Download PSS as an Alternative Hole Selective Contact for ITO

Highly Conductive PEDOT: PSS as an Alternative Hole
Selective Contact for ITO free High Efficiency Hybrid
Organic/Inorganic Solar Cell
Abstract: Organic-inorganic hybrid solar cells have been fabricated on textured n-type
crystalline silicon (n-Si) with highly conductive poly (3, 4-ethylenedioxythiophene): poly
(styrenesulfonate) (PEDOT: PSS) as a hole selective contact by spin coating. The
conductivity and wetability has been significantly improved by the addition of 5 wt% of cosolvents dimethyl sulfoxide (DMSO) and 0.1% capstone FS-31 with PEDOT: PSS solution.
Two products of PEDOT: PSS which are HTL solar and PH1000 has been used as an
alternative hole selective contact. The performance of HTL solar product has better than that
of PH1000. Transmission line measurement (TLM) reveals that the sheet resistance of
PEDOT:PSS layer and contact resistivity in between PEDOT: PSS and silver electrodes are
quite impressive to used as a hole transport layer on top of the hybrid solar cell without using
ITO. From the quasi-steady-state photo-conductance, revel the maximum implied VOC is 640
mV and minority carrier lifetime is 205 µs for HTL solar. The device eventually exhibits a
maximum power conversion efficiency of 11.64% with maximum cell area of 11 cm2 for
HTL solar.
Keywords: Textured silicon, PEDOT: PSS, Hybrid solar cell, TLM and QSS-PC
Introduction:
Hybrid photovoltaic devices best on inorganic semiconductor and organic conducting
polymers are currently great interest as an approach to next generation photovoltaics because
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of their potential to produce high efficiency solar cell at very low cost [1]. Also the organicinorganic hybrid (SOH) solar cells are much attractive due to its low temperature fabrication
process and using simple deposition such as spin coating [2]. One of the most important
advantages of organic-inorganic hybrid solar cell is that the organic materials are easily
deposited of different materials without any requirements of lattice matching.
Fabrication of conventional crystalline silicon solar cells are require very high process
temperature (>800 °C) to defuse their dopant in order to form p-n junctions [3]. To reduce the
process temperature and power generation cost of silicon solar cell an ultra thin (>5 nm)
amorphous silicon layers are deposited by plasma-enhanced chemical vapor deposition
(PECVD) on crystalline silicon to improve the conversion efficiency. The maximum
efficiency of heterojunction with intrinsic thin layer (HIT) cell is more than 25% and also the
process temperature is less than 200 °C [4]. But to deposit amorphous silicon on crystalline
silicon by PECVD method is very costly and need very high vacuum (>10-6) and also need
lot of toxic gases. Due to simplicity of fabrication process, different transition metal oxides
(TMOs) [5,6] and conducting polymers such as PEDOT: PSS [7] , P3HT [8] could help us as
an alternative hole selective contact for fabricating high efficiency silicon based hybrid solar
cell. The conducting polymers are easily deposited by spin coating or any other simple
chemical deposition process.
Poly (3, 4-ethyl- enedioxythiophene)/poly (styrenesulfonate) (PEDOT: PSS) has been
extensively investigated for hybrid solar cells due to its high transparency and high
conductivity. Various approaches have been made to enhance the performance of such kind
of the device, such as silicon surface passivation [9], surface morphology controlling [10] and
improvement of conductivity [11] of PEDOT: PSS etc. The nonionic surfactant or fluorosurfactant is used to improve the wettability of the PEDOT: PSS solution. Co-solvents, such
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as ethylene glycol (EG), [12] 6 dimethyl sulfoxide (DMSO), [13] and methano [14] have
been reported to be efficient additive agents for improving the conductivity of the coated
PEDOT: PSS layer.
Hong-Jhang Syu et al. [15] has been reported, the maximum PCE is up to 8.4% and
their active cell area is 7.42 mm2 with polished silicon. While Ken A. Nagamatsu et al. [16]
achieved PCE of 12 % of active cell area is 16 mm2. Recently Joseph Palathinkal Thomas et
al. [12] achieved maximum PEC is up to 13.3% of active cell area 39.69 mm2 using one side
polished silicon with the addition of different concentration of co-solvent and surfactant.
Here, we reported an organic-inorganic hybrid solar cell based on one side textured (random
pyramids) silicon surface. The conductivity of organic semiconductor, poly (3, 4- ethylene
dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has been improved by addition of 5%
of dimethyl sulfoxide (DMSO) and deposited by spin coating. Two different products of
PEDOT: PSS (HTL solar and PH1000) has been used as a hole selective contact. As a result,
our organic-inorganic hybrid solar cell with the structure of Ag/PEDOT: PSS/n-Si/Ti/Al
achieved maximum PCE of 11.64% with maximum cell area 100 mm2.
Experimental section:
Device fabrication:
Figure 1 (a) shows the schematic diagram of the PEDOT: PSS/c-Si hybrid solar cells.
One-side-textured n-type CZ Silicon (100) with resistivity of 2 -3Ω/cm and thickness of 280
µm was used as a substrate. Manufacturing steps are shown in figure 1(b). The textured c-Si
wafer was cleaned by standard RCA cleaning to remove the insoluble organic contaminants
and ionic or heavy metallic atomic contaminants. The back surface of the silicon wafer is the
first treatment of our manufacturing hybrid solar cell. It consists of three thin amorphous
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silicon layer deposited by plasma enhanced chemical vapor deposition (PECVD). This layer
was drilled with a laser to perform a doping diffusion inside the wafer and realize an efficient
localized back contact [17]. Dimethyl sulfoxide (DMSO) of 5 wt% was added to improve the
conductivity of the PEDOT: PSS solution. Capstone FS-31 of 0.1 wt. % was added as a
surfactant to improve the wetabality between PEDOT: PSS and silicon due to hydrophobic
silicon surface. After the metallisation on the back the PEDOT: PSS are deposited by spin
coating with 1000 rpm for 60 second and dried in air a few minutes. The deposited PEDOT:
PSS with n-Si heterojunction was thermally annealed on a hot plate at 130 ºC for 30 min at
nitrogen atmosphere to removing any residual moisture inside the PEDOT: PSS layer.
Finally, the fingers and busbar of almost 1µm was deposited using silver by thermal
evaporation. The outer area of the silver electrode was covered with an opaque mask to
eliminate the incident light. The silver electrode has resulted in a power loss of 4%–5% of the
incident light due to shad- owing.
Materials and device characterizations:
To study the structural and electrical properties of PEDOT: PSS composite films were
characterized by XPS, SEM, TLM and QSS-PC measurements. X-ray photoelectron
spectroscopy (XPS) patterns were recorded using XPS (Source: XR50, Sensor: phoibos 150
MCD-9). The surface and cross section morphology of PEDOT: PSS/n-Si solar cell was
studied using a scanning electron microscope (SEM, Model: Carl Zeiss NEON40). QuasiSteady-State Photoconductance (QSSPC) technique using a Sinton WCT-120 tool. Regarding
the structural and electrical characterization of finished devices, the current-voltage
characteristics were measured by means of a HP4142B DC source/monitor in dark and also
under illumination using a Newport solar simulator. Additionally, External Quantum
Efficiency (EQE) curves of the fabricated solar cells were obtained by means of QEX10 PV
Measurements equipment. In order to calculate the Internal Quantum Efficiency, the front
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reflectance spectra of the solar cells were also measured by means of a UV-visible-NIR
Shimadzu 3600 spectrophotometer.
Result and Discussions:
The surface and cross section morphology of two different PEDOT: PSS product has
been studied using SEM as shown in figure [3 (a) ‒ (e)] [20]. The surface topography of HTL
solar and PH100 are shown in figure [3(a) and 3(c)]. From the topography, it is observed that
the both PEDOT: PSS is continuous with homogeneously distributed all over the silicon
surface. From the cross view [shown in figure 3(b)] we assume that the PEDOT: PSS are
covering very nicely to the whole pyramids of silicon surface. To measuring the exert
thickness of PEDOT: PSS layer all over the pyramids we used filf cutting SEM image shown
in figure [3(d)]. From this cross section images we observed that the thickness of PEDOT:
PSS layer is not uniform all over the pyramids. The thickness is gradually decreased towards
the edge of the pyramids. The estimated average thickness is 60 nm to 70 nm at the bottom of
the pyramids shown in figure [3(e)]. Due to non-uniformity of PEDOT: PSS on textured
surface effects on film factor (FF) of our solar cell.
Transmission line measurement (TLM) are used to calculate the sheet resistance
(
), resistivity (ρ) and conductivity (σ) of the PEDOT: PSS layer and also calculate the
contact resistance
and contact resistivity (
in between PEDOT: PSS and sliver
electrode. TLM measurements are very useful as they allow separating easily the resistance
due to the semi-conductor and the resistance due to the contact with the metal electrode. It
consists in measuring the resistance between several parallel electrodes with different
distances from them. To measuring TLM on PEDOT: PSS layer we deposited it on glass
substrate by spin coating and annealed at 130 ºC for 30 min. Then evaporated silver (50 nm)
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on PEDOT: PSS layer as an electrode. From TLM measurement total resistance consist of
PEDOT: PSS and the electrodes are define as [21]
Where
is the resistance of silver electrode,
interface). Since
>>
so
is associated with the silver/PEDOT: PSS
is neglected.
