Synthetic Metals 212 (2016) 12–18
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Synthetic Metals
journal homepage: www.elsevier.com/locate/synmet
Effect of boric acid doped PEDOT:PSS layer on the performance of P3HT:
PCBM based organic solar cells
Özlem Yagcia , Serco Serkis Yesilkayaa , Süreyya Aydin Yüksela , Fatih Ongüla ,
Nurhan Mehmet Varalb , Mahmut Kusb , Serap Günesa , Orhan Icellia,*
a
b
Yıldız Technical University, Department of Physics, Istanbul, Turkey
Advanced Technology Research & Application Center, Konya, Turkey
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 12 August 2015
Received in revised form 7 November 2015
Accepted 10 November 2015
Available online xxx
In this study, we fabricated PEDOT:PSS film with different concentrations 0–5 mg/ml of boric acid
(H3BO3) doped PEDOT:PSS film as hole collector layer. Undoped and boric acid doped PEDOT:PSS films are
prepared with spin coating technique and characterized by XRD, UV, AFM, FTIR and electrical
conductivity measurements. We fabricated polymer solar cells in the form of ITO/PEDOT:PSS:H3BO3/
P3HT:PCBM/Al. Results show that the open-circuit voltage (Voc) and the fill factor (FF) increased by
introducing H3BO3 as dopant into the PEDOT:PSS layer due to an increase in the work function of PEDOT:
PSS layer and improvement of surface roughness, respectively. The solar cells without H3BO3 showed a
power conversion efficiency PCE of 1.79% whereas the cells with H3BO3 concentration of 1.25 mg/ml in
PEDOT:PSS, showed a higher performance resulting in a PCE of 2.14% under AM 1.5G illumination.
ã 2015 Elsevier B.V. All rights reserved.
Keywords:
PEDOT:PSS
Bulk heterojunction solar cells
Boric acid
1. Introduction
In recent years, there is an increasing level of interest in
organic-based solar cells as an alternative energy source. Organic
solar cell/photovoltaic (OPV) devices fabricated from the blends of
poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl C61-butyric acid
methyl ester (PCBM) are the most widely studied bulk heterojunction systems because of their relatively good photovoltaic (PV)
properties. Although the P3HT:PCBM devices exhibit excellent PV
properties compared to other bulk heterojunction OPV devices,
their power conversion efficiency is still too low compared to that
of the conventional silicon PV cells [1]. Annealing by a laser [2],
light harvesting by a use of a low band gap molecule [3],
incorporation of nanocrystals [4] or the use of crystallizable
solvent [5] have been foreseen as methods to improve the PV
efficiency of P3HT:PCBM based organic solar cells. The conventional polymer:fullerene solar cells include poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as hole collector
buffer layer because of its high work function, transparency in the
visible range, high conductivity, mechanical flexibility, and easy
processing ability which is inserted between transparent electrode, typically ITO and light-absorbing organic layer [6–11].
PEDOT:PSS is thought to be beneficial for device performance in a
* Corresponding author. Fax: +90 212 383 41 06.
E-mail address: [email protected] (O. Icelli).
http://dx.doi.org/10.1016/j.synthmet.2015.11.010
0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
number of ways. It not only decreases the surface roughness of
indium-tin oxide (ITO) transparent electrodes, reduces the short
cuts in the device, but also improves the selectivity of the anode
due to the higher work function relative to ITO, enhancing electron
blocking and therefore maintaining higher open-circuit voltage.
There are many approaches which demonstrated that properties
such as the electrical conductivity and the work function of PEDOT:
PSS can be changed by the addition of additives (e.g, WOx, MoOx,
V2O5, NiOx, Fe2O3 NPs, ethylene glycol (EG) and multi-walled
carbon nanotubes (MWCNT)) which in turn affects the device
performance significantly [7–13]. In these studies, the efficiency of
the organic solar cells increased with the addition ethylen glycol
(10%) [6], NiOx (5:1 ratio) [7], Fe2O3 NPs (0.7 wt%) [14], and
MWCNT (4%) [15] in PEDOT:PSS which used as hole collector layer,
29%, 10%, 32%, 23%, respectively, also observed that increases in FFs.
