Synergistic effect of polymer and small molecules for high-performance ternary organic solar cells

Optimization of Charge Management and Energy Loss in All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) have received tremendous attention in recent decades because of their advantages over their polymer counterparts. These advantages include well-defined chemical structures, easy purification, and negligible batch-to-batch variation. Remarkable progress with a power conversion efficiency (PCE) of over 17% has recently been achieved with improved charge management (FF × JSC) and reduced energy loss (Eloss). Morphology control is the key factor in the progress of ASM-OSCs, which remains a significant challenge because of the similarities in the molecular structures of the donors and acceptors. In this review, the effective strategies for charge management and/or Eloss reduction from the perspective of effective morphology control are summarized. The aim is to provide practical insights and guidance for material design and device optimization to promote further development of ASM-OSCs to a level where they can compete with or even surpass the efficiency of polymer solar cells.

High efficiency double light-emitting layer sky blue phosphor organic light-emitting diode with slow emission efficiency attenuation

In this study, by doping the iridium (III) double ((4,6-difluorobenzene) -pyridine-N, C2'Pyridinformate (FIrpic) into 4,4', 4″ three (carbazole) trioxilamine (TcTa) and 9 (4 tert-butyl-benzene) -3,6 double (triphenylsilyl) -9H carbazole (CzSi) layers as light emitting layers (EMLs) to achieve sky blue phosphor organic electroluminescence (EL) devices with very low efficiency roll-off. The dependence of the EL performance on the doping concentration of the light-emitting molecules and the different EML thicknesses are studied in detail. Experimental data show that the thin-film-doped TcTa layer is very important for the efficiency of the device. Finally, the optimal device achieves extremely high power efficiency, current efficiency, and external quantum efficiency (EQE), reaching 74.82 lm/W, 66.68 cd/A, and 31.9 %, respectively. At a certain luminance of 2000 cd/m2 (3.4 V), the same device retained power efficiency with current efficiency and external quantum efficiency of 57.71 lm/W, 62.46 cd/A and 29.3 %, respectively, based on theoretical discussion, the improved EL efficiency was attributed to the energy levels of the host and luminescent molecules, which helps to balance the charge carrier in the emitter molecule distribution and expand the recombination region.

Broadband Organic Ternary Bulk Heterojunctions Photodetector Based on Non-Fullerene Acceptor with Enhanced Flat-Spectrum Response Range from 200 to 1100 nm

Both flat-spectrum responsivity and high external quantum efficiency (EQE) of bulk heterojunction organic photodetectors (BHJ OPDs) are greatly in demand and still challenging to realize from the ultraviolet (UV) to near-infrared (NIR) regions. In this article, conjugated polymer donor poly(3-hexylthiophene) (P3HT) and PTB7-Th are blended with a low band gap nonfullerene acceptor (NFA) IEICO-4F to form a ternary BHJ active layer, thereby forming a BHJ OPD with a broadband responsivity spectrum from UV to visible light to NIR region (200-1100 nm). Under 6 V voltage and in the range from 280 to 810 nm, the ternary BHJ OPD shows a relatively flat responsivity spectrum, and the highest responsivity is 1.348 A/W, which is 1.34 times that of the binary BHJ OPD. Specifically, the ternary BHJ OPD achieved the highest EQE at 285 nm and as high as 449.31%. In addition, the ternary OPD detectivity (D*) is 2.65 times that of the binary BHJ OPD. Therefore, ternary BHJ as an active layer provides an effective method to develop BHJ OPDs with an expanded response range, higher responsivity, improved EQE, and broadband spectrum with flat spectral response from the UV to NIR region.

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.

Improving ternary blend morphology by adding a conjugated molecule into non-fullerene polymer solar cells

The use of ternary polymer solar cells (PSCs) is a promising strategy to enhance photovoltaic performance while improving the fill factor (FF) of a device, but is still a challenge due to the complicated morphology. Herein, ternary PSCs are fabricated via adding the conjugated small molecule p-DTS(FBTTh₂)₂ into a well-known blended film, PTB7-Th:IEICO-4F. The ternary blend morphology and device characterization reveal that the addition of p-DTS(FBTTh₂)₂ can improve crystallinity and optimize morphology, leading to the FF of the optimized device increasing to 73.69%. In combination with the advantages of an ultra-narrow bandgap material, IEICO-4F, with a broad optical absorption spectrum, the optimized ternary solar cell exhibits a high short-circuit current–density (J_SC) of 25.22 mA cm⁻². The best power conversion efficiency (PCE) is 12.84% for this optimized ternary device with 10 wt% p-DTS(FBTTh₂)₂ in the donors. This work indicates that incorporating a small molecule with high crystallinity into host binary non-fullerene PSCs would give an active layer with high crystallinity, thus greatly enhancing the FFs and PCEs of PSCs.

