High-Efficiency Thermal-Annealing-Free Organic Solar Cells Based on an Asymmetric Acceptor with Improved Thermal and Air Stability

Designing a Novel Wide Bandgap Small Molecule Guest for Enhanced Stability and Morphology Mediation in Ternary Organic Solar Cells with over 19.3% Efficiency

In this study, a novel wide-bandgap small molecule guest material, ITOA, designed and synthesized for fabricating efficient ternary organic solar cells (OSCs) ITOA complements the absorbance of the PM6:Y6 binary system, exhibiting strong crystallinity and modest miscibility. ITOA optimizes the morphology by promoting intensive molecular packing, reducing domain size, and establishing a preferred vertical phase distribution. These features contribute to improved and well-balanced charge transport, suppressed carrier recombination, and efficient exciton dissociation. Consequently, a significantly enhanced efficiency of 18.62% for the ternary device is achieved, accompanied by increased short-circuit current density (JSC), fill factor (FF), and open-circuit voltage (VOC). Building on this success, replacing Y6 with BTP-eC9 leads to an outstanding PCE of 19.33% for the ternary OSCs. Notably, the introduction of ITOA expedites the formation of the optimized morphology, resulting in an impressive PCE of 18.04% for the ternary device without any postprocessing. Moreover, the ternary device exhibits enhanced operational stability under maximum power point (MPP) tracking. This comprehensive study demonstrates that a rationally designed guest molecule can optimize morphology, reduce energy loss, and streamline the fabrication process, essential for achieving high efficiency and stability in OSCs, paving the way for practical commercial applications.

Stability of organic solar cells: toward commercial applications

Organic solar cells (OSCs) have attracted a great deal of attention in the field of clean solar energy due to their advantages of transparency, flexibility, low cost and light weight. Introducing them to the market enables seamless integration into buildings and windows, while also supporting wearable, portable electronics and internet-of-things (IoT) devices. With the development of photovoltaic materials and the optimization of fabrication technology, the power conversion efficiencies (PCEs) of OSCs have rapidly improved and now exceed 20%. However, there is a significant lack of focus on material stability and device lifetime, causing a severe hindrance to commercial applications. In this review, we carefully review important strategies employed to improve the stability of OSCs over the past three years from the perspectives of material design and device engineering. Furthermore, we analyze and discuss the current important progress in terms of air, light, thermal and mechanical stability. Finally, we propose the future research directions to overcome the challenges in achieving highly stable OSCs. We expect that this review will contribute to solving the stability problem of OSCs, eventually paving the way for commercial applications in the near future.

Ultra Robust and Highly Efficient Flexible Organic Solar Cells with Over 18 % Efficiency Realized by Incorporating a Linker Dimerized Acceptor

The wearable application of flexible organic solar cells (f-OSCs) necessitates high power conversion efficiency (PCE) and mechanical robustness. However, photoactive films based on efficient non-fullerene small molecule acceptors (NF-SMAs) are typically brittle, leading to poor mechanical stability in devices. In this study, we achieved a remarkable PCE of 18.06 % in f-OSCs while maintaining ultrahigh mechanical robustness (with a crack-onset strain (COS) of higher than 11 %) by incorporating a linker dimerized acceptor (DOY-TVT). Compared to binary blends, ternary systems exhibit reduced non-radiative recombination, suppressed crystallization and diffusion of NF-SMAs, and improved load distribution across the chain networks, enabling the dissipation of the load energy. Thus, the ternary f-OSCs developed in this study achieved, high PCE and stability, surpassing binary OSCs. Moreover, the developed f-OSCs retained 97 % of the initial PCE even after 3000 bending cycles, indicating excellent mechanical stability (9.1 % higher than binary systems). Furthermore, the rigid device with inverted structure based on the optimal active layer exhibited a substantial increase in efficiency retention, with 89.6 % after 865 h at 85 °C and 93 % after more than 1300 h of shelf storage at 25 °C. These findings highlight the potential of the linker oligomer acceptor for realizing high-performing f-OSCs with ultrahigh mechanical robustness.

