Non-fullerene Organic Solar Cells Research Articles

Molecular models of PM6 for non-fullerene acceptor organic solar cells: How DAD and ADA structures impact charge separation and charge recombination.

The quadrupole moment of a non-fullerene acceptor (NFA) generated by the constituent electron donor (D) and acceptor (A) units is a significant factor that affects the charge separation (CS) and charge recombination (CR) processes in organic photovoltaics (OPVs). However, its impact on p-type polymer domains remains unclear. In this study, we synthesized p-type molecules, namely acceptor-donor-acceptor (ADA) and donor-acceptor-donor (DAD), which are components of the benchmark PM6 polymer (D: benzodithiophene and A: dioxobenzodithiophene). Planar heterojunction films, a model of bulk heterojunction, were prepared using ADA, DAD, and PM6 as the bottom p-type layers and Y6 NFA as the top n-type layer. Flash-photolysis time-resolved microwave conductivity, femtosecond transient absorption spectroscopy, and quantum mechanical calculations were employed to probe the charge carrier dynamics. Our findings reveal that while the subtle difference in quadrupole moment and energy gradient of the p-type materials has a minimal influence on CS, the molecular type (ADA or DAD) significantly affects the bulk CR. This study expands the understanding of how the p-type component and its conformation at the p/n interface impact the CS and CR in OPVs, highlighting the critical role of molecular donors in optimizing device performance.

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.

Ionized Phenanthroline Derivatives Suppressing Interface Chemical Interactions with Active Layer for High-efficiency Organic Solar Cells with Exceptional Device Stability.

The contact interface between the charge transport interlayer and the active layer is crucial for the non-fullerene organic solar cells (NF OSCs) to achieve high efficiency and long-term stability. In this study, two novel phenanthroline (Phen) derivatives, tbp-Phen and tbp-PhenBr, are developed as efficient cathode interfacial materials (CIMs). The larger steric hindrance substituents and the ionization of nitrogen atoms on the Phen framework jointly enable the tbp-PhenBr CIM with a stable film morphology and immensely suppress the detrimental interface chemical interactions with the NF active layer. Consequently, tbp-PhenBr-based OSC achieves a higher efficiency (PCE=16.34%) than bathocuproine (BCP)-based control device (PCE=13.70%) using PM6:Y6 as the active layer. More importantly, the tbp-PhenBr-based device maintains 80% of its initial efficiency (T80) for 3264 h in dark conditions and 220 h after being heated at 85°C, significantly outperforming the BCP-based device. The tbp-PhenBr CIM also shows broad applicability across various binary and ternary active layer systems, affording a notable PCE of 19.49%. Additionally, the tbp-PhenBr CIM can be processed via a thermal evaporation technique and the prepared devices exhibit high reproducibility. This work provides innovative insights into the molecular design of the CIMs for stable and efficient NF OSCs.

Distributions and evolution of trap states in non-fullerene organic solar cells

Distributions and evolution of trap states in non-fullerene organic solar cells

Synergistic charge-transfer dynamics of novel benzothiadiazole-based donor materials for higher power conversion efficiency: From structural engineering to efficiency assessment in non-fullerene organic solar cells

In the realm of organic solar cell technology, current research is dedicated to enhancing the photovoltaic properties of donor-π-acceptor (D-π-A) materials to achieve higher power conversion efficiencies (PCE). This optimization focuses particularly on fine-tuning the conduction band and electrolytic characteristics to maximize performance. Addressing the growing demand for novel materials with enhanced optoelectronic properties in organic photovoltaic research, our proposed compound BT05, one of nine new benzothiadiazole-based D-π-A donor molecules (BT01-BT09), exhibits a power conversion efficiency (PCE) of 25%, surpassing the 18% PCE of the reference compound BTD-OMe. TD-DFT and DFT simulations illuminate how donor modifications enhance the photovoltaic characteristics of the proposed molecules. Higher open-circuit voltage (VOC) of 1.74-2.26 V, increase in binding energy (∼1.997), λmax (470-476 nm), reduction in energy gap (4.25-4.65 eV), also validates the PCE results and confirm the usefulness of designed molecules (BT01-BT09). Moreover, DHOMO and ALUMO, TDM, reorganization energy λe (0.0124-0.0134) and λh (0.0094-0.0098), and NPA results also confirm that BT01-BT09 molecules unlock the organic solar cell's potential and advance sustainable energy solutions through innovative technology. Among all developed compounds, BT05 displays higher VOC (2.26 V), 87% fill-factor, and 25% PCE; hence, it is recommended in future solar cell applications.

