Low temperature photoluminescence spectroscopy studies on sputter deposited CdS/CdTe junctions and solar cells
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Prakash Koirala Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, USA
Julio Soares Frederick-Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA
Robert Collins Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, USA
Angus Rockett Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA (Received 9 November 2015; accepted 15 December 2015)
Device quality CdS/CdTe heterostructures and completed solar cells (;12% efficient) have been studied using photoluminescence (PL) as a function of temperature and laser excitation power. The CdS/CdTe junctions were grown on transparent conducting oxide covered soda lime glass using radio frequency sputter deposition. In the current work we found that the PL spectra of sputtered and thermally evaporated CdTe absorber films share common features. It was found that the luminescence shifts from being dominated by sub-gap defect-mediated emission at lower excitation powers to near band edge excitonic emission at higher excitation powers. It was found that the presence of Cu suppresses the sub-band gap PL emissions. This effect was concluded to be due either to Cu occupying cadmium vacancies (VCd) or forming acceptor complexes with them. This points to a potential role of Cu in eliminating sub-band gap recombination routes and hence increasing the charge separation ability of the device.
I. INTRODUCTION
In recent years, there has been a shift of focus toward utilizing renewable sources of energy. Rising energy costs and increasing environmental concerns associated with fossil fuels are the major contributing factors. Photovoltaics have emerged as a major renewable energy source with Si, GaAs, CdTe, and Cu(In1xGax)Se2 as the major competing technologies. Recently, CdTe-based solar cells have demonstrated a record efficiency of 21.5%,1 making this technology a very promising candidate in photovoltaics. The performances of CdS/ CdTe heterojunction devices are primarily governed by various defect-mediated recombinations that are responsible for reducing the effective charge separation. These defect states have been attributed to Cd vacancies (VCd),2 complexes consisting of VCd and donor impurities such as Cl (VCd–ClTe),3 Cu on Cd sites (CuCd)4 and donor– acceptor (DA) complexes.5 Cl is incorporated in the CdTe layer post deposition, presumably in the form of
Contributing Editor: Xiaobo Chen a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.399 186
J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
http://journals.cambridge.org
Downloaded: 03 May 2016
chlorine on tellurium sites (ClTe), during annealing with CdCl2 vapor.6 This annealing is found to increase CdTe grain size.7 Cu is introduced, usually as a component of the back contact, and acts to enhance p-type doping of the CdTe and improve the ohmic behavior of back contacts.8 Photoluminescence (PL) is an important technique to probe states in the energy
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