Development Of Non-fullerene Acceptors Research Articles

Development of Non-Fullerene Acceptors with π-Extended Central Unit for Organic Photovoltaic Devices

Development of Non-Fullerene Acceptors with π-Extended Central Unit for Organic Photovoltaic Devices

High Detectivity Near Infrared Organic Photodetectors Using an Asymmetric Non-Fullerene Acceptor for Optimal Nanomorphology and Suppressed Dark Current.

Recently, the development of non-fullerene acceptors (NFAs) for near-infrared (NIR) organic photodetectors (OPDs) has attracted great interest due to their excellent NIR light absorption properties. Herein, we developed NFAs by substituting an electron-donating moiety (branched alkoxy thiophene (BAT)) asymmetrically (YOR1) and symmetrically (YOR2) for the Y6 framework. YOR1 exhibited nanoscale phase separation in a film blended with PTB7-Th. Moreover, substituting the BAT unit effectively extended the absorption wavelengths of YOR1 over 1000 nm by efficient intramolecular charge transfer and extension of the conjugation length. Consequently, YOR1-OPD exhibited significantly reduced dark current and improved responsivity by simultaneously satisfying optimal nanomorphology and significant suppression of charge recombination, resulting in 1.98 × 1013 and 3.38 × 1012 Jones specific detectivity at 950 and 1000 nm, respectively. Moreover, we successfully demonstrated the application of YOR1-OPD in highly sensitive photoplethysmography sensors using NIR light. This study suggests a strategic approach for boosting the overall performance of NIR OPDs targeting a 1000 nm light signal using an all-in-one (optimal morphology, suppressed dark current, and extended NIR absorption wavelength) NFA.

Star-shaped benzotriindole-based donor compounds for all–small–molecule non-fullerene organic solar cells

The recent success in the development of non-fullerene acceptors (NFAs) for organic solar cells (OSCs) encourages the search for novel efficient and complementary donor materials. Moreover, the in-depth understanding of the structure–property correlations for new generations of acceptor and donor materials is of great importance for the further progress in the field of OSCs. In this work the synthesis of a series of star-shaped small molecules based on benzotriindole (BTI) electron-donating core linked through bi– or terthiophene π-spacers to either hexyl dicyanovinyl or 2-ethylhexyl cyanoacetate electron-withdrawing terminal groups (EWGs) is reported. The thermal, optical and electrochemical properties of the molecules were carefully investigated and compared. The molecules were also explored as donor materials in non-fullerene (NF) OSCs in binary blends with well-known NFAs such as IDIC or Y6. The huge impact of the donor molecular structure on the device performance (up to 6 times difference in power conversion efficiency (PCE)) was evaluated due to significant difference in the active layer morphologies, which was successfully proved via scanning near-field infrared microscopy technique. The results obtained in this work demonstrate high potential of star-shaped BTI-based molecules to be used as donor materials with a new generation of NFAs and can be used for further molecular design of the highly efficient small molecule donors for NF OSCs.

The Dynamics of Delocalized Excitations in Organic Solar Cells with Nonfullerene Acceptors

Recently, the performance of organic solar cells has been markedly improved benefiting from the development of nonfullerene acceptors (NFAs) with acceptor-donor-acceptor structures. Arising from the intermolecular electronic interactions between the electron donating and accepting units, intramoiety and interfacial delocalized excitations make a substantial contribution to the photocurrent generation. In this Perspective, we discuss recent studies on the excited-state dynamics responsible for the working mechanism in NFA-based organic solar cells and emphasize the dynamics of delocalized excitations in charge generation and recombination processes. The intramoiety delocalized excitations in NFAs enable charge separation without forming interfacial charge-transfer excitons first, allowing efficient photocharge generation in planar heterojunctions with reduced interfacial energy loss. We suggest a few research directions in elucidating the performance-limited processes toward the further optimization of NFA-based devices.

