JOURNAL OF

LUMINESCENCE EISEVIER Journal of Luminescence 72-74 (1997) 1005-1006

Behavior of charge carriers and excitons in poly(methylphenylsilane) as investigated by electroluminescence from

single-layer-light-emitting diodes

Hiroyuki Suzuki* NIT Basic Research Laboratories, Atsugi, Kanagawa, 243-01, Japan

Abstract

The electroluminescent (EL) process of light-emitting diodes (LEDs) made from poly(methylphenylsilane) (PMPS) was studied. The current-voltage-EL intensity (I-V-EL) curves and the effects of defect concentrations at the interface between PMPS and electron-injecting electrodes (EIEs) on the EL characteristics were analyzed at temperatures between 28 and 294 K.

Keywords: Electroluminescence; Organic light-emitting diode; Polysilane; Space-charge-limited current

Recently, near-ultraviolet (NUV) EL was ob- served from PMPS-LEDs Cl, 21. The EL is emitted near the PMPS-EIE interface, and the defects at this interface play an important role in the EL process [2]. Details of the EL process are, however still unclear. In order to obtain further clarification, Z-I/-EL curves and the effects of defect concentra- tions on EL characteristics were analyzed at tem- peratures above 28 K. As PMPS is a good hole conductor, the LEDs exhibit hole current, and the EL intensity is determined by the electron supply. The EL is therefore used to study the behavior of electrons in PMPS, while hole behavior can be clarified by Z-I/ curve analyses [a].

Analyses of Z-T/-EL curves of LEDs with vac- uum-deposited Al contacts ( Al(vd)) revealed that they can be fitted by the power law (I cc I/“‘+ I) [3],

* Fax: +81-462-70-2353; e-mail: hiroyuki@will.brl.ntt.co.jp.

namely, the space-charge-limited current (SCLC) model (Fig. 1). The SCLC model predicts Z-I/ curves composed of (1) an ohmic region caused by thermal carrier generation (m = 0), (2) a square-law region caused by shallow trapping (m = l), (3) a trap-filling region (m > l), and (4) a trap-free square-law region (m = 1). Various parameters can be obtained by these analyses, as listed in Table 1 for temperatures at which the effective hole mobil- ity (p) was determined by the time-of-flight (TOF) technique [4].

The obtained trap depth agrees well with the activation energy for p in PMPS (0.36-0.37 eV) determined by the TOF technique [4,5]. The Z-I/ curve at 239 K exhibits an additional current in- crease in region (4) upon EL emission (Fig. 1). This is ascribed to the neutralization of positive space charges by the recombination of electrons with holes in the above traps, which accompanies the onset of NUV EL. At RT the EL starts in accord

0022-2313/97/$17.00 % 1997 Elsevier Science B.V. All rights reserved

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1006 H. Suzuki/Journal of Luminescence 72-74 (1997) 1005-1006

-4

-9 -3

0 0.2 0.4 0.6 0.6 1 1.2 1.4 1.6

WV)

Fig. 1. I-V-EL curves of ITO/PMPS/Al(vd). I. (0) RT, (m) 239 K; EL Intensity (A) RT, (v) 239 K. Fitted m values are given in parentheses.

Table 1 Parameters obtained from analyses using the SCLC model

Temperature (K) 294 239 Trap depth (eV) 0.40 0.41 Trap density (cm- 3, 3.5 x 10” 3.5 x 10” quasi-Fermi energy (eV) 0.43 0.44 Free-hole density (cmm3) 3.6 x 10” 4.4 x 1O’O Trapped-hole density (cm- 3, 9.2 x 1016 6.9 x lOI

with the onset of region (3) and accompanies no current increase in region (4), indicating that it is due to the recombination of electrons with holes in shallower traps at the PMPS-EIE interface be- cause p is larger at RT. This implies that the elec- tron-hole recombination sites depend on the temperature, and explains why the EL spectrum at RT is composed only of visible emission, whereas the NUV EL emerges at temperatures below 270 K [2]. Quantitative analyses of I-T/ data at lower temperatures are impossible due to the lack of p data. However, the Z-I/-EL curves can also be described qualitatively by the SCLC model with multiple shallower traps.

Fig. 2 shows EL spectra for LEDs with Mg:Ag (10: 1) contacts made by RF magnetron sputtering (Mg : Ag(sp)) [6]. Owing to its use of higher-energy metal particles, this technique increases defect con- centrations at the PMPS-EIE interface. The EL

- 3 cd

V

3 .- 2 2 + d

---------------II--u

350 400 450 500 550 600 Wavelength (nm)

Fig. 2. EL spectra of ITO/PMPS/Mg:Ag(sp). Inset shows the NUV EL spectra. (0) 28 K; (m) 67 K; (*) 117 K; (v) 157 K, (A) 199 K; (0) 244 K.

spectra are dominated by visible EL because these defects act as radiative trapping centers. They also act as energy acceptors for the one-dimensional excitons responsible for NUV EL. NUV EL is, for instance, detectable at 270 K with Al(vd) (lower defect concentration) compared with 244 K for Mg : Ag(sp) (higher defect concentration).

This study provides additional evidence that de- fects at the PMPS-EIE interface play a vital role in the EL process of PMPS-LEDs. Understanding of the EL process will be improved by using other polysilanes and/or improving device structures, both of which are now in progress.

References

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