Morphology Control Enables Efficient Ternary Organic Solar Cells

Using an external electric field to tune active layer morphology enabling high-efficiency organic solar cells via ambient blade coating

The nanoscale morphology of the photoactive layer notably impacts the performance of organic solar cells (OSCs). Conventional methods to tune the morphology are typically chemical approaches that adjust the properties (such as solubility and miscibility) of the active components including donor, acceptor, and/or additive. Here, we demonstrate a completely different approach by applying an external electric field (EEF) on the active layer during the wet coating. The EEF-coating method is perfectly compatible with an ambient blade coating using environmentally friendly solvents, which are essential requirements for industrial production of OSCs. A record 18.6% efficiency is achieved using the EEF coating, which is the best value for open-air, blade-coated OSCs to date. Our findings suggest broad material applicability and attribute-enhanced performance to EEF-induced fiber formation and long-range ordering of microstructures of acceptor domains. This technique offers an effective method for producing high-performance OSCs, especially suited for industry OSC production based on open-air printing.

Dual Function of the Third Component in Ternary Organic Solar Cells: Broaden the Spectrum and Optimize the Morphology

Ternary organic solar cells (T-OSCs) have attracted significant attention as high-performance devices. In recent years, T-OSCs have achieved remarkable progress with power conversion efficiency (PCE) exceeding 19%. However, the introduction of the third component complicates the intermolecular interaction compared to the binary blend, resulting in poor controllability of active layer and limiting performance improvement. To address these issues, dual-functional third components have been developed that not only broaden the spectral range but also optimize morphology. In this review, the effect of the third component on expanding the absorption range of T-OSCs is first discussed. Second, the extra functions of the third component are introduced, including adjusting the crystallinity and molecular stack in active layer, regulating phase separation and purity, altering molecular orientation of the donor or acceptor. Finally, a summary of the current research progress is provided, followed by a discussion of future research directions.

Non-halogenated Solvent-Processed Organic Solar Cells with Approaching 20 % Efficiency and Improved Photostability

The development of high-efficiency organic solar cells (OSCs) processed from non-halogenated solvents is crucially important for their scale-up industry production. However, owing to the difficulty of regulating molecular aggregation, there is a huge efficiency gap between non-halogenated and halogenated solvent processed OSCs. Herein, we fabricate o-xylene processed OSCs with approaching 20 % efficiency by incorporating a trimeric guest acceptor named Tri-V into the PM6:L8-BO-X host blend. The incorporation of Tri-V effectively restricts the excessive aggregation of L8-BO-X, regulates the molecular packing and optimizes the phase-separation morphology, which leads to mitigated trap density states, reduced energy loss and suppressed charge recombination. Consequently, the PM6:L8-BO-X:Tri-V-based device achieves an efficiency of 19.82 %, representing the highest efficiency for non-halogenated solvent-processed OSCs reported to date. Noticeably, with the addition of Tri-V, the ternary device shows an improved photostability than binary PM6:L8-BO-X-based device, and maintains 80 % of the initial efficiency after continuous illumination for 1380 h. This work provides a feasible approach for fabricating high-efficiency, stable, eco-friendly OSCs, and sheds new light on the large-scale industrial production of OSCs.

An Indacenodithienothiophene-Based Wide Bandgap Small Molecule Guest for Efficient and Stable Ternary Organic Solar Cells

The strategic and logical development of the third component (guest materials) plays a pivotal and intricate role in improving the efficiency and stability of ternary organic solar cells (OSCs). In this study, a novel guest material with a wide bandgap, named IDTR, is designed, synthesized, and incorporated as the third component. IDTR exhibits complementary absorption characteristics and cascade band alignment with the PM6:Y6 binary system. Morphological analysis reveals that the introduction of IDTR results in strong crystallinity, good miscibility, and proper vertical phase distribution, thereby realizing heightened and balanced charge transport behavior. Remarkably, the novel ternary OSCs have exhibited a significant enhancement in photovoltaic performance. Consequently, open-circuit voltage (VOC), short-circuit current (JSC), and fill factor (FF) have all witnessed substantial improvements with a remarkable power conversion efficiency (PCE) of 18.94% when L8-BO replaced Y6. Beyond the pronounced improvement in photovoltaic performance, superior device stability with a T80 approaching 400 h is successfully achieved. This achievement is attributed to the synergistic interplay of IDTR, providing robust support for the overall enhancement of performance. These findings offer crucial guidance and reference for the design and development of efficient and stable OSCs.

Binary Organic Solar Cells with over 19 % Efficiency and Enhanced Morphology Stability Enabled by Asymmetric Acceptors

The simultaneous improvement of efficiency and stability of organic solar cells (OSCs) for commercialization remains a challenging task. Herein, we designed asymmetric acceptors DT-C8Cl and DT-C8BTz with functional haloalkyl chains, in which the halogen atoms could induce noncovalent interactions with heteroatoms like O, S, and Se, etc., thus leading to appropriately manipulated film morphology. Consequently, binary devices based on D18: DT-C8Cl achieved a champion power conversion efficiency (PCE) of 19.40 %. The higher PCE of D18: DT-C8Cl could be attributed to the enhanced π-π stacking, improved charge transport, and reduced recombination losses. In addition, the noncovalent interactions induced by haloalkyl chains could effectively suppress unfavorable morphology evolutions and thereby reduce trap density of states, leading to improved thermal and storage stability. Overall, our findings reveal that the rational design of asymmetric acceptors with functional haloalkyl chains is a novel and powerful strategy for simultaneously enhancing the efficiency and stability of OSCs.