High Efficiency Transparent and Semi‐Transparent Photovoltaics Based on a Layer‐By‐Layer Deposition

Transparent and semitransparent photovoltaics offer an exciting opportunity to integrate existing infrastructure with renewable energy. Organic photovoltaics (OPVs) are key enablers for wavelength‐selective transparent photovoltaics (TPVs) because of their selective absorption in the near‐infrared (NIR) that enables simultaneously high power conversion efficiency (PCE) and average visible transmittance (AVT). The recent rise of OPVs and TPVs has been accelerated in large part by the development of nonfullerene acceptors (NFAs) as highly adaptable deep NIR harvesting materials. Herein, sequential layer‐by‐layer (LBL) deposition of a selectively NIR absorbing nontraditional acceptor polymer is paired with a NIR absorbing donor IEICO‐4F that is typically considered an NFA via solvent orthogonality. With detailed optimization of the active layers and top electrode, semi‐transparent photovoltaics with a PCE of 8.8%, AVT of 40.9%, and a light utilization efficiency of 3.6% are demonstrated. The LBL approach enables explicit optical modeling of the device structure to extract exciton diffusion lengths >100 nm for both the polymer and IEICO‐4F with a transition in charge collection length regimes dependent on the acceptor thickness. Furthermore, the LBL deposition technique enables an investigation of the full range of polymer thickness and its impact on power generation and optical performance.

Bay-Functionalized Perylene Diimide Derivative Cathode Interfacial Layer for High-Performance Organic Solar Cells.

The field of organic solar cells (OSCs) has acquired rapid progress with the development of nonfullerene acceptors. Interfacial engineering is also significant for the enhancement of the power conversion efficiency (PCE) in OSCs. Among the cathode interfacial materials (CIMs), perylene diimide (PDI) small molecules are promising owing to the excellent electron affinity and electron mobility. Although the well-known PDINN molecule has excellent properties, it has a high planarity formed by an extensive rigid π-conjugated backbone. Because the PDI molecular backbone has a strong tendency to aggregate, it causes the problem of excessive molecular aggregation and stacking, which directly leads to excessive crystallinity. Proper accumulation is beneficial for charge transport, but oversized crystals formed by overaggregation will hinder charge transport, ultimately affecting the film morphology and charge transport efficiency. Modifying the bay position of PDINN is an effective strategy to reduce the planarity, modulate the molecular aggregation, optimize the morphology, and enhance the charge-collecting efficiency. Therefore, PDINN-S was synthesized from PDINN by substituting the hydrogen with thiophene. The optimal PCE in the PM6:Y6 active layer was 16.18% and remained at 80% of the initial value after 720 h in a glovebox. This provides some guidance for exploring CIMs and preparing large-scale OSCs in the future.

Synthesis, Properties, and Photovoltaic Characteristics of Fluoranthenedione-containing Nonfullerene Acceptors for Organic Solar Cells

The progress of organic solar cells (OSCs) largely depends on the development of nonfullerene acceptors (NFAs) based on electron-accepting π-conjugated compounds. Therefore, the creation of its building unit is important to tune the properties and increase the OSC performance. In this contribution, a new electron-accepting building unit, fluoranthenedione (FDO), was designed by extending the π-conjugation of representative indene-1,3-dione (IDO) framework. The electron-accepting π-conjugated compound (FL-FDO) composed of thiophene-linked fluorene (FL) as a central unit and FDO as terminal units was designed and synthesized. Compared to the corresponding IDO-containing NFAs, the introduction of FDO led to the red-shifted absorption and increased frontier orbital energy levels. The OSCs based on FL-FDO and poly(3-hexylthiophene) (P3HT) as a donor showed improved power conversion efficiency, compared to the P3HT/FL-IDO-based devices. These results indicate that the FDO unit has the potential to act as a promising terminal unit of NFAs.

Renewed Prospects for Organic Photovoltaics.

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (Eloss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller Eloss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

PTB7-Th-Based Organic Photovoltaic Cells with a High VOC of over 1.0 V via Fluorination and Side Chain Engineering of Benzotriazole-Containing Nonfullerene Acceptors.

