Spray pyrolysed In2S3 thin films: A potential electron selective layer for large area inverted bulk-heterojunction polymer solar cells

Article abstract of DOI:10.1002/pssa.201127353

In this paper, we report the results of investigations on the potential of spray pyrolysis technique in depositing electron selective layer over larger area for the fabrication of inverted bulk-heterojunction polymer solar cells. The electron selective layer (In₂S₃) was deposited using spray pyrolysis technique and the linear heterojunction device thus fabricated exhibited good uniformity in photovoltaic properties throughout the area of the device. An MEH-PPV:PCBM inverted bulk-heterojunction device with In ₂S₃ electron selective layer (active area of 3.25×3.25cm²) was also fabricated and tested under indoor and outdoor conditions. From the indoor measurements employing a tungsten halogen lamp (50mW/cm² illumination), an open-circuit voltage of 0.41 V and a short-circuit current of 5.6 mA were obtained. On the other hand, the outdoor measurements under direct sunlight (74mW/cm²) yielded an open-circuit voltage of 0.46 V and a short-circuit current of 9.37 mA. Copyright

Full text of DOI:10.1002/pssa.201127353

Phys. Status Solidi A 209, No. 1, 199–203 (2012) / DOI 10.1002/pssa.201127353
a
p s s
www.pss-a.com
applications and materials science
Spray pyrolysed In S thin films:
2
3
A potential electron selective layer for
large area inverted bulk-heterojunction
polymer solar cells
1
2
2
1
*,
1
M. R. Rajesh Menon , M. V. Maheshkumar , K. Sreekumar , C. Sudha Kartha , and K. P. Vijayakumar
1
Department of Physics, Cochin University of Science and Technology, Cochin 22, India
Department of Applied Chemistry, Cochin University of Science and Technology, Cochin 22, India
2
Received 17 June 2011, revised 16 August 2011, accepted 31 August 2011
Published online 22 September 2011
Keywords indium sulphide, inverted bulk-heterojunctions, MEH-PPV, polymer solar cells
*
Corresponding author: e-mail kpv@cusat.ac.in, Phone: þ91 484 2577404, Fax: þ91 484 2577595
In this paper, we report the results of investigations on the device with In S electron selective layer (active area of
3
2
2
potential of spray pyrolysis technique in depositing electron 3.25 ꢀ 3.25 cm ) was also fabricated and tested under indoor
selective layer over larger area for the fabrication of inverted and outdoor conditions. From the indoor measurements employ-
2
bulk-heterojunction polymer solar cells. The electron selective ing a tungsten halogen lamp(50 mW/cm illumination), an open-
layer (In S ) was deposited using spray pyrolysis technique and circuit voltage of 0.41 V and a short-circuit current of 5.6 mA
2
3
the linear heterojunction device thus fabricated exhibited good were obtained. On the other hand, the outdoor measurements
2
uniformity in photovoltaic properties throughout the area of the under direct sunlight (74 mW/cm ) yielded an open-circuit
device. An MEH-PPV:PCBM inverted bulk-heterojunction voltage of 0.46 V and a short-circuit current of 9.37 mA.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
promising from the point of view of stability and ease of electron selective layers are ZnO or TiO thin films which are
Introduction Inverted polymer solar cells are quite
In inverted polymer solar cells, the most widely used
2
fabrication. The underlying mechanism is such that electrons processed from solutions. However, preparation of these thin
produced in the active layer, as a result of the photo-induced films is quite tedious and time consuming, even though they
charge separation, are collected by the transparent conduct- do not require any sophisticated equipments. For example,
ing front electrode, usually ITO, and the holes are collected TiO thin films are commonly prepared from a precursor
2
by the back electrode, usually silver. For the efficient solution which is coated onto ITO substrates and calcined
functioning of this structure, an ‘electron selective layer’ is at elevated temperatures (450–500 8C) for about 30–60 min
introduced between the active layer and ITO whereas an [4, 9–11]. ZnO thin films, on the other hand, are prepared
electron blocking layer is inserted between the metallic from their nano-particle solution/dispersion [3, 12], the
electrode and the active layer [1]. Advantage of the inverted preparation of which is again a tiresome process. Moreover a
structure is that it eliminates the use of low work function variety of preparation techniques are followed which points
cathode which is usually highly reactive and gets easily out the need for a standard and reliable process [10, 13–15].
oxidized in air. Furthermore, it avoids the possibility of an Furthermore, to accomplish large area devices, the depo-
ITO/PEDOT:PSS interface which is quite reactive and sition technique should be such that it ensures uniformity of
degrades rather quickly [2]. Such inverted structures were the film on the entire area in terms of its composition,
shown to have power conversion efficiencies greater than 3% morphology and thickness. Now from an industrial point of
[
fabricated on a laboratory scale with active areas less than is easy and less time consuming and which can be integrated
3, 4]. However in most of these reports, the devices were view, we must have a cost effective deposition process which
2
.5 cm . There have been only a few attempts on realizing onto a production line. It is at this point that we came across
the relevance of spray pyrolysis technique. Several other
0
such devices on a larger area [1, 5–8].
