Materials For Organic Solar Cells Research Articles

“Mutually Doped” Conjugated Polyelectrolyte‐Polyoxometalate Complex as Hole Transporting Material for Efficient Organic Solar Cells

“Mutually Doped” Conjugated Polyelectrolyte‐Polyoxometalate Complex as Hole Transporting Material for Efficient Organic Solar Cells

Theoretical screening of N-[5'-methyl-3'-isoxasolyl]-N-[(E)-1-(-2-thiophene)] methylidene]amine and its isoxazole based derivatives as donor materials for bulk heterojunction organic solar cells: DFT and TD-DFT investigation.

In the present work, the influence of aromatic ring substitution on a series of small-donor organic molecules (A, B, C, D, and E) with isoxazole cores was investigated for photovoltaic applications in organic solar cells. Frontier molecular orbital analysis, chemical reactivity descriptors, dipole moment, and population analysis showed that all the organic materials have intramolecular charge transfer abilities capable of donating electrons to the acceptor material (PCBM). The required photovoltaic parameters such as Voc, FF, Jsc, LHE, and other associated optoelectronic parameters are reported. The results demonstrate that aromatic ring substitution influences charge transfer and power conversion efficiencies of solar cells. That is, an increase in the aromatic character of a material increases its charge transfer, and as a result, its photovoltaic properties are increased. Additionally, all the investigated derivatives are good charge transporters with suitable electron reorganization energies, which are beneficial for minimizing energy loss. Hence, these organic derivatives with isoxazole backbones are promising materials and may provide fresh insights into the design of new materials for organic solar cell applications. All calculations were performed using DFT and the ORCA 4.1.0 program package as the main tool for geometry optimization and frequency calculations. The Avogadro 1.2.1 visualization tool was used to prepare all input files executed by ORCA 4.1.0. The BP86, B3LYP, and wB97M series of functionals coupled with the def2/TZVP basis set were employed for geometry optimization. All energy-related calculations were carried out using the M06-2x functional. Multiwfn version 3.7 was used for aromaticity and population analysis. Excited state and UV-visible spectra were simulated using the TD-DFT method at the CAM-B3LYP-D3, wB97X-D3, and PBE0-D3 coupled with the ma-def2-TZVP basis set. Moreover, solvent effects were incorporated using the SMD scheme as incorporated in the ORCA software. Lastly, the RIJCOSX approximations were used to speed up calculations while maintaining accuracy.

Optimizing Molecular Packing via Steric Hindrance for Reducing Non-Radiative Recombination in Organic Solar Cells.

Innovative molecule design strategy holds promise for the development of next-generation acceptor materials for efficient organic solar cells with low non-radiative energy loss (ΔEnr). In this study, we designed and prepared three novel acceptors, namely BTP-Biso, BTP-Bme and BTP-B, with sterically structured triisopropylbenzene, trimethylbenzene and benzene as side chains inserted into the shoulder of the central core. The progressively enlarged steric hindrance from BTP-B to BTP-Bme and BTP-Biso induces suppressed intramolecular rotation and altered the molecule packing mode in their aggregation states, leading to significant changes in absorption spectra and energy levels. By regulating the intermolecular π-π interactions, BTP-Bme possesses relatively reduced non-radiative recombination rate and extended exciton diffusion lengths. The binary device based on PB2:BTP-Bme exhibits an impressive power conversion efficiency (PCE) of 18.5% with a low ΔEnr of 0.19 eV. Furthermore, the ternary device comprising PB2:PBDB-TF:BTP-Bme achieves an outstanding PCE of 19.3%. The molecule design strategy in this study proposed new perspectives for developing high-performance acceptors with low ΔEnr in OSCs.