PTB7-Th is considered one of the most classic donor polymers for organic photovoltaic (OPV) cells. However, the power conversion efficiency (PCE) of PTB7-Th-based OPV is lagging behind that of other promising polymers mainly because of the relatively low open-circuit voltage (VOC). To increase the VOC and PCE of PTB7-Th-based OPV, the development of nonfullerene acceptors (NFAs) and studies of structure-property-performance relationship are vital. Here, three A2-A1-D-A1-A2-type acceptors, namely BTA45, F-BTA45, and F-BTA5, were developed by fluorination on the benzotriazole (BTA) unit and regulating alkoxy or alkyl phenyl side chains. Compared with BTA45, light absorption and π-π packing can be simultaneously improved for the two fluorinated BTA acceptors, resulting in an increased JSC and FF. Moreover, the F-BTA5-based blend film exhibits better phase separation morphology and electron transport than those of BTA45 and F-BTA45, which contribute to a device efficiency of 10.36% with a VOC of 1.03 V. In addition, the ΔE2 values of the three blends are less than 0.15 eV, together with their moderate ΔE3, efficiently decreasing their energy loss. These results highlight the importance of fluorination and side chain engineering for NFAs to boost the VOC and PCE for low-band gap photovoltaic polymers.

Toward high-performance organic photovoltaics: the new cooperation of sequential solution-processing and promising non-fullerene acceptors.

Organic photovoltaics (OPVs) have long been a hot topic due to their light weight, low cost, and flexibility. Simple blend-based OPVs have sufficient donor/acceptor (D/A) interfaces and high exciton dissociation efficiency, which result in certified high power conversion efficiency (PCE) exceeding 18%. However, the difficult morphology control and poor device stability limit further progress toward higher PCE and future application. Sequential solution-processing with tunable vertical phase distribution, D/A interfaces, and charge transportation pathways not only benefit device stability but can also overcome the up-scaling challenge. In recent years, the development of non-fullerene acceptors (NFAs) has been very rapid, which is attributed to their tunable energy levels, bandgaps, planarity, and crystallinity. In this minireview, the opportunities for the cooperation of sequential solution-processing and NFAs are revealed based on their characteristics, such as diverse molecular shapes, abundant functional groups and heteroatoms, and various aggregation states for NFAs; independent active layer processing, controllable D/A interfaces, and excellent device stability for sequential solution-processing. Few but important existing examples are discussed to display the prospects of sequential solution-processed fullerene-free OPVs toward high PCE, good device stability, high semitransparency, and large-area industrial manufacture. Finally, some possible research directions are predicted and the main issues that need to be overcome are proposed for sequential solution-processed fullerene-free OPVs toward higher performance.

Non-fullerene acceptor photostability and its impact on organic solar cell lifetime

The development of non-fullerene acceptors (NFAs) has facilitated the realization of efficient organic solar cells (OSCs) with minimal burn-in losses and excellent long-term stability. However, the role of NFA molecular structures on device stability remains unclear, limiting commercialization of NFA-based OSCs. Herein, the photostability of 10 OSC devices, fabricated with various NFAs (O-IDTBR, EH-IDTBR, ITIC, and ITIC-M) blended with donor polymers (PTB7-Th, PffBT4T-2OD, and PBDB-T), is investigated. O-IDTBR and EH-IDTBR form highly stable devices with all three polymers, whereas ITIC and ITIC-M devices suffer from burn-in losses and long-term degradation. Conformational instability is found to be responsible for the poor photostability of ITIC and ITIC-M, resulting in poor device stability. Twisting and potential breakage of the chemical bond that links the end group to the main backbone of ITIC and ITIC-M molecules causes undesirable conformational changes. Potential strategies to overcome such detrimental photo-induced conformational changes in NFAs are proposed.

