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:

kyuskim@samsung.com

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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