A Dual-Polythiophene Blending Strategy to Reduce the Efficiency-Stability-Cost Gap of Solar Cells

Benefiting from the photovoltaic material innovation and delicate device optimization, high-efficiency solar cells employing polymeric materials are thriving. Reducing the gap of cost, efficiency, and stability is the critical challenge faced by the emerging solar cells such as organics, quantum dots and perovskites. Poly(3-alkylthiophene) demonstrates great potential in organic solar cells and quantum dot solar cells as the active layer or the hole transport layer due to its large scalability, excellent photoelectric performance, and favorable hydrophobicity. The present low efficiency and insufficient stability, restrict its commercial application. In this work, a facile strategy of blending two simple polythiophenes is put forward to manipulate the film microstructure and enhance the device efficiency and thermal stability of solar cells. The introduction of P3PT can improve the power conversion efficiency (PCE) of a benchmark cost-effective blend P3HT:O-IDTBR to 7.41%, and the developed ternary solar cells also exhibit increased thermal stability. More strikingly, the quantum dot solar cells with the dual-polythiophene hole transport layer achieve the highest PCE of 10.51%, which is among the topmost efficiencies for quantum dots/polythiophene solar cells. Together, this work provides an effective route to simultaneously optimize the device efficiency and thermal stability of solar cells.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Fine-Tuned Active Layer Morphology for Bulk Heterojunction Organic Solar Cells with Indene-C60 Bisadduct as a Third Component

Active layer morphology is of vital importance for the photovoltaic performance of organic solar cells (OSCs). As fullerene derivatives and nonfullerene acceptors are highly complementary in many aspects, fullerene derivatives as a third component in nonfullerene OSCs could tune the blend morphology and improve the power conversion efficiency (PCE). Relative to PCBM, the indene-C60 bisadduct (IC60BA) as the third component in nonfullerene binary OSCs has not been extensively studied. Here, the fullerene derivative IC60BA is introduced into the PTZ1:IDIC blend system to finely tune the active layer morphology. Although the addition of IC60BA reduced the film absorption in the visible region and weakened the crystallinity, the more symmetric charge transport property, smaller domain size, and higher domain purity led to improved photovoltaic performance. This study indicates that IC60BA is a promising candidate to finely tune the morphology for achieving highly efficient OSCs.

Electrostatic force promoted intermolecular stacking of polymer donors toward 19.4% efficiency binary organic solar cells

Conjugated polymers are generally featured with low structural order due to their aromatic and irregular structural units, which limits their light absorption and charge mobility in organic solar cells. In this work, we report a conjugated molecule INMB-F that can act as a molecular bridge via electrostatic force to enhance the intermolecular stacking of BDT-based polymer donors toward efficient and stable organic solar cells. Molecular dynamics simulations and synchrotron X-ray measurements reveal that the electronegative INMB-F adsorb on the electropositive main chain of polymer donors to increase the donor-donor interactions, leading to enhanced structural order with shortened π-π stacking distance and consequently enhanced charge transport ability. Casting the non-fullerene acceptor layer on top of the INMB-F modified donor layer to fabricate solar cells via layer-by-layer deposition evidences significant power conversion efficiency boosts in a range of photovoltaic systems. A power conversion efficiency of 19.4% (certified 18.96%) is realized in PM6/L8-BO binary devices, which is one of the highest reported efficiencies of this material system. The enhanced structural order of polymer donors by INMB-F also leads to a six-fold enhancement of the operational stability of PM6/L8-BO organic solar cells.

Designing efficient A-D-A1-D-A type fullerene free acceptor molecules with enhanced power conversion efficiency for solar cell applications

The achievement of highly efficient power conversion efficiency (PCE) is a big concern for non-fullerene organic solar cells (NF-OSCs) because PCE can depend on numerous variables. Here, new five novel acceptor molecules without fullerenes were developed and investigated using DFT (density functional theory) and TD-DFT (time dependent-density functional theory). Compared to the recently synthesized molecule (PZ-dIDTC6), the developed molecules display a narrow optical band gap, exhibiting a red shift in the absorption spectrum. The developed molecules (YM1-YM5) express high mobility of electrons and holes in the active layer of OSCs (organic solar cells). In addition, high open-circuit voltage (V_oc) values with maximum charge density shifting are noted in designed molecules. YM1-YM5 is also associated with low binding energy and excitation energy. This work proves that noncovalent conformational locking is favourable for improving PCE devices.

