Y-Type Non-Fullerene Acceptors with Outer Branched Side Chains and Inner Cyclohexane Side Chains for 19.36% Efficiency Polymer Solar Cells

Enhanced Charge and Energy Transfer in All-Small-Molecule Ternary Organic Solar Cells: Transient Photocurrent and Photovoltage and Transient Photoluminescence Measurements

A donor-acceptor-donor (D-A-D) molecule, denoted as RC18, consisting of two nickel-porphyrin terminal donor units (D) and a selenophene-flanked diketopyrrolopyrrole central core, connected via an ethynylene linker has been synthesized. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels were measured showing values of -5.49 eV and -3.75 eV, respectively. We have utilized RC18 as donor along with two acceptors, DICTF and Y6, for OSCs and found that power conversion efficiencies were 12.10 % and 12.59 % for RC18:DICTF and RC18:Y6, respectively. The complementary absorption profiles of RC18, DICTF and Y6, along with the intermediate LUMO level of DICTF between RC18 and Y6, led to the fabrication of ternary organic solar cells. RC18:DICTF:Y6 based ternary attained power conversion efficiency of 16.06 %. The observed enhancement in the PCE is attributed to efficient exciton utilization through energy transfer from DICTF to Y6, increased donor-acceptor interfacial area, suppressed charge carrier recombination and improved molecular ordering. These all factors contribute to improvements in short-circuit current density (JSC) and fill factor (FF). Additionally, the open-circuit voltage (VOC) of the ternary OSC lies between those of the two binary OSCs indicating the formation of an alloy between the two acceptors.

Minimizing Energy Loss by Designing Multifunctional Solid Additives to Independent Regulation of Donor and Acceptor Layers for Efficient LBL Polymer Solar Cells

Solid additives are crucial in layer-by-layer (LBL) polymer solar cells (PSCs). Despite its importance, the simultaneous application of solid additives into both donor and acceptor layers has been largely overlooked. In this work, two multifunctional solid additives are actively designed, and investigated the synergistic effect on both donor and acceptor layers. Incorporating the multifunctional solid additives into the donor layer could effectively enhance the aggregation and molecular stacking of the donor polymer, leading to reduced energy disorder and minimizing ΔE2. When the multifunctional solid additives are introduced into the acceptor layer, they just play a role in optimizing the morphology, thereby reducing the ΔE3. Excitedly, the simultaneous addition of the multifunctional solid additives into both donor and acceptor layers produced a synergistic effect for decreasing ΔE2 and ΔE3 simultaneously, especially adding SA2, thus enabling an excellent power conversion efficiency (PCE) of 19.95% (certified as 19.68%) with an open-circuit voltage (Voc) of 0.921 V, a short circuit current density (Jsc) of 27.08 mA cm-2 and a fill factor (FF) of 79.98%. The work highlights the potential of multifunctional solid additives in independently regulating the properties of donor and acceptor layers, which is expected as a promising approach for further developing higher performance PSCs.

Near zero singlet-triplet gap through nonfullerene core modification with phenalene derivative building blocks

Recent advances in data-driven machine learning have highlighted the critical importance of the singlet-triplet gap (ΔEST = ES1 - ET1) in non-fullerene acceptor (NFA) molecules as a useful figure of merit to predict the efficiency of organic photovoltaic devices. By reducing ΔEST, the photovoltaic performance can be improved through the suppression of triplet state channels for non-geminate charge recombination. Encouraged by this strategy, we propose and theoretically explore the properties (particularly relative to ΔEST) of a new class of NFAs derived from modifications of the central core of the Y6 molecule (C82H86F4N8O2S5). The idea is to replace the benzothiadiazole chemical group by building blocks of phenalene derivatives, recognized for their unique inverted ΔEST. Using computational analysis that incorporates a double-hybrid exchange-correlation functional as a benchmark method, we anticipate a remarkable reduction of ΔEST upon phenalene derivative substitution, with some molecules achieving a near zero singlet-triplet gap. This is the first report that calls attention to new chemical strategies to synthesize NFA molecules with very low (eventually zero) ΔEST. Moreover, some modified molecules exhibited higher ET1 compared to Y6, which is interesting to mitigate non-geminate recombination. The molecular modifications also lead to a decrease in intramolecular reorganization energy, thereby lowering the energy barrier for electron transfer. Additionally, a significant increase in the quadrupole moment component along the π-π stacking direction was observed-an essential property for strengthening quadrupole-quadrupole intermolecular interactions, which play a crucial role in molecular packing and charge transport. Overall, our research yields valuable insights into optimizing NFAs, opening the possibility of alternative molecular architectures to further the development of high-efficiency organic photovoltaic devices.

Different Sized Cycloalkyl Chains on Non-Fullerene Acceptors Enhance Molecular Packing, Film Morphology and Charge Transport for 19.62% Efficiency Organic Solar Cells

This work addresses the challenge of achieving advanced fibril morphology of non-fullerene acceptors (NFAs) in layer-by-layer organic solar cels (LBL-OSCs) by cycloalkyl chain strategy, focusing on ta series of Y6-type NFAs, namely BTP-C6, BTP-C8 and BTP-C12, featured with cyclohexyl, cyclooctyl and cyclododecyl chains with increasing steric hindrance. These side chains influenced significantly molecular planarity, packing and film morphology, which are critical for device performance. BTP-C6 exhibits optimal molecular packing and fibril network morphology, enabling efficient exciton dissociation, charge transport and balanced carrier mobilities, finally achieving PCEs of 19.28% and 19.62% with chloroform- and toluene-cast acceptor layers, respectively. BTP-C8 featuring enhanced planarity (dihedral angle 8.27°) showed the loosest packing (packing coefficient 49.6%) due to the increased steric hindrance of side chains, limiting intermolecular charge transport. Conversely, BTP-C12 formed a high crystalline and tightly packed 3D network but suffered from reduced intramolecular charge transfer caused by severe molecular distortion (dihedral angle 27.27°). The findings in this work underscore the critical role of side-chain engineering in governing molecular packing and morphology, offering a systematic understanding of the relationships between steric hindrance, crystallinity and device performance, while providing a rational design strategy for next-generation NFAs to advance high-performance LBL-OSCs.

