Benzotriazole Based 2D-conjugated Polymer Donors for High Performance Polymer Solar Cells

Powerful Role of Benzotriazole Polymers and Small Molecules in Organic Solar Cells

In the dynamic landscape of renewable energy technologies, organic solar cells (OSCs) have emerged as frontrunners, offering a sustainable and promising alternative for harnessing solar energy. This review article delves into the recent strides made in leveraging the potential of the benzotriazole nucleus within the context of organic solar cells. The unique electronic properties of benzotriazole, coupled with its structural adaptability, position it as a key component in the pursuit of enhancing OSC performance. As researchers delve deeper into the intricacies of this compound, a clearer understanding of its impact on light absorption, charge transport, and overall device stability emerges. The exploration of recent literature in the last three years reveals a rich landscape with innovation and discovery, showcasing the diverse approaches taken to incorporate benzotriazole into different OSC architectures. From fundamental studies elucidating its electronic interactions to applied research refining its integration strategies, the potential of benzotriazole in advancing the capabilities of organic solar cells becomes increasingly evident, and showing that, with the most important advances in the last three years, is the main goal of this article.

Fluorinated Polymer Donors for Nonfullerene Organic Solar Cells

The rapid development of narrow-bandgap nonfullerene acceptors (NFAs) has boosted the efficiency of organic solar cells (OSCs) over 19 %. The new features of high-performance NFAs, such as visible-NIR light absorption, moderate the highest occupied molecular orbitals (HOMO), and high crystallinity, require polymer donors with matching physical properties. This emphasizes the importance of methods that can effectively tune the physical properties of polymers. Owning to very small atom size and strongest electronegativity, the fluorination has been proved the most efficient strategy to regulate the physical properties of polymer donors, including frontier energy level, absorption coefficient, dielectric constant, crystallinity and charge transport. Owing to the success of fluorination strategy, the vast majority of high-performance polymer donors possess one or more fluorine atoms. In this review, the fluorination synthetic methods, the synthetic route of well-known fluorinated building blocks, the fluorinated polymers which are categorized by the type of donor or acceptor units, and the relationships between the polymer structures, properties, and photovoltaic performances are comprehensively surveyed. We hope this review could provide the readers a deeper insight into fluorination strategy and lay a strong foundation for future innovation of fluorinated polymers.

A-A Strategy Enables Desirable Performance of All-Polymer Solar Cells Fabricated with Nonhalogenated Solvents

Most polymer acceptors have been designed by applying a D (electron-rich unit)-A (electron-deficient unit) strategy, which are principally processed with halogenated solvents to fabricate all-polymer solar cells (all-PSCs). Two novel polymer acceptors, containing an A-A type backbone, were designed and synthesized, which can be readily dissolved in o-xylene. The polymer PY-FBTA, comprising a Y6 derivative as the first A unit and a benzotriazole derivative as the second A unit, shows smaller dihedral angles in the backbone, stronger molecular interactions, higher LUMO level, more complementary absorption spectrum, and better morphology with PM6 than the polymer PY-DPP comprising a diketopyrrolopyrrole derivative as the second A unit. Accordingly, the PM6:PY-FBTA all-PSC achieves a higher PCE of 13.95% than the all-PSC based on PM6:PY-DPP (9.51%) for thoroughly improved Jsc (22.34 mA cm-2), Voc (0.963 V), and FF (64.84%) values, which are fabricated with o-xylene as the solvent. This work demonstrates that the A-A structure is a desirable strategy for designing polymer acceptors for efficient all-PSCs prepared with nonhalogenated solvents.

Asymmetric non-fullerene acceptors enable high photovoltaic performance via the synergistic effect of carbazole-terminated alkyl spacer and halogen substitution

A novel design approach of asymmetric NFA via synergetic alkyl spacer length and halogen substitution enables high photovoltaic performance.

Enhanced Photodynamic of Carriers and Suppressed Charge Recombination Enable Approaching 18% Efficiency in Nonfullerene Organic Solar Cells

Regulation of the exciton generation, diffusion, and carrier transport, as well as optimization of the non-radiative energy loss could further overcome the power conversion efficiency limitation of organic solar cells. However, the relationship between exciton properties and non-radiative energy loss has seldom been investigated. Herein, taking D18-series devices as the research model, the exciton diffusion length (L_D) and hole transfer dynamics can be remarkably improved by the variation of electron-withdrawing halogen and the non-radiative energy loss simultaneously can be suppressed. By combining the analysis results of hole transfer, exciton diffusion, charge separation, and recombination, this work demonstrates that the photo-induced exciton in the chlorinated polymer donor can diffuse to a longer distance within the effective exciton lifetime, suppress the exciton recombination, and enhance device performance. The results define the relationship between the exciton behaviors and non-radiative energy loss and further reveal the significance of controlling the bulk heterojunction with superior photo-physical properties.