Near-Infrared Acceptors with Imide-Containing End Groups for Organic Solar Cells

Near-infrared electron acceptors for organic solar cells (OSCs) mostly contain electron-withdrawing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end groups, which can be modified by but limited to phenyl, thienyl, and naphthyl units with halogenated, methyl, and methyloxy substitution. In this work, we employed an imide-containing unit to construct a new IC end group, based on which a series of new electron acceptors were synthesized. The strong electron-deficient nature of imide units enables the new acceptors to show efficient intramolecular charge transfer and hence red-shifted absorption spectra compared to their IC counterparts. These new electron acceptors were applied to OSCs, providing efficiencies of over 17% with a low voltage loss of 0.52 eV. These results demonstrate that the new imide-containing end groups are promising fragments for the construction of near-infrared electron acceptors for high-performance 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.

Novel A-π-D-π-A type non-fullerene acceptors of dithienyl diketopyrropopyrrole derivatives to enhance organic photovoltaic applications: a DFT study

To boost the photovoltaic attributes of organic photovoltaic cells, seven dithienyl diketopyrropopyrrole (TDPP) donor-based A-π-D-π-A (acceptor-bridge-donor-bridge-acceptor) type molecules (TM1-TM7) were formulated by modifying the electron accepting ends of the reference molecule (TMR). Optical and quantum chemical parameters of seven synthesized molecules were investigated using density functional theory with the MPW1PW91/6-31G(d,p) functional. Several parameters that can be used to measure and improve the efficiency of solar cells have been analyzed and summed up. These parameters include binding energy of exciton, excitation energy of electron, reorganization energies, dipole moment, molecular electrostatic potential, charge mobility, wavelength of maximum absorption, open circuit voltage, short circuit current, fill factor, density of states, transition density matrices, as well as iso-surface and non-covalent interactions. Thus, all of our proposed structures are perceived to be superior to the reference in terms of the maximum possible solar energy yield in solar cells with bulk heterojunctions, as determined by analyses of our designed molecules for the aforementioned parameters.

Organic Solar Cell With Efficiency Over 20% and VOC Exceeding 2.1 V Enabled by Tandem With All-Inorganic Perovskite and Thermal Annealing-Free Process

Organic solar cells (OSCs) based on polymer donor and non-fullerene acceptor achieve power conversion efficiency (PCE) more than 19% but their poor absorption below 550 nm restricts the harvesting of high-energy photons. In contrast, wide bandgap all-inorganic perovskites limit the absorption of low-energy photons and cause serious below bandgap loss. Therefore, a 2-terminal (2T) monolithic perovskite/organic tandem solar cell (TSC) incorporating wide bandgap CsPbI2 Br is demonstrated as front cell absorber and organic PM6:Y6 blend as rear cell absorber, to extend the absorption of OSCs into high-energy photon region. The perovskite sub-cell, featuring a sol-gel prepared ZnO/SnO2 bilayer electron transporting layer, renders a high open-circuit voltage (VOC ). The VOC is further enhanced by employing thermal annealing (TA)-free process in the fabrication of rear sub-cell, demonstrating a record high VOC of 2.116 V. The TA-free Ag/PFN-Br interface in organic sub-cell facilitates charge transport and restrains nonradiative recombination. Consequently, a remarkable PCE of 20.6% is achieved in monolithic 2T-TSCs configuration, which is higher than that of both reported single junction and tandem OSCs, demonstrating that tandem with wide bandgap all-inorganic perovskite is a promising strategy to improve the efficiency of OSCs.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

γ-Ester-Functionalized 1,1-Dicyanomethylene-3-indanone End-Capped Nonfullerene Acceptors for High-Performance, Annealing-Free Organic Solar Cells

