Efficient Organic Solar Cells Research Articles

Tuning of the Polymeric Nanofibril Geometry via Side-Chain Interaction toward 20.1% Efficiency of Organic Solar Cells.

Constructing fibril morphology has been believed to be an effective method of achieving efficient exciton dissociation and charge transport in organic solar cells (OSCs). Despite emerging endeavors on the fibrillization of organic semiconductors via chemical structural design or physical manipulation, tuning of the fibril geometry, i.e., width and length, for tailored optoelectronic properties remains to be studied in depth. In this work, a series of alkoxythiophene additives featuring varied alkyl side chains connected to thiophene are designed to modulate the growth of fibril aggregates in cutting-edge polymer donors PM6 and D18. Molecular dynamics simulations and morphological characterizations reveal that these additives preferentially locate near and entangle with the side chains of polymer donors, which enhance the conjugated backbone stacking of polymer donors to form nanofibrils with the width expanding from 12.6 to 21.8 nm and the length increasing from 98.3 to 232.7 nm. This nanofibril structure is feasible to acquire efficient exciton dissociation and charge transport simultaneously. By integrating the fibril PM6 and L8-BO as the donor and acceptor layers in pseudo-bulk heterojunction (p-BHJ) OSCs via layer-by-layer deposition, an improvement of power conversion efficiency (PCE) from 18.7% to 19.8% is observed, contributed by enhanced light absorption, charge transport, and reduced charge recombination. The versatility of these additives is also verified in D18:L8-BO OSCs, with enhanced PCE from 19.3% to 20.1%, which is among the highest values reported for OSCs.

Examining the Impact of 3D Multi-Arm Small Molecules on PM6 : Y6 Blend Reveals the Key Requirements for Their Electronic Properties.

While the emergence of PM6 : Y6 active layer re-energized the organic photovoltaic community, excessive aggregation of Y6 molecules induced by their strong intermolecular interactions has limited the performance of PM6 : Y6-based organic solar cells (OSCs). Adding 3D multi-arm small-molecule acceptors is an effective strategy to inhibit such aggregation. However, to maximize OSC efficiency, these molecules should also contribute to the electronic processes. Here, by taking a benzotriazole-based 3D four-arm small molecule (i.e., SF-BTA1) as representative example, we combine molecular dynamics simulations and density functional theory calculations to examine the molecular-scale impact of 3D multi-arm small molecules on morphological characteristics (especially at the nanoscale) and electronic properties of PM6 : Y6 blends. By considering the intermolecular packing distances, density, and patterns among PM6, Y6, and SF-BTA1 components, exciton transfer rates from SF-BTA1 to Y6 or PM6, charge transfer rates from Y6 or PM6 to SF-BTA1, electron/hole transfer rates among adjacent Y6/PM6 pairs, and radiative and non-radiative recombination processes, we draw a comprehensive picture that describes how 3D multi-arm small molecules improve morphological and electronic properties of PM6 : Y6 blends and thus the OSC efficiency. Furthermore, successful rationalization of these aspects allows us to point out key requirements regarding the electronic properties of 3D multi-arm small molecules.

Enhancing Mechanical Durability and Long-Term Stability in Organic Solar Cells via Flexible Linker-Sequential Block Copolymerized Donors.

Multi-component copolymerized donors (MCDs) hold great promise for improving both the efficiency and mechanical robustness of flexible organic solar cells (f-OSCs) owing to their facile molecular tunability and advantageous one-pot copolymerization. However, despite the excellent crystallinity imparted by their highly conjugated polymer backbone, MCDs often struggle to retain photovoltaic performance under large external deformations, limiting their applicability in wearable devices. Herein, we developed a novel series of flexible linker-sequential block MCDs (Fs-MCDs), specifically PM6-Cl0.8-b-D18-Cl0.2-BTB, PM6-Cl0.8-b-D18-Cl0.2-BTH, and PM6-Cl0.8-b-D18-Cl0.2-BTD, by precisely incorporating flexible functional groups into the conjugated polymer skeleton. This design strategy introduced highly effective tensile active sites, resulting in remarkable mechanical durability, with PM6-Cl0.8-b-D18-Cl0.2-BTD achieving crack-onset strain (COS) values of 49.88 % in pristine films and 31.29 % in blends. The nearly 50 % COS in pristine films represents one of the highest values reported for Fs-MCD-based OSCs, marking a significant milestone in advancing f-OSC. Additionally, PM6-Cl0.8-b-D18-Cl0.2-BTD demonstrated excellent photovoltaic performance, with efficiencies of 18.09 % in rigid binary and 19.05 % in ternary, as well as 16.63 % in flexible OSCs. It also showed impressive device stability in invert OSC (T80=9,078 h). This unique molecular design strategy provides a promising avenue for synergistically improving the photovoltaic performance, mechanical properties, and device stability of f-OSCs.

