Solution-processed Organic Photovoltaics Research Articles

Highly efficient solution-processed organic photovoltaics enabled by improving packing behavior of organic semiconductors

Highly efficient solution-processed organic photovoltaics enabled by improving packing behavior of organic semiconductors

From Solution to Thin Film: Molecular Assembly of π-Conjugated Systems and Impact on (Opto)electronic Properties.

The assembly of conjugated organic molecules from solution to solid-state plays a critical role in determining the thin film morphology and optoelectronic properties of solution-processed organic electronics and photovoltaics. During evaporative solution processing, π-conjugated systems can assemble via various forms of intermolecular interactions, forming distinct aggregate structures that can drastically tune the charge transport landscape in the solid-state. In blend systems composed of donor polymer and acceptor molecules, assembly of neat materials couples with phase separation and crystallization processes, leading to complex phase transition pathways which govern the blend film morphology. In this review, we provide an in-depth review of molecular assembly processes in neat conjugated polymers and nonfullerene small molecule acceptors and discuss their impact on the thin film morphology and optoelectronic properties. We then shift our focus to blend systems relevant to organic solar cells and discuss the fundamentals of phase transition and highlight how the assembly of neat materials and processing conditions can affect blend morphology and device performance.

Flexible Solution-Processed Electron-Transport-Layer-Free Organic Photovoltaics for Indoor Application

Organic photovoltaics (OPVs) have unique advantages of low weight, mechanical flexibility, and solution processability, which make them exceptionally suitable for integrating low-power Internet of Things devices. However, achieving improved operational stability together with solution processes that are applicable to large-scale fabrication remains challenging. Their major limitation arises due to the instable factors that occur both inside the thick active film and from the ambient environment, which cannot be completely resolved via the current encapsulation techniques used for flexible OPVs. Additionally, thin active layers are highly vulnerable to point defects, which result in low yield rates and impede the laboratory-to-industry translation. In this study, flexible fully solution-processed OPVs with improved indoor efficiency and long-term operational stability than that of conventional OPVs with evaporated electrodes are achieved. Benefiting from the oxygen and water vapor permeation barrier of the spontaneously formed gallium oxide layers on the exposed eutectic gallium-indium surface, fast degradation of the OPVs with thick active layers is prevented, maintaining 93% of its initial Pmax after 5000 min of indoor operation under 1000 lx light-emitting diode (LED) illumination. Additionally, by using the thick active layer, spin-coated silver nanowires could be directly used as bottom electrodes without complicated flattening processes, thereby substantially simplifying the fabrication process and proposing a promising manufacturing technique for devices with high-throughput energy demands.

High Performance Transparent Silver Grid Electrodes for Organic Photovoltaics Fabricated by Selective Metal Condensation.

We report silver grid electrodes on glass and flexible plastic substrates with performance that exceeds that of commercial indium-tin oxide (ITO) coated glass and show their suitability as a drop-in replacement for ITO glass in solution processed organic photovoltaics (OPVs). When supported on flexible plastic substrates these electrodes are stable towards repeated bending through a small radius of curvature over tens of thousands of cycles. The grid electrodes are fabricated by the unconventional approach of condensation coefficient modulation using a perfluorinated polymer shown to be far superior to the other compounds used for this purpose to date. The very narrow line-width and small grid pitch that can be achieved also opens the door to the possibility of using grid electrodes in OPVs without a conducting PEDOT:PSS layer to span the gaps between grid-lines. This article is protected by copyright. All rights reserved.

Progress in organic photovoltaics based on green solvents: from solubility enhancement to morphology optimization.

