Low dark current small molecule organic photodetectors with selective response to green light
Transcript of Low dark current small molecule organic photodetectors with selective response to green light
Low dark current small molecule organic photodetectors with selective response togreen lightDong-Seok Leem, Kwang-Hee Lee, Kyung-Bae Park, Seon-Jeong Lim, Kyu-Sik Kim, Yong Wan Jin, andSangyoon Lee Citation: Applied Physics Letters 103, 043305 (2013); doi: 10.1063/1.4816502 View online: http://dx.doi.org/10.1063/1.4816502 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mesa-isolated InGaAs photodetectors with low dark current Appl. Phys. Lett. 95, 031112 (2009); 10.1063/1.3184807 Carrier transport in multilayer organic photodetectors: I. Effects of layer structure on dark current andphotoresponse J. Appl. Phys. 95, 1859 (2004); 10.1063/1.1640453 Low dark current quantum-dot infrared photodetectors with an AlGaAs current blocking layer Appl. Phys. Lett. 78, 1023 (2001); 10.1063/1.1347006 Asymmetry in the dark current low frequency noise characteristics of B–B and B–C quantum well infraredphotodetectors from 10 to 80 K J. Appl. Phys. 87, 2400 (2000); 10.1063/1.372192 Very low dark current metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaNepitaxial layers Appl. Phys. Lett. 70, 1992 (1997); 10.1063/1.118777
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Low dark current small molecule organic photodetectors with selectiveresponse to green light
Dong-Seok Leem, Kwang-Hee Lee, Kyung-Bae Park, Seon-Jeong Lim, Kyu-Sik Kim,a)
Yong Wan Jin, and Sangyoon LeeEmerging Materials Research Center, Samsung Advanced Institute of Technology (SAIT), SamsungElectronics Co., San 14-1, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-712, South Korea
(Received 17 January 2013; accepted 8 July 2013; published online 24 July 2013)
We report green-sensitive organic photodetectors consisting of a bulk heterojunction blend of
N,N-dimethylquinacridone and dicyanovinyl-terthiophene. Devices incorporating a triphenylamine
derivative-based electron blocking layer and a molybdenum oxide hole extracting layer lead to
significantly low dark currents (Jd)� 6.41 nA/cm2 at �3 V and high external quantum efficiency of
55.2% at 540 nm wavelength with a narrow full width at half maximum of 146 nm, which is likely
to be applicable for full colour organic image sensors. Based on the interfacial energy barrier and
temperature dependent current-voltage characteristics, possible origins of the reverse Jd of devices
are further described. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816502]
Organic photodetectors (OPDs) have been widely used
in practical applications such as photo sensors and chemical
sensors.1–5 Recently, OPDs have also emerged as promising
candidates for x-ray,6 visible,7–12 and near-infrared13 image
sensor applications due to their ability to provide a large
spectral response, low dark current, high detectivity, and
flexibility, which compete with the performance of conven-
tional silicon-based image sensors.4,9
The early fabrication of organic image sensors (OISs)
has been achieved by using semiconducting polymers as
photosensitive materials.7,10,13 For instance, Heeger and co-
workers7 reported polymer bulk heterojunction poly(3-octyl
thiophene):[6,6]-phenyl-C61-butyric acid methyl ester
(PCBM)-based OPDs with dark current densities (Jd) less
than 10 nA/cm2 at �10 V and a high photoresponsivity (R)
of 0.2 A/W at 600 nm, consequently demonstrating the feasi-
bility of full color OISs. Ng et al.10 also investigated
poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]:
PCBM-based OPDs with low Jd values less than 1 nA/cm2
at �4 V and a moderate value of R of 0.14 A/W for mono
color OISs. However, creating full color images via above
approaches requires additional color filters in order to separate
the panchromatic photosensitivity of the polymers into
individual R/G/B signals,7,14,15 which limit the simplicity of
device fabrication and achieving higher resolutions of
images.14
In contrast to the panchromatic polymers, small organic
molecules that are orthogonally photosensitive to different
