Hybrid Cycloalkyl-Alkyl Chain-Based Symmetric/Asymmetric Acceptors with Optimized Crystal Packing and Interfacial Exciton Properties for Efficient Organic Solar Cells

Optical and crystalline properties of benzo[1,2-b:4,5-b']dithiophene derivatives

When designing solution-processable semiconducting molecules and polymers, benzo[1,2-b:4,5-b']dithiophene (BDT) derivatives are widely used because of their planar structures, superior optical properties, ease of synthesis and ease of modification. In this work, four BDT derivatives-BDTT, BDTT-Et, BDTT-OMe and BDTT-CH2-OMe-were designed and synthesized with different side chains, considering the important roles of side chains in the performance of organic semiconductors. Especially for BDTT-CH2-OMe, with a new methoxymethyl chain, it exhibited excellent optical properties and the deepest highest occupied molecular orbital energy level (E HOMO) among these derivatives. Moreover, it demonstrated strong intermolecular interactions and tight π-π stacking. The optical, electrochemical and crystalline properties suggested that BDTT-CH2-OMe could be further modified as a potential building block for the design of electron-donating small molecules (SMs) or polymers when used in organic electronics, such as bulk heterojunction organic solar cells (BHJ-OSCs).

Balanced Crystallization Enhances Morphology and Efficiency in Binary Organic Solar Cells

Achieving high-performance organic solar cells (OSCs) relies heavily on precise morphology optimization, a challenging task due to the intrinsic differences in crystallization kinetics and interfacial compatibility between polymer donors and small-molecule acceptors. In this work, 2,7-dibromonaphthalene (DBN) is introduced as an innovative solid additive that uniquely regulates crystallization in both donor and acceptor phases within the PM6:Y6 system. Unlike conventional liquid additives, which often induce excessive Y6 crystallization, DBN achieves a balanced crystallization, enhancing molecular order in PM6 while mitigating over-aggregation in Y6. This dual-phase effect improves light absorption, exciton generation and dissociation, charge transport, and reduces recombination losses. As a result, OSCs treated with DBN achieved a remarkable power conversion efficiency (PCE) of 18.5%, with an open-circuit voltage (VOC) of 0.848 V, a high short-circuit current density (JSC) of 28.15 mA cm-2, and an enhanced fill factor (FF) of 77.7%. Adding an anti-reflection MgF2 layer further boosts efficiency to 19.0%, setting a new benchmark for PM6:Y6 binary devices. This study establishes DBN as a promising crystallization regulator and presents a robust strategy for morphology control, advancing the development of high-performance photovoltaic applications.

Multilength-Scale Morphological Engineering for Stable Organic Solar Cells

Organic solar cells (OSCs) have garnered significant attention owing to the light weight, flexibility, and low cost. Continuous improvement in molecular design, morphology control, and device fabrication has propelled the power conversion efficiency of OSCs beyond 20%. While obtaining long-term device stability is still a critical obstacle for the commercialization of OSCs. The nano- and microstructural characteristics of the active layer morphology-including molecular stacking, phase separation, and domain sizes-play a pivotal role in determining device performance. Consequently, a comprehensive understanding of how film structure impacting device stability and the methods to control film morphology are vital for improving device lifetime. This review seeks to elucidate the structure-performance relationship between active layer morphology from the nanoscale to microscale and device stability. It can provide rational guidance to enhance device stability from morphology control, accelerating the commercialization of 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.

Morphology Regulation Is Achieved by Volatile Solid Additives in Halogen-Free Solvents to Fabricate Efficient Polymer Solar Cells

The meticulous control of micromorphology in high power conversion efficiency (PCE) of polymer solar cells (PSCs) typically relies on halogenated solvents, which pose serious threats to both environmental sustainability and human health. In this work, a green and efficient method for fabricating high PCE PSCs with halogen-free solvents is developed. By introducing volatile solid additives 1-bromo-2,6-dichlorobenzene (DIB) and 1-bromo-2,3,5-trichlorobenzene (TIB) into toluene solvents, the aggregation behaviors of PM6:L8-BO were meticulously regulated, forming distinct fibrous morphology; in detail, the micromorphology of vertical direction exhibited a distinct pattern of acceptor enrichment at the top and donor enrichment at the bottom, which leads to enhanced exciton dissociation efficiency, improved charge transport performance, significantly reducing charge recombination, and finally improved PCEs, as the maximum PCEs were 18.56 and 17.67%, respectively, which are notably higher than those of devices without additives. Furthermore, since the solid additives can be completely removed from the active layer, the additive-treated devices exhibit superior morphology and photovoltaic stability. This work, therefore, unveils a straightforward and environmentally friendly method for preparing efficient PSCs, which is instrumental in facilitating the large-scale commercialization of PSC technology.

