Crystallography, Morphology, Electronic Structure, and Transport in Non-Fullerene/Non-Indacenodithienothiophene Polymer:Y6 Solar Cells

CF3-Functionalized Side Chains in Nonfullerene Acceptors Promote Electrostatic Interactions for Highly Efficient Organic Solar Cells

The advent of next-generation nonfullerene acceptors (NFAs) has propelled major advances in organic solar cells (OSCs). Here we report an NFA design incorporating CF3-terminated side chains having varying N-(CH2)n-CF3 linker lengths (n = 1, 2, and 3) which introduce new intermolecular interactions, hence strong modulation of the photovoltaic response. We report a systematic comparison and contrast characterization of this NFA series with a comprehensive set of chemical/physical techniques versus the heavily studied third-generation NFA, Y6, revealing distinctive and beneficial properties of this new NFA series. Single-crystal diffraction analyses reveal unusual two-dimensional mesh-like crystal structures, featuring strong interactions between the side chain CF3-terminal and NFA core F substituents. These atomistic and morphological features contribute to enhanced charge mobility and significantly enhanced photovoltaic performance. We show that varying the CF3-terminated side chain linker length strongly modulates light harvesting efficiency as well as charge recombination and the photovoltaic bandgap. The CF3-(CH2)2-based OSC demonstrates the most balanced performance metrics, achieving a remarkable 19.08% power conversion efficiency and an exceptional 80.09% fill-factor. These results imply that introducing CF3-terminated side chains into other OSC conjugated constituents may accelerate next-generation solar cell development.

Molecular models of PM6 for non-fullerene acceptor organic solar cells: How DAD and ADA structures impact charge separation and charge recombination

The quadrupole moment of a non-fullerene acceptor (NFA) generated by the constituent electron donor (D) and acceptor (A) units is a significant factor that affects the charge separation (CS) and charge recombination (CR) processes in organic photovoltaics (OPVs). However, its impact on p-type polymer domains remains unclear. In this study, we synthesized p-type molecules, namely acceptor-donor-acceptor (ADA) and donor-acceptor-donor (DAD), which are components of the benchmark PM6 polymer (D: benzodithiophene and A: dioxobenzodithiophene). Planar heterojunction films, a model of bulk heterojunction, were prepared using ADA, DAD, and PM6 as the bottom p-type layers and Y6 NFA as the top n-type layer. Flash-photolysis time-resolved microwave conductivity, femtosecond transient absorption spectroscopy, and quantum mechanical calculations were employed to probe the charge carrier dynamics. Our findings reveal that while the subtle difference in quadrupole moment and energy gradient of the p-type materials has a minimal influence on CS, the molecular type (ADA or DAD) significantly affects the bulk CR. This study expands the understanding of how the p-type component and its conformation at the p/n interface impact the CS and CR in OPVs, highlighting the critical role of molecular donors in optimizing device performance.

Rebuilding Peripheral F, Cl, Br Footprints on Acceptors Enables Binary Organic Photovoltaic Efficiency Exceeding 19.7

Given homomorphic fluorine (F), chlorine (Cl) and bromine (Br) atoms are featured with gradually enlarged polarizability/atomic radius but decreased electronegativity, the rational screen of halogen species and locations on small molecular acceptors (SMAs) is quite essential for acquiring desirable molecular packing to boost efficiency of organic solar cells (OSCs). Herein, three isomeric SMAs (CH-F, CH-C and CH-B) are constructed by delicately rebuilding peripheral F, Cl, Br footprints on both central and end units. Such a re-permutation of peripheral halogens could not only maintain the structural symmetry of SMAs to the maximum, but also acquire extra asymmetric benefits of enhanced dipole moment and intramolecular charge transfer, etc. Moreover, central brominating enhances molecular crystallinity of CH-B without introducing undesirable steric hindrance on end groups, thus rendering a better balance between high crystallization and domain size control in PM6:CH-B blend. Further benefitting from the large dielectric constant, small exciton binding energy, optimized molecular packing and great electron transfer integral, CH-B affords the first class binary OSC efficiency of 19.78 %, moreover, the highest efficiency of 18.35 % thus far when increasing active layer thickness to ~300 nm. Our successful screening in rebuilding peripheral halogen footprints provides the valuable insight into further rational design of SMAs for record-breaking OSCs.

Delicate Regulation of Central Substituents Boosts Organic Photovoltaic Performance of Dimeric Acceptors

Dimeric acceptors are expected to satisfy both excellent power conversion efficiency (PCE) and operational stability of organic solar cells (OSCs). However, comparing to highly planar and symmetrical monomer-like acceptors, the quite different steric/spatial configurations of dimeric acceptors affect device outcomes greatly. Herein, on basis of the same dimeric molecular platform that constructed by bridging central units of two monomer-like acceptor, diverse substituents (─OCH3 for D1, ─CH3 for D2, and ─CF3 for D3) are grafted on central units to regulate the three dimensions (3D) geometries of dimeric acceptors delicately. A systematic investigation reveals the substituent-dependent variation of energy level, absorption, and molecular packing behavior. Consequently, D2 acceptor, characteristic of more favorable configuration, affords a superior film morphology and charge transfer/transport dynamics in resulting OSCs, thus yielding an excellent PCE of 17.50% along with a good long-term stability. This work manifests the crucially important role of central substituents in constructing high-performance dimeric acceptors.

Trifluoromethylation Enables Compact 2D Linear Stacking and Improves the Efficiency and Stability of Q-PHJ Organic Solar Cells

Compared to the bulk heterojunction (BHJ) devices, the quasiplanar heterojunction (Q-PHJ) exhibits a more stable morphology and superior charge transfer performance. To achieve both high efficiency and long-term stability, it is necessary to design new materials for Q-PHJ devices. In this study, QxIC-CF3 and QxIC-CH3 are designed and synthesized for the first time. The trifluoromethylation of the central core exerts a modulatory effect on the molecular stacking pattern, leveraging the strong electrostatic potential and intermolecular interactions. Compared with QxIC-CH3, the single crystal structure reveals that QxIC-CF3 exhibits a more compact 2D linear stacking behavior. These benefits, combined with the separated electron and hole transport channels in Q-PHJ device, lead to increased charge mobility and reduced energy loss. The devices based on D18/QxIC-CF3 exhibit an efficiency of 18.1%, which is the highest power conversion efficiency (PCE) for Q-PHJ to date. Additionally, the thermodynamic stability of the active layer morphology enhances the lifespan of the aforementioned devices under illumination conditions. Specifically, the T80 is 420 h, which is nearly twice that of the renowned Y6-based BHJ device (T80 = 220 h). By combining the advantages of the trifluoromethylation and Q-PHJ device, efficient and stable organic solar cell devices can be constructed.

