High Performance All-Polymer Solar Cells by Synergistic Effects of Fine-Tuned Crystallinity and Solvent Annealing

Fine Tuning the Glass Transition Temperature and Crystallinity by Varying the Thiophene-Quinoxaline Copolymer Composition

In recent years, the design and synthesis of high-performing conjugated materials for the application in organic photovoltaics (OPVs) have achieved lab-scale devices with high power conversion efficiency. However, most of the high-performing materials are still synthesised using complex multistep procedures, resulting in high cost. For the upscaling of OPVs, it is also important to focus on conjugated polymers that can be made via fewer simple synthetic steps. Therefore, an easily synthesised amorphous thiophene-quinoxaline donor polymer, TQ1, has attracted our attention. An analogue, TQ-EH that has the same polymer backbone as TQ1 but with short branched side-chains, was previously reported as a donor polymer with increased crystallinity. We have synthesised copolymers with varied ratios between octyloxy and branched (2-ethylhexyl)oxy-substituted quinoxaline units having the same polymer backbone, with the aim to control the aggregation/crystallisation behaviour of the resulting copolymers. The optical properties, glass transition temperatures and degree of crystallinity of the new copolymers were systematically examined in relation to their copolymer composition, revealing that the composition can be used to fine-tune these properties of conjugated polymers. In addition, multiple sub-Tg transitions were found from some of the polymers, which are not commonly or clearly seen in other conjugated polymers. The new copolymers were tested in photovoltaic devices with a fullerene derivative as the acceptor, achieving slightly higher performances compared to the homopolymers. This work demonstrates that side-chain modification by copolymerisation can fine-tune the properties of conjugated polymers without requiring complex organic synthesis, thereby expanding the number of easily synthesised polymers for future upscaling of OPVs.

The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push-pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT-GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes.

Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells

All-polymer solar cells (all-PSCs) offer improved morphological and mechanical stability compared with those containing small-molecule-acceptors (SMAs). They can be processed with a broader range of conditions, making them desirable for printing techniques. In this study, we report a high-performance polymer acceptor design based on bithiazole linker (PY-BTz) that are on par with SMAs. We demonstrate that bithiazole induces a more coplanar and ordered conformation compared to bithiophene due to the synergistic effect of non-covalent backbone planarization and reduced steric encumbrances. As a result, PY-BTz shows a significantly higher efficiency of 16.4% in comparison to the polymer acceptors based on commonly used thiophene-based linkers (i.e., PY-2T, 9.8%). Detailed analyses reveal that this improvement is associated with enhanced conjugation along the backbone and closer interchain π-stacking, resulting in higher charge mobilities, suppressed charge recombination, and reduced energetic disorder. Remarkably, an efficiency of 14.7% is realized for all-PSCs that are solution-sheared in ambient conditions, which is among the highest for devices prepared under conditions relevant to scalable printing techniques. This work uncovers a strategy for promoting backbone conjugation and planarization in emerging polymer acceptors that can lead to superior all-PSCs.

Energy level measurement for organic semiconductors

Along with the miniaturization and versatility of organic optoelectronic devices, it is desired to achieve a profound comprehension of the charge transport mechanism and even the basic device physics. The basis of these studies is the acquisition of relevant information about energy levels. This review provides a comprehensive analysis of five commonly-used techniques, including cyclic voltammetry, ultraviolet electron spectroscopy, inverse photoemission electron spectroscopy, low energy inverse photoemission spectroscopy and hot electron spectroscopy. According to the advantages and disadvantages, working mechanism, and application conditions, researchers will screen out a reliable and suitable characterization method, quickly and accurately. This should be beneficial for the efficient promotion of organic electronics and save valuable time for the related research studies.

Mitigating the Trade-Off between Non-Radiative Recombination and Charge Transport to Enable Efficient Ternary Organic Solar Cells

Ternary organic solar cells (OSCs) have attracted intensive studies due to their promising potential for attaining high-performing photovoltaics, whereas there has been an opening challenge in minimizing the open circuit voltage (Voc) loss while retaining the optimal carrier extraction in the multiple mixture absorbers. Here, we systemically investigate a ternary absorber comprised of two acceptors and a donor, in which the resultant Voc and fill factor are varied and determined by the ratios of acceptor components as a result of the unbalance of non-radiative recombination rates and charge transport. The transient absorption spectroscopy and electroluminescence techniques verify two distinguishable charge-transfer (CT) states in the ternary absorber, and the mismatch of non-radiative recombination rates of those two CT states is demonstrated to be associated with the Voc deficit, whilst the high-emissive acceptor molecule delivers inferior electron mobility, resulting in poor charge transport and a subpar fill factor. These findings enable us to optimize the mixture configuration for attaining the maximal-performing devices. Our results not only provide insight into maximizing the photovoltage of organic solar cells but can also motivate researchers to further unravel the photophysical mechanisms underlying the intermolecular electronic states of organic semiconductors.

