13.34 % Efficiency Non‐Fullerene All‐Small‐Molecule Organic Solar Cells Enabled by Modulating the Crystallinity of Donors via a Fluorination Strategy

Non‐fullerene all‐small‐molecule organic solar cells (NFSM‐OSCs) have shown potential as OSCs, owing to their high purity, easy synthesis and good reproducibility. However, challenges in the modulation of phase separation morphology have limited their development. Herein, two novel small molecular donors, BTEC‐1F and BTEC‐2F, derived from the small molecule DCAO3TBDTT, are synthesized. Using Y6 as the acceptor, devices based on non‐fluorinated DCAO3TBDTT showed an open circuit voltage (V_oc) of 0.804 V and a power conversion efficiency (PCE) of 10.64 %. Mono‐fluorinated BTEC‐1F showed an increased V_oc of 0.870 V and a PCE of 11.33 %. The fill factor (FF) of di‐fluorinated BTEC‐2F‐based NFSM‐OSC was improved to 72.35 % resulting in a PCE of 13.34 %, which is higher than that of BTEC‐1F (61.35 %) and DCAO3TBDTT (60.95 %). To our knowledge, this is the highest PCE for NFSM‐OSCs. BTEC‐2F had a more compact molecular stacking and a lower crystallinity which enhanced phase separation and carrier transport.     

High efficiency ternary organic solar cells enabled by compatible dual-donor strategy with planar conjugated structures

Ternary organic solar cells(OSCs) have received extensive attention for improving the power conversion efficiency(PCE) of organic photovoltaics(OPVs). In this work, a novel donor material(ECTBD) consisting of benzodithiophene(BDT) central electron donor unit was developed and synthesized. The small molecular donor has the same central unit as PM6. The addition of ECTBD into PM6:Y6 system could improve the morphology of active blend layer. In addition, ECTBD showed good morphologically compatibility when blending with PM6:Y6 host, resulting in the improvement of fill factor and current density. As a result, the ternary devices based on PM6:ECTBD:Y6 ternary system achieved a highest PCE of 16.51% with fill factor of 76.24%, which was much higher than that of the binary devices(15.7%). Overall, this work provided an effective strategy to fabricate highly efficient ternary organic solar cells through design of the novel small molecular donor as the third component.     

Potential of Nonfullerene Small Molecules with High Photovoltaic Performance

Over the past decades, fullerene derivatives have become the most successful electron acceptors in organic solar cells (OSCs) and have achieved great progress, with power conversion efficiencies (PCEs) of over 11 %. However, fullerenes have some drawbacks, such as weak absorption, limited energy‐level tunability, and morphological instability. In addition, fullerene‐based OSCs usually suffer from large energy losses of over 0.7 eV, which limits further improvements in the PCE. Recently, nonfullerene small molecules have emerged as promising electron acceptors in OSCs. Their highly tunable absorption spectra and molecular energy levels have enabled fine optimization of the resulting devices, and the highest PCE has surpassed 12 %. Furthermore, several studies have shown that OSCs based on small‐molecule acceptors (SMA) have very efficient charge generation and transport efficiency at relatively low energy losses of below 0.6 eV, which suggests great potential for the further improvement of OSCs. In this focus review, we analyze the challenges and potential of SMA‐based OSCs and discuss molecular design strategies for highly efficient SMAs.     

基于非富勒烯小分子受体Y6的有机太阳能电池

随着给/受体材料的不断发展,有机太阳能电池的器件效率不断取得进展.特别是非富勒受体分子Y6的出现,使单结有机太阳能电池的效率突破了15％.Y6已经应用到了有机太阳能电池各个方面并且极大提升了其性能.本综述主要总结了Y6在二元、三元和四元、逐层印刷、柔性、叠层和半透明等有机太阳能电池方面的研究情况,以及基于Y6三线态的有...     