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.

Asymmetric small-molecule acceptor enables suppressed electron-vibration coupling and minimized driving force for organic solar cells

Minimizing the energy loss, particularly the non-radiative energy loss (ΔEnr), without sacrificing the charge collection efficiency, is the key to further improve the photovoltaic performance of organic solar cells (OSCs). Herein, we proposed an asymmetric molecular design strategy, via developing alkyl/thienyl hybrid side chain based asymmetric small molecule acceptors (SMAs) BTP-C11-TBO and BTP-BO-TBO, to manipulate the intermolecular interactions to realize enhanced luminescence efficiency and reduced energy loss. Theoretical and experimental results indicate that compared to the three symmetric SMAs BTP-DC11, BTP-DTBO and BTP-DBO, the asymmetric SMAs BTP-C11-TBO and BTP-BO-TBO exhibit repressed electron-vibration coupling and reduced ΔEnr. Moreover, the asymmetric nature of BTP-BO-TBO allows the formation of multiple D:A interfacial conformations and interfacial energies, which have made the charge-transfer state energies closer to that of the strongly absorbing (and emitting) local-exciton state, thus gaining the low ΔEnr while maintaining efficient exciton dissociation. Consequently, the PM6:BTP-BO-TBO-based OSCs achieve a higher power conversion efficiency of 19.76%, with a high open circuit voltage of 0.913 V and an efficient fill factor of 81.17%, profiting from the more improved and balanced charge mobility and longer carrier lifetime. This work provides molecular design ideas to suppress nonradiative decay and paves the way to obtain high-performance OSCs.

Achieving 19.5% Efficiency via Modulating Electronic Properties of Peripheral Aryl-Substituted Small-Molecule Acceptors

The advancement of acceptors plays a pivotal role in determining photovoltaic performance. While previous efforts have focused on optimizing acceptor-donor-acceptor1-donor-acceptor (A-DA1-D-A)-typed acceptors by adjusting side chains, end groups, and conjugated extension of the electron-deficient central A1 unit, the systematic exploration of the impact of peripheral aryl substitutions, particularly with different electron groups, on the A1 unit and its influence on device performance is still lacking. In this study, three novel acceptors - QxTh, QxPh, and QxPy - with distinct substitutions on the quinoxaline (Qx) are designed and synthesized. Density functional theory (DFT) analyses reveal that QxPh, featuring a phenyl-substituted Qx, exhibits the smallest molecular binding energies and a tightest π···π stacking distance. Consequently, the PM6:QxPh device demonstrates a better power conversion efficiency (PCE) of 17.1% compared to the blends incorporating QxTh (16.4%) and QxPy (15.7%). This enhancement is primarily attributed to suppressed charge recombination, improved charge extraction, and more favorable molecular stacking and morphology. Importantly, introducing QxPh as a guest acceptor into the PM6:BTP-eC9 binary system yields an outstanding PCE of 19.5%, indicating the substantial potential of QxPh in advancing ternary device performance. The work provides deep insights into the expansion of high-performance organic photovoltaic materials through peripheral aryl substitution strategy.

The anti-correlation effect of alkyl chain size on the photovoltaic performance of centrally extended non-fullerene acceptors

Contrary to previous results, a unique anti-correlation effect of the alkyl chain size on the photovoltaic performance of acceptors was observed. For a centrally-extended acceptor, replacing linear alkyl chains (n-undecyl for CH-BBQ) on the thienothiophene unit with branched ones (2-butyloctyl for CH-BO) leads to a plunge in the power conversion efficiency of organic solar cells (18.12% vs. 11.34% for binary devices), while the largely shortened ones (n-heptyl for CH-HP) bring a surge in performance (18.74%/19.44% for binary/ternary devices). Compared with CH-BO, the more compact intermolecular packing of CH-HP facilitates carrier transport. The characterization of organic field effect transistors and carrier dynamics also echoes the above results. Molecular dynamics simulations indicate that the encounter of the branched alkyl chains and the extended central core hinders the effective interfacial interaction of polymer donors and acceptors, thus deteriorating the device performance. This work suggests that the conventional strategy for alkyl chain engineering of Y-series acceptors might need to be reconsidered in other molecular systems.

