Spatially Orthogonal 2D Sidechains Optimize Morphology in All-Small-Molecule Organic Solar Cells

Organic semiconductors consist of a conjugated backbone and flexible sidechains. Compared to the meticulous design of backbones, less attention has been paid to the investigation of sidechains, in particular their spatial orientation. Herein, three non-fullerene acceptors, anti-PDFC, syn-PDFC, and PDFC-Ph, are applied in all-small-molecule organic solar cells (ASM-OSCs) to reveal the varied effects of sidechains on morphology and device performance. With spatially orthogonal alkyl chains, anti-PDFC and syn-PDFC show unique bimodal lamellar packing and moderate crystallinity. When blending with an efficient binary BTR-Cl/Y6 system, anti-PDFC as well as syn-PDFC not only form their own crystal phase but also improve the packing order of BTR-Cl, consequently enhancing the power conversion efficiency (PCE) of ternary ASM-OSC to be 14.56%. However, although PDFC-Ph has an identical backbone with anti-PDFC, the alternated sidechains make it relatively amorphous, which is prone to damage the original packing of the host donor/acceptor, and thus deteriorating the device performance. When PC₇₁BM is added to optimize the morphology further, the triple-acceptor device involving anti-PDFC realizes a PCE of 15.67%, which is among the best efficiencies in ASM-OSCs. This study demonstrates that a multi-dimensional sidechain can optimize the morphology of a bulk heterojunction as effective as a conjugated backbone.     

Deciphering the Role of Side-Chain Engineering and Solvent Vapor Annealing for Binary All-Small-Molecule Organic Solar Cells

Fibrous interpenetrating network structure morphology is extremely crucial for all-small-molecule organic solar cells (ASM-OSCs) in achieving high power conversion efficiency (PCE). Rational molecular design and suitable posttreatment to the film are feasible methods to accomplish this goal. Herein, two small molecule donors, namely T4 and T6, with different substituents on their selenophene conjugated units, alkyl for T4 while trialkylsilyl for T6, are developed. Both as cast devices obtain poor PCEs (≈4.5%) when blending these two donors with N3 due to the oversize phase separation. Satisfactorily, the PCEs are dramatically increased after CS₂ annealing, which mainly originates from the favorable reorganization of donor and acceptor in the active layer, ultimately improving the phase separation and vertical electronic properties. As a result, the device based on trialkylsilyl-substituted T6 acquires a remarkable PCE of 16.03%, much higher than that of the blends of alkyl-substituted T4 and N3 (12.61%). The enhanced PCE of the T6-based device is attributed to the deeper HOMO energy levels, more obvious fibrous interpenetrating networks, and stronger molecular interaction between T6 and N3, as compared with T4-based ones. This study indicates that precise molecular design and the proper posttreatment process can be a brilliant approach for realizing highly efficient ASM-OSCs.     

Dual-Additive-Driven Morphology Optimization for Solvent-Annealing-Free All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs), which consist of small-molecule donors and acceptors, have recently been studied extensively to eliminate the batch-to-batch variation from polymer-based donor or acceptor. On the other hand, the control of their active layer morphology is more challenging due to the similar chemical structure and miscibility of small-molecule donor and small-molecule accepter. Hence, this study develops a dual-additive-driven morphology optimization method for ASM-OSCs based on BTR-Cl:Y6. One solid additive – 1,4-diiodobenzene (DIB) and one liquid additive – diiodomethane (DIM) are selected, making use of their distinct interaction mechanisms with Y6 and BTR-Cl. It is found that DIB can form a eutectic phase with Y6, which can increase the intermolecular interactions and modulate the acceptor phase separation, while the simultaneous volatilization of DIM suppresses the over-aggregation of BTR-Cl during the film casting process. As a result of the synergistic morphology tuning, the optimized device delivers a power conversion efficiency (PCE) as high as 15.2%, among the highest PCE reported to date for binary ASM-OSCs without solvent annealing treatment. This work demonstrates the potential of morphology tuning via the incorporation of dual additives into ASM-OSCs, enabling them to achieve comparable efficiencies to those of conventional polymer/small-molecule based OSCs.     

