Fine-tuning HOMO energy levels between PM6 and PBDB-T polymer donors via ternary copolymerization

To achieve high-efficiency polymer solar cells(PSCs), it is not only important to develop high-performance small molecule acceptors(SMAs) but also to find a matching polymer donor to achieve optimal morphology and matching electronic properties.Currently, state-of-the-art SMAs mostly rely on a donor polymer named PM6. However, as the family of SMAs continues to expend, PM6 may not be the perfect polymer donor due to the requirement of energy level matching. In this work, we tune the energy level of PM6 via the strategy of ternary copolymerization. We achieve two donor polymers(named PL-1 and PL-2) with upshifted HOMO(the highest occupied molecular orbital) energy level(compared with PM6), and can thus match with the SMAs with upshifted HOMO energy levels compared with Y6. These two copolymers exhibit slightly higher order of molecular packing and similar charge transport properties, which demonstrate that the method of ternary copolymerization can fine tune the HOMO level of donor polymers, while the morphology and mobility of the blend film remain mostly unaffected. Among them,the best device based on PL-1:Y6 exhibits power conversion efficiencies(PCEs) of 16.37% with lower open circuit voltage(Voc)but higher short circuit current voltage(Jsc) and fill factor(FF) than that of the device based on PM6:Y6. This work provides an effective approach to find polymer matches for the SMAs with upshifted HOMO levels.     

Ternary organic solar cells based on polymer donor,polymer acceptor and PCBM components

In this work,ternary organic solar cells(OSCs)combining a fullerene derivative PC71BM with a nonfullerene acceptor N2200-F blended with a polymer donor PM6 were reported.Compared with the binary systems,the highest power conversion efficiency(PCE)of 8.11%was achieved in ternary solar cells with 30 wt%N2200-F content,mainly due to the improved short-circuit current density(Jsc)and fill factor(FF).Further studies showed that the improved Jsc could attribute to the complementary abso rption of the two acceptors and the enhanced FF was originated from the higher hole mobility and the fine-tuned morphology in the ternary system.These results demonstrate that the combination of fullere ne and nonfullerene acceptors in ternary organic solar cells is a promising approach to achieve high-performance OSCs.     

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.     

Two star-shaped small molecule donors based on benzodithiophene unit for organic solar cells

Star-shaped small molecules have attracted great attention for organic solar cells(OSCs) because they have three-dimensional charge-transport characteristics, strong light absorption capacities and easily tunable energy levels. Herein, three-and four-armed star-shaped small molecule donors, namely BDT-3 Th and BDT-4 Th, respectively, have been successfully designed and synthesized, which used benzodithiophene(BDT) as the central unit. The two star-shaped intermediates(2 a and 2 b) could be simul...     

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.     

Improving open-circuit voltage by a chlorinated polymer donor endows binary organic solar cells efficiencies over 17%

Power conversion efficiency(PCE) of single-junction polymer solar cells(PSCs) has made a remarkable breakthrough recently.Plenty of work was reported to achieve PCEs higher than 16% derived from the PM6:Y6 binary system.To further increase the PCEs of binary OSCs incorporating small molecular acceptor(SMA) Y6,we substituted PM6 with PM7 due to the deeper highest occupied molecular orbital(HOMO) of PM7.Consequently,the PM7:Y6 has achieved PCEs as high as 17.0% by the hotcast method,due to the improved open-circuit voltage(V_(OC)).Compared with PM6,the lower HOMO of PM7 increases the gap between E_(LUMO-donor) and E_(HOMO-acceptor),which is proportional to V_(OC).This research provides a high PCE for single-junction binary PSCs,which is meaningful for device fabrication related to PM7 and commercialization of PSCs.     

High-performance all-polymer solar cells with only 0.47 eV energy loss

The field of all-polymer solar cells(all-PSCs) has experienced rapid development during the past few years, mainly driven by the development of efficient polymer acceptors. However, the power conversion efficiencies(PCEs) of the all-PSCs are still limited by insufficient light absorption of the donor/acceptor blend and large energy loss in devices. We herein designed a polymer acceptor PYT1 constructed n-type molecular acceptor Y5-C20 as the key building block and blended it with a polymer donor PM6 to obtain an all-polymer photoactive layer. The optimized PM6:PYT1 all-PSCs achieved a record higher PCE of 13.43% with a very low energy loss of 0.47 eV and a photoresponse of up to 900 nm compared with the Y5-C20 based device with a best PCE of 9.42%. Furthermore, the PCEs of the PM6:PYT1 all-PSCs are relatively insensitive to the 1-chloronaphthalene(CN)additive contents and active layer thickness. Our results also highlight the effect of CN additive on PM6:PYT1 morphology, i.e.,charge generation, and transport find an optimized balance, and radiative and non-radiative loss is simultaneously reduced in the blend. This work promotes the development of high-performance polymer acceptors and heralds a brighter future of all-PSCs for commercial applications.     

