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None-fullerene solar cells with ternary architecture have attracted much attention because it is an effective approach for boosting the device power conversion efficiency. Here, the crystalline polymer N2200 as the third component is integrated into J71: ITIC bulk heterojunction. A series of characterizations indicate that N2200 could increase photo-harvesting, balanced hole and electron mobilities, enhanced exciton dissociation, and suppressed charge recombination, which result in the comprehensive improvement of open circuit voltage, short circuit current and fill factor in the device. Moreover, after introduction of N2200, the morphology of the ternary active layer is optimized, and the film crystallinity is improved. This work demonstrates that adding a small quantity of high crystallization acceptor into non-fullerene donor: acceptor mixture is a promising strategy toward developing high-performance organic solar cells. Funding Agencies|National Science Foundation of China [11604099, 21875067, 51873138, 51811530011]; Shanghai Rising -Star [19QA1403100]; Shanghai Science and Technology Innovation Action Plan [17JC1402500]; National Key Project for Basic Research of China [2017YFA0303403]; Swedish Research Council [2016-05498]; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; STINT grant [CH2017-7163]

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is extensively used as an anode interfacial layer in optoelectronic devices due to its excellent hole-transporting properties and high work function (ϕF). In this study, we propose a simple and cost-effective method to controllably tune the ϕF of PEDOT:PSS from 5.1 eV to 3.95 eV by doping the polyethylenimine ethoxylated (PEIE). The PEIE-doped PEDOT:PSS films can function as a cathode interfacial layer in organic solar cells, showing the comparable performance to a conventional ZnO-based cell. The photoelectric characteristics, surface morphology, and electron transport ability of PEIE-doped PEDOT:PSS films were systematically investigated to elucidate the physical nature of the conversion from hole transport to electron transport. This strategy provides an effective charge transport layer for optoelectronic devices, which is suitable for large-scale, industrial applications.

While metal halide hybrid perovskite solar cells (PeSCs) are remarkably efficient, the rapid and undesirable degradation limit their practical applications for renewable energy sources. Herein, the evolution of perovskite decomposition under white light illumination has been investigated under the ultrahigh vacuum conditions, which can effectively rule out the influence of other factors such as oxygen or moisture. The chemical composition, electronic structure, surface morphology, and crystal structure of CH3NH3PbI3 perovskite films have been systematically characterized by X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), scanning electron microscopy, and X-ray diffraction techniques. The surface roughening with cracks and defects occurs in CH3NH3PbI3 perovskite films after a short-time white-light illumination in ultrahigh vacuum. In situ compositional analysis reveals the decomposition of the perovskite films with the appearance of metallic Pb feature, which is accompanied with the energy level shift towards higher binding energy during the illumination process. However, the decomposition rate is decreased and becomes saturated after exposed to light illumination for 24 h. These findings provide a direct evidence that CH3NH3PbI3 perovskite is photosensitive, and a light-induced degradation mechanism is proposed.

An electron-rich moiety, pyrrolo[3,2-b]pyrrole, has been used as a new donor block in organic lighting-emitting diodes (OLEDs). Combined with a strong electron-withdrawing group of 2,1,3-benzothiadiazole, a novel red fluorophore BTZPP was synthesized. The solution-processed red-emitting device based on BTZPP showed encouraging performances with external quantum efficiency of up to 3.4%. This evidently indicates that the pyrrolo[3,2-b]pyrrole group can be a promising electron-donating moiety for enriching the material types and promoting the development of OLEDs.

Despite the rapid development of high-efficiency perovskite light-emitting diodes (PeLEDs) via material design and morphology control, a comprehensive evaluation of the power loss among various optical modes has seldom been conducted for the precise design of device architectures. Here, a quantitative analysis of light outcoupling efficiency (ηout) of PeLEDs is presented with the theoretical simulations by taking into account the key factors of refractive index and film thickness of various functional layers, and the emitter dipole orientation. For a planar PeLED, a rational design to enhance the ηout is predicted by constructing multilayer stack in series of transparent electrode/high-index transport layer/perovskite emitter/low-index transport layer/reflective electrode. Furthermore, the role of photonic structures on the outcoupling management is explored, and the potential of a substantial increase in ηout is demonstrated with an enhancement factor of over 1.75 by implementing a moth-eye nanostructure into the emitter. We anticipate that the findings shown here will provide the design guidelines for the optical optimization of PeLEDs.