Considering two parallel electrodes on the front of the device and assuming a one
dimensional current flow confined in the channel between them leads to the simplification of
the equation above into [22]
Where l =distance between two electrodes, W= with of the electrode.
Current-Voltage (I-V) characteristic of PEDOT: PSS (HTL solar and PH1000) are shown in
figure 4 (insert). It can be seen that both contacts exhibit ohmic behaviour of PEDOT: PSS.
Plotting the values of measured
for different distances (l) should lead to a straight line, as
shown on Fig.4. The intercept of this straight at l =0 gives
slope. Once
while
is deduced from the
is known, the resistivity (ρ) and the conductivity (σ) of the PEDOT: PSS is
easily calculated through the following equations (where t is the total thickness of conductive
PEDOT: PSS):
To extrapolate back to the horizontal axis we calculate the transfer length (LT). The contact
resistance between PEDOT: PSS and silver depends on the size of the electrode. So the
dimension of the electrodes was kept 1 cm x 1 mm and the spacing between them ranged
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from 1 mm to 4 mm. The effective area of the contact can be treated as LTW (where LT is the
transfer length and W is the width of the electrode). The contact resistivity is than
W
The calculated results are shown in table 1. The contact resistance of HTL solar product is 10
times lower than PH 1000. Due to the influence of low contact resistivity between PEDOT:
PSS and fingers we obtained lower series resistance and better FF as compare to PH1000.
Before fabricating the device an alternative method based on measuring the photoconductance under steady- or quasi-steady-state illumination, we can estimate the open
circuit voltage, lifetime of minority charge carriers and short-circuit current. This method is
aimed at simplifying the determination of very low lifetimes, although it can also be used for
moderate and high effective lifetimes of our solar cell. To measure the effective
lifetime
, as a function of the excess minority carrier density (∆n) we used the quasi
steady- state photo conductance (QSS-PC) technique.
This quasi-steady-state photo-conductance data implicitly contain information about the
short-circuit current versus open-circuit voltage
‒
. The excess carrier density implies an
implied open circuit voltage, the separation of the quasi-Fermi levels. In fact, photoconductance and voltage are both measures of the same excess minority-carrier density for a
solar cell made on PEDOT: PSS/ n-Si heterojunction. Figure 5 shown implicit open-circuit
voltage vs illumination intensity curves at two different PEDOT: PSS (HTL solar and
PH1000) hybrid solar cell. The maximum implied open circuit voltage (
) before
fabricating any real contact cell is 640 mV for HTL solar PEDOT: PSS. So experimentally
we can achieve maximum 13% efficiency by HTL solar PEDOT: PSS.
The effective lifetime reflects the recombination processes both in the bulk crystalline silicon
( ) and at the surfaces of silicon ( ), is given by [23]
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To calculate
from
, two simplifying assumptions: (a) there is no recombination in the
bulk of the wafer, in other words, the bulk lifetime is infinite; and (b) the same PEDOT: PSS
layers are deposited on both side of c-Si because of the surface recombination velocity has
the same value at both interfaces (
=
=
), then [24]
Where W is the thickness of wafer. Figure 5 (insert) shows
as a function of
PEDOT: PSS deposited on n-type crystalline silicon. The effective lifetime (
for
) for both
PEDOT: PSS (HTL solar and PH1000) are 205 µs and 82 µs. The QSS-PS result suggests
that the solar HTL have higher implied open circuit voltage and higher effective minority
carrier lifetime.
Figure 6 represent the J-V characteristic of the device HTL solar and PH1000 in dark
condition. It can be observed that the saturation current density (Js) of HTL solar composition
is significantly higher than PH1000. The saturation current density is obtained according to
the thermionic emission model [25] as given by
) ‒ 1]
where J is the current density, V is the applied voltage, n is the ideal factor, T is the absolute
temperature(298K), k is the Boltzmann constant(1.3810‒23 J/K) and q is the electronic
charge(1.610‒19 C).