The use of ternary polymer solar cells (PSCs) is a promising strategy to enhance photovoltaic performance while improving the fill factor (FF) of a device, but is still a challenge due to the complicated morphology.

Enhanced and Balanced Charge Transport Boosting Ternary Solar Cells Over 17% Efficiency

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB-T-C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so-called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB-T-C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

High-Performance Ternary Organic Solar Cells with Controllable Morphology via Sequential Layer-by-Layer Deposition

Ternary blending of light-harvesting materials has been proven to be a potential strategy to improve the efficiency of solution-processed organic solar cells (OSCs). However, the optimization of a ternary system is usually more complicated than that of a binary one as the morphology of conventional ternary blend films is very difficult to control, thus undermining the potential of ternary OSCs. Herein, we report a general strategy for better control of the morphology of ternary blend films composed of a polymer donor and two nonfullerene small-molecule acceptors for high-performance OSCs using the sequential layer-by-layer (LbL) deposition method. The resulting LbL films form a bicontinuous interpenetrating network structure with high crystallinity of both the donor and acceptor materials, showing efficient charge generation, transport, and collection properties. In addition, the power conversion efficiencies (PCEs) of the ternary LbL OSCs are less sensitive to the blending ratio of the third component acceptor, providing more room to optimize the device performance. As a result, optimal PCEs of over 11, 13, and 16% were achieved for the LbL OSCs composed of PffBT4T-2OD/IEICO-4F:FBR, PBDB-T-SF/IT-4F:FBR, and PM6/Y6:FBR, respectively. Our work provides useful and general guidelines for the development of more efficient ternary OSCs with better controlled morphology.

Enhanced Vertical Charge Transport of Homo- and Blended Semiconducting Polymers by Nanoconfinement

The morphology of conjugated polymers has critical influences on electronic and optical properties of optoelectronic devices. Even though lots of techniques and methods are suggested to control the morphology of polymers, very few studies have been performed inducing high charge transport along out-of-plane direction. In this study, the self-assembly of homo- and blended conjugated polymers which are confined in nanostructures is utilized. The resulting structures lead to high charge mobility along vertical direction for both homo- and blended conjugated polymers. Both semicrystalline and amorphous polymers show highly increased population of face-on crystallite despite intrinsic crystallinity of polymers. They result in more than two orders of magnitude enhanced charge mobility along vertical direction revealed by nanoscale conductive scanning force microscopy and macroscale IV characteristic measurements. Moreover, blends of semicrystalline and amorphous polymers, which are known to show inferior optical and electrical properties due to their structural incompatibility, are formed into harmonious states by this approach. Assembly of blends of semicrystalline and amorphous polymers under nanoconfinement shows charge mobility in out-of-plane direction of 0.73 cm² V^-1 s^-1 with wide range of absorption wavelength from 300 to 750 nm demonstrating the synergistic effects of two different polymers.

Benzothiadiazole Based Cascade Material to Boost the Performance of Inverted Ternary Organic Solar Cells

A conjugated, ladder-type multi-fused ring 4,7-dithienbenzothiadiazole:thiophene derivative, named as compound ‘T’, was for the first time incorporated, within the PTB7:PC71BM photoactive layer for inverted ternary organic solar cells (TOSCs) realization. The effective energy level offset caused by compound T between the polymeric donor and fullerene acceptor materials, as well as its resulting potential as electron cascade material contribute to an enhanced exciton dissociation, electron transfer facilitator and thus improved overall photovoltaic performance. The engineering optimization of the inverted TOSC, ITO/PFN/PTB7:Compound T(5% v/v):PC71BM/MoO3/Al, resulted in an overall power conversion efficiency (PCE) of 8.34%, with a short-circuit current density (Jsc) of 16.75 mA cm−2, open-circuit voltage (Voc) of 0.74 V and a fill factor (FF) of 68.1%, under AM1.5G illumination. This photovoltaic performance was improved by approximately 12% with respect to the control binary device.

