Understanding the Interfacial Energy Structure and Electron Extraction Process in Inverted Organic Solar Cells with Phosphine-Doped Cathode Interlayers

Comprehensive Summary Cathode interlayers(CILs)play an essential role in achieving efficient organic solar cells(OSCs).However,the electronic structure at the electrode/CIL/active layer interfaces and the underlying mechanisms for electron collection remain unclear,which becomes a major obstacle to develop high-performance CILs.Herein,we investigate the relationship of the electron collection abilities of four cross-linked and n-doped CILs(c-NDI:P0,c-NDI:P1,c-NDI:P2,c-NDI:P3)with their electronic structure of space charge region at heterojunction interface.By accurately calculating the depletion region width according to the barrier height,doping density and permittivity,we put forward that the optimal thickness of CIL should be consistent with the depletion region width to realize the minimum energy loss.As a result,the depletion region width is largely reduced from 13 nm to 0.8 nm at the indium tin oxide(ITO)/c-NDI:P0 interface,resulting in a decent PCE of 17.7%for the corresponding inverted OSCs.

Naphthalene diimide-based cathode interlayer material enables 20.2%efficiency in organic photovoltaic cells

Cathode interlayer(CIL)materials play an important role in improving the power conversion efficiency(PCE)of organic photovoltaic(OPV)cells.However,the current understanding of the structure-property relationship in CIL materials is limited,and systematic studies in this regard are scarce.Here,two new CIL materials,NDI-PhC4 and NDI-Ph C6 were synthesized by varying the alkylamine chain length on the NDI-Ph core.Our investigation reveals a systematic variation in the physical and chemical properties of these materials with increasing alkylamine chain length.Specifically,we observe a sequential decrease in melting point and self-doping effect,accompanied by an enhancement in crystallinity.Among these CIL materials,NDI-PhC4 has a notable balance across various performance metrics.It also exhibits excellent surface modification capabilities,leading to a low surface roughness.Consequently,OPV cells based on NDI-PhC4 achieve a PCE of 20.2%,which is one of the highest reported efficiencies for OPV cells.In addition,the appropriate melting point of NDI-PhC4 contributes to the excellent stability of OPV cells.

Edge-functionalized graphene quantum dots as a thickness-insensitive cathode interlayer for polymer solar cells

A thickness-insensitive cathode interlayer （CIL） is necessary for large-area polymer solar cells （PSCs）, in which thickness variation is unavoidable. These C1L materials are typically based on n-type conjugated polymer/molecule backbones, which show strong light absorption in the visible/near-infrared （NIR） region. This interferes with the sunlight absorption by the active layer and deteriorates device efficiency. In this study, we developed graphene quantum dots functionalized with ammonium iodide （GQD-NI） at the edge as a thickness-insensitive CIL with high optical transparency. The peripheral ammonium iodide groups of GQD-NI formed the desired interfacial dipole with the cathode to decrease the work function. The graphene basal planes of GQD-NI with a lateral size of ca. 3 nm demonstrated a good conductivity of 3.56 ×10-6 S.cm-1 and high transparency in the visible/NIR region （λmax abs = 228 nm）. Moreover, GQD-NI was readily soluble in polar organic solvents, e.g., methanol, which enabled multilayer device fabrication with orthogonal solvent processing. As a result, the PSC device with GQD-NI as the CIL exhibited a power conversion efficiency （PCE） of 7.49%, which was much higher than that of the device without the CIL （PCE = 5.38%） or with calcium as the CIL （PCE = 6.72%）. Moreover, the PSC device performance of GQD-NI was insensitive to the GQD-NI layer thickness in the range of 2-22 nm. These results indicate that GQD-NI is a very promising material for application as a CIL in large-area printed PSCs.

Printable SnO2 cathode interlayer with up to 500 nm thickness-tolerance for high-performance and large-area organic solar cells

The printable electrode interlayer with excellent thickness tolerance is crucial for mass production of organic solar cells(OSCs)by solution-based print techniques. Herein, high-quality printable SnO2 films are simply fabricated by spin-coating or bladecoating the chemical precipitated SnO2 colloid precursor with post thermal annealing treatment. The SnO2 films possess outstanding optical and electrical properties, especially extreme thickness-insensitivity. The interfacial electron trap density of SnO2 cathode interlayers(CILs) are very low and show negligible increase as the thicknesses increase from 10 to 160 nm,resulting in slight change of the power conversion efficiencies(PCEs) of the PM6:Y6 based OSCs from 16.10% to 13.07%. For blade-coated SnO2 CIL, the PCE remains high up to 12.08% even the thickness of SnO2 CIL is high up to 530 nm. More strikingly, the large-area OSCs of 100 mm2 with printed SnO2 CILs obtain a high efficiency of 12.74%. To the best of our knowledge, this work presents the first example for the high-performance and large-area OSCs with the thickness-insensitive SnO2 CIL.

Perylene-diimide-based cathode interlayer materials for high performance organic solar cells

Organic solar cells(OSCs),benefiting from their significant advantages,such as light weight,flexibility,low cost,and large area manufacturing adaptability,are considered promising clean energy technologies.Currently,the power conversion efficiency(PCE)of state-of-the-art OSCs has reached over 18%through materials and device engineering.Specifically,cathode engineering with cathode interlayer materials(CIMs)is an important strategy to improve the PCEs and stability of OSCs.Among various CIMs reported in the literature,perylene diimides(PDIs)aremore appropriate for working as cathode interlayers in OSCs owing to their distinct advantages of suitable energy levels,high electron affinity,high electron mobility,and facile modification.In this review,the mechanism of cathode engineering is concisely summarized,and recent research progress on PDI derivatives working as CIMs in OSCs is systematically reviewed.Finally,prospects and suggestions are provided for the development of PDI-based CIMs for practical applications.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

An alcohol-soluble,environmentally friendly,and low-cost surfactant-encapsulated polyoxometalate complex[(C8H17)4N]4[SiW_(12)O40](TOASiW_(12))as a cathode interlayer(CIL)has exhibited excellent universality for various active layers and cathodes in nonfullerene polymer solar cells(NF-PSCs).In particular,incorporating TOASiW_(12) as the CIL enhanced power conversion efficiencies(PCEs)of the PM6:Y6-based NF-PSCs with Al or Ag cathode to 16.14%and 15.89%,respectively,and the PCEs of PM6:BTP-BO4Cl-based NF-PSCs with Al or Ag cathode to 17.04%and 17.00%,respectively.More importantly,the performances of the devices with TOASiW_(12) were insensitive to the TOASiW_(12) thickness from 3 to 33 nm.Furthermore,the NF-PSCs with TOASiW_(12) exhibited better device stability.Combined characterization of the photocurrent density versus effective voltage,capacitance versus voltage and electron mobility demonstrated that TOASiW_(12) as the CIL effectively promoted exciton dissociation,charge-carrier extraction,built-in potential,charge-carrier density,and electron mobility in the NF-PSCs.These findings suggest that TOASiW_(12) is a promising,competitive CIL for NF-PSCs fabricated by roll-to-roll processing.

Block copolymers as efficient cathode interlayer materials for organic solar cells

Emerging needs for the large-scale industrialization of organic solar cells require high performance cathode interlayers to facilitate the charge extraction from organic semiconductors.In addition to improving the efficiency,stability and processability issues are major challenges.Herein,we design block copolymers with well controlled chemical composition and molecular weight for cathode interlayer applications.The block copolymer coated cathodes display high optical transmittance and low work function.Conductivity studies reveal that the block copolymer thin film has abundant conductive channels and excellent longitudinal electron conductivity due to the interpenetrating networks formed by the polymer blocks.Applications of the cathode interlayers in organic solar cells provide higher power conversion efficiency and better stability compared to the most widelyapplied ZnO counterparts.Furthermore,no post-treatment is needed which enables excellent processability of the block copolymer based cathode interlayer.

Alcohol/water-soluble porphyrins as cathode interlayers in high-performance polymer solar cells

Three alcohol/water-soluble porphyrins, Zn-TPyPMeI： zinc（II） meso-tetra（N-methyl-4-pyridyl） porphyrin tetra-iodide, Zn- TPyPAdBr： zinc（II） meso-tetra[1-（1-adamantylmethyl ketone）-4-pyridyl] porphyrin tetra-bromide and MnC1-TPyPAdBr： man- ganese（III） meso-tetra[1-（1-adamantylmethyl ketone）-4-pyridyl] porphyrin tetra-bromide were employed as cathode interlayers to fabricate polymer solar cells （PSCs）. The PCvaBM （[6,6]-phenyl C71 butyric acid methyl ester） and PCDTBT （poly[N-9＂- hepta-decanyl-2,7-carbazole-alt-5,5-（4＇,7＇-di-2-thienyl-2＇,3＇-benzothiadiazole）]）-blend films were used as active layers in polymer solar cells （PSCs）. The PSCs with alcohol/water-soluble porphyrins interlayer showed obviously higher power con- version efficiency （PCE） than those without interlayers. The highest PCE, 6.86%, was achieved for the device with MnCl- TPyPAdBr as an interlayer. Ultraviolet photoemission spectroscopic （UPS）, carrier mobility, atomic force microscopy （AFM） and contact angle （0） characterizations demonstrated that the porphyrin molecules can result in the formation of interfacial dipole layer between active layer and cathode. The interfacial dipole layer can obviously improve the open-circuit voltage （Voc） and charge extraction, and sequentially lead to the increase of PCE.

Interface Engineering of Inverted Perovskite Solar Cells Using a Self-doped Perylene Diimide Ionene Terpolymer as a Thickness-Independent Cathode Interlayer

Inverted perovskite solar cells(PerSCs)are a highly promising candidate in the photovoltaic field due to their low-temperature fabrication process,negligible hysteresis,and easy integration with Si-based solar cells.A cathode interlayer(CIL)is necessary in the development of inverted devices to reduce the trap density and energy barrier between the electron transport layer(ETL)and the electrode.However,most CILs are highly thickness-sensitive due to low conductivity and poor film-forming.In this study,we report on a self-doping perylene imide-based ionene polymer(PNPDIN)used as CIL material to modify electrode in inverted PerSCs.PNPDIN exhibits high conductivity and a good solubility in polar solvent,which results in an improved power conversion efficiency(PCE)from 10.05%(device without a CIL)to 16.97%.When the blend of PNPDIN and Bphen was used as a mixed CIL,the PCE of PerSCs can be further increased to 21.28%owing to the excellent morphology and matched energy level.More importantly,the PCE of the device is highly tolerant to the thickness of the mixed CIL,which benefited from the high conductivity of PNPDIN.This development is expected to provide an excellent mixed CIL material for roll-to-roll processing efficient and stable inverted PerSCs.

Stable Radical TEMPO Terminated Perylene Bisimide(PBI) Based Small Molecule as Cathode Interlayer for Efficient Organic Solar Cells

By combining stable radical tetramethylpiperidine nitrogen oxide(TEMPO)as end groups and perylene bisimide(PBI)as the core,a small molecular cathode interlayer(CIL)(PBI-TEMPO)was synthesized.Detailed physical-chemical characterizations indicate that PBI-TEMPO can form smooth film,owns low unoccupied molecular orbital(LUMO)level of−3.67 eV and can reduce the work function of silver electrode.When using PBI-TEMPO as CIL in non-fullerene organic solar cells(OSCs),the PM6:BTP-4Cl based OSCs delivered high power conversion efficiencies(PCEs)up to 17.37%,higher than those using commercial PDINO CIL with PCEs of 16.95%.Further device characterizations indicate that PBI-TEMPO can facilitate more efficient exciton dissociation and reduce charge recombination,resulting in enhanced current density and fill factor.Moreover,PBI-TEMPO displays higher thermal stability than PDINO in solution.When PBI-TEMPO and PDINO solution were heated at 150℃ for 2 h and then were used as CIL in solar cells,PBI-TEMPO-based OSCs provided a PCE of 15%,while PDINO-based OSCs only showed a PCE of 10%.These results demonstrate that incorporating TEMPO into conjugated materials is a useful strategy to create new organic semiconductors for application in OSCs.

A lightweight nitrogen/oxygen dual-doping carbon nanofiber interlayer with meso-/micropores for high-performance lithium-sulfur batteries

Lithium-sulfur(Li-S) batteries are promising energy-storage devices for future generations of portable electronics and electric vehicles because of the outstanding energy density,low cost,and nontoxic nature of S.In the past decades,various novel electrodes and electrolytes have been studied to improve the performance of Li-S batteries.However,the very limited lifespan and rate performance of Li-S batteries originating from the dissolution and diffusion of long-chain polysulfides in liquid electrolytes,and the intrinsic poor conductivity of S severely hinder their practical application.Herein,an electrospinning method was developed to fabricate a thin conductive interlayer consisting of meso-/microporous N/O dual-doping carbon nanofiber(CNF).The freestanding 3 D interwoven structure with conductive pathways for electrons and ions can enhance the contact between polysulfides and N/O atoms to realize the highly robust trapping of polysulfides via the extremely polar interaction.Consequently,combining the meso-microporous N/O dual-doping CNF interlayer with a monodispersed S nanoparticle cathode results in a superior electrochemical performance of 862.5 mAh/g after 200 cycles at 0.2 C and a cycle decay as low as 0.08% per cycle.An area specific capacity of 5.22 mAh/cm^(2) can be obtained after 100 cycles at 0.1 C with a high S loading of 7.5 mg/cm^(2).

Enhanced High-Temperature Cycling Stability of Garnet-Based All Solid-State Lithium Battery Using a Multi-Functional Catholyte Buffer Layer

The pursuit of safer and high-performance lithium-ion batteries(LIBs)has triggered extensive research activities on solid-state batteries,while challenges related to the unstable electrode-electrolyte interface hinder their practical implementation.Polymer has been used extensively to improve the cathode-electrolyte interface in garnet-based all-solid-state LIBs(ASSLBs),while it introduces new concerns about thermal stability.In this study,we propose the incorporation of a multi-functional flame-retardant triphenyl phos-phate additive into poly(ethylene oxide),acting as a thin buffer layer between LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2)(NCM811)cathode and garnet electro-lyte.Through electrochemical stability tests,cycling performance evaluations,interfacial thermal stability analysis and flammability tests,improved thermal stability(capacity retention of 98.5%after 100 cycles at 60℃,and 89.6%after 50 cycles at 80℃)and safety characteristics(safe and stable cycling up to 100℃)are demonstrated.Based on various materials characterizations,the mechanism for the improved thermal stability of the interface is proposed.The results highlight the potential of multi-functional flame-retardant additives to address the challenges associated with the electrode-electrolyte interface in ASSLBs at high temperature.Efficient thermal modification in ASSLBs operating at elevated temperatures is also essential for enabling large-scale energy storage with safety being the primary concern.