However, these additives are more expensive and requires difficult
and long production processes [16]. The use of boric acid is an
alternative way for boron doping. Boric acid is one of the most
important boron compounds and has strategic and industrial
importance. It is a low-cost material and can be easily fabricated
[17]. It is mostly used in industrial applications, such as glass,
ceramics, textiles, detergents, and nuclear power, as well as in the
agricultural, medical, pharmaceutical and electronics related
sectors [18]. However, boric acid has not been widely used in
solar cell studies to best of our knowledge. In one of the studies on
boric acid, Ruan et al., examined the photoelectrochemical
properties of highly ordered titanium dioxide nanotube-array
O. Yagci et al. / Synthetic Metals 212 (2016) 12–18
2. Experimental
As substrates, 1.5 cm 1.5 cm ITO covered glass sheets with a
sheet resistance <15 V cm, from KINTEC company, were used as
substrates. The ITO was patterned by etching with an acid mixture
of HCl:HNO3:H2O (4.5 ml:1 ml:3 ml) for 30 min. The sheet resistance of the ITO substrate was 13 V cm. The part of the substrate
which forms the contact is covered with a scotch tape to prevent
etching. The tape was removed after etching and the substrate was
then cleaned using acetone and isopropanol in an ultrasonic bath.
The blends of P3HT: PCBM were prepared by dissolving 12 mg of
P3HT and 6.5 mg PCBM in 1 ml chlorobenzene. For the preparation
of the PEDOT:PSS:H3BO3 buffer layer, various ratios of H3BO3 (0–
5 mg/ml) was added in PEDOT:PSS solution. All the prepared
solutions were stirred in dark for 24 h.
For the preparation of the solar cells, PEDOT:PSS:H3BO3 was
spin coated on ITO substrates at 2000 rpm for 60 s and annealed at
150 C for 4 min under ambient conditions. The film thickness was
determined using Veeco Dektak profilometer. The film thickness
did not change very much with boric acid doping. The active layer
of P3HT:PCBM was spin cast onto PEDOT:PSS:H3BO3 film at
800 rpm for 60 s in glove box N2 atmosphere. Finally aluminum (Al)
contact metal was deposited by thermal evaporation on active
layer about 100 nm. The final device structure was in the form of
ITO/PEDOT:PSS:H3BO3/P3HT:PCBM/Al (see Fig. 1). A reference
device was fabricated without H3BO3 addition to check whether
H3BO3 had an effect on the PV performance of P3HT:PCBM based
Fig. 1. Schematic of the device architecture with PEDOT:PSS:H3BO3 buffer layer.
devices. Reference device structure was in the form of ITO/PEDOT:
PSS/P3HT:PCBM/Al.
The current–voltage (I–V) characteristics were measured with a
Keithley 2400 source meter under simulated 100 mW/cm2 (AM.
1.5G) irradiation. Optical characterizations of P3HT:PCBM active
layer, undoped and boric acid doped PEDOT:PSS films were
determined in the wavelength range of 300–1100 nm using
PerkinElmer Lambda2 UV–vis spectrophotometer. AFM measurements were performed using ezAFM Atomic Force Microscope
system for characterization of the film morphology. X-ray
diffraction studies were carried out using the Panalytical
Diffractometer with CuKa radiation of 1.5418 Å XRD system. The
resistivity of undoped and boric acid doped PEDOT:PSS films
coated on glass were measured by two probe method for
performed to compare the electrical properties of the undoped
and doped films. Electrical characterization of the resistance of the
film were obtained using Keithley 2400 sourcemeter. For fourier
transfer infrared spectrum (FTIR) of undoped and boric acid doped
PEDOT:PSS solution were recorded by a PerkinElmer spectrometerATR mode.
