Dithieno[3,2-b:2',3'-d]pyrrol-Fused Asymmetrical Electron Acceptors: A Study into the Effects of Nitrogen-Functionalization on Reducing Nonradiative Recombination Loss and Dipole Moment on Morphology

Inner/Outer Side Chain Engineering of Non-Fullerene Acceptors for Efficient Large-Area Organic Solar Modules Based on Non-Halogenated Solution Processing in Air

Achieving efficient and large-area organic solar modules via non-halogenated solution processing is vital for the commercialization yet challenging. The primary hurdle is the conservation of the ideal film-formation kinetics and bulk-heterojunction (BHJ) morphology of large-area organic solar cells (OSCs). A cutting-edge non-fullerene acceptor (NFA), Y6, shows efficient power conversion efficiencies (PCEs) when processed with toxic halogenated solvents, but exhibits poor solubility in non-halogenated solvents, resulting in suboptimal morphology. Therefore, in this study, the impact of modifying the inner and outer side-chains of Y6 on OSC performance is investigated. The study reveals that blending a polymer donor, PM6, with one of the modified NFAs, namely N-HD, achieved an impressive PCE of 18.3% on a small-area OSC. This modified NFA displays improved solubility in o-xylene at room temperature, which facilitated the formation of a favorable BHJ morphology. A large-area (55 cm2) sub-module delivered an impressive PCE of 12.2% based on N-HD using o-xylene under ambient conditions. These findings underscore the significant impact of the modified Y6 derivatives on structural arrangements and film processing over a large-area module at room temperature. Consequently, these results are poised to deepen the comprehension of the scaling challenges encountered in OSCs and may contribute to their commercialization.

A Deformable Additive on Defects Passivation and Phase Segregation Inhibition Enables the Efficiency of Inverted Perovskite Solar Cells over 24

The defects and phase segregation in perovskite will significantly reduce the performance and stability of perovskite solar cells (PSCs). In this work, a deformable coumarin is employed as a multifunctional additive for formamidinium-cesium (FA-Cs) perovskite. During the annealing process of perovskite, the partial decomposition of coumarin passivates the Pb2+ , iodine, and organic cation defects. Additionally, coumarin can affect colloidal size distributions, resulting in relatively large grain size and good crystallinity of target perovskite film. Hence, the carrier extraction/transport can be promoted, trap-assisted recombination is reduced, and energy levels are optimized in target perovskite films. Furthermore, the coumarin treatment can significantly release residual stress. As a result, the champion power conversion efficiencies (PCEs) of 23.18% and 24.14% are obtained for Br-rich (FA0.88 Cs0.12 PbI2.64 Br0.36 ) and Br-poor (FA0.96 Cs0.04 PbI2.8 Br0.12 ) based devices, respectively. The flexible PSCs based on Br-poor perovskite exhibit an excellent PCE of 23.13%, one of the highest values for flexible PSCs reported to date. Due to the inhibition of phase segregation, the target devices exhibit excellent thermal and light stability. This work provides new insights into the additive engineering of passivating defects, stress relief, and inhibition of phase segregation of perovskite films, offering a reliable method to develop state-of-the-art solar cells.

Structural Fusion Yields Guest Acceptors that Enable Ternary Organic Solar Cells with 18.77 % Efficiency

The design and selection of a suitable guest acceptor are particularly important for improving the photovoltaic performance of ternary organic solar cells (OSCs). Herein, we designed and successfully synthesized two asymmetric silicon-oxygen bridged guest acceptors, which featured distinct blue-shifted absorption, upshifted lowest unoccupied molecular orbital energy levels, and larger dipole moments than symmetric silicon-oxygen-bridged acceptor. Ternary devices with the incorporation of 14.2 wt % these two asymmetric guest acceptors exhibited excellent performance with power conversion efficiencies (PCEs) of 18.22 % and 18.77 %, respectively. Our success in precise control of material properties via structural fusion of five-membered carbon linkages and six-membered silicon-oxygen connection at the central electron-donating core unit of fused-ring electron acceptors can attract considerable attention and bring new vigor and vitality for developing new materials toward more efficient OSCs.

Optimized Morphology Enables High-Efficiency Nonfullerene Ternary Organic Solar Cells

Tuning the three-dimensional morphology in the active layer is an effective method to improve the performance of bulk heterojunction organic solar cells (OSCs). In this work, an acceptor-donor-acceptor structured small molecule ST10-CN-1 was synthesized and employed as the guest donor to fabricate ternary OSCs based on a PBDB-T:IT-M host binary system. The incorporation of ST10-CN-1 could broaden the active layer's absorption range of solar light thereby leading to a promotional short-circuit current. Moreover, adding an appropriate amount of ST10-CN-1 could effectively regulate the morphology of the active layer in both the lateral direction and vertical stratification. All of these morphological alterations helped to speed up the exciton dissociation, charge transit, and charge collecting processes, which in turn increased the power conversion efficiency. As a result, an excellent PCE of 11.5% for the ternary device based on PBDB-T:IT-M:ST10-CN-1 was obtained. The enhanced PCE was also linked to the formation of an alloylike state between PBDB-T and ST10-CN-1, as evidenced by the fact that the open circuit voltage of ternary OSCs lay between those for PBDB-T:IT-M (0.925 V) and ST10-CN-1:IT-M (1.064 V). This work illustrates that refining the morphology of the active layer by incorporating an appropriate third component is an effective way to further enhance the device's performance.

Achieving 19% Power Conversion Efficiency in Planar-Mixed Heterojunction Organic Solar Cells Using a Pseudosymmetric Electron Acceptor

A record power conversion efficiency (PCE) of over 19% is realized in planar-mixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe-4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/BS3TSe-4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe-4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE₂ ) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe-4F achieve a PCE of 18.48%. By incorporating another mid-bandgap acceptor Y6-O into D18/BS3TSe-4F to form a ternary PMHJ, a higher open-circuit voltage (V_OC ) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Direct Observation of the Charge Transfer States from a Non-Fullerene Organic Solar Cell with a Small Driving Force

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (E_CT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an E_CT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

Dodecacyclic-Fused Electron Acceptors with Multiple Electron-Deficient Units for Efficient Organic Solar Cells

Fused aromatic cores in non-fullerene electron acceptors (NFEAs) play a significant role in determining their optoelectronic properties and photovoltaic performance. In this work, a dodecacyclic-fused core with three electron-deficient units is synthesized through a double intramolecular Cadogan reduction cyclization. Terminal groups with different halogen substitution (F or Cl) are grafted onto the dodecacyclic-fused core to afford MS-4F and MS-4Cl, both of which showed strong and broad absorption, narrow bandgaps around 1.40 eV, and variable molecular packing model in pristine and blend films. Photovoltaic performance of solar cells containing MS-4F and MS-4Cl as NFEAs were investigated with resultant power conversion efficiencies (PCEs) of 11.75 % and 11.79 %, respectively. The mechanism study indicates that both of PBDB-T : MS-4F- and PBDB-T : MS-4Cl-based devices displayed high hole and electron mobility values, efficient charge transfer, and low charge recombination etc. These results indicate that designing multiple-fused aromatic cores with multiple electron-deficient units is a promising strategy to obtain high-performance NFEAs.

Fine-Tuning the Dipole Moment of Asymmetric Non-Fullerene Acceptors Enabling Efficient and Stable Organic Solar Cells

Modifying molecular conjugation has been demonstrated as an effective strategy to enhance the photovoltaic performance of the non-fullerene small molecule acceptors (SMAs), which would regulate the molecular packing and nanoscale morphology in the active layer of organic solar cells (OSCs). Here, two novel SMAs PTIC-4Cl and PT2IC-4Cl are designed and synthesized by expanding the core unit of TB-4Cl in one or two directions. The effects of how to expand the conjugation length on the absorption property, energy levels, dipole moment, and solubility are studied via theoretical calculation and experiments. Compared to PT2IC-4Cl, PTIC-4Cl with a more asymmetric structure exhibits the larger dipole moment and enhanced intermolecular packing. The PTIC-4Cl-based OSCs exhibit a favorable morphology and balanced charge transport, thereby leading to the highest power conversion efficiencies. In addition, PTIC-4Cl-based devices show outstanding thermal and air stability. These results reveal that fine-tuning the dipole moment via rationally expanding the conjugation in asymmetric A-D₁A'D₂-A-type non-fullerene acceptors is critical to achieve high-performance OSCs.

Understanding the Effect of Sequential Deposition Processing for High-Efficient Organic Photovoltaics to Harvest Sunlight and Artificial Light

As the market of the Internet of Things (IoT) increases, great attention has been paid to the development of high-efficient organic photovoltaics (OPVs) utilizing artificial light. However, in a real indoor condition, the power density contribution of the artificial light cannot exceed 35% in the combination of indoor and outdoor irradiation, which indicates that the illumination of sunlight cannot be ignored during daytime. Hence, it is urgent to develop high-efficient OPVs in indoor conditions taking into account both sunlight and artificial light. In this work, a novel asymmetric molecule TB-4F was synthesized to trade-off the absorption spectrum that can be applied under both artificial light and sunlight. In conventional bulk-heterojunction (C-BHJ), it was figured out that due to nonoptimal morphology some carriers failed to be efficiently collected. Herein, a sequential deposition bulk-heterojunction (SD-BHJ) as an alternative fabrication method successfully enhanced the performance of OPVs, under both artificial light and sunlight, which was attributed to the favorable microstructure being vertically distributed in the active layer. Notably, the PCE was significantly increased by 25% for SD-BHJ compared to C-BHJ under artificial light, owing to the strong effect of trap-assisted recombination and dark current on PCE in the condition of low carrier density. Our result indicates that an asymmetric molecule with a blue-shifted spectrum fabricated by SD-BHJ can be a promising candidate that can be applied in indoor environments to harvest sunlight and artificial light simultaneously.

