Organic Photovoltaic Materials Research Articles

Providing a Photovoltaic Performance Enhancement Relationship from Binary to Ternary Polymer Solar Cells via Machine Learning

Ternary polymer solar cells (PSCs) are currently the simplest and most efficient way to further improve the device performance in PSCs. To find high-performance organic photovoltaic materials, the established connection between the material structure and device performance before fabrication is of great significance. Herein, firstly, a database of the photovoltaic performance in 874 experimental PSCs reported in the literature is established, and three different fingerprint expressions of a molecular structure are explored as input features; the results show that long fingerprints of 2D atom pairs can contain more effective information and improve the accuracy of the models. Through supervised learning, five machine learning (ML) models were trained to build a mapping of the photovoltaic performance improvement relationship from binary to ternary PSCs. The GBDT model had the best predictive ability and generalization. Eighteen key structural features from a non-fullerene acceptor and the third components that affect the device’s PCE were screened based on this model, including a nitrile group with lone-pair electron, a halogen atom, an oxygen atom, etc. Interestingly, the structural features for the enhanced device’s PCE were essentially increased by the Jsc or FF. More importantly, the reliability of the ML model was further verified by preparing the highly efficient PSCs. Taking the PM6:BTP-eC9:PY-IT ternary PSC as an example, the PCE prediction (18.03%) by the model was in good agreement with the experimental results (17.78%), the relative prediction error was 1.41%, and the relative error between all experimental results and predicted results was less than 5%. These results indicate that ML is a useful tool for exploring the photovoltaic performance improvement of PSCs and accelerating the design and application with highly efficient non-fullerene materials.

19.36% Efficiency Organic Solar Cells Based on Low‐Cost Terpolymer Donors with Simple Molecular Structures

AbstractLow cost and high performance are key elements for the industrialization of organic solar cells (OSCs). In view of this, developing highly‐efficient organic photovoltaic molecules with simple chemical structure is one of the most effective countermeasures. Herein, three simple‐structure terpolymers PTQ12‐5, PTQ12‐10, and PTQ12‐15 are designed and synthesized, by embedding difluoro‐substituted bithiophene (2T‐2F) unit into the backbone of low‐cost and high‐efficiency polymer PTQ10. The synthesis of three terpolymers contains only two‐step synthetic routes and over 85% synthetic yields from cheap raw compounds, thus exhibiting distinctly low‐cost characteristic. Remarkably, PTQ12‐5 shows significantly higher absorbance coefficient and hole mobility than PTQ10. Consequently, PTQ12‐5‐based OSCs shows more efficient exciton dissociation and charge transfer, suppressed carrier recombination, tighter molecular π‐π stacking, and faster and more balanced charge transport, thus demonstrates a significantly improved efficiency of 18.77% than that of PTQ10‐based device (18.03%). Moreover, an outstanding efficiency of 19.36% is achieved in PTQ12‐5‐based ternary device by further modulating the energy level alignment and blending features of photoactive molecules, which is the highest efficiency of OSCs based on low‐cost polymers to date. This work has significant implications for guiding the design of low‐cost and high‐performance organic photovoltaic materials.

A Simple and Promising Prediction Model to Analyze the Optical Properties of Organic Photovoltaic Materials

Optimizing light utilization is crucial in organic photovoltaics. Understanding the intricate connection between the optical properties and the chemical structure of organic materials is pivotal yet challenging in this regard. Herein, over 2800 published reports with a database of ≈300 organic non‐fullerene acceptors (NFAs) are surveyed and a mathematical model suitable for predicting and analyzing the optical properties of organic photovoltaic materials is established. The model is used to predict the optical properties of representative NFAs within experimental error, including four newly synthesized organic materials to validate the model. In addition, the reliability and applicability of the model through data transformation are demonstrated and it is found that the addition of double bonds and asymmetry in the chemical structure does not necessarily reduce the optical bandgap of organic materials. Based on the model, the strong noncovalent interaction is more significant than the weak noncovalent interaction and asymmetry on the reduction of the bandgap, which provides new insights into the design and development of organic photovoltaic materials with tunable optical properties.