wavelengths of light with R/G/B colors have been recently
introduced as alternatives to photosensitive materials for
OISs.2,11,12 The vertical integration of these molecules in the
absence of conventional color separation filters has provided
proof-of-concept demonstrations of compact, lightweight,
and high resolution image sensors.11,12 Despite these advan-
tages, however, the characteristics of small molecule OPDs
are currently not as high as the polymer-based OPDs3,4,10
and even the properties of unit R/G/B OPDs are inconsis-
tent.11,12 For example, a previously reported green unit OPD
exhibited one order of magnitude higher reverse Jd and an
inferior photosensitivity compared to the blue and red unit
OPDs,11,12 impeding the achievement of uniform and high
quality images. In addition, each of the R/G/B OPDs requires
a highly selective response to each wavelength in order to
minimize the interference among the colors and the conse-
quent loss of photosensitivity of each of the OPDs.11,12,15
Thus, the development of high performance small molecule
OPDs with a narrow spectral response to green light is in
great demand for realizing high resolution full color stacked
OISs. In this study, we report low dark current small mole-
cule OPDs that selectively absorb green light by using bulk
heterojunction structures mixed with N,N-dimethylquinacri-
done (DMQA)11 and dicyanovinyl-substituted terthiophene
derivative (DCV3T),16 and an additional charge blocking
layer (see the inset of Fig. 1(d) for their chemical structures).
The OPDs were fabricated on ITO-coated glass by
sequentially depositing a 30 nm thick hole extraction layer
(HEL) composed of molybdenum oxide (MoOx),17 a 1:1
blend layer containing DMQA and DCV3T, and an Al
capping layer as depicted in Fig. 1(a). Three different thick-
nesses of the blend layer, including 50 nm, 70 nm, and
90 nm, were used in the OPDs and the devices obtained were
denoted as M3B5, M3B7, and M3B9, respectively. Also
shown in Fig. 1(b) is the modified OPD adopting a dual
buffer system consisting of an additional 30 nm thick elec-
tron blocking layer (EBL) of a triphenylamine derivative
(TPD15)18 and a 30 nm thick HEL of MoOx (hereinafter
referred to as T3M3B7). In order to elucidate the role of the
EBL, an OPD with a 60 nm thick single MoOx without the
EBL was also prepared (Fig. 1(c)) and denoted as M6B7. All
organic layers were thermally evaporated (<10�7 Torr) at
the rate of 0.1 nm/s. The pixel size, defined by the overlap of
the two electrodes, was 0.04 cm2. The device was finally
encapsulated with glass. The single carrier device consisting
a)Author to whom correspondence should be addressed. Electronic mail:
0003-6951/2013/103(4)/043305/5/$30.00 VC 2013 AIP Publishing LLC103, 043305-1
APPLIED PHYSICS LETTERS 103, 043305 (2013)
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of ITO/MoOx (10 nm)/TPD15 (100 nm)/Al (100 nm) was
fabricated to measure the hole mobility of TPD15 using the
space charge limited current method.19 The current-voltage
(J-V) characteristics of the devices were measured by a
Keithley K4200 parameter analyzer. The photocurrent
response was measured under illumination with a white
LED. The external quantum efficiency (EQE) was measured
using a setup illuminated by monochromatic light generated
by an ozone-free xenon lamp with a chopper frequency of
30 Hz. The monochromatic light intensity was calibrated
using a silicon photodiode (Hamamatsu, S1337). The corre-
sponding R (A/W) defined as the ratio of generated photocur-
rent (A) to incident light power (W) was converted from the
EQE using R ¼ EQE/hv, where hv is the energy of the inci-
dent photon in electron volts (eV).20 The absorption spectra
of the organic films deposited on glass were measured by a
UV-vis spectrophotometer (Shimadzu, UV-240). The highest
occupied molecular orbital (HOMO) levels of all organic
films were measured with an AC-2 photoelectron spectro-
photometer (Hitachi High Tech), and the lowest unoccupied
molecular orbital (LUMO) levels were then determined by
means of the optical band gap obtained from the edge of the
absorption spectrum.