All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies

The high efficiency all-small-molecule organic solar cells (OSCs) normally require optimized morphology in their bulk heterojunction active layers. Herein, a small-molecule donor is designed and synthesized, and single-crystal structural analyses reveal its explicit molecular planarity and compact intermolecular packing. A promising narrow bandgap small-molecule with absorption edge of more than 930 nm along with our home-designed small molecule is selected as electron acceptors. To the best of our knowledge, the binary all-small-molecule OSCs achieve the highest efficiency of 14.34% by optimizing their hierarchical morphologies, in which the donor or acceptor rich domains with size up to ca. 70 nm, and the donor crystals of tens of nanometers, together with the donor-acceptor blending, are proved coexisting in the hierarchical large domain. All-small-molecule photovoltaic system shows its promising for high performance OSCs, and our study is likely to lead to insights in relations between bulk heterojunction structure and photovoltaic performance.

Dual Role of Graphene Quantum Dots in Active Layer of Inverted Bulk Heterojunction Organic Photovoltaic Devices

Graphene quantum dots (GQDs) have shown broad application prospects in the field of photovoltaic devices due to their unique quantum confinement and edge effects. Here, we prepared GQDs by a photon-Fenton reaction as reported in our previous work, which has great advantage in the preparation scale. The photoelectric properties of the inverted hybrid solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C₆₁ butyric acid methylester (PCBM):GQDs and P3HT:GQDs with different contents of GQDs as the active layers are demonstrated, as well as their morphology and structure by atomic force microscopy images. Then, the different roles of GQDs played in the ternary (P3HT:PCBM:GQDs) and binary (P3HT:GQDs) hybrid solar cells are studied systematically. The results indicate that the GQDs provide an efficient excition separation interface and charge transport channel for the improvement of hybrid solar cells. The preliminary exploration and elaboration of the role of GQDs in hybrid solar cells will be beneficial to understand the interfacial procedure and improve device performance in the future.

Organic Photovoltaics with Multiple Donor-Acceptor Pairs

Compared with conventional organic solar cells (OSCs) based on single donor-acceptor pairs, terpolymer- and ternary-based OSCs featuring multiple donor-acceptor pairs are promising strategies for enhancing the performance while maintaining an easy and simple synthetic process. Using multiple donor-acceptor pairs in the active layer, the key photovoltaic parameters (i.e., short-circuit current density, open-circuit voltage, and fill factor) governing the OSC characteristics can be simultaneously or individually improved by positive changes in light-harvesting ability, molecular energy levels, and blend morphology. Here, these three major contributions are discussed with the aim of offering in-depth insights in combined terpolymers and ternary systems. Recent exemplary cases of OSCs with multiple donor-acceptor pairs are summarized and more advanced research and perspectives for further developments in this field are highlighted.

Achieving Over 15% Efficiency in Organic Photovoltaic Cells via Copolymer Design

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron-withdrawing unit, ester-substituted thiophene, is incorporated into a PBDB-TF polymer to downshift the molecular energy and broaden the absorption. Copolymer-based solar cells suitable for large-area devices can be fabricated by a blade-coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment-friendly processing conditions and show relatively low photostability compared to copolymer-based devices. These results not only demonstrate high-efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.

13%-Efficiency Quaternary Polymer Solar Cell with Nonfullerene and Fullerene as Mixed Electron Acceptor Materials

In this article, we report 13%-efficiency quaternary polymer solar cell. By introducing bis-PC₇₁BM:PC₇₁BM into a known nonfullerene system-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):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone-methyl))-5,5,11,11-tetrakis(4- n-hexylphenyl)-dithieno[2,3 d:2',3' d']- s-indaceno[1,2 b:5,6 b']dithiophene (PBDB-T:IT-M), the quaternary solar cell significantly outperforms the nonfullerene binary and the ternary (PBDB-T:IT-M:fullerene) devices with a significant increase in the short-circuit current-density (18.2 vs 16.5 and 16.8-17.5 mA/cm²) and the fill factor (0.73 vs 0.67 and 0.707-0.726), and hence, large power conversion efficiency (13% for quaternary vs 11% for the binary and 12% for the ternary). Grazing incidence wide-angle X-ray scattering data indicate that both the polymer and IT-M phase crystallinity becomes greater upon introduction of PC₇₁BM as the forth additive into the host ternary PBDB-T:IT-M:bis-PC₇₁BM, which results in an increase in both the electron and hole mobilities, contributing to the J_sc enhancement. Our results indicate that the use of the forth fullerene component provides more choices and more mechanisms than the ternary systems for tuning the photon-to-electron conversion; therefore, sheds light on the realization of high-efficiency polymer solar cells by designing the multiacceptor components with aligned energy levels, complementary absorption spectra, and improved film morphologies.

