Generally in semiconducting polymers

holes are majority carriers while electrons as minority

the carrier misbalance between electrons and holes further deteriorate in the fact that hole usually take possession of higher mobility and smaller injection barrier. To balance the electrons and holes in luminous layer

there are two commonly used stratagems. One is to improve the injection of electrons such as employ low work function metals such as barium and calcium as cathode though they are susceptible to degradation upon water vapor and oxygen or proposed to inser an insulating thin layer usually metal fluoride between polymer/electrode interfaces to build a bilayer cathode.Another is adding a layer into the polymer light-emitting diode such as hole buffer layer or electron-blocking layer.In this work

a layer of small organic molecule (CBP

dissolved in chloroform) was added into the polymer light-emitting diode between the anode (ITO) and luminous layer (MEH-PPV)

using spin-coating method. The influence of different CBP concentrations was investigated. A typical single-organic-layer polymer light-emitting diode device which were constituted with anode (ITO)

luminous layer (MEH-PPV) and cathode (LiF/Al) was employed as reference one. The layer of CBP acts as the hole buffer layer at low voltage and electron-blocking layer at high voltage. Comparing with the single-organic-layer (MEH-PPV) devices

the current efficiency of the composite structure devices is significantly enhanced. The experimental results showed that

below the voltage of 5 V

the current of the composite structure device is higher than that of the single-organic-layer device resulting in the reduction of the turn-on voltage of device. However

when the voltage is higher than 5 V

the current of the composite structure device is lower than that of the single-organic-layer device. That was because

in the low voltage

the CBP layer can slow down the injection speed of the holes into the luminous layer and restricted the holes in the CBP buffer layer. Thus

a built-in electric field was formed in the device

which could improve the transmission of electrons from the cathode to the anode

resulting in the increasing of current

reduce the turn-on voltage and improve the luminous efficiency. However

with increasing of voltage

the CBP layer acts as electron-blocking layer

which limits the electron leaking to the anode. Moreover

as the hole acts as majority carrier

the minority carrier

i.e. electrons

can still combine with holes efficiently which had little influence on the luminance intensity. Therefore

the electron-hole current density balance is improved

luminous efficiency is enhanced. When the CBP concentration is 20 mg/mL

the optimum of current efficiency and luminous intensity is achieved.