A Volatile Solid Additive Enables Oligothiophene All-Small-Molecule Organic Solar Cells with Excellent Commercial Viability

The commercial viability of all-small-molecule (ASM) organic solar cells (OSCs) requires high efficiency, long-term stability, and low-cost production. However, satisfying all these factors at the same time remains highly challenging. Herein, a volatile solid additive, namely, 1,8-dichloronaphthalene (DCN) is demonstrated to simultaneously enhance the power conversion efficiency (PCE) and the storage, thermal as well as photo stabilities of oligothiophene ASM-OSCs with concise and low-cost syntheses. The improved PCEs are mainly due to the DCN-induced morphology control with improved exciton dissociation and reduced non-geminate recombination. Notably, the PCE of 16.0% stands as the best value for oligothiophene ASM-OSCs and is among the top values for all types of binary ASM-OSCs. In addition, devices incorporating DCN have shown remarkable long-term stability, retaining over 90% of their initial PCE after dark storage aging of 3000 h and thermal or light stressing of 500 h. The findings demonstrate that the volatile-solid-additive strategy can be a simple yet effective method of delivering highly efficient and stable oligothiophene ASM-OSCs with excellent commercial viability.     

High-Performance Green Thick-Film Ternary Organic Solar Cells Enabled by Crystallinity Regulation

The power conversion efficiency (PCE) of organic solar cells (OSCs) has reached high values of over 19%. However, most of the high-efficiency OSCs are fabricated by spin-coating with toxic solvents and the optimal photoactive layer thickness is limited to 100 nm, limiting practical development of OSCs. It is a great challenge to obtain ideal morphology for high-efficiency thick-film OSCs when using non-halogenated solvents due to the unfavorable film formation kinetics. Herein, high-efficiency ternary thick-film (300 nm) OSCs with PCE of 15.4% based on PM6:BTR-Cl:CH1007 are fabricated by hot slot-die coating using non-halogenated solvent (o-xylene) in the air. Compared to PM6:BTR-Cl:Y6 blends, the stronger pre-aggregation of CH1007 in solution induces the earlier aggregation of CH1007 molecules and longer aggregation time, and thus results in high and balanced crystallinity of donors and acceptor in CH1007-based ternary film, which led to high-carrier mobility and suppressed charge recombination. The ternary strategy is further used to fabricate high-efficiency, thick-film, large-area, and flexible devices processed from non-halogenated solvents, paving the way for industrial development of OSCs.     

Additive-free organic solar cells with enhanced efficiency enabled by unidirectional printing flow of high shear rate

High-performance OSCs prepared by scalable techniques without additives are highly desirable because residual additives may cause gradual deterioration of the photoactive-layer morphology and device performance. Printing flows with high shear rate have the potential to replace additives by inducing higher degree of ordered stacking and crystallinity of organic molecules, as well as favorable phase separation. Here, PTQ10:Y6 organic solar cells (OSCs) without any additives were fabricated by a scalable and robust processing approach termed as soft porous blade printing (SPBP). The fluid flow and drying process of the wet films made by SPBP, blade coating and spin coating are visualized by high speed imaging, which reveals that the blade coating and SPBP introduce unidirectional flow while the wet film interference pattern of spin coating is irregular and random. The simplified flow model of SPBP suggests that the shear rate could be as high as ~1000 s⁻¹. The additive-free SPBP produces photoactive-layer with adequate morphology, which could be attributed to three intrinsic properties of SPBP: very high shear rate, flow assisted crystallizations induced by microstructures of the soft porous blade, and numerous nucleation sites generated as the liquid contact line follows the motion of the blade. The additive-free SPBP device demonstrates weaker charge recombination, higher and more balanced charge transport, and consequently better device performance than the spin-coated and blade-coated devices with 0.25 vol% 1,8-diioctane (DIO). SPBP achieved power conversion efficiency (PCE) of 16.45%, which is higher than those of spin-coated and blade-coated counterparts doped with DIO.     

