CdTe/CdS Solar Cells with Conducting Polymer Back ContactMichael Mount1, Naba Paudel2, Fernanda Duarte1, Yanfa Yan2, Weining Wang1

1Department of Physics, Seton Hall University, South Orange, NJ 07079 U. S. A.2Department 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 ContactIntroduction

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 2.2PEDOT 1 0.03 13.34 0.56 0.36 2.7PEDOT 2 0.24 15.76 0.70 0.46 5.1PEDOT 3 675 19.13 0.64 0.48 5.5PEDOT3 w. methanol treatment 675 21.42 0.71 0.60 9.1Cu/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)