Synthesis, Thermotropic Properties, and Applications of Porphyrin-based Liquid Crystals: A Comprehensive Review

Abstract: Research on novel discotic molecules, which consist of a rigid core with flexible peripheral chains, has gained much attention due to their crucial role as organic photovoltaic materials, organic field-effect transistors, and semiconductors for photocurrent generation, as well as the possibility of their other optoelectronic applications. This review article describes the developments in fundamental design ideas and synthetic approaches of porphyrin-based meso and beta-substituted liquid crystals. In addition, the current review highlights the various structural alterations made by the researchers in the field of porphyrin-based mesogens and changes in properties, both for materials intended for commercially successful liquid crystal displays, including other applications, and for more basic purposes of demonstrating structure-property relationships.

Indium(III) complexes: application as organic catalyst, precursor for chalcogenides nanoparticles and starting materials in the industry

AbstractTrivalent indium ions are not toxic like the common ions, and this implies that there are limited secondary pollution associated with their photoconversion processes. They are therefore a preferred choice for several applications. However, they are not used in their ionic form. Their salts are reacted with various organic ligands to form different trivalent complexes of indium for these applications. Herein, the use of different trivalent complexes as catalysts in organic synthesis are discussed. Their usage as a precursor of indium oxide, indium sulfide, indium tellurides and indium selenides are examined. In addition, their application as organic light-emitting diodes (OLEDs), sensor, photovoltaic and luminous materials are discussed. Finally, gaps and/or areas for future research are highlighted.

Toward Self-Driven Autonomous Material and Device Acceleration Platforms (AMADAP) for Emerging Photovoltaics Technologies.

ConspectusIn the ever-increasing renewable-energy demand scenario, developing new photovoltaic technologies is important, even in the presence of established terawatt-scale silicon technology. Emerging photovoltaic technologies play a crucial role in diversifying material flows while expanding the photovoltaic product portfolio, thus enhancing security and competitiveness within the solar industry. They also serve as a valuable backup for silicon photovoltaic, providing resilience to the overall energy infrastructure. However, the development of functional solar materials poses intricate multiobjective optimization challenges in a large multidimensional composition and parameter space, in some cases with millions of potential candidates to be explored. Solving it necessitates reproducible, user-independent laboratory work and intelligent preselection of innovative experimental methods.Materials acceleration platforms (MAPs) seamlessly integrate robotic materials synthesis and characterization with AI-driven data analysis and experimental design, positioning them as enabling technologies for the discovery and exploration of new materials. They are proposed to revolutionize materials development away from the Edisonian trial-and-error approaches to ultrashort cycles of experiments with exceptional precision, generating a reliable and highly qualitative data situation that allows training machine learning algorithms with predictive power. MAPs are designed to assist the researcher in multidimensional aspects of materials discovery, such as material synthesis, precursor preparation, sample processing and characterization, and data analysis, and are drawing escalating attention in the field of energy materials. Device acceleration platforms (DAPs), however, are designed to optimize functional films and layer stacks. Unlike MAPs, which focus on material discovery, a central aspect of DAPs is the identification and refinement of ideal processing conditions for a predetermined set of materials. Such platforms prove especially invaluable when dealing with "disordered semiconductors," which depend heavily on the processing parameters that ultimately define the functional properties and functionality of thin film layers. By facilitating the fine-tuning of processing conditions, DAPs contribute significantly to the advancement and optimization of disordered semiconductor devices, such as emerging photovoltaics.In this Account, we review the recent advancements made by our group in automated and autonomous laboratories for advanced material discovery and device optimization with a strong focus on emerging photovoltaics, such as solution-processing perovskite solar cells and organic photovoltaics. We first introduce two MAPs and two DAPs developed in-house: a microwave-assisted high-throughput synthesis platform for the discovery of organic interface materials, a multipurpose robot-based pipetting platform for the synthesis of new semiconductors and the characterization of thin film semiconductor composites, the SPINBOT system, which is a spin-coating DAP with the potential to optimize complex device architectures, and finally, AMANDA, a fully integrated and autonomously operating DAP. Notably, we underscore the utilization of a robot-based high-throughput experimentation technique to address the common optimization challenges encountered in extensive multidimensional composition and parameter spaces pertaining to organic and perovskite photovoltaics materials. Finally, we briefly propose a holistic concept and technology, a self-driven autonomous material and device acceleration platform (AMADAP) laboratory, for autonomous functional solar materials discovery and development. We hope to discover how AMADAP can be further strengthened and universalized with advancing development of hardware and software infrastructures in the future.

