High-Performance Simple Nonfused Ring Electron Acceptors with Diphenylamino Flanking Groups

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 (1.72 eV) and fill factor (0.9228). Energy loss of the model (R) is 0.57 eV 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.

Research Advances of Nonfused Ring Acceptors for Organic Solar Cells

The last few decades have witnessed the rapid development of organic solar cells (OSCs). High power conversion efficiencies (PCEs) of over 19% have been successfully achieved due to the emergence of fused-ring acceptors (FRAs). However, the high complexity and low yield for the material synthesis result in high production costs of FRAs, limiting the further commercial application of OSCs. In contrast, nonfused ring acceptors (NFRAs) with the merits of facile synthesis, high yield, and preferable stability can promote the development of low-cost OSCs. Currently, the PCEs of NFRAs-based OSCs have exceeded 17%, which is expected to reach efficiency comparable to that of the FRAs-based OSCs. This review describes the advantages of the recent advances in NFRAs, which emphasizes exploring how the chemical structures of NFRAs influence molecular conformation, aggregation, and packing modes. In addition, the further development of NFRA materials is prospected from molecular design, morphological control, and stability perspectives.

Halogenated Nonfused Ring Electron Acceptor for Organic Solar Cells with a Record Efficiency of over 17

Three nonfused ring electron acceptors (NFREAs), namely, 3TT-C2-F, 3TT-C2-Cl, and 3TT-C2, are purposefully designed and synthesized with the concept of halogenation. The incorporation of F or/and Cl atoms into the molecular structure (3TT-C2-F and 3TT-C2-Cl) enhances the π-π stacking, improves electron mobility, and regulates the nanofiber morphology of blend films, thus facilitating the exciton dissociation and charge transport. In particular, blend films based on D18:3TT-C2-F demonstrate a high charge mobility, an extended exciton diffusion distance, and a well-formed nanofiber network. These factors contribute to devices with a remarkable power conversion efficiency of 17.19%, surpassing that of 3TT-C2-Cl (16.17%) and 3TT-C2 (15.42%). To the best of knowledge, this represents the highest efficiency achieved in NFREA-based devices up to now. These results highlight the potential of halogenation in NFREAs as a promising approach to enhance the performance of organic solar cells.

Tailoring the Weight of Surface and Intralayer Edge States to Control LUMO Energies

The energies of the frontier molecular orbitals determine the optoelectronic properties in organic films, which are crucial for their application, and strongly depend on the morphology and supramolecular structure. The impact of the latter two properties on the electronic energy levels relies primarily on nearest-neighbor interactions, which are difficult to study due to their nanoscale nature and heterogeneity. Here, an automated method is presented for fabricating thin films with a tailored ratio of surface to bulk sites and a controlled extension of domain edges, both of which are used to control nearest-neighbor interactions. This method uses a Langmuir-Schaefer-type rolling transfer of Langmuir layers (rtLL) to minimize flow during the deposition of rigid Langmuir layers composed of π-conjugated molecules. Using UV-vis absorption spectroscopy, atomic force microscopy, and transmission electron microscopy, it is shown that the rtLL method advances the deposition of multi-Langmuir layers and enables the production of films with defined morphology. The variation in nearest-neighbor interactions is thus achieved and the resulting systematically tuned lowest unoccupied molecular orbital (LUMO) energies (determined via square-wave voltammetry) enable the establishment of a model that functionally relates the LUMO energies to a morphological descriptor, allowing for the prediction of the range of accessible LUMO energies.

Effects of Halogenation on Cyclopentadithiophenevinylene-Based Acceptors with Excellent Responses in Binary Organic Solar Cells