Theoretical Exploration of the Effects of Conjugated Side Chains on the Photoelectric Properties of Y6-Based Nonfullerene Acceptors

With the application of nonfullerene acceptors (NFAs) Y6 and its derivatives, the power conversion efficiencies (PCEs) of single-junction organic solar cells (OSCs) have exceeded 20%. Side-chain engineering has proven to be an important strategy for optimizing Y6-based NFAs. However, studies on the incorporation of conjugated side chains into Y6-based NFAs are still rare, and the corresponding underlying mechanisms are still not well understood. In this article, we systematically designed eight molecules based on modifications to the conjugated side chains of two reported Y6-based NFAs, involving alterations of branched alkyl chains at different positions on the thiophene, benzene, bithiophene, and benzene-thiophene moieties that serve as conjugated side chains. Using reliable density functional theory (DFT) and time-dependent DFT calculations, we obtained key photovoltaic parameters such as molecular planarity, dipole moments, electrostatic potential and corresponding fluctuations, frontier molecular orbitals, exciton binding energy (Eb), singlet-triplet energy differences (ΔEST), and UV-vis absorption spectra of these newly designed NFAs. The results show that the side conjugated rings and the positions of lateral alkyl chains attached to these rings exert noticeable influences on their photoelectric properties. Notably, compared to the prototype T3EH, 2T2EH, 2T3EH, PT2EH, PT3EH, and P2EH exhibit enhanced absorption (manifesting as increased total oscillator strength) and smaller Eb and ΔEST values, hinting at their promising potential as novel NFAs.

Green-Solvent-Processed All-Polymer Solar Cells with Enhanced Efficiency and Stability through Molecular Design and Side-Chain Engineering

Green-solvent-processed all-polymer solar cells (AP-SCs) are regarded as an excellent candidate for renewable energy due to their better stability and eco-friendly features. Two polymers, PYF-U and PYF-BO, have been designed by introducing a Y-series derivative with difluoro-substituted dicyanindenone units and a difluorobenzotriazole derivative as the first and second electron-deficient (A) units, respectively. The introduction of two additional F atoms on dicyanindenone units leads to a more coplanar backbone because of noncovalent interactions. Compared with the polymer PYF-U with undecyl chains on thiophene, the polymer PYF-BO with 2-butyloctyl chains exhibits stronger intermolecular aggregation during the film-forming process, more dominant face-on molecular packing, and higher crystallinity in films. Therefore, the PM6:PYF-BO AP-SC achieves an efficiency of 15.38%, outperforming that of the PM6:PYF-U device (14.27%). Moreover, the former exhibits a longer T80 lifetime (1789 h) than the latter (826 h) under thermal aging at 65 °C because of better molecular packing and morphology. Our research demonstrates that combining noncovalent interactions to enhance the coplanarity of the polymeric backbone with side-chain engineering to optimize molecular packing and blend-film morphology is one of the efficient strategies for developing high-performance polymer acceptors.

Non-fullerene acceptors with high crystallinity and photoluminescence quantum yield enable >20% efficiency organic solar cells

The rational design of non-fullerene acceptors (NFAs) with both high crystallinity and photoluminescence quantum yield (PLQY) is of crucial importance for achieving high-efficiency and low-energy-loss organic solar cells (OSCs). However, increasing the crystallinity of an NFA tends to decrease its PLQY, which results in a high non-radiative energy loss in OSCs. Here we demonstrate that the crystallinity and PLQY of NFAs can be fine-tuned by asymmetrically adapting the branching position of alkyl chains on the thiophene unit of the L8-BO acceptor. It was found that L8-BO-C4, with 2-butyloctyl on one side and 4-butyldecyl on the other side, can simultaneously achieve high crystallinity and PLQY. A high efficiency of 20.42% (certified as 20.1%) with an open-circuit voltage of 0.894 V and a fill factor of 81.6% is achieved for the single-junction OSC. This work reveals how important the strategy of shifting the alkyl chain branching position is in developing high-performance NFAs for efficient OSCs.

Significant Efficiency Enhancements in Non-Y Series Acceptors by the Addition of Outer Side Chains

Most current highly efficient organic solar cells utilize small molecules like Y6 and its derivatives as electron acceptors in the photoactive layer. In this work, a small molecule acceptor, SC8-IT4F, is developed through outer side chain engineering on the terminal thiophene of a conjugated 6,12-dihydro-dithienoindeno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IDTT) central core. Compared to the reference molecule C8-IT4F, which lacks outer side chains, SC8-IT4F displays notable differences in molecule geometry (as shown by simulations), thermal behavior, single-crystal packing, and film morphology. Blend films of SC8-IT4F and the polymer donor PM6 exhibit larger carrier mobilities, longer carrier lifetimes, and reduced recombination compared to C8-IT4F, resulting in improved device performance. Binary photovoltaic devices based on the PM6:SC8-IT4F films reveal an optimal efficiency over 15%, which is one of the best values for non-Y type small molecule acceptors (SMAs). The resultant devices also show better thermal and operational stability than the control PM6:L8-BO devices. SC8-IT4F and its blend exhibit a higher relative degree of crystallinity and π coherence length, compared to C8-IT4F samples, beneficial for charge transport and device performance. The results indicate that outer side chain engineering on existing small electron acceptors can be a promising molecular design strategy for further pursuing high-performance organic solar cells.