A Facile Low Prevacuum Treatment to Enhance the Durability of Nonfullerene Organic Solar Cells

Herein, a straightforward vacuum‐assisted method is introduced to enhance the stability of nonfullerene organic solar cells (OSCs). The method, termed “prevacuum” involves subjecting the active layer (D18:Y6) to a low‐pressure vacuum (−1 bar) before thermal annealing at 100 °C. Compared to untreated devices, prevacuum‐treated OSCs exhibit a notable increase in power conversion efficiency from 13.71% to 14.90%. This enhancement is attributed to improved light absorption and charge extraction, as evidenced by external quantum efficiency measurements. Moreover, prevacuum treatment significantly improves device stability under operational conditions, with a 30% power loss occurring after 8.25 h compared to 4.5 h for untreated devices. This improvement is attributed to the removal of volatile components and impurities during the vacuum process, leading to a more hydrophobic and stable active layer. The study demonstrates the efficacy of prevacuum treatment as a simple and accessible method for enhancing the performance and longevity of OSCs, paving the way for their broader application in sustainable energy technologies.

Photoinduced charge transfer in small-molecule organic solar cells dependent on external electric field

In this paper, MTDATA is selected as the electron donor and BPhen as the acceptor. The trianiline structure of the donor molecule provides a variety of charge transfer pathways for charge separation. Based on Marcus theory, the photoinduced charge transfer properties of non-fullerene acceptor organic solar cells (NFA-OSCs) in external electric field (Fext) were studied. By analyzing the changes of reorganization energy (λ), Gibbs free energy (ΔG), electron transfer integral (Vda) and charge transfer rate under the influence of Fext, it was found that Fext can effectively regulate the charge transfer. Moreover, the change of charge transfer rate under Fext is similar to Vda, indicating that Vda is the main factor affecting charge transfer rate. To a certain extent, this study provides theoretical support for improving the photoelectric performance of NFA-OSCs.

Temperature-dependent charge transport measurements unveil morphological insights in non-fullerene organic solar cells

The morphological analysis of bulk heterojunction (BHJ) active layer stands as a critical imperative for advancing the performance of future organic solar cells. Conventional characterization tools employed for morphological investigation often require substantial resources, both in cost and physical space, thereby imposing restraints on research endeavors in this domain. Here, we extend the application of charge carrier transport characterization beyond conventional mobility assessments, utilizing it as a table-top method for preliminary morphological screening in organic thin films. The investigation focuses on several high-performance BHJ systems that utilize typical “Y” non-fullerene acceptors. It involves in-depth transport studies, including temperature- and field-dependent transport characterizations. The resulting transport data are analyzed in detail using the Gaussian disorder model to extract key transport parameters, specifically the high-temperature limited mobility (μ∞) and positional disorder (∑). Integrating these transport parameters with morphological insights obtained through various characterization tools—including x-ray scattering, sensitive spectroscopy, and quantum chemistry simulation—provides a deep understanding of the intricate interplay between charge transport properties and morphological characteristics. The results reveal explicit relationships, associating μ∞ with the degree of molecular stacking in BHJs and ∑ with the structural disorder in molecule skeleton. Our findings point to the promising potential of utilizing a simple transport characterization technique for the early stage evaluation of thin film packing and geometric properties of organic materials.

