Efficient Charge Transfer and Fine-Tuned Energy Level Alignment in a THF-Processed Fullerene-Free Organic Solar Cell with 11.3% Efficiency

Role of Charge-Carrier Dynamics Toward the Fabrication of Efficient Air-Processed Organic Solar Cells

Over the past couple of decades, immense research has been carried out to understand the photo-physics of an organic solar cell (OSC) that is important to enhance its efficiency and stability. Since OSCs undergoes complex photophysical phenomenon, studying these factors has led to designing new materials and implementing new strategies to improve efficiency in OSCs. In this regard, the invention of the non-fullerene acceptorshas greatly revolutionized the understanding of the fundamental processes occurring in OSCs. However, such vital fundamental research from device physics perspectives is carried out on glovebox (GB) processed OSCs and there is a scarcity of research on air-processed (AP) OSCs. This review will focus on charge carrier dynamics such as exciton diffusion, exciton dissociation, charge-transfer states, significance of highest occupied molecular orbital-offsets, and hole-transfer efficiencies of GB-OSCs and compare them with the available data from the AP-OSCs. Finally, key requirements for the fabrication of efficient AP-OSCs will be presented from a charge-carrier dynamics perspective. The key aspects from the charge-carrier dynamics view to fabricate efficient OSCs either from GB or air are provided.

Progress in organic photovoltaics based on green solvents: from solubility enhancement to morphology optimization

Solution-processed organic photovoltaics (OPVs) is one of the most promising photovoltaic technologies in the energy field, due to their clean and renewable low-cost manufacturing potential. OPV has rapidly developed with the design and synthesis of highly efficient photovoltaic materials and the development of smart device engineering. To date, the majority of advanced OPV devices have been prepared using halogenated solvents, achieving power conversion efficiencies (PCE) exceeding 19% on a laboratory scale. However, for industrial-scale production, less toxic manufacturing processes and environmental sustainability are the key considerations. Therefore, this review summarizes recent advances in green solvent-based approaches for the preparation of OPVs, highlighting material design (including polymer donors and small molecule acceptors) and device engineering (co-solvent methods, additive strategies, post-treatment methods, and regulation of coating method), emphasizing crucial factors for achieving high performance in green solvent-processed OPV devices. This review presents potential future directions for green solvent-based OPVs, which may pave the way for future industrial development.

A polymer donor based on difluoro-quinoxaline with a naphthalimide substituent unit exhibits a low-lying HOMO level for efficient non-fullerene polymer solar cells

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (VOC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Origin of Photovoltaics in Organic Solar Cells at Negligible Energy Level Offsets─An Insight of the Charge Accumulation Effect

Reducing the energy level offset is one of the key elements of low open-circuit voltage loss in organic solar cells. However, the origin of charge separation driving force at negligible energy level offsets still remains unexplained. Herein, from the perspective of built-in potential caused by charge accumulation, we discuss the nonequilibrium energy level displacement as current passing with distinct variable current densities. Due to the different carrier mobilities of electrons and holes in organic semiconductor materials, carriers with high mobility will be rapidly transmitted to the electrode, while those with low mobility will remain in the materials, resulting in the accumulation of corresponding charges. It is suggested that the higher the carrier mobility, the better the efficiency of photovoltaic devices, with the balance of the charge transport.

The role of structural parameters on efficiency and transparency of semi-transparent non-fullerene organic solar cell

Semitransparent organic solar cells have become attractive recently because of their photon harvesting in the near-infrared and ultraviolet range and passing in the visible light region. Semitransparent organic solar cells with ITO/ZnO/PBDB-T:ITIC/MoO3/Ag/MoO3 structure have been studied in this work and the effects of PBDB-T:ITIC active layer thicknesses and the transparent top electrode, MoO₃/Ag/MoO₃, thickness on the solar cell performance such as I-V characteristics, the power conversion efficiency, the average visible transmittance, and the color coordinates in the CIE color space are investigated. The drift–diffusion model, including the density of exactions, and their displacement is used to model the devices. The model is examined with experimentally reported devices, where there is a very good agreement between them, then is applied to the new structures. The obtained results show that the average visible transmittance of more than 45% is achievable for these structures with reasonable power conversion efficiency.