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
p s s
2
00
2 3
M. R. R. Menon et al.: Spray pyrolysed In S thin films
techniques such as ink jet printing, screen printing, etc. have
already been demonstrated in this direction [1, 16]. The
uniqueness of spray technique is that it is scalable, less
time consuming and requires no elaborate procedures for
preparingthe precursor solution. Recently, Dedova et al. [17]
have shown that nano-structured films could also be prepared
using this technique.
In the present work, we report the use of spray pyrolysis
technique for depositing indium sulphide (In S ) as electron
2
3
selective layer over larger areas for inverted polymer solar
cells. In our earlier publication we had already reported the
feasibility of spray deposited In S as an electron selective
2
3
layer by fabricating In S /MEH-PPV heterojunctions [18].
2
3
An inverted bulk heterojunction configuration is tried in this
0
work by using a blend of poly (2-methoxy-5-[2 -ethylhexyl-
oxy]-1,4-phenylene vinylene) (MEH-PPV) with [6,6]
phenyl C61 butyric acid methyl ester (PCBM). We have
used MEH-PPV for ease and simplicity although it is
well acknowledged that it is quite susceptible to photo-
degradation under continuous illumination [19].
Figure 1 (online colour at: www.pss-a.com) (a) Device structure,
b) schematic of the top of the device and (c) HOMO-LUMO energy
levelsofMEH-PPVandPCBM, andthepositionofconductionband
minimum and valence band maximum of In
(
2
ITO coated glass substrates (thickness 2000 A, sheet
Experimental In S thin films were deposited on
2 3
˚
2 3
S .
2
resistance 10 V/cm ) by using the spray pyrolysis technique.
In this method, aqueous solution containing indium chloride
and thiourea was sprayed at a rate of 2 ml/min on to the from the top were taken by depositing silver electrodes
heated substrate kept at 350 ꢁ 5 8C. At the substrate surface, through vacuum evaporation (at a pressure less than
ꢂ5
the spray droplet vaporizes and leaves a dry precipitate, 10 Torr) using a shadow mask. Except for electrode
which instantly decomposes to form In S . An indigenously deposition, all the other deposition and characterization
2
3
developed automated spray pyrolysis unit was used for the processes were carried out under ambient air atmosphere.
deposition process. All the deposition parameters such as The device structure is depicted in Fig. 1a.
spray rate, substrate temperature, pressure of carrier gas,
Dark and illuminated J–V characteristics of the devices
distance between spray head and substrate, can be controlled were measured using the Keithley SMU, K236 and Metric’s
in this unit. More details related to automated spray unit and Interactive Characterization Software (ICS). The devices
CSP technique has been described elsewhere [20, 21]. In the were illuminated using a tungsten halogen lamp, with an
2
present work, we prepared In S thin films with In/S ratio of intensity of 50 mW/cm , on the substrate surface.
2
3
1
.2/8, 2/3, 2.5/3 and 2/2 in the spray solution. This was
achieved by varying the molarity of the precursors keeping
the volume of solution fixed. Here an indium concentration fabricated polymer heterojunction using In S with In/S ratio
3 Results and discussion In our previous study, we
2
3
of ‘2’ corresponds to 0.025 M InCl . All the ratios are derived of 1.2/8 in the precursor solution [18]. This bilayer device
3
from this fixed molarity value for InCl . A total volume of exhibits good photo-response compared to the single layer
3
2
0 ml was sprayed in all the cases. Thickness of the films was device employing the polymer alone which showed no
measured using the Stylus Profilometer (Dektak-6M). photoactivity. Now as a continuation to this work, we varied
Resistivity and photosensitivity [22] of the films prepared the In/S ratio in the precursor solution used for preparing
on glass substrates was measured using the two probe In S and studied its effect on the photovoltaic performance
2
3
method employing a Keithley Source Measure Unit of the heterojunction. In S thin films on ITO substrates were
3
2
(
SMU, K236). prepared using precursor ratios such as 1.2/8, 2/3, 2.5/3 and
Linear heterojunction was prepared by spin coating the 2/2 keeping the volume of solution sprayed at 20 ml. The
solution of MEH-PPV in chlorobenzene (5 mg/ml), on top of films were found to be polycrystalline in nature except
the In S film deposited on ITO substrates. MEH-PPV was for the 2/2 ratio which was found to be amorphous.