(Invited) Influence of Vibronic Interaction of Charge Transfer Excitons in PTB7/BTA-Based Nonfullerene Organic Solar Cells

For the design of rational D/A interfaces, it is important to elucidate the dynamic processes of excitons generated at the D/A stacking structure interface and the photoelectric conversion mechanism. We predict that the charge-transfer excitons initially formed are weakly bound electron-hole polaron pairs, which dissociate without relaxing to the charge-transfer state and diffuse as free electron polarons and hole polarons. We consider this to be a nonadiabatic process due to oscillatory interactions in the initial charge-transfer excitonic state. In this study, we use time-dependent DFT to investigate the electronic structure, electron-hole distance, electron coupling in charge transfer state generated by photoexcitaion and the Huang- Rhys factors of the D/A complex BTAx, PCTBT/BTAx and PDCBT/BTAx (x = 1, 3)) in the excited state and the exciton-phonon coupling and nonadiabatic process. Based on the above, we aim to elucidate the role of vibronic interactions in CT excitonic states and the control of JSC and electron-hole recombination.

The Double-Cross of Benzotriazole-Based Polymers as Donors and Acceptors in Non-Fullerene Organic Solar Cells.

Organic solar cells (OSCs) are considered a very promising technology to convert solar energy to electricity and a feasible option for the energy market because of the advantages of light weight, flexibility, and roll-to-roll manufacturing. They are mainly characterized by a bulk heterojunction structure where a polymer donor is blended with an electron acceptor. Their performance is highly affected by the design of donor-acceptor conjugated polymers and the choice of suitable acceptor. In particular, benzotriazole, a typical electron-deficient penta-heterocycle, has been combined with various donors to provide wide bandgap donor polymers, which have received a great deal of attention with the development of non-fullerene acceptors (NFAs) because of their suitable matching to provide devices with relevant power conversion efficiency (PCE). Moreover, different benzotriazole-based polymers are gaining more and more interest because they are considered promising acceptors in OSCs. Since the development of a suitable method to choose generally a donor/acceptor material is a challenging issue, this review is meant to be useful especially for organic chemical scientists to understand all the progress achieved with benzotriazole-based polymers used as donors with NFAs and as acceptors with different donors in OSCs, in particular referring to the PCE.

Inhibiting interfacial transport loss for efficient organic nonfullerene solar cells and photodetectors

Nonfullerene organic solar cells (OSCs) and photodetectors have received tremendous interest due to their rapidly progressed power conversion efficiency (PCE) and wide range photoresponse to near-infrared region, respectively. Further optimization of the interfacial transport layer is one of the key factors toward enhanced performance. Herein, we reported a general multi-component electron transport layer (ETL) strategy to achieve better energy level alignments and interfacial contact for both OSCs and photodetectors. The binary polymer:molecule blend based ETL can overcome low crystallinity and self-aggregation issue in neat polymer and molecule ETL, respectively. The mixed blend provides a more tunable platform to optimize the interfacial morphology and creates more efficient charge-transporting pathways. We showcase that the PNDIT-F3N:PDINN binary ETL exhibits its strength in a series of nonfullerene OSCs with enhanced fill factor and current density, achieving a champion PCE approaching 19%. Additionally, self-powered organic photodetectors with lower dark current and high detectivity were achieved with the same binary ETL strategy. Detailed morphology and device characterizations reveal that the binary ETL modulates the interfacial interface to deliver a more favorable energy level alignment, facilitating carrier extraction and transport. We believe these findings could provide insight into the design of ETL with sufficient interfacial tunability for organic optoelectronic devices.

A Theoretical Investigation for Exploring the Potential Performance of Non-Fullerene Organic Solar Cells Through Side-Chain Engineering Having Diphenylamino Groups to Enhance Photovoltaic Properties.