Effects of adjusting nickel pulse count on NiOx films prepared by atomic layer deposition

The paper describes the preparation of NiOx films using atomic layer deposition (ALD) and analyzes their hole transport properties. During the ALD process, NiOx films with varying properties were fabricated by adjusting the number of nickel pulses in the reaction. Various characterization techniques were employed to investigate the morphology, composition, optical, and electrical properties of the films prepared with different numbers of nickel pulses. The study reveals that as the number of Ni pulses increases, the content of Ni metal and Ni(OH)2 in the NiOx films changes, and post-annealing treatment can significantly enhance the performance of the NiOx films. Finally, NiOx was used as a hole transport layer to successfully fabricate silicon solar cells, resulting in an increase in power conversion efficiency (PCE) from 17.89% to 18.89% compared to untreated cells.

Nitrogen-Bridged Fused-Ring Nonacyclic and Heptacyclic A-D-A Acceptors for Organic Photovoltaics

In this work, we designed two nitrogen-bridged fluorene-based heptacyclic FNT and nonacyclic FNTT ladder-type structures, which were constructed by one-pot palladium-catalyzed Buchwald-Hartwig amination. FNT and FNTT were further end-capped by FIC acceptors to form two FNT-FIC and FNTT-FIC non-fullerene acceptors (NFAs), respectively. The two NFAs exhibit more red-shifted absorption and higher crystallinity compared to those of the corresponding carbon-bridged FCT-FIC and FCTT-FIC counterparts. Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements reveal that the 2-butyloctyl groups on the nitrogen in the convex region of FNT-FIC interdigitate with the dioctyl groups on the fluorene in the concave region of another FNT-FIC, resulting in a lamellar packing structure with a d spacing of 13.27 Å. As a consequence, the PM6:FNT-FIC (1:1 wt %) device achieved a power conversion efficiency (PCE) of only 6.60%, primarily due to the highly crystalline nature of FNT-FIC, which induced significant phase separation between PM6 and FNT-FIC in the blended film. However, FNTT-FIC, featuring 2-butyloctyl groups positioned on the nitrogen within the concave region of its curved skeleton, exhibits improved donor-acceptor miscibility, thereby promoting a more favorable morphology. As a result, the PM6:FNTT-FIC (1:1.2 wt %) device exhibited a higher PCE of 12.15% with an exceptional Voc of 0.96 V. This research demonstrates that placing alkylamino moieties within the concave region of curved A-D-A NFAs leads to a better molecular design.

Constructing Multiscale Fibrous Morphology to Achieve 20% Efficiency Organic Solar Cells by Mixing High and Low Molecular Weight D18

This study underscores the significance of precisely manipulating the morphology of the active layer in organic solar cells (OSCs). By blending polymer donors of D18 with varying molecular weights, a multiscale interpenetrating fiber network structure within the active layer is successfully created. The introduction of 10% low molecular weight D18 (LW-D18) into high molecular weight D18 (HW-D18) produces MIX-D18, which exhibits an extended exciton diffusion distance and orderly molecular stacking. Devices utilizing MIX-D18 demonstrate superior electron and hole transport, improves exciton dissociation, enhances charge collection efficiency, and reduces trap-assisted recombination compared to the other two materials. Through the use of the nonfullerene acceptor L8-BO, a remarkable power conversion efficiency (PCE) of 20.0% is achieved. This methodology, which integrates the favorable attributes of high and low molecular weight polymers, opens a new avenue for enhancing the performance of OSCs.

Small-Molecule Mixed Ionic-Electronic Conductors for Efficient N-Type Electrochemical Transistors: Structure-Function Correlations

The fundamental challenge in electron-transporting organic mixed ionic-electronic conductors (OMIECs) is simultaneous optimization of electron and ion transport. Beginning from Y6-type/U-shaped non-fullerene solar cell acceptors, we systematically synthesize and characterize molecular structures that address the aforementioned challenge, progressively introducing increasing numbers of oligoethyleneglycol (OEG; g) sidechains from 1 g to 3 g, affording OMIECs 1gY, 2gY, and 3gY, respectively. The crystal structure of 1gY preserves key structural features of the Yn series: a U-shaped/planar core, close π-π molecular stacking, and interlocked acceptor groups. Versus inactive Y6 and Y11, all of the new glycolated compounds exhibit mixed ion-electron transport in both conventional organic electrochemical transistor (cOECT) and vertical OECT (vOECT) architectures. Notably, 3gY with the highest OEG density achieves a high transconductance of 16.5 mS, an on/off current ratio of ~106, and a turn-on/off response time of 94.7/5.7 ms in vOECTs. Systematic optoelectronic, electrochemical, architectural, and crystallographic analysis explains the superior 3gY-based OECT performance in terms of denser ngY OEG content, increased crystallite dimensions with decreased long-range crystalline order, and enhanced film hydrophilicity which facilitates ion transport and efficient redox processes. Finally, we demonstrate an efficient small-molecule-based complementary inverter using 3gY vOECTs, showcasing the bioelectronic applicability of these new small-molecule OMIECs.

Multi-arm quinoxaline-based acceptors formed by π-conjugation extension for efficient organic solar cells

Manipulating the conjugated backbone of small molecule acceptors (SMAs) is of particular importance in developing efficient organic solar cells (OSCs). Recently, trimers and other multi-arm SMAs have been found to be able to provide more intermolecular interaction, demonstrating excellent molecular stacking and device performance. However, the synthesis of this type of SMA usually relies on tristin or polystin compounds. Instead, expanding multiple arms in the central cores of SMAs is relatively simple and not restricted by tin compounds. Based on the quinoxaline core, two kinds of multi-arm SMAs, FQx-IC and TQx-IC with 4 and 3 arms, have been developed in this work. Compared to FQx-IC, TQx-IC exhibits an ordered face-on molecular orientation, appropriate film-forming process, and more favorable phase separation morphology and balanced charge transport. When blended with the polymer donor D18, OSCs based on TQx-IC achieve a power conversion efficiency (PCE) of 17.36%, which is superior to the device based on D18:FQx-IC (16.24%). In addition, using the ternary strategy of incorporating the TQx-IC into the D18:Y6 system, an excellent PCE of 18.82% is achieved. Therefore, this multi-arm molecular design strategy has great potential for regulating molecular stacking, absorption, and the corresponding device performance.

Non-conventional bulk heterojunction nanoparticle photocatalysts for sacrificial hydrogen evolution from water

Photocatalyst systems combining donor polymers with acceptor molecules have shown the highest evolution rates for sacrificial hydrogen production from water for organic systems to date. Here, new donor molecules have been designed and synthesised taking inspiration from the structure-performance relationships which have been established in the development of non-fullerene acceptors. While a conventional bulk heterojunction (BHJ) pairing consists of a donor polymer and acceptor small molecule, here we have successfully reversed this approach by using new p-type small molecules in combination with a n-type conjugated polymer to produce non-conventional BHJ (ncBHJ) nanoparticles. We have applied these ncBHJs as photocatalysts in the sacrificial hydrogen evolution from water, and the best performing heterojunction displayed high activity for sacrificial hydrogen production from water with a hydrogen evolution rate of 22 321 μmol h-1 g-1 which compares well with the state-of-the-art for conventional BHJ photocatalyst systems.