Enhanced performance of PTB7-Th:PCBM based active layers in ternary organic solar cells

The present study aims at understanding the role of incorporating Cu₂S nanocrystals (NCs) as a third component in ternary organic solar cells.

Ternary organic solar cells: using molecular donor or acceptor third components to increase open circuit voltage

The addition of donor or acceptor type molecular semiconductors to PBDB-T:PC₆₀BM based organic photovoltaics leads to increases in open circuit-voltages and overall power conversion efficiencies.

Achieving Balanced Crystallinity of Donor and Acceptor by Combining Blade-Coating and Ternary Strategies in Organic Solar Cells

As a prototype tool for slot-die coating, blade-coating exhibits excellent compatibility with large-area roll-to-roll coating. A ternary organic solar cell based on PBDB-T:PTB7-Th:FOIC blends is fabricated by blade-coating and exhibits a power conversion efficiency of 12.02%, which is one of the highest values for the printed organic solar cells in ambient environment. It is demonstrated that blade-coating can enhance crystallization of these three materials, but the degree of induction is different (FOIC > PBDB-T > PTB7-Th). Thus, the blade-coated PBDB-T:FOIC device presents much higher electron mobility than hole mobility due to the very high crystallinity of FOIC. Upon the addition of PTB7-Th into the blade-coated PBDB-T:FOIC blends, the crystallinity of FOIC decreases together with the corresponding electron mobility, due to the better miscibility between PTB7-Th and FOIC. The ternary strategy not only maintains the well-matched crystallinity and mobilities, but also increases the photocurrent with complementary light absorption as well as the Förster resonant energy transfer. Furthermore, small domains with homogeneously distributed nanofibers are observed in favor of the exciton dissociation and charge transport. This combination of blade-coating and ternary strategies exhibits excellent synergistic effect in optimizing morphology, showing great potential in the large-area fabrication of highly efficient organic solar cells.

Pyrene-Imidazole Based Aggregation Modifier Leads to Enhancement in Efficiency and Environmental Stability for Ternary Organic Solar Cells

A novel pyrene-imidazole derivative (PyPI), which can form effcient π-π stacking in solid film, has been utilized in organic solar cells (OSCs). The stacking of small a molecule PyPI can facilitate a charge transfer and suppress fullerene aggregation. As a result, PTB7-Th: PyPI: PC₇₁BM based ternary OSC exhibits a high power conversion efficiency (PCE) of 10.36%, which presents a 15.88% increase from the binary device (8.94%). Concurrently, the ternary OSC shows a much better thermal and light illumination stability. Under continuous 60°C annealing for 3 h, in atmosphere, the device still remains at 94.13% efficiency more than the pristine state, while the control device remains at 52.47% PCE. Constant illumination under Air Mass (AM) 1.5G irradiation (100 mW cm⁻²) in atmosphere, the PCE of OSC remains at 72.50%. The high conversion efficiency and excellent environmental stability of the PyPI based ternary OSC, has narrowed the gap between laboratory investigation and industrial production.

Functional Scanning Force Microscopy for Energy Nanodevices

Energy nanodevices, including energy conversion and energy storage devices, have become a major cross-disciplinary field in recent years. These devices feature long-range electron and ion transport coupled with chemical transformation, which call for novel characterization tools to understand device operation mechanisms. In this context, recent developments in functional scanning force microscopy techniques and their application in thin-film photovoltaic devices and lithium batteries are reviewed. The advantages of scanning force microscopy, such as high spatial resolution, multimodal imaging, and the possibility of in situ and in operando imaging, are emphasized. The survey indicates that functional scanning force microscopy is making significant contributions in understanding materials and interfaces in energy nanodevices.

Ternary organic solar cells with a phase-modulated surface distribution via the addition of a small molecular luminescent dye to obtain a high efficiency over 10.5

Incorporation of a ternary organic component is an effective strategy to enhance the performance of bulk heterojunction (BHJ) organic solar cells (OSCs). In this study, a small molecule luminescent dye, C545T, was first doped into blends of PTB7-Th/PC71BM and PTB7/PC71BM as a third component to fabricate ternary OSCs. It is demonstrated that C545T can disrupt the severe vertical distribution in the binary blend and effectively modulate the novel surface chemical configuration by improving the self-assembly process of the polymer donor, as a result of the good miscibility among the active layer materials and the π-π interactions between PC71BM and C545T. The obtained homogeneously bicontinuous BHJ with numerous interpenetrating nanofibers optimizes the domain size of exciton diffusion and the length of charge transfer. The energy transfer between C545T and polymers changes the transmission path of photo-generated excitons, which together improve the exciton dissociation process and reduce the recombination loss. Champion power conversion efficiencies (PCEs) of 10.69% and 9.42% were achieved by the ternary blends of PTB7-Th/C545T/PC71BM and PTB7/C545T/PC71BM, respectively, which correspond to a nearly 20% enhancement over their binary counterparts.