The development of ecofriendly fabrication phenomenon is essential requirement for commercialization of non-fullerene acceptors. Recently, end-capped modeling is employed for computational design of five non-fullerene acceptors to elevate various photovoltaic properties. All new molecules are formulated by altering the peripheral acceptors of CH3-2F and DFT methodology is employed to explore the opto-electronic, morphological and charge transfer analysis. From the computational investigation, all reported molecules manifested red shifted absorption with remarkable reduced band gap. Among investigated molecules, FA1-FA3 evinced effectively decreased value of band gaps and designed molecules have low excitation energy justifying proficient charge transference. The lower values of binding energy of FA1 and FA2 suggest their facile exciton dissociation leading to improved charge mobility. By blending with J61 donor, FA4 have sufficiently enhanced value of VOC (1.72eV) and fill factor (0.9228). Energy loss of the model (R) is 0.57eV and statistical calculation demonstrate that all our modified molecules except FA3 has profoundly reduced energy loss compelling in its pivotal utilization. From accessible supportive outcomes of recent investigation, it is recommended that our modified chromophore exhibit remarkable noteworthy applications in solar cells for forthcoming innovations.

Central aromatic-core fluorination in non-fullerene acceptor advances binary organic solar cells to a record efficiency of 19.7%

Central aromatic-core fluorination in non-fullerene acceptor advances binary organic solar cells to a record efficiency of 19.7%

Wide Bandgap Polymer Donors Based on Succinimide-Substituted Thiophene for Nonfullerene Organic Solar Cells.

The advent of nonfullerene acceptors (NFAs) has greatly improved the photovoltaic performance of organic solar cells (OSCs). However, to compete with other solar cell technologies, there is a pressing need for accelerated research and development of improved NFAs as well as their compatible wide bandgap polymer donors. In this study, a novel electron-withdrawing building block, succinimide-substituted thiophene (TS), is utilized for the first time to synthesize three wide bandgap polymer donors: PBDT-TS-C5, PBDT-TSBT-C12, and PBDTF-TSBT-C16. These polymers exhibit complementary bandgaps for efficient sunlight harvesting and suitable frontier energy levels for exciton dissociation when paired with the extensively studied NFA, Y6. Among these donors, PBDTF-TSBT-C16 demonstrates the highest hole mobility and a relatively low highest occupied molecular orbital (HOMO) energy level, attributed to the incorporation of thiophene spacers and electron-withdrawing fluorine substituents. OSC devices based on the blend of PBDTF-TSBT-C16:Y6 achieve the highest power conversion efficiencyof 13.21%, with a short circuit current density (Jsc) of 26.83mA cm-2, an open circuit voltage (Voc) of 0.80V, and a fill factorof 0.62. Notably, the Voc × Jsc product reaches 21.46mW cm-2, demonstrating the potential of TS as an electron acceptor building block for the development of high-performance wide bandgap polymer donors in OSCs.

Designing efficient materials for high-performance of non-fullerene organic solar cells through side-chain engineering on DBT-4F derivatives by non-fused-ring electron acceptors.

For the advancement in fields of organic and perovskite solar cells, various techniques of structural alterations are being employed on previously reported chromophores. In this study, the end-capped engineering is carried out on DBT-4F (R) by modifying terminal acceptors to improve optoelectronic and photovoltaic attributes. Seven molecules (AD1-AD7) are modeled using different push-pull acceptors. DFT/B3LYP/6-31G along with its time-dependent approach (TD-DFT) are on a payroll to investigate ground state geometries, absorption maxima (λmax), energy gap (Eg), excitation energy (Ex), internal reorganization energy, light harvesting efficiency (LHE), dielectric constant, open circuit voltage (VOC), fill factor (FF), etc. of OSCs. AD1 displayed the lowest band gap (1.76eV), highest λmax (876nm), lowest Ex (1.41eV), and lowest binding energy (0.21eV). Among various calculated parameters, all of the sketched molecules demonstrated greater dielectric constant when compared to R. The highest dielectric constant was exhibited by AD3 (56.26). AD5 exhibited maximum LHE (0.9980). Lower reorganization energies demonstrated improved charge mobility. AD5 and AD7 (1.63 and 1.68eV) have higher values of VOC than R (1.51eV). All novel molecules having outperforming attributes will be better candidates to enhance the efficacy of OSCs for future use. Precisely, a DFT and TD-DFT analysis on all of the proposed organic molecules were conducted, using the functional MPW1PW91 at 6-31G (d,p) basis set to examine their optoelectronic aspects, additionally the solvent-state computations were studied with a TD-SCF simulation. For all these simulations, Guassian 09 and GuassView 5.0 were employed. Moreover, the Origin 6.0, Multiwfn 3.8, and PyMOlyze 1.1 software were utilized for the visual depiction of the graphs of absorption, TDM, and DOS, respectively of the studied molecules. A number of crucial aspects such as FMOs, bandgaps, light-harvesting efficiency, electrostatic potential, dipole moment, ionization potential, open-circuit voltage, fill factor, binding energy, interaction coefficient, chemical hardness-softness, and electrophilicity index were also investigated for the studied molecules.