What is the Limit Size of 2D Conjugated Extension on Central Units of Small Molecular Acceptors in Organic Solar Cells?

2D conjugated extension on central units of small molecular acceptors (SMAs) has gained great successes in reaching the state-of-the-art organic photovoltaics. Whereas the limit size of 2D central planes and their dominant role in constructing 3D intermolecular packing networks are still elusive. Thus, by exploring a series of SMAs with gradually enlarged central planes, it is demonstrated that, at both single molecular and aggerated levels, there is an unexpected blue-shift for their film absorption but preferable reorganization energies, exciton lifetimes and binding energies with central planes enlarging, especially when comparing to their Y6 counterpart. More importantly, the significance of well-balanced molecular packing modes involving both central and end units is first disclosed through a systematic single crystal analysis, indicating that when the ratio of central planes area/end terminals area is no more than 3 likely provides a preferred 3D intermolecular packing network of SMAs. By exploring the limit size of 2D central planes, This work indicates that the structural profiles of ideal SMAs may require suitable central unit size together with proper heteroatom replacement instead of directly overextending 2D central planes to the maximum. These results will likely provide some guidelines for future better molecular design.

White-light activatable organic NIR-II luminescence nanomaterials for imaging-guided surgery

While second near-infrared (NIR-II) fluorescence imaging is a promising tool for real-time surveillance of surgical operations, the previously reported organic NIR-II luminescent materials for in vivo imaging are predominantly activated by expensive lasers or X-ray with high power and poor illumination homogeneity, which significantly limits their clinical applications. Here we report a white-light activatable NIR-II organic imaging agent by taking advantages of the strong intramolecular/intermolecular D-A interactions of conjugated Y6CT molecules in nanoparticles (Y6CT-NPs), with the brightness of as high as 13315.1, which is over two times that of the brightest laser-activated NIR-II organic contrast agents reported thus far. Upon white-light activation, Y6CT-NPs can achieve not only in vivo imaging of hepatic ischemia reperfusion, but also real-time monitoring of kidney transplantation surgery. During the surgery, identification of the renal vasculature, post-reconstruction assessment of renal allograft vascular integrity, and blood supply analysis of the ureter can be vividly depicted by using Y6CT-NPs with high signal-to-noise ratios upon clinical laparoscopic LED white-light activation. Our work provides efficient molecular design guidelines towards white-light activatable imaging agent and highlights an opportunity for precision imaging theranostics.

Recent Progress of Oligomeric Non-Fullerene Acceptors for Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) have achieved remarkable power conversion efficiencies (PCEs) of over 19 % in the past few years due to the rapid development of non-fullerene acceptors (NFAs). However, the operational stability remains a great challenge that inhibits their commercialization. Recently, oligomeric NFAs (ONFAs) have attracted great attention, which not only can deliver excellent device performance, but also improve the thermal-/photo- stability of OSCs. This is attributed to the suppressed molecular diffusion of ONFAs associated with their high glass-transition temperature (Tg) and improved thermodynamic properties of ONFAs. Herein, we focus on investigating the correction between the ONFA chemical structure, material properties, device performance, and stability. In addition, we also try to point out the challenges in synthesizing ONFAs and provide potential directions for future ONFA designs.

High Open-Circuit Voltage Organic Solar Cells with 19.2% Efficiency Enabled by Synergistic Side-Chain Engineering

Restricted by the energy-gap law, state-of-the-art organic solar cells (OSCs) exhibit relatively low open-circuit voltage (VOC) because of large nonradiative energy losses (ΔEnonrad). Moreover, the trade-off between VOC and external quantum efficiency (EQE) of OSCs is more distinctive; the power conversion efficiencies (PCEs) of OSCs are still <15% with VOCs of >1.0 V. Herein, the electronic properties and aggregation behaviors of non-fullerene acceptors (NFAs) are carefully considered and then a new NFA (Z19) is delicately designed by simultaneously introducing alkoxy and phenyl-substituted alkyl chains to the conjugated backbone. Z19 exhibits a hypochromatic-shifted absorption spectrum, high-lying lowest unoccupied molecular orbital energy level and ordered 2D packing mode. The D18:Z19-based blend film exhibits favorable phase separation with face-on dominated molecular orientation, facilitating charge transport properties. Consequently, D18:Z19 binary devices afford an exciting PCE of 19.2% with a high VOC of 1.002 V, surpassing Y6-2O-based devices. The former is the highest PCE reported to date for OSCs with VOCs of >1.0 V. Moreover, the ΔEnonrad of Z19- (0.200 eV) and Y6-2O-based (0.155 eV) devices are lower than that of Y6-based (0.239 eV) devices. Indications are that the design of such NFA, considering the energy-gap law, could promote a new breakthrough in OSCs.

Customizable Organic Charge-Transfer Cocrystals for the Dual-Mode Optoelectronics in the NIR (II) Window

Organic molecules have been regarded as ideal candidates for near-infrared (NIR) optoelectronic active materials due to their customizability and ease of large-scale production. However, constrained by the intricate molecular design and severe energy gap law, the realization of optoelectronic devices in the second near-infrared (NIR (II)) region with required narrow band gaps presents more challenges. Herein, we have originally proposed a cocrystal strategy that utilizes intermolecular charge-transfer interaction to drive the redshift of absorption and emission spectra of a series BFXTQ (X = 0, 1, 2, 4) cocrystals, resulting in the spectra located at NIR (II) window and reducing the optical bandgap to ∼0.98 eV. Significantly, these BFXTQ-based optoelectronic devices can exhibit dual-mode optoelectronic characteristics. An investigation of a series of BFXTQ-based photodetectors exhibits detectivity (D*) surpassing 1013 Jones at 375 to 1064 nm with a maximum of 1.76 × 1014 Jones at 1064 nm. Moreover, the radiative transition of CT excitons within the cocrystals triggers NIR emission over 1000 nm with a photoluminescence quantum yield (PLQY) of ∼4.6% as well as optical waveguide behavior with a low optical-loss coefficient of 0.0097 dB/μm at 950 nm. These results promote the advancement of an emerging cocrystal approach in micro/nanoscale NIR multifunctional optoelectronics.