Ternary Blend Organic Solar Cells: Understanding the Morphology from Recent Progress

Ternary blend organic solar cells (TB-OSCs) incorporating multiple donor and/or acceptor materials into the active layer have emerged as a promising strategy to simultaneously improve the overall device parameters for realizing higher performances than binary devices. Whereas introducing multiple materials also results in a more complicated morphology than their binary blend counterparts. Understanding the morphology is crucially important for further improving the device performance of TB-OSC. This review introduces the solubility and miscibility parameters that affect the morphology of ternary blends. Then, this review summarizes the recent processes of morphology study on ternary blends from the aspects of molecular crystallinity, molecular packing orientation, domain size and purity, directly observation of morphology, vertical phase separation as well as morphological stability. Finally, summary and prospects of TB-OSCs are concluded.

Promoting the Efficiency and Stability of Nonfullerene Organic Photovoltaics by Incorporating Open-Cage [60]Fullerenes in the Nonfullerene Nanocrystallites

The device efficiency of PM6:Y6-based nonfullerene organic solar cells is fast advanced recently. To maintain organic solar cells (OSCs) with high power conversion efficiency over 16% in long-term operation, however, remains a challenge. Here, a novel non-volatile additive, an open-cage [60]fullerene (8OC₆₀Me), is incorporated into PM6:Y6-based OSCs for high-performance with high durability. With optimized addition of 1.0 wt % 8OC₆₀Me, the PCE value of PM6:Y6/8OC₆₀Me OSCs can be promoted to 16.5% from 15.0%. Most strikingly, such a high PCE performance can maintain nearly 100% for over 500 h at room temperature; at an elevated operation temperature of 80 °C, the PCE can be stabilized above 15.0% after 45 h of operation. Grazing incidence small- and wide- angle X-ray scattering studies reveal improved orientation and crystallinity of Y6 in a fractal-like network structure of PM6 in PM6:Y6/8OC₆₀Me films under in situ annealing, parallel to the enhanced electron mobility. Analysis of charge distributions lines up possible van der Waals interaction between the thienyl/carbonyl moiety of 8OC₆₀Me and difluorophenyl-based FIC-end groups of Y6. This result is of great contrast to those devices with the best-selling PC₆₁BM as the additives─8OC₆₀Me might be of interest to be incorporated into future Y6-based OSCs for concomitantly improved PCE and excellent stability.

Process-Aid Solid Engineering Triggers Delicately Modulation of Y-Series Non-Fullerene Acceptor for Efficient Organic Solar Cells

Volatile solids with symmetric π-backbone are intensively implemented on manipulating the nanomorphology for improving the operability and stability of organic solar cells. However, due to the isotropic stacking, the announced solids with symmetric geometry cannot modify the microscopic phase separation and component distribution collaboratively, which will constrain the promotion of exciton splitting and charge collection efficiency. Inspired by the superiorities of asymmetric configuration, a novel process-aid solid (PAS) engineering is proposed. By coupling with BTP core unit in Y-series molecule, an asymmetric, volatile 1,3-dibromo-5-chlorobenzene solid can induce the anisotropic dipole direction, elevated dipole moment, and interlaminar interaction spontaneously. Due to the synergetic effects on the favorable phase separation and desired component distribution, the PAS-treated devices feature the evident improvement of exciton splitting, charge transport, and collection, accompanied by the suppressed trap-assisted recombination. Consequently, an impressive fill factor of 80.2% with maximum power conversion efficiency (PCE) of 18.5% in the PAS-treated device is achieved. More strikingly, the PAS-treated devices demonstrate a promising thickness-tolerance character, where a record PCE of 17.0% is yielded in PAS devices with a 300 nm thickness photoactive layer, which represents the highest PCE for thick-film organic solar cells.

Rylene-Fullerene Hybrid an Emerging Electron Acceptor for High-Performing and Photothermal-Stable Ternary Solar Cells

Molecular carbon imides, especially extended perylene diimides (PDIs) have been the best wide-band-gap nonfullerene acceptors. Despite their excellent photothermal/chemical stability, flexible reaction sites, and unique photoelectronic properties, there is still a lack of fundamental understanding of their molecular characteristics as a third component. Here, generations of PDIs with distinctive molecular architecture, are deliberately screened out as the third component to PM6:Y6. Only a rylene-fullerene hybrid, S-Fuller-PMI, surprisingly boosts the fill factor (FF) of ternary organic solar cells (OSCs) to 0.77 from 0.72 for PM6:Y6 binary ones, and therefore the power conversion efficiency (PCE) of ternary cells is enhanced from 15.3% to 16.2%. Compared with highly-flexible rylene dimer and rigid multimer, S-Fuller-PMI exhibits higher electron mobility, favorable surface tension, and, therefore tailored compatibility with Y6. These formed Y6:S-Fuller-PMI alloys play as a morphological controller to improve charge separation and transport process. Simultaneously, the suppressed photothermal-induced traps, along with inherent enlarged entropy effect, endow the ternary OSCs still with ≈70% of initial PCE even after 500 h continuous illumination, whereas only 53% is left in their binary counterparts. These results provide new insight into the molecular design principle for distinctive molecular carbon imides as the third component for efficient and durable PM6:Y6-based OSCs.

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.