Facile,Versatile and Stepwise Synthesis of High-Performance Oligomer Acceptors for Stable Organic Solar Cells

Oligomer acceptors have recently emerged as promising photovoltaic materials for achieving high power conversion efficiency (PCE) and long-term stability in organic solar cells (OSCs). However, the limited availability of diverse acceptors, resulting from the sole synthetic approach, has hindered their potential for future industrialization. In this study, we present a facile and effective stepwise approach that utilizes two consecutive Stille coupling reactions for the synthesis of oligomer acceptors. To demonstrate the feasibility of the novel approach, we successfully synthesize a trimer acceptor, Tri-Y6-OD, and further systematically investigate the impact of oligomerization on device performance and stability. The results reveal that this approach has significant advantages compared to the conventional method, including reduced formation of unwanted by-products and lower difficulties in purification. Remarkably, the OSC based on PM6 : Tri-Y6-OD achieves an impressive PCE of 18.03 % and maintains 80 % of the initial PCE (T₈₀) for 1523 h under illumination, surpassing the performance of the corresponding small molecule acceptor Y6-OD-based device. Furthermore, the versatility of the synthetic strategy in obtaining diverse acceptors is further demonstrated. Overall, our findings provide a facile, versatile and stepwise way for synthesizing oligomer acceptors, thereby facilitating the development of stable and efficient OSCs.     

13.34 % Efficiency Non-Fullerene All-Small-Molecule Organic Solar Cells Enabled by Modulating the Crystallinity of Donors via a Fluorination Strategy

Non-fullerene all-small-molecule organic solar cells (NFSM-OSCs) have shown potential as OSCs, owing to their high purity, easy synthesis and good reproducibility. However, challenges in the modulation of phase separation morphology have limited their development. Herein, two novel small molecular donors, BTEC-1F and BTEC-2F, derived from the small molecule DCAO3TBDTT, are synthesized. Using Y6 as the acceptor, devices based on non-fluorinated DCAO3TBDTT showed an open circuit voltage (V_oc) of 0.804 V and a power conversion efficiency (PCE) of 10.64 %. Mono-fluorinated BTEC-1F showed an increased V_oc of 0.870 V and a PCE of 11.33 %. The fill factor (FF) of di-fluorinated BTEC-2F-based NFSM-OSC was improved to 72.35 % resulting in a PCE of 13.34 %, which is higher than that of BTEC-1F (61.35 %) and DCAO3TBDTT (60.95 %). To our knowledge, this is the highest PCE for NFSM-OSCs. BTEC-2F had a more compact molecular stacking and a lower crystallinity which enhanced phase separation and carrier transport.     

Single-wall carbon nanotube-containing cathode interfacial materials for high performance organic solar cells

Water/alcohol soluble cathode interfacial materials(CIMs)are playing important roles in optoelectronic devices such as organic light emitting diodes,perovskite solar cells and organic solar cells(OSCs).Herein,n-doped solution-processable single-wall carbon nanotubes(SWCNTs)-containing CIMs for OSCs are developed by dispersing SWCNTs to the typical CIMs perylene diimide(PDI)derivatives PDIN and PDINO.The Raman and X-ray photoelectron spectroscopy(XPS)measurement results illustrate the ndoped behavior of SWCNTs by PDIN/PDINO in the blend CIMs.The blended and n-doped SWCNTs can tune the work function and enhance the conductivity of the PDI-derivative/SWCNT(PDI-CNT)composite CIMs,and the composite CIMs can regulate and down-shift the work function of cathode,reduce the charge recombination,improve the charge extraction rate and enhance photovoltaic performance of the OSCs.High power conversion efficiency(PCE)of 17.1%and 17.7%are obtained for the OSCs based on PM6:Y6 and ternary PM6:Y6:PC₇₁ BM respectively with the PDI-CNTcomposites CIMs.These results indicate that the ndoped SWCNT-containing composites,like other n-doped nanomaterials such as zero dimensional fullerenes and two dimensional graphenes,are excellent CIMs for OSCs and could find potential applications in other optoelectronic devices.     