High-Performance Ternary Nonfullerene Polymer Solar Cells with Both Improved Photon Harvesting and Device Stability

Efficiency and stability of polymer solar cells (PSCs) are the two most significant decisive factors for the purpose of actual applications. Here, highly efficient and stable ternary PSCs were fabricated by incorporating two well-compatible polymer donors (poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b;4,5- b0]dithiophene-2,6-diyl- alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4- b]thiophene-)-2-carboxylate-2-6-diyl] and poly[[9-(1-octylnonyl)-9 H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]) with one narrow band gap nonfullerene acceptor (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene)). It is found that Förster resonance energy transfer acts as an efficient pathway to further strengthen photon harvesting in this ternary system, which results in a significant improvement in current density ( J_SC) without sacrificing the strong absorption of binary blends in the near-infrared region. Meanwhile, both of the inverted and conventional ternary PSCs exhibit better stability compared with the related binary PSCs in air condition because of the interlocked morphology in ternary films. The optimized ternary PSCs exhibit an outstanding power conversion efficiency (PCE) of 9.53% resulting from the synchronous improvements in J_SC and fill factor. Moreover, this ternary strategy can be further confirmed by the use of an ultranarrow-band gap nonfullerene acceptor IEICO-4F, and the champion PCE of ternary PSCs reaches to 12.15%.

Inverted P3HT:PC61BM organic solar cells incorporating a π-extended squaraine dye with H- and (or) J-aggregation

In this report, we investigated the impact of adding a large π-extended squaraine dye to the classic bulk heterojunction P3HT:PC61BM system. The compound, SQIQ-A, is a bis-indole squaraine dye appended with acetylene-indoloquinoxaline end-capping units that exhibits strong optical absorption in thin film from 650 to 750 nm, beyond the absorption wavelength cut-off for P3HT:PC61BM films (ca. ∼650 nm). The dye SQIQ-A can form H- or J-aggregates (blue or red shifting the optical absorption profiles) via solvent vapour annealing with CHCl3 or THF, respectively, thus providing a simple method to control the morphology and optical properties of the molecule. Ternary blended films composed of P3HT:PC61BM:SQIQ-A (1:1:0.3 mass ratio) were investigated as-cast, after solvent vapour annealing, and after thermal annealing. Films were characterized using optical absorption spectroscopy, X-ray diffraction, atomic force microscopy, and polarized light microscopy. Solar cells were fabricated using an inverted architecture in air and analysed using current–voltage and external quantum efficiency measurements. It was found that solar cells thermally annealed at 130 °C for 10 min gave good power conversion efficiencies of 3%, similar to the control P3HT:PC61BM but with improved fill factors and noticeable photocurrent generation from 650 to 760 nm where the SQIQ-A molecule absorbs. Exposure to solvent vapour (CHCl3 or THF) results in aggregation of all components but specifically leads to the formation of micron-sized domains, lowering the overall photovoltaic performance.

A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary-Blend Organic Solar Cells with Efficiency over 13

A new acceptor-donor-acceptor (A-D-A) type nonfullerene acceptor, 3TT-FIC, which has three fused thieno[3,2-b]thiophene as the central core and difluoro substituted indanone as the end groups, is designed and synthesized. 3TT-FIC exhibits broad and strong absorption with extended onset absorption to 995 nm and a low optical bandgap of 1.25 eV. The binary device based on 3TT-FIC and the polymer PTB7-Th exhibits a power conversion efficiency (PCE) of 12.21% with a high short circuit current density (   Jsc) of 25.89 mA cm-2. To fine-tune the morphology and make full use of the visible region sunlight, phenyl-C71-butyricacid-methyl ester (PC71BM) is used as the third component to fabricate ternary devices. In contrast to the binary devices, the ternary blend organic solar cells show significantly enhanced EQE ranging from 300 to 700 nm and thus an improved  Jsc with a high value of 27.73 mA cm-2. A high PCE with a value of 13.54% is achieved for the ternary devices, which is one of the highest efficiencies in single junction organic solar cells reported to date. The results provide valuable insight for the ternary devices in which the external quantum efficiency (EQE) induced by the third component is evidently observed and directly contributed to the enhancement of the device efficiency.