Enhanced Efficiency and Excellent Thermostability in Organic Photovoltaics via Ternary Strategy with Twisted Conjugated Compound

The thermal stability of high-efficiency organic photovoltaics (OPVs) is critical to the manufacturing of this technology. Therefore, targeted strategies and approaches shall be developed to improve efficiency and stability, simultaneously. Herein, a seleno twisted benzodiperylenediimides (TBD-PDI-Se) acceptor-doping strategy is taken advantage of to demonstrate the ternary bulk heterojunction OPVs with excellent stability, achieving outstanding power conversion efficiency of 14.43% and 17.25% based on PM6:IT-4F and PM6:Y6 ternary blend devices, respectively, which are superior to the corresponding binary devices. As evidenced by the active layer morphology, exciton dynamic study and the characterizations of the enabled device, the ternary blend device keeps nearly 90% original efficiency (t = 1000 h) under continuous constant heating at 140 °C. Furthermore, the application of acceptor as the third component in PBDB-T:ITIC, J71:ITIC, and PBDB-T:PC₇₁ BM systems is also verified, proving the good universality of acceptor-doping ternary strategy.

Perylene Diimide Hexamer Based on Combination of Direct and Indirect Linkage Manners for Non-fullerene Organic Solar Cells

Perylene diimide (PDI) is one of the most intensively studied building blocks for the construction of non-fullerene acceptors (NFAs). In this contribution, based on combination of the direct and indirect linkage manners of PDI units at the bay position, a propeller-shaped PDI hexamer T-DPDI was designed and synthesized. The singly bonded PDI dimer DPDI and the benzene ring cored PDI trimer TPDI were synthesized for comparison. The photovoltaic performances of these three PDI derivatives were investigated using the commercially available PTB7-Th as electron donor. A best power conversion efficiency (PCE) of 6.58% was obtained for T-DPDI based organic solar cells (OSCs), which is higher than those of DPDI and TPDI based ones. The superior photovoltaic performance of T-DPDI can be ascribed to its stronger absorption and more favorable morphology. This study presents an interesting example of improving the photovoltaic performances of PDI based NFAs by hybridizing the direct and indirect linkage manners.

Binary and Ternary Polymer Solar Cells Based on a Wide Bandgap D-A Copolymer Donor and Two Nonfullerene Acceptors with Complementary Absorption Spectral

A new wide-bandgap conjugated D-A polymer denoted as P106 with a medium acceptor dithieno [2,3-e;3'2'-g]isoindole-7,9 (8H) (DTID) unit and strong 2-dodecylbenzo[1,2-b:3,4-b':6,5-b"]trithiophene (3TB) donor units shows an optical bandgap of 2.04 and highest occupied molecular orbital energy level of -5.56 eV. P106 is used as the donor and two nonfullerene acceptors-medium bandgap DBTBT-IC and narrow band Y18-DMO-are used as acceptors for the construction of binary and ternary bulk heterojunction polymer solar cells. The optimized polymer solar cells based on P106 : DBTBT-IC and P106 : Y18-DMO exhibit power conversion efficiencies of 11.76 % and 14.07 %, respectively. The short-circuit current density (22.78 mA cm^-2 ) for the P106 : Y18-DMO device is higher than that for P106 : DBTBT-IC (18.56 mA cm^-2 ) one, which could be attributed to the more photon harvesting efficiency of the P106 : Y18-DMO active layer. In light of the high short-circuit current densities and fill factors for the Y18-DMO based device and the high value of open circuit voltage of the DBTBT-IC based device, ternary polymer solar cells are fabricated by using ternary active layer (P106 : DBTBT-IC : Y18-DMO) and achieve a power conversion efficiency of 16.49 % with low energy loss of 0.47 eV.

Ultrafast Fluctuations in PM6 Domains of Binary and Ternary Organic Photovoltaic Thin Films Probed with Two-Dimensional White-Light Spectroscopy

We present two-dimensional white-light spectroscopy (2DWL) measurements of binary and ternary bulk heterojunctions of the polymer donor PM6 mixed with state-of-the-art nonfullerene acceptors Y6 or IT4F. The ternary film has a shorter lifetime and faster spectral diffusion than either of the binary films. 2D line shape analysis of the PM6 ground state bleach with a Kubo model determines that all three films have similar amplitudes of fluctuations (Δ = 0.29 fs^-1) in their transition frequencies, but different relaxation times (ranging from 102 to 24 fs). The ternary film exhibits faster dynamics than either of the binary films. The short lifetime of the ternary blend is consistent with increased photoexcitation transfer and the fast frequency fluctuations are consistent with structural dynamics of aliphatic side chains. These results suggest that the femtosecond fluctuations of PM6 are impacted by the choice of the acceptor molecules. We hypothesize that those dynamics are either indicative, or perhaps the initial source, of structural dynamics that ultimately contribute to solar cell operation.

Capture the high-efficiency non-fullerene ternary organic solar cells formula by machine-learning-assisted energy-level alignment optimization

Appropriate energy-level alignment in non-fullerene ternary organic solar cells (OSCs) can enhance the power conversion efficiencies (PCEs), due to the simultaneous improvement in charge generation/transportation and reduction in voltage loss. Seven machine-learning (ML) algorithms were used to build the regression and classification models based on energy-level parameters to predict PCE and capture high-performance material combinations, and random forest showed the best predictive capability. Furthermore, two sets of verification experiments were designed to compare the experimental and predicted results. The outcome elucidated that a deep lowest unoccupied molecular orbital (LUMO) of the non-fullerene acceptors can slightly reduce the open-circuit voltage (V_OC) but significantly improve short-circuit current density (J_SC), and, to a certain extent, the V_OC could be optimized by the slightly up-shifted LUMO of the third component in non-fullerene ternary OSCs. Consequently, random forest can provide an effective global optimization scheme and capture multi-component combinations for high-efficiency ternary OSCs.