The Effect of Fluorinated Benzothiadiazole-dicyanovinyl Acceptors on the Dielectric Constants of Organic Photovoltaic Materials

The Effect of Fluorinated Benzothiadiazole-dicyanovinyl Acceptors on the Dielectric Constants of Organic Photovoltaic Materials

2,5-dichloro-3,4-diiodothiophene as a versatile solid additive for high-performance organic solar cells

Volatile solid additive (SA) with diverse chemical structures has been proved to be an alternative to prepare high-efficient organic solar cells (OSCs). While, various organic photovoltaic materials raise challenges in selecting suitable SAs during the device optimization. Herein, based on the simple and commonly used thiophene unit, we proposed a perhalogenated 2,5-dichloro-3,4-diiodothiophene (SA-T5) as the SA for the device optimizations. As results, the capacity of SA-T5 in optimizing molecular packings of small-molecular acceptor L8-BO, dimeric acceptor DL8, and polymeric acceptor PY-DT based blend films has been demonstated. Their optimized morphologies assisted by SA-T5 contribute to the suppressed charge recombination and enhanced charge transportation, resulting in their improved short-circuit current density and fill factor, simultaneously. In comparsion with as-cast devices, SA-T5-treated devices showed significantly improved power conversion efficiencies (PCEs) of 18.8 % for PM6:L8-BO, 17.8 % for PM6:DL8, and 17.6 % for PM6:PY-DT, respectively. Furthermore, the potential of SA-T5 in obtaining high-performance ternary OSCs was also confirmed, for example, a 18.1 % efficiency was realized in PM6:DL8:L8-BO based device. To our knowledge, the performance of these SA-T5-treated devices reaches the best results of different acceptor-based OSCs. Importantly, this work provides a rare and successful case to develop versatile SAs for high-performance OSCs.

Tetracyclic aromatic lactam-substituted tin-free donors for high-efficiency ternary organic solar cells via regulation of pre-aggregation and crystallization kinetics

Organic solar cells (OSCs) are rapidly developed for next-generation photovoltaic technologies due to low-cost and solution-processability etc. However, so far high-efficiency organic photovoltaic materials are typically prepared by multi-step Stille reactions, which are neither cost-efficient nor atom-economic, even raise safety concerns and incompatible with large-scale process. Large-scale industrial production of photovoltaic materials through simple, cost-effective, and green chemistry remains to be addressed. In this work, we present a case where two isomeric small-molecule donors based on polycyclic aromatic lactams (PAL) unit, named oPh2F and pPh2F with wide-bandgap and high-crystallinity, were designed and synthesized by atom-economic direct C-H activation strategy, modulates pre-aggregation and crystallization kinetics of host system PM6:Y6, induce distinct morphology evolution and effectively boost photovoltaic performance. The best power conversion efficiency (PCE) of 18.40 % for pPh2F-based ternary devices are found to be a fruit of improved light-harvesting ability, favorable vertical phase separation with enhanced crystallinity, the separated crystallization process of host materials and the higher and more balanced charge transport. To our knowledge, this study achieved the highest PCE among PM6:Y6-based ternary OSCs containing tin-free small-molecule donor, which not only highlights the great potential of PAL-based photovoltaic materials, but also promotes the integration of renewable energy with green chemistry.

Designing Thiadiazoloquinoxaline-Based Conjugated Polymers for Efficient Organic Photovoltaics: A DFT/TDDFT Study.