Optimizing Molecular Packing via Steric Hindrance for Reducing Non‐Radiative Recombination in Organic Solar Cells

Innovative molecule design strategy holds promise for the development of next‐generation acceptor materials for efficient organic solar cells with low non‐radiative energy loss (ΔEnr). In this study, we designed and prepared three novel acceptors, namely BTP‐Biso, BTP‐Bme and BTP‐B, with sterically structured triisopropylbenzene, trimethylbenzene and benzene as side chains inserted into the shoulder of the central core. The progressively enlarged steric hindrance from BTP‐B to BTP‐Bme and BTP‐Biso induces suppressed intramolecular rotation and altered the molecule packing mode in their aggregation states, leading to significant changes in absorption spectra and energy levels. By regulating the intermolecular π‐π interactions, BTP‐Bme possesses relatively reduced non‐radiative recombination rate and extended exciton diffusion lengths. The binary device based on PB2:BTP‐Bme exhibits an impressive power conversion efficiency (PCE) of 18.5% with a low ΔEnr of 0.19 eV. Furthermore, the ternary device comprising PB2:PBDB‐TF:BTP‐Bme achieves an outstanding PCE of 19.3%. The molecule design strategy in this study proposed new perspectives for developing high‐performance acceptors with low ΔEnr in OSCs.

Design of non-fused ring electron acceptors with “embracing-type” side chains for organic solar cells

Backbone planarity is an inherent structural feature for fused ring electron acceptors and one of important prerequisites for their high photovoltaic performance. However, for non-fused ring electron acceptors (NFREAs), it is a challenging task to achieve it. In this work, two NFREAs (Ph2TIC and BT2TIC) containing thiophene-benzene-thiophene or thiophene-benzothiadiazole-thiophene three non-fused aromatic rings in central scaffold were designed with an attempt to use a pair of 2,4,6-triisopropylphenyl substituents (referred as “embracing”-type side chains) at 3,3′-positions of the two thiophene units to lock the backbone in a planar conformation. X-ray crystallography revealed that Ph2TIC has a quasi-planar backbone, whereas BT2TIC shows a twisted one, but both tend to form two-dimensional layered arrays through intermolecular π-π stacking involving terminal acceptor moieties. UV–vis light-absorption spectroscopy showed that both compounds have similar optoelectronic properties either in dilute solutions or in solid film state, thus confirming the conformation-lock function of the “embracing”-type side chains. While for photophysical properties, BT2TIC displays a red-shifted broader light absorption spectrum as compared to Ph2TIC due to the introduction of an additional acceptor moiety in the structure. Ph2TIC and BT2TIC were evaluated as the acceptor materials in organic solar cells in combination with PM6, a typical donor polymer, and delivered the optimized power conversion efficiencies of 4.31 % and 3.78 %, respectively. Although the photovoltaic performances are modest, the work provides a new thought for maintaining the backbone planarity in NFREAs and would be useful in their future design.

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.

Triflic Acid-Assisted Regioselective Bromination of Quinoxaline Derivatives Enables a Facile Synthesis of Polymer PTQ10.

Poly[(thiophene)-alt-(6,7-difluoro-2(2-hexyldecyloxy)quinoxaline)] (PTQ10) emerges as a promising candidate for donor materials in organic solar cells (OSCs) due to its high efficiency, simplified synthesis, and cost-effectiveness. The acceptor unit of PTQ10 is derived from the alkylation of 5,8-dibromo-6,7-difluoroquinoxaline-2-ol, emphasizing the importance of its economical synthesis for commercial viability. This study investigates triflic acid-assisted regioselective bromination of quinoxaline derivatives and proposes an alternative synthetic pathway for PTQ10. The developed route benefits from concise synthetic steps, a dependable procedure, and high overall yield. Starting with the condensation of 4,5-difluorobenzene-1,2-diamine with ethyl oxoacetate to yield 6,7-difluoroquinoxaline-2-ol, subsequent triflic acid-assisted regioselective bromination produces 5,8-dibromo-6,7-difluoroquinoxaline-2-ol in high yield. Alkylation under Mitsunobu reaction conditions yields 5,8-dibromo-6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline, followed by polymerization with 2,5-distannylated thiophene under Stille reaction conditions to afford PTQ10. This research provides insights into efficient synthetic strategies for PTQ10, advancing its potential for commercial application in OSCs.