Anthracene-Assisted Morphology Optimization in Photoactive Layer for High-Efficiency Polymer Solar Cells

Currently, morphology optimization methods for the fused-ring nonfullerene acceptor-based polymer solar cells (PSCs) empirically follow the treatments originally developed in fullerene-based systems, being unable to meet the diverse molecular structures and strong crystallinity of the nonfullerene acceptors. Herein, a new and universal morphology controlling method is developed by applying volatilizable anthracene as solid additive. The strong crystallinity of anthracene offers the possibility to restrict the over aggregation of fused-ring nonfullerene acceptor in the process of film formation. During the kinetic process of anthracene removal in the blend under thermal annealing, donor can imbed into the remaining space of anthracene in the acceptor matrix to form well-developed nanoscale phase separation with bi-continuous interpenetrating networks. Consequently, the treatment of anthracene additive enables the power conversion efficiency (PCE) of PM6:Y6-based devices to 17.02%, which is a significant improvement with regard to the PCE of 15.60% for the reference device using conventional treatments. Moreover, this morphology controlling method exhibits general application in various active layer systems to achieve better photovoltaic performance. Particularly, a remarkable PCE of 17.51% is achieved in the ternary PTQ10:Y6:PC₇₁BM-based PSCs processed by anthracene additive. The morphology optimization strategy established in this work can offer unprecedented opportunities to build state-of-the-art PSCs.     

All Slot-Die Coated Non-Fullerene Organic Solar Cells with PCE 11%

Slot-die (SD) coating is used to fabricate fully solution processed organic solar cells (OSCs) based on a blend of high performance donor polymer (PTB7-Th) and a non-fullerene acceptor (IEICO-4F) for stable devices over extended periods of operation. The optimization of a sequential deposition process of transport and active layers, under ambient conditions, enable high efficiency slot-die coated solar cells with remarkable power conversion efficiencies (PCE) > 11.0% to bridge the gap between lab-to-fab. Fully slot-die coated inverted OSCs are demonstrated with efficiencies reaching 11% along with 1 cm² devices, proving the scalability and reproducibility of the proposed technique. Further, replacing the evaporated Ag electrode with solution processed Ag nanowire (AgNW) electrodes shows the highest light utilization efficiency of 5.26% for semi-transparent OSC with a PCE of 9.07% and average visible transmission of 58%.     

Releasing Acceptor from Donor Matrix to Accelerate Crystallization Kinetics with a Second Donor toward High-Efficiency Green-Printable Organic Photovoltaics

The state-of-the-art power conversion efficiency (PCE) of organic solar cells (OSCs) is typically achieved in the devices fabricated by toxic halogen solvents with complex post-treatment processes in strictly inert atmosphere. Developing suitable processing method for printing in ambient air using eco-friendly solvents with continuous solution supply and fabricating efficient devices without any post-treatment are intensively desired. Controlling the crystallization kinetics to fine-tune the acceptor's assembly behavior with a second donor for favorable morphological evolution is an effective approach to achieve above requirements. Herein, a kinetics-controlling strategy is implemented by introducing a strong crystalline small molecule, BTR-Cl, to enhance the crystallinity of acceptors. The combined in situ spectra characterizations revealed that the earlier aggregation of acceptor and modulation in conformation of PM6 can be achieved. This unique aggregation behavior facilitated enhanced film crystallization with reduced paracrystallinity of π–π stacking, resulting in improved charge transport and inhibited charge recombination. An outstanding PCE of 17.50% is obtained for the device processed with o-xylene via ambient air printing without any post-treatment. More significantly, efficient all-printed inverted devices and large-area modules are prepared. The generalization of this strategy has been confirmed in other efficient systems, suggesting a great potential for universally fabricating high-efficiency and eco-friendly OSCs.     