The absorption spectra of the DMQA and DCV3T used
in this study as the electron donor and electron acceptor,
respectively, are shown in Fig. 1(d). The DMQA film exhib-
ited two sharp absorption peaks at 507 nm and 540 nm with
high selectivity to green light, while the DCV3T film
showed relatively broader absorption peaks at 530 nm and
570 nm with a higher absorption coefficient when compared
to that of the DMQA film. Blending two materials in the
ratio of 1:1 produced an intermediate absorption curve,
giving rise to a highly selective absorption characteristic of
the green region.
Figures 2(a) and 2(b) show the spectral response curves
of the OPDs measured at a reverse bias of �3 V. All the
devices showed selective responses to green light, exhibiting
peak EQEs at the wavelength of 540 nm along with a drastic
decrease in the EQEs at longer wavelengths of over 600 nm
and at shorter wavelengths below 450 nm. These results are
in agreement with the absorption spectrum of the
DMQA:DCV3T blend system (shown in Fig. 1(d)). In the
case of OPDs consisting of a 30 nm thick single MoOx HEL,
the EQEs proportionally increased with an increase in the
blend layer thickness mainly due to the enhanced absorption
by the blend layer. Peak EQEs of 44.5%, 54.1%, and 64.1%
were obtained for M3B5, M3B7, and M3B9, respectively
(Fig. 2(a)). The efficiency of OPDs with a 70 nm thick blend
layer remained unchanged by modification with either
increase in the thickness of MoOx HEL up to 60 nm (M6B7)
or inserting an additional TPD15 blocking layer (T3M3B7),
exhibiting peak EQEs of 53.6% and 55.2%, respectively, as
shown in Fig. 2(b).
We note that although the M3B9 device exhibited the
highest EQE value, the spectral response curves were signifi-
cantly broadened, deteriorating the selectivity to green light.
This is liable to induce spectral crosstalk between the adjacent
colors, i.e., blue and red when full color OISs using small
molecule R/G/B OPDs are fabricated.21 Further device
xx
x
FIG. 1. (a)-(c) Schematic diagrams of OPDs with different buffer layers. (d)
UV-Vis absorption spectra of the organic molecules. The inset shows chemi-
cal structures of organic molecules.
FIG. 2. Spectral response curves of the OPDs with (a) 30 nm thick single
buffer layer and (b) 60 nm thick dual buffer layer measured at �3 V. (c)
Integral R values and (d) FWHM and selectivity of the OPDs to green
light.
043305-2 Leem et al. Appl. Phys. Lett. 103, 043305 (2013)
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characterization with respect to the R values integrated at blue
(420–490 nm), green (510–570 nm), and red (590–650 nm)
regions21 (individually) clearly showed that M3B9 indeed had
the highest integral R value in the green region in addition to
red and blue regions (Fig. 2(c)) and consequently large full
width at half maximum (FWHM) of the spectral curve as high
as 167 nm (Fig. 2(d)). The OPDs consisting of a 70 nm thick
blend layer (i.e., M3B7, M6B7, and T3M3B7), by contrast,
showed a moderate drop in the R values in the green region
(Rgreen) of <19% and a relatively large drop in Rblue and Rred
values of over 30% in comparison to the integral R in the
green, red, and blue regions of the M3B9, leading to smaller
values of FWHM of 142–151 nm. As a result, the OPDs with
a 70 nm thick blend layer exhibited higher selectivity towards
green light when compared to the OPD with a 90 nm thick
blend layer, as shown in Fig. 2(d). This implies that the con-
trol of the blend layer thickness is crucial in determining the
color selectivity.