•

ML assists in analyzing energy-level alignment of non-fullerene ternary blends

•

Random forest approach provides the best predictive capability

•

The effective global optimization scheme in material selection is provided

Introducing a third component into a binary blend to fabricate the ternary organic solar cells (OSCs) is a common practice to enhance light harvest and reduce energy loss of the photoactive blends, especially the non-fullerene ternary OSCs, which showed thrilling power conversion efficiencies improvement. A proper energy-level alignment in ternary blends, promoting the device charge generation, transport, and extraction, is of importance to maximize the short-circuit current and open-circuit voltage simultaneously. The machine-learning (ML) technique is a powerful tool for processing complex data from previous research to find the underlying mechanisms. In this work, we built regression and classification models, aiming to find the relationship between molecular energy levels and device performances. The results demonstrated that random forest is an effective method to assess the energy-level alignment, providing guidelines for the design of high-performance non-fullerene ternary OSCs.

Regulating the Aggregation of Unfused Non-Fullerene Acceptors via Molecular Engineering towards Efficient Polymer Solar Cells

Tuning molecular aggregation via structure design to manipulate the film morphology still remains as a challenge for polymer solar cells based on unfused non-fullerene acceptors (UF-NFAs). Herein, a strategy was developed to modulate the aggregation patterns of UF-NFAs by systematically varying the π-bridge (D) unit and central core (A') unit in A-D-A'-D-A framework (A and D refer to electron-withdrawing and electron-donating moieties, respectively). Specifically, the quantified contents of H- or J-aggregation and crystallite disorder of three UF-NFAs (BDIC2F, BCIC2F, and TCIC2F) were analyzed via UV/Vis spectrometry and grazing incidence X-ray scattering. The results showed that the H-aggregate-dominated BCIC2F with less crystallite disorder exhibited a more favorable blend morphology with polymer donor PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione)]) relative the other two UF-NFAs, resulting in improved exciton dissociation and charge tranport. Consequently, photovoltaic devices based on BCIC2F delivered a promising power conversion efficiency of 12.4 % with an exceptionally high short-circuit current density of 22.1 mA cm^-2 .

Light intensity dependence of organic solar cell operation and dominance switching between Shockley-Read-Hall and bimolecular recombination losses

We investigated the variation of current density-voltage (J-V) characteristics of an organic solar cell (OSC) in the dark and at 9 different light intensities ranging from 0.01 to 1 sun of the AM1.5G spectrum. All three conventional parameters, short-circuit currents (Jsc), open-circuit voltage (Voc), and Fill factor (FF), representing OSC performance evolved systematically in response to light intensity increase. Unlike Jsc that showed quasi-linear monotonic increase, Voc and FF showed distinctive non-monotonic variations. To elucidate the origin of such variations, we performed extensive simulation studies including Shockley-Read-Hall (SRH) recombination losses. Simulation results were sensitive to defect densities, and simultaneous agreement to 10 measured J-V curves was possible only with the defect density of [Formula: see text]. Based on analyses of simulation results, we were able to separate current losses into SRH- and bimolecular-recombination components and, moreover, identify that the competition between SRH- and bimolecular-loss currents were responsible for the aforementioned variations in Jsc, Voc, and FF. In particular, we verified that apparent demarcation in Voc, and FF variations, which seemed to appear at different light intensities, originated from the same mechanism of dominance switching between recombination losses.

A Well-Mixed Phase Formed by Two Compatible Non-Fullerene Acceptors Enables Ternary Organic Solar Cells with Efficiency over 18.6

The ternary strategy, introducing a third component into a binary blend, opens a simple and promising avenue to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The judicious selection of an appropriate third component, without sacrificing the photocurrent and voltage output of the OSC, is of significant importance in ternary devices. Herein, highly efficient OSCs fabricated using a ternary approach are demonstrated, wherein a novel non-fullerene acceptor L8-BO-F is designed and incorporated into the PM6:BTP-eC9 blend. The three components show complementary absorption spectra and cascade energy alignment. L8-BO-F and BTP-eC9 are found to form a homogeneous mixed phase, which improves the molecular packing of both the donor and acceptor materials, and optimizes the ternary blend morphology. Moreover, the addition of L8-BO-F into the binary blend suppresses the non-radiative recombination, thus leading to a reduced voltage loss. Consequently, concurrent increases in open-circuit voltage, short-circuit current, and fill factor are realized, resulting in an unprecedented PCE of 18.66% (certified value of 18.2%), which represents the highest efficiency values reported for both single-junction and tandem OSCs so far.

