Materials For Organic Solar Cells Research Articles

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.

A Three-Dimensional Non-Fullerene Acceptor with Contorted Hexabenzocoronene and Perylenediimide for Organic Solar Cells.

Although fullerene derivatives such as [6,6]-phenyl-C61/C71-butyric acid methyl ester (PC61BM/PC71BM) have dominated the the photoactive acceptor materials in bulk heterojunction organic solar cells (OSCs) for decades, they have several drawbacks such as weak absorption, limited structural tunability, prone to aggregation, and high costs of production. Constructing non-fullerene small molecules with three-dimensional (3D) molecular geometry is one of the strategies to replace fullerenes in OSCs. In this study, a 3D molecule, contorted hexa-cata-hexabenzocoronene tetra perylenediimide (HBC-4-PDI), was designed and synthesized. HBC-4-PDI shows a wide and strong light absorption in the whole UV-vis region as well as suitable energy levels as an acceptor for OSCs. More importantly, the 3D construction effectively reduced the self-aggregation of c-HBC, leading to an appropriate scale phase separation of the blend film morphology in OSCs. A preliminary power conversion efficiency of 2.70 % with a champion open-circuit voltage of 1.06 V was obtained in OSCs with HBC-4-PDI as the acceptor, which was the highest among the previously reported OSCs based on c-HBC derivatives. The results indicated that HBC-4-PDI may serve as a good non-fullerene acceptor for OSCs.

Propeller-shaped NI isomers of cathode interfacial material for efficient organic solar cells

Propeller-shaped NI isomers of cathode interfacial material for efficient organic solar cells

Graphene and Graphene Derived Nanomaterials as Versatile Candidates for Organic Solar Cells and Smart Windows Applications – A Review

AbstractGraphene and related materials have been employed in several applications till date due to the unique set of properties. Research in the area of organic solar cells and smart windows are important to meet the increasing global energy demands and to realize the future goal of zero energy buildings. Here, we present a summary of the important research progress in application of graphene and related materials in organic solar cells and smart windows. This review starts by introducing the need of photovoltaic technology including organic solar cells and smart windows. The remaining sections are broadly divided into two sections – the first one briefly introduces organic cells and their different types, and the next section focuses on the different chromogenic technologies used in smart windows. After a brief overview of graphene, the application of graphene and related materials in organic cells and smart windows are presented. In the end, some of the major challenges and possible research directions are also outlined. Hopefully, this review will shed light on the recent trends and directions to achieve enhancement in performance of organic solar cells and smart windows with integration of graphene and provide overview for researchers about this novel field.

Ternary polythiophene enables over 17% efficiency organic solar cells

Two ternary polythiophenes (PTs), PT-CN and PT-2CN, as donors are developed to deepen energy levels, reduce miscibility and regulate crystallinity. PT-CN:BTP-eC9 based OSCs gain an impressive PCE of 17.27%, which is the record for PT-based OSCs.

Conjugated backbone optimization of an all-fused-ring acceptor for efficient and stable organic solar cells.

Three all-fused-ring acceptors with different conjugated backbone are designed as acceptor materials for organic solar cells. It is found that the horizontally extended conjugation of all-fused-ring acceptors leads to better photovoltaic performance compared to the vertically extended analogue. The results provide a guideline for designing high-performance and stable all-fused-ring acceptor materials.

Additive-assisted strategy for high-efficiency organic solar cells

Herein, we summarize the recent advances of the additive-assisted strategy with solvent/solid additives and binary additives and their working mechanism in OSCs.

Flexible, transparent, and sustainable cellulose-based films for organic solar cell substrates.

Cellulose, often considered a highly promising substitute for petroleum-based plastics, offers several compelling advantages, including abundant availability, cost-effectiveness, environmental friendliness, and biodegradability. However, its inherent highly crystalline structure and extensive hydrogen-bonded network pose challenges for processing and recycling. In this study, we introduce the concept of cellulose vitrimers (CVs), wherein dynamic bonds are incorporated to reconfigure the hydrogen-bonded network, resulting in a mechanically robust, highly transparent material. CVs exhibit exceptional malleability, thermal stability, and noteworthy resistance to water and solvents. Due to the dynamic bond disassociation, CVs can be effectively chemically recycled using a well-established "dissolution-and-reforming" process. Moreover, CVs have proven successful as flexible substrate materials for organic solar cells, outperforming traditional petroleum-based polyethylene naphthalate (PEN). Given these advantages, CVs have the potential to replace conventional petroleum-based materials as recyclable and environmentally friendly alternatives, particularly within the realm of electronic devices and displays.

Combinatorial screening of wide band-gap organic solar cell materials with open-circuit voltage between 1.1 and 1.4 V

Organic solar cells with open circuit voltages close to 1.4 V are found through the combinatorial screening of three donors and three wide band acceptors, with some blends retaining values of 1.2 V under indoor illumination.