2
3
synthesized as described elsewhere [23]. Layer thickness of Thickness, resistivity and photosensitivity of the films were
MEH-PPV was approximately 100 nm. For fabricating also measured and are given in Table 1.
the bulk-heterojunction devices, a chlorobenzene solution
containing MEH-PPV and PCBM (purchased from Solenne a 100 nm thick MEH-PPV layer over the In S films. The
Heterojunctions were fabricated by spin coating
2
3
BV) in the ratio 1:5 (wt:wt) and having a concentration of J–V characteristics of these devices are presented in Fig. 2
0 mg/ml was spin coated on top of In S . Layer thickness in and the photovoltaic parameters are given in Table 2. From
this case was measured to be 120 nm. Electrical contacts Fig. 2, it can be seen that an increase in open-circuit voltage
2
2
3
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
Phys. Status Solidi A 209, No. 1 (2012)
201
Table 1 Thickness, resistivity and photosensitivity of In
films.
2
S
3
thin the characteristic around V ¼ 0. Now, the drop in V has the
oc
effect that the fill factor also decreases resulting in decrease
in efficiency of the device.
In/S
ratio solution (ml) (nm)
volume of
thickness resistivity photosensitivity
(kV cm)
It may also be argued that an increase in thickness of
In S might also have contributed to improvement in V .
2
3
oc
1
2
2
2
.2/8 20
/3 20
.5/3 20
/2 20
200
330
350
210
238
342
416
161
100
99
557
116
Hence in order to clarify this, we decreased the thickness of
In S keeping In/S ratio at 2.5/3. Now reducing the thickness
2
3
by decreasing the volume of solution below 20 ml will result
in discontinuous films and hence shorting of the device.
Hence to overcome this problem, the volume was kept fixed
at 20 ml and the molarity of the precursor was reduced to half.
The deposited films were having a thickness of 195 nm. The
resistivity and photosensitivity were 182 kV cm and 119,
respectively. It may be noted that the film properties are
comparable to that prepared with 2/2 ratio. This is also
reflected in the J–V characteristic of the heterojunction
fabricated using this film (see Fig. 3). This might be due to
the fact that reduction in molarity keeping the volume fixed,
causes lesser chance of sulphur being incorporated leading to
a nature similar to that exhibited by 2/2 ratio. Hence it may be
concluded that the optimum performance has been obtained
for devices with the 350 nm thick film. Hence it has been used
for fabricating devices for further studies.
In all these studies, the active area of the devices
2
fabricated were only 0.12 cm , i.e. only a very small area
2
compared to the actual area of the sample (2.5 cm ). Hence
the variation of the photovoltaic parameters over the entire
area of the sample was evaluated by measuring the J–V
characteristics from 15 different points on the sample. For
Figure 2 (onlinecolour at: www.pss-a.com) J–Vcharacteristics of
the linear heterojunctions employing In
In/S ratios.
2
S
3
thin films with different
2
this, fifteen small electrodes of area 0.04 cm were deposited
at these positions as depicted in Fig. 1b. This helps to map the
variation of the photovoltaic parameters over the entire area
Voc) has been achieved by using an In/S ratio of 2.5/3. This of the sample. The results are presented in Fig. 4.
(
increase in Voc without any decrease in short-circuit current In the figure, the X- and Y-positions represent theposition
density (J ) may be attributed to the increase in both of the electrodes along the width and length of the sample.
sc
resistivity and photosensitivity of In2S3 (see Table 1). The Voc was observed to be fairly uniform throughout the
Typically, an increase in resistivity will naturally improve entire area of the sample. Out of the 15 points selected, 12
the open-circuit voltage, but at the expense of J . This can showed Voc in the range of 0.67–0.71 V. Maximum Voc
sc
be seen in the case of 2/3 ratio in which J decreases to
sc
2
.1 mA/cm . For the 2.5/3 ratio, however, it is apparent that
0
the increase in photosensitivity could have saved Jsc from
dropping due to increased resistivity of the film. In the case of
device with 2/2 ratio, a reverse nature can be observed in
which the Voc drops due to the reduction in shunt resistance
as a result of the decrease in resistivity of In S [24]. The
2
3
decrease in shunt resistance is evident from the nature of
2 3
Table 2 Photovoltaic parameters of the devices using In S of
different ratios.
In/S
ratio
Voc
(V)
Jsc
(mA/cm )
fill factor,
FF (%)
efficiency,
h (%)
2
1
2
2
2
.2/8
/3
.5/3
/2
0.63
0.68
0.71
0.46
0.133
0.10
0.14
0.14
44.9
49.0
45.4
30.3
0.075
0.07
0.09
Figure 3 (onlinecolour at: www.pss-a.com) J–Vcharacteristics of
the devices with different In S thickness (2.5/3 ratio).