π-Extended Nonfullerene Acceptor for Compressed Molecular Packing in Organic Solar Cells To Achieve over 20% Efficiency

Organic photovoltaics (OPVs) suffer from a trade-off between efficient charge transport and suppressed nonradiative recombination due to the aggregation-induced luminance quenching of organic semiconductors. To resolve this grand challenge, a π-extended nonfullerene acceptor (NFA) B6Cl with large voids among the honeycomb network is designed and introduced into photovoltaic systems. We find that the presence of a small amount of (i.e., 0.5 or 1 wt %) B6Cl can compress the molecular packing of the host acceptor L8-BO, leading to shortened π-π stacking distance from 3.59 to 3.50 Å (that will improve charge transport) together with ordered alkyl chain packing (that will inhibit nonradiative energy loss due to the suppressed C-C and C-H bonds vibrations), as validated by high-energy X-ray scattering measurements. This morphology transformation ultimately results in simultaneously improved JSC, FF, and VOC of OPVs. As a result, the maximum PCEs of PM6:L8-BO and D18:L8-BO are increased from 19.1 and 19.3% to 19.8 and 20.2%, respectively, which are among the highest values for single-junction OPVs. The university of B6Cl to increase the performance of OPVs is further evidenced in a range of polymer:NFA OPVs.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

New Avenues for Organic Solar Cells Using Intrinsically Charge-Generating Materials

The molecular electron acceptor material Y6 has been a key part of the most recent surge in organic solar cell sunlight-to-electricity power conversion efficiency, which is now approaching 20%. Numerous studies have sought to understand the fundamental photophysical reasons for the exceptional performance of Y6 and its growing family of structural derivatives. Though significant uncertainty about several details remains, many have concluded that initially photogenerated excited states rapidly convert into electron-hole charge pairs in the neat material. These charge pairs are characterized by location of the electron and hole on different Y6 molecules, in contrast to the Frenkel excitons that dominate the behavior of most organic semiconductor materials. Here, we summarize the current state of knowledge regarding Y6 photophysics and the key observations that have led to it. We then link this understanding to other advances, such as the role of quadrupolar fields in donor-acceptor blends, and the importance of molecular interactions and organization in providing the structural basis for Y6's properties. Finally, we turn our attention to ways of making use of the new photophysics of Y6, and suggest molecular doping, crystal structure tuning, and electric field engineering as promising avenues for future exploration.

Enhanced Photocatalytic and Photovoltaic Performance Arising from Unconventionally Low Donor-Y6 Ratios

Development of both organic photovoltaics (OPVs) and organic photocatalysts has focused on utilizing the bulk heterojunction (BHJ). The BHJ promotes charge separation and enhances the carrier lifetime, but may give rise to increased charge traps, hindering performance. Here, high photocatalytic and photovoltaic performance is displayed by electron donor-acceptor (D-A) nanoparticles (NPs) and films, using the nonfullerene acceptor Y6 and polymer donor PIDT-T8BT. In contrast to conventional D-A systems, the charge generation in PIDT-T8BT:Y6 NPs is mainly driven by Y6, allowing a high performance even at a low D:A mass ratio of 1:50. The high performance at the low mass ratio is attributed to the amorphous behavior of PIDT-T8BT. Low ratios are generally thought to yield lower efficiency than the more conventional ≈1:1 ratio. However, the OPVs exhibit peak performance at a D:A ratio of 1:5. Similarly the NPs used for photocatalytic hydrogen evolution show peak performance at the 1:6.7 D:A ratio. Interestingly, for the PIDT-T8BT:Y6 system, as the polymer proportion increases, a reduced photocatalytic and photovoltaic performance is observed. The unconventional D:A ratios provide lower recombination losses and increased charge-carrier lifetime with undisrupted ambipolar charge transport in bulk Y6, enabling better performance than conventional ratios. This work reports novel light-harvesting materials in which performance is reduced due to unfavorable morphology as D:A ratios move toward conventional ratios of 1:1.2-1:1.

Assessing intra- and inter-molecular charge transfer excitations in non-fullerene acceptors using electroabsorption spectroscopy

Organic photovoltaic cells using Y6 non-fullerene acceptors have recently achieved high efficiency, and it was suggested to be attributed to the charge-transfer (CT) nature of the excitations in Y6 aggregates. Here, by combining electroabsorption spectroscopy measurements and electronic-structure calculations, we find that the charge-transfer character already exists in isolated Y6 molecules but is strongly increased when there is molecular aggregation. Surprisingly, it is found that the large enhanced charge transfer in clustered Y6 molecules is not due to an increase in excited-state dipole moment, Δμ, as observed in other organic systems, but due to a reduced polarizability change, Δp. It is proposed that such a strong charge-transfer character is promoted by the stabilization of the charge-transfer energy upon aggregation, as deduced from density functional theory and four-state model calculations. This work provides insight into the correlation between molecular electronic properties and charge-transfer characteristics in organic electronic materials.

Organic Donor-Acceptor Systems for Photocatalysis

Organic semiconductor materials are considered to be promising photocatalysts due to their excellent light absorption by chromophores, easy molecular structure tuning, and solution-processable properties. In particular, donor-acceptor (D-A) type organic photocatalytic materials synthesized by introducing D and A units intra- or intermolecularly, have made great progress in photocatalytic studies. More and more studies have demonstrated that the D-A type organic photocatalytic materials combine effective carrier separation, tunable bandgap, and sensitive optoelectronic response, and are considered to be an effective strategy for enhancing light absorption, improving exciton dissociation, and optimizing carrier transport. This review provides a thorough overview of D-A strategies aimed at optimizing the photocatalytic performance of organic semiconductors. Initially, essential methods for modifying organic photocatalytic materials, such as interface engineering, crystal engineering, and interaction modulation, are briefly discussed. Subsequently, the review delves into various organic photocatalytic materials based on intramolecular and intermolecular D-A interactions, encompassing small molecules, conjugated polymers, crystalline polymers, supramolecules, and organic heterojunctions. Meanwhile, the energy band structures, exciton dynamics, and redox-active sites of D-A type organic photocatalytic materials under different bonding modes are discussed. Finally, the review highlights the advanced applications of organic photocatalystsand outlines prospective challenges and opportunities.

Molecular-Level Insight into Impact of Additives on Film Formation and Molecular Packing in Y6-based Organic Solar Cells

Additive engineering is widely utilized to optimize film morphology in active layers of organic solar cells (OSCs). However, the role of additive in film formation and adjustment of film morphology remains unclear at the molecular level. Here, taking high-efficiency Y6-based OSC films as an example, this work thus employs all-atom molecular-dynamics simulations to investigate how introduction of additives with different π-conjugation degree thermodynamically and dynamically impacts nanoscale molecular packings. These results demonstrate that the van der Waals (vdW) interactions of the Y6 end groups with the studied additives are strongest. The larger the π-conjugation degree of the additive molecules, the stronger the vdW interactions between additive and Y6 molecules. Due to such vdW interactions, the π-conjugated additive molecules insert into the neighboring Y6 molecules, thus opening more space for relaxation of Y6 molecules to trigger more ordered packing. Increasing the interactions between the Y6 end groups and the additive molecules not only accelerates formation of the Y6 ordered packing, but also induces shorter Y6-intermolecular distances. This work reveals the fundamental molecular-level mechanism behind film formation and adjustment of film morphology via additive engineering, providing an insight into molecular design of additives toward optimizing morphologies of organic semiconductor films.