Star-shape non-fullerene acceptor featuring an aza-triangulene core for organic solar cells

We present the simple synthesis of a star-shape non-fullerene acceptor (NFA) for application in organic solar cells. This NFA possesses a D(A)3 structure in which the electron-donating core is an aza-triangulene unit and we report the first crystal structure for a star shape NFA based on this motive. We fully characterized this molecule's optoelectronic properties in solution and thin films, investigating its photovoltaic properties when blended with PTB7-Th as the electron donor component. We demonstrate that the aza-triangulene core leads to a strong absorption in the visible range with an absorption edge going from 700 nm in solution to above 850 nm in the solid state. The transport properties of the pristine molecule were investigated in field effect transistors (OFETs) and in blends with PTB7-Th following a Space-Charge-Limited Current (SCLC) protocol. We found that the mobility of electrons measured in films deposited from o-xylene and chlorobenzene are quite similar (up to 2.70 × 10-4 cm2 V-1 s-1) and that the values are not significantly modified by thermal annealing. The new NFA combined with PTB7-Th in the active layer of inverted solar cells leads to a power conversion efficiency of around 6.3% (active area 0.16 cm2) when processed from non-chlorinated solvents without thermal annealing. Thanks to impedance spectroscopy measurements performed on the solar cells, we show that the charge collection efficiency of the devices is limited by the transport properties rather than by recombination kinetics. Finally, we investigated the stability of this new NFA in various conditions and show that the star-shape molecule is more resistant against photolysis in the presence and absence of oxygen than ITIC.

Manipulating Polymer Backbone Configuration via Halogenated Asymmetric End-Groups Enables Over 18% Efficiency All-Polymer Solar Cells

High-performance all-polymer solar cells (all-PSCs) deeply rely on the joint contributions of desirable optical absorption, adaptive energy levels, and appropriate morphology. Herein, two structural analogous polymerized small-molecule acceptors (PSMAs), PYFCl-T and PYF&PYCl-T, are synthesized, and then incorporated into the PM6:PY-IT binary blends to construct ternary all-PSCs. Due to the superior compatibility of PY-IT and PYFCl-T, the ternary all-PSC based on PM6:PY-IT:PYFCl-T with 10 wt% PYFCl-T, presents higher and more balanced charge mobility, suppressed charge recombination, and faster charge-transfer kinetics, resulting in an outstanding power conversion efficiency (PCE) of 18.12% with enhanced J_sc and FF, which is much higher than that (PCE of 16.09%) of the binary all-PSCs based on PM6:PY-IT. Besides, the ternary all-PSCs also exhibit improved photostability. The conspicuous performance enhancement principally should give the credit to the miscibility-driven phase optimization of the donor and acceptor. These findings highlight the significance of polymer-backbone configuration modulation of PSMAs in morphology optimization toward boosting the device properties of all-PSCs.

Molecular symmetry effect on the morphology and self-aggregation of semiconducting polymers

We investigate molecular symmetricity on the physical and photoelectric properties of semiconducting polymers by selectively incorporating different numbers of fluorine atoms at different positions in the donor unit of NDI-based copolymers.

Achieving efficient and stabilized organic solar cells by precisely controlling the proportion of copolymerized units in electron-rich polymers

A series of random polymers based on the donor polymer PM6 were designed from the perspective of regulating the surface electrostatic potential (ESP) distribution of the polymers and applied in organic solar cells (OSCs). Random polymers with different ESPs were obtained by introducing structural units of polymer PM6 into the polymer structure as the third unit. The simulation results showed that the random polymers feature a wider electron-donating region after the introduction of BDT units, indicating a more efficient charge generation probability. Benefiting from the optimized morphology of the active layer and the stronger interaction between the donor and the acceptor in the active layer, the device exhibited the best charge transport efficiency and lower charge recombination after the introduction of 5% BDT units, and a high power conversion efficiency (PCE) of 16.76% was achieved. In addition, OSC devices based on random polymers incorporating 5% BDT units exhibit excellent device stability. In contrast, the devices based on random polymers after the introduction of BDD units showed a much lower PCE of around 13% due to the inferior charge generation and charge transport. This work not only provides a new perspective for the molecular design of efficient random polymers, but also demonstrates that the OSC devices based on random polymers can still achieve better stability.

Modification of the Surface Composition of PTB7-Th: ITIC Blend Using an Additive

We investigated the effect of adding p-anisaldehyde (AA) solvent to the ink containing poly[[2,60-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7-Th) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene(ITIC) on the morphology of the active layer. The present study focuses on determining the effect of the additive on the compositions at the surface of the PTB7-Th: ITIC composite and its morphology, forming one side of the interface of the blend with the MoO_X electrode, and the influence of the structural change on the performance of devices. Studies of device performance show that the addition of the additive AA leads to an improvement in device performance. Upon the addition of AA, the concentration of PTB7-Th at the surface of the bulk heterojunction (BHJ) increases, causing an increase in surface roughness of the surface of the BHJ. This finding contributes to an understanding of the interaction between the donor material and high work function electrode/interface material. The implications for the interface are discussed.