Application of Buffer Layer to Improve the Efficiency of Organic Solar Cells

Organic solar cells (OSCs) are widely studied for their advantages such as simple production, low cost, good flexibility, and large‐area printing. The buffer layer between electrodes and the photoactive layer has a significant impact on the efficiency and stability of OSCs. In order to lower the energy barrier height at the interface, provide an Ohmic contact with lower series resistance, and increase the charge collection efficiency of the corresponding electrodes for either holes or electrons, suitable electrode buffer materials can be chosen. The selection of electrode buffer layer has an important effect on the photoelectric conversion efficiency. Therefore, the study of buffer layer characteristics has certain guiding significance for the improvement of device structure and performance optimization. At present, the buffer layer materials used in OSCs are mainly organic polymer materials, organic small molecule materials, metal fluoride, metal oxides, and so on. Herein, the structure and working principle of OSCs are introduced first. Then, the properties and function of buffer layer in OSCs are summarized and discussed. Finally, the application of common buffer layer materials in OSCs is summarized, and how to improve the performance of the device is described.

Biaxially Conjugated Materials for Organic Solar Cells.

Organic solar cells (OSCs) represent one of the most important emerging photovoltaic technologies that can implement solar energy conversion efficiently. The chemical structure of organic semiconductors deployed in the active layer of OSCs plays a critical role in the photovoltaic performance and chemical/physical stability of relevant devices. With the structure innovation of organic semiconductors, especially nonfullerene acceptors (NFAs), the performance of OSCs have been promoted rapidly in recent years, with state-of-the-art power conversion efficiencies (PCEs) exceeding 19.5%. Compared with other photovoltaics like perovskite, the shortcoming of OSCs mainly lies in the high nonradiative recombination loss. However, the photocurrent density is superior in OSCs owing to the easy modulation of the NFA band gap toward the near-infrared region. In these regards, the effort to further boost the PCE of OSCs to achieve a milestone >21% should be devoted to reducing the nonradiative loss while further broadening the absorption band. Developing organic semiconductors with biaxially extended conjugated structures has provided a potential solution to achieve these goals. Herein, we summarize the design rules and performance progress of biaxially extended conjugated materials for OSCs. The descriptions are divided into two major categories, i.e., polymers and NFAs. For p-type polymers, we focus on the biaxial conjugation on some representative building blocks, e.g., polythiophene, triphenylamine, and quinoxaline. Whereas for n-type polymers, some structures with large conjugated planes in the normal direction are presented. We also elaborate on the biaxial conjugation strategies in NFAs with modification site at either the π-core or side-group. The general structure-property relationships are further retrieved within these materials, with focus on the short-wavelength absorption and nonradiative energy loss. Finally, we provide an outlook for the further structure modification strategies of biaxially conjugated materials toward highly efficient, stable, and industry-compatible OSCs.

Regulation of photoinduced charge transfer in all-small-molecule organic solar cells through the synergistic effect of external electric field and solvent

The donor (BTR-Cl) and acceptor (BTP-FCl-FCl) have well-defined small molecule properties and excellent repeatability, and they can form charge transfer complexes with a wide spectral absorption range. Using density functional theory (DFT), we simulate the photoinduced charge transfer of bulk-heterojunction (BHJ) organic solar cell (OSC) materials modulated by the external electric field (Fext) at the microscopic level. The excited-state properties, reorganization energy (λ), Gibbs free energy (ΔG), electron coupling matrix elements (VDA) and intermolecular charge transfer (ICT) rate dependent on Fext are systematically analyzed. The results manifest that Fext has apparent positive regulation on the charge separation rate (KCS), and VDA is the main factor affecting KCS. The synergistic effect of solvent and Fext on charge transfer process is further investigated. It is found that the charge transfer rate in solvent is lower than that in solvent-free conditions, and the charge transfer rate in chlorobenzene (CB) solvent does not change significantly under the control of Fext. At the same time, Fext has a positive effect on the charge transfer rate in tetrahydrofuran (THF) solvent. Considering the charge transfer rate under different solvents and Fext intensities, it is found that the solvent and Fext play an essential role in determining the KCS and charge recombination rate (KCR).

Exploring the potential of porphyrin-based materials for organic solar cells supported on carbon: A quantum chemistry approach

In this work, novel solar cell materials are studied by using first-principle calculations to improve the electronic structure properties and thus enhance the hypothetical efficiency in the active layer of dye-sensitized solar cells (DSSCs). The modeling of 75 composite systems is explored, based on the interaction of three molecular structures; namely graphene, a linker, and metalated porphyrins (a base species and the coordinated cases with Mg, Cu, Ni, and Zn). The stability and electronic structure properties of 10 composite systems, screened from the original set, were analyzed as potential candidates to be implemented in solar cell devices. All calculations were performed using density functional theory (DFT). This study is intended to screen systems that broaden the range of the absorption spectra in the composite materials of the form porphyrin/carbon by improving the electron transfer properties. The methodology to obtain the final set of selected composite materials may be considered a guide in the design of novel DSSC systems.