Selectively Modulating Componential Morphologies of Bulk Heterojunction Organic Solar Cells

Achieving precise control over the nanoscale morphology of bulk heterojunction films presents a significant challenge for the conventional post-treatments employed in organic solar cells (OSCs). In this study, a near-infrared photon-assisted annealing (NPA) strategy is developed for fabricating high-performance OSCs under mild processing conditions. It is revealed a top NIR light illumination, together with the bottom heating, enables the selective tuning of the molecular arrangement and assembly of narrow bandgap acceptors in polymer networks to achieve optimal morphologies, as well as the acceptor-rich top surface of active layers. The derived OSCs exhibit a remarkable power conversion efficiency (PCE) of 19.25%, representing one of the highest PCEs for the reported binary OSCs so far. Moreover, via the NPA strategy, it has succeeded in accessing top-illuminated flexible OSCs using thermolabile polyethylene terephthalate from mineral water bottles, displaying excellent mechanical stabilities. Overall, this work will hold the potential to develop organic solar cells under mild processing with various substrates.

Nonfullerene Acceptor Featuring Unique Self-Regulation Effect for Organic Solar Cells with 19 % Efficiency

The blend nanomorphology of electron-donor (D) and -acceptor (A) materials is of vital importance to achieving highly efficient organic solar cells. Exogenous additives especially aromatic additives are always needed to further optimize the nanomorphology of blend films, which is hardly compatible with industrial manufacture. Herein, we proposed a unique approach to meticulously modulate the aggregation behavior of NFAs in both crystal and thin film nanomorphology via self-regulation effect. Nonfullerene acceptor Z9 was designed and synthesized by tethering phenyl groups on the inner side chains of the Y6 backbone. Compared with Y6, the tethered phenyl groups participated in the molecular aggregation via the π-π stacking of phenyl-phenyl and phenyl-2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) groups, which induced 3D charge transport with phenyl-mediated super-exchange electron coupling. Moreover, ordered molecular packing with suitable phase separation was observed in Z9-based blend films. High power conversion efficiencies (PCEs) of 19.0 % (certified PCE of 18.6 %) for Z9-based devices were achieved without additives, indicating the great potential of the self-regulation strategy in NFA design.

Propeller vs Quasi-Planar 6-Cantilever Small Molecular Platforms with Extremely Two-Dimensional Conjugated Extension

Two exotic 6-cantilever small molecular platforms, characteristic of quite different molecular configurations of propeller and quasi-plane, are established by extremely two-dimensional conjugated extension. When applied in small molecular acceptors, the only two cases of CH25 and CH26 that could contain six terminals and such broad conjugated backbones have been afforded thus far, rendering featured absorptions, small reorganization and exciton binding energies. Moreover, their distinctive but completely different molecular geometries result in sharply contrasting nanoscale film morphologies. Finally, CH26 contributes to the best device efficiency of 15.41 % among acceptors with six terminals, demonstrating two pioneered yet highly promising 6-cantilever molecular innovation platforms.

A Polymer Acceptor with Grafted Small Molecule Acceptor Unit for Efficient All Polymer Organic Solar Cells

A polymer acceptor, named PX-1, is  designed and synthesized using a polymerization strategy with grafted small molecule acceptors. This design approach allows for the freedom of end groups while maintaining efficient terminal packing, enhancing π-π interactions, and facilitating charge transport. All-polymer organic solar cells based on PM6: PX-1 demonstrate a promising efficiency of 13.55%. The result presents an alternative pathway for the design of high-efficiency polymer acceptors through the careful regulation of small molecule acceptor monomers and linker units.

Bulk Heterojunction Upconversion Thin Films Fabricated via One-Step Solution Deposition

Upconversion of near-infrared light into the visible has achieved limited success in applications due to the difficulty of creating solid-state films with high external quantum efficiency (EQE). Recent developments have expanded the range of relevant materials for solid-state triplet-triplet annihilation upconversion through the use of a charge-transfer state sensitization process. Here, we report the single-step solution-processed deposition of a bulk heterojunction upconversion film using organic semiconductors. The use of a bulk heterojunction thin film enables a high contact area between sensitizer and annihilator materials in this interface-triplet-generation mechanism and allows for a facile single-step deposition process. Demonstrations of multiple deposition and patterning methods on glass and flexible substrates show the promise of this materials system for solid-state upconversion applications.

A high-performance dual-functional organic upconversion device with detectivity approaching 1013 Jones and photon-to-photon efficiency over 20

Organic upconversion devices (UCDs) are a cutting-edge technology and hot topic because of their advantages of low cost and convenience in the important applications of near-infrared (NIR) detection and imaging. However, to realize utilization of triplet excitons (T1), previous UCDs have the drawback of heavily relying on toxic and costly heavy-metal-doped emitters. More importantly, due to poor performance of the detecting unit and/or emitting unit, improving their detectivity (D*) and photon-to-photon conversion efficiency (ηp-p) is still a challenge for real applications. Here, we report a high-performance dual-functional purely organic UCD that has an outstanding D* approaching 1013 Jones and a high ηp-p of 20.1% in the NIR region, which are some of the highest values among those reported for UCDs. The high performance is credited to the excellent D* of the detecting unit, exceeding 1014 Jones, and is also attributed to efficient T1 utilization via a dual reverse intersystem crossing channel and high optical out coupling achieved via a high horizontal dipole ratio in the emitting unit. The high D* and ηp-p enable the UCD to detect 850 nm light at as little as 0.29 μW cm-2 and with a high display contrast of over 70 000 : 1, significantly improving the potential of practical applications of UCDs in NIR detection and imaging. Furthermore, a fast rise time and fall time of 8.9 and 14.8 μs are also achieved. Benefiting from the high performance, consequent applications of low-power pulse-state monitoring and fine-structure bio-imaging are successfully realized with high quality results by using our organic UCDs. These results demonstrate that our design not only eliminates dependence of UCDs on heavy-metal emitters, but also takes their performance and applications to a high level.

The central core size effect in quinoxaline-based non-fullerene acceptors for high VOC organic solar cells

For organic solar cells (OSCs), obtaining a high open circuit voltage (VOC) is often accompanied by the sacrifice of the circuit current density (JSC) and filling factor (FF), and it is difficult to strike a balance between VOC and JSC × FF. The trade-off of these parameters is often the critical factor limiting the improvement of the power conversion efficiency (PCE). Extended backbone conjugation and side chain engineering of non-fullerene acceptors (NFAs) are effective strategies to optimize the performance of OSCs. Herein, based on the quinoxaline central core and branched alkyl chains at the β position of the thiophene unit, we designed and synthesized three NFAs with different sized cores. Interestingly, Qx-BO-3 with a smaller central core showed better planarity and more appropriate crystallinity. As a result, PM6:Qx-BO-3-based devices obtained more suitable phase separation, more efficient exciton dissociation, and charge transport properties. Therefore, the OSCs based on PM6:Qx-BO-3 yielded an outstanding PCE of 17.03%, significantly higher than the devices based on PM6:Qx-BO-1 (10.57%) and PM6:Qx-BO-2 (11.34%) although the latter two devices have lower VOC losses. These results indicated that fine-tuning the central core size can effectively optimize the molecular geometry of NFAs and the film morphology of OSCs. This work provides an effective method for designing high-performance NFA-OSCs with high VOCs.