Naphthalene diimide-based random terpolymer acceptors for constructing all-polymer solar cells with enhanced fill factors

All-polymer solar cells (all-PSCs) with mechanical and thermal stability have potential for applications in flexible devices. Polymer acceptors based on naphthalene diimide (NDI) have been widely studied because of their strong electron affinity, high electron mobility, and high mechanical reliability. However, controlling the film morphology of the polymer–polymer blends of NDI-based all-PSCs is difficult. Consequently, all-PSCs based on NDI building blocks exhibit a low fill factor (FF) and a lower power-conversion efficiency (PCE) than state-of-the-art polymer solar cells. In this work, we added a small amount of dicyanodistyrylbenzene (DCB) unit to the NDI-based polymer acceptor N2200 through random copolymerization and synthesized a series of NDI-based terpolymer acceptors PNDIx, where x is the molar concentration of DCB units relative to NDI units. PNDI5 and PNDI10, corresponding to 5% and 10% molar concentrations of DCB, respectively, showed lower crystallization and good miscibility with PBDB-T, a widely used electron-donating copolymer, than the terpolymer based on DCB-free N2200. Moreover, compared to the PBDB-T:N2200 device, the PNDI5-based device exhibited a much higher PCE (8.01%), and an enhanced FF of 0.75 in all-PSCs. These results indicate that ternary random copolymerization is a convenient and effective strategy for optimizing the film morphology of NDI-based polymers, and that the resulting terpolymer acceptor is a promising n-type acceptor for constructing high-performance all-PSCs.

We developed two terpolymer acceptors based on N2200 through random copolymerization for all polymer solar cells with enhanced fill factors and photovoltaic performance.

High-Performance, Flexible NO2 Chemiresistors Achieved by Design of Imine-Incorporated n-Type Conjugated Polymers

Flexible and mechanically robust gas sensors are the key technologies for wearable and implantable electronics. Herein, the authors demonstrate the high-performance, flexible nitrogen dioxide (NO2 ) chemiresistors using a series of n-type conjugated polymers (CPs: PNDIT2/IM-x) and a polymer dopant (poly(ethyleneimine), PEI). Imine double bonds (C = N) are incorporated into the backbones of the CPs with different imine contents (x) to facilitate strong and selective interactions with NO2 . The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO2 sensors with PNDIT2/IM-0.1 and PEI (1:1 by weight ratio) exhibit outstanding sensing performances, such as excellent sensitivity (ΔR/Rb = 240% @ 1 ppm), ultralow detection limit (0.1 ppm), high selectivity (ΔR/Rb < 8% @ 1 ppm of interfering analytes), and high stability, thereby outperforming other state-of-the-art CP-based chemiresistors. Furthermore, the thin film of PNDIT2/IM-0.1 and PEI blend is stretchable and mechanically robust, providing excellent flexibility to the NO2 sensors. Our study contributes to the rational design of high-performance flexible gas sensors.

Domain size control in all-polymer solar cells

In all polymer solar cells (all-PSCs), the domain size is critical for device performance. In highly crystalline polymer blends, however, precisely adjusting the domain size remains a significant challenge because of the simultaneous crystallization of both components. Herein, a strategy that promotes acceptor and donor to crystallize separately was proposed. Take PBDB-T/N2200 blends for instance; the solution state and confined crystallization were combined, which induced the crystallization of N2200, and PBDB-T occurred during the film-forming process and at thermal annealing stage. This separated crystallization process lowers the driving force of phase separation without affecting the degree of crystallinity of the blends. Thus, an interpenetrating network with high crystallinity and proper domain size was obtained, which boosted the power conversion efficiency to 7.59%. Importantly, the relation between pre-aggregation and domain size was established, which may be a guide to precisely adjust the active layer’s domain size in all-PSCs.

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This strategy decreases domain size without sacrificing crystallinity

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A phase diagram about solution state and domain size was proposed

Polymerized Small Molecular Acceptor with Branched Side Chains for All Polymer Solar Cells with Efficiency over 16.7

The power conversion efficiencies (PCEs) of small molecule acceptor (SMA)-based organic solar cells have already exceeded 18%. However, the development of polymer acceptors still lags far behind their SMA counterparts mainly due to the lack of efficient polymer acceptors. Herein, a series of polymer acceptors named PY-X (with X being the branched alkyl chain) are designed and synthesized by employing the same central core with the SMA L8-BO but with different branched alkyl chains on the pyrrole motif. It is found that the molecular packing of SMA-HD featuring 2-hexyldecyl side chain used in the synthesis of PY-HD is similar to L8-BO, in which the branched alkyl chains lead to condensed and high-order molecular assembly in SMA-HD molecules. When combined with PM6, PY-HD-based all polymer solar cell (all-PSC) exhibits a high PCE of 16.41%, representing the highest efficiency for the binary all-PSCs. Moreover, the side-chain modification on the pyrrole site position further improves the performance of the all-PSCs, and the PY-DT-based device delivers a new record high efficiency of 16.76% (certified as 16.3%). The work provides new insights for understanding the structure-property relationship of polymer acceptors and paves a feasible avenue to develop efficient conjugated polymer acceptors.

Polymer Acceptors with Flexible Spacers Afford Efficient and Mechanically Robust All-Polymer Solar Cells

High efficiency and mechanical robustness are both crucial for the practical applications of all-polymer solar cells (all-PSCs) in stretchable and wearable electronics. In this regard, a series of new polymer acceptors (P_A s) is reported by incorporating a flexible conjugation-break spacer (FCBS) to achieve highly efficient and mechanically robust all-PSCs. Incorporation of FCBS affords the effective modulation of the crystallinity and pre-aggregation of the P_A s, and achieves the optimal blend morphology with polymer donor (P_D ), increasing both the photovoltaic and mechanical properties of all-PSCs. In particular, an all-PSC based on PYTS-0.3 P_A incorporated with 30% FCBS and PBDB-T P_D demonstrates a high power conversion efficiency (PCE) of 14.68% and excellent mechanical stretchability with a crack onset strain (COS) of 21.64% and toughness of 3.86 MJ m^-3 , which is significantly superior to those of devices with the P_A without the FCBS (PYTS-0.0, PCE = 13.01%, and toughness = 2.70 MJ m^-3 ). To date, this COS is the highest value reported for PSCs with PCEs of over 8% without any insulating additives. These results reveal that the introduction of FCBS into the conjugated backbone is a highly feasible strategy to simultaneously improve the PCE and stretchability of PSCs.