2000
(d)
1600
Intensity (a.u.)
photoanodes, fabricated by anodization of titanium in a nitric acid/
hydrofluoric acid electrolyte, with and without the addition of
boric acid. They reported the presence of boric acid in the
electrolyte resulted in a TiO2 nanotube-array possessing significantly greater photoelectrochemical properties, under both UV–vis
spectrum illumination [17]. Rahman et al., investigated the effect
of boric acid composition on the morphology, thickness, elemental
composition, optical absorption, structure, photoluminescence of
ZnO nanotubes and the performance of the DSSC utilizing the ZnO
samples. They reported that the diameter and thickness of ZnO
nanotubes decreased with the increasing composition of boric
acid. The DSSC utilizing ZnO nanotubes prepared at 2 wt% boric
acid demonstrated the highest Jsc and h of 2.67 mA/cm2 and 0.29%,
respectively [19]. The use of boric acid as a dopant in titanium
dioxide nanotubes based dye sensitized solar cells have been
investigated by Subramanian et al., They introduced boron into the
interstitial sites of TiO2 lattice and contributed to the shift of
conduction band. The boron-doped TiO2 nanotube arrays showed
an enhanced performance compare to those of undoped TiO2
nanotubes [20].
In this work, we introduced a new boron doped PEDOT:PSS
layer to improve the PV performance of P3HT:PCBM based organic
solar cells. We have shown that boron can be used as a dopant to
the PEDOT:PSS films for organic solar cells as hole collector layer. It
has been shown that the devices employing boron showed a better
FF and efficiency.
13
1200
(c)
800
(b)
400
(a)
0
10
15
20
25
30 35 40
2Theta (o)
45
50
55
60
Fig. 2. XRD diffraction pattern of (a) PEDOT:PSS, (b) 1.25 mg/ml boric acid doped
PEDOT:PSS, (c) 5.00 mg/ml boric acid doped PEDOT:PSS, (d) ITO substrate.
Table 1
Photovoltaic parameters of organic solar cell prepared with different concentrations of boric acid in PEDOT:PSS buffer layer.
Boric acid concentration in PEDOT:PSS (mg/ml)
Film thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF%
PCE%
Rs (kV)
Rsh (kV)
0
0.625
1.25
2.50
3.75
5.00
56.85
56.33
56.08
57.19
55.67
56.23
0.535
0.525
0.602
0.612
0.620
0.640
8.75
8.00
7.57
6.37
7.00
5.10
38.3
36.5
46.9
45.8
42.2
39.0
1.79
1.53
2.14
1.78
1.83
1.27
0.486
0.680
0.625
0.600
0.600
1.400
3.017
2.981
11.47
9.153
8.000
9.803
14
O. Yagci et al. / Synthetic Metals 212 (2016) 12–18
3. Results and discussion
Fig. 2 shows the XRD pattern of undoped, boric acid doped
PEDOT:PSS films and bare ITO substrate. A weak and broad peak
was observed between 2u = 25–26 which belongs to (0 2 0) plane
of PEDOT:PSS polymer [15,21,22].
The broadness of the peaks reflects the semi-crystalline nature
of PEDOT:PSS. This 2u value corresponds to the d-spacing of
approximately 3.56 Å, which can be attributed to the inter-chain
distance of PEDOT [23]. When closely examined, the broad peaks of
the undoped and doped PEDOT:PSS are slightly shifted with
respect to one another. The peak positions of PEDOT:PSS, PEDOT:
PSS (1.25 mg/ml) and PEDOT:PSS (5 mg/ml) have been extracted
using a Gaussian peak fitting after a linear background subtraction.
This yields the 2u values of the peaks as 25.04, 25.28, 25.47,
corresponding to the d-spacing values of 3.56, 3.53 and 3.50 Å,
respectively. The lowest D-spacing is obtained from the films of
PEDOT:PSS (5 mg/ml), which indicates the closest packing of the
PEDOT chains.
As also can be seen from Fig. 2, the ITO peak at 2u = 30
disappears for the boric acid doped PEDOT:PSS films that is
attributed to the homogeneous film formation on the ITO substrate
which is in accordance with the AFM studies.