In recent years, non-fused non-fullerene acceptors (NFAs) have attracted increasing consideration due to several advantages, which include simple preparation, superior yield, and low cost. In the work reported here, we designed and synthesized three new NFAs with the same cyclopentadithiophenevinylene (CPDTV) trimer as the electron-donating unit and different terminal units (IC for FG10, IC-4F for FG8, and IC-4Cl for FG6). Both halogenated NFAs, i.e., FG6 and FG8, show red-shifted absorption spectra and higher electron mobilities (more pronounced for FG6) in comparison with FG10. Moreover, the dielectric constants of these materials also increased upon halogenation of the IC terminal units, thus leading to a reduction in the exciton binding energy, which is favorable for dissociation of excitons and subsequent charge transfer despite the driving force (highest occupied molecular orbital and lowest unoccupied molecular orbital offsets) being very small. The organic solar cells (OSCs) constructed using these acceptors and PBDB-T, as the donor, showed PCE values of 15.08, 12.56, and 9.04% for FG6, FG8, and FG10, respectively. The energy loss for the FG6-based device was the lowest (0.45 eV) of all the devices, and this may be attributed to it having the highest dielectric constant, which leads to a reduction in the binding energy of exciton and a small driving force for hole transfer from FG6 to PBDB-T. The results indicate that the NFA containing the CPDTV oligomer core and halogenated terminal units can efficiently spread the absorption spectrum to the NIR zone. Non-fused NFAs have a bright future in the quest to obtain efficient OSCs with low cost for marketable purposes.

Advances in organic photovoltaic cells: a comprehensive review of materials, technologies, and performance

This paper provides a comprehensive overview of organic photovoltaic (OPV) cells, including their materials, technologies, and performance. In this context, the historical evolution of PV cell technology is explored, and the classification of PV production technologies is presented, along with a comparative analysis of first, second, and third-generation solar cells. A classification and comparison of PV cells based on materials used is also provided. The working principles and device structures of OPV cells are examined, and a brief comparison between device structures is made, highlighting their advantages, disadvantages, and key features. The various parts of OPV cells are discussed, and their performance, efficiency, and electrical characteristics are reviewed. A detailed SWOT analysis is conducted, identifying promising strengths and opportunities, as well as challenges and threats to the technology. The paper indicates that OPV cells have the potential to revolutionize the solar energy industry due to their low production costs, and ability to produce thin, flexible solar cells. However, challenges such as lower efficiency, durability, and technological limitations still exist. Despite these challenges, the tunability and versatility of organic materials offer promise for future success. The paper concludes by suggesting that future research should focus on addressing the identified challenges and developing new materials and technologies that can further improve the performance and efficiency of OPV cells.

Large Steric Hindrance Enhanced Molecular Planarity for Low-Cost Non-Fused Electron Acceptors

Designing efficient non-fused ring electron acceptors is of great importance in decreasing the material cost of organic photovoltaic cells (OPVs). It is a challenge to construct a planar molecular skeleton in non-fused molecules as there are many torsions between adjacent units. Here, we design two non-fused electron acceptors based on bithieno[3,2-b]thiophene units as core structures and study the impact of steric hindrance of substituents on molecular planarity. We use 2,4,6-triisopropylphenyl and 4-hexylphenyl groups to prepare ATTP-1 and ATTP-2, respectively. Our results suggest that the enhanced steric hindrance is beneficial for obtaining a more planar molecular configuration, which significantly increases the optical absorption and charge transport properties. The power conversion efficiency (PCE) of PBDB-TF:ATTP-1 combination (11.3%) is superior to that of PBDB-TF:ATTP-2 combination (3.7%). In addition, an impressive PCE of 10.7% is recorded in ATTP-1-based devices when a low-cost polythiophene donor PDCBT is used, which is an outstanding value in OPVs fabricated by non-fused donor/acceptor combinations. Our work demonstrates that modulation of the steric hindrance effect is of great significance to control the molecular planarity and thus obtain excellent photovoltaic performance of low-cost non-fused electron acceptors.

Unsymmetrically Chlorinated Non-Fused Electron Acceptor Leads to High-Efficiency and Stable Organic Solar Cells

Searching the cost-effective organic semiconductors is strongly needed in order to facilitate the practice of organic solar cells (OSCs), yet to be fulfilled. Herein, we have succeeded in developing two non-fused ring electron acceptors (NFREAs), leading to the highest efficiency of 16.2 % for the NFREA derived OSCs. These OSCs exhibit the superior operational stabilities under one sun equivalent illumination without ultraviolet (UV) filtration. It is revealed that the modulation of halogen substituents on aromatic side chains, as the new structural tool to tune the intermolecular interaction and optoelectronic properties of acceptors, not only promotes the interlocked tic-tac-toe frame of three-dimensional stacks in solid, but also improves charge dynamics of acceptors to enable high-performance and stable OSCs.