Optimizing Device Structure of PTB7-Th:PNDI-T10 Bulk Heterojunction Polymer Solar Cells by Enhancing Optical Absorption

Using the optical transfer matrix method, we optimized the layered structure of a conventional and an inverted BHJ OSC with the active layer made of blended PTB7-Th:PNDI-T10 by maximizing the optical absorption and, hence, the JSC. The maximum JSC thus obtained from the optimised structure of the inverted OSC was 139 Am−2 and that of the conventional OSC was 135 Am−2. Simulation of the electric field distribution in both inverted and conventional OSCs showed that the formation of a single CIP was obtained in the active layer of thickness 105 nm in both OSCs. As the light incidents from the ITO side, it was found that excitons were generated more closely to ITO electrode, which favors the efficient charge transport and collection at the opposite electrodes in the inverted OSC, which produces higher JSC.

Enhancement of All-Polymer Solar Cells by Addition of a Chlorinated Polymer and Formation of an Energy Cascade in a Nonhalogenated Solvent

Ternary organic solar cells (OSCs) containing a three-component photoactive layer with cascading energy alignments could benefit the charge transfer and improve the open-circuit voltage and power conversion efficiency. Herein, we report the incorporation of a derived chlorinated polymer, J52-Cl, as a guest donor into the donor J52 and acceptor N2200 blend film. The lowest unoccupied molecular orbital and the highest occupied molecular orbital levels of J52-Cl are between the corresponding energy levels of J52 and N2200, and this leads to generation of a cascade of energy levels. Photoluminescence measurements and the J-V of devices containing the donors indicated that this incorporation of J52-Cl could promote the charge transfer of the solar cells. The contribution from J52-Cl reduced the energy loss of J52-based binary devices significantly from 0.932 to 0.797 eV and the nonradiative energy loss from 0.399 to 0.269 eV, leading to an enhancement of V_oc from 0.79 to 0.93 V. This introduction of chlorinated polymers also improves the intermolecular interactions and leads to a favorable morphology with appropriate phase separation and interpenetrating networks. As expected, the power conversion efficiency of ternary all-polymer solar cells (all-PSCs) processed in o-xylene solvent was increased from 8.55 to 11.02%. These results indicate that the ternary devices with the appropriate cascade of energy levels can fine tune the device's open-circuit voltage and finally improve the photovoltaic performance of OSCs.

Cyano-Functionalized Quinoxaline-Based Polymer Acceptors for All-Polymer Solar Cells and Organic Transistors

Quinoxaline (Qx) derivatives are promising building units for efficient photovoltaic polymers owing to their strong light absorption and high charge-transport abilities, but they have been used exclusively in the construction of polymer donors. Herein, for the first time, Qx-based polymer acceptors (P_A s) were developed by introducing electron-withdrawing cyano (CN) groups into the Qx moiety (QxCN). A series of QxCN-based P_A s, P(QxCN-T2), P(QxCN-TVT), and P(QxCN-T3), were synthesized by copolymerizing the QxCN unit with bithiophene, (E)-1,2-di(thiophene-2-yl)ethene, and terthiophene, respectively. All of the P_A s exhibited unipolar n-type characteristics with organic field-effect transistor (OFET) mobilities of around 10^-2  cm²  V^-1  s^-1 . In space-charge-limited current devices, P(QxCN-T2) and P(QxCN-TVT) exhibited electron mobilities greater than 1.0×10^-4  cm²  V^-1  s^-1 , due to the well-ordered structure with tight π-π stacking. When the P_A s were applied in all-polymer solar cells (all-PSCs), the highest performance of 5.32 % was achieved in the P(QxCN-T2)-based device. These results demonstrate the significant potential of Qx-based P_A s for high-performance all-PSCs and OFETs.

Narrow-Bandgap Single-Component Polymer Solar Cells with Approaching 9% Efficiency

Two narrow-bandgap block conjugated polymers with a (D1-A1)-(D2-A2) backbone architecture, namely PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6, are designed and synthesized for single-component organic solar cells (SCOSCs). Both polymers contain same donor polymer, PBDB-T, but different polymerized nonfullerene molecule acceptors. Compared to all previously reported materials for SCOSCs, PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 exhibit narrower bandgap for better light harvesting. When incorporated into SCOSCs, the short-circuit current density (J_sc ) is significantly improved to over 15 mA cm^-2 , together with a record-high power conversion efficiency (PCE) of 8.64%. Moreover, these block copolymers exhibit low energy loss due to high charge transfer (CT) states (E_ct ) plus small non-radiative loss (0.26 eV), and improved stability under both ambient condition and continuous 80 °C thermal stresses for over 1000 h. Determination of the charge carrier dynamics and film morphology in these SCOSCs reveals increased carrier recombination, relative to binary bulk-heterojunction devices, which is mainly due to reduced ordering of both donor and acceptor fragments. The close structural relationship between block polymers and their binary counterparts also provides an excellent framework to explore further molecular features that impact the photovoltaic performance and boost the state-of-the-art efficiency of SCOSCs.