Atomic force microscopy images of undoped and boric acid
doped PEDOT:PSS films are shown in Fig. 3. The peak to valley
height values were 13.40, 11.61 and 8.68 for undoped PEDOT:PSS,
PEDOT:PSS (1.25 mg/ml boric acid), PEDOT:PSS (5 mg/ml boric
acid)/PEDOT:PSS films, respectively whereas the RMS values were
1.37 nm, 1.36 nm, 1.15 nm, respectively. By the help of the AFM
images, it has been observed that the additional H3BO3 into the
PEDOT:PSS layer led to a smoother surface and therefore a better
morphology. A better film forming property led to an improved the
fill factor of the solar cells.
The results of resistivity of undoped and doped PEDOT:PSS films
are shown in Fig. 4. They demonstrate the reduction of sheet
resistance by 5–20% compared to undoped PEDOT:PSS film with
increased boric acid concentration up to 1.75 mg/ml. The amount
of doping concentration is increased above 1.75 mg/ml increasing
resistance with the B
O peak observed in FTIR spectra shown in
Fig. 5.
Fig. 5 shows the FTIR spectra of undoped and boric acid doped
PEDOT:PSS at room temperature. On the PEDOT:PSS films FTIR
spectra can clearly see one peak at about 1400 cm1 caused by
asymmetric B
O stretching [24] with boric acid added after
1.25 mg/ml boric acid doping concentration. We observed that
decreased resistivity with boric acid doping up to 1.75 mg/ml
concentration than increased resistance with increasing doping
Fig. 3. The AFM images of PEDOT:PSS, PEDOT:PSS (1.25 mg/ml boric acid), PEDOT:PSS (1.25 mg/ml boric acid)/PEDOT:PSS on ITO glass.
O. Yagci et al. / Synthetic Metals 212 (2016) 12–18
Fig. 4. Variation of resistivity of undoped PEDOT:PSS and PEDOT:PSS films with
different concentrations of boric acid.
T%
additional H3BO3 on the performance of the organic solar cells. The
calculated cell parameters in order to determine the effect of boric
acid on the characteristics of photovoltaic cells are given in Table 1.
The reference device where no H3BO3 is employed in the device
exhibited a short-circuit current density (Jsc) of 8.75 mA/cm2 and
an open-circuit voltage (Voc) of 535 mV. A fill factor of 0.39 was
calculated which led to a power conversion efficiency of 1.79%. For
the solar cells employing 1.25 mg/ml of H3BO3 in PEDOT:PSS layer
exhibited a Jsc of 7.57 mA/cm2 and a Voc of 602 mV. A fill factor of
0.46 was calculated which led to a PCE of 2.14%. It has been
observed that at a specific concentration of H3BO3 (1.25 mg/ml) the
solar cells showed the best PV performance. From photovoltaic
parameters point of view, the main effect of the additional H3BO3
was to increase the Voc and the fill factor. The increase in the Voc is
explained in terms of the work function difference (Fig. 11)
whereas the increase in the fill factor is attributed to the better
morphology (Fig. 3) achieved by an additional H3BO3 into the
PEDOT:PSS layer.
The power conversion efficiency of an organic solar cell is
calculated using the following formula:
h¼
100
90
80
70
60
50
84.9
40
84.4
30
20
83.9
1350
10
1400
1450
Undoped
0.625 mg/ml Boric acid doped
1.25 mg/ml Boric acid doped
2.50 mg/ml Boric acid doped
3.75 mg/ml Boric acid doped
5.00 mg/ml Boric acid doped
0
600
1100
1600
2100
2600
3100
3600
Wave Number (cm-1)
Fig. 5. FTIR spectra of undoped PEDOT:PSS and PEDOT:PSS with different
concentrations of boric acid.
15
Isc V oc FF
Pin
ð1Þ
where Isc is the short-circuit current, Voc is the open-circuit voltage
and FF is the fill factor. The open-circuit voltage is a sensitive
function of energy levels of the used materials as well as their
interfaces [25]. It has been previously demonstrated that interfacial effects at the metal/organic semiconductor interface (such as
oxide formation) change the work function of the electrodes and
influence the open-circuit voltage [26,27] which has been also
supported by KPFM results in this study.