π-π stacking induced high current density and improved efficiency in ternary organic solar cells

Ternary blend systems have been used to enhance the short-circuit current density (JSC) and fill factor (FF) of organic solar cells (OSCs). However, it is still a challenge to find suitable third components that concurrently possess complementary light absorption and well-matched energy levels. Here, a small organic molecule, 4,4'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(N,N-bis(4-(pyren-1-yl)phenyl)anili-ne) (DFNPy), which contains a triphenylamine core and bulky pyrene rings, was designed and used to construct ternary blend OSCs. DFNPy shows complementary absorption spectra in the 350-450 nm shortwave band, which has seldom been reported in the field of ternary OSCs. Furthermore, the bulky pyrene rings aggregate via π-π stacking to promote charge transfer. As a result, a high power conversion efficiency (PCE) of 10.59% with an enhanced JSC of 19.72 mA cm-2 was realized in PTB7-Th:DFNPy:PC71BM-based ternary OSCs. The addition of DFNPy was found to modulate the film morphology by improving the film phase separation and crystallinity, which can facilitate charge generation and decrease charge recombination, resulting in enhanced mobility. The results demonstrate an effective strategy for improving the photovoltaic performance of OSCs.

Critical Role of Vertical Phase Separation in Small-Molecule Organic Solar Cells

An inverted device structure is a more stable configuration than a regular device structure for solution-processed organic solar cells (OSCs). However, most of the solution-processed small-molecule OSCs (SM-OSCs) reported in the literature used the regular device structure, and a regular device normally exhibits a higher efficiency than an inverted device. Herein, a representative small-molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage ( V_oc) and fill factor (FF) in the inverted device were studied. The reduced V_oc and FF is due to the vertical phase separation with excess [6,6]-phenyl-C₇₁-butyric acid methyl ester near the air/blend surface, which leads to a reduction in build-in voltage and unbalanced charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of the blend film and the increased population of face-on crystallites in the bottom layer of the blend film. This study illustrates that the morphology plays a key role in photovoltaic performance difference between regular and inverted devices and provides useful guidelines for further optimization of the morphology of solution-processed SM-OSCs.

High-Performance Organic Bulk-Heterojunction Solar Cells Based on Multiple-Donor or Multiple-Acceptor Components

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next-generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple-donor or multiple-acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple-donor or multiple-acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.

Ternary Nonfullerene Polymer Solar Cells with 12.16% Efficiency by Introducing One Acceptor with Cascading Energy Level and Complementary Absorption

A novel small-molecule acceptor, (2,2'-((5E,5'E)-5,5'-((5,5'-(4,4,9,9-tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-(2-ethylhexyl)thiophene-5,2-diyl))bis(methanylylidene)) bis(3-hexyl-4-oxothiazolidine-5,2-diylidene))dimalononitrile (ITCN), end-capped with electron-deficient 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile groups, is designed, synthesized, and used as the third component in fullerene-free ternary polymer solar cells (PSCs). The cascaded energy-level structure enabled by the newly designed acceptor is beneficial to the carrier transport and separation. Meanwhile, the three materials show a complementary absorption in the visible region, resulting in efficient light harvesting. Hence, the PBDB-T:ITCN:IT-M ternary PSCs possess a high short-circuit current density (J_sc ) under an optimal weight ratio of donors and acceptors. Moreover, the open-circuit voltage (V_oc ) of the ternary PSCs is enhanced with an increase of the third acceptor ITCN content, which is attributed to the higher lowest unoccupied molecular orbital energy level of ITCN than that of IT-M, thus exhibits a higher V_oc in PBDB-T:ITCN binary system. Ultimately, the ternary PSCs achieve a power conversion efficiency of 12.16%, which is higher than the PBDB-T:ITM-based PSCs (10.89%) and PBDB-T:ITCN-based ones (2.21%). This work provides an effective strategy to improve the photovoltaic performance of PSCs.

Suppressing charge recombination in small-molecule ternary organic solar cells by modulating donor–acceptor interfacial arrangements

Introducing a proper amount of the third component in ternary blend organic solar cells can result in a significant decrease of docking PC₇₁BM with the central electron-donating unit of electron donors and thus suppress charge recombination.