Naphthalene diimides: perspectives and promise

In this review, we describe the developments in the field of naphthalene diimides (NDIs) from 2016 to the presentday. NDIs are shown to be an increasingly interesting class of molecules due to their electronic properties, large electron deficient aromatic cores and tendency to self-assemble into functional structures. Almost all NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. NDIs have also been extensively studied due to their potential real-world uses across a wide variety of applications including supramolecular chemistry, sensing, host-guest complexes for molecular switching devices, such as catenanes and rotaxanes, ion-channels, catalysis, and medicine and as non-fullerene accepters in solar cells. In recent years, NDI research with respect to supramolecular assemblies and mechanoluminescent properties has also gained considerable traction. Thus, this review will assist a wide range of readers and researchers including chemists, physicists, biologists, medicinal chemists and materials scientists in understanding the scope for development and applicability of NDI dyes in their respective fields through a discussion of the main properties of NDI derivatives and of the status of emerging applications.

Materials Design and Optimization for Next-Generation Solar Cell and Light-Emitting Technologies

We review some of the most potent directions in the design of materials for next-generation solar cell and light-emitting technologies that go beyond traditional solid-state inorganic semiconductor-based devices, from both the experimental and computational standpoints. We focus on selected recent conceptual advances in tackling issues which are expected to significantly impact applied literature in the coming years. Specifically, we consider solution processability, design of dopant-free charge transport materials, two-dimensional conjugated polymeric semiconductors, and colloidal quantum dot assemblies in the fields of experimental synthesis, characterization, and device fabrication. Key modeling issues that we consider are calculations of optical properties and of effects of aggregation, including recent advances in methods beyond linear-response time-dependent density functional theory and recent insights into the effects of correlation when going beyond the single-particle ansatz as well as in the context of modeling of thermally activated fluorescence.

To Reveal the Importance of the Crystallization Sequence on Micro-Morphological Structures of All-Crystalline Polymer Blends by In Situ Investigation

In crystalline/crystalline polymer blend systems, complex competition and coupling of crystallization and morphology usually happen due to the different crystal nucleation and growth processes of polymers, making the morphology and crystallization behavior difficult to control. Herein, we probe the crystallization sequence during the film formation process (crystallize simultaneously, component A crystallizes prior to B or inverse) to illustrate the micro-morphology evolution process in poly(3-hexylthiophene) (P3HT) and poly[[N,N-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5, 5'-(2,2'-bithiophene)] (N2200) blend using in situ UV-vis absorption spectra and in situ two-dimensional grazing incidence X-ray diffraction (2D GIXRD). When P3HT and N2200 crystallize simultaneously, a large-sized morphology structure is formed. When strengthening the solution aggregation of P3HT by increasing the solvent-polymer interaction, P3HT crystallizes prior to N2200. A P3HT-based micro-morphology structure is obtained. As the molecular weight of N2200 increases to a critical value (72.0 kDa), the crystallization of N2200 dominates the film formation process. A N2200-based micro-morphology is formed guided by N2200 domains. The results confirm that the crystallization sequence is one of the most important factors to determine the micro-morphology structure in all-crystalline polymer blends.

Novel High-Efficiency Polymer Acceptors via Random Ternary Copolymerization Engineering Enables All-Polymer Solar Cells with Excellent Performance and Stability

Continuous breakthroughs have been achieved in improving the efficiency of all-polymer solar cells (all-PSCs) using diimide-based polymer acceptors, and their easy-to-synthesize, low-cost, and high stability attributes make them potential candidates for use in commercial all-PSCs. However, their low light absorption coefficient, strong aggregation, and poor adaptability with high-efficient polymer donors still limit further improvements in the device performance. Here, we combine the advantages of fluorinated bithiophene and rhodanine dye molecules to create low-cost diimide-based polymer acceptors, PNDI-2FT-TR10 and PNDI-2FT-TR20, by random copolymerization for achieving highly efficient and stable all-PSCs. The synergistic effects of fluorine atoms and rhodanine dye molecules not only significantly improve the absorption coefficient but also enable enhanced miscibility and stability of the blend film. When blended with a PM6 donor, the PNDI-2FT-TR10-based device exhibits a notable power conversion efficiency (PCE) of 10.71% with a short-circuit current (J_SC) of 17.32 mA cm^-2. Note that both the PCE and J_SC show outstanding values for diimide-based all-PSCs, and this is the first report on blending diimide-based polymer acceptors with the PM6 donor to achieve high-performance all-PSCs. Moreover, the favorable morphology of the active layer enables the device to have good thickness tolerance and thermal stability. The results demonstrate that the absorption coefficients, blend morphology, and photovoltaic properties of all-PSCs could be rationally optimized by a random copolymer.