A New Conjugated Polymer that Enables the Integration of Photovoltaic and Light-Emitting Functions in One Device

Exploring the intriguing bifunctional nature of organic semiconductors and investigating the feasibility of fabricating bifunctional devices are of great significance in realizing various applications with one device. Here, the design of a new wide-bandgap polymer named PBQx-TCl (optical bandgap of 2.05 eV) is reported, and its applications in photovoltaic and light-emitting devices are studied. By fabricating devices with nonfullerene acceptors BTA3 and BTP-eC9, it is shown that the devices exhibit a high power conversion efficiency (PCE) of 18.0% under air mass 1.5G illumination conditions and an outstanding PCE of 28.5% for a 1 cm² device and 26.0% for a 10 cm² device under illumination from a 1000 lux light-emitting diode. In addition, the PBQx-TCl:BTA3-based device also demonstrates a moderate organic light-emitting diode performance with an electroluminescence external quantum efficiency approaching 0.2% and a broad emission range of 630-1000 nm. These results suggest that the polymer PBQx-TCl-based devices exhibit outstanding photovoltaic performance and potential light-emitting functions.

Simultaneous Optimization of Donor/Acceptor Pairs and Device Specifications for Nonfullerene Organic Solar Cells Using a QSPR Model with Morphological Descriptors

Optimally efficient organic solar cells require not only a careful choice of new donor (D) and/or acceptor (A) molecules but also the fine-tuning of experimental fabrication conditions for organic solar cells (OSCs). Herein, a new framework for simultaneously optimizing D/A molecule pairs and device specifications of OSCs is proposed, through a quantitative structure-property relationship (QSPR) model built by machine learning. Combining the device bulk properties with structural and electronic properties, the built QSPR model achieved unprecedentedly high accuracy and consistency. Additionally, a large chemical space of 1 942 785 D/A pairs is explored to find potential synergistic ones. Favorable device bulk properties such as the root-mean-square of surfaces roughness for D/A blends and the D/A weight ratio are further screened by grid search methods. Overall, this study indicates that the simultaneous optimization of D/A molecule pairs and device specifications by theoretical calculations can accelerate the improvement of OSC efficiencies.

A Simple Dithieno[3,2-b:2',3'-d]pyrrol-Rhodanine Molecular Third Component Enables Over 16.7% Efficiency and Stable Organic Solar Cells

Organic solar cells (OSCs) can achieve greatly improved power conversion efficiency (PCE) by incorporating suitable additives in active layers. Their structure design often faces the challenge of operation generality for more binary blends. Herein, a simple dithieno[3,2-b:2',3'-d]pyrrole-rhodanine molecule (DR8) featuring high compatibility with polymer donor PM6 is developed as a cost-effective third component. By employing classic ITIC-like ITC6-4Cl and Y6 as model nonfullerene acceptors (NFAs) in PM6-based binary blends, DR8 added PM6:ITC6-4Cl blends exhibit significantly promoted energy transfer and exciton dissociation. The PM6:ITC6-4Cl:DR8 (1:1:0.1, weight ratio) OSCs contribute an exciting PCE of 14.94% in comparison to host binary devices (13.52%), while PM6:Y6:DR8 (1:1.2:0.1) blends enable 16.73% PCE with all simultaneously improved photovoltaic parameters. To the best of the knowledge, this performance is among the best for ternary OSCs with simple small molecular third components in the literature. More importantly, DR8-added ternary OSCs exhibit much improved device stability against thermal aging and light soaking over binary ones. This work provides new insight on the design of efficient third components for OSCs.

Mechanism of the Alcohol-Soluble Ionic Organic Interlayer in Organic Solar Cells

In this article combining density functional theory (DFT) calculations and corresponding experimental measurements, the adsorption behaviors and working mechanism of the alcohol-soluble ionic organic interlayer on different electrode substrates were studied. The results suggest that, when the ionic organic bipyridine salt interlayer (FPyBr) is adsorbed on the Ag surface, Br^- will break away from molecule chains and form new chemical bonds with the Ag substrate, as confirmed by both the X-ray photoelectron spectroscopy (XPS) study and DFT study for the first time. Charges are further found to transfer to the Ag substrate from the new interlayer molecular structure without Br^-, resulting in adsorption dipoles directed from Ag to the interlayer. Moreover, the direction of the intrinsic dipole of the molecule itself on the Ag substrate is also verified, which is the same as that of the adsorption dipole. Subsequently, the superposition of the two dipoles results in a large reduction of the Ag substrate work function. In addition, the dipole formation mechanism of the interlayer on the ITO surface was also studied. The change in the work function of the ITO substrate by this interlayer is found to be smaller than that of Ag as confirmed by both a DFT study and scanning Kelvin probe microscopy (SKPM) results, which is mainly due to the reversed direction of the molecular intrinsic dipole with respect to the interfacial dipole. The worst device performance of organic solar cells based on the ITO-FPyBr substrate is considered to be one of the consequences of the feature. The findings here are of great importance for the study of the mechanism of the ionic organic interlayer in organic electronic devices, providing insightful understandings on how to further improve the material and device performance.