Solution-processed organic photovoltaics (OPVs) is one of the most promising photovoltaic technologies in the energy field, due to their clean and renewable low-cost manufacturing potential. OPV has rapidly developed with the design and synthesis of highly efficient photovoltaic materials and the development of smart device engineering. To date, the majority of advanced OPV devices have been prepared using halogenated solvents, achieving power conversion efficiencies (PCE) exceeding 19% on a laboratory scale. However, for industrial-scale production, less toxic manufacturing processes and environmental sustainability are the key considerations. Therefore, this review summarizes recent advances in green solvent-based approaches for the preparation of OPVs, highlighting material design (including polymer donors and small molecule acceptors) and device engineering (co-solvent methods, additive strategies, post-treatment methods, and regulation of coating method), emphasizing crucial factors for achieving high performance in green solvent-processed OPV devices. This review presents potential future directions for green solvent-based OPVs, which may pave the way for future industrial development.

Halogen-Free -Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility.

In -conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a -conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (VOC s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and VOC s of more than 0.84V are observed, both of which are among the top values reported so far for "halogen-free" polymers. This study will serve as an important reference for designing -conjugated polymers to achieve highly efficient and solution-processable OPVs.

Solution-Processed Electron-Transport Layer-free Organic Photovoltaics with Liquid Metal Cathodes

Flexible, lightweight, and large-area solar cells provide new power supply opportunities in the renewable energy field and facilitate the supply of power to internet-of-things devices and wearable devices. The choice of printing process technologies is a key parameter for such flexible power sources because of their energy-saving process technology and high throughput rate. In addition to selecting the appropriate printing method for the active and charge transport layers, the development of printed electrodes is critical. Numerous printable materials have been developed to replace conventional evaporated top electrodes. However, achieving fully solution-processed organic photovoltaics (OPVs) with power conversion efficiency (PCE) comparable to OPVs with vacuum-deposited transparent and top electrodes is challenging. This is because of the difficulty of forming a uniform interface between the top solution-processed electrode and the active layers while preventing deterioration. In this study, an electron transport layer-free, eutectic gallium-indium (EGaIn) top-cathode strategy was developed and a record PCE of 12.7% in fully solution-processed, flexible OPVs was achieved. Direct coating of EGaIn on the active layer, in a nitrogen atmosphere, is conducive for energy band matching and obtaining physically perfect interfaces without any penetrations or voids. An average PCE of 14.1% and enhanced operating stability, comparable to conventional OPVs, were achieved with indium tin oxide transparent electrodes by eliminating the electron-transport layer. The fully solution-processed flexible OPVs fabricated with the embedded silver nanowire strategy in ultrathin transparent polyimide, achieved an average PCE of 12.7%, representing a promising technique to meet green and high-throughput energy demands.

Real-time views of morphological evolution in solution-processed organic photovoltaics

This review focuses on the morphological evolution of films for organic photovoltaics from the perspective of in situ measurements, showing factors governing the final morphology during liquid–solid transitions.

Synthesis and investigation of a hexyl substituted thieno-fused BODIPY derivative as a versatile near-IR fluorophore.

The synthesis, photophysical and electrochemical properties of hexyl thiophene substituted thieno[b]-fused BODIPY structure are reported within this work. One such derivative, HTFBod, has been studied, which is one of the rare compounds that has a maximum absorbance wavelength greater than 750 nm among fused BODIPY compounds. In addition, it preserves its significant spectral properties such as high molar absorptivity and fluorescence quantum yield. Electrochemical characterizations indicated that the compound could be a successful candidate for an organic solar cell donor compound due to its low band gap, which is known to improve the short-circuit current (Jsc) values. The compound could also harvest the near-IR portion of the solar irradiance where maximum solar photon flux exists. In addition, its low LUMO value of -4.13 eV contributes to its air stability. Hexyl substituents provide greater solubility which is required in solution-processable organic photovoltaics and further bathochromic shift when compared to similar compounds. Next, the synthesis was accomplished in five steps with higher yields compared to similar compounds in the literature. Heavy atom-free singlet oxygen production analysis has also been performed while it has been shown that the compound cannot produce singlet oxygen, while this property could be acquired via halogen substitution.