High Performance Roll‐to‐Roll Produced Fullerene‐Free Organic Photovoltaic Devices via Temperature‐Controlled Slot Die Coating

Solution‐processed organic photovoltaics (OPVs) have continued to show their potential as a low‐cost power generation technology; however, there has been a significant gap between device efficiencies fabricated with lab‐scale techniques—i.e., spin coating—and scalable deposition methods. Herein, temperature‐controlled slot die deposition is developed for the photoactive layer of OPVs. The influence of solution and substrate temperatures on photoactive films and their effects on power conversion efficiency (PCE) in slot die coated OPVs using a 3D printer‐based slot die coater are studied on the basis of device performance, molecular structure, film morphology, and carrier transport behavior. These studies clearly demonstrate that both substrate and solution temperatures during slot die coating can influence device performance, and the combination of hot substrate (120 °C) and hot solution (90 °C) conditions result in mechanically robust films with PCE values up to 10.0% using this scalable deposition method in air. The efficiency is close to that of state‐of‐the‐art devices fabricated by spin coating. The deposition condition is translated to roll‐to‐roll processing without further modification and results in flexible OPVs with PCE values above 7%. The results underscore the promising potential of temperature‐controlled slot die coating for roll‐to‐roll manufacturing of high performance OPVs.     

Modulation of Alkyl Chain Length on the Thiazole Side Group Enables Over 17% Efficiency in All-Small-Molecule Organic Solar Cells

Molecular innovation is highly desirable to achieve efficient all-small-molecule organic solar cells (SM-OSCs). Herein, three small-molecule donors (SMDs) with alkylated thiazole side groups (namely BO-1, HD-1, and OD-1), which differ only in the alkyl side chain are reported. Although these SMDs possess similar absorption profiles and molecular energy levels, their crystallinity and miscibility with BTP-eC9 slightly decrease along with the elongation of the alkyl side chain. After blending with BTP-eC9, different miscibility leads to different degrees of phase separation. Among these SM-OSCs, the HD-1-based device shows a decent bulk-heterojunction (BHJ) morphology with proper phase separation and more dynamic carrier behavior. Thus, compared to the BO-1 and OD-1-based devices, the HD-1-based device achieves a higher short-circuit current of 26.04 mA cm⁻² and a fill factor of 78.46%, leading to an outstanding PCE of 17.19%, which is one of the highest values among SM-OSCs. This work provides a rational design strategy of SMDs for highly efficient SM-OSCs.     

Highly efficient inverted rapid-drying blade-coated organic solar cells

Efficient inverted bulk-heterojunction (BHJ) poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic solar cells fabricated by rapid-drying blade-coated were demonstrated. Optimized self-organization interpenetration networks and donor/acceptor domain sizes were obtained while maintaining the smooth surface morphology. By integrating with low-temperature-processed sol-gel ZnO electron extraction layer, power conversion efficiency (PCE) up to 4.4% under AM1.5G 1 sun illumination is achieved, compared to fast drying but low efficiency (1.2%) and high efficiency but with long-time solvent annealing treatment (4.3%) control cells deposited by spin coating in chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) solution, respectively. The novel deposition technique reveals a promising process for highly efficient, high throughput, stable morphology organic solar cells fabrication.     

High-Efficiency All-Small-Molecule Organic Solar Cells Based on New Molecule Donors with Conjugated Symmetric/Asymmetric Hybrid Cyclopentyl-Hexyl Side Chains