Electron-Rich Heptacyclic S,N Heteroacene Enabling C-Shaped A-D-A-type Electron Acceptors With Photoelectric Response beyond 1000 Nm for Highly Sensitive Near-Infrared Photodetectors

A highly electron-rich S,N heteroacene building block is developed and condensed with FIC and Cl-IC acceptors to furnish CT-F and CT-Cl, which exhibit near-infrared (NIR) absorption beyond 1000 nm. The C-shaped CT-F and CT-Cl self-assemble into a highly ordered 3D intermolecular packing network via multiple π-π interactions in the single crystal structures. The CT-F-based organic photovoltaic (OPV) achieved an impressive efficiency of 14.30% with a broad external quantum efficiency response extending from the UV-vis to the NIR (300-1050 nm) regions, outperforming most binary OPVs employing NIR A-D-A-type acceptors. CT-Cl possesses a higher surface energy than CT-F, promoting vertical phase segregation and resulting in its preferential accumulation near the bottom interface of the blend. This arrangement, combined with the lower HOMO energy level of CT-Cl, effectively reduces undesired hole and electron injection under reverse voltage. The PM6:CT-Cl-based organic photodetectors (OPDs) devices achieved an ultra-high shot-noise-limited specific detectivity (Dsh*) values exceeding 1014 Jones in the NIR region from 620 to 1000 nm, reaching an unprecedentedly high value of 1.3 × 1014 Jones at 950 nm. When utilizing a 780 nm light source, the PM6:CT-Cl-based OPDs show record-high rise/fall times of 0.33/0.11 µs and an exceptional cut-off frequency (f-3dB) of 590 kHz at -1 V.

Manipulating Aggregation Kinetics toward Efficient All-Printed Organic Solar Cells

The power conversion efficiencies (PCEs) of all-printed organic solar cells (OSCs) remain inferior to those of spin-coated devices, primarily due to morphological variations within the bulk heterojunction processed via diverse coating/printing techniques. Herein, cyclohexyl is introduced as outer side chains to formulate a non-fullerene acceptor, BTP-Cy, aimed at modulating the molecular aggregation in solution and subsequent film formation kinetics during printing. Investigations demonstrate that BTP-Cy molecule with cyclohexyl side chains exhibits enhanced intermolecular π-π stacking, optimal solution aggregation size, and favorable phase separation. Consequently, PB3:FTCC-Br:BTP-Cy-based OSCs achieve remarkable PCEs of 20.2% and 19.5% via spin-coating and blade-coating, respectively. Furthermore, a 23.6 cm2 module exhibits a remarkable efficiency of 16.7%. This study offers a fresh perspective on tailoring the film formation kinetics of photoactive materials during printing through molecular design, paving a novel path to enhance the efficiency of all-printed OSCs.

Fluorinated and methylated ortho-benzodipyrrole-based acceptors suppressing charge recombination and minimizing energy loss in organic photovoltaics

The elimination of the A' unit from -type Y6-derivatives has led to the development of a new class of ortho-benzodipyrrole (o-BDP)-based A-DNBND-A-type NFAs. In this work, two new A-DNBND-A-type NFAs, denoted as CFB and CMB, are designed and synthesized, where electron-withdrawing fluorine atoms and electron-donating methyl groups are substituted on the benzene ring of the o-BDP moiety, respectively. CFB exhibits a blue-shifted absorption spectrum, stronger intermolecular interactions, shorter π-π stacking distances, and more ordered 3D intermolecular packing in the neat and blend films, enabling it to effectively suppress charge recombination in the PM6:CFB device showing a higher PCE of 16.55% with an FF of 77.45%. CMB displays a higher HOMO/LUMO energy level, a smaller optical bandgap, and a less ordered 3D packing, which contributes to its superior ability to suppress energy loss in the PM6:CMB device with a high V oc of 0.90 V and a PCE of 16.46%. To leverage the advantages of CFB and CMB, ternary PM6:Y6-16:CFB and PM6:Y6-16:CMB devices are fabricated. The PM6:Y6-16:CFB device exhibits the highest PCE of 17.83% with an increased V oc of 0.86 V and a J sc of 27.32 mA cm-2, while the PM6:Y6-16:CMB device displayed an elevated V oc of 0.87 V and an improved FF of 74.71%, leading to a PCE of 17.44%. The high PCE was achieved using the non-halogenated greener solvent o-xylene, highlighting their potential for facilitating more eco-friendly processing procedures. C-shaped disubstituted o-BDP-based A-D-A type acceptors open up new avenues for tailoring electronic properties and molecular self-assembly, achieving higher OPV performance with enhanced charge recombination suppression and reduced energy loss.

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

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

Individually Tunable Energy Levels of Oligomers Based on N-B←N Units

Opto-electronic properties and device performance of organic semiconductors are mainly determined by energy levels of their frontier molecular orbitals, e.g. lowest unoccupied molecular orbital (ELUMO) and highest occupied molecular orbital (EHOMO) in the ground state, first singlet state (ES1) and first triplet state (ET1) in the excited state. These energy levels are always intricately intertwined. Herein, we report a series of monodisperse oligomers based on double B←N bridged bipyridine (BNBP) units. With the increasing number of repeating units, the oligomers exhibit gradually downshifted ELUMO and nearly unchanged EHOMO due to the different distribution of the frontier molecular orbitals of the oligomers. Moreover, the oligomers exhibit gradually decreasing ES1 and nearly unchanged ET1 because of the different contributions of the charge transfer component in the excited state. This work provides new insight into energy level tuning of organic semiconductors, which is important for high-performance organic opto-electronic devices.

Boosting organic solar cell efficiency via tailored end-group modifications of novel non-fused ring electron acceptors

In this study, we designed and synthesized two NFREAs, 2BTh-3F and 2BTh-CN, incorporating distinct substituents to modulate their electron-withdrawing properties. We meticulously explore the distinct impacts of these substituents on NFREA performance. Our investigation revealed that the introduction of 3,5-difluoro-4-cyanophenyl in 2BTh-CN significantly enhanced electron withdrawal and intramolecular charge transfer, leading to a red-shifted absorption spectrum and optimized energy levels. Consequently, organic solar cells (OSCs) utilizing 2BTh-CN demonstrate a notable power conversion efficiency (PCE) of 15.07%, outperforming those employing 2BTh-3F (PCE of 9.34%). Moreover, by incorporating 2BTh-CN into the D18:2BTh-C2 system as a third component, we achieve a PCE exceeding 17% in a high-performing ternary OSC, ranking among the most efficient NFREA-based OSCs reported to date. Overall, our study underscores the potential of deliberate design and optimization of non-fused ring acceptor molecular structures to attain outstanding photovoltaic performance.

Constructing efficient organic solar cells by highly volatile solid additives with controlled phase morphology

We synthesized two highly volatile and low-cost solid additives, PT and TFT. The inclusion of PT and TFT effectively influences the aggregation behavior of D18: L8-BO during the film-forming process. Consequently, PT and TFT-treated D18: L8-BO-based OSCs achieved power conversion efficiencies of 18.28% and 19.19%, respectively. This study provides a straightforward approach for achieving efficient photovoltaic performance of OSCs.