Fused-Ring Electron-Acceptor Single Crystals with Chiral 2D Supramolecular Organization for Anisotropic Chiral Optoelectronic Devices

Supramolecular chiral organization gives π-conjugated molecules access to fascinating specific interactions with circularly polarized light (CPL). Such a feature enables the fabrication of high-performance chiral organic electronic devices that detect or emit CPL directly. Herein, it is shown that chiral fused-ring electron-acceptor BTP-4F single-crystal-based phototransistors demonstrate distinguished CPL discrimination capability with current dissymmetry factor exceeding 1.4, one of the highest values among state-of-the-art direct CPL detectors. Theoretical calculations prove that the chirality at the supramolecular level in these enantiomeric single crystals originates from chiral exciton coupling of a unique quasi-2D supramolecular organization consisting of interlaced molecules with opposite helical conformation. Impressively, such supramolecular organization produces a higher dissymmetry factor along the preferred growth direction of the chiral single crystals in comparison to that of the short axis direction. Furthermore, the amplified, inverted, and also anisotropic current dissymmetry compared to optical dissymmetry is studied by finite element simulations. Therefore, a unique chiral supramolecular organization that is responsible for the excellent chiroptical response and anisotropic electronic properties is developed, which not only enables the construction of high-performance CPL detection devices but also allows a better understanding of the structure-property relationships in chiral organic optoelectronics.

Complete Peripheral Fluorination of the Small-Molecule Acceptor in Organic Solar Cells Yields Efficiency over 19

Due to the intrinsically flexible molecular skeletons and loose aggregations, organic semiconductors, like small molecular acceptors (SMAs) in organic solar cells (OSCs), greatly suffer from larger structural/packing disorders and weaker intermolecular interactions comparing to their inorganic counterparts, further leading to hindered exciton diffusion/dissociation and charge carrier migration in resulting OSCs. To overcome this challenge, complete peripheral fluorination was performed on basis of a two-dimensional (2D) conjugation extended molecular platform of CH-series SMAs, rendering an acceptor of CH8F with eight fluorine atoms surrounding the molecular backbone. Benefitting from the broad 2D backbone, more importantly, strengthened fluorine-induced secondary interactions, CH8F and its D18 blends afford much enhanced and more ordered molecular packings accompanying with enlarged dielectric constants, reduced exciton binding energies and more obvious fibrillary networks comparing to CH6F controls. Consequently, D18:CH8F-based OSCs reached an excellent efficiency of 18.80 %, much better than that of 17.91 % for CH6F-based ones. More excitingly, by employing D18-Cl that possesses a highly similar structure to D18 as a third component, the highest efficiency of 19.28 % for CH-series SMAs-based OSCs has been achieved so far. Our work demonstrates the dramatical structural multiformity of CH-series SMAs, meanwhile, their high potential for constructing record-breaking OSCs through peripheral fine-tuning.

Molecular Stacking and Aggregation Optimization of Photoactive Layer through Solid Additive Enables High-Performance Organic Solar Cells

Regulating molecular packing and aggregation of photoactive layer is a critical but challenging issue in developing high-performance organic solar cells. Herein, two structurally similar analogues of anthra[2,3-b : 6,7-b']dithiophene (ADT) and naphtho[1,2-b : 5,6-b']dithiophene (NDT) are developed as solid additive to exploit their effect in regulating the molecular aggregation and π-stacking of photoactive layer. We clarify that the perpendicular arrangements of NDT can enlarge the molecular packing space and improve the face-on stacking of Y6 during the film formation, favoring a more compact and ordered long-range π-π stacking in the out-of-plane direction after the removal of NDT under thermal annealing. The edge-to-face stacked herringbone-arrangement of ADT along with its non-volatilization under thermal annealing can induce the coexistence of face-on and edge-on stacking of blend film. As a result, the NDT treatment shows encouraging effect in improving the photovoltaic performance of devices based on various systems. Particularly, a remarkable PCE of 18.85 % is achieved in the PM6 : L8-BO-based device treated by NDT additive, which is a significant improvement with regard to the PCE of 16.41 % for the control device. This work offers a promising strategy to regulate the molecular packing and aggregation of photoactive layer towards significantly improved performance and stability of organic solar cells.

The State-of-the-Art Solution-Processed Single Component Organic Photodetectors Achieved by Strong Quenching of Intermolecular Emissive State and High Quadrupole Moment in Non-Fullerene Acceptors

A bulk-heterojunction (BHJ) blend is commonly used as the photoactive layer in organic photodetectors (OPDs) to utilize the donor (D)/acceptor (A) interfacial energetic offset for exciton dissociation. However, this strategy often complicates optimization procedures, raising serious concerns over device processability, reproducibility, and stability. Herein, highly efficient OPDs fabricated with single-component organic semiconductors are demonstrated via solution-processing. The non-fullerene acceptors (NFAs) with strong intrinsic D/A character are used as the photoactive layer, where the emissive intermolecular charge transfer excitonic (CTE) states are formed within <1 ps, and efficient photocurrent generation is achieved via strong quenching of these CTE states by reverse bias. Y6 and IT-4F-based OPDs show excellent OPD performances, low dark current density (≈10-9 A cm-2 ), high responsivity (≥0.15 A W-1 ), high specific detectivity (>1012 Jones), and fast photo-response time (<10 µs), comparable to the state-of-the-art BHJ OPDs. Together with strong CTE state quenching by electric field, these excellent OPD performances are also attributed to the high quadrupole moments of NFA molecules, which can lead to large interfacial energetic offset for efficient CTE dissociation. This work opens a new way to realize efficient OPDs using single-component systems via solution-processing and provides important molecular design rules.

Extending Se substitution to the limit: from 5S to 5Se in high-efficiency non-fullerene acceptors

Based on the newly synthesized seleno[3,2-b]selenophene unit, two near-infrared non-fullerene acceptors (NFAs) of 4Se and 5Se are constructed by replacing four or all sulfurs with selenium in high-efficiency Y-series NFAs. Consequently, binary devices based on 4Se and 5Se afford PCEs of 15.17% and 15.23%, respectively, with a photoelectric response approaching 1000 nm. More excitingly, the energy loss of the 5Se-based device was as low as 0.477 eV along with almost the smallest non-radiative loss of ∼0.15 eV thus far.