Benzo[1,2-b:4,5-b']difuran Polymer-Based Non-Fullerene Organic Solar Cells: The Roles of Non-Fullerene Acceptors and Molybdenum Oxide on Their Ambient Stabilities and Processabilities

The ambient stability and processability of organic solar cells (OSCs) are important factors for their commercialization. Herein, we selected four benzo[1,2-b:4,5-b']difuran (BDF) polymers and two electron acceptors to examine the role of photovoltaic materials in the ambient stability. The investigations revealed that the MoO_x layer is the detrimental factor for the ambient stabilities. The penetration of MoO_x into the active layer and their interactions will strengthen the interface and form a favorable contact, hence leading to the increased photovoltaic performance, in which the efficiency loss induced by air was balanced out. As such, these BDF polymer-based non-fullerene (NF) OSCs possessed very promising ambient stabilities even after ∼1000 h with the almost maintained power conversion efficiencies (PCEs). These results drive us to further investigate the ambient processability of these NF-OSCs. The PCEs from the devices processed under ambient condition only possessed 0.3-2% loss compared to those devices under inert conditions, which suggest the significant potentials of BDF polymers to develop highly efficient and stable NF-OSCs for the practical applications.

Recent advances, challenges and prospects in ternary organic solar cells

The past decade has seen a tremendous development of organic solar cells (OSCs). To date, high-performance OSCs have boosted power conversion efficiencies (PCEs) over 17%, showing bright prospects toward commercial applications. Compared with binary OSCs, ternary OSCs, by introducing a third component as a second donor or acceptor into the active layer, have great potential in realizing outstanding photovoltaic performance. Herein, a comprehensive review of the recent advances of ternary solar cells is presented. According to the chemical components of active layer materials, we classify the ternary systems into four categories, including polymer/small molecule/small molecule, polymer/polymer/small molecule, all-polymer and all-small-molecule types. The relationships among the photovoltaic materials structure and weight ratio, active layer morphology and photovoltaic performance are systematically analyzed and summarized. The features and design strategies of each category are also discussed and summarized. Key issues and challenges faced in ternary OSCs are pointed out, and potential strategies and solutions are proposed. This review may provide guidance for the field of ternary OSCs.

From Y6 to BTPT-4F: a theoretical insight into the influence of the individual change of fused-ring skeleton length or side alkyl chains on molecular arrangements and electron mobility

Organic solar cells (OSCs) based on non-fullerene acceptor (NFA) Y6 have drawn tremendous attention due to the great progress in their power conversion efficiencies (PCEs).

Characterization of optical manipulation using microlens arrays depending on the materials and sizes in organic photovoltaics

Various physical structures have improved light-harvesting and power-conversion efficiency in organic photovoltaic devices, and optical simulations have supported the improvement of device characteristics. Herein, we experimentally investigated how microlens arrays manipulate light propagation in microlens films and material stacks for organic photovoltaics to understand the influence of the constituent materials and sizes of the microlens. As materials to fabricate a microlens array, poly(dimethylsiloxane) and Norland Optical Adhesive 63 were adopted. The poly(dimethylsiloxane) microlens array exhibited higher total transmittance and higher diffuse transmittance, further enhancing the effective optical path and light extinction in material stacks for organic photovoltaics. This resulted in more current generation in an organic photovoltaic device with a poly(dimethylsiloxane) microlens array than in a Norland Optical Adhesive 63 microlens array. The sizes of the microlenses were controlled from 0.5 to 10 μm. The optical characteristics of microlens array films and material stacks with microlenses generally increased with size of the microlens, leading to a 10.6% and 16.0% improvement in the light extinction and power-conversion efficiency, respectively. In addition, electron and current generation in material stacks for organic photovoltaics were calculated from light extinction. The theoretical current generation matched well with experimental values derived from organic photovoltaic devices. Thus, the optical characterization of physical structures helps to predict how much more current can be generated in organic photovoltaic cells with a certain physical structure; it can also be used for screening the physical structures of organic photovoltaic cells.

The influence of constituent materials and sizes of a microlens was experimentally and theoretically explored.

Isomeric Effect of Wide Bandgap Polymer Donors with High Crystallinity to Achieve Efficient Polymer Solar Cells

Two highly crystalline polymer donors (PBTz4T2C-a, PBTz4T2C-b) with isomers (4T2C-a, 4T2C-b) are synthesized and applied in polymer solar cells. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor IT2F. It is interesting that the photophysical properties, crystallinity, and active layer morphology characteristic can be significantly changed by just slightly regulating the substitution position of the carboxylate groups. A series of simulation calculations of the two isomers are conducted in the geometry and electronic properties to explore the difference induced by the position adjustment of carboxylate groups. The results decipher that 4T2C-b moiety features much stronger intramolecular noncovalent S⋯O interactions compared to that of 4T2C-a, implying a higher coplanarity and much stronger crystallinity, and leading to excessive phase separation in PBTz4T2C-b:IT2F blend film. In contrast, PBTz4T2C-a with 4T2C-a moiety exhibits suitable crystallinity with a lower the highest occupied molecular orbital level, higher film absorption coefficient, and charge mobilities, resulting in a much higher power conversion efficiency of 11.02%. This research demonstrates that the molecular conformation is of great importance to be considered for developing high-performance polymer donors.