Photooxidation Analysis of Two Isomeric Nonfullerene Acceptors: A Systematic Study of Conformational, Morphological, and Environmental Factors

Despite the striking progress toward improving the efficiencies of organic solar cells (OSCs) resulting from the development of nonfullerene acceptors (NFAs), there is a rising requirement of investigation of the molecular design for achieving photochemically stable NFAs. Herein, applying two isomeric NFA molecules a‐IDTBTRh and l‐IDTBTRh based on angular‐indaceno[2,1‐b:6,5‐b′]dithiophene (a‐IDT) and linear‐IDT (l‐IDT) as the central cores, the effects of molecular conformation, aggregation behavior, and environmental factors on their photooxidation degradation processes are systematically studied. It is found that tuning regioisomeric central cores on the molecular skeleton influences the backbone conformation and conjugation, resulting in the different optoelectronic properties and molecular stacking characteristics. A strong molecular aggregation dependence of the optical losses and degradation mechanics in the annealed neat films is also observed, and it is shown that l‐IDTBTRh is much more photostable than its less‐ordered core derivative, a‐IDTBTRh. Furthermore, the effects of external environmental stresses on photobleaching behaviors of the annealed thin films are studied systematically, highlighting the role of concentrated light in accelerating photooxidation of organic materials. The results not only provide a comprehensive insight into the photooxidation properties and attenuation mechanisms of organic materials, but also suggest guidelines to rationally select materials for stable OSCs.

Degradation Behavior of Scalable Nonfullerene Organic Solar Cells Assessed by Outdoor and Indoor ISOS Stability Protocols

The development of nonfullerene acceptors (NFAs) has led to dramatic improvements in the device efficiencies of organic photovoltaic (OPV) cells. To date it is, however, still unclear how those laboratory‐scale efficiencies transfer to commercial modules, and how stable these devices will be when processed via industrially compatible methods. Herein, the degradation behavior of lab‐scale and scalable OPV devices using similar nonfullerene‐based active layers is assessed. It is demonstrated that the scalable NFA OPV exhibits completely reversible degradation when assessed in ISOS‐O‐1 outdoor conditions, which is in contrast to the laboratory‐scale devices assessed via the indoor ISOS‐L‐2 protocol. Results from transient photovoltage (TPV) indicate the presence of charge trap formation, and a number of potential mechanisms are proposed for the selective occurrence of this in laboratory‐scale devices tested in ISOS‐L laboratory conditions—ultimately concluding that it has its origins in the different device architectures used. The study points at the risk of assessing active layer stability from laboratory‐scale devices and degradation studies alone and highlights the importance of using a diverse range of testing conditions and ISOS protocols for such assessment.

The effect of halogenation on PBDTT-TQxT based non-fullerene polymer solar cells – Chlorination vs fluorination

The rapid advancement in the development of non-fullerene acceptors has led to single-junction polymer solar cells with efficiencies over 18%. Even with these novel acceptor materials, the choice of the donor polymer remains important. Tuning of the donor and acceptor compatibility in terms of absorption, frontier orbital energy levels, mixing enthalpy and charge carrier mobility is routinely performed by side chain variation. Fluorination presents an additional powerful approach to optimize these parameters. Although significantly less studied, chlorination can give rise to similar effects, while donor-acceptor phase separation due to fluorophobic interactions is less of an issue. Moreover, from a material synthesis point of view, the introduction of chlorine groups is in many cases much more straightforward. In this work, we present a series of push-pull type benzo[1,2-b:4,5-b']dithiophene-alt-quinoxaline donor polymers and compare the behavior of the non-halogenated, fluorinated and chlorinated derivatives in polymer solar cells when combined with small molecule and polymer type non-fullerene acceptors. The solar cell efficiencies vary from 2.4 to 8.4%, elucidating the large impact of these small structural variations. Best results are achieved for the chlorinated donor polymer, affording a high open-circuit voltage, balanced charge carrier mobilities and favorable donor-acceptor interactions. Combined with the easier synthesis of chlorinated materials, this suggests that more emphasis should be put on chlorination as a valuable approach to tune the properties of organic semiconductors for solar cell blends (and other optoelectronic applications).

