Asymmetric electron and hole transport in a high-mobility n-type conjugated polymer

Electron- and hole-transport properties of the n-type copolymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2, 6-diyl]-alt-5,5′-(2,2′-dithiophene)} [P(NDI2OD-T2), PolyeraActivInk™ N2200] are investigated. Electron- and hole-only devices with Ohmic contacts are demonstrated, exhibiting trap-free space-charge-limited...

Non-radiative recombination losses in polymer light-emitting diodes

We present a quantitative analysis of the loss of electroluminescence in light-emitting diodes (LEDs) based on poly[2-methoxy-5-(2′-ethylhexyloxy)- p-phenylenevinylene] (MEH-PPV) due to the combination of non-radiative trap-assisted recombination and exciton quenching at the metallic cathode. It is demonstrated that for an MEH-PPV LED the...

IdenTifying the nature of charge recombination in organic solar cells from charge-transfer state electroluminescence

Charge-transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free-carrier bimolecular recombination at the donor-acceptor interface, rather than a charge-trap...

Unification of trap-limited electron transport in semiconducting polymers

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking....

Origin of the dark-current ideality factor in polymer:fullerene bulk heterojunction solar cells

In organic bulk heterojunction solar cells, a deviation of the ideality factor of the dark current from unity is commonly put forward as evidence for the presence of trap-assisted recombination. We demonstrate that the non-ideality of the dark characteristics is determined by deeply trapped carriers in the transport-dominating constituent of the...

Determination of the trap-assisted recombination strength in polymer light emitting diodes

The recombination processes in poly(p -phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered...

The effect of ketone defects on the charge transport and charge recombination in polyfluorenes

The effect of on-chain ketone defects on the charge transport of the polyfluorene derivative poly(9,9-dioctylfluorene) (PFO) is investigated. Using MoO3 as ohmic hole contact, the hole transport in a pristine PFO diode is observed to be limited by space-charge, whereas fluorenone contaminated PFO (PFO-F) is shown to be trap limited by the...

Optical detection of deep electron traps in poly(p-phenylene vinylene) light-emitting diodes

The trap-limited electron currents in poly(p-phenylene vinylene) (PPV) derivatives can be modeled using a Gaussian trap distribution that is positioned approximately 0.75 eV below the lowest unoccupied molecular orbital (LUMO) of PPV. Photothermal deflection spectroscopy measurements and internal photo-emission spectroscopy measurements confirm...

Polymer light-emitting diodes with doped hole-transport layers

We demonstrate a solution processed bi-layer PLED based on poly(p-phenylene vinylene) derivatives using orthogonal solvents. To lower the voltage drop the hole transport layer (HTL) based on poly[2,5-bis(2-ethylhexyloxy)-co-2,5- bis(butoxy)-1,4-phenylenevinylene] (BEH/BB-PPV (1:3)) is doped with tetracyano-tetrafluoro-quinodimethane (F4TCNQ)....

Trap-assisted and Langevin-type recombination in organic light-emitting diodes

Trapping of charges is known to play an important role in the charge transport of organic semiconductors, but the role of traps in the recombination process has not been addressed. Here we show that the ideality factor of the current of organic light-emitting diodes (OLEDs) in the diffusion-dominated regime has a temperature-independent value of...

Trap-assisted recombination in disordered organic semiconductors

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μp. We demonstrate that the rate limiting step for this mechanism is the...

Hysteresis-free electron currents in poly(p-phenylene vinylene) derivatives

The interpretation of electron currents in conjugated polymers is strongly hindered by the occurrence of hysteresis. We investigate the transport of electrons in electron-only devices based on derivatives of poly(p-phenylene vinylene) (PPV) for various hole-blocking bottom electrodes as well as purification of the polymer. The use of a variety...