143115-2
Tang et al.
Appl. Phys. Lett. 95, 143115 ͑2009͒
FIG. 2. ͑Color online͒ I-V characteristics of the bistable devices containing
FIG. 3. ͑Color online͒ I-V characteristics of the device based on the blends
of Cu2S and MEH-PPV ͑1:1 in mass ratio͒ for different amplitude of sweep-
ing voltages.
different mass ratios of Cu S to MEH-PPV: ͑a͒ 1:2, ͑b͒ 1:1, and ͑c͒ 3:1.
2
Arrows represent the direction of the sweeping voltages.
shows a photograph of Cu S nanocrystals dispersed in chlo-
the NDR region ͑from 0 to Ϫ5 V͒, the current increases
rapidly to its peak value. The NDR behavior has been re-
cently reported in organic bistable devices with metal nano-
particles capped by alkanethiol molecules, and the NDR be-
havior is associated with the trapped charges in the
2
roform at room temperature, showing a brown color due to
their strong absorbance for visible light.
A schematic diagram of the bistable device in our work
is shown in Fig. 1͑d͒. The bistable devices were fabricated
on glass substrates precoated with an indium tin oxide
anode ͑sheet resistance 20 ⍀/sq͒. The substrates were
cleaned with deionized water, acetone, and isopropanol se-
quentially, and then treated in ultraviolet ozone before spin-
coating a layer of poly͑3,4-ethylenedioxythiophene͒:poly-
nanoparticles. As to our work, we believe that the traps of
the nanoparticles become filled with the injected electrons in
the negative voltages region, which leads to the formation of
Figure 3 gives the I-V curves of the bistable device con-
͑
styrene-sulfonate͒ ͑PEDOT:PSS͒. After spin-coating and
taining 50 wt % Cu S concentrations in MEH-PPV by vary-
2
annealing PEDOT:PSS layer at 150 °C, the blends contain-
ing the amplitude of maximum sweeping voltages ͑Ϫ5 to 5
V, Ϫ10 to 10 V, Ϫ15 to 15 V, and Ϫ20 to 20 V͒. The results
indicate that electrical hysteresis behavior is repeatable, and
the magnitude of the I-V hysteresis increases with increasing
amplitude of the maximum sweeping voltages. Furthermore,
as shown in Fig. 3, in the different negative sweeping volt-
ages region, the NDR characteristics are also observed. This
suggests that the repeated observation of NDR behavior can
be reliably regarded as an evidence of charge trapping and
ing Cu S nanocrystals and MEH-PPV were spin-coated from
2
chloroform solution onto the top of PEDOT:PSS layer, in
which the mass ratios of Cu S to MEH-PPV are 1:2, 1:1, and
2
3:1, respectively. Finally, a top Al electrode layer with a
thickness of 100 nm was deposited using high vacuum ther-
mal evaporation.
To clearly understand the role of Cu S nanocrystals in
2
the bistable devices studied in our work, we compared the
I-V characteristics for the devices with different mass ratios
detrapping in charge-trap centers of Cu S nanocrystals. Be-
2
of Cu S to MEH-PPV. Figure 2 gives the I-V curves of the
cause the application of different sweeping voltages lead to
NDR effects in I-V characteristics and produce different con-
ducting states, these electronic properties of the fabricated
2
devices with different Cu S concentrations measured from
2
Ϫ15 to 0, 0 to 15, 15 to 0, and 0 to Ϫ15 V. It is clearly
observed an electrical hysteresis with different currents of a
high-conducting state ͑ON state͒ and a low-conducting state
devices result in the multilevel memory capability.
To further understand the current switching from the ON
state to the OFF state, we have tried to fit the I-V character-
istics in both states in terms of different theoretical models.
Figure 4 show the I-V characteristics on a double logarithmic
scale for both states. The I-V curve for the off state can fit to
͑
OFF state͒ at the same sweeping voltages. Such an electrical
hysteresis behavior is an essential feature for a bistable
device.
For the bistable device with 50 wt % Cu S nano-
2
crystals concentration, the amplitude of the hysteresis of the
I-V curve is larger than that for the other two devices. The
ratio between the ON state and OFF state at the same sweep-
ing voltage ͑the ON/OFF ratio͒ is the highest at an optimum
mass ratio of Cu S to MEH-PPV. It is noted that the ON/OFF
2
ratio for the device with 50 wt % Cu S concentration can
2
4
reach 10 at Ϫ1 V, which is equivalent to a reading process
in a digital memory cell. However, for the devices with
3.3 and 75 wt % Cu S concentrations, the ON/OFF ratios
3
2
2
are about 10 , which is two orders smaller than that for the
device with 50 wt % Cu S concentration. On the other hand,
2
the device without Cu S nanocrystals in the polymer did
2
not exhibit the current hysteresis. This indicates that the
n-dodecanethiol capped Cu S nanocrystals play an important
2
role in the electrical bistability of the bistable device. In ad-
dition, the application of a negative sweeping voltage region
leads to a significant NDR in the I-V characteristics of the
FIG. 4. ͑Color online͒ I-V curves of the Cu S:MEH-PPV ͑1:1 in mass ratio͒
2
bistable device on a double logarithmic scale. The scatters are experimental
data, and the straight lines show linear fits to the OFF state and ON state
with slopes of 1.9 and 1.4, respectively.
devices with different mass ratios of Cu S to MEH-PPV. In
2
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