Controllable synthesis and n-doping of HMoOx nanoparticle inks through simple photoreduction for solution-processed organic photovoltaics

Solution-processable hole transporting layer (HTL) is one basis for the full-solution-processable fabrication of the organic solar cells (OSCs). Molybdenum oxide (MoOx) is a widely used HTL and has the possibility of solution processing. Controlling the valence state of molybdenum (Mo) in MoOx HTL is critical for OSCs performance. In this work, we developed a novel methode based on photochemical reduction to synthesize HMoOx nanoparticle ink for OSCs. HMoO3 nanoparticle was firstly obtained through oxidaiton of metal molybdenum. Afterward, the HMoO3 precursor nanoparticles were continuously illuminated to induce the reduction of Mo(VI) to Mo(V) to form HMoOx. Such a process could proceed under the illumination of different light sources, even sunlight. In addition, photoelectron spectroscopy (XPS) and absorption spectra demonstrated such a photoreduction process was well controllable by illumination intensity and illumination time. With this HMoOx HTL for the inverted OSCs, a high efficiency of 13.17% and 16.64% for the PM7:IT-4F and PM6:Y6 OSCs was obtained, which is comparable with the evaporated MoO3 HTL-based OSCs. Furthermore, based on the Ag nanowires (AgNWs) electrode, the performances of all-solution-processed PM6:Y6 semitransparent device also reach 10.52%. This work provides a simple and feasible route to fabricate hole transporting layer for solution-processable OSCs.

Influence of alkyne spacers on the performance of thiophene-based donors in bulk-heterojunction organic photovoltaic cells

The ever-increasing demand for renewable energy has led to the intensification of research focused upon harnessing the abundant energy from sunlight. Solution processed organic solar cells fabricated using conjugated polymer donor materials dominated the early development of organic photovoltaics (OPV). More recently, organic solar cells fabricated using small molecule donors have become increasingly attractive due to the advantages these systems have over their polymeric counterparts. These include reproducible synthesis and the ability to more accurately develop structure-activity relationships. Here, we report the synthesis of thiophene based small molecule donors that have been used to investigate the influence of alkyne π-spacers on the optical, electronic and photovoltaic properties of solution-processable bulk-heterojunction (BHJ) organic OPV cells. Additionally, we compare the addition of 3,4-ethylenedioxythiophene (EDOT) units in place of the end-thiophene of these derivatives. We found that the compounds featuring alkyne moieties displayed poor charge transfer and blue-shifted UV–vis spectra, whereas the incorporation of the EDOT units increased charge transfer for the non-alkyne derivative resulting in red-shifted absorbance. DFT calculations predicted higher charge separation/mobility and lower charge generation for the compounds without alkyne units. Indeed, greater Jsc values and higher PCEs were found for the compounds lacking alkyne spacers. EDOT containing compounds showed poor performance which is likely due to an unfavourable morphology and higher HOMO level, leading to lower Voc. Hence, the simplest 5T-1 molecule showed the highest performance of 3.24%, which is c.a. 1.6x that of the alkyne-containing 5TA-1 and c.a. 2.9x that of the EDOT-containing derivative. This study demonstrates that alkyne spacers can be detrimental to orbital interactions, charge transfer and OPV performance, and that the EDOT unit can be added to provide greater charge transfer due to its auxiliary donating ability. However, careful design is required to ensure higher Voc values and a favourable morphology in thiophene-based solution-processable OPVs.