Effect of Benzothiadiazole-Based π-Spacers on Fine-Tuning of Optoelectronic Properties of Oligothiophene-Core Donor Materials for Efficient Organic Solar Cells: A DFT Study.

In this work, five novel A-π-D-π-A type molecules D1-D5 were designed by adding unusual benzothiadiazole derivatives as π-spacer blocks to the efficient reference molecule DRCN5T for application as donor materials in organic solar cells (OSCs). Based on a density functional theory approach, a comprehensive theoretical study was performed with different functionals (B3LYP, B3LYP-GD3, B3LYP-GD3BJ, CAM-B3LYP, M06, M062X, and wB97XD) and with different solvent types (PCM and SMD) at the extended basis set 6-311+g(d,p) to evaluate the structural, optoelectronic, and intramolecular charge transfer properties of these molecules. The B3LYP-GD3BJ hybrid functional was used to optimize the studied molecules in CHCl3 solution with the SMD model solvent as it provided the best results compared to experimental data. Transition density matrix maps were simulated to examine the hole-electron localization and the electronic excitation processes in the excited state, and photovoltaic parameters including open-circuit photovoltage and fill factor were investigated to predict the efficiency of these materials. All the designed materials showed promising optoelectronic and photovoltaic characteristics, and for most of them, a red shift. Out of the proposed molecules, [1,2,5]thiadiazolo[3,4-d]pyridazine was selected as a promising π-spacer block to evaluate its interaction with PC61BM in a composite to understand the charge transfer between the donor and acceptor subparts. Overall, this study showed that adding π-spacer building blocks to the molecular structure is undoubtedly a potential strategy to further enhance the performance of donor materials for OSC applications.

Dependence of Exciton Binding Energy on Bandgap of Organic Semiconductors.

Strongly bound excitons crucially affect the operation of organic optoelectronic devices. Nevertheless, precise experimental data on the exciton binding energy of organic semiconductors are lacking. In this study, we determine the exciton binding energy as the difference between the optical and transport bandgaps with a precision of 0.1 eV. In particular, electron affinities with a precision higher than 0.05 eV determined by low-energy inverse photoelectron spectroscopy allow us to determine the transport gap and the exciton binding energies with such high precision. Through a systematic comparison of a wide range of organic semiconductors, including 42 organic solar cell materials (15 nonfullerene acceptors, 4 fullerene acceptors, 13 low-bandgap polymers, 7 organic light-emitting diode materials, and 3 crystalline materials), we found that the exciton binding energy is one-quarter of the transport gap regardless of the materials. We interpret this unexpected relation from a hydrogen atom-like model, i.e., the quantized energy levels in a Coulomb potential between the positive and the negative charges.

Naphthodithiophene Diimide (NDTI)‐Based Cathode Interlayer Material Enables 19% Efficiency Binary Organic Solar Cells

AbstractA fused naphthodithiophene diimide (NDTI) derivative is first used as cathode interlayer materials (CIMs) in organic solar cells, by introducing two dimethylamine‐functionalized fluorenes on both sides, namely NDTI1. Meanwhile, two non‐fused naphthalene diimide (NDI) derivatives are synthesized as the control CIMs to validate the design strategy of fused NDI. All three CIMs show high thermal stability, robust adhesion, and strong electrode modification capability. Compared with two NDI‐based materials, NDTI1 possesses excellent film‐forming capacity and strong crystallinity, simultaneously. Besides, NDTI1 presents a strong self‐doping effect and distinct intermolecular interaction with non‐fullerene acceptors. As expected, the NDTI1‐based OSCs achieve a power conversion efficiency (PCE) of 18.02% using the PM6:Y6 active layer and a champion PCE of 19.01% employing the active layer PM6:L8‐BO, which is attributed to improve charge transport and extraction, and suppressive charge recombination. More importantly, NDTI1 retains 91% of the optimal PCE when the film thickness increases from 7 to 20 nm. Furthermore, NDTI1 also exhibits satisfactory universality for different active layer materials and excellent device stability.