Clean and renewable energy development is becoming frontier research for future energy resources, as renewable energy offers sustainable and environmentally friendly alternatives to non-renewable sources such as fossil fuels. Among various renewable energy sources, tremendous progress has been made in converting solar energy to electric energy by developing efficient organic photovoltaics. Organic photovoltaic materials comprising conjugated polymers (CP) with narrow optical energy gaps are promising candidates for developing sustainable sources due to their potentially lower manufacturing costs. Organic semiconductor materials with a high electron affinity are required for many optoelectronic applications. We have designed a series of organic semiconductors comprised of cyclopentadithiophene as a donor and thiadiazoloquinoxaline (TQ) as an acceptor, varying the π-conjugation and TQ-derivatives. We have employed density functional theory (DFT) and time-dependent DFT (TDDFT) to evaluate the designed CP's optoelectronic properties, such as optical energy gap, dipole moment, and absorption spectra. Our DFT/TDDFT result shows that the energy gap of CPs is lowered and redshifted in the absorption spectra if there is no insertion of conjugation units such as thiophene and selenophene between donor and acceptor. In addition, selenophene shows relatively better redshift behavior compared to thiophene. Our work also provides rational insight into designing donor/acceptor-based CPs for organic solar cells.

Molecular Design of Fully Nonfused Acceptors for Efficient Organic Photovoltaic Cells.

The nonfused thiophene-benzene-thiophene (TBT) unit offers advantages in obtaining low-cost organic photovoltaic (OPV) materials due to its simple structure. However, OPV cells, including TBT-based acceptors, exhibit significantly lower energy conversion efficiencies. Here, we introduce a novel approach involving the design and synthesis of three TBT-based acceptors by substituting different position-branched side chains on the TBT unit. In comparison to TBT-10 and TBT-11, TBT-13, which exclusively incorporates α-position branched side chains with a large steric hindrance, demonstrates a more planar and stable conformation. When blended with the donor PBQx-TF, TBT-13-based blend film achieves favorable π-π stacking and aggregation characteristics, resulting in excellent charge transfer performance in the corresponding device. Due to the simultaneous enhancements in short-circuit current density and fill factor, the TBT-13-based OPV cell obtains an outstanding efficiency of 16.1%, marking the highest value for the cells based on fully nonfused acceptors. Our work provides a practical molecular design strategy for high-performance and low-cost OPV materials.

Achieving High Fill Factor in Organic Photovoltaic Cells by Tuning Molecular Electrostatic Potential Fluctuation

AbstractIn the field of organic photovoltaics (OPVs), significant progress has been made in tailoring molecular structures to enhance the open‐circuit voltage and the short‐circuit current density. However, there remains a crucial gap in the development of coordinated material design strategies focused on improving the fill factor (FF). Here, we introduce a molecular design strategy that incorporates electrostatic potential fluctuation to design organic photovoltaic materials. By reducing the fluctuation amplitude of IT‐4F, we synthesized a new acceptor named ITOC6‐4F. When using PBQx‐TF as a donor, the ITOC6‐4F‐based cell shows a markedly low recombination rate constant of 0.66×10−14 cm3 s−1 and demonstrates an outstanding FF of 0.816, both of which are new records for binary OPV cells. Also, we find that a small fluctuation amplitude could decrease the energetic disorder of OPV cells, reducing energy loss. Finally, the ITOC6‐4F‐based cell creates the highest efficiency of 16.0 % among medium‐gap OPV cells. Our work holds a vital implication for guiding the design of high‐performance OPV materials.

Achieving High Fill Factor in Organic Photovoltaic Cells by Tuning Molecular Electrostatic Potential Fluctuation.

In the field of organic photovoltaics (OPVs), significant progress has been made in tailoring molecular structures to enhance the open-circuit voltage and the short-circuit current density. However, there remains a crucial gap in the development of coordinated material design strategies focused on improving the fill factor (FF). Here, we introduce a molecular design strategy that incorporates electrostatic potential fluctuation to design organic photovoltaic materials. By reducing the fluctuation amplitude of IT-4F, we synthesized a new acceptor named ITOC6-4F. When using PBQx-TF as a donor, the ITOC6-4F-based cell shows a markedly low recombination rate constant of 0.66×10-14 cm3 s-1 and demonstrates an outstanding FF of 0.816, both of which are new records for binary OPV cells. Also, we find that a small fluctuation amplitude could decrease the energetic disorder of OPV cells, reducing energy loss. Finally, the ITOC6-4F-based cell creates the highest efficiency of 16.0 % among medium-gap OPV cells. Our work holds a vital implication for guiding the design of high-performance OPV materials.