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-equivalent D-A copolymerization strategy towards highly efficient polymer donor for polymer solar cells

D-A copolymerization is a broadly utilized molecular design strategy to construct high efficiency photovoltaic materials for polymer solar cells (PSCs),and all the D-A copolymer donors reported till now are the alternate D-A copolymers with equal D-and A-units.Here,we first propose a non-equivalent D-A copolymerization strategy with unequal D-and A-units,and develop three novel non-equivalent D-A copolymer donors (PM6-D1,PM6-D2 and PM6-D3 with D/A unit ratio of 1.1:0.9,1.2:0.8 and1.3:0.7,respectively) by inserting more D units into the alternate D-A copolymer PM6 backbone to finely tune the physicochemical and photovoltaic properties of the polymers.The three non-equivalent D-A copolymers show the down-shifted highest occupied molecular orbital (HOMO) energy levels,higher hole mobility,higher degree of molecular self-assembly and higher molecular crystallinity with the increase of D-unit ratio in comparison with the alternate D-A copolymer PM6.As a result,all the three non-equivalent D-A copolymer-based PSCs with Y6 as acceptor achieve improved power conversion efficiency (PCE)with higher V_(oc),larger J_(sc)and higher FF simultaneously.Particularly,the PM6-D1:Y6 based PSC achieved a high PCE of17.71%,which is significantly higher than that (15.82%) of the PM6:Y6 based PSC and is one of the highest performances in the binary PSCs.     

A chlorinated non-fullerene acceptor for efficient polymer solar cells

After additive and thermal annealing treatment, the PM6:Y15 based device obtains a high power conversion efficiency of 14.13%.     

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.     

Effects of subtle change in side chains on the photovoltaic performance of small molecular donors for solar cells

Small-molecule organic solar cells (SMOSCs) have attracted considerable attention owing to the merits of small molecules, such as easy purification, well-defined chemical structure. To achieve high-performance SMOSCs, the rational design of well-matched donor and acceptor materials is extremely essential. In this work, two new small molecular donor materials with subtle change in the conjugated side thiophene rings are synthesized. The subtle change significantly affects the photovoltaic performance of molecular donors. Compared with chlorinated molecule MDJ-Cl, the non-chlorinated analogue MDJ exhibits decreased miscibility with the non-fullerene acceptor Y6, can more efficiently quench the excitons of Y6. As a result, a improved PCE of 11.16% is obtained for MDJ:Y6 based SMOSCs. The results highlight the importance of fine-tuning the molecular structure to achieve high-performance SMOSCs.     

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.     

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...     

Synthesis of D-A copolymers based on thiadiazole and thiazolothiazole acceptor units and their applications in ternary polymer solar cells

We have designed and synthesized two wide bandgap new donor-acceptor (D-A) copolymers consisting of the same alkylthiazole-substituted benzo[1,2-b;4,5-b′]dithiophene (BDTTz) donor unit and but different acceptor units, i.e., thiazolo[5,4-d]thiazole (TT_Z) ( P122 ) and 1,3,-4 thiadiazole (TDz) ( P123 ) and investigated their optical and electrochemical properties. We have employed these copolymers as donor and fullerene (PC ₇₁ BM) and narrow bandgap non-fullerene (Y6) as acceptor, to fabricate binary and ternary bulk heterojunction polymer solar cells (PSCs). The overall power conversion efficiency (PCE) of optimized binary bulk heterojunction PSCs based on P122 :Y6 and P123 :Y6 is 12.60% and 13.16%, respectively. The higher PCE for PSCs based on P123 than P122 counterparts may be associated with the broader absorption profile of the P123 and more charge carrier mobilities than that for the P122 active layer. With the incorporation of small amount of PC₇₁BM into either P122 :Y6 or P123 :Y6 binary blend, the corresponding ternary PSCs showed an overall PCE of 14.89% and 15.52%, respectively, which is higher than the binary counterparts using either Y6 or PC₇₁BM as acceptor. Incorporating the PC₇₁BM in the binary host blend increases the absorption in the 300–500 nm wavelength region, generating more excitons in the active ternary layer and helping to dissociate the excitons into free charge carriers more effectively. The more appropriate nanoscale phase separation in the active ternary layer than the binary counterpart may be one of the reasons for higher PCE.     

A chlorinated non-fullerene acceptor for efficient polymer solar cells

Improving the performance and reducing the manufacturing costs are the main directions for the development of organic solar cells in the future. Here, the strategy that uses chemical structure modification to optimize the photoelectric properties is reported. A new narrow bandgap (1.30 eV) chlorinated non-fullerene electron acceptor (Y15), based on benzo[d][1,2,3] triazole with two 3-undecyl-thieno[2′,3′:4,5] thieno[3,2-b] pyrrole fused -7-heterocyclic ring, with absorption edge extending to the near-infrared (NIR) region, namely A-DA'D-A type structure, is designed and synthesized. Its electrochemical and optoelectronic properties are systematically investigated. Benefitting from its NIR light harvesting, the fabricated photovoltaic devices based on Y15 deliver a high power conversion efficiency (PCE) of 14.13%, when blending with a wide bandgap polymer donor PM6. Our results show that the A-DA'D-A type molecular design and application of near-infrared electron acceptors have the potential to further improve the PCE of polymer solar cells (PSCs).     