A naphthodithieno[3,2-b]thiophene-based copolymer as a novel third component in ternary polymer solar cells with a simultaneously enhanced open circuit voltage, short circuit current and fill factor

Ternary polymer solar cells with simultaneously improved V_OC, J_SC and FF have been achieved by doping PV12 as a third component.

Design Rule for Improved Open-Circuit Voltage in Binary and Ternary Organic Solar Cells

Mixing different compounds to improve functionality is one of the pillars of the organic electronics field. Here, the degree to which the charge transport properties of the constituent materials are simply additive when materials are mixed is quantified. It is demonstrated that in bulk heterojunction organic solar cells, hole mobility in the donor phase depends critically on the choice of the acceptor material, which may alter the energetic disorder of the donor. The same holds for electron mobility and disorder in the acceptor. The associated mobility differences can exceed an order of magnitude compared to pristine materials. Quantifying these effects by a state-filling model for the open-circuit voltage (V_OC) of ternary Donor:Acceptor1:Acceptor2 (D:A₁:A₂) organic solar cells leads to a physically transparent description of the surprising, nearly linear tunability of the V_OC with the A₁:A₂ weight ratio. It is predicted that in binary OPV systems, suitably chosen donor and acceptor materials can improve the device power conversion efficiency (PCE) by several percentage points, for example from 11 to 13.5% for a hypothetical state-of-the-art organic solar cell, highlighting the importance of this design rule.

Fullerene-Free Organic Solar Cells with an Efficiency of 10.2% and an Energy Loss of 0.59 eV Based on a Thieno[3,4-c]Pyrrole-4,6-dione-Containing Wide Band Gap Polymer Donor

Although the combination of wide band gap polymer donors and narrow band gap small-molecule acceptors achieved state-of-the-art performance as bulk heterojunction (BHJ) active layers for organic solar cells, there have been only several of the wide band gap polymers that actually realized high-efficiency devices over >10%. Herein, we developed high-efficiency, low-energy-loss fullerene-free organic solar cells using a weakly crystalline wide band gap polymer donor, PBDTTPD-HT, and a nonfullerene small-molecule acceptor, ITIC. The excessive intermolecular stacking of ITIC is efficiently suppressed by the miscibility with PBDTTPD-HT, which led to a well-balanced nanomorphology in the PBDTTPD-HT/ITIC BHJ active films. The favorable optical, electronic, and energetic properties of PBDTTPD-HT with respect to ITIC achieved panchromatic photon-to-current conversion with a remarkably low energy loss (0.59 eV).

Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting Greater Than 10% Power Conversion Efficiency and a High Fill Factor of 75

Ternary bulk heterojunction (BHJ) is a brilliant photovoltaic technology for improving the performance of organic solar cells (OSCs), because the light absorption range can be significantly extended by using multiple donors or acceptor materials. In this paper, coumarin7 (C7), a small organic molecule typical led used in organic light-emitting diodes, was initially exploited as second electron-donor component in ternary bulk heterojunction OSCs along with conventional blend system spolythieno[3,4-b]-thiophene/benzodithiophene(PTB7) and [6,6]-phenyl-C71 -butyric acid methyl(PC₇₁ BM). A champion PCE value of 10.28% was realized in the ternary OSCs when incorporated with 10 wt % C7 doping ratio in the donors, corresponding to about 35% enhancement compared with the PTB7:PC₇₁BM-based OSCs, a high fill factor (FF) of 75.03%, a short-circuit currentdensity (J_sc) of 18.72 mA cm^-2 and an open-circuit voltage (V_oc) of 0.73 V. The enhanced performance of the ternary OSCs can be attributed to the simultaneous improvement of the FF and the J_sc. In addition to extended light absorption, a perfect nanofiber filament active layer morphology is obtained due to the good compatibility between C7 and PTB7, which facilitates the balance of charge transportation and the suppression of charge recombination. This investigation suggests that coumarin derivatives, which have completely different structure with polymer donors, can also be used to fabricate ternary solar cells and have the potential applications to obtain amazing performance after further device engineering and optimization.

