T
D
and LED is:
V
DD min
¼ V
DS min
þ V
F
ð3Þ
where V
DD
is determined by the highest grey level and
remains unchanged at lower grey levels. Taking an instance in
Fig. 3, the operation current decreases from the highest grey
level(themiddlesolidblackcurve)toalowerone(thelowest
solid black curve). We can observe that the intersection of the
blue curve and the solid black curve is right-shifted, indicating
a decrease in V
F
and an increase in V
DS
. The intersection point
still dwells in the saturation region. The red curve in Fig. 3
depicts the I-V
F
characteristics of the μLED. We can see that
the behaviour of the μLED disp lay is the same as that of the
OLED display, except for a lower V
F
.
Notably, the V
F
values of the μLED chip are lower than
those of the OLED; this result is widely observed in the J-
V
F
characteristics. The relationship between the current
density of μLED (J
μLED
) and V
F
can be described by the
Shockley model
54,55
:
J
μLED
¼ J
s
e
V
F
=nV
T
1
ð4Þ
where J
s
, V
T
and n stand for the saturation current density,
the thermal voltage and the ideality factor, respectively. On
the other hand, because of the small intrinsic charge density
in organic materials, the current density of the OLED (J
OLED
)
is space-charge limited
16,17,56
. According to the space-charge-
limited-current (SCLC) model, the J-V
F
characteristic of
OLEDs follows the famous Mott-Gurney law
57
:
J
OLED
¼
9
8
ε
0
ε
r
μ
V
2
F
d
3
ð5Þ
Here, ε
0
is the vacuum permittivity, ε
r
is the relative
permittivity of the OLED material, and d is the distance
between the OLED electrodes. In Eq. (5), the free carrier
mobility (μ) is a function of the electric field (E = V
F
/d)
58
:
μ ¼ μ
0
e
0:89β
ffiffiffi
E
p
ð6Þ
where μ
0
is the carrier mobility at a zero electric field and
β is the Poole-Frenkel factor. Because of its much lower
mobility, the OLED exhibits a higher threshold voltage
and lower J-V
F
curve slope than the μLED, leading to a
higher operation voltage. Exemplary calculations are given
in the Supplementary Information.
From Eq. (1), we find that the power consumption
ratio between the TFT and LED is equal to V
DS
/V
F
.
From Fig. 3,thehighV
DS
/V
F
ratio indicates that the
TFT may not be an efficient driver for the mL ED/μLE D
displays. In the experiment, we also confirmed that
TFTs could consume more power than LED chips in an
mLED/μLED display. Later, in this section, we will
discuss how to reduce P
TFT
.
Apart from P
static
, the charge and discharge in C
s
and
the parasitic capacitance of data/scan lines in Fig. 2a
generate the dynamic power consumption (P
dyn
)
55
.
However, because P
dyn
is much smaller than P
static
, the
power evaluation in this part will only consider P
static
.
In a full-colour display, the driving voltage is deter-
mined by the following procedures: First, we determine V
F
and I for each RGB chip according to LED L-I-V char-
acteristics and panel specifications. Next, we adopt the
proper TFT type and W
T
/L
T
value to provide the required
I with a reasonable V
DS_min
(Eq. (2)) and V
DD_min
(Eq. (3)).
Last, because the j = R, G, B subpixels are integrated in a
single panel, the common voltage (V
DD,W
)is
V
DD;W
¼ maxðV
DD min;j
Þð7Þ
Apart from the power consumption on each subpixel, in
AM panels, scan drivers and source drivers are employed for
updating the driving current of the emissive device, as Fig. 4a
Power source
Source driver
Timing
controller
Scan driver
Scan driver
Active matrix
pixel array
Control
signal
ab
Power source
V
DD,W
+ (I
W
·R)
V
DD,W
+ (I
W
·R)
V
DD,W
+ 0.5(N – 1)N·(I
W
·R)
+ 2(I
W
·R)
V
DD,W
2·I
W
3·I
W
(N – 1)·I
W
N·I
W
I
W
Fig. 4 Illustration of V
DD
voltage drop. a System schematic of an AM panel. b Voltage drop on a V
DD
line
Huang et al. Light: Science & Applications (2020) 9:105 Page 4 of 16
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