All small molecule organic solar cells (ASM-OSCs) have numerous advantages but lower power conversion efficiencies (PCEs) than their polymer equivalents, which is largely due to the suboptimal nanoscale network structure in a bulk heterojunction (BHJ). Herein, new small molecule donors with symmetric/asymmetric hybrid cyclopentyl-hexyl side chains are designed, accounting for manipulated intermolecular interactions and BHJ morphology. Theoretical and experimental results reveal that the asymmetric cyclopentyl-hexyl side chains modification has a significant influence on potential energy surface and intermolecular interaction that can ensure preferable molecular assembly and regulate the D/A interfacial energetics, thus boosting the exciton dissociation and charge transport when pairing with a wide-used acceptor L8-BO. Concurrently, a nanoscale bicontinuous interpenetrating network with optimal domain size can be fully evolved in the BHJ layer. As a consequence, the As-TCp-based binary device achieves a superior PCE of 16.46% in comparison to that of the controlled symmetric counterparts S-BF (14.92%) and A-TCp (15.77%), and ranks one of best performance among ASM-OSCs. This study demonstrates that precise manipulation of the cyclo-alkyl chain in combination with the asymmetric 2D side chain strategy is an effective synergistic approach to control intermolecular interaction and nanoscale bicontinuous phase separation for achieving high-performance ASM-OSCs.     

Regulating crystallization to maintain balanced carrier mobility via ternary strategy in blade-coated flexible organic solar cells

Regulating the crystallization of donor and acceptor to maintain balanced carrier mobility is of great importance to fabricate efficient organic solar cells (OSCs). Herein, the balanced crystallinity between donor and acceptor was finely controlled in blade-coated OSCs. By adding high crystalline FOIC into PBDB-T:ITIC system, a balanced carrier mobility was achieved, resulting in the much improved fill factor. The optimized ternary device exhibits an increased current density, due to the enhanced light-harvesting efficiency with complementary absorption and the morphology change. Morphology characterization demonstrated that the ternary film exhibits a highly balanced crystallinity between the donor and acceptor on account of the formation of acceptor alloy. Moreover, the ternary film not only possesses a small domain size, but also exhibits a high domain purity as compared to both binary films. Encouragingly, a highest power conversion efficiency (PCE) of 10.68% was obtained for the blade-coated ternary OSCs. In addition, the blade-coated flexible large-area (105 mm²) OSC based on PBDB-T:ITIC:FOIC ternary system also exhibits a high PCE of 9.81%, showing great potential in the high-throughput fabrication of OSCs.     

Isomeric Small Molecule Donor with Terminal Branching Position Directly Attached to the Backbone Enables Efficient All-Small-Molecule Organic Solar Cells with Excellent Stability

All-small-molecule organic solar cells (ASM-OSCs) are challenging for their inadequate efficiency and device stability due to their more susceptive morphology. Herein, a family of isomeric small molecule donors (SMDs) is synthesized based on the benzodithiophene–terthiophene core with linear, 1st carbon, and 2nd carbon position branched butyl-based rhodanine for ASM-OSCs, respectively. The single crystal of thiophene-substituted model T-s-Bu forms a more compact intermolecular packing with herringbone structure than slip-layered packing-based T-n-Bu and T-i-Bu . SM-i-Bu and SM-s-Bu show slightly blue-shifted absorption and deepened HOMO levels in the neat film compared to SM-n-Bu . SM-s-Bu:BO-4Cl blend films have distinct face-on packing orientations and suitable fibrous phase separation along with more apparent microcrystals. Finally, SM-s-Bu : BO-4Cl -based device yields an improved power conversion efficiency of 16.06% compared to 15.12% and 8.22% for SM-n-Bu : BO-4Cl and SM-i-Bu : BO-4Cl , which is one of the top-ranked results for BTR-series SMDs in binary ASM-OSCs. More importantly, the excellent storage stability with a T₈₀ lifetime of over 1700 h and decent thermal stability is realized in SM-s-Bu : BO-4Cl . This work highlights that the isomeric terminal alkyl with a branching point directly connected to the backbone for SMDs is a promising strategy for improving the crystal packing and film morphology and achieving highly efficient and stable ASM-OSCs.     