A rare case of brominated small molecule acceptors for high-efficiency organic solar cells

Given that bromine possesses similar properties but extra merits of easily synthesizing and polarizing comparing to homomorphic fluorine and chlorine, it is quite surprising very rare high-performance brominated small molecule acceptors have been reported. This may be caused by undesirable film morphologies stemming from relatively larger steric hindrance and excessive crystallinity of bromides. To maximize the advantages of bromides while circumventing weaknesses, three acceptors (CH20, CH21 and CH22) are constructed with stepwise brominating on central units rather than conventional end groups, thus enhancing intermolecular packing, crystallinity and dielectric constant of them without damaging the favorable intermolecular packing through end groups. Consequently, PM6:CH22-based binary organic solar cells render the highest efficiency of 19.06% for brominated acceptors, more excitingly, a record-breaking efficiency of 15.70% when further thickening active layers to ~500 nm. By exhibiting such a rare high-performance brominated acceptor, our work highlights the great potential for achieving record-breaking organic solar cells through delicately brominating.

Organic Solar Cells with Over 19% Efficiency Enabled by a 2D-Conjugated Non-Fullerene Acceptor Featuring Favorable Electronic and Aggregation Structures

The π-expansion of non-fullerene acceptors is a promising method for boosting the organic photovoltaic performance by allowing the fine-tuning of electronic structures and molecular packing. In this work, highly efficient organic solar cells (OSCs) are fabricated using a 2D π-expansion strategy to design new non-fullerene acceptors. Compared with the quinoxaline-fused cores of AQx-16, the π-expanded phenazine-fused cores of AQx-18 induce more ordered and compact packing between adjacent molecules, affording an optimized morphology with rational phase separation in the blend film. This facilitates efficient exciton dissociation and inhibited charge recombination. Consequently, a power conversion efficiency (PCE) of 18.2% with simultaneously increasing Voc , Jsc , and fill factor is achieved in the AQx-18-based binary OSCs. Significantly, AQx-18-based ternary devices fabricated via a two-in-one alloy acceptor strategy exhibit a superior PCE of 19.1%, one of the highest values ever reported for OSCs, along with a high Voc of 0.928 V. These results indicate the importance of the 2D π-expansion strategy for the delicate regulation of the electronic structures and crystalline behaviors of the non-fullerene acceptors to achieve superior photovoltaic performance, aimed at significantly promoting further development of OSCs.

Intrinsic Role of Volatile Solid Additive in High-Efficiency PM6:Y6 Series Nonfullerene Solar Cells

Organic nonfullerene solar cells (ONSCs) have made unprecedented progress; however, morphology optimization of ONSCs is proven to be particularly challenging relative to classical fullerene-based devices. Here, a novel volatile solid additive (VSA), 2-hydroxy-4-methoxybenzophenone (2-HM), is reported for achieving high-efficiency ONSCs. 2-HM functions as a universal morphology-directing agent for several well-known PM6:Y6 series nonfullerene blends, viz. PM6:Y6, PM6:BTP-eC9, PM6:L8-BO, leading to a best efficiency of 18.85% at the forefront of reported binary ONSCs. VSAs have recently emerged, while the intrinsic kinetics is still unclear. Herein, a set of in situ and ex situ characterizations is employed to first illustrate the molecule-aggregate-domain transition dynamic process assisted by the VSA. More specifically, the role of 2-HM in individual donor PM6 and acceptor Y6 systems is unlocked, and the function of 2-HM in altering the PM6:Y6 bulk heterojunction blends is further revealed for enhanced photovoltaic performance. It is believed that the achievement brings not only a deep insight into emerging volatile solid additive, but also a new hope to further improve the molecular ordering, film microstructure, and relevant performance of ONSCs.

Geometry design of tethered small-molecule acceptor enables highly stable and efficient polymer solar cells

With the power conversion efficiency of binary polymer solar cells dramatically improved, the thermal stability of the small-molecule acceptors raised the main concerns on the device operating stability. Here, to address this issue, thiophene-dicarboxylate spacer tethered small-molecule acceptors are designed, and their molecular geometries are further regulated via the thiophene-core isomerism engineering, affording dimeric TDY-α with a 2, 5-substitution and TDY-β with 3, 4-substitution on the core. It shows that TDY-α processes a higher glass transition temperature, better crystallinity relative to its individual small-molecule acceptor segment and isomeric counterpart of TDY-β, and a more stable morphology with the polymer donor. As a result, the TDY-α based device delivers a higher device efficiency of 18.1%, and most important, achieves an extrapolated lifetime of about 35000 hours that retaining 80% of their initial efficiency. Our result suggests that with proper geometry design, the tethered small-molecule acceptors can achieve both high device efficiency and operating stability.

Increasing Charge Carrier Mobility through Modifications of Terminal Groups of Y6: A Theoretical Study

The applications of non-fullerene acceptor Y6 with a new type of A1-DA2D-A1 framework and its derivatives have increased the power conversion efficiency (PCE) of organic solar cells (OSCs) up to 19%. Researchers have made various modifications of the donor unit, central/terminal acceptor unit, and side alkyl chains of Y6 to study the influences on the photovoltaic properties of OSCs based on them. However, up to now, the effect of changes of terminal acceptor parts of Y6 on the photovoltaic properties is not very clear. In the present work, we have designed four new acceptors-Y6-NO2, Y6-IN, Y6-ERHD, and Y6-CAO-with different terminal groups, which possess diverse electron-withdrawing ability. Computed results show that with the enhanced electron-withdrawing ability of the terminal group, the fundamental gaps become lower; thus, the wavelengths of the main absorption peaks of UV-Vis spectra red-shifts and total oscillator strength increase. Simultaneously, the electron mobility of Y6-NO2, Y6-IN, and Y6-CAO is about six, four, and four times faster than that of Y6, respectively. Overall, Y6-NO2 could be a potential NFA because of its longer intramolecular charge-transfer distance, stronger dipole moment, higher averaged ESP, enhanced spectrum, and faster electron mobility. This work provides a guideline for the future research on modification of Y6.

13% Single-Component Organic Solar Cells based on Double-Cable Conjugated Polymers with Pendent Y-Series Acceptors

Double-cable conjugated polymers with pendent electron acceptors, including fullerene, rylene diimides, and nonfused acceptors, have been developed for application in single-component organic solar cells (SCOSCs) with efficiencies approaching 10%. In this work, Y-series electron acceptors have been firstly incorporated into double-cable polymers in order to further improve the efficiencies of SCOSCs. A highly crystalline Y-series acceptor based on quinoxaline core and the random copolymerized strategy are used to optimize the ambipolar charge transport and the nanophase separation of the double-cable polymers. As a result, an efficiency of 13.02% is obtained in the random double-cable polymer, representing the highest performance in SCOSCs, while the regular double-cable polymer only provides a low efficiency of 2.75%. The significantly enhanced efficiencies are attributed to higher charge carrier mobilities, better ordering conjugated backbones and Y-series acceptors in random double-cable polymers.

Molecular orientation-dependent energetic shifts in solution-processed non-fullerene acceptors and their impact on organic photovoltaic performance

The non-fullerene acceptors (NFAs) employed in state-of-art organic photovoltaics (OPVs) often exhibit strong quadrupole moments which can strongly impact on material energetics. Herein, we show that changing the orientation of Y6, a prototypical NFA, from face-on to more edge-on by using different processing solvents causes a significant energetic shift of up to 210 meV. The impact of this energetic shift on OPV performance is investigated in both bilayer and bulk-heterojunction (BHJ) devices with PM6 polymer donor. The device electronic bandgap and the rate of non-geminate recombination are found to depend on the Y6 orientation in both bilayer and BHJ devices, attributed to the quadrupole moment-induced band bending. Analogous energetic shifts are also observed in other common polymer/NFA blends, which correlates well with NFA quadrupole moments. This work demonstrates the key impact of NFA quadruple moments and molecular orientation on material energetics and thereby on the efficiency of high-performance OPVs.

Correlation of Local Isomerization Induced Lateral and Terminal Torsions with Performance and Stability of Organic Photovoltaics

Organic photovoltaics (OPVs) have achieved great progress in recent years due to delicately designed non-fullerene acceptors (NFAs). Compared with tailoring of the aromatic heterocycles on the NFA backbone, the incorporation of conjugated side-groups is a cost-effective way to improve the photoelectrical properties of NFAs. However, the modifications of side-groups also need to consider their effects on device stability since the molecular planarity changes induced by side-groups are related to the NFA aggregation and the evolution of the blend morphology under stresses. Herein, a new class of NFAs with local-isomerized conjugated side-groups are developed and the impact of local isomerization on their geometries and device performance/stability are systematically investigated. The device based on one of the isomers with balanced side- and terminal-group torsion angles can deliver an impressive power conversion efficiency (PCE) of 18.5%, with a low energy loss (0.528 V) and an excellent photo- and thermal stability. A similar approach can also be applied to another polymer donor to achieve an even higher PCE of 18.8%, which is among the highest efficiencies obtained for binary OPVs. This work demonstrates the effectiveness of applying local isomerization to fine-tune the side-group steric effect and non-covalent interactions between side-group and backbone, therefore improving both photovoltaic performance and stability of fused ring NFA-based OPVs.

The Multiplicity of π-π Interactions of Fused-Ring Electron Acceptor Polymorphs on the Exciton Migration and Charge Transport

Efficient long-range exciton migration and charge transport are the key parameters for organic photovoltaic materials, which strongly depend on the molecular stacking modes. Herein, we extracted the stacked structures of the archetype fused-ring electron acceptor molecule, ITIC, based on the information on four polymorphic crystals and investigated the relationship between molecular stacking modes and exciton migration/charge transport properties through the intermolecular Coulomb coupling and charge transfer integral calculation. Experimentally, the thin film texture is crystallized through a post-annealing treatment through grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, which lead to the enhanced exciton migration through exciton-exciton annihilation in the femtosecond transient absorption (fs-TA) measurements. This work demonstrates the relationship between the molecular arrangement and the exciton migration and electron transport and highlights the significance of optimizing molecular stacking for the development of high-performance electron acceptor materials.

Semi-Transparent, Chiral Organic Photodiodes with Incident Direction-Dependent Selectivity for Circularly Polarized Light

Detection of the circular polarization of light is possible using chiral semiconductors, yet the mechanisms remain poorly understood. Semi-transparent chiral photodiodes allow for a simple experiment to investigate the basis of their selectivity: changing the side from which the diode is illuminated. A reversal of circular selectivity is observed in photocurrent generation when changing the direction of illumination on organic, bulk-heterojunction cells. The change in selectivity can be explained by a space-charge limitation on the collection of photocarriers in combination with preferential absorption of one of the circular polarizations of near-infrared light by the chiral non-fullerene acceptor. The space-charge limitation is supported by detailed measurements of frequency and intensity dependence of dc and ac photocurrents.

Polaritons in a Polycrystalline Layer of Non-fullerene Acceptor

Non-fullerene acceptor molecules developed for organic solar cells feature a very intense absorption band in the near-infrared. In the solid phase, the strong interaction between light and the transition dipole moment for molecular excitation should induce formation of polaritons. The reflection spectra for polycrystalline films of a non-fullerene acceptor with a thienothienopyrrolo-thienothienoindole core of the so-called Y6 type indeed show a signature of polaritons. A local minimum in the middle of the reflection band is associated with the allowed molecular transition. The minimum in reflection allows efficient entry of light into the solid, resulting in a local maximum in external quantum efficiency of a photovoltaic cell made of the pure acceptor.

Quasi-Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

Picosecond Charge-Transfer-State Dynamics in Wide Band Gap Polymer-Non-Fullerene Small-Molecule Blend Films Investigated via Transient Infrared Spectroscopy

Organic solar cells based on wide band gap polymers and nonfullerene small-molecule acceptors have demonstrated remarkably good device performances. Nevertheless, a thorough understanding of the charge-transfer process in these materials has not been achieved yet. In this study, we use Fano resonance signals caused by the interaction of broad electronic charge carrier absorption and the molecular vibrations of the electron acceptor molecule to monitor the charge-transfer state dynamics. In our time-resolved infrared spectroscopy experiments, we find that in the small-molecule acceptor, they have additional dynamics on the order of a few picoseconds. A change in the solvent used in thin film deposition, leading to different morphologies, influences this time further. We interpret our findings as the dynamics of the charge-transfer state at the interface of the electron donor and the electron- acceptor. The additional mid-infrared transient signal is generated in this state, as both electron and hole polarons can interact with small-molecule acceptor vibrational modes.

Organic Solar Cell With Efficiency Over 20% and VOC Exceeding 2.1 V Enabled by Tandem With All-Inorganic Perovskite and Thermal Annealing-Free Process

Organic solar cells (OSCs) based on polymer donor and non-fullerene acceptor achieve power conversion efficiency (PCE) more than 19% but their poor absorption below 550 nm restricts the harvesting of high-energy photons. In contrast, wide bandgap all-inorganic perovskites limit the absorption of low-energy photons and cause serious below bandgap loss. Therefore, a 2-terminal (2T) monolithic perovskite/organic tandem solar cell (TSC) incorporating wide bandgap CsPbI2 Br is demonstrated as front cell absorber and organic PM6:Y6 blend as rear cell absorber, to extend the absorption of OSCs into high-energy photon region. The perovskite sub-cell, featuring a sol-gel prepared ZnO/SnO2 bilayer electron transporting layer, renders a high open-circuit voltage (VOC ). The VOC is further enhanced by employing thermal annealing (TA)-free process in the fabrication of rear sub-cell, demonstrating a record high VOC of 2.116 V. The TA-free Ag/PFN-Br interface in organic sub-cell facilitates charge transport and restrains nonradiative recombination. Consequently, a remarkable PCE of 20.6% is achieved in monolithic 2T-TSCs configuration, which is higher than that of both reported single junction and tandem OSCs, demonstrating that tandem with wide bandgap all-inorganic perovskite is a promising strategy to improve the efficiency of OSCs.