Exploring structure-property landscape of non-fullerene acceptors for organic solar cells.

We present a comprehensive analysis of the structure-property relationship in small molecule non-fullerene acceptors (NFAs) featuring an acceptor-donor-acceptor configuration employing state-of-the-art quantum chemical computational methods. Our focus lies in the strategic functionalization of halogen groups at the terminal positions of NFAs as an effective means to mitigate non-radiative voltage losses and augment photovoltaic and photophysical properties relevant to organic solar cells. Through photophysical studies, we observe a bathochromic shift in the visible region for all halogen-functionalized NFAs, except type-2, compared to the unmodified compound. Most of these functionalized compounds exhibit exciton binding energies below 0.3eV and ΔLUMO less than 0.3eV, indicating their potential as promising candidates for organic solar cells. Selected candidate structures undergo an analysis of charge transport properties using the semi-classical Marcus theory based on hopping transport formalism. Molecular dynamics simulations followed by charge transport simulations reveal an ambipolar nature of charge transport in the investigated NFAs, with equivalent hole and electron mobilities compared to the parent compound. Our findings underscore the crucial role of end-group functionalization in enhancing the photovoltaic and photophysical characteristics of NFAs, ultimately improving the overall performance of organic solar cells. This study advances our understanding of the structure-property relationships in NFAs and provides valuable insights into the design and optimization of organic solar cell materials.

Designing of low band gap organic semiconductors through data mining from multiple databases and machine learning assisted property prediction

Bandgap is a key parameter for selecting suitable materials for a broad range of applications. Organic solar cells (OSCs) are emerging as powerful devices due to their low-cost solution processing. Developing OSCs necessitates producing effective materials in a computationally cost-effective and rapid manner. Machine learning has become popular and well-recognized among researchers to screen and design high performance materials for OSCs. Machine learning models require data from the literature (reported studies or databases) to effectively predict targeted properties. To unveil the hidden dataset patterns, a thorough data visualization analysis is conducted. Importantly, multiple database mining is performed for designing low band gap organic semiconductors. Molecular descriptors are utilized to train machine learning models. Importantly, about 22 different machine learning models are tested. Among all models, extra trees regressor shows higher predictive capability. Residuals, learning curve and validation curve are also drawn for extra trees regressor. Feature importance analysis determines the significance of the features. Moreover, library enumeration and similarity analysis further facilitate designing of high-performance semiconductor materials. This work may help in screening and designing efficient semiconductors having low band gap for increasing the efficiency of OSCs.

Graphene oxide-titanium oxide-polyaniline hybrid materials for high-performance dye-sensitized solar cells

A new solar cell that comprises a graphene oxide and titanium oxide nanoparticle (NP) composite with graphitized polyaniline (PANI) has been tested in polymer solar cells (PSCs). In this study, a series of conjugated polymers were synthesized and evaluated as the absorber and hole-transport layer materials in organic solar cells, demonstrating the significant impact of the chemical structure of the polymers on solar cell performance. The incorporation of graphene oxide (GOX) and titanium oxide TiO2 NP in conjugated polymers, particularly when graphitized within the PANI layer, led to improved open-circuit voltages compared to structurally similar polymers without NP. The morphology of the composite films was analyzed and related to the photovoltaic performance of polymers synthesized with polyaniline PANI-base and PANI-salt. Optimal NP addition in PANI-salt, PANI-base, PANI-base-TiO2, and PANI-base-GOX resulted in open-circuit voltages and efficiencies of 433 mV and 3.67%, 250 mV and 0.595%, 310 mV and 1.358%, and 520 mV and 2.366%, respectively. This study underscores the importance of polymer film uniformity in the performance of PSCs, with the highest efficiency obtained using a conjugated polymer with the best PANI-salt and PANI-base-GOX films. The research sheds light on the relationship between polymer structure and device photovoltaic performance, which is crucial for the design of new polymeric materials for efficient use and stable OSCs and PSCs.