Semi-transparent organic photovoltaic cells (STOPVs) hold the great potential as a solar conversion source installed on the glass curtain walls. The cell performance of STOPVs is to some extent balanced among the absorption, transmission and reflection of the incident solar radiation. For the practical application with a defined transparency, the efficiency of STOPVs can be optimized by maximizing the light absorption as well as minimizing the reflection that contribute to the light pollution. Herein, a low-reflectance STOPV is presented with relatively high efficiency, high transmittance, and suitable color rendering property. A 15.9% increase in photocurrent and a 28% decrease in reflection are realized with the implementation of biomimetic moth-eye nanostructures into the dielectric-metal-dielectric transparent electrodes. For an optimized PTB7:PC71BM-based STOPV on flexible plastic substrate, the average transmittance in the visible region is kept at 46.1% and the average reflectance is reduced to 10.3%, leading a power conversion efficiency of 3.4%. The pathway described here is promising for opening up opportunities of reliable STOPVs towards the future commercial applications.

Interfacial compatibility between the electrode and organic semiconductor plays a critical role in controlling the charge transport and hence efficiency of organic solar cell. Here, we introduce a novel small-molecule zwitterionic electrolyte (S1) combined with ZnO as electron transporting interlayer employed for the inverted PTB7:PC71BM bulk heterojunction solar cell. The resulting device with the S1/ZnO stacked interlayer achieves a high PCE of 8.59%, obtaining a 16.2% improvement over the control device performance of 7.4% without the S1 attributed to the significant increased short-circuit current density and fill factor. The interfacial properties are investigated. It is found that the S1/ZnO interlayer possess an ultralow work function of 3.6 eV, which originates from the interfacial double dipole step induced by the zwitterionic side chain electrostatic realignment at interface. The S1/ZnO interlayer exhibits the excellent charge extraction ability, suppresses the charge recombination loss and decreases the series resistance at the active layer/electrode contact.

Three D-σ-A type thermally activated delayed fluorescent materials (TADF), AQ-S-Cz, AQ-S-DPA and AQ-S-DMAC, with sulfur atom connect donors and acceptor, were designed and synthesized. All of the emitters exhibit small ΔEST (

An intermediate connector of Cs 2 CO 3 /Al/MoO 3 used in tandem organic light-emitting diodes (OLEDs) was investigated in this work. Here, an ultrathin Cs 2 CO 3 /Al was used as an electron-injection layer (EIL) from MoO 3 to the adjacent electron transporting layer (ETL). To verify the function of this intermediate connector, the device performances were evaluated through current density-voltage-luminance characteristics, current density-efficiency curves, and EL spectra. Additionally, the effect of photon energy on carriers behavior in the Cs 2 CO 3 /Al/MoO 3 connector is also estimated. The electrical properties and EL spectra of tandem OLEDs show that the Cs 2 CO 3 /Al/MoO 3 can function well for charge generation and transport, and the current density-voltage curves of Cs 2 CO 3 /Al/MoO 3 based special multilayer device shows the photovoltaic effect as a photovoltaic cell.

We report computational and experimental studies (i) on the Fermi level (EF) pinning phenomena in weakly interacting electrode-organic semiconductor systems with pentacene, N, N-Di(naphthalene-l-yl)-N, N’-diphenyl-benzidine (α-NPD) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)], and (ii) to elucidate a common reason of the pinning phenomena which have been widely observed for various organic materials on inert electrode surfaces. For (i), the computed results on the electrode-dependence of EF-HOMO and EF-LUMO distance agreed excellently with UPS and Kelvin-probe results. For (ii) we found theoretically that the pinning phenomena occur at systems even without any electronic states in the HOMO-LUMO gap, and thereby indicate that this is a universal Fermi-level pinning-like phenomena for various band gap materials, even if specific interface states do not exist upon contact. We further obtained for Gaussian-distributed HOMO and LUMO that the minimal hole and electron injection barriers are quantitatively determined by degree of standard deviations of HOMO and LUMO bands, which are in excellent agreement with the experimental results. Furthermore, our results provide a guideline for approaching the “zero” injection barriers.