An efficient nonfullerene acceptor for all-small-molecule solar cells with versatile processability in environmentally benign solvents

A new structure of dicyanodistyrylbenzene-naphthalimide-based nonfullerene acceptor NIDCSN was synthesized and characterized to have a favorable electron accepting property and versatile processability in various organic solvents. The nonfullerene all-small-molecule solar cells comprising p-DTS(FBTTh₂)₂ as the donor and NIDCSN as the acceptor exhibited a maximum power conversion efficiency of 3.45% with a remarkable open-circuit voltage of 1.04 V, together with similar device performances when fabricated in five different solvents including environmentally benign non-halogenated ones.     

Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V_oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E_loss) of 0.521 eV. Such small E_loss is the best record for polymer solar cells with PCEs over 16% to date.     

High efficiency arrays of polymer solar cells fabricated by spray‐coating in air

We present bulk heterojunction organic solar cells fabricated by spray‐casting both the PEDOT:PSS hole‐transport layer (HTL) and active PBDTTT‐EFT:PC₇₁BM layers in air. Devices were fabricated in a (6 × 6) array across a large‐area substrate (25 cm²) with each pixel having an active area of 6.45 mm². We show that the film uniformity and operational homogeneity of the devices are excellent. The champion device with spray cast active layer on spin cast PEDOT:PSS had an power conversion efficiency (PCE) of 8.75%, and the best device with spray cast active layer and PEDOT:PSS had a PCE of 8.06%. The impacts of air and light exposure of the active layer on device performance are investigated and found to be detrimental. Copyright © 2015 John Wiley & Sons, Ltd.     

Annealing‐Free High Efficiency and Large Area Polymer Solar Cells Fabricated by a Roller Painting Process

Polymer solar cells are fabricated by a novel solution coating process, roller painting. The roller‐painted film – composed of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) – has a smoother surface than a spin‐coated film. Since the roller painting is accompanied by shear and normal stresses and is also a slow drying process, the process effectively induces crystallization of P3HT and PCBM. Both crystalline P3HT and PCBM in the roller‐painted active layer contribute to enhanced and balanced charge‐carrier mobility. Consequently, the roller‐painting process results in a higher power conversion efficiency (PCE) of 4.6%, as compared to that for spin coating (3.9%). Furthermore, annealing‐free polymer solar cells (PSCs) with high PCE are fabricated by the roller painting process with the addition of a small amount of octanedi‐1,8‐thiol. Since the addition of octanedi‐1,8‐thiol induces phase separation between P3HT and PCBM and the roller‐painting process induces crystallization of P3HT and PCBM, a PCE of roller‐painted PSCs of up to 3.8% is achieved without post‐annealing. A PCE of over 2.7% can also be achieved with 5 cm² of active area without post‐annealing.     

Naphthodithiophene Diimide (NDTI)-Based Cathode Interlayer Material Enables 19% Efficiency Binary Organic Solar Cells

A fused naphthodithiophene diimide (NDTI) derivative is first used as cathode interlayer materials (CIMs) in organic solar cells, by introducing two dimethylamine-functionalized fluorenes on both sides, namely NDTI1 . Meanwhile, two non-fused naphthalene diimide (NDI) derivatives are synthesized as the control CIMs to validate the design strategy of fused NDI. All three CIMs show high thermal stability, robust adhesion, and strong electrode modification capability. Compared with two NDI-based materials, NDTI1 possesses excellent film-forming capacity and strong crystallinity, simultaneously. Besides, NDTI1 presents a strong self-doping effect and distinct intermolecular interaction with non-fullerene acceptors. As expected, the NDTI1 -based OSCs achieve a power conversion efficiency (PCE) of 18.02% using the PM6:Y6 active layer and a champion PCE of 19.01% employing the active layer PM6:L8-BO, which is attributed to improve charge transport and extraction, and suppressive charge recombination. More importantly, NDTI1 retains 91% of the optimal PCE when the film thickness increases from 7 to 20 nm. Furthermore, NDTI1 also exhibits satisfactory universality for different active layer materials and excellent device stability.