A theoretical investigation for improving the performance of non-fullerene organic solar cells through side-chain engineering of BTR non-fused-ring electron acceptors

In the current quantum chemical study, indacenodithiophene donor core-based the end-capped alterations of the reference chromophore BTR drafted eight A2-A1-D-A1-A2 type small non-fullerene acceptors. All the computational simulations were executed under MPW1PW91/6-31G (d, p) level of DFT. The UV–Vis absorption, open circuit voltage, electron affinity, ionization potential, the density of states, reorganization energy, orbital analysis, and non-covalent interactions were studied and compared with BTR. Several molecules of our modeled series BT1-BT8 have shown distinctive features that are better than those of the BTR. The open circuit voltage (VOC) of BT5 has a favorable impact, allowing it to replace BTR in the field of organic solar cells. The charge carrier motilities for proposed molecules generated extraordinary findings when matched to the reference one (BTR). Further charge transmission was confirmed by creating the complex with a PM6 donor molecule. The remarkable dipole moment contributes to the formation of non-covalent bond interactions with chloroform, resulting in superior charge mobility. Based on these findings, it can be said that every tailored molecule has the potential to surpass chromophore molecule (BTR) in OSCs. So, all tailored molecules may enhance the efficiency of photovoltaic cells due to the involvement of potent terminal electron-capturing acceptor2 moieties. Considering these obtained results, these newly presented molecules can be regarded for developing efficient solar devices in the future.

A 3,3'-Difluoro-2,2'-Bithiophene Based Donor Polymer Realizing High Efficiency (>17%) Single Junction Binary Organic Solar Cells.

In this study, two novel donor-acceptor (D-A) copolymers are designed and synthesized, DTBT-2T and DTBT-2T2F with 2,2'-bithiophene or 3,3'-difluoro-2,2'-bithiophene as the donor unit and dithienobenzothiadiazole as the acceptor unit, and used them as donor materials in non-fullerene organic solar cells (OSCs). Due to enhanced planarity of polymer chains resulted by the intramolecular F···S noncovalent interactions, the incorporation of 3,3'-difluoro-2,2'-bithiophene unit instead of 2,2'-bithiophene into the polymers can enhance their molecular packing, crystallinity and hole mobility. The DTBT-2T:L8-BO based binary OSCs deliver a power conversion efficiency (PCE) of only 9.71% with a Voc of 0.78V, a Jsc of 20.69mAcm-2 , and an FF of 59.67%. Moreover, the introduction of fluoro atoms can lower the highest occupied molecular orbital levels. As a result, DTBT-2T2F:L8-BO based single-junction binary OSCs exhibited less recombination loss, more balanced charge mobility, and more favorable morphology, resulting in an impressive PCE of 17.03% with a higher Voc of 0.89V, a Jsc of 25.40mAcm-2, and an FF of 75.74%. These results indicate that 3,3'-difluoro-2,2'-bithiophene unit can be used as an effective building block to synthesize high performance polymer donor materials. This work greatly expands the selection range of donor units for constructing high-performance polymers.