Out-of-Plane Alignment of Conjugated Semiconducting Polymers by Horizontal Rotation in a High Magnetic Field

The effective control of film morphology and molecular packing in the out-of-plane direction of semiconductor polymers plays a critical role in governing charge carrier transport in the direction perpendicular to the substrate. In this study, a highly out-of-plane alignment of the n-type polymer P(NDI2OD-T2) film has been successfully achieved by horizontal rotation in a high magnetic field (HR-HMF). The out-of-plane alignment of the P(NDI2OD-T2) film has showed a change from 72% face-on to 98.2% face-on lamellar texture as well as a 1.6-fold increase of the π-π stacking crystalline correlation length compared with that of as-cast polymer films without HR-HMF-induced alignment. Meanwhile, the film with near-perfect face-on molecular packing exhibited more than 18-fold enhancement of electron mobility compared to the unaligned film. The excellent electrical performance achieved with the HR-HMF process indicates its application potential for fabricating high-performance sandwich-type organic electronic devices, such as solar cells and light-emitting diodes.

Nonconjugated Terpolymer Acceptors with Two Different Fused-Ring Electron-Deficient Building Blocks for Efficient All-Polymer Solar Cells

The ternary polymerization strategy of incorporating different donor and acceptor units forming terpolymers as photovoltaic materials has been proven advantageous in improving power conversion efficiencies (PCEs) of polymer solar cells (PSCs). Herein, a series of low band gap nonconjugated terpolymer acceptors based on two different fused-ring electron-deficient building blocks (IDIC16 and ITIC) with adjustable photoelectric properties were developed. As the third component, ITIC building blocks with a larger π-conjugation structure, shorter solubilizing side chains, and red-shifted absorption spectrum were incorporated into an IDIC16-based nonconjugated copolymer acceptor PF1-TS4, which built up the terpolymers with two conjugated building blocks linked by flexible thioalkyl chain-thiophene segments. With the increasing ITIC content, terpolymers show gradually broadened absorption spectra and slightly down-shifted lowest unoccupied molecular orbital levels. The active layer based on terpolymer PF1-TS4-60 with a 60% ITIC unit presents more balanced hole and electron mobilities, higher photoluminescence quenching efficiency, and improved morphology compared to those based on PF1-TS4. In all-polymer solar cells (all-PSCs), PF1-TS4-60, matched with a wide band gap polymer donor PM6, achieved a similar open-circuit voltage (Voc) of 0.99 V, a dramatically increased short-circuit current density (Jsc) of 15.30 mA cm-2, and fill factor (FF) of 61.4% compared to PF1-TS4 (Voc = 0.99 V, Jsc = 11.21 mA cm-2, and FF = 55.6%). As a result, the PF1-TS4-60-based all-PSCs achieved a PCE of 9.31%, which is ∼50% higher than the PF1-TS4-based ones (6.17%). The results demonstrate a promising approach to develop high-performance nonconjugated terpolymer acceptors for efficient all-PSCs by means of ternary polymerization using two different A-D-A-structured fused-ring electron-deficient building blocks.

Bis-acenaphthoquinone diimides with high electron deficiency and good coplanar conformation

A series of novel bis-acenaphthoquinone diimides featuring a highly electron-deficient bis-acenaphthoquinone core are facilely synthesized via Knoevenagel condensation reaction. The diimides show high electron deficiency and good coplanar conformation, together with one of them having a maximum electron mobility up to 0.038 cm2 V-1 s-1.

Random Polymerization Strategy Leads to a Family of Donor Polymers Enabling Well-Controlled Morphology and Multiple Cases of High-Performance Organic Solar Cells

Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D-A1-D-A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs.

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.

The Crystallinity Control of Polymer Donor Materials for High-Performance Organic Solar Cells

Bulk heterojunction (BHJ) organic solar cells (OSCs) can be regarded as one of the most promising energy generation technologies for large-scale applications. Despite their several well-known drawbacks, the devices where polymers are employed as the donor are still leading the OSC universe in terms of performance. Such performance generally depends upon various critical factors such as the crystallinity of the material, the crystallization process during the film formation, and also the final film morphology. Despite a few reviews on the structure of the polymer donor materials and device performance, not enough attention has been paid toward the crystallinity problem. Herein, the structure and crystallinity of the representative polymer donor materials and the corresponding device properties have been briefly reviewed. Furthermore, several typical methods for controlling the crystallinity of materials have been summarized and illustrated as well. Moreover, the obstacles lying in the way of successful commercialization of such polymer solar cells have been systematically discussed. The in-depth interpretation of the crystallinity of the polymer donors in this article may stimulate novel ideas in material design and device fabrication.

Improving the all-polymer solar cell performance by adding a narrow bandgap polymer as the second donor

Ternary all-polymer solar cells are fabricated using an N2200 acceptor and two donor polymers (PF2 and PM2) with complementary absorption. The major donor PF2 is a relatively wide bandgap polymer that contributes the most photon absorption in the UV-vis region while the second donor PM2 improves the light harvesting due to its strong absorption in the near-IR region. By carefully tuning the ratio of two donor polymers, the best ratio of 9 : 1 : 5 (PF2 : PM2 : N2200) is achieved and shows a PCE of 6.90%, which is better than two binary devices. This work demonstrates an effective strategy of utilizing a narrow bandgap donor polymer as the second donor to improve the performance of all-polymer solar cells.