Nonfullerene Ternary Organic Solar Cell with Effective Charge Transfer between Two Acceptors.

High power conversion efficiency can be realized by using a ternary bulk heterojunction with complementary absorption spectra in organic solar cells. However, as the development of nonfullerene acceptors with a broad absorption spectrum makes the absorption efficiency of the photovoltaic devices close to optimal, such a strategy needs modifying. In particular, charge transfer between the two acceptors is necessary to be considered. Herein, we purposely design a ternary system based on PTB7-Th:COi8DFIC:ITIC-4F. Though the presence of ITIC-4F in PTB7-Th:COi8DFIC could not broaden the absorption spectrum obviously, the formed cascade-energy-level alignment is beneficial for promoting and balancing exciton separation and charge transport between the donor and two acceptors and even between the acceptors. Insights into the charge transport route in the completed system are provided via using the techniques including photoluminescence spectroscopy and pump-probe photoconductivity spectroscopy. This work provides a new idea for designing highly efficient ternary organic solar cells.

Dual non-fullerene acceptors based high efficiency ternary organic solar cells

&lt;sec&gt;Organic photovoltaics (OPVs) have been considered as one of the preliminary candidates for the third-generation solar cells due to their particularly advantages, such as light-weight, low cost, solution processability and mechanical flexibility. In recent years, the power conversion efficiency (PCE) of OPVs has achieved remarkable progress with the development of non-fullerene acceptors (NFAs), which exhibit stronger capability of light absorption and stability than the fullerene acceptors. However, the narrow absorption properties of organic materials still restrict the further breakthrough of the performance of OPVs. It is difficult for the binary heterojunction films to realize the effective wide spectrum utilization of solar energy. Ternary strategy, which consists of one donor and two acceptors or two donors and one acceptor in a bulk-heterojunction, has proven to be an effective and facile way to enhance performances of OPVs. The emergence of new NFAs such as ITIC, IT-4F, Y6 &lt;i&gt;etc.&lt;/i&gt; greatly increases the selectivity of acceptors in a ternary system. It is necessary to investigate the compatibility of latest NFA materials and corresponding ternary device performance.&lt;/sec&gt; &lt;sec&gt;In this article, we report a new ternary OPV system with dual NFAs and a polymer donor. The excellent NFA material (Y6), which was first reported in 2019, is used as the third component. Different concentrations of Y6 are introduced into the binary system based on PBDB-T-2F:IT-4F. The compatibility between materials and device performances are investigated through absorption capability of blend films, AFM, water contact angle, GISAXS, and corresponding electrical properties of devices. The 12.48% PCE is achieved from ternary OPVs with PBDB-T:IT-4F as the active layers containing 20wt% Y6 in acceptors, resulting from the enhanced &lt;i&gt;J&lt;/i&gt;&lt;sub&gt;sc&lt;/sub&gt; of 22.09 mA/cm&lt;sup&gt;2&lt;/sup&gt;, &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;oc&lt;/sub&gt; of 0.83 V and &lt;i&gt;FF&lt;/i&gt; of 68.45%. The origin of the improvement of the ternary OPVs is summarized below. Firstly, an apparently complementary absorption spectrum is obtained through the introduction of Y6, which has a stronger photo harvesting capability in the spectral range from 750 to 950 nm than IT-4F, and higher short current density (&lt;i&gt;J&lt;/i&gt;&lt;sub&gt;sc&lt;/sub&gt;) is observed in the ternary devices than in the binary device. Secondly, a rougher surface of the active layer is observed by increasing the Y6 concentration, which may result in an inferior exciton dissociation and charge transport process, the existence of larger-scaled crystal is proved by the GISAXS technology. Thirdly, the introduced Y6 can help to suppress the bimolecular recombination, which is in favor of the incremental device performance. Approximately 17.85% &lt;i&gt;PCE&lt;/i&gt; improvement is obtained in comparison with PBDB-T-2F:IT-4F based binary OPVs. &lt;/sec&gt;