Enhancing predictive modeling of photovoltaic materials’ solar power conversion efficiency using explainable AI

We present a study on Explainable AI-based prediction of power conversion efficiency (PCE) of organic solar cells, conducted on a dataset of 566 small-molecule organic solar cell materials samples with varying donor and acceptor species combinations. This research uncovers an interesting phenomenon, the first of its kind to be reported, of PCE quantization, where the PCE values increase in steps with the increase in feature values. Our findings have significant implications for the development of efficient organic solar cells, as they provide a better understanding of the factors that influence PCE, and highlight the feature value ranges for which more efficient PCE would be achieved. Our study demonstrates the power of XAI techniques in uncovering hidden patterns in scientific datasets and highlights the importance of interdisciplinary research in the field of materials science.

Single‐Layer Carbon Nitride as an Efficient Metal‐Free Organic Electron‐Transport Material with a Tunable Work Function

AbstractSingle‐layer carbon nitride (SLCN) is introduced as a metal‐free organic electron transport material (ETL) for organic solar cells, delivering a performance comparable to that of the benchmark nanocrystalline ZnO. SLCN is produced in the form of stable aqueous inks and can serve as an efficient electron transport layer for three different types of active layers, showing high stability to photodegradation. A lower conductivity of SLCN films as compared to ZnO is counter‐balanced by their higher transparency and lower light scattering losses. The SLCN ETL provides a unique opportunity to tune the work function from ca. −4.1 to −4.6 eV by varying the annealing temperature due to partial thermal oxidation of the SLCN film. The PL band position follows the oxidation‐induced changes of the work function allowing PL properties of SLCN to be used to predict the PV efficiency. The 2D structure of SLCN coupled with unique structural and compositional variability, and tunability of the work function show a high promise for emerging organic PV devices.

Triselenasumanene-based organic molecules as donor and hole-transporting materials for next-generation solar cells

The optoelectronic and charge transport properties of triselenasumanene-based organic molecules were studied using electronic structure calculations to explore their potential as a hole-transporting material (HTM) in perovskite solar cells (PSCs) and as electron donors in organic solar cells (OSCs). Based on the recently synthesized triselenasumanene (SESUM) molecule, nine molecules (SESUMXn, where X = anthracene (A), tetracene (T) and pentacene (P), n = number of attached A, T and P, i.e. 1, 2 and 3) were designed by extending the π-conjugation. The newly designed molecules possessed suitable energy-level alignment with the energy bands of commonly used perovskite material, CH3NH3PbI3, in PSCs, and are suitable as electron-donor material with the commonly used acceptor material, PC61BM, in OSCs. The calculated ionic state properties and electrophilicity index also showed that the studied molecules were potential donor materials for OSCs. Further, the calculated open circuit voltage and fill factor of SESUM and anthracene- and tetracene-substituted SESUM molecules were comparable with those of recently reported donor molecules. The calculated absorption spectra, effective hole transfer integral and stacking angle distribution showed the suitability of SESUMX2 and SESUMX3 molecules as HTM in PSCs. The results of periodic density functional theory calculations showed that the tri-substituted SESUM molecules, SESUMX3 (where, X = A, T and P), were effectively adsorbed on the CH3NH3PbI3 surface and abstracted the hole carrier from the perovskite surface.