Effect of Steric Hindrance at the Anthracene Core on the Photovoltaic Performance of Simple Nonfused Ring Electron Acceptors

Solving the contradiction between good solubility and dense packing is a challenge in designing high-performance nonfullerene acceptors. Herein, two simple nonfused ring electron acceptors (o-AT-2Cl and m-AT-2Cl) carrying ortho- or meta-substituted hexyloxy side chains can be facilely synthesized in only three steps. The two ortho-substituted phenyl side chains in o-AT-2Cl cannot freely rotate due to a big steric hindrance, which endows the acceptor with good solubility. Moreover, o-AT-2Cl displays a more ordered packing than m-AT-2Cl as revealed by the absorption measurement. When blended with polymer donor D18 for the fabrication of organic solar cells (OSCs), o-AT-2Cl-based devices exhibit a favorable morphology, more efficient exciton dissociation, and better charge transport. Consequently, the optimal OSCs based on D18:o-AT-2Cl exhibit a power conversion efficiency (PCE) of 12.8%, which is significantly higher than the moderate PCE (7.66%) for D18:m-AT-2Cl-based devices. Remarkably, o-AT-2Cl shows a higher figure-of-merit value compared with classic high-efficiency fused ring electron acceptors. As a result, our research succeeds in obtaining nonfused ring acceptors with cost-effective photovoltaic performance and provides a valuable experience for simultaneously improving solubility as well as ensuring ordered packing of acceptors through regulating the steric hindrance via changing the position of substituents.

Self-Aggregated Light-Trapping Nanodots for Highly Efficient Organic Solar Cells

The typical thickness of the photoactive layer in organic solar cells (OSCs) is around 100 nm, which limits the absorption efficiency of the incident light and the power conversion efficiency (PCE) of OSCs. Therefore, light-trapping schemes to reduce the optical losses in the thin photoactive layers are critically important for efficient OSCs. Herein, light-trapping and electron-collection dual-functional small organic molecules, N,N,N',N'-tetraphenyloxalamide (TPEA) and N,N,N',N'-tetraphenylmalonamide (TPMA), are designed and synthesized by a one-step acylation reaction. Driven by strong intermolecular force, TPEA and TPMA tend to self-aggregate into hemispherical light-trapping nanodots on the photoactive layer, resulting in enhanced light harvesting. Meanwhile, TPEA and TPMA demonstrate high electron mobility and excellent electron-collection ability.  Compared with the device without cathode buffer layer (CBL, PCE = 14.09%), PM6:BTP-eC9 based OSCs with TPEA and TPMA light-trapping CBLs demonstrate greatly enhanced PCE of 16.21% and 17.85%, respectively. Furthermore, a record PCE of 19.02% can be achieved for PM6:BTP-eC9:PC₇₁ BM based ternary OSC with TPMA light-trapping CBL. Moreover, TPMA exhibits a low synthesis cost of only 0.61 $ g^-1 with high yield. These findings could open a window for the rational design of multifunctional CBLs for efficient and stable OSCs.

Molecular-Shape-Controlled Nonfused Ring Electron Acceptors for High-Performance Organic Solar Cells with Tunable Phase Morphology

Two nonfused ring electron acceptors (NFREAs), BTh-OC8-2F and DTh-OC8-2F, with different molecular shapes are designed and synthesized. Both acceptors can form planar molecular shapes by the assistance of S···O intramolecular interactions. Differently, BTh-OC8-2F, with a linear molecular backbone and two trans-arranged side chains at the core unit, exhibits much stronger crystallinity than DTh-OC8-2, with a C-shape molecular shape and two cis-arranged steric side chains at the core unit. Thus, the DTh-OC8-2F based blend film displays a better nanoscale phase separation, more suppressed charge recombination, more efficient exciton dissociation, and lower nonradiative energy loss. Organic solar cells based on DTh-OC8-2F can deliver a power conversion efficiency of 14.13%, which is much higher than BTh-OC8-2F based ones (11.95%) and is also one of the highest values reported for organic solar cells based on NFREAs.