Dual-Donor-Induced Crystallinity Modulation Enables 19.23% Efficiency Organic Solar Cells

Article HighlightsBy modulating the crystalline properties of the active layer with dual donors, the efficiency of organic solar cells reaches 19.23%.The introduction of PTzBI-dF suppresses the accumulation of traps and charge recombination, allowing ternary devices to maintain 82% of their initial power conversion efficiency (PCE) after illumination for 800 h.The dual-donor strategy for modulating the crystallinity of the active layer is applicable to a variety of Y6 derivatives, and the increase in PCE exceeds 1%.

U-Shaped Dimeric Acceptors for Balancing Efficiency and Stability in Organic Solar Cells.

Despite significant improvements in power conversion efficiencies (PCEs) of organic solar cells (OSCs), achieving excellent stability remains a great challenge to their commercial feasibility. Here, U-shaped dimeric acceptors (5-IDT and 6-IDT) with different molecular lengths are introduced into the binary OSCs as a third component, respectively. The introduction of the third component effectively reduces the energetic disorder and non-radiative voltage losses and improves the exciton dissociation and charge transport of the devices. Consequently, the PCEs of the 6-IDT- and 5-IDT-treated OSCs are significantly improved to 19.32% and 19.96%, respectively, which is the highest PCE for oligomeric acceptors-based ternary OSCs to date. Meanwhile, the thermal stability of the treated devices is dramatically improved, with the initial efficiency retention of the 6-IDT- and 5-IDT-treated devices increasing from 18% to 32% and 75%, respectively, after 1000h of thermal stress. This is mainly attributed to the ability of the smaller molecular length of 5-IDT to stabilize the phase-separated morphology of the polymeric donor and small molecular acceptor, rather than the high glass transition temperature and low diffusion coefficient.

Improving Molecular Arrangement and Alleviating Nonradiative Energy Loss Using a Chlorinated Pyrido[3,4‐b]Quinoxaline‐Core‐Based Acceptor for High‐Performance Organic Solar Cells

AbstractThe electron‐deficient A1 unit in A‐DA1D‐A structured acceptors is critical for optimizing the efficiency of organic solar cells (OSCs). Drawing inspiration from the high performance of the previously reported pyrido[2,3‐b]quinoxaline‐core acceptors, Py6, an isomer of Py1 is designed with a repositioned pyridine nitrogen atom, and further modified it by chlorinating Py6 to create Py7. Theoretical calculations show that chlorine incorporation strengthens intermolecular non‐covalent interactions and promotes the tighter molecular stacking, as confirmed by grazing‐incidence wide‐angle X‐ray scattering. Consequently, D18/Py7 device delivers the enhanced fill factor and short‐circuit current density, compared to D18/Py1 and D18/Py6 device. Notably, D18/Py7 device also yields a higher open‐circuit voltage of 0.871 V, significantly outperforming Py1 (0.764 V) and Py6 (0.723 V), due to the low nonradiative energy losses. Further studies reveal that introducing Cl directs hole density toward the central pyrido[3,4‐b]quinoxaline unit and decreases the charge transfer state ratio of D18/acceptor. This prompts triplet‐to‐singlet conversion and reduces non‐radiative recombination losses. Additionally, using a mutual donor–acceptor dilution strategy, the (D18:1wt.% Py7)/(Py7:1wt.% D18) device achieves an impressive efficiency of 19.60%. This work emphasizes the great potential of the Py‐series acceptors and demonstrates that chlorine incorporation effectively reduces non‐radiative losses.