Boosting the Efficiency of Perovskite/Organic Tandem Solar Cells via Enhanced Near-Infrared Absorption and Minimized Energy Losses

The compatibility of perovskite and organic photovoltaic materials in solution processing provides a significant advantage in the fabrication of high-efficiency perovskite/organic tandem solar cells. However, additional recombination losses can occur during exciton dissociation in organic materials, leading to energy losses in the near-infrared region of tandem devices. Consequently, a ternary organic rear subcell is designed containing two narrow-bandgap non-fullerene acceptors to enhance the absorption of near-infrared light. Simultaneously, a unique diffusion-controlled growth technique is adopted to optimize the morphology of the ternary active layer, thereby improving exciton dissociation efficiency. This innovation not only broadens the absorption range of near-infrared light but also facilitates the generation and effective dissociation of excitons. Owing to these technological improvements, the power conversion efficiency (PCE) of organic solar cells increased to 19.2%. Furthermore, a wide-bandgap perovskite front subcell is integrated with a narrow-bandgap organic rear subcell to develop a perovskite/organic tandem solar cell. Owing to the reduction in near-infrared energy loss, the PCE of this tandem device significantly improved, reaching 24.5%.

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

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

Inner/Outer Side Chain Engineering of Non-Fullerene Acceptors for Efficient Large-Area Organic Solar Modules Based on Non-Halogenated Solution Processing in Air

Achieving efficient and large-area organic solar modules via non-halogenated solution processing is vital for the commercialization yet challenging. The primary hurdle is the conservation of the ideal film-formation kinetics and bulk-heterojunction (BHJ) morphology of large-area organic solar cells (OSCs). A cutting-edge non-fullerene acceptor (NFA), Y6, shows efficient power conversion efficiencies (PCEs) when processed with toxic halogenated solvents, but exhibits poor solubility in non-halogenated solvents, resulting in suboptimal morphology. Therefore, in this study, the impact of modifying the inner and outer side-chains of Y6 on OSC performance is investigated. The study reveals that blending a polymer donor, PM6, with one of the modified NFAs, namely N-HD, achieved an impressive PCE of 18.3% on a small-area OSC. This modified NFA displays improved solubility in o-xylene at room temperature, which facilitated the formation of a favorable BHJ morphology. A large-area (55 cm2) sub-module delivered an impressive PCE of 12.2% based on N-HD using o-xylene under ambient conditions. These findings underscore the significant impact of the modified Y6 derivatives on structural arrangements and film processing over a large-area module at room temperature. Consequently, these results are poised to deepen the comprehension of the scaling challenges encountered in OSCs and may contribute to their commercialization.

Improving Miscibility of Polymer Donor and Polymer Acceptor by Reducing Chain Entanglement for Realizing 18.64 % Efficiency All Polymer Solar Cells

All-polymer solar cells have experienced rapid development in recent years by the emergence of polymerized small molecular acceptors (PSMAs). However, the strong chain entanglements of polymer donors (PDs) and polymer acceptors (PAs) decrease the miscibility of the resulting polymer mixtures, making it challenging to optimize the blend morphology. Herein, we designed three PAs, namely PBTPICm-BDD, PBTPICγ-BDD and PBTPICF-BDD, by smartly using a BDD unit as the polymerized unit to copolymerize with different Y-typed non-fullerene small molecular acceptors (NF-SMAs), thus achieving a certain degree of distortion and giving the polymer system enough internal space to reduce the entanglements of the polymer chains. Such effects increase the chances of the PD being interspersed into the acceptor material, which improve the solubility between the PD and PA. The PBTPICγ-BDD and PBTPICF-BDD displayed better miscibility with PBQx-TCl, leading to a well optimized morphology. As a result, high power conversion efficiencies (PCEs) of 17.50 % and 17.17 % were achieved for PBQx-TCl : PBTPICγ-BDD and PBQx-TCl : PBTPICF-BDD devices, respectively. With the addition of PYFT-o as the third component into PBQx-TCl : PBTPICγ-BDD blend to further extend the absorption spectral coverage and finely tune microstructures of the blend morphology, a remarkable PCE of 18.64 % was realized finally.

Accomplishing High-Performance Organic Solar Sub-Modules (≈55 cm2) with >16% Efficiency by Controlling the Aggregation of an Engineered Non-Fullerene Acceptor

The fabrication of environmentally benign, solvent-processed, efficient, organic photovoltaic sub-modules remains challenging due to the rapid aggregation of the current high performance non-fullerene acceptors (NFAs). In this regard, design of new NFAs capable of achieving optimal aggregation in large-area organic photovoltaic modules has not been realized. Here, an NFA named BTA-HD-Rh is synthesized with longer (hexyl-decyl) side chains that exhibit good solubility and optimal aggregation. Interestingly, integrating a minute amount of new NFA (BTA-HD-Rh) into the PM6:L8-BO system enables the improved solubility in halogen-free solvents (o-xylene:carbon disulfide (O-XY:CS2)) with controlled aggregation is found. Then solar sub-modules are fabricated at ambient condition (temperature at 25 ± 3 °C and humidity: 30-45%). Ultimately, the champion 55 cm2 sub-modules achieve exciting efficiency of >16% in O-XY:CS2 solvents, which is the highest PCE reported for sub-modules. Notably, the highest efficiency of BTA-HD-Rh doped PM6:L8-BO is very well correlated with high miscibility with low Flory-Huggins parameter (0.372), well-defined nanoscale morphology, and high charge transport. This study demonstrates that a careful choice of side chain engineering for an NFA offers fascinating features that control the overall aggregation of active layer, which results in superior sub-module performance with environmental-friendly solvents.