In the ideal, loss free contacts, the short-circuit current, Isc, is
determined by the product of the photoinduced charge carrier
density and the charge carrier mobility within the organic
semiconductors:
Isc ¼ nemE
ð2Þ
where n is the density of charge carriers, e is the elementary
charge, m is the mobility, and E is the electric field. Assuming the
100% efficiency for the photoinduced charge generation in a bulk
heterojunction mixture, n is the number of absorbed photons per
unit volume. For a given absorption profile of a given material, the
bottleneck is the mobility of charge carriers. Mobility is not a
material parameter but a device parameter. It is sensitive to the
nanoscale morphology of the organic semiconductor thin film
[28–33]. In a van der Waals crystal, the final nanomorphology
depends on film preparation. Parameters such as solvent type, the
solvent evaporation (crystallization) time, the temperature of the
Fig. 6. I–V curves of the investigated devices in linear scale.
amount with the B
O stretching peak has appeared as shown in
FTIR spectra.
Fig. 6 shows the current–voltage (I–V) characteristics of solar
cells with different H3BO3 concentration (0–5 mg/ml) in PEDOT:
PSS layer measured in dark and under illumination. A reference
solar cell in the form of ITO/PEDOT:PSS/P3HT:PCBM/Al was
fabricated without any H3BO3 to better analyze the effect of
Fig. 7. Absorption of PEDOT:PSS/P3HT:PCBM with (a) undoped, (b) 1.25 mg/ml
doped PEDOT:PSS, (c)5 mg/ml doped PEDOT:PSS.
16
O. Yagci et al. / Synthetic Metals 212 (2016) 12–18
Fig. 8. The external quantum efficiency of organic solar cells with (1) undoped PEDOT:PSS, (2) 1.25 mg/ml doped PEDOT:PSS, (3) 5 mg/ml doped PEDOT:PSS.
substrate, and/or the deposition method can change the nanomorphology [34,35].
In this study it has been observed that the blend film of PEDOT:
PSS and boric acid improves the interfacial effects between the ITO
and PEDOT:PSS as compared to bare PEDOT:PSS films and therefore
improves the open-circuit voltage of the herein investigated
devices however, the prize paid is the complicated nanomorphology of the blend that is difficult to optimize and control which
results in a lower short-circuit current density. Since the efficiency
is directly proportional to the product of Voc and Isc, although the Isc
of the devices involving boric acid decreased non-dramatically, the
overall efficiency increased upon additional boric acid. What
matters for the performance of an organic solar cell is the overall
power conversion efficiency.
Also, as it is seen in Fig. 4 the reduction in the resistivity of the
PEDOT:PSS films with boric acid doping confirms the improvementin serial-shunt resistance of the cells.
An increase of Rsh and a decrease of Rs or both can cause an
increase in FF for photo voltaic devices. The Rsh values for cells
which contained boric acid in PEDOT:PSS increased from 3 kV to
11.47 kV. The increase in the Rsh led to a higher FF and PCE% upon
additional boric acid.
Fig. 7 shows the absorption graph of PEDOT:PSS/P3HT:PCBM
layers with different concentration of H3BO3 in PEDOT:PSS. It has
been seen that the additional H3BO3 in PEDOT:PSS leads to a small
red shift in the absorption. It is well known that, the PEDOT:PSS
layer is almost transparent in the visible region of the solar
spectrum. The major role of the conducting PEDOT:PSS layer in
organic solar cells is to modify and increase the work function of
Fig. 9. The relationship between the photovoltaic parameters of the solar cell and boric acid concentration.
Current Density (mA/cm2)
O. Yagci et al. / Synthetic Metals 212 (2016) 12–18
10
9
8
7
6
5
4
3
2
1
0
Undoped
1.25 mg/ml boric acid doped
0
100
200
300
400
500
600
Time (Hour)
Fig. 10. Life time of OPV cell prepared with undoped and 1.25 mg/ml boric acid
doped.