Use of BODIPY and BORANIL Dyes to Improve Solar Conversion in the Fabrication of Organic Photovoltaic Cells Through the Co-Sensitization Method

This work addresses the implementation of the co-sensitization technique to increase the energy efficiency of organic dye-sensitized solar cells (DSSCs). Fluorescent dyes derived from boron complexes— (BORANIL) and (BODIPY)— were successfully synthesized and used as co-sensitizers in different volume percentage ratios to verify the most effective concentration for photon capture through these sensitizers. The dyes were optically characterized using ultraviolet–visible spectroscopy (UV-VIS) and Fourier transform infrared (FTIR), analyzing them through the optical performance of each hybrid combination of dyes, an optimization of the photon collection capacity in the tests performed in a volume percentage ratio of 25:75 or 1:3. The morphology and surface roughness of the electrodes were analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. Through electrochemical characterizations, it was found that the highest photovoltaic conversion efficiency was obtained with the ATH1005 (D) dye mixed with ATH032 (G) in the proportion of 25%:75% or DG 1:3, with efficiency (η) of 3.45%, against 2.43% and 1.90% for DG 1:1 and DG 3:1 cells, respectively. Cells with BODIPY dyes also present higher conversion efficiencies compared to BORANIL cells. The results corroborate the presentation of organic solar cells as a viable option for electricity generation.

Does the substrate and a buffer layer have a greater influence on poly(3-hexylthiophene) films deposited by the chronocoulometry technique?

Abstract In this study, we investigate the interface morphology and optical properties of electrochemical poly(3-hexylthiophene) (P3HT) films deposited by electrochemical synthesis using the chronocoulometry technique on tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) and how the substrate used influences in the deposition of the films and their optical and morphological properties, as well as the buffer layer and the interface effect, with and without spin-coating films of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). P3HT polymeric films were synthesized and deposited via the oxidation of the 3-hexylthiophene (3-HT) monomer and characterized via Raman spectroscopy. The morphology and thickness of the P3HT layer present in the samples ITO/P3HT, FTO/P3HT, ITO/PEDOT:PSS/P3HT, and FTO/PEDOT:PSS/P3HT were carried out using atomic force microscopy (AFM). The optical and electronic gap energy of P3HT were calculated from UV‒Vis spectra and cyclic voltammetry curves, respectively. The photoluminescence (PL) spectra showed broad and asymmetrical line shapes fitted by multi-Gaussian functions identifying different emission species. Emission ellipsometry spectroscopy was performed to study the energy transference between adjacent polymer chains. Our results show a higher linear polarized light emitted by ITO/PEDOT:PSS/P3HT film, ∼37%, thus demonstrating a significant decrease in the energy transfer. Based on these results, the efficiency of organic solar cells can be improved via the interaction of the polymer/polymer interface.

Y6-Derived Non-fullerene Acceptors with Unsaturated Alkyl Side Chains Enabling Improved Molecular Packing for Highly Efficient Additive-Free Organic Solar Cells

Y6-Derived Non-fullerene Acceptors with Unsaturated Alkyl Side Chains Enabling Improved Molecular Packing for Highly Efficient Additive-Free Organic Solar Cells

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

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

Reducing Non-Radiative Energy Losses in Non-Fullerene Organic Solar Cells.

With the rapid advancement of non-fullerene acceptors (NFAs), the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 20 % threshold, highlighting their considerable potential as next-generation energy conversion devices. In comparison to inorganic or perovskite solar cells, the open-circuit voltage (Voc) of OSCs is constrained by substantial non-radiative energy losses (ΔEnr), leading to values notably below those anticipated by the Shockley-Queisser limit. In OSCs, non-radiative energy losses are intimately associated with the electroluminescent quantum efficiency (EQEEL) of charge transfer states, which is in turn directly affected by the photoluminescence quantum yield (PLQY) of acceptor materials. Consequently, enhancing the PLQY of low-bandgap acceptor materials has emerged as a pivotal strategy to effectively mitigate ΔEnr. This review article delves into the intrinsic correlation between molecular structure and PLQY from the vantage point of acceptor material design. It further explores methodologies for designing acceptor materials exhibiting high PLQY, with the ultimate goal of realizing OSCs that combine high efficiency with minimal ΔEnr.

Heterojunction Organic Solar Cells with Efficient Charge Mobility and Separation Capabilities Studied by DFT.

The properties of the active layer materials play a decisive role in determining the power conversion efficiency of organic solar cells (OSCs). Chlorophyll and its derivatives are abundant and environmentally friendly functional organic molecular materials. Using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), we have calculated the absorption spectra and their excited state properties based on optimized ground state structures. It was found that bacteriochlorin exhibits superior structural properties, a smaller energy gap and hole reorganization energy, redshifted absorption spectra, and higher hole mobility compared to the donor D18. This suggests that bacteriochlorin exhibits superior donor properties. Comparative studies between o-AT-2Cl and m-AT-2Cl showed that o-AT-2Cl had superior acceptor properties, implying that differences in substitution positions can influence the physicochemical properties of non-fullerene acceptors (NFAs). Subsequently, six bulk heterojunctions (BHJs) were constructed by combining three donors with nonfused ring electron acceptors, o-AT-2Cl and m-AT-2Cl. The bacteriochlorin-based BHJs performed well among them, with BChl3/o-AT-2Cl and BChl4/o-AT-2Cl having the largest interfacial charge separation rate. The results suggested that BHJs composed of bacteriochlorin and NFAs can improve OSCs' photovoltaic performance, providing a feasible scheme for designing efficient OSCs.