Enhanced Photovoltaic Performance of Asymmetrical Benzo Dithiophene Homopolymer Donor Materials in Nonfullerene Acceptor-Based Organic Photovoltaics.

Although much promising synthetic progress in conjugated polymer-based organic solar cells (OSCs) has resulted in significant improvement in power conversion efficiencies (PCEs) of from over 15 to >19.0% in the last five years, the sophisticated and complex reactions from at least two families' monomers with remarkably different electron push-pull effects could still pose an unavoidable material burden for the commercialization of OSCs in the coming future. Therefore, the method of preparing a homopolymer from a sole monomer would significantly reduce the synthetic steps and costs in order to pave the way for the large-scale production of OSC materials. Therefore, alkylthio-thiophenyl-substituted benzo[1,2-b;4,5-b']dithiophene (BDTTS) as the sole and key structural moiety with dihalogen and distannyl functional groups was designed and synthesized, respectively, in this study, for facile monomer syntheses and polymerizations to achieve three wide-bandgap homopolymer donors of BDTTS-alt-BDTT-Cl (P13), BDTTS-alt-BDTT (P15), and BDTTS (P14), respectively. The structural symmetry dependency on their physical, electrochemical, and optical properties, thin-film morphologies, and photovoltaic (PV) performance was investigated in detail. As a result, OSCs based on the asymmetric polymer P15, paired with BTP-eC9 as the electron acceptor, presented the best PV performance, with a PCE of 11.5%, a fill factor (FF) of 65.87%, and a short-circuit current (JSC) of 22.04 mA·cm-2, respectively. This PCE value is among the highest ones reported for BDT-type homopolymer donor-based OPVs, providing us with knowledge for obtaining promising PV performance from devices made of P15-like materials.

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT.

Conjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties.

Organic Photovoltaic Materials for Solar Fuel Applications: A Perfect Match?

This work discusses the use of donor and acceptor materials from organic photovoltaics in solar fuel applications. These two routes to solar energy conversion have many shared materials design parameters, and in recent years there has been increasing overlap of the molecules and polymers used in each. Here, we examine whether this is a good approach, where knowledge can be translated, and where further consideration to molecular design is required.

Real-time monitoring polymerization degree of organic photovoltaic materials toward no batch-to-batch variations in device performance

Polymerization degree plays a vital role in material properties. Previous methodologies of molecular weight control generally cannot suppress or alleviate batch-to-batch variations in device performance, especially in polymer solar cells. Herein, we develop an in-situ photoluminescence system in tandem with a set of analysis and processing procedures to track and estimate the polymerization degree of organic photovoltaic materials. To support the development of this protocol, we introduce polymer acceptor PYT constructed by near-infrared Y-series small molecule acceptors via Stille polymerization, and shed light on the correlations between molecular weight, spectral parameters, and device efficiencies that enable the design of the optical setup and confirm its feasibility. The universality is verified in PYT derivatives with stereoregularity and fluoro-substitution as well as benzo[1,2-b:4,5-b’]dithiophene-based polymers. Overall, our result provides a tool to tailor suitable conjugated oligomers applied to polymer solar cells and other organic electronics for industrial scalability and desired cost reduction.

