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Low dark leakage current in organic planar heterojunction photodiodesHimanshu Shekhar, Olga Solomeshch, Dan Liraz, and Nir Tessler
Citation: Appl. Phys. Lett. 111, 223301 (2017);View online: https://doi.org/10.1063/1.4996826View Table of Contents: http://aip.scitation.org/toc/apl/111/22Published by the American Institute of Physics
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Low dark leakage current in organic planar heterojunction photodiodes
Himanshu Shekhar, Olga Solomeshch, Dan Liraz, and Nir Tesslera)
Microelectronic & Nanoelectronic Centers, Electrical Engineering Department, Technion Israel Instituteof Technology, Haifa 32000, Israel
(Received 19 July 2017; accepted 14 November 2017; published online 30 November 2017)
It is often suggested that the dark leakage current of organic photodiodes is due to extrinsic leakage
paths that do not involve the electronic junction. By studying a series of devices, where the acceptor
is kept constant (C70) and the donor material is varied, we find a direct correlation between the
strength of the sub-gap signature of the charge-transfer states and the leakage current. Attributing the
differences in the sub-gap absorption to the donor’s sub-gap states suggests that the donor’s side of
the junction should be made longer, to push the Fermi level at V¼ 0 towards the acceptor’s LUMO,
and thus, an optimized value of 800 Pacm�2 at V ¼ �1 V is reported. Published by AIP Publishing.https://doi.org/10.1063/1.4996826
Organic photodiodes (OPDs)1–4 are being presented as a
promising complementary or an alternative to inorganic based
photodetectors for detection/imaging applications.5–8 A major
challenge preventing polymer or small molecule based OPDs
from producing a high-quality photodetector is their dark cur-
rent level under reverse bias, which is usually several orders
of magnitude higher than the required value. Low reverse bias
dark current, often referred to as leakage current, is essential
for higher specific detectivity D* and large dynamic ranges
which are the key figure of merits of any photodetector. We
note that we are interested in the actual reverse bias dark cur-
rent and are not to be confused with the ideal-diode’s reverse
dark saturation current density, J0, which is the preexponent
factor of the forward current. It has been shown in some
works that by employing thick active layers,9 electron/hole
blocking layers,10–12 and different donor active materials,13
the leakage current to some extent can be suppressed. Despite
these successes, it remains a challenge because the physical
processes which govern the losses and determine the dark
leakage current are relatively unclear, and hence, it is still an
active area of research. Also, not always care is taken that
while minimizing the reverse (leakage) current the forward
diode current will not be compromised.
Here, we report the dependence of the reverse bias dark
current of an acceptor C70 based planar heterojunction (PHJ)
diode on different donor molecules [copper phthalocyanine
(CuPc), boron subphthalocyanine chloride (SubPc), 1,1-bis
[(di-4-tolylamino) phenyl] cyclohexane (TAPC), tetraphenyl-
dibenzoperiflanthene (DBP), and chloroaluminum phthalocya-
nine (ClAlPc)]. Similar comparisons were studied before but
in the context of solar cells’ open circuit voltage14,15 and less
in the context of the reverse bias dark current. It is common
(to many technologies) that reverse leakage current has a low
dependence on both voltage and temperature.16,17 This has led
to a range of explanations and models some of which are con-
cerned with the junction itself18–20 while others deal with
extrinsic factors and thus often lump it under the notion of
parallel or shunt resistance.15,16,21 To decide whether our
diodes belong to the former or latter group, we study identical
device structures and use the dark current at �1 V as an indi-
cation for the level of leakage current. We found that the leak-
age current reduced from CuPc/C70 to TAPC/C70 and to
SubPc/C70 with an overall reduction of 2 orders of magnitude.
To study the physical mechanism behind this result, we per-
formed both temperature dependent J-V and subgap external
quantum efficiency (EQE) analyses. Finally, as TAPC lends
itself easier (compared to SubPC) for device optimization, we
report a value of 800 pA cm�2 at �1 V for PHJ TAPC/C70
devices.