Unfused Acceptors Matching π-Bridge Blocks with Proper Frameworks Enable Over 12% As-Cast Organic Solar Cells

Emerging unfused-ring acceptors (UFAs) have been explored in pursuit of low-cost high-efficient organic solar cells (OSCs). Assembling unfused building blocks into proper frameworks are challenging for the molecular design of UFAs. The authors report herein four UFAs adopting either dithiophene cyclopentadiene (DTC) or dithieno[3,2-b:2',3'-d]pyrrole (DTP) as π-bridge units with different molecular frameworks for high-efficient as-cast OSCs. All these acceptors exhibit strong near-infrared absorption and narrow optical band gap (E_g ^opt  < 1.50 eV). DTC-bridged symmetric and DTP-bridged asymmetric UFAs exhibit higher planar conformation as well as suitable miscibility and homogeneous phase separation when blending with polymer donor PBDB-T to promote efficient charge transport in the blends. Their blends with PBDB-T contribute optimal PCE of 12.17% and 11.92% in as-cast OSCs, among the highest values for UFAs based as-cast devices in the literature. Experimental and theoretical simulations systematically reveal the impact of manipulating the molecular framework of UFAs on their conformation, optoelectronic, and photovoltaic performance. The results indicate the matching π-bridge units with molecular frameworks as an attractive approach to design UFAs for high-performance as-cast OSCs.

Designing High-Performance Nonfused Ring Electron Acceptors via Synergistically Adjusting Side Chains and Electron-Withdrawing End-Groups

Three nonfused ring electron acceptors, Hexyl-0F, Isopropyl-0F, and Isopropyl-2F, are designed and synthesized. Unlike Hexyl-0F, Isopropyl-0F with two sterically hindered 2,4,6-triisopropyl-phenyl groups is highly soluble, which provides a good opportunity for solution processability. Compared with Isopropyl-0F, Isopropyl-2F with fluorinated end-groups exhibits red-shifted absorption. Due to these synergistic adjustment, Isopropyl-2F-based devices displayed a high power conversion efficiency of 12.55%, higher than that of Isopropyl-0F (9.49%). The result demonstrates that the introduction of large steric substituents in the π-bridge units and electron-withdrawing end-groups plays a positive role in the construction of high-efficiency nonfused ring electron acceptors.

Influence of Large Steric Hinderance Substituent Position on Conformation and Charge Transfer Process for Non-Fused Ring Acceptors

To obtain stable and planar molecular geometry in non-fused electron acceptors, A4T-25 and A4T-26 are designed and synthesized by introducing the bulk 2,4,6-triisopropylphenyl side groups onto different positions of the central two thiophene units. A4T-25 and A4T-26 both show a narrow-bandgap of 1.39 and 1.46 eV, with highest occupied molecular orbital/lowest unoccupied molecular orbital levels of -5.56/-3.81 and -5.65/-3.83 eV, respectively, and the electrostatic potential distributions imply that they have strong electron-accepting capability. The single crystal structure analysis and the transfer integral calculation demonstrate that the much more compact π-π stacking in A4T-26 can promote efficient charge transportation compared to that in A4T-25. Therefore, the electron mobility of A4T-26 is obviously higher and more balanced than that of A4T-25. When blending the two acceptors with the same polymer donor PBDB-TF, the photovoltaic cell based on PBDB-TF:A4T-25 has an inadequate power conversion efficiency (PCE) of 7.83%, while the PBDB-TF:A4T-26-based one yields an enhanced PCE of 12.1%. Overall, this study offers an insight into the relationship between the fine-tuning of the molecular structure of non-fused ring acceptors and the corresponding charge transfer process in organic solar cells.

Terminal Groups Plays an Important Role in Enhancing the Performance of Organic Solar Cells Based on Non-Fused Electron Acceptors

Non-fused-ring electron acceptors (NFRAs) have made great progress resulting in their property of cheap and efficient in organic solar cells (OSCs). In order to solve the disadvantage of low device...