Achieving and Understanding of Highly Efficient Ternary Organic Photovoltaics: From Morphology and Energy Loss to Working Mechanism

Ternary strategy, adding an additional donor (D) or acceptor (A) into conventional binary D:A blend, has shown great potential in improving photovoltaic performances of organic photovoltaics (OPVs) for practical applications. Herein, this review is presented on how efficient ternary OPVs are realized from the aspects of morphology, energy loss, and working mechanism. As to morphology, the role of third component on the formation of preferred alloy-like-phase and vertical-phase, which are driven by the miscibility tuning, is discussed. For energy loss, the effect of the third component on the luminescence enhancement and energetic disordering suppression, which lead to favorable increase of voltage, is presented. Regarding working mechanism, dilution effect and relationships between two acceptors or donor/acceptor, which explain the observed device parameters variations, are analyzed. Finally, some future directions concerning ternary OPVs are pointed out. Therefore, this review can provide a comprehensive understanding of working principles and effective routes for high-efficiency ternary systems, advancing the commercialization of OPVs.

Fluorinated Perylene-Diimides: Cathode Interlayers Facilitating Carrier Collection for High-Performance Organic Solar Cells

Organic solar cells (OSCs) have experienced rapid progress with the innovation of near-infrared (NIR)-absorbing small-molecular acceptors (SMAs), while the unique electronic properties of the SMAs raise new challenges in relation to cathode engineering for effective electron collection. To address this issue, two fluorinated perylene-diimides (PDIs), PDINN-F and PDINN-2F, are synthesized by a simple fluorination method, for application as cathode interlayer (CIL) materials. The two bay-fluorinated PDI-based CILs possess a lower lowest unoccupied molecular orbital (LUMO) energy level of ≈-4.0 eV, which improves the energy level alignment at the NIR-SMAs (such as BTP-eC9)/CIL for a favorable electron extraction efficiency. The monofluorinated PDINN-F shows higher electron mobility and better improved interfacial compatibility. The PDINN-F-based OSCs with PM6:BTP-eC9 as active layer exhibit an enhanced fill factor and larger short-circuit current density, leading to a high power conversion efficiency (PCE) exceeding 18%. The devices with PDINN-F CIL retain more than 80% of their initial PCE after operating at the maximum power point under continuous illumination for 750 h. This work prescribes a facile, cost-effective, and scalable method for the preparation of stable, high-performance fluorinated CILs, and instilling promise for the NIR-SMAs-based OSCs moving forward.

A New Diazabenzo[k]fluoranthene-based D-A Conjugated Polymer Donor for Efficient Organic Solar Cells

The development of wide-bandgap polymer donors having complementary absorption and compatible energy levels with near-infared (NIR) absorbing nonfullerene acceptors is highly important for realizing high-performance organic solar cells (OSCs). Herein, a new thiophene-fused diazabenzo[k]fluoranthene derivative has been successfully synthesized as the electron-deficient unit to construct an efficient donor-acceptor (D-A) type alternating copolymer donor, namely PABF-Cl, using the chlorinated benzo[1,2-b:4,5-b']dithiophene as the copolymerization unit. PABF-Cl exhibits a wide optical bandgap of 1.93 eV, a deep highest occupied molecular level of -5.36 eV, and efficient hole transport. As a result, OSCs with the best power conversion efficiency of 11.8% has been successfully obtained by using PABF-Cl as the donor to blend with a NIR absorbing BTP-eC9 acceptor. Our work thus provides a new design of electron-deficient unit for constructing high performance D-A type polymer donors. This article is protected by copyright. All rights reserved.

Synthesis of a Low-Cost Thiophene-Indoloquinoxaline Polymer Donor and Its Application to Polymer Solar Cells

A simple wide-bandgap conjugated polymer based on indoloquinoxaline unit (PIQ) has been newly designed and synthesized via cheap and commercially available starting materials. The basic physicochemical properties of the PIQ have been investigated. PIQ possesses a broad and strong absorption band in the wavelength range of 400~660 nm with a bandgap of 1.80 eV and lower-lying highest occupied molecular orbital energy level of −5.58 eV. Polymer solar cells based on PIQ and popular acceptor Y6 blend display a preliminarily optimized power conversion efficiency of 6.4%. The results demonstrate indoloquinoxaline is a promising building unit for designing polymer donor materials for polymer solar cells.

Domain size control in all-polymer solar cells

In all polymer solar cells (all-PSCs), the domain size is critical for device performance. In highly crystalline polymer blends, however, precisely adjusting the domain size remains a significant challenge because of the simultaneous crystallization of both components. Herein, a strategy that promotes acceptor and donor to crystallize separately was proposed. Take PBDB-T/N2200 blends for instance; the solution state and confined crystallization were combined, which induced the crystallization of N2200, and PBDB-T occurred during the film-forming process and at thermal annealing stage. This separated crystallization process lowers the driving force of phase separation without affecting the degree of crystallinity of the blends. Thus, an interpenetrating network with high crystallinity and proper domain size was obtained, which boosted the power conversion efficiency to 7.59%. Importantly, the relation between pre-aggregation and domain size was established, which may be a guide to precisely adjust the active layer’s domain size in all-PSCs.