Dual Function of the Third Component in Ternary Organic Solar Cells: Broaden the Spectrum and Optimize the Morphology

Ternary organic solar cells (T-OSCs) have attracted significant attention as high-performance devices. In recent years, T-OSCs have achieved remarkable progress with power conversion efficiency (PCE) exceeding 19%. However, the introduction of the third component complicates the intermolecular interaction compared to the binary blend, resulting in poor controllability of active layer and limiting performance improvement. To address these issues, dual-functional third components have been developed that not only broaden the spectral range but also optimize morphology. In this review, the effect of the third component on expanding the absorption range of T-OSCs is first discussed. Second, the extra functions of the third component are introduced, including adjusting the crystallinity and molecular stack in active layer, regulating phase separation and purity, altering molecular orientation of the donor or acceptor. Finally, a summary of the current research progress is provided, followed by a discussion of future research directions.

Spectrally Resolved Exciton Polarizability for Understanding Charge Generation in Organic Bulk Hetero-Junction Diodes

Despite decades of research, the dominant charge generation mechanism in organic bulk heterojunction (BHJ) devices is not completely understood. While the local dielectric environments of the photoexcited molecules are important for exciton dissociation, conventional characterizations cannot separately measure the polarizability of electron-donor and electron-acceptor, respectively, in their blends, making it difficult to decipher the spectrally different charge generation efficiencies in organic BHJ devices. Here, by spectrally resolved electroabsorption spectroscopy, we report extraction of the excited state polarizability for individual donors and acceptors in a series of organic blend films. Regardless of the donor and acceptor, we discovered that larger exciton polarizability is linked to larger π-π coherence length and faster charge transfer across the heterojunction, which fundamentally explains the origin of the higher charge generation efficiency near 100% in the BHJ photodiodes. We also show that the molecular packing of the donor and acceptor influence each other, resulting in a synergetic enhancement in the exciton polarizability.

Understanding Causalities in Organic Photovoltaics Device Degradation in a Machine-Learning-Driven High-Throughput Platform

Organic solar cells (OSCs) now approach power conversion efficiencies of 20%. However, in order to enter mass markets, problems in upscaling and operational lifetime have to be solved, both concerning the connection between processing conditions and active layer morphology. Morphological studies supporting the development of structure-process-property relations are time-consuming, complex, and expensive to undergo and for which statistics, needed to assess significance, are difficult to be collected. This work demonstrates that causal relationships between processing conditions, morphology, and stability can be obtained in a high-throughput method by combining low-cost automated experiments with data-driven analysis methods. An automatic spectral modeling feeds parametrized absorption data into a feature selection technique that is combined with Gaussian process regression to quantify deterministic relationships linking morphological features and processing conditions with device functionality. The effect of the active layer thickness and the morphological order is further modeled by drift-diffusion simulations and returns valuable insight into the underlying mechanisms for improving device stability by tuning the microstructure morphology with versatile approaches. Predicting microstructural features as a function of processing parameters is decisive know-how for the large-scale production of OSCs.

Optimizing Alkyl Side Chains in Difluorobenzene-Rhodanine Small-Molecule Acceptors for Organic Solar Cells

A series of small molecules, T-2FB-T-ORH, T-2FB-T-BORH, and T-2FB-T-HDRH, were synthesized to have a thiophene-flanked difluorobenzene (T-2FB-T) core and alkyl-substituted rhodanine (RH) end groups for their use as nonfullerene acceptors (NFAs) in organic solar cells (OSCs). Octyl, 2-butyloctyl (BO), and 2-hexyldecyl (HD) alkyl side chains were introduced into RHs to control the material's physical properties based on the length and size of the alkyl chains. The optical properties of the three NFAs were found to be almost the same, irrespective of the alkyl chain length, whereas the molecular crystallinity and material solubility significantly differed depending on the alkyl side chains. Owing to the sufficient solubility of T-2FB-T-HDRH, OSCs based on PTB7-Th and T-2FB-T-HDRH were fabricated. A power conversion efficiency of up to 4.49% was obtained by solvent vapor annealing (SVA). The AFM study revealed that improved charge mobility and a smooth and homogeneous film morphology without excessive aggregation could be obtained in the SVA-treated film.

High-Efficiency Ternary Polymer Solar Cells with a Gradient-Blended Structure Fabricated by Sequential Deposition

Acquiring the ideal blend morphology of the active layer to optimize charge separation and collection is a constant goal of polymer solar cells (PSCs). In this paper, the ternary strategy and the sequential deposition process were combined to make sufficient use of the solar spectrum, optimize the energy-level structure, regulate the vertical phase separation morphology, and ultimately enhance the power conversion efficiency (PCE) and stability of the PSCs. Specifically, the donor and acceptor illustrated a gradient-blended distribution in the sequential deposition-processed films, thus resulting in facilitated carrier characteristics in the gradient-blended devices. Consequently, the PSCs based on D18-Cl/Y6:ZY-4Cl have achieved a device efficiency of over 18% with the synergetic improvement of open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). Therefore, this work reveals a facile approach to fabricating PSCs with improved performance and stability.

Design of Chlorinated Indaceno[1,2-b:5,6-b']dithiophene Acceptors toward Efficient Organic Photovoltaics

Chlorinated modifications have been extensively employed to modulate the optoelectronic properties of π-conjugated materials. Herein, the Cl substitution in designing nonfullerene acceptors (NFAs) with various bandgaps is studied. Four narrow-bandgap electron acceptors (GS-40, GS-41, GS-42, and GS-43) were synthesized by tuning the electrostatic potential distributions of the molecular conjugated backbones. The optical absorption onset of these NFAs ranges from 900 to 1030 nm. Compared to the nonchlorinated analogue, the introduction of Cl atoms on the core of indaceno[1,2-b:5,6-b'] dithiophene (IDT) and π spacer results in an upward shift of the lowest unoccupied molecular orbital levels and induces a blue shift in the absorption spectra of the NFAs. This alteration facilitates achieving appropriate energy-level alignment and favorable bulk heterojunction morphology when blended with the widely used donor PBDB-TF. The PBDB-TF:GS-43-based solar cells show an optimal power conversion efficiency of 13.3%. This work suggests the potential of employing chlorine-modified IDT and thiophene units as fundamental building blocks for developing high-performance photoactive materials.