The role of transition metal oxides (TMOs) based intermediate connectors in tandem organic light emitting diodes (OLEDs) has been studied via capacitance-voltage and current-voltage characteristics, in order to elucidate the dynamic processes of charges generation and transport within externally applied voltages. The TMO-based intermediate connectors are composed of molybdenum trioxide (MoO3) and cesium azide (CsN3)-doped-4, 7–diphenyl-1, 10-phenanthroline (BPhen) layers, where MoO3 and CsN3 are used due to low deposition temperatures. From the obtained results of capacitance and current density, charges generation in CsN3:BPhen/MoO3/NPB is proposed to the defect states in thermally evaporated MoO3, which offers a minimal energy offset for charges generation. Moreover, our results clearly indicate that charges generation efficiency is not only relying on the MoO3-NPB interface, but also influenced by CsN3:BPhen-MoO3 interface. CsN3 doped BPhen layer further improves charges separation efficiency, which finally results in favorable charges transport into the adjacent layers and ensure to function efficiently for tandem OLEDs.

Nanocomposite buffer layer based on metal oxide and polymer is merging as a novel buffer layer for organic solar cells, which combines the high charge carrier mobility of metal oxide and good film formation properties of polymer. In this work, a nanocomposite of zinc oxide and a commercialized available polyethylenimine (PEI) was developed and used as the cathode buffer layer (CBL) for the inverted organic solar cells and p-i-n heterojunction perovskite solar cells. The cooperation of PEI in nano ZnO offers a good film forming ability of the composite material, which is an advantage in device fabrication. In addition, power conversion efficiency (PCE) of the ZnO:PEI CBL based device was also improved when compared to that of ZnO-only and PEI-only devices. The highest PCE of P3HT:PC 61 BM and PTB7-Th:PC 61 BM devices reached to 3.57% and 8.16%, respectively. More importantly, there is no obvious device performance loss with the increase of the layer thickness of ZnO:PEI CBL to 60 nm in organic solar cells, which is in contrast to the PEI based devices, whose device performance decreases dramatically when the PEI layer thickness is higher than 6 nm. Such a nano composite material is also applicable in inverted heterojunction perovskite solar cells. A PCE of 11.76% was achieved for the perovskite solar cell with a thick ZnO:PEI CBL (150 nm) CBL, which is around 1.71% higher than that of the reference cell without CBL, or with ZnO CBL. In addition, stability of the organic and perovskite solar cells having ZnO:PEI CBL was also found to be improved in comparison with that of PEI based device.

Perovskites light emitting diodes (PeLEDs) are promising candidates for next-generation display devices because of their attractive properties of low-cost, solution processability and excellent color purity. For metal halide perovskite materials, the emission tuning can be easily achieved by altering halide composition. However, blue PeLEDs with chlorine incorporation usually suffer from the severe phase segregation and ion migration, resulting in the red-shifted electroluminescence and the efficiency degradation. Here, the partial substitution of Pb2+ ions with non-toxic Sr2+ is adopted to enhance the formation energy of Sr-alloyed perovskite films, leading to the negligible electroluminescence in electrically driven blue PeLEDs. The maximum brightness and efficiency are optimized simultaneously. Further, such a substitution can greatly enhance the thermal and operational stabilities of PeLEDs.

A photochromic diarylethene-based compound BMTA, which undergoes a reversible conversion between ring- open and closed isomers by alternating UV and visible light illumination, has been designed and synthesized. By utilizing a mask-free Digital Micro-mirror Device (DMD) micro-lithography system, arbitrary micro-photopatterning in polymer films doped with BMTA can be easily obtained with UV light writing. This recorded photo information can easily be erased by further visible light irradiation. The reversible and rewritable optical storage is based on photo-switched intermolecular energy transfer between the emissive host and the ring-closed isomer c -BMTA. Furthermore, the solution-processable organic light-emitting devices (OLEDs) with the single emitting layer doped with BMTA were fabricated, which exhibit rewritable memory behavior with light control. The luminescence and current density decrease significantly upon UV light irradiation, and recover by further visible light illumination. This is because the hole trapping is much facilitated in closed-ring isomer based devices, due to elevated HOMO level of c -BMTA. Without incorporating any cross-linking layer, the maximum luminescence and current density on/off ratios of this solution-processable and simple-structured device are 1.9 × 10 3 and 1.4 × 10 2 , respectively. Arbitrary micro-photolithography of OLEDs by DMD system has also been demonstrated, which shows great prospects in large-scale production of high resolution OLED displays.

We report the effect of gap states on energy level alignment in a typical organic charge generation interface of N,N-bis(1-naphthyl)-N,N-diphenyl-1,1-biphenyl-4,4-diamine (α-NPD)/hexaazatriphenylene−hexacarbonitrile [HAT(CN) 6 ] by using ultraviolet and X-ray photoemission spectroscopy. The gap states tailed from the highest occupied molecular orbital (HOMO) onset of α-NPD dominate the Fermi level pinning at the α-NPD( 6 interface, which is favorable for charges generation upon bias operation and facilitates the electron injection from the HOMO-tail region of α-NPD to the lowest unoccupied molecular orbital (LUMO) region of HAT(CN) 6 .

Morphology optimization of donor–acceptor bulk heterojunctions at microscopic scale is critical for improving performance of organic photovoltaic devices. Here, effects of thermal annealing on phase separation processes in small-molecular bulk heterojunctions with different geometrical structures (i.e., PTCDA, TiOPc, CuPc and C60) are investigated with ultraviolet and X-ray photoemission spectroscopies. It was identified that post-annealing treatment caused the different degrees of vertical diffusions at the bulk heterojunctions, leading to non-uniform composition distributions. Variations in phase separations are mainly due to the differences in surface energy of the involved materials, which play a crucial role in the intermolecular interactions and the molecular diffusion. Low-surface-energy materials were found to segregate preferentially on the surface for minimizing total energy of the systems.

C60-based organic thin film transistors (OTFTs) with high electron mobility and high oper- ational stability are achieved with (1 1 1) oriented C60 films grown by using template effects of diindenoperylene (DIP) under layer on the SiO2 gate insulator. The electron mobility of the C60 transistor is significantly increased from 0.21 cm 2

The doping effect of cesium compounds (i.e., Cs 2 CO 3 , CsN 3 and CsF) doped electron injection layer (EIL) on charge transport properties and operational stability of organic light-emitting diodes (OLEDs) was systematically investigated in this work. It has been found that device characteristics and lifetime are highly dependent on the doping constituent materials. The doping of cesium compounds in EIL can improve the charge injection and transport of OLEDs, due to the increase in conductivity and reduction in electron injection barrier. Apart from the difference in electrical characteristics, the operational stability of OLEDs is strongly influenced by the doping mechanism of different cesium compounds in the EILs. The OLED device using Cs 2 CO 3 as the n-type dopant for the EIL shows a superiority in both electrical property and operational lifetime.

We report on the solution-processed mixture of Au nanoparticles (NPs) and MoOX as an inter-layer in organic light-emitting devices (OLEDs), leading to the enhanced light emission and good stability. An impressive enhancement in current efficiency and power efficiency is achieved up to 70% and 100%, respectively. A systematic study to the effect of the Au NPs:MoOX inter-layer on OLEDs demonstrates that the improved electrical properties is mainly ascribed to the enhanced hole injection due to the high work function of MoOX and the good conductivity of Au NPs, and the enhanced optical properties is mainly attributed to the localized surface plasmon induced by Au NPs, which makes a great contribution to the improved efficiency. Besides, Au NPs:MoOX inter-layer also behaves superior to the frequently-used polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in device stability. The decay ratio for Au NPs:MoOX based device is 60% after 80 h, while it is nearly dying out for the device with PEDOT:PSS inter-layer.

High performance inverted polymer solar cell is demonstrated by introducing a nanostructured backscattering rear electrode, which is fabricated by embedding silver nanoparticle (NP) arrays into the MoO3 hole extraction layer. As verified by characterizing and simulating the electrical and optical properties, such a nanostructured rear electrode can achieve an improved cell performance by maintaining simultaneously high open-circuit voltage and fill factor values, while providing excellent short-circuit current enhancement through efficient backscattering-induced light trapping. A careful optimization of the nanostructured rear electrode can result in polymer solar cells with an enhanced power conversion efficiency of 7.21%, as compared to 6.26% of the reference cell with a flat electrode. It is noteworthy that the method described here offers a convenient and scalable way for inexpensive and high-performance polymer solar cell designs.

An efficient inverted polymer solar cell (PSC) is reported by integrating a small molecular electron collection layer (ECL) between indium tin oxide (ITO) cathode and the photoactive layer of blended poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM). The ECL is composed of a cesium carbonate-doped tris(8-hydroxyquinolinato) aluminum (Cs2CO3:Alq3) layer. As determined by photoelectron spectroscopy and electrical measurements, the Cs2CO3 doping induces suitable energy level alignment at the ITO/Cs2CO3:Alq3/PCBM interface and the increase in bulk conductivity of organic ECL, which are favorable to electron extraction through Cs2CO3:Alq3 to ITO cathode. In addition, optical simulation indicates that the Cs2CO3:Alq3 layer can act as an optical spacer to modulate the region of highest incident light intensity within the photoactive layer, where absorption and charge dissociation are efficient. The inverted PSC with an optimized Cs2CO3:Alq3 ECL exhibits a power conversion efficiency of 4.83%. The method reported here provides a facile approach to achieve high-performance inverted PSCs at low processing temperature.

We report on the role of cesium fluoride (CsF) doping on the enhanced electron transport properties of tris-(8-hydroxyquinolin) aluminum (Alq 3 ) for organic light-emitting diodes. The electronic structures of CsF-doped Alq 3 layers with various doping concentration are characterized by in situ ultraviolet and X-ray photoelectron spectroscopies, showing an n-type electrical doping effect with Fermi level shift towards unoccupied molecular orbital and the formation of chemistry-induced gap-states. The increase in conductivity and reduction in electron injection barrier in CsF-doped Alq 3 layer with optimal doping concentration lead to the enhanced electron injection and transport, which are consistent with the improved electrical characteristics of OLEDs.

We report on a high-quality hybrid intermediate connector (IC) used in tandem organic light-emitting diodes (OLEDs), which is composed of an ultrathin MoO 3 interlayer sandwiched between a n-type Cs 2 CO 3 -doped 4,7-diphenyl-1,10-phenanthroline (BPhen) layer and a p-type MoO 3 -doped N , N ′-diphenyl- N , N′ -bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) layer. The charge generation characteristics for light emission in tandem OLEDs have been identified by studying the interfaces and the corresponding devices. The hybrid IC structure exhibits superior charge generation capability, and its interfacial electronic structures are beneficial to the generation and injection of electrons and holes into bottom and top emission units, respectively. Compared to the organic-TMO bilayer and doped p–n junction structures, the hybrid IC structure combining MoO 3 -based interlayer and p-type doping can effectively decrease the driving voltage and improve the current efficiency of tandem devices due to the increased bulk heterojunction-like charge generation interfaces. Our results indicate that the TMO-based hybrid IC structure can be a good structure in the fabrication of high-efficiency tandem OLEDs.

An efficient inverted polymer solar cell is enabled by incorporating an n-type doped wide-gap organic electron transporting layer (ETL) between the indium tin oxide cathode and the photoactive layer for electron extraction. The ETL is formed by a thermal-deposited cesium carbonate-doped 4,7-diphenyl-1,10-phenanthroline (Cs 2 CO 3 :BPhen) layer. The cell response parameters critically depended on the doping concentration and film thickness of the Cs 2 CO 3 :BPhen ETL. Inverted polymer solar cell with an optimized Cs 2 CO 3 :BPhen ETL exhibits a power conversion efficiency of 4.12% as compared to 1.34% for the device with a pristine BPhen ETL. The enhanced performance in the inverted device is associated with the favorable energy level alignment between Cs 2 CO 3 :BPhen and the electron-acceptor material, as well as increased conductivity in the doped organic ETL for electron extraction. The method reported here provides a facile approach to optimize the performance of inverted polymer solar cells in terms of easy control of film morphology, chemical composition, conductivity at low processing temperature, as well as compatibility with fabrication on flexible substrates.

An efficient inverted polymer solar cell (PSC) is reported by employing an atomic layer deposited (ALD) cadmium sulfide (CdS) film between the indium tin oxide (ITO) cathode and the photoactive layer as the electron collection layer (ECL), on which a active layer is composed of a blended poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction. As determined by photoelectron spectroscopy, the sulfur vacancy induces an n-type semiconducting property in the ALD-grown CdS films, and suitable energy level alignment at the ITO/CdS/PCBM interface is favorable to electron extraction through CdS to the ITO electrode. With the optimized CdS film thickness, the power conversion efficiency increases to 3.33%, with short-circuit current of 8.94 mA/cm2, open-circuit voltage of 0.61 V, and fill factor of 61.1% under AM1.5G 100 mW/cm2 irradiation.

Interfacial electronic structures of WO3-based intermediate connectors used in tandem organic light-emitting diodes were studied with photoemission spectroscopy. Important factors controlling their functional effectiveness for charge carrier generation and injection were analyzed. By systematically characterizing the electronic properties and chemical structures for various intermediate connectors, it is identified that the WO3 interlayer is capable of pronouncedly modifying the energy level alignment of the intermediate connectors and thereby the adjacent emission units. Substantial interface dipole and energy level bending caused by the oxide connector are shown to be beneficial to carrier injection into the suitable molecular energy levels of adjoining emission units.