Highly Efficient Non-Fused-Ring Electron Acceptors Enabled by the Conformational Lock and Structural Isomerization Effects

Structural modification of A-C-A configured X-PCIC acceptor molecule for efficient photovoltaic properties with low energy loss in organic solar cells

Modification of terminal acceptors of non-fullerene organic solar cell molecule with different terminal acceptors can help in screening of molecules to develop organic photovoltaic cells with improved performance. Thus, in this work, seven new molecules with an unfused core have been designed and thoroughly investigated. DFT/TD-DFT simulations were performed on studied molecules to explore the ground and excited state characteristics. UV-Visible analysis revealed the red shift in the absorption spectrum (reaching 781 nm) owing to their smaller energy gap up to 1.94 eV. Furthermore, transition density matrix analysis demonstrated that peripheral acceptors extract the electron density from the core effectively. The effectiveness of our investigated molecules as materials for high-performing organic photovoltaic cells has been shown by an examination of their electron and hole mobilities for fast charge transfer. When combined with PTB7-Th, all molecules displayed high open circuit voltage. XP5 molecule exhibited highest open circuit voltage (1.70 eV) and lowest energy loss of 0.30 eV. All designed molecules exhibit the improved aforementioned parameters, which shows that these molecules can be used to develop competent solar devices in future.

Invariant Charge Carrier Dynamics Using a Non-Planar Non-Fullerene Acceptor across Multiple Processing Solvents

Conventional non-fullerene acceptors (NFAs) typically have planar structures that can enable improved electron mobility and produce more efficient organic photovoltaic devices. A relatively simple A-D-A'-D-A type NFA specifically designed to match with poly(3-hexylthiophene-2,5-diyl) (P3HT) for green-absorbing agrivoltaic applications has been examined using a variety of techniques: microsecond transient absorption spectroscopy, atomic force microscopy, and photoluminescence. Relatively invariant charge carrier decay dynamics are observed in the blend films across a variety of processing solvents. Raman spectroscopy in conjunction with computational studies reveals that this NFA is non-planar and that multiple conformations are present in films, while preserving the crystalline nature of P3HT. The non-planarity of the NFA therefore creates a dispersive acceptor environment, irrespective of processing solvent, and this leads to the observed relative invariance in charge carrier decay dynamics and high tolerance to morphological variation. The findings presented in this work highlight the potential of non-planar materials as acceptors in organic photovoltaic devices.

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.

Designing Thieno[3,4-c]pyrrole-4,6-dione Core-Based, A2-D-A1-D-A2-Type Acceptor Molecules for Promising Photovoltaic Parameters in Organic Photovoltaic Cells

Nonfullerene-based organic solar cells can be utilized as favorable photovoltaic and optoelectronic devices due to their enhanced life span and efficiency. In this research, seven new molecules were designed to improve the working efficiency of organic solar cells by utilizing a terminal acceptor modification approach. The perceived A2-D-A1-D-A2 configuration-based molecules possess a lower band gap ranging from 1.95 to 2.21 eV compared to the pre-existing reference molecule (RW), which has a band gap of 2.23 eV. The modified molecules also exhibit higher λmax values ranging from 672 to 768 nm in the gaseous and 715-839 nm in solvent phases, respectively, as compared to the (RW) molecule, which has λmax values at 673 and 719 nm in gas and chloroform medium, respectively. The ground state geometries, molecular planarity parameter, and span of deviation from the plane were analyzed to study the planarity of all of the molecules. The natural transition orbitals, the density of state, molecular electrostatic potential, noncovalent interactions, frontier molecular orbitals, and transition density matrix analysis of all studied molecules were executed to validate the optoelectronic properties of these molecules. Improved charge mobilities and dipole moments were observed, as newly designed molecules possessed lower internal reorganization energies. The open circuit voltage (Voc) of W4, W5, W6, and W7 among newly designed molecules was improved as compared to the reference molecule. These results elaborate on the superiority of these novel-designed molecules over the pre-existing (RW) molecule as potential blocks for better organic solar cell applications.

Conformational Locking Control of 2D Outer Side Chains via Fluorine Atom Positioning for Improving the Thermal Stability of Organic Solar Cells

Alongside high power conversion efficiencies (PCEs), device stability, especially thermal issues, is another key factor for the successful commercialization of nonfullerene acceptor (NFA)-based organic solar cells (OSCs). Considering the significant effects of the side-chain engineering of NFAs on molecular packing and/or locking strongly associated with the thermal stability of OSCs, herein, we present two new isomeric NFAs with 4-fluoro- and 2-fluoro-substituted hexylphenyl two-dimensional (2D) outer side chains (4FY and 2FY, respectively). In contrast with the 2FY having a horizontal stretching conformation, 4FY exhibits a diagonal stretching conformation of the 2D outer side chains and a higher dipole moment, resulting in a huge difference in their crystalline/aggregation characteristics, i.e., 4FY possesses a higher crystallinity with a denser molecular packing than the 2FY neat film, as evidenced by thermal and morphological characterizations. Encouragingly, relative to the one based on 2FY, the OSC based on 4FY delivers a PCE as high as 16.4%, together with excellent thermal stability (88.4% PCE retention under 85 °C for 360 h), which is attributed to a more optimal and robust blend morphology induced by its better compatibility into the used donor component and stronger crystallinity. This work demonstrates that in addition to the improved photovoltaic property, the appropriate F-positioning on the 2D outer side chains can play a key role in controlling their conformations, which can promote the increase of the thermal stability of OSCs.

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.

Binary Blend All-Polymer Solar Cells with a Record Efficiency of 17.41% Enabled by Programmed Fluorination Both on Donor and Acceptor Blocks

Despite remarkable breakthrough made by virtue of "polymerized small-molecule acceptor (PSMA)" strategy recently, the limited selection pool of high-performance polymer acceptors and long-standing challenge in morphology control impede their further developments. Herein, three PSMAs of PYDT-2F, PYDT-3F, and PYDT-4F are developed by introducing different fluorine atoms on the end groups and/or bithiophene spacers to fine-tune their optoelectronic properties for high-performance PSMAs. The PSMAs exhibit narrow bandgap and energy levels that match well with PM6 donor. The fluorination promotes the crystallization of the polymer chain for enhanced electron mobility, which is further improved by following n-doping with benzyl viologen additive. Moreover, the miscibility is also improved by introducing more fluorine atoms, which promotes the intermixing with PM6 donor. Among them, PYDT-3F exhibits well-balanced high crystallinity and miscibility with PM6 donor; thus, the layer-by-layer processed PM6/PYDT-3F film obtains an optimal nanofibril morphology with submicron length and ≈23 nm width of fibrils, facilitating the charge separation and transport. The resulting PM6/PYDT-3F devices realizes a record high power conversion efficiency (PCE) of 17.41% and fill factor of 77.01%, higher than the PM6/PYDT-2F (PCE = 16.25%) and PM6/PYDT-4F (PCE = 16.77%) devices.

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.

Estimating donor:acceptor compatibility for polymer solar cells through nonfused-ring acceptors with benzoxadiazole core and different halogenated terminal groups

Novel nonfused-ring electron acceptors based on a benzoxadiazole-derived core are developed to estimate different miscibility-driven morphologies and donor:acceptor compatibilities.