ITO electrode, for facilitating charge transfer between ITO and
organic active layer. In this study, the transparency of the PEDOT:
PSS layer in the visible portion of the solar spectrum is diminished
upon addition of H3BO3 which in turn leads to a decrease in the
short-circuit current density.
Fig. 8 shows the external quantum efficiency (EQE) of OSCs
measurement. We compared the curve of the external quantum
efficiency (EQE) between PEDOT:PSS and PEDOT:PSS:H3BO3
(different concentration) device. The spectrum span between
300 and 750 nm and is in accordance with the photovoltaic data.
Fig. 9 shows the H3BO3 concentration dependence on the
principal cell parameters of the device such as the open-circuit
voltage (Voc), the short-circuit current density (Jsc), FF and power
conversion efficiency. It is clear that the doping H3BO3 has an
influence on all the device parameters. With increasing H3BO3
concentration the device fill factor and open-circuit voltage
increased despite the decreased the short-circuit current density.
The maximum PCE and FF were obtained at the 1.25 mg/ml
concentration ratio of H3BO3.
Fig. 10 shows lifetime of OPV cell prepared with undoped and
1.25 mg/ml boric acid doped PEDOT:PSS film was investigated in a
glove box. At the end of 600 h it has been observed that boric acid
doped solar cells still have a good stability.
Energy level alignment is crucial for the performance of organic
solar cells and depends on the work function of the individual
ITO
PEDOT undoped
1.25 mg/ml boric acid doped
5.00 mg/ml boric acid doped
12
Magnitute (nA)
10
8
(d)
6
4
(c)
2
(b)
0
(a)
-2
-1
0
1
2
Surface Potential (V)
Fig. 11. Magnitude–surface potential curves of (a) ITO (b) undoped PEDOT:PSS film,
(c) 1.25 mg/ml boric acid doped PEDOT:PSS, and (d) 5 mg/ml boric acid doped
PEDOT:PSS films.
17
electrodes [36,37]. In order to understand the influence of H3BO3
blending in PEDOT:PSS on the contact potential difference, we used
scanning Kelvin probe microscopy (SKPM). Local contact potential
difference (CPD) between doped and undoped PEDOT:PSS films
formed on ITO substrates and AFM conductive tip (TiN) was
investigated. The local contact potential difference between TiN tip
and bare-ITO was measured as 0.281 V and it was obtained as
0.0375 V for undoped PEDOT:PSS whereas 0.12 V and 0.124 V
were obtained for 1.25 mg/ml and 5 mg/ml doped PEDOT:PSS,
respectively. The values obtained from KPFM measurements
clarified that the work function of the ITO surface increased as a
result of the doping of PEDOT:PSS with boric acid. One of the major
roles of the PEDOT:PSS layer is to increase the work function of the
ITO to facilitate the charge transfer from the organic layer to the
ITO (see Fig. 11). The increase in the work function leads to an
increase in the Voc. The increase in the Voc is attributed to the
higher work function of 1.25 mg/ml boric acid doped PEDOT:PSS.
4. Conclusion
We have investigated the influence of addition of H3BO3 in the
PEDOT:PSS solution and its consequence on the performance of
P3HT:PCBM based photovoltaic devices. Results show that the
power conversion efficiency was significantly improved by
inserting H3BO3 in the PEDOT:PSS layer using as hole collector
layer for OPV. An increase in power conversion efficiency and FF
approximately 20% and 23% were obtained between the bare
device and optimized device due to the addition of H3BO3. The
main effect of H3BO3 in the device structure was the increase in Voc
and FF. With the help of the Kelvin probe microscopy studies the
increase in the Voc is attributed to an increase in the work function
of ITO upon additional boric acid. The increase in the fill factor is
attributed to the better morphology of the herein investigated
films achieved by an additional boric acid in PEDOT:PSS layer.
Acknowledgement
This work was supported by TUBITAK (The Scientific and
Technological Research Council of Turkey) with project no 113F391.
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