Modifying the end-capping groups in nonfullerene acceptors (NFAs) is an effective strategy for modulating their properties and that of the entire NFAs. This study reports the synthesis of a novel γ-ester-functionalized IC end-capping group (IC-γe) and its incorporation into the benzothiadiazole-fused central core, yielding isomer-free IC-γe end-capped NFAs, such as Y-IC-γe, Y-FIC-γe, and Y-ClIC-γe. The resultant NFAs exhibited similar absorption profiles but upshifted the lowest unoccupied molecular orbital energy level compared with those of the ester-free analogues, such as Y6 and Y7. Without thermal annealing, an excellent power conversion efficiency (PCE) of 16.4% is realized in the annealing-free OSC based on Y-FIC-γe with the PM6 donor polymer, which outperforms the OSCs based on Y-IC-γe and Y-ClIC-γe. In addition, the OSCs based on asymmetric Y-FIC-γe and Y-ClIC-γe have higher thermal stability with more than 83% PCE retention at an elevated temperature after 456 h than the symmetric Y-IC-γe case. In this study, we not only establish the structure-property relationship regarding the ester functionality and symmetricity tuning on the NFAs but also diagnose the reasons for the best-performing Y-FIC-γe-based OSCs, providing useful information for a novel high-performing NFA design strategy.

Effect of Electron-Withdrawing Chlorine Substituent on Morphological and Photovoltaic Properties of All Chlorinated D-A-Type Quinoxaline-Based Polymers

The choice of the chlorine (Cl) atom as an electron-withdrawing substituent in conjugated polymers leads to a higher potential in the commercialization of polymer solar cells than its fluorine counterpart because of the versatility and cost-effectiveness of the chlorination process. In addition, the population and location of Cl substituents can significantly influence the photovoltaic characteristics of polymers. In this study, three chlorinated quinoxaline-based polymers were invented to examine the numerical and positioning effects of the Cl atom on their photovoltaic characteristics. The number of Cl substituents in the reference polymer, PBCl-Qx, was adjusted to three: two Cl atoms in the benzodithiophene-type D unit and one Cl atom in the quinoxaline-type A unit. Subsequently, two more Cl atoms were selectively introduced at the 4- and 5-positions of the alkylated thiophene moieties at the 2,3-positions of the quinoxaline moiety in PBCl-Qx to obtain the additional polymers PBCl-Qx4Cl and PBCl-Qx5Cl, respectively. The conventional PBCl-Qx4Cl device exhibited a better power conversion efficiency (PCE) of 12.95% as compared to those of PBCl-Qx (12.44%) and PBCl-Qx5Cl (11.82%) devices. The highest PCE of the device with PBCl-Qx4Cl was ascribed to an enhancement in the open-circuit voltage and fill factor induced by the deeper energy level of the highest occupied molecular orbital and the favorable morphological features in its blended film with Y6.

Enhanced photovoltaic performance of donor polymers effected by asymmetric π-bridges

Asymmetric π-bridge-based donor polymers produced via a simple one-pot chemical synthesis method exhibit enhanced photovoltaic performance.

Dual Interface Protection for High Performance and Excellent Long-Term Stability of Organic Solar Cells

Stability is still the main barrier to the commercial application of organic solar cells (OSCs), although the maximal power conversion efficiency (PCE) value has exceeded 19%. The encapsulation technique is an effective and vital way to guarantee the long-term stabilities of OSCs, but it can only avoid the penetration of water and oxygen from the environment. Herein, we introduced a structure that provides dual interface protection by using commercially available and chemically stable polyvinylidene fluoride (PVDF) as the cathode interface protection layer working as the cathode interlayer (CIL) and poly(styrene-comethyl-methacrylate) (PS-r-PMMA) as the anode interface protection layer between the poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) and the active layer. With this structure, both the migration of impurities caused by degradation of the interfacial layer and the infiltration of oxygen and water in the air can be prevented. PVDF can effectively provide optimal electron transfer by improving the surface potential of active layers and lowering the work function of the Al electrode. PS-r-PMMA can improve the hydrophobicity of PEDOT:PSS and induce optimized phase separation, facilitating charge transfer. After storage in an air environment with a humidity of approximately 60% for 3600 h, the device based on the PM6:IT-4F blend film with dual interface protection showed a decrease in its PCE value from 13.43 to 10.90%, retaining 81.2% of its original PCE value, in contrast to the sharp decrease in the PCE value from 13.66 to 0.74% of the device without dual interface protection. The dual interface protection design could also be useful in the high-performance PM6:Y6 system, which shows a champion PCE of 15.39% and shows potential for the effective fabrication of stable OSCs in the future.