Rhodanine-based nonfullerene acceptors for organic solar cells

Significant progress on the development of non-fullerene acceptors (NFAs) for organic solar cells (OSCs) has been made in the past several years, and the power conversion efficiency (PCE) exceeding 17% has been already realized based on a tandem non-fullerene device. To date, NFAs with a linearly fused acceptor-donor-acceptor (A-D-A) structure are of great interest, due to their attracting synthetic flexibility and high photovoltaic performance. Rhodanine is one of the most studied electron-withdrawing moieties to construct such A-D-A type NFAs, and the resulting single-junction OSCs have produced PCEs of ~10%. More interestingly, those rhodanine-based NFAs have demonstrated a particularly excellent compatibility with well-known P3HT donor, enabling respectable PCEs over 7%. Thus in this review, we summarize the important advances on rhodanine-based NFAs with a main focus on discussing the molecular design strategies, providing a better understanding of the structure-property relationship for those rhodanine-based NFAs.

Alkyl Chain Length Effects of Polymer Donors on the Morphology and Device Performance of Polymer Solar Cells with Different Acceptors

AbstractThe development of nonfullerene acceptors has brought polymer solar cells into a new era. Maximizing the performance of nonfullerene solar cells needs appropriate polymer donors that match with the acceptors in both electrical and morphological properties. So far, the design rationales for polymer donors are mainly borrowed from fullerene‐based solar cells, which are not necessarily applicable to nonfullerene solar cells. In this work, the influence of side chain length of polymer donors based on a set of random terpolymers PTAZ‐TPD10‐Cn on the device performance of polymer solar cells is investigated with three different acceptor materials, i.e., a fullerene acceptor [70]PCBM, a polymer acceptor N2200, and a fused‐ring molecular acceptor ITIC. Shortening the side chains of polymer donors improves the device performance of [70]PCBM‐based devices, but deteriorates the N2200‐ and ITIC‐based devices. Morphology studies unveil that the miscibility between donor and acceptor in blend films depends on the side chain length of polymer donors. Upon shortening the side chains of the polymer donors, the miscibility between the donor and acceptor increases for the [70]PCBM‐based blends, but decreases for the N2200‐ and ITIC‐based blends. These findings provide new guidelines for the development of polymer donors to match with emerging nonfullerene acceptors.

Perylene-Diimide Based Donor–Acceptor–Donor Type Small-Molecule Acceptors for Solution-Processable Organic Solar Cells

Development of nonfullerene acceptors plays an important role in the commercial availability of plastic solar cells. We report herein synthesis of bay-substituted donor–acceptor–donor (D–A–D)-type perylene diimide (PDI)-based small molecules (SM-1 to SM-4) by Suzuki coupling method and their use as acceptors in bulk heterojunction organic solar cells (BHJ-OSCs) with poly(3-hexylthiophene) (P3HT) polymer donor. We varied the number of electron-rich thiophene units and the solubilizing side chains and also evaluated the optical and electrochemical properties of the small molecules. The synthesized small molecules were confirmed by Fourier-transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and high-resolution mass spectroscopy (HR-MS). The small molecules showed extensive and strong absorption in the ultraviolet–visible (UV–Vis) region up to 750 nm, with bandgap ( $$E_{\rm{g}}^{\rm{opt}} $$ ) reduced below <2 eV. The energy levels of small molecules SM-1 to SM-4 were suitable for use as electron-accepting materials. The small molecules showed good thermal stability up to 300°C. BHJ-OSCs with SM-1 and P3HT polymer donor showed maximum power conversion efficiency (PCE) of 0.19% with V oc of 0.30 V, J sc of 1.72 mA cm−2, and fill factor (FF) of 37%. The PCE decreased with the number of thiophene units. The PCE of SM-2 was lower than that of SM-1. This difference in PCE can be explained by the higher aggregation tendency of the bithiophene compared with the thiophene unit. Introduction of the solubilizing group in the bay position increased the aggregation property, leading to much lower PCE than for the small molecules without solubilizing group.