The impact of ester groups on 1,8-naphthalimide electron transport material in organic solar cells

Recently, 1,8-naphthalimide (NI) molecules have emerged as a new category of organic electron transport materials (ETMs) employed in organic solar cells (OSCs). Among them, NI molecules with a "NI-π-NI" molecular structure commonly exhibit over aggregation in films owing to their planar molecular structures, which is unfavorable for solution-processed film fabrication in OSC devices. In this work, we successfully synthesized a new NI molecule named NDT-2, employing a strategy that incorporates a solubilizing ester group modification to induce twisted molecular planarity, thereby reducing intermolecular over-aggregation and optimizing quality of the film. Interestingly, the NDT-2 modified by ester groups as a π bridge exhibits an enlarged molecular dihedral angle, enhanced alcohol solubility, a smoother film surface morphology, and improved film forming quality. Ultimately, OSCs incorporating NDT-2 as the ETM achieved a notable power conversion efficiency (PCE) of 17.1%. This study introduces a design strategy that utilizes ester functional groups to improve film forming quality of ETM, offering valuable insights for the development of advanced, alcohol-soluble NI molecules as efficient ETMs in OSCs.

Hybrid Organic-Inorganic ZnO/PCBM Electron Transport Layer with Organic Lead Halide Perovskite Solar Cell Materials CH3NH3PbIX2, X is Cl or I

An experimental investigation has been undertaken to explore a variety of properties of organic perovskite solar cells (OPSCs) that are distinct in their photovoltaic response. In this study, OPSCs were constructed using zinc oxide, ZnO and [6,6]-phenylC[Formula: see text]-acid methyl ester, PCBM, as electron transfer materials (ETM), poly(3-hexylthiophene), P3HT, as hole transport material (HTM), and CH3NH3PbIX2 as a photosensitizer, where X is either Cl or I. The structural and optical characteristics of the ETM and HTM layers and the perovskite materials of OPSCs were investigated. The [Formula: see text]–[Formula: see text] and [Formula: see text]–[Formula: see text] curves of each solar cell prepared in OPSCs were measured using an integrated cell tester. This comprises a photovoltaic [Formula: see text]–[Formula: see text] measurement system and a solar simulation system. As a result of these results, the OPSC’s performance is evaluated. The calculations in this work suggest that the highest power conversion efficiency (PCE = 6.31%) was achieved in photovoltaic devices using CH3NH3PbI3 as the effective absorber layer and ZnO with PCBM as the ETM layer. At a light intensity of 100 mW/cm2, each OPSC cell was performed. Under the same conditions, the [Formula: see text]–[Formula: see text] curves were evaluated for each OPSC device. By improving the selection of organic and inorganic active materials in the ETM layer, OPSC performance may be improved.

Development of fullerene acceptors and the application of non-fullerene acceptors in organic solar cells

Organic solar cells (OSCs) enjoy a huge market and public attention as an emerging renewable and clean energy source due to their accessibility, sustainability, translucency, good flexibility, non-toxicity, and low preparation cost. However, the choice of an acceptor material is now the pivotal factor restricting the development of OSCs. To continuously improve stability and increase power conversion efficiency (PCE) for better performance, high-performance acceptor materials are an important part of enabling OSCs. From fullerenes and their derivatives and non-fullerenes, we sum up the latest research progress on high-performance acceptor materials for OSCs and then introduce the synthesis methods of non-fullerenes. The latest strategies to enhance the performance of organic solar cells and the wide range of applications of non-fullerene acceptors (NFAs) on different OSCs are also discussed. In addition, challenges faced by OSCs in improving their performance and the prospects for future development are revealed, providing new ideas for designing the next generation of high-performance OSCs.

Rational molecular engineering towards efficient heterojunction solar cells based on organic molecular acceptors

The selection of photoactive layer materials for organic solar cells (OSCs) is essential for the photoelectric conversion process. It is well known that chlorophyll is an abundant pigment in nature and is extremely valuable for photosynthesis. However, there is little research on how to improve the efficiency of chlorophyll-based OSCs by matching chlorophyll derivatives with excellent non-fullerene acceptors to form heterojunctions. Therefore in this study we utilize a chlorophyll derivative, Ce6Me3, as a donor material and investigate the performance of its heterojunction with acceptor materials. Through density functional theory, the photoelectric performances of acceptors, including the fullerene derivative PC71BM and the terminal halogenated non-fullerene DTBCIC series, are compared in detail. It is found that DTBCIC-Cl has better planarity, light absorption, electron affinity, charge reorganization energy and charge mobility than others. Ce6Me3 has good energy level matching and absorption spectral complementarity with the investigated acceptor molecules and also shows good electron donor properties. Furthermore, the designed Ce6Me3/DTBCIC interfaces have improved charge separation and reorganization rates (K CS/K CR) compared with the Ce6Me3/PC71BM interface. This research provides a theoretical basis for the design of photoactive layer materials for chlorophyll-based OSCs.