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Mixed-flow design for microfluidic printing of two-component polymer semiconductor systems

The rational creation of two-component conjugated polymer systems with high levels of phase purity in each component is challenging but crucial for realizing printed soft-matter electronics. Here, we report a mixed-flow microfluidic printing (MFMP) approach for two-component π-polymer systems that significantly elevates phase purity in bulk-heterojunction solar cells and thin-film transistors. MFMP integrates laminar and extensional flows using a specially microstructured shear blade, designed with fluid flow simulation tools to tune the flow patterns and induce shear, stretch, and pushout effects. This optimizes polymer conformation and semiconducting blend order as assessed by atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing incidence wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (R-SoXS), photovoltaic response, and field effect mobility. For printed all-polymer (poly[(5,6-difluoro-2-octyl-2H-benzotriazole-4,7-diyl)-2,5-thiophenediyl[4,8-bis[5-(2-hexyldecyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-2,5-thiophenediyl]) [J51]:(poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}) [N2200]) solar cells, this approach enhances short-circuit currents and fill factors, with power conversion efficiency increasing from 5.20% for conventional blade coating to 7.80% for MFMP. Moreover, the performance of mixed polymer ambipolar [poly(3-hexylthiophene-2,5-diyl) (P3HT):N2200] and semiconducting:insulating polymer unipolar (N2200:polystyrene) transistors is similarly enhanced, underscoring versatility for two-component π-polymer systems. Mixed-flow designs offer modalities for achieving high-performance organic optoelectronics via innovative printing methodologies.

High-Performance All-Polymer Solar Cells Enabled by n-Type Polymers with an Ultranarrow Bandgap Down to 1.28 eV

Compared to organic solar cells based on narrow-bandgap nonfullerene small-molecule acceptors, the performance of all-polymer solar cells (all-PSCs) lags much behind due to the lack of high-performance n-type polymers, which should have low-aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near-infrared region. Herein, two novel polymer acceptors, DCNBT-TPC and DCNBT-TPIC, are synthesized with ultranarrow bandgaps (ultra-NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm² V^-1 s^-1 obtained for DCNBT-TPC. Blended with a polymer donor, PBDTTT-E-T, the resultant all-PSCs based on DCNBT-TPC and DCNBT-TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short-circuit currents up to 19.44 and 22.52 mA cm^-2 , respectively. This is the first example that a PCE of over 10% can be achieved using ultra-NBG polymer acceptors with a photoresponse reaching 950 nm in all-PSCs. These results demonstrate that ultra-NBG polymer acceptors, in line with nonfullerene small-molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all-PSCs.

Introducing Porphyrin Units by Random Copolymerization Into NDI-Based Acceptor for All Polymer Solar Cells

Naphthalene diimide (NDI)-based polymer N2200 is a promising organic polymer acceptor for all-polymer solar cells (all-PSCs), but its inherent shortcomings like poor extinction coefficient and strong aggregation limit further performance optimization of all-PSCs. Here, a series of random copolymers, PNDI-Px, were designed and synthesized by introducing porphyrin unit into NDI-based polymer as acceptors for all-PSCs. These random copolymers show a higher absorption coefficient and raised the lowest unoccupied molecular orbital (LUMO) energy levels compared to N2200. The crystallinity can also be fine-tuned by regulation of the content of porphyrin unit. The random copolymers are matched with polymer donor PBDB-T for the application in all-polymer solar cells. The best power conversion efficiency (PCE) of these PNDI-Px-based devices is 5.93%, ascribed to the overall enhanced device parameters compared with the N2200-based device. These results indicate that introducing porphyrin unit into polymer is a useful way to fine-tune the photoelectric performance for efficient all-PSCs.

Ternary All-Polymer Solar Cells With 8.5% Power Conversion Efficiency and Excellent Thermal Stability

All-polymer solar cells (all-PSCs) composed of polymer donors and acceptors have attracted widespread attention in recent years. However, the broad and efficient photon utilization of polymer:polymer blend films remains challenging. In our previous work, we developed NOE10, a linear oligoethylene oxide (OE) side-chain modified naphthalene diimide (NDI)-based polymer acceptor which exhibited a power conversion efficiency (PCE) of 8.1% when blended with a wide-bandgap polymer donor PBDT-TAZ. Herein, we report a ternary all-PSC strategy of incorporating a state-of-the-art narrow bandgap polymer (PTB7-Th) into the PBDT-TAZ:NOE10 binary system, which enables 8.5% PCEs within a broad ternary polymer ratio. We further demonstrate that, compared to the binary system, the improved photovoltaic performance of ternary all-PSCs benefits from the combined effect of enhanced photon absorption, more efficient charge generation, and balanced charge transport. Meanwhile, similar to the binary system, the ternary all-PSC also shows excellent thermal stability, maintaining 98% initial PCE after aging for 300 h at 65°C. This work demonstrates that the introduction of a narrow-bandgap polymer as a third photoactive component into ternary all-PSCs is an effective strategy to realize highly efficient and stable all-PSCs.