Noncovalently Fused-Ring Electron Acceptors with C2v Symmetry for Regulating the Morphology of Organic Solar Cells

Four noncovalently fused-ring electron acceptors p-DOC6-2F, o-DOC6-2F, o-DOC8-2F, and o-DOC2C6-2F have been designed and synthesized. p-DOC6-2F and o-DOC6-2F have the same molecular backbone but different molecular shapes and symmetries. p-DOC6-2F has an S-shaped molecular backbone and C_2h symmetry, whereas o-DOC6-2F possesses a U-shaped molecular backbone and C_2v symmetry. The molecular shape and symmetry can influence the dipole moment, solubility, optical absorption, energy level, molecular packing, and film morphology. Compared with the corresponding p-DOC6-2F, o-DOC6-2F exhibits better solubility, a wider band gap, and a larger dipole moment. When blended with the donor polymer PBDB-T, the C_2v symmetric o-DOC6-2F can form an appropriate active layer morphology, whereas the C_2h symmetric p-DOC6-2F forms oversized domains. Organic solar cells (OSCs) based on p-DOC6-2F, o-DOC6-2F, o-DOC8-2F, and o-DOC2C6-2F obtained power conversion efficiencies of 9.23, 11.87, 11.23, and 10.80%, respectively. The result reveals that the molecular symmetry can facilely regulate the performance of OSCs.

Effects of heteroatoms in π-conjugated linkers on the optical and electronic properties of modified triphenylamine based dyes: towards DSSCs' applications

Optoelectronic properties of triphenylamine dyes arising from the embedded five-membered π-linkers C₄H₄X (X = O, NH, S, Se, Te) and varying anchoring groups, cyanoacrylic acid and hydantoin, in D-π-π-A model are examined. The reported properties for both, isolated dyes and dye@TiO₂ complexes, are realized through density functional theory (DFT) and time-dependent DFT. The study reveals that chalcogen doping (X = S, Se, Te) enhances absorption and fluorescent emission spectra in the visible and NIR regions. The adsorption of the dyes on the TiO₂ cluster has been simulated. Alteration of the UV-Vis spectra and electron density redistribution for the complexes from individual dyes are examined and analyzed. The binding energies relate to the nature of the heteroatoms X; the complexes dye@TiO₂ with heavier heteroatoms Se and Te demonstrate stronger binding. Graphical abstract.

Effects of the Isomerized Thiophene-Fused Ending Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells

Recently, benefiting from the merits of small-molecule acceptors (NFAs), polymer solar cells (PSCs) have achieved tremendous advances. From the perspective of the structural characteristics of the π-conjugated acceptor-donor-acceptor (A-D-A) type of organic molecules, the backbone's planarity and the terminal groups and their substituents have strong influences on the performances of the constructed NFAs. Through enlarging the dihedral angle of the conjugated main chain of NFAs, a certain degree of enhancement of photovoltaic parameters has been achieved. To further probe the influences of ending groups on the performances of nonplanar NFAs, we synthesized two new NFAs i-cc23 and i-cc34 with isomerized thiophene-fused ending groups and a twisted π-conjugated main chain. Compared to i-cc23 containing the 2-(6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene)malononitrile ending group, the acceptor i-cc34 containing 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile has a relatively higher molar extinction coefficient, bathochromic-shifted absorption spectrum, and deepened energy levels. When mixed with PBDB-T in solar cells, the i-cc23-based device achieved an excellent open-circuit voltage (V_OC) of 1.10 V and a moderate power conversion efficiency of 7.34%. Although the V_OC of the i-cc34-related device was decreased to 0.96 V, the short-circuit current density and fill factor were improved, giving rise to an enhanced efficiency of 9.51%. Apart from the distinct photovoltaic performances, the two isomer-based devices exhibit a high radiative efficiency of 8 × 10^-4, leading to a very small nonradiative loss of 0.19 V. Our results emphasize the importance of the isomerized thiophene-fused ending groups on the performances of nonplanar NFA-based PSCs.

Recent advances in non-fullerene organic solar cells: from lab to fab

The key factors for OSC materials toward application mainly include high performance, thickness tolerance, low cost, simple fabrication processing, high stability, and an environmentally-friendly nature.