All devices were fabricated on top of an indium tin
oxide (ITO) coated glass substrate. The ITO substrates were
cleaned & dried as described elsewhere.22 Ultra-high purity
(UHP) grade organic materials CuPc, C70 (CreaPhys),
SubPc, TAPC, DBP, and ClAlPc (Lumtec) were used as
received. The first device structure used consists of a pat-
terned ITO anode, a 70 nm solution deposited thick film of
copper thiocyanate (CuSCN, Sigma 99%) as the hole trans-
port layer (HTL), a 20 nm thick film of donor, a 40 nm thick
film of C70 as an acceptor, a 8 nm thick film of bathocuproine
(BCP, Lumtec 99.99%) as the hole/exciton blocking layer,
and 30 nm thick Mg covered by a 60 nm thick Ag as the cath-
ode. To avoid perimeter leakage, the edges of the patterned
ITO were covered by a 100 nm polyimide layer, and the
device area was 18.4 mm2 (variation of leakage current
density between 1, 4, and 18 mm2 devices was less than a
factor of 2). The dark current-voltage of all the devices was
characterized using a semiconductor parameter analyzer
(B1500 A, Agilent Technologies) inside a nitrogen filled glo-
vebox. Temperature dependent dark J-V characteristics were
analyzed using a K-20 temperature controller (MMR
Technologies), and the signal was read using a Keithley
2612 source meter. Spectrally resolved external quantum
efficiency (EQE) was obtained outside the glove box with
measured samples kept in the nitrogen atmosphere inside a
holder. Light from the monochromator (Oriel, CS130) was
chopped at 80 Hz, and the signal was amplified (Low-Noise
Current Preamplifier SR570) and finally read using a lock-in
amplifier (EG&G 7265). The samples for cyclic voltammetry
(CV) measurements were prepared as evaporated films on
a)Author to whom correspondence should be addressed: nir@ee.technion.
ac.il. URL: http://webee.technion.ac.il/orgelect/.
0003-6951/2017/111(22)/223301/5/$30.00 Published by AIP Publishing.111, 223301-1
APPLIED PHYSICS LETTERS 111, 223301 (2017)
ITO substrates with ITO acting as the working electrode. All
measurements were made inside a glove box in an inert
atmosphere of N2.
Figure 1(a) shows the molecular structural formula of
the active materials used in this study, and the energy level
diagram of all the materials is shown in Fig. 1(b). Figure
1(c) shows the schematic of the investigated PHJ device in a
stack ITO/CuSCN (70 nm)/Donor (20 nm)/C70 (40 nm)/BCP
(8 nm)/Mg (30 nm)/Ag (60 nm), using the donor materials
CuPc, TAPC, SubPc, DBP, and ClAlPc. Because of the
higher and extended absorption spectrum in the visible
range, C70 was used in place of the commonly used C60
acceptor. To place the HOMO/LUMO values in the proper
context, Fig. 1(d) shows the cyclic voltammogram of two of
the active materials. It shows that when using the tangential
to estimate the onset, the two materials are assigned the
same energy level. However, as was shown for fullerene,22 it
is also clear that this simple procedure does not capture the
entire picture or it may explain the range of values reported
in the literature.
The measured dark and light performance characteristics
of the devices are shown in Fig. 2. Organic photovoltaics
(OPVs) with the structure similar to those studied here, using
SubPc/C70 and CuPc/C70, have been reported before.23–25
The solar cell characteristic parameters such as open circuit
voltage (Voc), short circuit current (Jsc), and power conver-
sion efficiency (PCE) of these devices are consistent (listed
in Table I) with the literature values. The dark J-V character-
istics (see Fig. 2) show typical diode behavior, with the leak-
age current part at low forward voltage being clearly visible
for TAPC & SubPc donor devices. The ideal-diode’s reverse
dark saturation current density, J0, of the devices was
obtained by extrapolating the exponential part, which is lin-
ear on the semi-log scale, and finding the intercept on the y-
axis (see the inset in Fig. 2). The actual slope defines the ide-
ality factor, n.
In this paper, the term leakage current is used for cur-
rents that are not part of the main (“ideal”) diode characteris-
tics. As discussed in the introduction, such leakage currents
could be due to shunt paths,16,26 thermal activation of car-
riers at the D/A interface,15 or thermally and/or electrically
activated injection of carriers at the contacts.27,28 To quantify
the leakage, we defined the actual dark leakage currents as
the value measured under a reverse bias of �1 V. The
extracted currents’ values are also collected in Table I.
Since, in the ideal case, there is a direct relationship between
kT�log(J0) and the open circuit voltage (VOC),29 we added
the VOC values to Table I too.
Examining the obtained values in Table I, we note that
there is a rough correlation between kTlog(J0), VOC, and the
change in the energy difference between acceptor LUMO
and the donor’s HOMO. Namely, J0 is relatively well
behaved and can be safely associated with excitation across
the junction. The actual leakage current (Jleak) is however
2–8 orders of magnitudes higher. Specifically, the change
from CuPc to TAPC reduced Jleak only by a factor of �15,
compared to 107 in J0. Namely, while by going from CuPc to
TAPC and SubPc, there was a significant improvement in
the leakage current, there is still significant room for
improvement. This is especially true for the TAPC based
device which happens to also have the lowest ideality factor.