Effects of a Fluorinated Donor Polymer on the Morphology, Photophysics, and Performance of All-Polymer Solar Cells Based on Naphthalene Diimide-Arylene Copolymer Acceptors

Naphthalene diimide (NDI)-biselenophene copolymer (PNDIBS), NDI-selenophene copolymer (PNDIS), and the fluorinated donor polymer PM6 were used to investigate how a fluorinated polymer component affects the morphology and performance of all-polymer solar cells (all-PSCs). Although the PM6:PNDIBS blend system exhibits a high open-circuit voltage (V_oc = 0.925 V) and a desired low optical bandgap energy loss (E_loss = 0.475 eV), the overall power conversion efficiency (PCE) was 3.1%. In contrast, PM6:PNDIS blends combine a high V_oc (0.967 V) with a high fill factor (FF = 0.70) to produce efficient all-PSCs with 9.1% PCE. Furthermore, the high-performance PM6:PNDIS all-PSCs could be fabricated by various solution processing approaches and at active layer thickness as high as 300 nm without compromising photovoltaic efficiency. The divergent photovoltaic properties of PNDIS and PNDIBS when paired respectively with PM6 are shown to originate from the starkly different blend morphologies and blend photophysics. Efficient PM6:PNDIS blend films were found to exhibit a vertical phase stratification along with lateral phase separation, while the molecular packing had a predominant face-on orientation. Bulk lateral phase separation with both face-on and edge-on molecular orientations featured in the poor-performing PM6:PNDIBS blend films. Enhanced charge photogeneration and suppressed geminate and bimolecular recombinations with 99% charge collection probability found in PM6:PNDIS blends strongly differ from the poor charge collection probability (66%) and high electron-hole pair recombination seen in PM6:PNDIBS. Our findings demonstrate that beyond the generally expected enhancement of V_oc, a fluorinated polymer component in all-PSCs can also exert a positive or negative influence on photovoltaic performance via the blend morphology and blend photophysics.

Optimizing the Phase-Separated Domain Size of the Active Layer via Sequential Crystallization in All-Polymer Solar Cells

The proper domain size of the active layer plays a key role in determining the exciton dissociation and charge transport in all-polymer solar cells (all-PSCs). However, fine-tuning the domain size remains challenging due to low glass transition temperature and negligible mixing entropy in polymer blends. Herein, we systematically studied the influence of "crystallization kinetics" on the domain size and proposed that if the donor and acceptor crystallize simultaneously, they are prone to form a large domain, while if sequential crystallization of the donor and acceptor occurs, a fine phase separation structure with the proper domain size can be obtained. Taking PBDB-T/PNDI blends for instance, the domain size was decreased by using sequential crystallization; meanwhile, the crystallinity and molecular orientation were also optimized, boosting the power conversion efficiency from 6.55% to 7.78%. This work provides a novel way to finely tune the heterojunction phase separation structures.

Manipulating nanoscale structure to control functionality in printed organic photovoltaic, transistor and bioelectronic devices

Printed electronics is simultaneously one of the most intensely studied emerging research areas in science and technology and one of the fastest growing commercial markets in the world today. For the past decade the potential for organic electronic (OE) materials to revolutionize this printed electronics space has been widely promoted. Such conviction in the potential of these carbon-based semiconducting materials arises from their ability to be dissolved in solution, and thus the exciting possibility of simply printing a range of multifunctional devices onto flexible substrates at high speeds for very low cost using standard roll-to-roll printing techniques. However, the transition from promising laboratory innovations to large scale prototypes requires precise control of nanoscale material and device structure across large areas during printing fabrication. Maintaining this nanoscale material control during printing presents a significant new challenge that demands the coupling of OE materials and devices with clever nanoscience fabrication approaches that are adapted to the limited thermodynamic levers available. In this review we present an update on the strategies and capabilities that are required in order to manipulate the nanoscale structure of large area printed organic photovoltaic (OPV), transistor and bioelectronics devices in order to control their device functionality. This discussion covers a range of efforts to manipulate the electroactive ink materials and their nanostructured assembly into devices, and also device processing strategies to tune the nanoscale material properties and assembly routes through printing fabrication. The review finishes by highlighting progress in printed OE devices that provide a feedback loop between laboratory nanoscience innovations and their feasibility in adapting to large scale printing fabrication. The ability to control material properties on the nanoscale whilst simultaneously printing functional devices on the square metre scale is prompting innovative developments in the targeted nanoscience required for OPV, transistor and biofunctional devices.

Significantly Improved Morphology and Efficiency of Nonhalogenated Solvent-Processed Solar Cells Derived from a Conjugated Donor-Acceptor Block Copolymer

A highly crystalline conjugated donor (D)-acceptor (A) block copolymer (PBDT2T-b-N2200) that has good solubility in nonhalogenated solvents is successfully synthesized. PBDT2T-b-N2200 shows a broad complementary absorption behavior owing to a wide-band gap donor (PBDT2T) present as a D-block and a narrow-band gap acceptor (N2200) present as an A-block. Polymer solar cells (PSCs) with conjugated block copolymer (CBCP) are fabricated using a toluene solution and PSC created with an annealed film showing the highest power conversion efficiency of 6.43%, which is 2.4 times higher than that made with an annealed blend film of PBDT2T and N2200. Compared to the blend film, the PBDT2T-b-N2200 film exhibits a highly improved surface and internal morphology, as well as a faster photoluminescence decay lifetime, indicating a more efficient photoinduced electron transfer. In addition, the PBDT2T-b-N2200 film shows high crystallinity through an effective self-assembly of each block during thermal annealing and a predominant face-on chain orientation favorable to a vertical-type PSC. Moreover, the CBCP-based PSCs exhibit an excellent shelf-life time of over 1020 h owing to their morphological stability. From these results, a D-A block copolymer system is one of the efficient strategies to improve miscibility and morphological stability in all polymer blend systems.