Silver-nanowire-based lamination electrode for a fully vacuum-free and solution-processed organic photovoltaic cell

A silver nanowire (AgNW)-based stacked lamination electrode was investigated for application as the top electrode in fully vacuum-free and solution-processed organic photovoltaic (OPV) fabrication. AgNW layers were stacked with an ethylene-glycol-doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer for conductivity enhancement and uniform contact creation, and an ethylene vinyl acetate (EVA) adhesive and a polyethylene terephthalate (PET) supporting film with subsequent hot-pressing were used for complete attachment (good electrical contact). During hot-pressing at 100 °C, the melted EVA adhesive was found to create a concrete electrical contact between the lamination electrode and the underlying polymer semiconducting layer. To verify the performance of the lamination electrode, fully vacuum-free and solution-processed OPVs with the lamination electrode were fabricated and compared to OPVs with a reference evaporated metal electrode. The results revealed that the OPVs with lamination electrodes provided the best fill factor (FF) of 53% and a final photoconversion efficiency (PCE) of 2.61%, whereas the OPVs with metal electrodes provided the best FF of 64% and a PCE of 3.41%. Analytical calculations indicate that the FF loss of lamination electrode OPVs was due to higher series resistance (11.4 Ω-cm2 vs. 15.1 Ω-cm2) and higher recombination at the interface, which can be considered an ideality factor (1.7 vs. 2.7).

Solution-processed, semitransparent organic photovoltaics integrated with solution-doped graphene electrodes

In this work, by applying a transfer method simultaneously with a solution doping process for graphene as top electrodes, we demonstrate a solution-processed semitransparent organic photovoltaics (OPV). The work function of doped graphene under various doping conditions was investigated via photoemission spectroscopy. The transparent device was fabricated using PEDOT-doped graphene as electrodes, which provide an energetically favorable band alignment for carrier extractions. The solution-processed semitransparent organic photovoltaics exhibit the power conversion efficiency (PCE) of 4.2%, which is 85.7% of the PCE of control devices based on metallic reflecting electrodes, while maintaining good transparency at most visible wavelengths.

Azadipyrromethene-Based Materials: From Synthesis to Electronic and Photovoltaic Properties

Azadipyrromethene (ADP, see image) is an aromatic monoanionic bidentate ligand whose core resembles half of a phthalocyanine ligand. Complexes of ADP have strong visible to near-IR absorption, high electron affinity and easy synthesis, making them attractive for a variety of applications. We have demonstrated that homoleptic zinc(II) complexes of ADP with phenylethynyl pyrrolic substituents (Zn(WS3)2, see image), are promising candidates as non-fullerene acceptors for solution-processed organic photovoltaics (OPVs). The complexe’s distorted tetrahedral structure (non-planar) facilitates favorable phase separation from the common conjugated polymer donor, poly(3-hexylthiophene) (P3HT), yielding good performance in OPVs. The phenylethynyl groups also contribute to increase the electron affinity of the complex. To further increase electron affinity, we installed electronegative fluorine at various positions of the ADP ligands. To our surprise, the electrochemical properties are relatively unaffected by the substitutions, but the electron mobility and performance in OPVs improves. On the other hand, substitution with nitrile groups anodically shifts the redox potentials and thus increases electronegativity. The substitutions also improve electron mobility in diodes as compared to the un-substituted analogue. However, OPV performance is poor, possibly due to unfavorable blend morphology with P3HT, demonstrating the difficulties in optimizing all properties simultaneously.To further enhance charge transport and OPV performance, we investigated replacing the phenyl in the phenylethynyl pyrrolic substituents with larger aromatic groups such as naphthyl and phenanthrenyl groups. Because these larger aromatic groups reduced solubility in organic solvents, we added hexyl solubilizing groups at the para position of the proximal phenyls. We found that the hexyl and the larger aromatic groups both contribute to improving crystallinity of the zinc(II) complexes, electron mobility and OPV performance. The best performance was obtained with 1-naphthyl substitution, Zn(L2)2 shown in the image. OPVs using P3HT as the donor gave a high power conversion efficiency (PCE) of 5.5% and the PCE was not very sensitive to the P3HT:Zn(L2)2 weight ratio. Additionally, Zn(L2)2 can be synthesized on the gram scale using inexpensive starting materials without chromatography column purification, increasing its industrial accessibility. Due to its non-planar structure, Zn(L2)2 shows isotropic charge transport. Interestingly, it had high hole mobility, making it a good candidate as donor for OPVs. This combination of properties demonstrates the potential of these types of molecules for optoelectronic devices. Selected references: Z. Mao; W. Senevirathna; J. Y. Liao; J. Gu; S. V. Kesava; C. Guo; E. D. Gomez; G. Sauvé, “Azadipyrromethene-Based Zn(II) Complexes as Nonplanar Conjugated Electron Acceptors for Organic Photovoltaics.” Adv. Mater. 26 (2014), 6290-6294. S. Pejić; A. M. Thomsen; F. S. Etheridge; R. Fernando; C. Wang; G. Sauvé, “Fluorination Increases the Electron Mobility of Zinc Azadipyrromethene-Based Electron Acceptors and Enhances the Performance of Fullerene-Free Organic Solar Cells.” J. Mater. Chem. C 6 (2018), 3990-3998. G. Sauvé, “Designing Alternative Non-Fullerene Molecular Electron Acceptors for Solution-Processable Organic Photovoltaics.” Chem. Rec. 19 (2019), 1-16. C. Wang; P. Wei; J. H. L. Ngai; A. L. Rheingold; T. G. Gray; Y. Li; E. Pentzer; R. Li; L. Zhu; G. Sauvé, “A Zinc(Ii) Complex of Di(Naphthylethynyl)Azadipyrromethene with Low Synthetic Complexity Leads to OPV with High Industrial Accessibility.” J. Mater. Chem. A 7 (2019), 24614-24625. C. Wang, C. Daddario, S. Pejić, G. Sauvé, “Synthesis and Properties of Azadipyrromethene-based Complexes with Nitrile Substitution” Submitted to Eur. JOC. Figure 1