Figure 3 shows the J-V characteristics of the OPDs meas-
ured in the dark. The OPDs with a 30 nm thick HEL of MoOx
and thin blend layers of 50-70 nm exhibited a steep increase
in the reverse Jd with increase in the applied bias (Fig. 3(a))
due to the electric field driven charge injection from the elec-
trode to the blend layer under reverse bias condition.22 For
instance, the M3B5 and M3B7 samples exhibited relatively
high Jd values of 9.96� 10�6 A/cm2 and 3.48� 10�7 A/cm2
at a reverse bias of �3 V, respectively. The Jd, however,
decreased with increase in the blend layer thickness up to
90 nm, resulting in low Jd of 1.76� 10�8 A/cm2 at �3 V for
the M3B9. The modified OPD with a 60 nm thick MoOx HEL
(M6B7) produced a slightly reduced Jd of 7.92� 10�8 A/cm2
at �3 V when compared to the Jd of the M3B7 with a 30 nm
thick MoOx (3.48� 10�7 A/cm2) as shown in Fig. 3(b).
Employing a dual buffer consisting of a 30 nm thick TPD15
and a 30 nm thick MoOx (i.e., T3M3B7 sample), however,
led to a significant decrease in the reverse Jd to the nA/cm2
range, exhibiting a low Jd of 6.41� 10�9 A/cm2 at �3 V,
which is an order of magnitude lower than the Jd value of the
M6B7. The T3M3B7 device, meanwhile, experienced a large
drop in the forward Jd, indicative of retardation of hole injec-
tion (transport) by the TPD15 that is liable to hinder the effi-
cient extraction of photogenerated charges especially under
zero-biased operation.23,24 Note that inferior hole injection
(transport) of the TPD15 is mainly affected by hole mobility
as low as 7.2� 10�7 cm2/Vs at 0.5 MV/cm.
Based on the Jd and EQE values of OPDs, a widely
used figure of merit known as specific detectivity (D*)3 was
evaluated as shown in Fig. 3(c) using the expression of
D*¼R/(2qJd)0.5, where R is the photoresponsivity, Jd is the
dark current density, and q is the electronic charge. Note that
shot noise from the dark current is considered as the major
contributor to the total noise of the OPD.3 Notably the M3B9
and T3M3B7 OPDs exhibited higher D* values of
2.41� 1012 and 3.05� 1012 cm Hz1/2=W, respectively,
mainly due to lower reverse Jd as well as higher R values.
We now further focus on the modified OPDs (i.e.,
T3M3B7, M6B7) due to their low reverse Jd in addition to
high EQEs with good selectivity to green light. Figure 4
shows the dependence of the photocurrent density (Jph) of
OPDs with increasing the light intensity as well as the
reverse bias. At zero-biased condition, the T3M3B7 exhib-
ited slightly lower Jph compared to the M6B7 under most of
the light intensity attributed to unfavorable hole extraction,
i.e., hole trapping23 by the TPD15 layer, but the Jph of the
T3M3B7 notably increased at high reverse biases over �3 V
due to the enhanced charge extraction with the aid of the
electric field applied in the device and the minimization of
charge recombination loss.22,23 We further calculated the lin-
earity of Jph, namely, the linear dynamic range (LDR)3 using
FIG. 3. Current density-voltage characteristics of the OPDs with (a) 30 nm
thick buffer layer and (b) 60 nm thick buffer layer measured in the dark.
(c) Calculated D* values of OPDs. R and Jd measured at �3 V are also
shown.
FIG. 4. Current density-voltage characteristics of the OPDs with 60 nm thick
(a) single MoOx and (b) dual TPD15/MoOx buffer layer under illumination.