Balancing Solvent Polarity and Drying‐Kinetics for Processing Nonfullerene Acceptors Toward High‐Performance Organic Solar Cells

In recent years, the emergence of high‐efficiency nonfullerene acceptors (NFAs) has pushed the power conversion efficiency (PCE) beyond 19% in organic solar cells (OSCs). For solution processing of the photoactive layers, the study of solvent properties on device performance has become a hot research topic toward up‐scaling. Herein, the key roles of balancing solvent polarity and drying kinetics for processing of small molecular NFAs toward high‐performance OSCs are revealed. It is demonstrated that the synergistic effect of solvent polarity and drying kinetics significantly affects the multilength scale morphology of solid films and photovoltaic performance. Furthermore, the self‐assembly of NFAs in solution and the resulting thin film microstructures induced by electrostatic interactions with polarized additives are even more sensitive to solvent polarity and drying kinetics. Finally, the optimized devices achieve an outstanding PCE of 18.54% by fine‐tuning the microstructure with polar solvent and fast‐drying kinetics to form improved molecular packing and structure size. The work provides a novel view and deeper insights into the general selection rules of solvents toward the solution processing of high‐performance nonfullerene OSCs.

Critical role of the end-group acceptor in enhancing the efficiency of indacenodithiophene-benzothiadiazole-linked nonfullerene organic solar cells through morphology optimization

Solution-processed organic solar cells (OSCs) have great promise for next-generation solar energy conversion technologies. Innovative acceptors are constantly being designed to improve device efficiency. In this work, we designed and synthesized a series of A–π–D–π–A type non-fullerene acceptor (NFA) molecules, named IB-TOT, IB-TOM, IB-IOO, and IB-IOM for their applications in OSCs. All molecules showed high thermal stability, broad photoabsorption, and relatively lower highest occupied molecular orbital (HOMO) energy levels from −5.48 eV to −5.58 eV. The optimized OSCs based on these NFAs and PTB7-Th donor deliver power conversion efficiencies (PCEs) of 8.19%, 5.50%, 2.42%, and 3.10% for IB-TOT, IB-TOM, IB-IOO, and IB-IOM molecules, respectively. The higher PCE of IB-TOT-based OSCs is attributed to their excellent miscibility and superior interpenetrating network morphology compared to the other three compounds. These results demonstrate that using the 3-ethylrhodanine acceptor unit to construct the NFA molecule is a successful approach for outstanding achievement in OSCs.

CPE-Na-Based Hole Transport Layers for Improving the Stability in Nonfullerene Organic Solar Cells: A Comprehensive Study.

Organic photovoltaic (OPV) cells have experienced significant development in the last decades after the introduction of nonfullerene acceptor molecules with top power conversion efficiencies reported over 19% and considerable versatility, for example, with application in transparent/semitransparent and flexible photovoltaics. Yet, the optimization of the operational stability continues to be a challenge. This study presents a comprehensive investigation of the use of a conjugated polyelectrolyte polymer (CPE-Na) as a hole layer (HTL) to improve the performance and longevity of OPV cells. Two different fabrication approaches were adopted: integrating CPE-Na with PEDOT:PSS to create a composite HTL and using CPE-Na as a stand-alone bilayer deposited beneath PEDOT:PSS on the ITO substrate. These configurations were compared against a reference device employing PEDOT:PSS alone, as the HTL increased efficiency and fill factor. The instruments with CPE-Na also demonstrated increased stability in the dark and under simulated operational conditions. Device-based PEDOT:PSS as an HTL reached T80 after 2500 h while involving CPE-Na in the device kept at T90 in the same period, evidenced by a reduced degradation rate. Furthermore, the impedance spectroscopy and photoinduced transient methods suggest optimized charge transfer and reduced charge carrier recombination. These findings collectively highlight the potential of CPE-Na as a HTL optimizer material for nonfluorine OPV cells.