Fluorine/bromine/selenium multi-heteroatoms substituted dual-asymmetric electron acceptors for o-xylene processed organic solar cells with 19.12% efficiency

AbstractThe development of high-performance near-infrared (NIR) absorbing electron acceptors is a major challenge in achieving high short-circuit current density (JSC) to increase power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, three new multi-heteroatomized Y-series acceptors (bi-asy-Y-Br, bi-asy-Y-FBr, and bi-asy-Y-FBrF) were developed by combining dual-asymmetric selenium-fused core and brominated end-groups with different numbers of fluorine substitutions. With gradually increasing fluorination, three acceptors exhibit red-shift absorption. Among them, bi-asy-Y-FBrF presents planar molecular geometry, the maximum average electrostatic potential, and the minimum molecular dipole moment, which are conducive to intramolecular packing and charge transport. Moreover, D18:bi-asy-Y-FBrF active layer presents higher crystallinity, more suitable phase separation, and reduced charge recombination compared to D18:bi-asy-Y-Br and D18:bi-asy-Y-FBr blends. Consequently, among theses binary OSCs, D18:bi-asy-Y-FBrF device achieves a higher PCE of 15.74% with an enhanced JSC of 26.28 mA cm−2, while D18:bi-asy-Y-Br device obtains a moderate PCE of 15.04% with the highest open-circuit voltage (VOC) of 0.926 V. Inspired by its high VOC and complementary absorption with NIR-absorbing BTP-eC9 as acceptor, bi-asy-Y-Br is introduced into binary D18:BTP-eC9 to construct ternary OSCs, achieving a further boosted PCE of 19.12%, which is among the top values for the reported green solvent processed OSCs.

Efficient and Stable Organic Solar Cells Achieved by Synergistic Optimization of Extended End‐Capped Groups and Fluorinated Quinoxaline Central Cores in Nonfullerene Acceptors

Efficient and Stable Organic Solar Cells Achieved by Synergistic Optimization of Extended End‐Capped Groups and Fluorinated Quinoxaline Central Cores in Nonfullerene Acceptors

Integrated Omnidirectional Design of Non‐Volatile Solid Additive Enables Binary Organic Solar Cells with Efficiency Exceeding 19.5 %

AbstractSolid additives have drawn great attention due to their numerous appealing benefits in enhancing the power conversion efficiencies (PCEs) of organic solar cells (OSCs). To date, various strategies have been reported for the selection or design of non‐volatile solid additives. However, the lack of a general design/evaluation principles for developing non‐volatile solid additives often results in individual solid additives offering only one or two efficiency‐boosting attributes. In this work, we propose an integrated omnidirectional strategy for designing non‐volatile solid additives. By validating the method on the 4,5,9,10‐pyrene diimide (PyDI) system, a novel non‐volatile solid additive named PyMC5 was designed. PyMC5 is capable of enhancing device performance by establishing synergistic dual charge transfer channels, forming appropriate interactions with active layer materials, reducing non‐radiative voltage loss and optimizing film morphology. Notably, the binary device (PM6 : L8‐BO) treated by PyMC5 achieved a PCE over 19.5 %, ranking among the highest reported to date. In addition, the integration of PyMC5 mitigated the degradation process of the devices under photo‐ and thermal‐stress conditions. This work demonstrates an efficient integrated omnidirectional approach for designing non‐volatile solid additives, offering a promising avenue for further advancements in OSC development.

Integrated Omnidirectional Design of Non-Volatile Solid Additive Enables Binary Organic Solar Cells with Efficiency Exceeding 19.5 .

