Position on Conformation and Charge Transfer Process for Non-Fused Ring Acceptors

To obtain stable and planar molecular geometry in non-fused electron acceptors, A4T-25 and A4T-26 are designed and synthesized by introducing the bulk 2,4,6-triisopropylphenyl side groups onto different positions of the central two thiophene units. A4T-25 and A4T-26 both show a narrow-bandgap of 1.39 and 1.46 eV, with highest occupied molecular orbital/lowest unoccupied molecular orbital levels of -5.56/-3.81 and -5.65/-3.83 eV, respectively, and the electrostatic potential distributions imply that they have strong electron-accepting capability. The single crystal structure analysis and the transfer integral calculation demonstrate that the much more compact π-π stacking in A4T-26 can promote efficient charge transportation compared to that in A4T-25. Therefore, the electron mobility of A4T-26 is obviously higher and more balanced than that of A4T-25. When blending the two acceptors with the same polymer donor PBDB-TF, the photovoltaic cell based on PBDB-TF:A4T-25 has an inadequate power conversion efficiency (PCE) of 7.83%, while the PBDB-TF:A4T-26-based one yields an enhanced PCE of 12.1%. Overall, this study offers an insight into the relationship between the fine-tuning of the molecular structure of non-fused ring acceptors and the corresponding charge transfer process in organic solar cells.

Symmetrically Fluorinated Benzo[1,2-b:4,5-b']dithiophene-Cored Donor for High-Performance All-Small-Molecule Organic Solar Cells with Improved Active Layer Morphology and Crystallinity

Side-chain engineering is an efficient molecular design strategy for morphology optimization and performance improvement of organic solar cells (OSCs). Herein, a novel small-molecule donor C-2F, which owns a benzo[1,2-b:4,5-b']dithiophene (BDT) central unit with a symmetrically difluorinated benzene ring as a conjugated side chain, has been synthesized. The conjugated side chain possesses both the symmetry and halogenation effect in novel small molecular donor material. The photovoltaic devices were fabricated with N3 as an acceptor. C-2F:N3 based devices achieved an outstanding power conversion efficiency of 14.64% with a J_sc of 24.87 mA/cm², a V_oc of 0.85 V, and an FF of 69.33%. Then, we investigated the basic material properties, photovoltaic mechanism, and active layer morphology, and the results show that this molecular design strategy of the symmetrically difluorinated moiety as the conjugated side chain provides an effective method for fine-tuning the molecular stacking pattern and active layer phase separation morphology, to improve the all-small-molecule (ASM) OSCs' performances.

Novel Pyrrolo [3,4-b] dithieno [3, 2-f:2",3"-h] quinoxaline-8,10 (9H)-dione Based Wide Bandgap Conjugated Copolymers for Bulk Heterojunction Polymer Solar Cells

Two D-A copolymers consisted of fused ring pyrrolo-dithieno-quinoxaline acceptor are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donor and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells based on P134:Y6 attained the power conversion efficiency of 15.42%, which is higher than P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 could be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for polymer solar cells. This article is protected by copyright. All rights reserved.

Electrohydrodynamic-Jet-Printed Phthalimide-Derived Conjugated Polymers for Organic Field-Effect Transistors and Logic Gates

A π-conjugated polymer semiconductor, PBDTTTffPI, was synthesized for use as an organic semiconductor suitable for electrohydrodynamic (EHD) jet printing technology. Bulky alkylation of the polymer gave PBDTTTffPI good solubility in several organic solvents. EHD jet printing using PBDTTTffPI ink produced direct patterns of polymer semiconductors while maintaining smooth surface morphologies and crystal structures similar to those of spin-coated PBDTTTffPI films. EHD-jet-printed PBDTTTffPI was appropriate for use as a semiconductor layer in organic field-effect transistors (OFETs) and logic gates. OFETs that used EHD-jet-printed PBDTTTffPI had better electrical characteristics than devices that used spin-coated semiconductor films. When a dielectric material (Al₂O₃) with a high dielectric constant was introduced, the jet-printed PBDTTTffPI operated well at low voltages. Integrated devices such as inverters, NAND gates, and NOR gates were fabricated by printing PBDTTTffPI patterns and showed good switching behaviors. Therefore, the use of printable PBDTTTffPI provides an advance toward fabrication of practical integrated arrays in next-generation devices.

High Miscibility Compatible with Ordered Molecular Packing Enables an Excellent Efficiency of 16.2% in All-Small-Molecule Organic Solar Cells

In all-small-molecule organic solar cells (ASM-OSCs), a high short-circuit current (J_sc ) usually needs a small phase separation, while a high fill factor (FF) is generally realized in a highly ordered packing system. However, small domain and ordered packing always conflicted each other in ASM-OSCs, leading to a mutually restricted J_sc and FF. In this study, alleviation of the previous dilemma by the strategy of obtaining simultaneous good miscibility and ordered packing through modulating homo- and heteromolecular interactions is proposed. By moving the alkyl-thiolation side chains from the para- to the meta-position in the small-molecule donor, the surface tension and molecular planarity are synchronously enhanced, resulting in compatible properties of good miscibility with acceptor BTP-eC9 and strong self-assembly ability. As a result, an optimized morphology with multi-length-scale domains and highly ordered packing is realized. The device exhibits a long carrier lifetime (39.8 μs) and fast charge collection (15.5 ns). A record efficiency of 16.2% with a high FF of 75.6% and a J_sc of 25.4 mA cm^-2 in the ASM-OSCs is obtained. These results demonstrate that the strategy of simultaneously obtaining good miscibility with high crystallinity could be an efficient photovoltaic material design principle for high-performance ASM-OSCs.

Accelerated Explore of Efficient Ternary Solar Cell with PTB7:PC71BM:SMPV1 Using Machine-Learning Methods

Machine learning(ML) provides an efficient tool for predicting the photoelectric conversion efficiency(PCE) of organic solar cells(OSCs). In this paper, random forest (RF), K-nearest neighbors , and support vector machine in...

Efficient all-small-molecule organic solar cells based on a fluorinated small-molecule donor

A fluorinated donor with a deep HOMO energy level enables efficient all-small-molecule organic solar cells.

Controlling solid-state structure and film morphology in non-fullerene organic photovoltaic devices

Organic solar cells (OSCs) have long promised to provide renewable energy in a scalable, cost-effective way; however, for years, their relatively low efficiency has been a significant barrier to commercialization. Recent progress on cell efficiency means that OSCs are now much more competitive with other established technologies. These key advancements have come from better understanding and controlling the molecular structure, solid-state packing, and film morphology of the light absorbing layer. This focused review will explore the different ways that the solid-state structure and film morphology of the light absorbing layer can be controlled. It will examine the key features of an efficient light absorbing layer and present guiding principles for creating efficient OSCs. The future directions and remaining research questions of this field will be briefly discussed.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.