Temperature- and field-dependent electron and hole mobilities in polymer light-emitting diodes

Abstract

We have studied the transport properties of electron-and hole-dominated MEH-PPV, poly͑2-methoxy,5-͑2Ј-ethyl-hexoxy͒-p-phenylene vinylene͒, devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (ϭ 0 e ␥ͱE) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility 0 and electric-field coefficient ␥ are temperature dependent with an activation energy of the hole and electron mobility of 0.38Ϯ0.02 and 0.34Ϯ0.02 eV, respectively. At 300 K, we find a zero-field mobility 0 on the order of 1Ϯ0.5ϫ10 Ϫ7 cm 2 /V s and an electric-field coefficient ␥ of 4.8Ϯ0.3 ϫ10 Ϫ4 (m/V) 1/2 for holes. For electrons, we find a 0 an order of magnitude below that for holes but a larger ␥ of 7.8Ϯ0.5ϫ10 Ϫ4 (m/V) 1/2. Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.

Diffusion-enhanced hole transport in thin polymer light-emitting diodes

“…10 Furthermore, the middle of the highest occupied molecular orbital ͑HOMO͒ is located at around 5.35 eV below the vacuum level. 11 Together with the work function of PEDOT:PSS of 5.0 eV, as measured by a Kelvin probe, this gives a hole density of 5 ϫ 10 23 m −3 at the injecting PEDOT:PSS/MEH-PPV interface, as shown in Fig. 4.…”

Thickness scaling of the space-charge-limited current in poly(p-phenylene vinylene)

“…In numerous experimental and model studies of LEDs, the possible role of this effect was not considered. 4,[8][9][10] In the present study we have investigated the thickness dependence of the SCL current, which can be used to discriminate the effect of a field from that of density-dependent mobility. The enhanced thickness dependence of the experimental SCL current provides a direct proof that the carrier-density-dependent mobility dominates the hole transport in PPV-based diodes.…”

Charge-carrier concentration dependence of the hopping mobility in organic materials with Gaussian disorder

“…Hopping mobilities in structurally disordered organic materials are often many orders of magnitude smaller than the mobilities due to transport via delocalized band states in crystalline inorganic semiconductors. For example, the room temperature hole mobility in typical polyphenylene-vinylene ͑PPV͒ polymer films such as used in polymer LEDs is of the order of 10 −11 to 10 −10 m 2 /V s, 5,6 eight to ten orders of magnitude smaller than the hole mobility in single crystalline Si.…”