Chemical similarity-based design of materials for organic solar cells: Visualizing the generated chemical space of polymers

Potential candidates for photovoltaic applications can be found using chemical similarity analysis. Compounds with similar structural characteristics may perform similarly. Designing of materials for organic solar cells (OSCs) is a difficult and time-consuming process. An existing polymer database is mined to find polymers for OSCs, three polymers (PM6, PBT7-Th, and D18) are used as standard. However, good polymers are not found from this database. A large database of polymers is generated using an automatic method that is implemented on RDkit. Synthetic accessibility of generated polymers is calculated. Similarity analysis is conducted to search the polymers from the generated database. Similarity analysis is performed using Tanimoto index. Large number of potential candidates for polymer solar cells are found. Structure Activity Landscape Index (SALI) plot is used to quantitatively characterize activity landscapes.

Tunning interfacial interaction strategy by asymmetric configuration to construct self-assembly materials for efficient organic solar cells

Developing highly ordered self-assembled monolayer is very critical and challenging for realizing high-performance organic solar cells. Herein, we develop an asymmetric carbazole derived molecule 1Cl-PACz by the introduction of single Cl atom to modulate energy level alignment and induce densely assembly. 1Cl-PACz exhibits an improved molecular dipole moment, thus modulating the work function of indium tin oxide (ITO) electrodes to facilitate charge extraction. Moreover, 1Cl-PACz is proven to have strengthened π-π interaction observed in its single crystal, forming densely assembly on the substrates. Excitingly, a champion power conversion efficiency of 19.0 % can be obtained when 1Cl-PACz is evaluated as hole-extraction channel, among the highest values for the SAMs derived cells to date. This study opens new avenues for exploring high-performance SAMs materials to boost the performance of OSC devices.

A Scalable and Chromatography‐Free Synthesis of N,N‐Bis(9,9‐dimethyl‐9H‐fluoren‐2‐yl)‐3′,3′,4′,7′‐tetramethyl‐2′,3′‐dihydrospiro[fluorene‐9,1′‐indene]‐2‐amine, a new Hole Transport Material for Organic Solar Cells

AbstractThe title triarylamine N,N‐bis(9,9‐dimethyl‐9H‐fluoren‐2‐yl)‐3′,3′,4′,7′‐tetramethyl‐2′,3′‐dihydrospiro[fluorene‐9,1′‐indene]‐2‐amine is a new hole transport material for organic solar cells. After investigating different discovery approaches (Schemes 1 + 2), a multi gram‐scale synthetic sequence was developed (Scheme 4). The key intermediate 2‐bromo‐9‐(2,5‐dimethylphenyl)‐9H‐fluorene was accessible from 2‐bromo‐9‐fluorenone by either the sequence Grignard reaction, Et3SiH/BF3 reduction or by direct arylation of the corresponding 2‐bromo‐9‐fluorenol. Alkylation at C(9) of 2‐bromo‐9‐(2,5‐dimethylphenyl)‐9H‐fluorene with methallyl chloride and cyclization by an intramolecular Friedel‐Crafts alkylation led to the key building block 2‐bromo‐3′,3′,4′,7′‐tetramethyl‐2′,3′‐dihydrospiro[fluorene‐9,1′‐indene] (Scheme 4). A Buchwald‐Hartwig coupling was employed (Scheme 3 + 4) for the assembly of the final triarylamines. The developed gram‐scale synthesis of the title compound is scalable and chromatography‐free with an overall yield >25 % over 5 steps.