Solid additives have drawn great attention due to their numerous appealing benefits in enhancing the power conversion efficiencies (PCEs) of organic solar cells (OSCs). To date, various strategies have been reported for the selection or design of non-volatile solid additives. However, the lack of a general design/evaluation principles for developing non-volatile solid additives often results in individual solid additives offering only one or two efficiency-boosting attributes. In this work, we propose an integrated omnidirectional strategy for designing non-volatile solid additives. By validating the method on the 4,5,9,10-pyrene diimide (PyDI) system, a novel non-volatile solid additive named PyMC5 was designed. PyMC5 is capable of enhancing device performance by establishing synergistic dual charge transfer channels, forming appropriate interactions with active layer materials, reducing non-radiative voltage loss and optimizing film morphology. Notably, the binary device (PM6 : L8-BO) treated by PyMC5 achieved a PCE over 19.5 %, ranking among the highest reported to date. In addition, the integration of PyMC5 mitigated the degradation process of the devices under photo- and thermal-stress conditions. This work demonstrates an efficient integrated omnidirectional approach for designing non-volatile solid additives, offering a promising avenue for further advancements in OSC development.

Conjugated Group Tuning of Self‐Assembled Monolayer for Efficient Hole‐Transport Layer in Organic Solar Cells

AbstractP‐type carbazole‐derived self‐assembled monolayers (SAMs) have garnered significant attention as promising hole transport layers (HTLs) in the development of highly efficient organic solar cells (OSCs). However, it still lacks the effective navigation to modulate the terminal functional groups of SAMs to achieve a compromise between the highest occupied molecular orbital (HOMO) energy levels and self‐aggregation behavior. Herein, the terminal functional groups are adjusted and three SAMs are synthesized, namely, t‐Bu‐3PACz, Ph‐3PACz, and Bz‐3PACz to comprehensively investigate their intrinsic properties and influence on photovoltaic performance. Among them, Ph‐3PACz featuring an exceptionally suitable conjugated region and steric hindrance exhibits the best compatibility with the active layer, superior electrical conductivity, HOMO energy level aligning with polymer donor, and ordered film packing. As a result, the photovoltaic devices based on Ph‐3PACz exhibit an open‐circuit voltage (VOC) of 0.850 V, a short‐circuit current density (JSC) of 28.7 mA cm−2, and a fill factor (FF) of 78.5%, thus resulting in a remarkable power conversion efficiency (PCE) of 19.2%. This work provides an effective and easily navigable method to modulate the molecular packing and energy levels of SAMs, thereby achieving highly efficient OSCs.

Fluorination or Not in Small Molecule Solar Cells: Achieving a Higher Efficiency with Halogen-Free End Group.

Fluorination is an efficient strategy for improving organic solar cells (OSCs) efficiency, particularly by fluorinating the end group of emerging nonfullerene acceptors. Here, the fluorination effect was investigated by using small molecule donors with fluorine-free (SBz) and fluorinated (SBz-F) end groups, paired with the emerging nonfullerene acceptor Y6. Interestingly and unexpectedly, fluorination of the end group negatively affects OSCs efficiency, with fluorine-free SBz:Y6 OSCs achieving a higher power conversion efficiency (PCE) of 11.05 % compared to the fluorine-containing SBz-F:Y6 blends (PCE=9.61 %). Analysis of space-charge limited currents reveals lower and unbalanced hole/electron mobility in SBz-F:Y6 compared to the SBz:Y6 blends. These findings are further supported by charge recombination dynamics and donor-acceptor miscibility analyses.

Regiospecific Incorporation of Fluorine Atoms in Polythiophene Derivatives for Efficient Organic Solar Cells.

Derivatives of polythiophene (PT) have garnered considerable attention in organic solar cells (OSCs) because of their relatively uncomplicated molecular structures and cost-effective synthesis. Herein, we have developed two regioisomeric fluorinated PT donors, PEI3T-FITVT and PEI3T-FOTVT, to realize efficient OSCs. PEI3T-FITVT and PEI3T-FOTVT are strategically designed with different fluorine atom arrangements on thiophene-vinyl-thiophene (TVT) units. Notably, PEI3T-FOTVT possesses enhanced backbone planarity induced by F···S noncovalent interactions between two constituent building blocks. Consequently, PEI3T-FOTVT with the higher aggregation and crystalline properties leads to a 2.5-fold increase in hole mobility over PEI3T-FITVT (from 1.4 × 10-4 to 3.6 × 10-4 cm2 V-1 s-1). Furthermore, PEI3T-FOTVT exhibits higher domain purity than PEI3T-FITVT, leading to faster charge transport and reduced charge recombination in OSC devices. These characteristics lead to a higher power conversion efficiency of 14.4% for PEI3T-FOTVT-based OSCs, compared to 12.9% for PEI3T-FITVT-based OSCs.

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

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