18.6% Efficiency All-Polymer Solar Cells Enabled by a Wide Bandgap Polymer Donor Based on Benzo[1,2-d:4,5-d']bisthiazole

The limited selection of wide bandgap polymer donors for all-polymer solar cells (all-PSCs) is a bottleneck problem restricting their further development and remains poorly studied. Herein, a new wide bandgap polymer, namely PBBTz-Cl, is designed and synthesized by bridging the benzobisthiazole acceptor block and chlorinated benzodithiophene donor block with thiophene units for application as an electron donor in all-PSCs. PBBTz-Cl not only possesses wide bandgap and deep energy levels but also displays strong absorption, high-planar structure, and good crystallinity, making it a promising candidate for application as a polymer donor in organic solar cells. When paired with the narrow bandgap polymer acceptor PY-IT, a fibril-like morphology forms, which facilitates exciton dissociation and charge transport, contributing to a power conversion efficiency (PCE) of 17.15% of the corresponding all-PSCs. Moreover, when introducing another crystalline polymer acceptor BTP-2T2F into the PBBTz-Cl:PY-IT host blend, the absorption ditch in the range of 600-750 nm is filled, and the blend morphology is further optimized with the trap density reducing. As a result, the ternary blend all-PSCs achieve a significantly improved PCE of 18.60%, which is among the highest values for all-PSCs to date.

Halogenated Nonfused Ring Electron Acceptor for Organic Solar Cells with a Record Efficiency of over 17

Three nonfused ring electron acceptors (NFREAs), namely, 3TT-C2-F, 3TT-C2-Cl, and 3TT-C2, are purposefully designed and synthesized with the concept of halogenation. The incorporation of F or/and Cl atoms into the molecular structure (3TT-C2-F and 3TT-C2-Cl) enhances the π-π stacking, improves electron mobility, and regulates the nanofiber morphology of blend films, thus facilitating the exciton dissociation and charge transport. In particular, blend films based on D18:3TT-C2-F demonstrate a high charge mobility, an extended exciton diffusion distance, and a well-formed nanofiber network. These factors contribute to devices with a remarkable power conversion efficiency of 17.19%, surpassing that of 3TT-C2-Cl (16.17%) and 3TT-C2 (15.42%). To the best of knowledge, this represents the highest efficiency achieved in NFREA-based devices up to now. These results highlight the potential of halogenation in NFREAs as a promising approach to enhance the performance of organic solar cells.

19.32% Efficiency Polymer Solar Cells Enabled by Fine-Tuning Stacking Modes of Y-Type Molecule Acceptors: Synergistic Bromine and Fluorine Substitution of the End Groups

The success of Y6-type nonfullerene small molecule acceptors (NF-SMAs) in polymer solar cells (PSCs) can be attributed to their unique honeycomb stacking style, which leads to favorable thin-film morphologies. The intermolecular interactions related to the crystallization tendency of these NF-SMAs is closely governed by their electron accepting end groups. For example, the high performance Y6 derivative L8-BO (BTP-4F) presents three types of stacking modes in contrast to two stacking modes of Y6. Hence, it is ultimately interesting to obtain more insight on the packing properties and the preferences influenced by chemical modifications such as end group engineering. This work designs and synthesizes asymmetric and symmetric L8-BO derivatives with brominated end groups and explores the stacking preferences in various modes. The asymmetric BTP-3FBr displays an optimized crystallization tendency and thin film morphology, leading to a decent power conversion efficiency (PCE) of 18.34% in binary devices and a top PCE of 19.32% in ternary devices containing 15 wt% IDIC as the second acceptor.

Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium-Substituted Acceptor for Efficient Binary Organic Solar Cells

Both the regional isomerization and selenium-substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S-CSeF, A-ISeF, and A-OSeF) via subtly manipulating the mono-selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A-OSeF presents a closer intermolecular π-π stacking and more ordered 3-dimensional network packing and efficient charge-hopping pathways. With the successive out-shift of the mono-selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A-OSeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A-OSeF-based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium-containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium-based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top-ranked selenium-containing SMAs-based OSCs.

Nonfused Ring Electron Acceptors for Ternary Polymer Solar Cells with Low Energy Loss and Efficiency Over 18

Ternary polymer solar cells(PSCs) have been identified as an effective approach to improving power conversion efficiency (PCE) of binary PSCs. However, most of the third component, especially Y-series non-fullerene acceptors, is a fused ring acceptor which often requires a rather tedious synthesis and the use of hazardous organostannane reagents. In this work, two nonfused ring acceptors IOEH-4F and IOEH-N2F are synthesized by a green synthetic route and incorporated into PM6:Y6 blend. Encouragingly, the IOEH-4F and IOEH-N2F-based ternary PSCs exhibited more efficient charge transfer, exciton separation, and lower energy loss than PM6:Y6-based PSCs. And the IOEH-4F and IOEH-N2F-based ternary PSCs achieved an impressive PCE of 17.80% and 18.13%, respectively, which are higher than that of PM6:Y6 based PSCs (16.18%). Notably, these PCE values are also the highest PCEs for ternary PSCs including non-fused ring acceptors. Importantly, even when the IOEH-N2F:Y6 ratios increased from 0.05:1.2 to 0.50:1.2, the PCE of IOEH-N2F-based ternary PSCs (16.70%) are still higher than that of PM6:Y6 based PSCs, indicating the great potential for cost saving. It is believed that the findings will help the design of better nonfused ring acceptors and the optimization of corresponding ternary PSCs with cost-saving advantage.

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

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

High-Performance Nonfused Electron Acceptor with Precisely Controlled Side Chain Fluorination

Modification of the molecular packing of nonfullerene acceptors through fluorination represents one of the most promising strategies to achieve highly efficient organic solar cells (OSCs). In this work, three nonfused electron acceptors, namely, DTCBT-Fx (x = 0, 5, 9) with precisely controlled amounts of fluorine atoms in the side chains are designed and synthesized, and the effect of side chain fluorination is systematically studied. The results demonstrate that the light absorption, energy levels, molecular ordering, and film morphology could be effectively tuned by precisely controlling the side chain fluorination. DTCBT-F5 with an appropriate fluorine functionalization exhibits suitable miscibility with the donor polymer (PM6), leading to diminished charge recombination and improved charge carrier mobility. Consequently, a promising power conversion efficiency of 12.7% was obtained for DTCBT-F5-based solar cells, which outperforms those OSCs based on DTCBT-F0 (11.4%) and DTCBT-F9 (11.6%), respectively. This work demonstrates that precise control of the fluorine functionalization in side chains of nonfused electron acceptors is an effective strategy for realizing highly efficient OSCs.