Different agglomeration properties of PC61BM and PC71BM in photovoltaic inks - a spin-echo SANS study.

Fullerene derivatives are used in a wide range of applications including as electron acceptors in solution-processable organic photovoltaics. We report agglomeration of fullerene derivatives in optically opaque solutions of PC61BM and PC71BM, with concentrations ranging from 30 mg mL−1 up to 90 mg mL−1, in different solvents with relevance to organic photovoltaics, using a novel neutron scattering technique, Spin-Echo Small Angle Neutron Scattering (SESANS). From SESANS, agglomerates with correlation lengths larger than 1 μm are found in some PC61BM solutions, in contrast no agglomerates are seen in PC71BM solutions. These results clearly show that PC71BM is fundamentally more soluble than PC61BM in the solvents commonly used in photovoltaic inks and corroborating similar observations previously achieved using other experimental techniques. Computer models are presented to study the energetics of solution and agglomeration of both species, ascribing the difference to a kinetic effect probably related to the larger anisotropy of PC71BM. Also, this work showcases the power of SESANS to probe agglomerates of fullerene derivatives in completely opaque solutions for agglomerates of the order of one to several microns.

All solution and ambient processable organic photovoltaic modules fabricated by slot-die coating and achieved a certified 7.56% power conversion efficiency

To accelerate the commercialization of organic photovoltaics (OPVs), researchers should use scalable coating approaches compatible with roll-to-roll production in understanding the performance for OPV module manufacturing. The performance of OPV module is strongly changed by the interfacial materials, thin film deposition methods, and processing atmosphere with the film-forming properties of the bulk heterojunction (BHJ) layer influenced intimately to the efficiency of the devices. Herein, we present a reliable process flow and module architecture to demonstrate an air-stable, large-area, and solution-processable OPV module, which the electron transporting layer (ETL), BHJ layer, and hole transporting layer (HTL) are all deposited by slot-die coating and processed in the ambient. Thus prepared module achieved a certified power conversion efficiency (PCE) of 7.56% with an active area of 23.7 cm2 and FF up to 65% under 100 mW/cm2 AM1.5G irradiation. The cell to module loss is found to be 5.5% only in PCE. The encapsulated module also performed superior stability, the performance remained almost unchanged, and PCE can still maintain about 91.7% of the initial value after 1000 h light soaking. We believe this module performed an extremely high PCE reported for OPV module fabricated by ambient slot-die coating in single junction architecture, inferring that the industrialization of OPVs is promising and expectable.

