Electrical characteristics and efficiency of single-layer organic light-emitting diodes

Abstract

We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly͓2-methoxy-5-͑2-ethylhexoxy͒-1,4-phenylene vinylene͔ ͑MEH-PPV͒, with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current. ͓S0163-1829͑98͒52844-5͔Organic light-emitting diodes ͑OLED's͒ have emerged over the past ten years as viable candidates for application in display technologies. 1 In their simplest configuration, a fluorescent semiconducting polymer is sandwiched between two metal electrodes, an anode with a high and a cathode with a low work function. Under the application of an electric field, holes and electrons are injected into the valence and the conduction band of the polymer, respectively. A fraction of these charges combine to form excitons that decay radiatively, giving rise to light emission. While the technology of OLED's is advancing rapidly, fundamental studies of the device operation are lagging behind. Even in PPV derivatives, which were the first polymers to show electroluminescence 2,3 and are by far the best studied, the relative importance of charge injection as opposed to charge transport as the factor limiting the efficiency of OLED's is still under debate. [4][5][6] For the case of large barriers at the cathode ͑anode͒, inefficient electron ͑hole͒ injection is the limiting process. 4,7 However, since the trap-free drift mobilities are not known for both carriers, it is not clear whether the efficiency of the best devices is limited by injection or by bulk transport. One experimental degree of freedom is the electrode work function which one can change to alter the barrier for electron or hole injection into the polymer, thus changing the magnitude of the electron ͑hole͒ current. Parker 4 has performed a systematic study of ͑mostly unipolar͒ devices with different electrode combinations.In the case of bipolar devices, where there is a significant difference between the work functions of the anode and the cathode, a built-in potential (V bi ) is established in the polymer layer at zero bias ͑see Fig. 1͒. 8 This built-in potential fundamentally affects the operating characteristics of the diode: For applied bias (V appl ) less than V bi the electric field inside the polymer opposes charge injection and forward drift current. ͑Current may flow by diffusion.͒ In the simplest picture, where the bands of the polymer remain rigid, V bi is equal to the work-function difference ͑⌬͒ between the anode and the cathode. The above picture is surely rather simplistic: Instead of extended bands, the electronic levels of conjugated polymers are best described as a ͑Gaussian͒ distribution of localized states. Charge transp...

Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control

“…3), which implies that the current is space-charge-limited. The high mobility of the carrier-transporting materials facilitates a high current density ( 1 A cm 22 ) at a fairly low bias ( 4.5 V) in these devices (Fig. 3).…”

“…A representative current-voltage characteristic (corrected for the built-in potential 22 ) reveals a single operation regime with a slope of 2.1 (Fig. 3), which implies that the current is space-charge-limited.…”

Organic magnetoelectroluminescence for room temperature transduction between magnetic and optical information

“…PEDOT is commonly employed for this purpose in organic light-emitting diode devices. 18 We chose poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as the luminescent polymer, as it is widely used as a red emitter in OLEDs 19 . The MEHPPV layer (55 nm) was deposited by spin-coating from toluene solution.…”

Enhanced power conversion efficiency in iridium complex-based terpolymers for polymer solar cells

“…The SCLC mobilities were calculated through MOTT-Gurney equation. 49,50 Time-resolved PL measurement For time-resolved PL, the laser source was used by a Coherent Libra TM regenerative amplifier (50 fs, 1 KHz, 800 nm), which was seeded by a Coherent Vitesse TM oscillator (50 fs, 80 MHz). Wavelength laser pulses of 800 nm were obtained from the regenerative amplifier while 400 nm laser pulses were achieved from a BBO doubling crystal.…”