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MoO3 back contact for CuInSe2-based thin film solar cells

Published online by Cambridge University Press:  28 August 2013

Hamed Simchi ,
Brian E. McCandless ,
T. Meng ,
Jonathan H. Boyle  and
William N. Shafarman
Hamed Simchi
Affiliation:
Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A. Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, U.S.A.
Brian E. McCandless
Affiliation:
Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A.
T. Meng
Affiliation:
Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A.
Jonathan H. Boyle
Affiliation:
Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A. Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, U.S.A.
William N. Shafarman
Affiliation:
Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A. Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, U.S.A.
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Abstract

MoO3 films with a high work function (5.5 eV), high transparency, and a wide bandgap (3.0 - 3.4 eV) are a potential candidate for the primary back contact of Cu(InGa)Se2 thin film solar cells. This may be advantageous to form ohmic contact in superstrate devices where the back contact will be deposited after the Cu(InGa)Se2 layer and MoSe2 layer doesn’t form during Cu(InGa)Se2 deposition. In addition, the MoO3 may be incorporated in a transparent back contact in tandem or bifacial cells. In this study, MoO3 films for use as a back contact for Cu(In,Ga)Se2 thin film solar cells were prepared by reactive rf sputtering with O2/(O2+Ar) = 35%. The effect of post processing on the structural properties of the deposited films were investigated using x-ray diffraction and scanning electron microscopy. Annealing resulted in crystallization of the films to the α-MoO3 phases at 400°C. Increasing the oxygen partial pressure had no significant effect on optical transmittance of the films, and bandgaps in the range of 2.6-2.9 eV and 3.1-3.4 eV were obtained for the as deposited and annealed films, respectively. Cu(In,Ga)Se2 thin film solar cells prepared using an as-deposited Mo-MoO3 back contact yielded an efficiency of >14% with VOC = 647 (mV), JSC = 28.4 (mA), and FF. = 78.1%. Cells with ITO-MoO3 back contact showed an efficiency of ∼12% with VOC = 642 (mV), JSC = 26.8 (mA), and FF. = 69.2%. The efficiency of cells with an annealed MoO3 back contact was limited to 4%, showing a blocking diode behavior in the forward bias J-V curve. This may be caused by the presence of a barrier between the valence bands of the Cu(In,Ga)Se2 and MoO3, due to the higher bandgap of the annealed MoO3 films. SEM cross section studies showed uniform coverage of the as-deposited MoO3 layer and formation of voids for the annealed MoO3 film. Structural orientation of the Cu(In,Ga)Se2 absorber layer was also altered by the MoO3 film and less-oriented films were observed for either cases.

Keywords

oxidephotovoltaicthin film
Type
Articles
Information
MRS Online Proceedings Library (OPL) , Volume 1538: Symposium C – Compound Semiconductors: Thin-Film Photovoltaics, LEDs and Smart Energy Controls , 2013 , pp. 173 - 178
Copyright
Copyright © Materials Research Society 2013 

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