Tackling Energy Loss in Organic Solar Cells via Volatile Solid Additive Strategy

The energy loss induced open-circuit voltage (VOC) deficit hampers the rapid development of state-of-the-art organic solar cells (OSCs), therefore, it is extremely urgent to explore effective strategies to address this issue. Herein, a new volatile solid additive 1,4-bis(iodomethyl)cyclohexane (DIMCH) featured with concentrated electrostatic potential distribution is utilized to act as a morphology-directing guest to reduce energy loss in multiple state-of-art blend system, leading to one of highest efficiency (18.8%) at the forefront of reported binary OSCs. Volatile DIMCH decreases radiative/non-radiative recombination induced energy loss (ΔE2/ΔE3) by rationally balancing the crystallinity of donors and acceptors and realizing homogeneous network structure of crystal domain with reduced D-A phase separation during the film formation process and weakens energy disorder and trap density in OSCs. It is believed that this study brings not only a profound understanding of emerging volatile solid additives but also a new hope to further reduce energy loss and improve the performance of OSCs.

Correlating Aggregation Ability of Polymer Donors with Film Formation Kinetics for Organic Solar Cells with Improved Efficiency and Processability

Film formation kinetics significantly impact molecular processability and power conversion efficiency (PCE) of organic solar cells. Here, two ternary random copolymerization polymers are reported, D18─N-p and D18─N-m, to modulate the aggregation ability of D18 by introducing trifluoromethyl-substituted pyridine unit at para- and meta-positions, respectively. The introduction of pyridine unit significantly reduces material aggregation ability and adjusts the interactions with acceptor L8-BO, thereby leading to largely changed film formation kinetics with earlier phase separation and longer film formation times, which enlarge fiber sizes in blend films and improve carrier generation and transport. As a result, D18─N-p with moderate aggregation ability delivers a high PCE of 18.82% with L8-BO, which is further improved to 19.45% via interface engineering. Despite the slightly inferior small area device performances, D18─N-m shows improved solubility, which inspires to adjust the ratio of meta-trifluoromethyl pyridine carefully and obtain a polymer donor D18─N-m-10 with good solubility in nonhalogenated solvent o-xylene. High PCEs of 13.07% and 12.43% in 1 cm2 device and 43 cm2 module fabricated with slot-die coating method are achieved based on D18─N-m-10:L8-BO blends. This work emphasizes film formation kinetics optimization in device fabrication via aggregation ability modulation of polymer donors for efficient devices.

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.

The Influence of Donor/Acceptor Interfaces on Organic Solar Cells Efficiency and Stability Revealed through Theoretical Calculations and Morphology Characterizations

End-groups halogenation strategies, generally refers to fluorination and chlorination, have been confirmed as simple and efficient methods to regulate the photoelectric performance of non-fullerene acceptors (NFAs), but a controversy over which one is better has existed for a long time. Here, two novel NFAs, C9N3-4F and C9N3-4Cl, featured with different end-groups were successfully synthesized and blended with two renowned donors, D18 and PM6, featured with different electron-withdrawing units. Detailed theoretical calculations and morphology characterizations of the interface structures indicate NFAs based on different end-groups possess different binding energy and miscibility with donors, which shows an obvious influence on phase-separation morphology, charge transport behavior and device performance. After verified by other three pairs of reported NFAs, a universal conclusion obtained as the devices based on D18 with fluorination-end-groups-based NFAs and PM6 with chlorination-end-groups-based NFAs generally show excellent efficiencies, high fill factors and stability. Finally, the devices based on D18: C9N3-4F and PM6: C9N3-4Cl yield outstanding efficiency of 18.53 % and 18.00 %, respectively. Suitably selecting donor and regulating donor/acceptor interface can accurately present the photoelectric conversion ability of a novel NFAs, which points out the way for further molecular design and selection for high-performance and stable organic solar cells.

Central Core Substitutions and Film-Formation Process Optimization Enable Approaching 19% Efficiency All-Polymer Solar Cells

Molecular interactions and film-formation processes greatly impact the blend film morphology and device performances of all-polymer solar cells (all-PSCs). Molecular structure, such as the central cores of polymer acceptors, would significantly influence this process. Herein, the central core substitutions of polymer acceptors are adjusted and three quinoxaline (Qx)-fused-core-based materials, PQx1, PQx2, and PQx3 are synthesized. The molecular aggregation ability and intermolecular interaction are systematically regulated, which subsequently influence the film-formation process and determine the resulting blend film morphology. As a result, PQx3, with favorable aggregation ability and moderate interaction with polymer donor PM6, achieves efficient all-PSCs with a high power conversion efficiency (PCE) of 17.60%, which could be further improved to 18.06% after carefully optimizing device annealing and interface layer. This impressive PCE is one of the highest values for binary all-PSCs based on the classical polymer donor PM6. PYF-T-o is also involved in promoting light utilization, and the resulting ternary device shows an impressive PCE of 18.82%. In addition, PM6:PQx3-based devices exhibit high film-thickness tolerance, superior stability, and considerable potential for large-scale devices (16.23% in 1 cm2 device). These results highlight the importance of structure optimization of polymer acceptors and film-formation process control for obtaining efficient and stable all-PSCs.

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.

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.