High electron mobility fluorinated indacenodithiophene small molecule acceptors for organic solar cells

Indacenodithiophene (IDT) derivatives are kinds of the most representative and widely used cores of small molecule acceptors (SMAs) in organic solar cells (OSCs). Here we systematically investigate the influence of end-group fluorination density and position on the photovoltaic properties of the IDT-based SMAs IDIC-nF (n = 0, 2, 4). The absorption edge of IDIC-nF red-shifts with the π-π stacking and crystallinity improvement, and their electronic energy levels downshift with increasing n. Due to the advantages of J_sc and FF as well as acceptable V_oc, the difluorinated IDIC-2F acceptor based OSCs achieve the highest power conversion efficiency (PCE) of 13%, better than the OSC devices based on IDIC and IDIC-4F as acceptors. And the photovoltaic performance of the PTQ10: IDIC-2F OSCs is insensitive to the active layer thickness: PCE still keep high values of 12.00% and 11.46% for the devices with active layer thickness of 80 and 354 nm, respectively. This work verifies that fine and delicate modulation of the SMAs molecular structure could optimize photovoltaic performance of the corresponding OSCs. Meanwhile, the thickness-insensitivity property of the OSCs has potential for large-scale and printable fabrication technology.     

Altering alkyl-chains branching positions for boosting the performance of small-molecule acceptors for highly efficient nonfullerene organic solar cells

The emergence of the latest generation of small-molecule acceptor(SMA) materials,with Y6 as a typical example,accounts for the surge in device performance for organic solar cells(OSCs).This study proposes two new acceptors named Y6-C2 and Y6-C3,from judicious alteration of alkyl-chains branching positions away from the Y6 backbone.Compared to the Y6,the Y6-C2 exhibits similar optical and electrochemical properties,but better molecular packing and enhanced crystallinity.In contrast,the Y6-C3 shows a significant blue-shift absorption in the solid state relative to the Y6 and Y6-C2.The as-cast PM6:Y6-C2-based OSC yields a higher power conversion efficiency(PCE) of 15.89% than those based on the Y6(15.24%) and Y6-C3(13.76%),representing the highest known value for as-cast nonfullerene OSCs.Prominently,the Y6-C2 displays a good compatibility with the PC_(71)BM.Therefore,a ternary OSC device based on PM6:Y6-C2:PC_(71)BM(1.0:1.0:0.2) was produced,and it exhibits an outstanding PCE of 17.06% and an impressive fill factor(FF) of 0.772.Our results improve understanding of the structureproperty relationship for state-of-the-art SMAs and demonstrate that modulating the structure of SMAs via fine-tuning of alkylchains branching positions is an effective method to enhance their performance.     

Ternary solvent-processed efficient organic solar cells based on a new A-DA'D-A acceptor derivative employing the 3rd-position branching side chains on pyrroles

Recent advances in non-fullerene acceptors(NFAs),typically Y6,have driven power conversion efficiency(PCE) of single-junction orga nic solar cells(OSCs) over 16%.Mea nwhile,it becomes essential to know how to adopt simple strategies to further improve device performance.In this work,a new A-DA'D-A acceptor derivative,Y19-N3 employing 3-ethylheptyl branched at the 3^rd-position instead of 2-ethylhexyl on the pyrroles of Y19 is reported.The selection of an appropriate solvent in casting device is implemented to maximize the photovoltaic performance.PBDB-T:Y19-N3-based OSCs treated with a ternary solvent of CF/CB(1:3,v/v) and 0.8% DIO exhibit the optimal PCE of 13.77% here,with the significantly improved V_oc(0.78 V) and FF(0.72) as well as the high J_sc(24.46 mA/cm²).Further characterizations indicate that this ternary solvent-treated PBDB-T/Y19-N3 film exhibits the more appropriate morphological features with the highly efficient charge generation and collection as well as the more balanced electron and hole mobilities.This work combines molecular design and device engineering to improve the photovoltaic properties,which is important to the development of OSCs.     

Organic solar cells based on small molecule donor and polymer acceptor

Small molecule donor/polymer acceptor(SD/PA)-type organic solar cells(OSCs) have attracted widespread attention in recent years due to the continuing power conversion efficiency(PCE) growth, near 10%, and the excellent thermal stability for the practical applications. However, the development of SD/PA-type OSCs lags far behind that of polymer donor/small molecule acceptor(PD/SA)-type OSCs, which are also based on the combination of small molecule and polymer, with the PCEs exceeding 18%. The rea...     

Unidirectional Sidechain Engineering to Construct Dual-Asymmetric Acceptors for 19.23 % Efficiency Organic Solar Cells with Low Energy Loss and Efficient Charge Transfer

Achieving both high open-circuit voltage (V_oc) and short-circuit current density (J_sc) to boost power-conversion efficiency (PCE) is a major challenge for organic solar cells (OSCs), wherein high energy loss (E_loss) and inefficient charge transfer usually take place. Here, three new Y-series acceptors of mono-asymmetric asy-YC11 and dual-asymmetric bi-asy-YC9 and bi-asy-YC12 are developed. They share the same asymmetric D₁AD₂ (D₁=thieno[3,2-b]thiophene and D₂=selenopheno[3,2-b]thiophene) fused-core but have different unidirectional sidechain on D₁ side, allowing fine-tuned molecular properties, such as intermolecular interaction, packing pattern, and crystallinity. Among the binary blends, the PM6 : bi-asy-YC12 one has better morphology with appropriate phase separation and higher order packing than the PM6 : asy-YC9 and PM6 : bi-asy-YC11 ones. Therefore, the PM6 : bi-asy-YC12-based OSCs offer a higher PCE of 17.16 % with both high V_oc and J_sc, due to the reduced E_loss and efficient charge transfer properties. Inspired by the high V_oc and strong NIR-absorption, bi-asy-YC12 is introduced into efficient binary PM6 : L8-BO to construct ternary OSCs. Thanks to the broadened absorption, optimized morphology, and furtherly minimized E_loss, the PM6 : L8-BO : bi-asy-YC12-based OSCs achieve a champion PCE of 19.23 %, which is one of the highest efficiencies among these annealing-free devices. Our developed unidirectional sidechain engineering for constructing bi-asymmetric Y-series acceptors provides an approach to boost PCE of OSCs.     

Giant Molecule Acceptor Enables Highly Efficient Organic Solar Cells Processed Using Non-halogenated Solvent

High efficiency organic solar cells (OSCs) based on A-DA′D-A type small molecule acceptors (SMAs) were mostly fabricated by toxic halogenated solvent processing, and power conversion efficiency (PCE) of the non-halogenated solvent processed OSCs is mainly restricted by the excessive aggregation of the SMAs. To address this issue, we developed two vinyl π-spacer linking-site isomerized giant molecule acceptors (GMAs) with the π-spacer linking on the inner carbon (EV-i) or out carbon (EV-o) of benzene end group of the SMA with longer alkyl side chains (ECOD) for the capability of non-halogenated solvent-processing. Interestingly, EV-i possesses a twisted molecular structure but enhanced conjugation, while EV-o shows a better planar molecular structure but weakened conjugation. The OSC with EV-i as acceptor processed by the non-halogenated solvent o-xylene (o-XY) demonstrated a higher PCE of 18.27 % than that of the devices based on the acceptor of ECOD (16.40 %) or EV-o (2.50 %). 18.27 % is one of the highest PCEs among the OSCs fabricated from non-halogenated solvents so far, benefitted from the suitable twisted structure, stronger absorbance and high charge carrier mobility of EV-i. The results indicate that the GMAs with suitable linking site would be the excellent candidates for fabricating high performance OSCs processed by non-halogenated solvents.     

Diazabicyclic Electroactive Ionenes for Efficient and Stable Organic Solar Cells

Electroactive ionenes combining caged-shaped diazabicyclic cations and aromatic diimides were developed as interlayers in organic solar cells (OSCs). These ionenes reduce the work-function of air-stable metal electrodes (e.g., Ag, Cu and Au) by generating strong interfacial dipoles, and their optoelectronic and morphological characters can be modulated by aromatic diimides, leading to high conductivity and good compatibility with active layers. The optimal ionene exhibits superior charge-transport, desirable crystallinity, and weak visible-absorption, boosting the efficiency of benchmark PM6 : Y6-based OSCs up to 17.44 %. The corresponding normal devices show excellent stability at maximum power point test under one sun illumination for 1000 h. Replacing Y6 with L8-BO promotes the efficiency to 18.43 %, one of the highest in binary OSCs. Notably, high efficiencies >16 % are maintained as the interlayer thickness increasing to 105 nm, the best result with interlayer-thickness over 100 nm.     

Fluorination strategy enables greatly improved performance for organic solar cells based on polythiophene derivatives

The power conversion efficiencies(PCEs) of organic solar cells(OSCs) have reached 18% recently,which have already met the demand of practical application.However,these outstanding results were generally achieved with donor-acceptor(D-A) type copolymer donors,which can hardly fulfill the low-cost largescale production due to their complicated synthesis processes.Therefore,developing polymer donors with simple chemical structures is urgent for realizing low-cost OSCs.Polythiophene(PT) derivatives are currently regarded as promising candidates for such kind of donor materials,which has been illustrated in many works.In this work,two new alkylthio substituted PT derivatives,P301 and P302,were synthesized and tested as donors in the OSCs using Y5 as the accepto r.In comparison,the introduction of fluorine atoms on the backbone of P302 can not only downshift the energy levels,but also greatly improve the phase separation morphologies of the active layers,which is ascribed to the enhanced aggregation effect and the reduced miscibility with the non-fullerene acceptor.As a result,the P302:Y5-based OSC exhibits a significantly improved PCE of 9.65% than that of P301:Y5-based one,indicating the important role of fluorination in the construction of efficient PT derivative donors.     

Benzotriazole-Based 3D Four-Arm Small Molecules Enable 19.1 % Efficiency for PM6 : Y6-Based Ternary Organic Solar Cells

A third component featuring a planar backbone structure similar to the binary host molecule has been the preferred ingredient for improving the photovoltaic performance of ternary organic solar cells (OSCs). In this work, we explored a new avenue that introduces 3D-structured molecules as guest acceptors. Spirobifluorene (SF) is chosen as the core to combine with three different terminal-modified (rhodanine, thiazolidinedione, and dicyano-substituted rhodanine) benzotriazole (BTA) units, affording three four-arm molecules, SF-BTA1, SF-BTA2, and SF-BTA3, respectively. After adding these three materials to the classical system PM6 : Y6, the resulting ternary devices obtained ultra-high power-conversion efficiencies (PCEs) of 19.1 %, 18.7 %, and 18.8 %, respectively, compared with the binary OSCs (PCE=17.4 %). SF-BTA1-3 can work as energy donors to increase charge generation via energy transfer. In addition, the charge transfer between PM6 and SF-BTA1-3 also acts to enhance charge generation. Introducing SF-BTA1-3 could form acceptor alloys to modify the molecular energy level and inhibit the self-aggregation of Y6, thereby reducing energy loss and balancing charge transport. Our success in 3D multi-arm materials as the third component shows good universality and brings a new perspective. The further functional development of multi-arm materials could make OSCs more stable and efficient.     

Non-fullerene acceptor fibrils enable efficient ternary organic solar cells with 16.6% efficiency

Optimizing the components and morphology within the photoactive layer of organic solar cells(OSCs) can significantly enhance their power conversion efficiency(PCE). A new A-D-A type non-fullerene acceptor IDMIC-4F is designed and synthesized in this work, and is employed as the third component to prepare high performance ternary solar cells. IDMIC-4F can form fibrils after solution casting, and the presence of this fibrillar structure in the PBDB-T-2F:BTP-4F host confines the growth of donors and acceptors into fine domains, as well as acting as transport channels to enhance electron mobility. Single junction ternary devices incorporating 10 wt% IDMIC-4F exhibit enhanced light absorption and balanced carrier mobility, and achieve a maximum PCE of 16.6% compared to 15.7% for the binary device, which is a remarkable efficiency for OSCs reported in literature. This non-fullerene acceptor fibril network strategy is a promising method to improve the photovoltaic performance of ternary OSCs.     

Novel Small Four-armed Molecules with Triphenylamine-bridged Structure for Organic Solar Cells Featuring High Open-circuit Voltage

In view of few attention on star-shaped molecules containing triphenylamine(TPA) unit as π-linker, a series of small four-armed molecules, consisting of octyloxy-substituted 2,1,3-benzothiadiazole(DOBT) or 4-octyl-2-thienyl functionalized DOBT as the core, TPA as π-bridge and 4-methylphenyl or 4-methoxyphenyl groups as terminal units, was designed and synthesized. The effects of π-bridges and substitute groups on molecular photoelectric performance and photovoltaic performance were fully explored. With the help of the additional thiophene-linkers incorporation, 3-octylthienyl substituted molecule with end-capping 4-methylphenyl(T-BTTPAM) and 3-octylthienyl substituted molecule with end-capping 4-methoxyphenyl(T-BTTPAOM) showed stronger and broader absorption, as well as higher charge mobilities compared to the molecules without thiophene-linkers(BTTPAM and BTTPAOM). Additionally, changing substitute groups from methyl to methoxy helped BTTPAOM and T-BTTPAOM achieve better absorption properties than BTTPAM and T-BTTPAM, respectively. When paired with PC₆₁BM as the electron acceptor to fabricate solution-processed photovoltaic devices, the four materials gave high open-circuit voltage(V_oc) values over 0.90 V. These results demonstrate that our materials are promising candidates as donor materials for organic solar cells(OSCs), and further device optimization is in progress in our laboratory.     

A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss

A new acceptor-donor-acceptor(A-D-A) type small-molecule acceptor NCBDT-4 Cl using chlorinated end groups is reported.This new-designed molecule demonstrates wide and efficient absorption ability in the range of 600–900 nm with a narrow optical bandgap of 1.40 eV. The device based on PBDB-T-SF:NCBDT-4 Cl shows a power conversion efficiency(PCE) of 13.1%without any post-treatment, which represents the best result for all as-cast organic solar cells(OSCs) to date. After device optimizations, the PCE was further enhanced to over 14% with a high short-circuit current density(Jsc) of 22.35 m A cm-2 and a fill-factor(FF) of 74.3%. The improved performance was attributed to the more efficient photo-electron conversion process in the optimal device. To our knowledge, this outstanding efficiency of 14.1% with an energy loss as low as 0.55 eV is among the best results for all single-junction OSCs.     

High-efficiency organic solar cells with low voltage-loss of 0.46 V

The recent evolution of active components yielded brilliant progresses for organic solar cells (OSCs), yet the mechanism is needed to be clearly understood. In this work, two electron acceptors, a linear SN6-2Br and a V-shaped BTP-2Br, are developed with nitrogen atoms introduced to replace the traditional sp³-hybridized carbon in the fused ring. BTP-2Br possesses an electron-deficient central core, which exhibits slightly blue-shifted absorption as well as deepened HOMO-level compared with SN6-2Br. The corresponding photovoltaic performance from V-shaped BTP-2Br based devices exhibit superior performance especially in short-circuit current (J_sc), despite an enhanced absorption and charge carrier mobilities for SN6-2Br. The primary reason for the higher J_sc from BTP-2Br is faster exciton diffusion and dissociation in blends, than those of SN6-2Br. As a result, PBDB-TF:BTP-2Br based devices achieve a power conversion efficiency (PCE) of 13.84% with an voltage-loss of only 0.46 V, which is one of the lowest values ever reported. Moreover, we fabricated semitransparent OSCs that exhibit an excellent PCE of 9.62% with average visible transparency of 20.1%.