Ternary Solar Cells Based on Two Small Molecule Donors with Same Conjugated Backbone: The Role of Good Miscibility and Hole Relay Process

Ternary organic solar cells (OSCs) are very attractive for further enhancing the power conversion efficiencies (PCEs) of binary ones but still with a single active layer. However, improving the PCEs is still challenging because a ternary cell with one more component is more complicated on phase separation behavior. If the two donors or two acceptors have similar chemical structures, good miscibility can be expected to reduce the try-and-error work. Herein, we report ternary devices based on two small molecule donors with the same backbone but different substituents. Whereas both binary devices show PCEs about 9%, the PCE of the ternary cells is enhanced to 10.17% with improved fill factor and short-circuit current values and external quantum efficiencies almost in the whole absorption wavelength region from 440 to 850 nm. The same backbone enables the donors miscible at molecular level, and the donor with a higher HOMO level plays hole relay process to facilitate the charge transportation in the ternary devices. Since side-chain engineering has been well performed to tune the active materials' energy levels in OSCs, our results suggest that their ternary systems are promising for further improving the binary cells' performance although their absorptions are not complementary.

Amorphous Ternary Charge-Cascade Molecules for Bulk Heterojunction Photovoltaics

Ternary bulk heterojunctions with cascade-type energy-level configurations are of significant interest for further improving the power conversion efficiency (PCE) of organic solar cells. However, controlling the self-assembly in solution-processed ternary blends remains a key challenge. Herein, we leverage the ability to control the crystallinity of molecular semiconductors via a spiro linker to demonstrate a simple strategy suggested to drive the self-assembly of an ideal charge-cascade morphology. Spirobifluorene (SF) derivatives with optimized energy levels from diketopyrrolopyrrole (DPP) or perylenediimide (PDI) components, coded as SF-(DPP)₄ and SF-(PDI)₄, are synthesized and investigated for application as ternary components in the host blend of poly(3-hexylthiophene-2,5-diyl):[6,6]phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). Differential scanning calorimetry and X-ray/electron diffraction studies suggest that at low loadings (up to 5 wt %) the ternary component does not perturb crystallization of the donor:acceptor host blend. In photovoltaic devices, up to 36% improvement in the PCE (from 2.5% to 3.5%) is found when 1 wt % of either SF-(DPP)₄ or SF-(PDI)₄ is added, and this is attributed to an increase in the fill factor and open-circuit voltage, while at higher loadings, the PCE decreased because of a lower short-circuit current density. A comparison of the quantum efficiency measurements [where light absorption of SF-(DPP)₄ was found to give up to 95% internal conversion] suggests that improvement due to enhanced light absorption or to better exciton harvesting via resonance energy transfer is unlikely. These data, together with the crystallinity results, support the inference that the SF compounds are excluded to the donor:acceptor interface by crystallization of the host blend. This conclusion is further supported by impedance spectroscopy and a longer measured charge-carrier lifetime in the ternary blend.

Two Well-Miscible Acceptors Work as One for Efficient Fullerene-Free Organic Solar Cells

High-performance ternary organic solar cells are fabricated by using a wide-bandgap polymer donor (bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer, J52) and two well-miscible nonfullerene acceptors, methyl-modified nonfullerene acceptor (IT-M) and 2,2'-((2Z,2'Z)-((5,5'-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short-circuit current density of 19.7 mA cm^-2 and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.

Layer-by-Layer-Processed Ternary Organic Solar Cells Using Perylene Bisimide as a Morphology-Inducing Component

A perylene bisimide (PBI-C4) with strong self-aggregation ability was used with polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) to form an underlying morphology-inducing layer in organic solar cells. The PTB7:PC₇₁BM ([6,6]-phenyl C₇₁ butyric acid methyl ester) spin-coated atop the preprocessed morphology-inducing thin layer showed an enhanced domain size with better phase separation compared with the conventional spin-coating of PTB7:PC₇₁BM on ZnO directly. Proper aggregation morphology of the active layer shows higher electrical properties resulting in a 14% improvement in the power conversion efficiency (PCE) (8.96% vs 7.85%). After replacing the ZnO cathode interlayer with a photoconductive interlayer (ZnO:PBI-H), a maximum PCE of 9.47% is obtained, which is among the best device performance based on PTB7:PC₇₁BM systems.

Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells

Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.

High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π-π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices.

Achieving High-Performance Ternary Organic Solar Cells through Tuning Acceptor Alloy

Acceptor alloys based on n-type small molecular and fullerene derivatives are used to fabricate the ternary solar cell. The highest performance of optimized ternary device is 10.4%, which is the highest efficiency for one donor/two acceptors-based ternary systems. Three important parameters, J_SC , V_OC , and FF, of the optimized ternary device are all higher than the binary reference devices.