FIG. 1. (a) Molecular structure of
organic molecules used in this study.
(b) Energy levels of all the materials
used in this work. (c) Schematics of the
investigated device structure. (d) Cyclic
voltammetry of TAPC and SubPc.
FIG. 2. Current density vs voltage characteristics in the dark (closed sym-
bols) and under one sun, AM 1.5G, illumination (open symbols) of investi-
gated devices for different donors. The built-in voltage values for CuPc,
TAPC, and SubPc donor devices are marked with black arrows. The device
structure is shown in Fig. 1. The inset shows the extrapolation method used
to obtain the ideal-diode leakage current for CuPc, TAPC, and SubPc donor
devices.
223301-2 Shekhar et al. Appl. Phys. Lett. 111, 223301 (2017)
In an attempt to better identify the physical origin of the
currents at low bias, we obtained the temperature dependence
of the dark J-V between 200 k and 300 k. The ideal-diode’s
reverse dark saturation current density, J0, is thermally acti-
vated and can be expressed as17
J0¼ J00 exp�/B
KT
� �; (1)
where /B is the activation energy, J00 is the prefactor, and
KT is the thermal energy. It is also accepted that for a diode
with an ideality factor n, the actual energy barrier DE is DE
¼ /B�n.17 We found that close to room temperature, the acti-
vation energies of J0 for (CuPc, TAPC, and SubPc) devices
are /B ¼ (0.45 eV, 0.5 eV, and 0.65 eV), and using the n val-
ues and Eq. (1), we get for the energy barrier DE ¼ (0.75 eV,
0.65 eV, and 1.0 eV). Namely, there is relative correlation
between the activation energies and VOC, suggesting that
both are determined by the same processes at the junction.
Returning to the main concern of this paper, the measured
dark currents at�1 V and �0.3 V, Jleak, and the activation ener-
gies close to room temperature are found to be /B_leak(�1 V)
¼ (0.45 eV, 0.28 eV, and 0.16 eV) and /B_leak(�0.3 V)
¼ (0.46 eV, 0.30 eV, and 0.18 eV) for (CuPc, TAPC, and SubPc)
devices, respectively. It seems strange that the device show-
ing the highest leakage also shows the highest activation
energy. However, this very device also exhibits a tempera-
ture dependent activation energy which reduces to �0.25 eV
below 250 k.
Turning the focus to our main interest, dark current
under reverse bias, Jleak, we note that the activation energies
are rather low. They are low compared to any energy differ-
ence that can be evaluated using the energetics presented in
Fig. 1(b), be the HOMO-LUMO differences or reverse injec-
tion barriers. Studies of inorganic planar heterojunctions
have shown that if the junction is the source of leakage, low
activation energies are attributed to tunneling being part of
the generation/recombination process and that it requires the
presence of deep sub-gap states and/or defects in the vicinity
of the junction.17,19,30 With the above in mind, we decided to
look into the sub-gap range using EQE spectra extending to
long wavelengths.
Figure 3(a) shows the zero bias spectral photocurrent
response of all the donor devices studied in this work. From
the measured spectral photocurrent response, two regions are
clearly distinguishable. A relatively flatter part, in the higher
energy region, corresponds to bulk absorption in the donor
and\or acceptor. The declining range, lower energy region,
corresponds to the excitation via sub-gap states.31 The photo-
current response observed in the lower energy part is often
referred to as charge transfer (CT) excitation and serves as a
measure for the absorption of these states.32,33 The primary
interest from this measurement was the estimation of the
subgap density of states in different donor’s based devices
and to reach energies below those of the CT state associated
with J0 (note the new feature that is revealed beyond
1200 nm or below 1 eV). The results in Fig. 3(a) can be
divided into three groups. In the first, there is the CuPc/C70
device showing a very broad response. In the second are
grouped the TAPC, DBP, and ClAlPc which show a second
feature beyond 1200 nm. The SubPc donor devices have the
lowest absorption tail, and beyond 1000 nm, it is below our
detection limit. Since C70 is the common acceptor in all the
devices, we attribute the pronounced subgap absorption to
the donor material in all devices. We note that the relative
strength of the sub-gap states [Fig. 3(a)] and that of the leak-
age currents [Fig. 3(b)] are correlated and are grouped
according to the low energy sub-gap states. This suggests
that the leakage current is associated with excitations across
the D/A interface through sub-gap or CT states. To test this
hypothesis, we inserted a thin (7 nm) layer of TAPC between
CuPc and C70. The EQE of this triple active layer is shown
as the dashed black line in Fig. 3(a), and its J-V curve is
shown (symbols) in Fig. 3(b). We note that both the sub-gap
EQE and the reverse leakage current changed to match those
of the TAPC/C70 bi-layer device. Careful examination of the
sub-gap EQE of the triple active layer (dashed line) shows
some remains of CuPc at �1100 nm and close to our set-up
noise margin, indicating the quality of the surface coverage
of CuPc by TAPC.