An Optimized Fibril Network Morphology Enables High-Efficiency and Ambient-Stable Polymer Solar Cells

Morphological stability is crucially important for the long-term stability of polymer solar cells (PSCs). Many high-efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene-4,8-dione (T1-Cl) via a random copolymerization approach. It is found that all the copolymers can self-assemble into a fibril nanostructure in films. By altering the T1-Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT-4F, O-INIC3, EH-INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high-efficiency and ambient-stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Polyethylenimine-Ethoxylated Interfacial Layer for Efficient Electron Collection in SnO2-Based Inverted Organic Solar Cells

In this work, we studied inverted organic solar cells based on bulk heterojunction using poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PCBM) as an active layer and a novel cathode buffer bilayer consisting of tin dioxide (SnO2) combined with polyethylenimine-ethoxylated (PEIE) to overcome the limitations of the single cathode buffer layer. The combination of SnO2 with PEIE is a promising approach that improves the charge carrier collection and reduces the recombination. The efficient device, which is prepared with a cathode buffer bilayer of 20 nm SnO2 combined with 10 nm PEIE, achieved Jsc = 7.86 mA/cm2, Voc = 574 mV and PCE = 2.84%. The obtained results exceed the performances of reference solar cell using only a single cathode layer of either SnO2 or PEIE.

Facile Method of Solvent-Flushing To Building Component Distribution within Photoactive Layers for High-Performance Organic Solar Cells

Suitable donor and acceptor distribution in the blended photoactive layer benefits the charge transfer and exciton separation to boost the performance of organic solar cells (OSCs). Herein, we propose a universal solvent-flushing method for building component distribution in photoactive layers based on the different solubilities of the donor and acceptor in acetylacetone (Acac). The donor and acceptor concentration distribution through the photoactive layers is investigated by the time-of-flight secondary-ion mass spectroscopy, and the surface concentration changes are examined by contact angle measurements and atomic force microscopy tests. The charge-transfer properties of OSCs before and after Acac flushing are further investigated by the rectification ratio and light intensity-dependent J_sc and V_oc measurements. For inverted OSCs based on PBDB-TF:IT-4F, the power conversion efficiency (PCE) enhances from 12.87 to 14.05%, and for a PBDB-TF:Y6-based device, the PCE also significantly increases from 15.40 to 16.51% because of greatly enhanced J_sc and FF, benefited from enhanced charge transport and suppressed charge recombination by solvent-flushing. Our findings suggest that solvent-flushing is a simply processed and easily controlled method to achieve vertical component distribution in photoactive layers to boost the performance of OSCs.

Improved Performance of Ternary Solar Cells by Using BODIPY Triads

Two boron dipyrromethene (BODIPY) triads, namely BODIPY-1 and BODIPY-2, were synthesized and incorporated with poly-3-hexyl thiophene: (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) P3HT:PCBM. The photovoltaic performance of BODIPY:P3HT:PCBM ternary solar cells was increased, as compared to the control binary solar cells (P3HT:PCBM). The optimized power conversion efficiency (PCE) of BODIPY-1:P3HT:PCBM was improved from 2.22% to 3.43%. The enhancement of PCE was attributed to cascade charge transfer, an improved external quantum efficiency (EQE) with increased short circuit current (Jsc), and more homogeneous morphology in the ternary blend.

Optimized active layer morphology toward efficient and polymer batch insensitive organic solar cells

Morphology control in laboratory and industry setting remains as a major challenge for organic solar cells (OSCs) due to the difference in film-drying kinetics between spin coating and the printing process. A two-step sequential deposition method is developed to control the active layer morphology. A conjugated polymer that self-assembles into a well-defined fibril structure is used as the first layer, and then a non-fullerene acceptor is introduced into the fibril mesh as the second layer to form an optimal morphology. A benefit of the combined fibril network morphology and non-fullerene acceptor properties was that a high efficiency of 16.5% (certified as 16.1%) was achieved. The preformed fibril network layer and the sequentially deposited non-fullerene acceptor form a robust morphology that is insensitive to the polymer batches, solving a notorious issue in OSCs. Such progress demonstrates that the utilization of polymer fibril networks in a sequential deposition process is a promising approach towards the fabrication of high-efficiency OSCs.

Reliably controlling the morphology in organic solar cells is desired for up-scaling. Here Weng et al. combine the advantages of the fibril network donor and the state of the art Y6 acceptor in a two-step approach to deliver a high efficiency of 16% without batch-to-batch variation.

Progress in Stability of Organic Solar Cells

The organic solar cell (OSC) is a promising emerging low-cost thin film photovoltaics technology. The power conversion efficiency (PCE) of OSCs has overpassed 16% for single junction and 17% for organic-organic tandem solar cells with the development of low bandgap organic materials synthesis and device processing technology. The main barrier of commercial use of OSCs is the poor stability of devices. Herein, the factors limiting the stability of OSCs are summarized. The limiting stability factors are oxygen, water, irradiation, heating, metastable morphology, diffusion of electrodes and buffer layers materials, and mechanical stress. The recent progress in strategies to increase the stability of OSCs is surveyed, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation materials. The International Summit on Organic Photovoltaic Stability guidelines are also discussed. The potential research strategies to achieve the required device stability and efficiency are highlighted, rendering possible pathways to facilitate the viable commercialization of OSCs.