Recent Advance in Solution‐Processed Hole Transporting Materials for Organic Solar Cells

AbstractSolution‐processed hole transporting layers (HTLs) not only play a crucial role in realizing high performance of organic solar cells (OSCs), but also possess excellent compatibility with low‐cost and large‐area processing methods of industrialized productions. However, the number and species of HTL materials are obviously fewer than that of electron‐transporting materials, which limits the development and application of OSCs. In particular, the large energy level difference between anode and organic active layer leads to the serious energy barrier for hole collection, bringing much difficulty in developing efficient HTL materials. In this review, it is focused on the recent advances in solution‐processed HTLs in OSCs. Initially, the working mechanism, property requirement, and existing issues of solution‐processed HTLs are systematically analyzed. Afterward, the main classes of solution‐processed HTL materials are discussed, including PEDOT:PSS, conjugated polyelectrolytes (CPEs), TMOs, and others. The structure‐property relationships of solution‐processed HTL materials are analyzed, and some important design rules for such materials toward efficient and stable OSCs are presented. Finally, a brief summary is presented along with some perspectives to help researchers understanding the challenges and opportunities in this field.

Strengthening the Hetero-Molecular Interactions in Giant Dimeric Acceptors Enables Efficient Organic Solar Cells.

Giant dimeric acceptor (G-Dimer) is becoming one of the most promising organic solar cell (OSC) materials because of its definite structure, long-term stability, and high efficiency. Strengthening the hetero-molecular interactions by monomer modification greatly influences the morphology and thus the device performance, but lacks investigation. Herein, two novel quinoxaline core-based G-Dimers, Dimer-QX and Dimer-2CF, are synthesized. By comparing trifluoromethyl-substituted Dimer-2CF and non-substituted Dimer-QX, the trifluoromethylation effect on the G-Dimer is investigated and revealed. The trifluoromethyl with strong electronegativity increases electrostatic potential and reduces surface energy of the G-Dimer, weakening the homo-molecular ordered packing but reinforcing the hetero-molecular interaction with the donor. The strong hetero-molecular interaction suppresses the fast assembly during the film formation, facilitating small domains with ordered molecular packing in the blend, which is a trade-off in conventional morphology control. Together with favorable vertical phase separation, efficient charge generation, and reduced bimolecular recombination are concurrently obtained. Hence, the Dimer-2CF-based OSCs obtain a cutting-edge efficiency of 19.02% with fill factor surpassing 80%, and an averaged extrapolated T80 of ≈12000h under continuous 80 °C heating. This study emphasizes the importance of hetero-molecular interaction and trifluoromethylation strategy, providing a facile strategy for designing highly efficient and stable OSC materials.

Iron complex/graphene oxide hybrid modification of ZnO surface for achieving stable inverted organic photovoltaics

We synthesized a four-coordinated semi-cyclic iron complex and composited it with graphene oxide (GO) to form a hybrid iron complex/GO-based material for organic solar cells (OSC). X-ray diffraction (XRD) analysis showed increased spacing between the GO sheets, indicating that the iron complex successfully intercalated or exfoliated the GO and deposited it on the surface. The as-prepared functionalized graphene oxide (FGO) also exhibits superior dispersion, which can avoid severe aggregation during fabrication. We demonstrated the impact of utilizing FGO as an interlayer to modify the ZnO-based OSC. Through X-ray Photoelectron Spectroscopy (XPS) and Photoluminescence (PL) analysis, we confirmed that FGO effectively reduces oxygen defects on the ZnO surface, leading to a reduction in the rate of carrier recombination and promoting the efficiency (up to 17.04 %) and ambient-stability of PM6:Y6-based OSCs. This discovery holds tremendous potential for advancing the development of high-performance OSCs with exceptional long-term stability.

Theoretical study of the effect of D-A type polymer donors on the performance of Y6-based organic solar cells

Non-fullerene organic solar cells (OSCs) based on Y6 molecule have developed rapidly in recent years, and the power conversion efficiency (PCE) varies greatly when the OSC device constructed by Y6 and donor molecules with small structural difference. In this work, the influence of six donor-acceptor (D-A) polymer donors on Y6-based OSCs were focused. The ground-state properties (planarity, frontier molecular orbital energy levels, driving force), excited-state properties of donors (primary indicators, Coulomb attraction) and donor/acceptor interfaces (Frenkel excitation (FE) and charge transfer (CT) states, electron coupling and charge separation rates) were discussed. The results show that symmetrical alkyl chains in the acceptor unit is favorable for maintaining planarity of molecule, while fluorination induces twisting in the thiophene bridge and acceptor unit. And the investigated high-performance molecules have desirable driving force, smaller Coulomb attraction and significant degree of holes and electrons delocalization. Also, the highest-performance system has multiple charge transfer pathways, which can improve charge separation efficiency. At the same time, high-performance systems often have better planarity and more favorable substituent positions. The high-performance donor/Y6 systems have more advantage in charge separation than the low-performance systems. This work provides a fundamental understanding of the impact of D-A type donor materials on the cell performance and theoretical guidance for further optimization of polymer donor materials in Y6-based OSCs.