1. CdTe/CdS Solar Cells with Conducting Polymer Back Contact
Michael Mount1
, Naba Paudel2
, Fernanda Duarte1
, Yanfa Yan2
, Weining Wang1
1
Department of Physics, Seton Hall University, South Orange, NJ 07079 U. S. A.
2
Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606 U.S.A.
Abstract
Acknowledgements: Cottrell College Science Award from Research Corporation for Science Advancement, SHU-Research Council, Clare
Boothe Luce Foundation, and SHU-New Jersey Space Grant Consortium
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was studied as the back contact of Cadmium telluride (CdTe) solar cells and was compared with conventional Cu-based back contact. A
series of PEDOT:PSS aqueous solutions with different conductivities were spin coated onto the glass/SnO2:F/SnO2/CdS/CdTe structures as back contact, and the PEDOT:PSS conductivity dependence of device
performance was studied. It was found that PEDOT:PSS back contact with higher conductivity produces devices with lower series resistance and higher shunt resistance, leading to higher fill factor and higher
device efficiencies. As the conductivity of PEDOT:PSS increased from 0.03 S/cm to 0.24 S/cm, the efficiency of the solar cell increased from 2.7% to 5.1%. Methanol cleaning also played an important role in
increasing the device performance. The efficiency of our best device with PEDOT:PSS back contact has reached 9.1%, approaching those with conventional Cu/Au back contact (12.5%).
.
TEC
Module
In this work, Pulsed Laser
Deposition (PLD) was utilized
to deposit a ZnO layer upon an
indium-tin oxide (ITO)
covered glass slide. A laser of
wavelength 248 nm was
directed at a ZnO target that in
turn generated a ZnO plasma
plume that allowed the
deposition of ZnO.
J-V Characteristics of CdTe with Different Back Contact
Introduction
Conclusion
Sun Light
Au (50 nm)
p-CdTe (4-5µm)
n-CdS (80-120 nm)
n-
PEDOT:PSS (0.2 µm)
SnO2:F/SnO2
Glass
Sun Light
p-CdTe (4-5µm)
n-CdS (80-120 nm)
n-
Cu (4-5 nm)
SnO2:F/SnO2
Au (40 nm)
Glass
Back Contact PEDOT Conductivity
(S/cm)
Jsc (mA/cm2
) Voc (V) FF Efficiency
(%)
Au --- 7.82 0.57 0.49 2.2
PEDOT 1 0.03 13.34 0.56 0.36 2.7
PEDOT 2 0.24 15.76 0.70 0.46 5.1
PEDOT 3 675 19.13 0.64 0.48 5.5
PEDOT3 w. methanol treatment 675 21.42 0.71 0.60 9.1
Cu/Au --- 21.89 0.81 0.71 12.5
CdTe Solar Cell Structures with Different Back Contact Conclusion
Fig.3. Current density vs. voltage (J-V) characteristics of CdTe solar cells with different
back contacts: 1) Au (50 nm); 2) PEDOT 1 ( ≈ 0.03 S/cm); 3) PEDOT 2 ( ≈ 0.24 S/cm);
4) PEDOT3 ( ≈675 S/cm); 5) PEDOT3 (on methanol-cleaned CdTe structure); 6) Au (40
nm)/Cu (4-5µm)
Table 1. Photovoltaic performance of the glass/SnO2:F/SnO2/CdS/CdTe structures with
different back contacts.
CdTe
PEDOT:PSS
Fig.2. Field emission scanning electron microscope (FE-SEM) image of the cross section of
CdTe/PEDOT:PSS. Acceleration voltage: 5 kV. Scale bar: 500 nm.
Fig. 1. Device structure for glass/SnO2:F/SnO2/CdS/CdTe with PEDOT:PSS back contact (left) and
conventional Cu/Au back contact (right). The glass/SnO2:F/SnO2/CdS/CdTe structures were prepared
by close space sublimation (CSS) technique and treated with chloride
CdTe/CdS solar cell is one of the most promising thin film technologies and its highest
efficiency has reached 21%. One of the most important technical problems of fabricating CdTe/CdS
solar cells is the fabrication of a good ohmic back contact on p-type CdTe. CdTe has a high electron
affinity (about 4.5 eV), so a metal with high work function is needed to form a good ohmic contact
with CdTe. However, most metals do not have high enough work functions. As a result a Schottky
barrier is usually formed at the CdTe/metal junction. This Schottky barrier is opposite to the
CdTe/Cds p-n junction, causing non-idealities in the cell characteristics. As a result, it causes
decrease in fill factor thus reduces efficiency of the solar cells.
Conducting polymers are good candidates as back contact for CdTe because conducting
polymers have high work functions and high conductivities, are easy to process, and cost less,
meeting all the requirements of a good ohmic back contact for CdTe. Moreover, the common
problem of lattice mismatching for inorganic/inorganic junction does not exist for polymer/inorganic
heterojunction. Because there will be less likely broken covalent bond at the CdTe/conducting
polymer back contact interface, trapping and recombination will be minimized.
Among the most studied conducting polymers, poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) is promising because of its reported high work function,
ranging from 4.7 eV to 5.4 eV, and high conductivity achieved after mixing with various solvents
such as dimethyl sulfoxide (DMSO). PEDOT has been widely used in OPVs and commonly used as
a hole-injecting layer (HIL) between ITO and hole-transport layer (HTL) in OLEDs. Cook, etc has
shown that PEDOT:PSS with higher work function and resistivity gives higher device efficiency
when used as a HIL in OLEDs. Jarkov, etc have suggested that organic semiconductors with higher
work function could be promising for effective back contact for CdTe solar cells. However, there are
only sporadic studies on CdTe solar cells with polymer back contact.
In this work, we fabricated CdTe solar cells based on spin-coated PEDOT:PSS as the back
contact, and compared their device performances with those with conventional Cu-based back
contact. We also studied the effect of PEDOT:PSS conductivity on the performance of the CdTe
solar cells for the first time.
In summary, we have demonstrated that conducting polymers such as PEDOT:PSS can
be used as the back contact of CdTe solar cells, and the conductivity of PEDOT:PSS is of
great importance in determining device characteristics. PEDOT:PSS with higher
conductivity has been shown to produce devices with lower series resistance and higher
shunt resistance, resulting better device performances. Methanol treatment of CdTe
structures before the polymer back contact deposition also helped improve the efficiency of
the solar cells. The efficiency of our best device has reached 9.1%, approaching the
efficiency of those devices based on conventional Cu-based back contact (12.5%). For
future studies, conducting polymers with higher conductivity and higher work function
could be promising as the back contact for CdTe solar cells.
“PEDOT:PSS as Back Contact for CdTe Solar Cells and the Effect of PEDOT:PSS
Conductivity on Device Performance”, Weining Wang, Naba Raj Paudel, Yanfa Yan,
Fernanda Duarte, Michael Mount, Journal of Material Science: Materials in Electronics
(2015)