Optimization and Efficiency Enhancement of Modified Polymer Solar Cells

In this study, an organic bulk heterojunction (BHJ) solar cell with a spiro OMeTAD as a hole transport layer (HTL) and a PDINO as an electron transport layer (ETL) was simulated through the one-dimensional solar capacitance simulator (SCAPS-1D) software to examine the performance of this type of organic polymer thin-film solar cell. As an active layer, a blend of polymer donor PBDB-T and non-fullerene acceptor ITIC-OE was used. Numerical simulation was performed by varying the thickness of the HTL and the active layer. Firstly, the HTL layer thickness was optimized to 50 nm; after that, the active-layer thickness was varied up to 80 nm. The results of these simulations demonstrated that the HTL thickness has rather little impact on efficiency while the active-layer thickness improves efficiency significantly. The temperature effect on the performance of the solar cells was considered by simulations performed for temperatures from 300 to 400 K; the efficiency of the solar cell decreased with increasing temperature. Generally, polymer films are usually full of traps and defects; the density of the defect (Nt) value was also introduced to the simulation, and it was confirmed that with the increase in defect density (Nt), the efficiency of the solar cell decreases. After thickness, temperature and defect density optimization, a reflective coating was also applied to the cell. It turned out that by introducing the reflective coating to the back side of the solar cell, the efficiency increased by 2.5%. Additionally, the positive effects of HTL and ETL doping on the efficiency of this type of solar cells were demonstrated.

Unidirectional Sidechain Engineering to Construct Dual-Asymmetric Acceptors for 19.23 % Efficiency Organic Solar Cells with Low Energy Loss and Efficient Charge Transfer

Achieving both high open-circuit voltage (Voc ) and short-circuit current density (Jsc ) to boost power-conversion efficiency (PCE) is a major challenge for organic solar cells (OSCs), wherein high energy loss (Eloss ) and inefficient charge transfer usually take place. Here, three new Y-series acceptors of mono-asymmetric asy-YC11 and dual-asymmetric bi-asy-YC9 and bi-asy-YC12 are developed. They share the same asymmetric D1 AD2 (D1 =thieno[3,2-b]thiophene and D2 =selenopheno[3,2-b]thiophene) fused-core but have different unidirectional sidechain on D1 side, allowing fine-tuned molecular properties, such as intermolecular interaction, packing pattern, and crystallinity. Among the binary blends, the PM6 : bi-asy-YC12 one has better morphology with appropriate phase separation and higher order packing than the PM6 : asy-YC9 and PM6 : bi-asy-YC11 ones. Therefore, the PM6 : bi-asy-YC12-based OSCs offer a higher PCE of 17.16 % with both high Voc and Jsc , due to the reduced Eloss and efficient charge transfer properties. Inspired by the high Voc and strong NIR-absorption, bi-asy-YC12 is introduced into efficient binary PM6 : L8-BO to construct ternary OSCs. Thanks to the broadened absorption, optimized morphology, and furtherly minimized Eloss , the PM6 : L8-BO : bi-asy-YC12-based OSCs achieve a champion PCE of 19.23 %, which is one of the highest efficiencies among these annealing-free devices. Our developed unidirectional sidechain engineering for constructing bi-asymmetric Y-series acceptors provides an approach to boost PCE of OSCs.

Improving Cooperative Interactions Between Halogenated Aromatic Additives and Aromatic Side Chain Acceptors for Realizing 19.22% Efficiency Polymer Solar Cells

Processing additive plays an important role in the standard operation procedures for fabricating top performing polymer solar cells (PSCs) through efficient interactions with key photovoltaic materials. However, improving interaction study of acceptor materials to high performance halogenated aromatic additives such as diiodobenzene (DIB) is a widely neglected route for molecular engineering toward more efficient device performances. In this work, two novel Y-type acceptor molecules of BTP-TT and BTP-TTS with different aromatic side chains on the outer positions are designed and synthesized. The resulting aromatic side chains significantly enhanced the interactions between the acceptor molecules and DIB through an arene/halogenated arene interaction, which improved the crystallinity of the acceptor molecules and induced a polymorph with better photovoltaic performances. Thus, high power conversion efficiencies (PCEs) of 18.04% and 19.22% are achieved in binary and ternary blend devices using BTP-TTS as acceptor and DIB as additive. Aromatic side chain engineering for improving additive interactions is proved to be an effective strategy for achieving much higher performance photovoltaic materials and devices.

Double-Dipole Induced by Incorporating Nitrogen-Bromine Hybrid Cathode Interlayers Leads to Suppressed Current Leakage and Enhanced Charge Extraction in Non-Fullerene Organic Solar Cells

The cathode interlayer plays a vital role in organic solar cells, which can modify the work function of electrodes, lower the electron extraction barriers, smooth the surface of the active layer, and remove solvent residuals. However, the development of organic cathode interlayer lags behind the rapidly improved organic solar cells because their intrinsic high surface tension can lead to poor contact with the active layers. Herein, a double-dipole strategy is proposed to enhance the properties of organic cathode interlayers, which is induced by incorporating nitrogen- and bromine-containing interlayer materials. To verify this approach, the state-of-the-art active layer composed of PM6:Y6 and two prototypical cathode interlayer materials, PDIN and PFN-Br is selected. Using the cathode interlayer PDIN: PFN-Br (0.9:0.1, in wt.%) in the devices can reduce the electrode work function, suppress the dark current leakage, and improve charge extractions, leading to enhanced short circuit current density and fill factor. The bromine ions tend to break from PFN-Br and form a new chemical bond with the silver electrode, which can adsorb extra dipoles directed from the interlayer to silver. These findings on the double-dipole strategy provide insights into the hybrid cathode interlayers for efficient non-fullerene organic solar cells.