11.2% Efficiency all-polymer solar cells with high open-circuit voltage

Herein,we fabricated all-polymer solar cells(all-PSCs)based on a fluorinated wide-bandgap p-type conjugated polymer PM6 as the donor,and a narrow bandgap n-type conjugated polymer PZ1 as the acceptor.In addition to the complementary absorption and matching energy levels,the optimized blend films possess high cystallinity,predominantly face-on stacking,and a suitable phase separated morphology.With this active layer,the devices exhibited a high V_(oc)of 0.96 V,a superior J_(sc)of 17.1 mA cm~(-2),a fine fill factor(FF)of 68.2%,and thus an excellent power conversion efficiency(PCE)of 11.2%,which is the highest value reported to date for single-junction all-PSCs.Furthermore,the devices showed good storage stability.After 80 d of storage in the N_2-filled glovebox,the PCE still remained over 90%of the original value.Large-area devices(1.1 cm~2)also demonstrated an outstanding performance with a PCE of 9.2%,among the highest values for the reported large-area all-PSCs.These results indicate that the PM6:PZ1 blend is a promising candidate for scale-up production of large area high-performance all-PSCs.     

Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency

A high performance polymer solar cells(PSCs) based on polymer donor PM6 containing fluorinated thienyl benzodithiophene unit and n-type organic semiconductor acceptor IT-4 F containing fluorinated end-groups were developed. In addition to complementary absorption spectra(300–830 nm) with IT-4 F, the PM6 also has a deep HOMO(the highest occupied molecular) level(-5.50 e V), which will lower the open-circuit voltage(V_(oc)) sacrifice and reduce the E_(loss) of the IT-4 F-based PSCs. Moreover, the strong crystallinity of PM6 is beneficial to form favorable blend morphology and hence to suppress recombination. As a result, in comparison with the PSCs based on a non-fluorinated D/A pair of PBDB-T:ITIC with a medium PCE of 11.2%, the PM6:IT-4 Fbased PSCs yielded an impressive PCE of 13.5% due to the synergistic effect of fluorination on both donor and acceptor, which is among the highest values recorded in the literatures for PSCs to date. Furthermore, a PCE of 12.2% was remained with the active layer thickness of up to 285 nm and a high PCE of 11.4% was also obtained with a large device area of 1 cm~2. In addition, the devices also showed good storage, thermal and illumination stabilities with respect to the efficiency. These results indicate that fluorination is an effective strategy to improve the photovoltaic performance of materials, as well as the both fluorinated donor and acceptor pair-PM6:IT-4 F is an ideal candidate for the large scale roll-to-roll production of efficient PSCs in the future.     

Binary All-polymer Solar Cells with a Perhalogenated-Thiophene-Based Solid Additive Surpass 18 % Efficiency

Morphological control of all-polymer blends is quintessential yet challenging in fabricating high-performance organic solar cells. Recently, solid additives (SAs) have been approved to be capable in tuning the morphology of polymer: small-molecule blends improving the performance and stability of devices. Herein, three perhalogenated thiophenes, which are 3,4-dibromo-2,5-diiodothiophene (SA-T1), 2,5-dibromo-3,4-diiodothiophene (SA-T2), and 2,3-dibromo-4,5-diiodothiophene (SA-T3), were adopted as SAs to optimize the performance of all-polymer organic solar cells (APSCs). For the blend of PM6 and PY-IT, benefitting from the intermolecular interactions between perhalogenated thiophenes and polymers, the molecular packing properties could be finely regulated after introducing these SAs. In situ UV/Vis measurement revealed that these SAs could assist morphological character evolution in the all-polymer blend, leading to their optimal morphologies. Compared to the as-cast device of PM6 : PY-IT, all SA-treated binary devices displayed enhanced power conversion efficiencies of 17.4–18.3 % with obviously elevated short-circuit current densities and fill factors. To our knowledge, the PCE of 18.3 % for SA-T1-treated binary ranks the highest among all binary APSCs to date. Meanwhile, the universality of SA-T1 in other all-polymer blends is demonstrated with unanimously improved device performance. This work provide a new pathway in realizing high-performance APSCs.     

Effective design of A-D-A small molecules for high performance organic solar cells via F atom substitution and thiophene bridge

Novel A-D-A-type small molecule donors employ thiophene bridge and F-substitution to improve the power conversion efficiency in organic solar cell.