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This strategy decreases domain size without sacrificing crystallinity

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A phase diagram about solution state and domain size was proposed

Origin of the Additive-Induced VOC Change in Non-Fullerene Organic Solar Cells

Additives are often used to adjust the morphology of the active layer to improve the performance of organic solar cells (OSCs). Here, taking typical high-efficiency non-fullerene systems as examples, the effect of the additive on the device performance in non-fullerene OSCs is systematically investigated. Surprisingly, an unpresented V_OC change is observed in the opposite direction of the two typical systems (PM6:Y6 and PTB7-Th: ITIC) appearing after the incorporation of the additive DIO, which can be affected by the morphological differences as indicated by the several morphological studies. The bewildering V_OC change caused by the additive in different material systems is supposed to originate from the different energy level variations as verified by the energy level studies. Molecular dynamic (MD) and density functional theory (DFT) calculations are also included to get an insight into the dynamic of the additive-induced morphological differences that are supposed to contribute to the energy level changes. Combining a series of morphological and energic studies as well as the theoretical calculations, the origin of unforeseeable V_OC changes caused by additives in non-fullerene OSCs is clarified, and provides in-depth insights into the effects of additives on device performance.

A High-Performance Nonfused Wide-Bandgap Acceptor for Versatile Photovoltaic Applications

Wide-bandgap (WBG) nonfullerene acceptors (NFAs) with nonfused conjugated structures play a critical role in organic photovoltaic (OPV) cells. Here, NFAs named GS-OEH, GS-OC6, and GS-ISO, with optical bandgaps larger than 1.70 eV, are synthesized without using the fused ring structures. Compared with GS-OEH and GS-OC6, GS-ISO exhibits much stronger crystallinity, leading to a smaller energetic disorder and a larger exciton diffusion coefficient. GS-ISO also possesses a higher electroluminescence external quantum efficiency of 1.0 × 10^-2 . The OPV cell based on PBDB-TF:GS-ISO demonstrates a power conversion efficiency (PCE) of 11.62% under the standard one sun illumination. Besides, the PBDB-TF:GS-ISO-based cell with effective area of 1.0 cm²  exhibits a PCE of 28.37% under 2700 K illumination of 500 lux. A tandem OPV cell using PBDB-TF:GS-ISO as the front subcell shows an outstanding efficiency of 19.10%. Importantly, the GS-ISO-based OPV cell exhibits promising stability under the continuous illumination of simulated sunlight. This study indicates that the molecular design strategy demonstrated in this work has great superiority in developing nonfused NFAs and also that GS-ISO is a promising WBG acceptor for versatile photovoltaic applications.

Boosting the Efficiency of Non-fullerene Organic Solar Cells via a Simple Cathode Modification Method

This work demonstrates a simple yet effective method to significantly improve the power conversion efficiency (PCE) of highly efficient non-fullerene organic solar cells by mixing two electron transport materials. The new electron transport layer shows an energy level better aligned with the active layer and an improved morphology that could reduce the active layer-electrode contact. These improvements lead to enhanced charge extraction, better charge selectivity, suppressed exciton recombination, and finally a boosted PCE in the PM6:Y6-based solar cells. When applied in conjunction with the non-halogenated solvent-processed PM6:PY-IT-based active layer, the mixed ETL also gives rise to a leading result for binary all-polymer solar cells (PCE of >16%) with a concurrent increase in V_OC, J_SC, and FF.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Indacenodithiophene-based small-molecule donor with strong crystallinity for efficient organic solar cells

Two indacenodithiophene (IDT)-based small-molecule analogues (IDBT and IDBT-Cl) are designed as donor materials for organic solar cells. Relative to the amorphous IDBT-Cl, the IDBT with strong crystallinity shows overall better photovoltaic performance when blended with a Y6 acceptor. The results demonstrate the great potential of IDT units in designing efficient small-molecule donors.

Boosting Highly Efficient Hydrocarbon Solvent-Processed All-Polymer-Based Organic Solar Cells by Modulating Thin-Film Morphology

Many highly efficient all-polymer-based organic solar cells (OSCs) have been achieved owing to material design and device engineering. However, most of them were achieved by using halogenated solvents to process the active layers, being not beneficial to its nature of green energy technology. In this work, we compared chloroform- and toluene-processed PM6:PY-IT-based all-polymer devices with the same blend solution recipe, same film formation speed, and same postcast treatment. The film cast from toluene exhibited weaker crystallinity. For device performance, toluene enabled a better power conversion efficiency (PCE) of 15.51%, outperforming that of chloroform (15.00%), and it is the highest value for non-halogenated solvent-cast all-polymer-based OSCs to date. Toluene's morphology tuning effect was characterized to increase and balance the charge transport and then suppress the exciton recombination and improve the charge extraction, considered to be the reason for efficiency enhancement. Besides, the toluene-cast active layer-based devices showed slightly better photostability than the chloroform-driven ones. This work provided a new direction for building low-toxicity solvent-treated all-polymer OSCs with cutting-edge performance.