Highly Stable Organic Solar Cells Based on an Ultraviolet-Resistant Cathode Interfacial Layer

Although the photovoltaic efficiency of organic solar cells(OSCs)has exceeded 17%,poor lifetime excludes OSCs from practical use.In particular,UV rays in sunlight may cause the decomposition of organic photovoltaic materials,which has been proved to be the main reason for the efficiency decay.At present,there is still no effective approach to substantially improve the device stability.Herein,we fabricate a highly efficient OSC with exceptional stability under sunlight illumination by incorporating a UV-resistant cathode interlayer(CIL),namely(sulfobetaine-N,Ndimethylamino)propyl naphthalene diimide(NDI-B).NDI-B was designed and synthesized based on the naphthalene diimide(NDI)unit,thereby exhibiting excellent capability of electron collection.Moreover,NDI-B shows strong absorption in the UV region and has good UV resistance.Devices using NDI-B as a CIL exhibited a photovoltaic efficiency of 17.2%,representing the state-of-the-art photovoltaic performance of OSCs.Notably,the NDI-B-modified OSC exhibited a T80 of over 1800 h under full-sun AM 1.5 G illumination(100 mW cm^(−2)),which represents the best stability for OSCs.We demonstrate that the unique ability of the NDI-B interlayer to convert UV light to an additional photocurrent can effectively protect photovoltaic materials from UV-induced decomposition,which is the key to obtain high OSC stability under operational conditions.

Intrinsically inert hyperbranched interlayer for enhanced stability of organic solar cells

Device stability becomes one of the most crucial issues for the commercialization of organic solar cells(OSCs) after high power conversion efficiencies have been achieved. Besides the intrinsic stability of photoactive materials, the chemical/catalytic reaction between interfacial materials and photoactive materials is another critical factor that determines the stability of OSC devices. Herein, we design and synthesize a reaction-inert rylene diimide-embedded hyperbranched polymer named as PDIEIE, which effectively reduces the work function of indium tin oxide electrode from 4.62 to 3.65 eV. Meanwhile, PDIEIE shows negligible chemical reaction with high-performance photoactive materials and no catalytic effect under strong ultraviolet illumination, resulting in much better photo-stability of OSCs with PDIEIE cathode interlayer(CIL), relative to the traditional CILs, including most-widely used metal oxides and polyethyleneimine derivatives.

Perylene-diimide derived organic photovoltaic materials

In organic solar cells(OSCs), the material design on photovoltaic layers and interlayers has significantly contributed to the rapid progress of the device performance. Perylene-diimides(PDIs), owing to their distinct advantages of high electron affinity, high electron mobility and facial chemical modification, are being widely studied in OSCs, especially designed as photovoltaic acceptors and cathode interlayers. In this review, recent progress on those PDI derived photovoltaic materials is systematically summarized. Due to the different working mechanism in devices, the design strategies on modification of the parent PDI units towards their application as acceptors and cathode interlayers are explained. After disclosing the fundamental structure-property relationships, we disclose some common features in the design of those tailor-made PDI-based photovoltaic materials, and we also highlight the challenges and opportunities in improving their device performance in the future.

Microwave-assisted One-pot Three-component Polymerization of Alkynes, Aldehydes and Amines toward Amino-functionalized Optoelectronic Polymers

We present a microwave-assisted one-pot polymerization with three-components of alkynes, aldehydes and amines for the synthesis of new amino-functionalized optoelectronic polymers. The polymerization of diynes （la-lc）, dialdehydes （2a and 2b） and dibenzylamine catalyzed by InC13 was carried out smoothly within 1 h under microwave radiation, yielding four soluble polymers with high molecular weights. The resulting polymers P1 and P2 could be easily dissolved in alcohol and thus utilized as the cathode interlayer for polymer solar cells （PSCs）. Compared with the control device, the PSCs with P1 and P2 as the cathode interlayer and PTB7-Th：PC71BM as the photoactive layer exhibited significantly higher power conversion efficiencies （PCEs） of 9.49% and 9.16%, respectively. These results suggest that this polycoupling reaction is an efficient approach to construct three-component polymers for the practical applications.