The first diode shows the ideality factor is almost n=1 at the current in between 10-2 to 10-3
A/cm2 but in the second diode shows the non-ideality (n>2) nature at lower current region
(10-5 to 10-6 A/cm2). The nom-idealistic behaviour indicates the surface defect states at
PEDOT/n-Si interface. Also the second diode is called recombination diode. Due to
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formation of this type of recombination diode the leakage current is very high and formed an
s-shape nature in J-V curve. As a result film factor (FF) is decreased and efficiency is goes
down. In the other hand the interface in between silicon and PEDOT: PSS is unable to form a
perfect p-n junction. Sometimes it’s formed a schottky like junction between silicon and
PEDOT: PSS.
Figure 7 shows the J-V characteristics under the illumination of AM1.5G 100 mW/cm2
simulated solar light. The photovoltaic parameters, i.e., open circuit voltage
current density
, fill factor
, short circuit
, and , are summarized for the devices with two different
chemical compound of PEDOT: PSS percents in Table I. The devices for the HTL solar
PEDOT: PSS achieved maximum efficiency 11%–12% of a maximum active cell area (100
cm2).
Figure 8 EQE
Conclusions:
In conclusion, we fabricate a heterojunction solar cell with organic PEDOT: PSS and
inorganic crystalline n-type silicon textured substrate. The 5% DMSO and 0.1% Capstone
FZ-31 addition to PEDOT: PSS to significantly improved conductivity and adhesion on
hydrophobic c-Si wafer. It is found that HTL solar product has better performance than that
of PH1000 due to its very low contact resistivity and lower series resistance. Also HTL
product has better implied open circuit voltage and minority carrier lifetime. As a
consequence, a highly efficiency hybrid organic-inorganic solar cell we are obtained with the
highest power conversion efficiency of 11.64% with a Jsc of 33.54 mA/cm2, Voc of 569 mV,
and a FF of 61.02%. Such organic-inorganic hybrid cells can potentially further deliver high
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conversion efficiency combining with this textured structure and the creation of ohmic
junction at the rear surface
Acknowledgement
The authors would like to thank Montserrat Dominguez for the XPS analyses and
Trifon Trifonov for the SEM measurements. This work has been supported to the Erasmus
Mundus Action 2 AREAS + projects. We are also thankful the Determent of Applied Physics
Indian School of Mines Dhanbad (IIT Dhanbad) for their supports.
PEDOT:
PSS
Sheet
resistance
(Ω/sq)
Contact
resistance
( Ω)
Transfer
length
(cm)
Contact
resistivity
(Ω-cm2)
Conductivity
(S-cm‒1)
Resistivity
(Ω-cm)
HTL solar
190.38
4.93
5.8 10 ‒3
2.8 10 ‒2
875.44
11.4 10 ‒4
PH 1000
122.22
20.16
3.9 10 ‒2
7.9 10 ‒1
1363.66
7.3  10 ‒4
PEDOT:PSS
Cell
Voc
(mV)
FF
(%)
Jsc
(mAcm-2)
PCE
(%)
Solar HIT
569
61.02
33.53
11.64
Series
resistance
(Ω-cm2)
3.68
PH1000
545
49.60
31.54
8.52
6.05
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Figures
Figure 1: (a) Architecture of organic-inorganic hybrid solar cell (b) Summary of the
manufacturing steps (c) Band alignment of PEDOT: PSS/n-Si hybrid solar cell under the
illumination.
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Figure 2: De-convolution of S 2p core level from the XPS analysis of both HTL solar and
PH1000 PEDOT: PSS films.
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Figure 3: Surface and cross section images of hybrid solar cell: (a) HTL solar and (c) PH
1000 compound of PEDOT: PSS layer covering random-pyramid on silicon surface. (b)
Cross section view of HTL solar PEDOT: PSS solar cell (d)-(f) Filf cutting images of PH
1000 PEDOT: PSS solar cell to measuring the thickness.
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Figure 4: The plot of RT vs transfer length, by extrapolating back to the horizontal axis, where
the intercept = –2LT. Thus we calculate to find sheet resistance (Rsh) and contact resistance
(Rc) (Insert) Current-voltage measurements with different transfer length.
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Figure 5: Implicit open-circuit voltage vs illumination intensity curves at two different
PEDOT: PSS products during a hybrid solar cell fabricated on n-type c-Si wafer (Insert)
Effective lifetime vs minority-carrier density both PEDOT: PSS sample
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Figure 6: Current density vs voltage characteristic of the hybrid solar cells in dark.
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Figure 7: Current density vs voltage characteristic of the hybrid solar cells under the 100
mW/cm2 illumination (AM1.5)
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Figure 8: The convolution of the EQE curves with the AM1.5 spectrum over the whole
wavelength range EQE spectra and reflectance of hybrid solar cells.
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