Development of non-fullerene electron acceptors for efficient organic photovoltaics

Compared to fullerene based electron acceptors, n-type organic semiconductors, so-called non-fullerene acceptors (NFAs), possess some distinct advantages, such as readily tuning of optical absorption and electronic energy levels, strong absorption in the visible region and good morphological stability for flexible electronic devices. The design and synthesis of new NFAs have enabled the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) devices to increase to around 19%. This review summarises the important breakthroughs that have contributed to this progress, focusing on three classes of NFAs, i.e. perylene diimide (PDI), diketopyrrolopyrrole (DPP) and acceptor–donor–acceptor (A-D-A) based NFAs. Specifically, the PCEs of PDI, DPP, and A-D-A series based non-fullerene OPVs have been reported up to 11%, 13% and 19%, respectively. Structure–property relationships of representative NFAs and their impact on OPV performances are discussed. Finally, we consider the remaining challenges and promising directions for achieving high-performing NFAs.

Quinoxaline-Based D-A Copolymers for the Applications as Polymer Donor and Hole Transport Material in Polymer/Perovskite Solar Cells

Polymer solar cells (PSCs) have achieved great progress recently, benefiting from the rapid development of narrow bandgap small molecule acceptors and wide bandgap conjugated polymer donors. Among the polymer donors, the D-A copolymers with quinoxaline (Qx) as A-unit have received increasing attention since the report of the low-cost and high-performance D-A copolymer donor based on thiophene D-unit and difluoro-quinoxalline A-unit in 2018. In addition, the weak electron-deficient characteristic and the multiple substitution positions of the Qx unit make it an ideal A-unit in constructing the wide bandgap polymer donors with different functional substitutions. In this review article, recent developments of the Qx-based D-A copolymer donors, including synthetic method of the Qx unit, backbone modulation, side chain optimization, and functional substitution of the Qx-based D-A copolymers, are summarized and discussed. Furthermore, the application of the Qx-based D-A copolymers as hole transport material in perovskite solar cells (pero-SCs) is also introduced. The focus mainly on the molecular design strategies and structure-properties relationship of the Qx-based D-A copolymers, aiming to provide a guideline for developing high-performance Qx-based D-A copolymers for the applications as donor in PSCs and as hole transport material in pero-SCs.

Naphthobisthiadiazole-Based π-Conjugated Polymers for Nonfullerene Solar Cells: Suppressing Intermolecular Interaction Improves Photovoltaic Performance

Naphthobisthiadiazole has been known as a promising building unit of π-conjugated polymers for organic photovoltaics (OPVs). Here, we synthesized new NTz-based polymers that were combined with a benzodithiophene (BDT) unit having alkylthienyl substituents in the polymer backbone, named PNTzBDT, and PNTzBDT-F and PNTzBDT-Cl with fluorine and chlorine groups in the substituents, respectively. The polymers had significantly improved solubility than the previously reported NTz-based polymer (PNTz4T), most likely due to the torsion of the alkylthienyl substituents with respect to the BDT moiety, which suppresses the intermolecular interaction between the backbones. Despite the lower intermolecular interaction and thereby lower crystallinity, these polymers, in particular PNTzBDT and PNTzBDT-F, exhibited higher photovoltaic performances, with power conversion efficiencies as high as 13.3%, than PNTz4T in the cells that used Y6 as the acceptor material. The improved performance was ascribed to the enhanced miscibility of the polymers with the nonfullerene acceptor due to the increased solubility, which in addition led to the better charge generation and reduced charge recombination. These results indicate that NTz-based π-conjugated polymers have high potential for nonfullerene-based OPVs.

A novel energy level detector for molecular semiconductors

The multifunction of molecule-based devices is always achieved by improving their charge transport characteristics. These characteristics depend strongly on the energy levels of molecular semiconductors, which fundamentally govern the working principle and device performance. Therefore, an accurate measurement of these energy levels is crucial for evaluating the availability of the prepared materials and thus optimizing the device performance. Here, an easy-to-operate three-terminal hot electron transistor has been developed, which comprises a molecular optoelectronic device that records the charge transport. It achieves exceptional properties including the lowest unoccupied molecular orbit level, highest occupied molecular orbit level, higher energy states, and higher electronic bandgap. When compared with existing techniques such as cyclic voltammetry, inverse photoemission spectroscopy, and ultraviolet photoemission spectroscopy, the hot electron transistor provides in-situ characterization and categorizes the measured energy information as intrinsic properties of the molecular semiconductor. Furthermore, we provide an in-depth understanding of the fundamental device-physics, which provides promising guidance for performance optimization.

Spiroconjugated Tetraaminospirenes as Donors in Color-Tunable Charge-Transfer Emitters with Donor-Acceptor Structure

Charge-transfer emitters are attractive due to their color tunability and potentially high photoluminescence quantum yields (PLQYs). We herein present tetraaminospirenes as donor moieties, which, in combination with a variety of acceptors, furnished 12 charge-transfer emitters with a range of emission colors and PLQYs of up to 99 %. The spatial separation of their frontier molecular orbitals was obtained through careful structural design, and two DA structures were confirmed by X-ray crystallography. A range of photophysical measurements supported by DFT calculations shed light on the optoelectronic properties of this new family of spiro-NN-donor-acceptor dyes.

Rod-Coil Block Copolymer: Fullerene Blend Water-Processable Nanoparticles: How Molecular Structure Addresses Morphology and Efficiency in NP-OPVs

The use of water-processable nanoparticles (WPNPs) is an emerging strategy for the processing of organic semiconducting materials into aqueous medium, dramatically reducing the use of chlorinated solvents and enabling the control of the nanomorphology in OPV active layers. We studied amphiphilic rod-coil block copolymers (BCPs) with a different chemical structure and length of the hydrophilic coil blocks. Using the BCPs blended with a fullerene acceptor material, we fabricated NP-OPV devices with a sustainable approach. The goal of this work is to clarify how the morphology of the nanodomains of the two active materials is addressed by the hydrophilic coil molecular structures, and in turn how the design of the materials affects the device performances. Exploiting a peculiar application of TEM, EFTEM microscopy on WPNPs, with the contribution of AFM and spectroscopic techniques, we correlate the coil structure with the device performances, demonstrating the pivotal influence of the chemical design over material properties. BCP5, bearing a coil block of five repeating units of 4-vinilpyridine (4VP), leads to working devices with efficiency comparable to the solution-processed ones for the multiple PCBM-rich cores morphology displayed by the blend WPNPs. Otherwise, BCP2 and BCP15, with 2 and 15 repeating units of 4VP, respectively, show a single large PCBM-rich core; the insertion of styrene units into the coil block of BCP100 is detrimental for the device efficiency, even if it produces an intermixed structure.

Recent advances in non-fullerene organic photovoltaics enabled by green solvent processing

Solution-processed organic photovoltaic (OPV) as a new energy device has attracted much attention due to its huge potential in future commercial manufacturing. However, so far, most of the studies on high-performance OPV have been treated with halogenated solvents. Halogenated solvents not only pollute the environment, but are also harmful to human health, which will negatively affect the large-scale production of OPV in the future. Therefore, it is urgent to develop low-toxic or non-toxic non-halogen solvent-processable OPV. Compared with conventional fullerene OPVs, non-fullerene OPVs exist with stronger absorption, better-matched energy levels and lower energy loss. Processing photoactive layers with non-fullerenes as the acceptor material has broad potential advantages in non-halogenated solvents. This review introduces the research progress of non-fullerene OPV treated by three different kinds of green solvents as the non-halogenated and aromatic solvent, the non-halogenated and non-aromatic solvent, alcohol and water. Furthermore, the effects of different optimization strategies on the photoelectric performance and stability of non-fullerene OPV are analyzed in detail. The current optimization strategy can increase the power conversion efficiency of non-fullerene OPV processed with non-halogen solvents up to 17.33%, which is close to the performance of processing with halogen-containing solvents. Finally, the commercial potential of non-halogen solvent processing OPVs is discussed. The green solvent processing of non-fullerene-based OPVs will become a key development direction for the future of the OPV industry.

Developing Efficient Small Molecule Acceptors with sp2 -Hybridized Nitrogen at Different Positions by Density Functional Theory Calculations, Molecular Dynamics Simulations and Machine Learning

Chemical structure of small molecule acceptors determines their performance in organic solar cells. Multiscale simulations are necessary to avoid trial-and-error based design, ultimately to save time and resources. In current study, the effect of sp² -hybridized nitrogen substitution at the inner or the outmost position of central core, side chain, and terminal group of small molecule acceptors is investigated using multiscale computational modelling. Quantum chemical analysis is used to study the electronic behavior. Nitrogen substitution at end-capping has significantly decreased the electron-reorganization energy. No big change is observed in transfer integral and excited state behavior. However, nitrogen substitution at terminal group position is good way to improve electron-mobility. Power conversion efficiency (PCE) of newly designed acceptors is predicted using machine learning. Molecular dynamics simulations are also performed to explore the dynamics of acceptor and their blends with PBDB-T polymer donor. Florgy-Huggins parameter is calculated to study the mixing of designed small molecule acceptors with PBDB-T. Radial distribution function has indicated that PBDB-T has a closer packing with N3 and N4. From all analysis, it is found that nitrogen substitution at end-capping group is a better strategy to design efficient small molecule acceptors.

Q-Force: Quantum Mechanically Augmented Molecular Force Fields

The quality of molecular dynamics simulations strongly depends on the accuracy of the underlying force fields (FFs) that determine all intra- and intermolecular interactions of the system. Commonly, transferable FF parameters are determined based on a representative set of small molecules. However, such an approach sacrifices accuracy in favor of generality. In this work, an open-source and automated toolkit named Q-Force is presented, which augments these transferable FFs with molecule-specific bonded parameters and atomic charges that are derived from quantum mechanical (QM) calculations. The molecular fragmentation procedure allows treatment of large molecules (>200 atoms) with a low computational cost. The generated Q-Force FFs can be used at the same computational cost as transferable FFs, but with improved accuracy: We demonstrate this for the vibrational properties on a set of small molecules and for the potential energy surface on a complex molecule (186 atoms) with photovoltaic applications. Overall, the accuracy, user-friendliness, and minimal computational overhead of the Q-Force protocol make it widely applicable for atomistic molecular dynamics simulations.

Numerical Investigation for the Resin Filling Behavior during Ultraviolet Nanoimprint Lithography of Subwavelength Moth-Eye Nanostructure

Accurate analysis of the resin filling process into the mold cavity is necessary for the high-precision fabrication of moth-eye nanostructure using the ultraviolet nanoimprint lithography (UV-NIL) technique. In this research, a computational fluid dynamics (CFD) simulation model was proposed to reveal resin filling behavior, in which the effect of boundary slip was considered. By comparison with the experimental results, a good consistency was found, indicating that the simulation model could be used to analyze the resin filling behavior. Based on the proposed model, the effects of process parameters on resin filling behavior were analyzed, including resin viscosity, inlet velocity and resin thickness. It was found that the inlet velocity showed a more significant effect on filling height than the resin viscosity and thickness. Besides, the effects of boundary conditions on resin filling behavior were investigated, and it was found the boundary slip had a significant influence on resin filling behavior, and excellent filling results were obtained with a larger slip velocity on the mold side. This research could provide guidance for a more comprehensive understanding of the resin filling behavior during UV-NIL of subwavelength moth-eye nanostructure.

High-Efficiency Thickness-Insensitive Organic Solar Cells with an Insulating Polymer

Achieving high-efficiency thick-film bulk heterojunction (BHJ) organic solar cells (OSCs) with thickness-independent power conversion efficiencies (PCEs) in a wide thickness range is still a challenge for the roll-to-roll printing techniques. The concept of diluting the transport sites within BHJ films with insulating polymers can effectively eliminate charge trapping states and optimize the charge transport. Herein, we first adopted the concept with insulating polypropylene (PP) in the efficient non-fullerene system (PM6:Y6) and demonstrated its potential to fabricate thick-film OSCs. The PP can form an insulating matrix prior to PM6 and Y6 within the BHJ film, resulting in an enhanced molecular interaction and isolated charge transport by expelling Y6 molecules. We thus observed reduced trap state density and improved charge transport properties in the PP-blended device. At around 300 nm, the PM6:Y6:PP device enjoys a high PCE of 15.5% and achieves over 100% of the efficiency of the optimal thin-film device, which is significantly improved compared to the binary PM6:Y6 counterpart. This research promotes an effective strategy with insulating polymers and provides knowledge of commercial production with response to the roll-to-roll technique demands.

Aggregation Controlled Charge Generation in Fullerene Based Bulk Heterojunction Polymer Solar Cells: Effect of Additive

Optimization of charge generation in polymer blends is crucial for the fabrication of highly efficient polymer solar cells. While the impacts of the polymer chemical structure, energy alignment, and interface on charge generation have been well studied, not much is known about the impact of polymer aggregation on charge generation. Here, we studied the impact of aggregation on charge generation using transient absorption spectroscopy, neutron scattering, and atomic force microscopy. Our measurements indicate that the 1,8-diiodooctane additive can change the aggregation behavior of poly(benzodithiophene-alt-dithienyl difluorobenzotriazole (PBnDT-FTAZ) and phenyl-C61-butyric acid methyl ester (PCBM)polymer blends and impact the charge generation process. Our observations show that the charge generation can be optimized by tuning the aggregation in polymer blends, which can be beneficial for the design of highly efficient fullerene-based organic photovoltaic devices.

Eco-Friendly Polymer Solar Cells: Advances in Green-Solvent Processing and Material Design

Despite the recent breakthroughs of polymer solar cells (PSCs) exhibiting a power conversion efficiency of over 17%, toxic and hazardous organic solvents such as chloroform and chlorobenzene are still commonly used in their fabrication, which impedes the practical application of PSCs. Thus, the development of eco-friendly processing methods suitable for industrial-scale production is now considered an imperative research focus. This Review provides a roadmap for the design of efficient photoactive materials that are compatible with non-halogenated green solvents (e.g., xylenes, toluene, and tetrahydrofuran). We summarize the recent development of green processing solvents and the processing methods to match with the efficient photoactive materials used in non-fullerene solar cells. We further review progress in the use of more eco-friendly solvents (i.e., water or alcohol) for achieving truly sustainable and eco-friendly PSC fabrication. For example, the concept of water- or alcohol-dispersed nanoparticles made of conjugated materials is introduced. Also, recent important progress and strategies to develop water/alcohol-soluble photoactive materials that completely eliminate the use of conventional toxic solvents are discussed. Finally, we provide our perspectives on the challenges facing the current green processing methods and materials, such as large-area coating techniques and long-term stability. We believe this Review will inform the development of PSCs that are truly clean and renewable energy sources.

Orthogonal Printable Reduced Graphene Oxide 2D Materials as Hole Transport Layers for High-Performance Inverted Polymer Solar Cells: Sheet Size Effect on Photovoltaic Properties

Creating an orthogonal printable hole-transporting layer (HTL) without damaging the underlying layer is still a major challenge in fabricating large-area printed inverted polymer solar cells (PSCs). In this study, we prepared orthogonal-processable fluorine-functionalized reduced graphene oxide (FrGO) series with various two-dimensional sheet sizes such as large-sized FrGO (1.1 μm), medium-sized FrGO (0.7 μm), and small-sized FrGO (0.3 μm) and systematically investigated the size effect of FrGOs on the hole transport properties of PSCs. The FrGOs exhibit highly stable dispersion without change over 90 days in 2-propanol solvent, indicating very high dispersion stability. Decreasing the sheet size of FrGOs enhanced hole-transporting properties, resulting in power conversion efficiencies (PCEs) of 9.27 and 9.02% for PTB7-Th:EH-IDTBR- and PTB7-Th:PC₇₁BM-based PSCs, respectively. Compared to devices with solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a 14% enhancement of PCEs was achieved. Interestingly, the PCEs of devices with the smallest FrGO sheet are higher than the PCE of 8.77% of a device with vacuum-deposited MoO₃. The enhancement in the performance of PSCs is attributed to the enhanced charge collection efficiency, decreased leakage current, internal resistance, and minimized charge recombination. Finally, small-sized FrGO HTLs were successfully coated on the photoactive layer using the spray coating method, and they also exhibited PCEs of 9.22 and 13.26% for PTB7-Th:EH-IDTBR- and PM6:Y6-based inverted PSCs, respectively.

Efficient charge generation at low energy losses in organic solar cells: a key issues review

Light absorption generates strongly bound excitons in organic solar cells (OSCs). To obtain efficient charge generation, a large driving force is required, which causes a large energy loss (E _loss) and severely hinders the improvement in the power conversion efficiencies (PCEs) of OSCs. Recently, the development of non-fullerene OSCs has seen great success, and the resulting OSCs can yield highly efficient charge generation with a negligible driving force, which raises a fundamental question about how the excitons split into free charges. From a chemical structure perspective, the molecular electrostatic potential differences between donors and acceptors may play a critical role in facilitating charge separation. Although the E _loss caused by charge generation has been suppressed, charge recombination, particularly via non-radiative pathways, severely limits further improvements in the PCEs. In OSCs with negligible driving forces, the lowest excited state, a hybrid local exciton-charge transfer state, is believed to have a strong association with the non-radiative E _loss. This review discusses the efficient charge generation at low E _loss values in highly efficient OSCs and highlights the issues that should be tackled to further improve the PCEs to new levels (∼20%).

Ultrafast Hole Transfer and Carrier Transport Controlled by Nanoscale-Phase Morphology in Nonfullerene Organic Solar Cells

Nonfullerene acceptors (NFAs) have attracted great attention in high-efficiency organic solar cells (OSCs). While the effect of molecular properties including structures and energetics on charge transfer has been extensively investigated, the effect of macroscopic-phase properties is yet to be revealed. Here we have performed a correlation study of the nanoscale-phase morphology on the photoexcited hole transfer (HT) process and photovoltaic performance by combining ultrafast spectroscopy with high temporal resolution and photo-induced force microscopy (PiFM) with high spatial and chemical resolution. In PM6/IT-4F, we observe biphasic HT behavior with a minor ultrafast (<100 fs) interfacial process and a major diffusion-mediated HT process until ∼100 ps, which depends strongly on phase segregation. Because of the interplay between charge transfer and transport, a compromised domain size of 20-30 nm for NFAs shows the best performance. This study highlights the critical role of phase morphology in high-efficiency OSCs.

Spiroconjugated Donor-σ-Acceptor Charge-Transfer Dyes: Effect of the π-Subsystems on the Optoelectronic Properties

Charge-transfer-based materials with intramolecular donor-acceptor structures are attractive for technological applications. Herein, a series of donor-σ-acceptor dyes has been prepared in a modular approach. The design of these intramolecular charge-transfer dyes is based on the concept of spiroconjugation, which leads to unique materials with special optical properties. The optical transitions are based on intramolecular charge transfer, as shown by solvatochromic measurements and density functional theory (DFT) calculations. Crystallographic, computational, electrochemical, and optical studies were performed to clarify the effect of different perpendicular π-moieties on the optoelectronic properties. Our molecular tuning allowed for the synthesis of molecules exhibiting strong visible-range absorption. The compounds are not fluorescent due to structural changes in the excited state, as revealed by DFT calculations. Finally, our study describes enantiomerically pure spiroconjugated absorber molecules using 1,1'-binaphthyl-2,2'-diol (BINOL) units on the donor part.

Color-Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

Future lightweight, flexible, and wearable electronics will employ visible-light-communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >10² mA W^-1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals.

Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.

It has been commonly believed that the driving force at the donor-acceptor heterojunction is vital to efficient charge separation in organic solar cells. Here Zhong et al. show that the driving force can be as small as 0.05 eV without compromising the charge transfer rate and efficiency.

Recent progress in wide bandgap conjugated polymer donors for high-performance nonfullerene organic photovoltaics

This feature article summarizes our recent achievements in the development of wide bandgap polymer donors as high-performance organic photovoltaics.

Insights into ultrafast charge-pair dynamics in P3HT:PCBM devices under the influence of static electric fields

Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor–acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor–acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices.

Electric field effects in P3HT:PCBM solar cell result in polaron-pair-like secondary photoexcitation species showing slower and bimolecular decay characteristics.

Star-Shaped Non-Fullerene Small Acceptors for Organic Solar Cells

Over the past decade, organic solar cells (OSCs) have received considerable attention from the scientific community and are considered one of the most important sources of low-cost electricity production. Recently, OSC-based on star-shaped small-molecule (SM) non-fullerene acceptors (NFAs) have developed rapidly, and the highest power conversion efficiency (PCE) has exceeded 10 %. The star-shaped SM NFAs not only have three-dimensional charge-transport characteristics similar to fullerenes but also have a strong light absorption capacities and easily tunable energy levels. They are potential candidates as outstanding acceptor materials. In this Review, research progress in of star-shaped SM NFAs OSCs is reviewed specifically. Moreover, the influence of molecular structure, central unit, and peripheral linking group on OSC performance has been evaluated systematically. This Review could stimulate inspiration for designing high-performance OSC acceptor materials in the future.

Two Well-Compatible Acceptors with Efficient Energy Transfer Enable Ternary Organic Photovoltaics Exhibiting a 13.36% Efficiency

Organic photovoltaics (OPVs) are fabricated with PM6 as donor and T6Me, IT-2F, or their mixture as acceptor. A 13.36% power conversion efficiency (PCE) is achieved from the optimized ternary OPVs with 50 wt% IT-2F in acceptors, which is attributed to the enhanced photon harvesting of ternary active layers and improved exciton utilization efficiency through energy transfer from IT-2F to T6Me. The efficient energy transfer from IT-2F to T6Me can be confirmed from the photoluminescence spectra of neat and blend films, which may provide additional channels to enhance exciton utilization efficiency for achieving short-circuit current density (J_SC ) improvement of ternary OPVs. It should be highlighted that the fill factor (FF) of ternary OPVs can be monotonously increased along with the incorporation of IT-2F, indicating the gradually optimized phase separation degree of ternary active layers. The third component IT-2F plays a key role in optimizing phase separation as a morphology regulator. Over 8% PCE improvement is achieved in the optimized ternary OPVs compared with the over 12% PCEs of the corresponding binary OPVs, respectively. This work indicates that the performance of ternary OPVs can be well optimized by carefully picking materials with good compatibility and complementary absorption spectra, as well as the appropriate energy levels.

Efficient and thermally stable organic solar cells based on small molecule donor and polymer acceptor

Efficient organic solar cells (OSCs) often use combination of polymer donor and small molecule acceptor. Herein we demonstrate efficient and thermally stable OSCs with combination of small molecule donor and polymer acceptor, which is expected to expand the research field of OSCs. Typical small molecule donors show strong intermolecular interactions and high crystallinity, and consequently do not match polymer acceptors because of large-size phase separation. We develop a small molecule donor with suppressed π-π stacking between molecular backbones by introducing large steric hindrance. As the result, the OSC exhibits small-size phase separation in the active layer and shows a power conversion efficiency of 8.0%. Moreover, this OSC exhibits much improved thermal stability, i.e. maintaining 89% of its initial efficiency after thermal annealing the active layer at 180 °C for 7 days. These results indicate a different kind of efficient and stable OSCs.

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.

Introducing Fluorine and Sulfur Atoms into Quinoxaline-Based p-type Polymers To Gradually Improve the Performance of Fullerene-Free Organic Solar Cells

Three quinoxaline-based "D-π-A" conjugated polymers, named as PE61, PE62, and PE63, are utilized to investigate the effect of introducing fluorine and sulfur atoms into the thiophene side chains on the photovoltaic performance when paired with a nonfullerene Y6. The open-circuit voltage (V_OC) and power conversion efficiency (PCE) can be improved from 0.66 V and 8.61% for PE61:Y6 to 0.78 V and 12.02% for PE62:Y6, and then to 0.83 V and 13.10% for PE63:Y6, respectively. The results provide a simple and effective strategy to fine-tune the optoelectronic properties and thus improve the photovoltaic performance.

Local Excitation/Charge-Transfer Hybridization Simultaneously Promotes Charge Generation and Reduces Nonradiative Voltage Loss in Nonfullerene Organic Solar Cells

High power conversion efficiencies in state-of-the-art nonfullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low nonradiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to the fact that the CT states can borrow considerable oscillator strengths from the energy-close local excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor-acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce nonradiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures to further improve the NF OSC performance.

Improved Charge Transport and Reduced Non-Geminate Recombination in Organic Solar Cells by Adding Size-Selected Graphene Oxide Nanosheets

Size-selected graphene oxide (GO) nanosheets were used to modify the bulk heterojunction (BHJ) morphology and electrical properties of organic photovoltaic (OPV) devices. The GO nanosheets were prepared with sizes ranging from several hundreds of nanometers to micrometers by using a physical sonication process and were then incorporated into PTB7:PC₇₁BM photoactive layers. Different GO sizes provide varied portions of the basal plane where aromatic sp²-hybridized regions are dominant and edges where oxygenated functional groups are located; thus, GO size distributions affect the GO dispersion stability and morphological aggregation of the BHJ layer. Electron delocalization by sp²-hybridization and the electron-withdrawing characteristics of functional groups p-dope the photoactive layer, giving rise to increasing carrier mobilities. Hole and electron mobilities are maximized at GO sizes of several hundreds of nanometers. Consequently, non-geminate recombination is significantly reduced by these facilitated hole and electron transports. The addition of GO nanosheets decreases the recombination order of non-geminate recombination and increases the generated carrier density. This reduction in the non-geminate recombination contributes to an increased power conversion efficiency of PTB7:PC₇₁BM OPV devices as high as 9.21%, particularly, by increasing the fill factor to 70.5% in normal devices and 69.4% in inverted devices.

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.