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.

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA'D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π-π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve J_sc and FF without sacrificing V_oc .

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.

Intermolecular Interaction Control Enables Co-optimization of Efficiency, Deformability, Mechanical and Thermal Stability of Stretchable Organic Solar Cells

Promoting efficiency, deformability, and life expectancy of stretchable organic solar cells (OSCs) have always been key concerns that researchers are committed to solving. However, how to improve them simultaneously remains challenging, as morphology parameters, such as ordered molecular arrangement, beneficial for highly efficient devices actually limits mechanical stability and deformability. In this study, the unfavorable trade-off among these properties has been reconciled in an all-polymer model system utilizing a mechanically deformable guest component. The success of this strategy stems from introducing a highly ductile component without compromising the pristine optimized morphology. Preferable interaction between two donors can maintain the fiber-like structure while enhancing the photocurrent to improve efficiency. Morphology evolution detected via grazing incidence X-ray scattering and in situ UV-vis absorption spectra during stretching have verified the critical role of strengthened interaction on stabilizing morphology against external forces. The strengthened interaction also benefits thermal stability, enabling the ternary films with small efficiency degradation after heating 1500 h under 80 °C. This work highlights the effect of morphology evolution on mechanical stability and provides new insights from the view of intermolecular interaction to fabricate highly efficient, stable, and stretchable/wearable OSCs.

Novel High-Efficiency Polymer Acceptors via Random Ternary Copolymerization Engineering Enables All-Polymer Solar Cells with Excellent Performance and Stability

Continuous breakthroughs have been achieved in improving the efficiency of all-polymer solar cells (all-PSCs) using diimide-based polymer acceptors, and their easy-to-synthesize, low-cost, and high stability attributes make them potential candidates for use in commercial all-PSCs. However, their low light absorption coefficient, strong aggregation, and poor adaptability with high-efficient polymer donors still limit further improvements in the device performance. Here, we combine the advantages of fluorinated bithiophene and rhodanine dye molecules to create low-cost diimide-based polymer acceptors, PNDI-2FT-TR10 and PNDI-2FT-TR20, by random copolymerization for achieving highly efficient and stable all-PSCs. The synergistic effects of fluorine atoms and rhodanine dye molecules not only significantly improve the absorption coefficient but also enable enhanced miscibility and stability of the blend film. When blended with a PM6 donor, the PNDI-2FT-TR10-based device exhibits a notable power conversion efficiency (PCE) of 10.71% with a short-circuit current (J_SC) of 17.32 mA cm^-2. Note that both the PCE and J_SC show outstanding values for diimide-based all-PSCs, and this is the first report on blending diimide-based polymer acceptors with the PM6 donor to achieve high-performance all-PSCs. Moreover, the favorable morphology of the active layer enables the device to have good thickness tolerance and thermal stability. The results demonstrate that the absorption coefficients, blend morphology, and photovoltaic properties of all-PSCs could be rationally optimized by a random copolymer.

Simple-Structured Blue Thermally Activated Delayed Fluorescence Emitter for Solution-Processed Organic Light-Emitting Diodes with External Quantum Efficiency of over 20

Solution-processed organic light-emitting diodes (OLEDs) are much preferred for the manufacture of low-temperature, low-cost, large-area, and flexible lighting and displaying devices. However, these devices with high external quantum efficiency are still limited, especially for blue ones. In addition, the molecular configurations of emitters are usually complicated, indicative of high costs. In this study, two simple-structured thermally activated delayed fluorescent emitters M1 and its polymer P1 were synthesized with acridine as a donor and benzophenone as an acceptor. Solution-processed OLEDs were prepared based on M1 and P1 as doped light-emitting layer, and M1-based doped device could achieve maximum external quantum efficiency of up to 20.6% with blue-light emission.