Unraveling the Role of Non-Fullerene Acceptor with High Dielectric Constant in Organic Solar Cells

Increasing the relative dielectric constant is a constant pursuit of organic semiconductors, but it often leads to multiple changes in device characteristics, hindering the establishment of a reliable relationship between dielectric constant and photovoltaic performance. Herein, a new non-fullerene acceptor named BTP-OE is reported by replacing the branched alkyl chains on Y6-BO with branched oligoethylene oxide chains. This replacement successfully increases the relative dielectric constant from 3.28 to 4.62. To surprise, BTP-OE offers consistently lower device performance relative to Y6-BO in organic solar cells (16.27% vs 17.44%) due to the losses in open-circuit voltage and fill factor. Further investigations unravel that BTP-OE has resulted in reduced electron mobility, increased trap density, enhanced first order recombination, and enlarged energetic disorder. These results demonstrate the complex relationship between dielectric constant and device performance, which provide valuable implications for the development of organic semiconductors with high dielectric constant for photovoltaic application.

Eco-Friendly Solvent-Processed Dithienosilicon-Bridged Carbazole-Based Small-Molecule Acceptors Achieved over 25.7% PCE in Ternary Devices under Indoor Conditions

Terminal acceptor atoms and side-chain functionalization play a vital role in the construction of efficient nonfullerene small-molecule acceptors (NF-SMAs) for AM1.5G/indoor organic photovoltaic (OPV) applications. In this work, we report three dithienosilicon-bridged carbazole-based (DTSiC) ladder-type (A-DD'D-A) NF-SMAs for AM1.5G/indoor OPVs. First, we synthesize DTSiC-4F and DTSiC-2M, which are composed of a fused DTSiC-based central core with difluorinated 1,1-dicyanomethylene-3-indanone (2F-IC) and methylated IC (M-IC) end groups, respectively. Then, alkoxy chains are introduced in the fused carbazole backbone of DTSiC-4F to form DTSiCODe-4F. From solution to film absorption, DTSiC-4F exhibits a bathochromic shift with strong π-π interactions, which improves the short-circuit current density (J_sc) and the fill factor (FF). On the other hand, DTSiC-2M and DTSiCODe-4F display up-shifting lowest unoccupied molecular orbital (LUMO) energy levels, which enhances the open-circuit voltage (V_oc). As a result, under both AM1.5G/indoor conditions, the devices based on PM7:DTSiC-4F, PM7:DTSiC-2M, and PM7:DTSiCOCe-4F show power conversion efficiencies (PCEs) of 13.13/21.80%, 8.62/20.02, and 9.41/20.56%, respectively. Furthermore, the addition of a third component to the active layer of binary devices is also a simple and efficient strategy to achieve higher photovoltaic efficiencies. Therefore, the conjugated polymer donor PTO2 is introduced into the PM7:DTSiC-4F active layer because of the hypsochromically shifted complementary absorption, deep highest occupied molecular orbital (HOMO) energy level, good miscibility with PM7 and DTSiC-4F, and optimal film morphology. The resulting ternary OSC device based on PTO2:PM7:DTSiC-4F can improve exciton generation, phase separation, charge transport, and charge extraction. As a consequence, the PTO2:PM7:DTSiC-4F-based ternary device achieves an outstanding PCE of 13.33/25.70% under AM1.5G/indoor conditions. As far as we know, the obtained PCE results under indoor conditions are one of the best binary/ternary-based systems processed from eco-friendly solvents.

Benzo[d]thiazole Based Wide Bandgap Donor Polymers Enable 19.54% Efficiency Organic Solar Cells Along with Desirable Batch-to-Batch Reproducibility and General Applicability

The limited selection pool of high-performance wide bandgap (WBG) polymer donors is a bottleneck problem of the nonfullerene acceptor (NFA) based organic solar cells (OSCs) that impedes the further improvement of their photovoltaic performances. Herein, a series of new WBG polymers, namely PH-BTz, PS-BTz, PF-BTz, and PCl-BTz, are developed by using the bicyclic difluoro-benzo[d]thiazole (BTz) as the acceptor block and benzo[1,2-b:4,5-b']dithiophene (BDT) derivatives as the donor units. By introducing S, F, and Cl atoms to the alkylthienyl sidechains on BDT, the resulting polymers exhibit lowered energy levels and enhanced aggregation properties. The fluorinated PBTz-F not only exhibits a low-lying HOMO level, but also has stronger face-on packing order and results in more uniform fibril-like interpenetrating networks in the related PF-BTz:L8-BO blend. A high-power conversion efficiency (PCE) of 18.57% is achieved. Moreover, PBTz-F also exhibits a good batch-to-batch reproducibility and general applicability. In addition, ternary blend OSCs based on the host PBTz-F:L8-BO blend and PM6 guest donor exhibits a further enhanced PCE of 19.54%, which is among the highest values of OSCs.

Finely Tuned Molecular Packing Realized by a New Rhodanine-Based Acceptor Enabling Excellent Additive-Free Small- and Large-Area Organic Photovoltaic Devices Approaching 19 and 12.20% Efficiencies

A new nonfullerene acceptor (NFA), BTA-ERh, was synthesized and integrated into a PM6:Y7:PC₇₁BM ternary system to regulate the blend film morphology for enhanced device performance. Due to BTA-ERh's good miscibility with host active blend films, an optimized film morphology was obtained with appropriate phase separation and fine-tuning of film crystallinity, which ultimately resulted in efficient exciton dissociation, charge transport, lower recombination loss, and decreased trap-state density. The resulting additive-free quaternary devices achieved a remarkable efficiency of 18.90%, with a high voltage, fill factor, and current density of 0.87 V, 76.32%, and 28.60 mA cm^-2, respectively. By adding less of a new small molecule with high crystallinity, the favorable nanomorphology shape of blend films containing NFAs might be adjusted. Consequently, this strategy can enhance photovoltaic device performance for cutting-edge NFA-based organic solar cells (OSCs). In contrast, the additive-free OSCs exhibited good operational stability. More importantly, large-area modules with the quaternary device showed a remarkable efficiency of 12.20%, with an area as high as 55 cm² (substrate size, 100 cm²) in an air atmosphere via D-bar coating. These results highlight the enormous research potential for a multicomponent strategy for future additive-free OSC applications.