0.039
2
3
www.pss-a.com
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
p s s
2
02
2 3
M. R. R. Menon et al.: Spray pyrolysed In S thin films
Figure 5 (onlinecolour at: www.pss-a.com) J–Vcharacteristics of
the bulk-heterojunction device with an active area of 0.12 cm at
2
2
0 mW/cm .
5
Figure 4 (online colour at: www.pss-a.com) 2D mapping of the
photovoltaicparameters ofthelinear heterojunction (a)Voc (inmV),
(
2
b) Jsc (in mA/cm ), (c) fill factor (%) and (d) efficiency (%).
heterojunction device with the same metal electrode
deposited under similar conditions. Hence it may be alleged
attained was 0.71 V with Jsc of 0.12 mA/cm , fill factor of that a counter acting diode is responsible for this inflection in
2
43.5% and efficiency of 0.75%. Significant variation in Voc the device characteristics. In our device structure, we used
was found only towards the two corners of the sample. silver as the anode. It has already been found to serve as good
Similar is the case with other parameters such as J , fill positive contact [18]. However when used directly over the
sc
factor and efficiency. This result also indicates that the In2S3 blend, both the donor and acceptor phases come in direct
layer is quite uniform and is free from pin holes or cracks. contact with the anode and leads to a counter injecting
Further, any significant variation in the device properties behaviour for the device. This can be overcome by using a
could occur only at the edges and corners since the substrate hole collecting layer between the active layer and the metal
temperature may drop quite easily at these places during the anode. Weickert et al. [26] have shown that a significant
spraying process. This is also reflected in the photovoltaic improvement in the fill factor and efficiency could be
properties of the device depicted in Fig. 4.
It may be noted that even though the V is fairly high, all PEDOT:PSS.
achieved by introducing an hole transporting layer such as
oc
the devices were having a very low value for J leading to
sc
Now a large area bulk-heterojunction device of dimen-
2
2
very small efficiencies. This is because only the In S /MEH- sion 4.5 ꢀ 4.5 cm with an active area of 3.25 ꢀ 3.25 cm was
2
3
PPV interface contributes to the photocurrent generation and fabricated in order to demonstrate the potential of our spray
that the series resistance of the device is too high technique to achieve large area uniform coating of thin films
3
2
(
blending the polymer with an electron acceptor such as is given in Fig. 6a. The cells were then tested both
ꢃ10 V cm ). An improvement in J can be achieved by for polymer solar cells. The photograph of the fabricated cell
sc
PCBM. Figure 5 shows the J–V characteristics of such an under indoor as well as outdoor conditions. As before, the
MEH-PPV:PCBM bulk-heterojunction device of active area indoor measurements were carried out using the Keithley
2
.12 cm , again employing In S as electron selective layer. SMU. Figure 6b shows the semi-logarithmic plot of the
0
The J of this device was in fact significantly improved. It I–V characteristics of this large area cell under dark and
2
3
sc
2
2
2
increased to 0.87 mA/cm at 50 mW/cm , i.e. there is almost 50 mW/cm illumination. From the indoor measurements,
a sixfold increase in the current density. This increase in Jsc we obtained a short-circuit current (I ) of 5.6 mA (current
sc
2
leads to an increase in efficiency of the device to 0.25%. The density of 0.53 mA/cm ) and an open-circuit voltage of
open-circuit voltage of this device was found to be 0.54 V.
From Fig. 5, it can be seen that there is a point of calculated to be 27% and 0.11%, respectively.
inflection near Voc and subsequent concavity in the fourth On the other hand, the outdoor measurements yielded a
0.41 V for this device. The fill factor and efficiency were
quadrant of the characteristic. This negative curvature causes short-circuit current of 9.37 mA and an open-circuit voltage
a substantial reduction in the fill factor of the device. The fill of 0.46 V. The light intensity at the time of measurement was
2
factor decreases to around 27% for the bulk-heterojunction only 74 mW/cm . It should be noted that the large area device
device. Gupta et al. [25] relates this concavity to a non- has smaller Voc and Jsc compared to the small area device
uniform contact or an incomplete coverage of metal over the resulting in a decrease in the efficiency. A decrease in J is
sc
polymer surface. However, this may not be true in the present expected since the series resistance will increase due to the
case, since no such concavity was observed for the linear collection of carriers using two contacts at the corners of the
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
Phys. Status Solidi A 209, No. 1 (2012)
203
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Figure 6 (online colour at: www.pss-a.com) (a) Photograph of the
large area device fabricated, (b) semi-logarithmic plot of the
2
I–V characteristics of the device under 50 mW/cm illumination
measured indoors, (c) and (d) photographs of the device under
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2
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this technique and inverted polymer devices were success-
fully fabricated using it. The device performance however
has to be improved further by introducing a hole transporting
[
[
2 2 3
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Acknowledgements One of the authors (MRRM)
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ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim