Baofu Ding

Dual Plasmonic Nanostructures for High Performance Inverted Organic Solar Cells

Polymer-fullerene-based bulk heterojunction (BHJ) solar cells have many advantages, including low-cost, low-temperature fabrication, semi-transparency, and mechanical fl exibility. [ 1 , 2 ] However, there is a mismatch between optical absorption length and charge transport scale. [ 3 , 4 ] These factors lead to recombination losses, higher series resistances, and lower fi ll factors. Attempts to optimize both the optical and electrical properties of the photoactive layer in organic solar cells (OSCs) inevitably result in a demand to develop a device architecture that can enable effi cient optical absorption in fi lms thinner than the optical absorption length. [ 5 , 6 ] Here, we report the use of two metallic nanostructures to achieve broad light absorption enhancement, increased shortcircuit current ( J sc ), and improved fi ll factor ( FF ) simultaneously based on the new small-bandgap polymer donor poly{[4,8-bis(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b ′ ]dithiophene2,6-diyl]alt -[2-(2 ′ -ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-C-T) in BHJ cells. [ 7 ] The dual metallic nanostructure consists of a metallic nanograting electrode as the back refl ector and metallic nanoparticles (NPs) embedded in the active layer. Consequently, we achieve the high power conversion effi ciency (PCE) of 8.79% for a single-junction BHJ OSC. Recently, plasmonic nanostructures have been introduced into solar cells for highly effi cient light harvesting. [ 5 , 8–17 ] Two types of plasmonic resonances, surface plasmonic resonances (SPRs) [ 18–22 ] and localized plasmonic resonances (LPRs), [ 11–14 ] can be used for enhancing light absorption. Metallic gratingbased light-trapping schemes have been investigated in traditional inorganic photovoltaic cells. [ 18–20 ] For metallic nanogratings, which can support SPRs, it is still challenging to experimentally demonstrate the enhancement of PCE in OSCs owing to the obvious issue of solution processing of

Mechanism of charge generation in p -type doped layer in the connection unit of tandem-type organic light-emitting devices

A p-type doped organic layer combined with a hole-blocking layer has been experimentally demonstrated to serve as the charge generation unit in tandem-type organic light-emitting devices. The p-type layer functions as the source of both holes and electrons. Charge separation is explained by the tunneling model that the hole-blocking layer reduces the energy barrier for the electrons generated in the p-type layer to tunnel through into one light-emitting unit, while the holes generated in the p-type layer can transport to the other light-emitting unit easily under operation voltage.

Delayed-switch-on effect in metal-insulator-metal organic memories

We report a delayed-switch-on effect in organic memories; i.e., the organic memory devices can automatically switch from off state to on state after a certain period of time when biased at voltages below the threshold voltage. Meanwhile, the lower the voltage is, the longer the switching time will be. The time scales from milliseconds to about 104s with decreasing voltage. Moreover, by applying a certain voltage between threshold voltage and Vmax, intermediate states are also obtained. The existence of filamentary microconducting channels in the organic layer is proposed to be responsible for the observed switching phenomenon.

Investigation of the behaviour of electronic resistive switching memory based on MoSe2-doped ultralong Se microwires

MoSe2-doped ultralong Se microwires of length/diameter ratio in the order of ∼240 are synthesized by hydrothermal method. An electronic resistive switching memory (ERSM) device using a single MoSe2-doped ultralong Se microwire is attained. The ERSM exhibits stable resistance ratio of ∼102 for 5000 s, highly stable performance during 500 stressing cycles, and excellent immunity to the frequency of the driving voltage. By investigating the dynamic processes of trap filling, de-trapping, and free-charge migration, trap-controlled space-charge-limited current mechanism is found to dominate the observed ERSM behaviour.

Magnetic field modulated exciton generation in organic semiconductors: An intermolecular quantum correlated effect

Magnetoelectroluminescence (MEL) of organic semiconductor has been experimentally tuned by adopting blended emitting layer consisting of hole transporting material and electron transporting material. Theory based on Hubbard model fits experimental MEL well, which reveals two findings: (1) spin scattering and spin mixing, respectively, dominate MEL in low-field and high-field region. (2) Blended ratio, and thus the mobility, determines the value of the relative change in the EL in a given magnetic field. Finally successful prediction about the increase in singlet excitons in low field with little change in triplet exciton population further confirms the first finding.

Magnetic field effects on the electroluminescence of organic light emitting devices: A tool to indicate the carrier mobility

The magnetoelectroluminescence (MEL) of organic light emitting devices with a N,N′-bis(l-naphthyl)-N,N′-diphenyl-1,l′-biphentl-4,4′-diamine:tris-(8-hydroxyquinoline) aluminum (NPB:Alq3) mixed emission layer (EML) has been investigated. We find that MEL is maximized when the volume ratio of NPB of the mixed EML reaches 30% and the EML thickness is 40 nm. The features of MEL under various magnetic field strengths are insensitive to the change in EML thickness and mixing ratio. Meanwhile, MEL has a close relationship with the carrier mobility. We have conducted a theoretical study to further verify the relationship. Our experimental and theoretical results confirm that MEL can function as a tool to indicate the mobility.

Electroluminescence and magnetoresistance of the organic light-emitting diode with a La0.7Sr0.3MnO3 anode

Electroluminescence (EL) with brightness up to 300 cd m2 is observed from organic light-emitting diodes fabricated on oxygen-treated La0.7 Sr0.3 Mn O 3 anodes. An external magnetic field of 150 mT ...

A combined theoretical and experimental investigation on the transient photovoltage in organic photovoltaic cells

We present a time-dependent device model, describing the dynamical processes of both exciton induced by light illumination and charge carriers created from the exciton dissociation, to calculate the transient photovoltage (TPV) in single-layer organic photovoltaic cells. With reasonable parameters for the specific ITO (indium tin oxide)/CuPc (copper phthalocyanine)/Al (aluminum) structure, we could obtain the TPV well fitted with previous experimental observation by adjusting only the intensity of input laser pulse. Further, we saw a saturation of this TPV by changing the intensity of laser pulse from the calculation, which has been confirmed by the experimental measurement on ITO/NPB [N,N-bis(l-naphthyl)-N,N-diphenyl-1,l-biphentl-4,4-diamine]/Al structure. The saturated TPV value is found to be sensitive to the mobility of minority carriers, which might be useful in the estimation of mobilities.

Dissociation of excitons in the C60 film studied by transient photovoltage measurements

The dissociation of excitons at indium tin oxide (ITO)∕C60 interface is studied by means of transient photovoltage measurements. An abnormal polarity change of transient photovoltage from positive to negative upon pulsed laser irradiation is observed, indicating that the exciton dissociation at ITO∕C60 interface results in holes injected into ITO and electrons left in the C60 film, opposite to that occurring at ITO/NPB and ITO/CuPc interfaces. It is confirmed that C60 has a moderately strong ability of donating holes to ITO during the dissociation process of the excitons at the ITO∕C60 interface. Moreover the long term transient photovoltage (t>10ns) and its polarity can be tuned by applying external bias on the device, which further proves the validity of the model proposed to explain the polarity change of the transient photovoltage.

Interfacial reactions at Al/LiF and LiF/Al

High-resolution synchrotron radiation photoemission spectroscopy was used to investigate the chemical properties of Al–LiF interfaces. An electronic state appeared at the Al/LiF interface with a binding energy 4.8 eV higher than that of the metallic Al 2p core level, but the state was hardly found to be present at the LiF/Al interface. This indicates that intensive chemical reaction could occur at the Al/LiF interface, while the reaction occurring at the LiF/Al interface would be weak. This result explains well the unsymmetrical electron injection from different sides of the symmetrical device of indium-tin-oxide\Al\LiF\tris(8-hydroxyquinoline) aluminum\LiF\Al showing unsymmetrical current-voltage characteristics.