TABLE I. Dark and light characteristic parameters for devices, shown in Fig. 1, with various donors.
Donor HOMO (eV) J0 (A/cm2) n Jleak at �1 V (A/cm2) VOC (V) JSC (mA/cm2) PCE (%) FF (%)
CuPc 5.3 2 � 10�8 1.7 1 � 10�6 0.52 4.84 1.33 53
TAPC 5.5 2 � 10�16 1.3 6 � 10�8 0.98 5.24 1.83 36
SubPc 5.5 1 � 10�16 1.55 1 � 10�8 1.12 5.21 1.96 34
FIG. 3. (a) External quantum efficiency (EQE), extended to the sub gap
range, for devices of different donors. (b) J-V curves PHJ based on different
donors. Both (a) and (b) also show a CuPc/C70 device where a thin (7 nm)
TAPC layer was inserted inbetween (black, dashed line). (c) Sketch of phys-
ical mechanisms at play at the donor-acceptor interface in the measured
devices.
223301-3 Shekhar et al. Appl. Phys. Lett. 111, 223301 (2017)
Figure 3(c) shows schematically the physical picture we
suggest for the source of the leakage current in the above
diodes. Moving from CuPc to TAPC and to SubPc, the den-
sity of the donor’s tail states reduces and consequently the
probability for low energy CT states that could promote exci-
tation across the D/A interface reduces. Namely, we claim
that the increased leakage current is due to a higher genera-
tion/recombination at the junction and that the latter is
enhanced by the donor’s deep sub-gap states tailing from the
HOMO level.
If the main issues are the tail states of the donor’s
HOMO level, then it would be beneficial to position the
Fermi level at V¼ 0 between those states and the acceptors
LUMO. This can be achieved by lengthening the hole’s side
of the junction. Attempting to optimize the SubPc/C70 cell
was not successful due to the SubPc properties being highly
dependent on the film thickness.23 The optimized TAPC/C70
device structure was ITO/CuSCN (150 nm)/TAPC (60 nm)/
C70 (40 nm)/BCP (8 nm)/Mg (30 nm)/Ag (60 nm) which
consisted primarily of enhancing the thickness of a high
mobility hole transporting layer. The J-V characteristics of
this device are shown in Fig. 4 along with those of a device
having about half the CuSCN film thickness. Also com-
pared to the previous figures, we note that Jleak, at �1 V,
has reduced to 800 pA cm�2, while the exponential part of
J-V in the forward direction is not affected. Statistic across
3 substrates or 6 devices showed the ranges to be 3–8 nA
cm�2 and 800–1100 pA cm�2.
In conclusion, different donor based planar heterojunc-
tion devices are studied for their reverse bias leakage current.
We find that temperature dependent dark J-V exhibits activa-
tion energies that are too low to be correlated with common
energetics of the materials used to construct the devices (i.e.,
HOMO-LUMO differences or reverse injection barriers). We
have shown that the deep tail states at the donor/acceptor
junction are efficient generation-recombination centers and
are one of the main sources of the leakage current. Also, as
our study indicated that most of the subgap tail states are due
to the donor material, we found that positioning the Fermi
level (at V¼ 0) through device design can farther suppress
the leakage current. From the practical point of view, we
show that by minimizing the tail state density, i.e., by using
a less electronically disordered material, leakage current can
be greatly reduced. Also, by designing the device such that
most of the built-in voltage drops across the donor side, an
ultralow leakage current of 800 pA cm�2 at �1 V for TAPC/
C70 is obtained. Deciphering the details of the leakage mech-
anism at the junction requires further studies and will be
reported at a later stage.
This research was supported by the Israel Science
Foundation (Grant No. 488/16) and the Adelis Foundation for
renewable energy research within the framework of the Grand
Technion Energy Program (GTEP). O.S. acknowledges the
support by the Center for Absorption in Science of the
Ministry of Immigrant Absorption under the framework of the
KAMEA Program. Part of the device fabrication process was
performed at the Technion’s micro-nanofabrication & printing
unit (MNF&PU).
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