Wide‐Bandgap Polymers with a C(sp3)─F Polyfluoride Backbone Enable High‐Efficient Ternary Organic Solar Cells

AbstractFluorination strategy is demonstrated to be a successful approach for optimizing the electron distribution and morphology of organic photovoltaic materials. The previous works focus on introducing only a few C(sp2)─F bonds into conjugated backbone or C(sp3)─F bonds into sidechains. Herein, a new strategy by introducing C(sp3)─F polyfluoride unit into the backbone is proposed, wherein the fluorine atoms are not involved into the conjugation but can promote the intermolecular interaction between backbones. Two wide‐bandgap fluoropolymers are prepared and employed as the third component for ternary organic solar cells. As expected, even if there are six fluorine atoms in a single repeat unit, the relevant fluoropolymers possess complementary absorption and aligned energy levels. More importantly, the polyfluoride backbone affords adequate non‐covalent interactions, consequently enhancing the polymer aggregation and packing order, which is verified by a fibril‐like morphology in the blend film with the host polymer PM6 and only 10 wt.% fluoropolymer. In addition, the decreased surface energy caused by polyfluoride unit is beneficial for the improvement of domain purity and the formation of nanoscale phase separation between donor and acceptor materials. As a result, the fluoropolymer‐assisted ternary device displays a higher efficiency of 18.74% compared with the binary device (17.39%).

Suppressing Exciton‐Vibration Coupling to Prolong Exciton Lifetime of Nonfullerene Acceptors Enables High‐Efficiency Organic Solar Cells

AbstractThe limited exciton lifetime (τ, generally <1 ns) leads to short exciton diffusion length (LD) of organic semiconductors, which is the bottleneck issue impeding the further improvement of power conversion efficiencies (PCEs) for organic solar cells (OSCs). However, efficient strategies to prolong intrinsic τ are rare and vague. Herein, we propose a facile method to efficiently reduce vibrational frequency of molecular skeleton and suppress exciton‐vibration coupling to decrease non‐radiative decay rate and thus prolong τ via deuterating nonfullerene acceptors. The τ remarkably increases from 0.90 ns (non‐deuterated L8‐BO) to 1.35 ns (deuterated L8‐BO‐D), which is the record for organic photovoltaic materials. Besides, the inhibited molecular vibration improves molecular planarity of L8‐BO‐D for enhanced exciton diffusion coefficient. Consequently, the LD increases from 7.9 nm (L8‐BO) to 10.7 nm (L8‐BO‐D). The prolonged LD of L8‐BO‐D enables PM6 : L8‐BO‐D‐based bulk heterojunction OSCs to acquire higher PCEs of 18.5 % with more efficient exciton dissociation and weaker charge carrier recombination than PM6 : L8‐BO‐based counterparts. Moreover, benefiting from the prolonged LD, D18/L8‐BO‐D‐based pseudo‐planar heterojunction OSCs achieve an impressive PCE of 19.3 %, which is among the highest values. This work provides an efficient strategy to increase the τ and thus LD of organic semiconductors, boosting PCEs of OSCs.

Suppressing Exciton-Vibration Coupling to Prolong Exciton Lifetime of Nonfullerene Acceptors Enables High-Efficiency Organic Solar Cells.

The limited exciton lifetime (τ, generally <1 ns) leads to short exciton diffusion length (LD ) of organic semiconductors, which is the bottleneck issue impeding the further improvement of power conversion efficiencies (PCEs) for organic solar cells (OSCs). However, efficient strategies to prolong intrinsic τ are rare and vague. Herein, we propose a facile method to efficiently reduce vibrational frequency of molecular skeleton and suppress exciton-vibration coupling to decrease non-radiative decay rate and thus prolong τ via deuterating nonfullerene acceptors. The τ remarkably increases from 0.90 ns (non-deuterated L8-BO) to 1.35 ns (deuterated L8-BO-D), which is the record for organic photovoltaic materials. Besides, the inhibited molecular vibration improves molecular planarity of L8-BO-D for enhanced exciton diffusion coefficient. Consequently, the LD increases from 7.9 nm (L8-BO) to 10.7 nm (L8-BO-D). The prolonged LD of L8-BO-D enables PM6 : L8-BO-D-based bulk heterojunction OSCs to acquire higher PCEs of 18.5 % with more efficient exciton dissociation and weaker charge carrier recombination than PM6 : L8-BO-based counterparts. Moreover, benefiting from the prolonged LD , D18/L8-BO-D-based pseudo-planar heterojunction OSCs achieve an impressive PCE of 19.3 %, which is among the highest values. This work provides an efficient strategy to increase the τ and thus LD of organic semiconductors, boosting PCEs of OSCs.