Liquid-chalk painted perovskite films toward low-cost photovoltaic devices

Abstract New fabrication techniques for solution-processed organic photovoltaics are more and more desirable due to greater demand for affordable method to make large area. Different from traditional spin-coating, here, we report a new fabrication technique inspired by “liquid-chalk”. Solution can be added into liquid-chalk, and is almost all coated on the devices, while spin-coating will waste most of solution. In addition, large area could be easily achieved with it. Meanwhile, the thickness and roughness could be adjusted by optimizing speed, time of annealing and amount of the solution added. We demonstrated solar cells based on liquid-chalk-coated perovskite photoactive layer (CH3NH3PbI3) exhibit comparatively high power con-version efficiency (PCE) of 14.83% on average. A champion PCE of 16.04% was attained. Similarly, applying the method in pure inorganic system (CsPbBrI2), we also successfully fabricated devices with a highest PCE of 9.17%. The results indicate that using liquid-chalk to coat is an effective new way to fabricate perovskite solar cells, and also an ideal method to make large-area and flexible devices.

Realizing Efficient Charge/Energy Transfer and Charge Extraction in Fullerene-Free Organic Photovoltaics via a Versatile Third Component.

Solution-processed organic photovoltaics (OPVs) based on bulk-heterojunctions have gained significant attention to alleviate the increasing demend of fossil fuel in the past two decades. OPVs combined of a wide bandgap polymer donor and a narrow bandgap nonfullerene acceptor show potential to achieve high performance. However, there are still two reasons to limit the OPVs performance. One, although this combination can expand from the ultraviolet to the near-infrared region, the overall external quantum efficiency of the device suffers low values. The other one is the low open-circuit voltage (VOC) of devices resulting from the relatively downshifted lowest unoccupied molecular orbital (LUMO) of the narrow bandgap. Herein, the approach to select and incorporate a versatile third component into the active layer is reported. A third component with a bandgap larger than that of the acceptor, and absorption spectra and LUMO levels lying within that of the donor and acceptor, is demonstrated to be effective to conquer these issues. As a result, the power conversion efficiencies (PCEs) are enhanced by the elevated short-circuit current and VOC; the champion PCEs are 11.1% and 13.1% for PTB7-Th:IEICO-4F based and PBDB-T:Y1 based solar cells, respectively.

Designing Alternative Non-Fullerene Molecular Electron Acceptors for Solution-Processable Organic Photovoltaics.

Until recently, solution-processable organic photovoltaics (OPVs) mainly relied on fullerene derivatives as the n-type material, paired with a p-type conjugated polymer. However, fullerene derivatives have disadvantages that limit OPV performance, thus fueling research of non-fullerene acceptors (NFAs). Initially, NFAs showed poor performance due to difficulties in obtaining favorable blend morphologies. One example is our work with 2,6-dialkylamino core-substituted naphthalene diimides. Researchers then learned to control blend morphology by NFA molecular design. To limit miscibility with polymer while preventing excessive self-aggregation, non-planar, twisted or 3D structures were reported. An example of a 3D structure is our work with homoleptic zinc(II) complexes of azadipyrromethene. The most recent design is a planar A-D-A conjugated system where the D unit is rigid and has orthogonal side chains to control aggregation. These have propelled power conversion efficiencies (PCEs) to ∼14 %, surpassing fullerene-based OPVs. These exciting new developments prompt further investigations of NFAs and provide a bright future for OPVs.

Organic energy-harvesting devices achieving power conversion efficiencies over 20% under ambient indoor lighting

The ability of solution-processed organic photovoltaics (OPVs) based on new small-molecule semiconductors, 1DTP-ID and 2DTP-ID, for indoor dim-light energy harvesting is reported.