043305-3 Leem et al. Appl. Phys. Lett. 103, 043305 (2013)
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LDR¼ 20 log Jph*/Jd, where Jph
* is measured at �3 V bias
under the light intensity of 1 mW/cm2.3 The M6B7 OPD
showed a moderate LDR of 65.1 dB, whereas the T3M3B7
OPD produced a considerably enhanced LDR of 87.6 dB
mainly due to the reduced Jd.
We describe the possible origins of the reduced Jd of
OPDs by adopting the TPD15 blocking layer as follows. For
the EBL-free device (M6B7), both electrons and holes can
penetrate from the ITO anode and Al cathode, respectively,
under reverse bias condition (see the inset of Fig. 5), which
contributes to reverse Jd.24 It is likely, however, the reverse
leakage is primarily dominated by electrons rather than holes
since the electron barrier (ca. 0.57 eV) between the work
function of ITO (/ITO) and LUMO level of DCV3T acceptor
(LUMODCV3T) in blend system is much smaller than the hole
barrier (ca. 1.05 eV) between the HOMO level of DMQA do-
nor (HOMODMQA) and /Al. We further note that although the
MoOx buffer is inserted to the M6B7, the electron barrier
may not be notably affected due to the deep-lying conduction
band edge of MoOx (5.5 eV).17,25 We verified this by fabricat-
ing the MoOx-free OPD (i.e., M0B7) and examined similar
dark J-V characteristics (2.70� 10�7 A/cm2 at �3 V) with
the exception of a reduced forward current compared to the
M6B7 device (not shown). This suggests that the MoOx
buffer indeed exerted little effect on the electron blocking
characteristics, albeit more effectively acted as hole injection
(extraction) layer.17,25 Including the TPD15 layer in the de-
vice, by contrast, increased the effective barrier for electron
leakage due to its low-lying LUMO level22,24 of 2.33 eV,
which caused a remarkable decrease in the reverse Jd of the
OPD as shown in Fig. 3(b).
Further insight into the reverse leakage of OPDs can be
obtained from dependence of the reverse dark J-V character-
istics (at �3 V bias) on temperature as shown in Fig. 5. The
M6B7 device clearly showed temperature dependent reverse
Jd characteristics. The activation energy (Ea) determined
from the slope of Arrhenius plots22,26 was calculated with
Ea� 0.48 eV, which is similar to the theoretical barrier of ca.
0.57 eV (/ITO - LUMODCV3T) for the electron-leak pathway.
This result implies that the Jd of the M6B7 may be dominated
by the thermally activated charge injection (transport).22,26
By contrast, the T3M3B7 exhibited relatively weak tempera-
ture dependence of the Jd with smaller Ea� 0.23 eV
compared to the M6B7, which is also significantly lower than
the ideal barrier of ca. 2.3 eV (/ITO - LUMOTPD15), suggest-
ing that the T3M3B7 had different dark injection mecha-
nisms. Thus, we interpret that leakage paths contributing to
the Jd of the T3M3B7 are preferentially suppressed by large
interfacial barrier over 2.3 eV (/ITO - LUMOTPD15) and then
additionally governed by thermally assisted tunneling24,26
through a small activation barrier of 0.23 eV under reverse
bias condition. The exact mechanisms are, however, under
further investigation.
In summary, we have investigated highly green-
sensitive OPDs using a bulk heterojunction blend of DMQA
and DCV3T for organic image sensor applications. The opti-
mized device adopting a dual buffer layer of TPD15/MoOx
exhibited a high EQE value of 55.2% at 540 nm with a nar-
row FWHM of 146 nm and a significantly low reverse Jd of
6.41 nA/cm2 at �3 V. We interpreted that low reverse Jd of
the OPD was primarily attributed to the suppression of elec-
tron injection under reverse bias via the TPD15 layer with
low-lying LUMO energy level, which was also additionally
governed by thermally assisted tunneling with a small activa-
tion energy.
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043305-4 Leem et al. Appl. Phys. Lett. 103, 043305 (2013)
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