Identification of Defect Levels in Copper Indium Diselenide (CuInSe 2 ) Thin Films via Photoluminescence Studies
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.556
Identification of Defect Levels in Copper Indium Diselenide (CuInSe2) Thin Films via Photoluminescence Studies Niraj Shrestha, Dhurba R. Sapkota, Kamala K. Subedi, Puja Pradhan, Prakash Koirala, Adam B. Phillips, Robert W. Collins, Michael J. Heben, and Randy J. Ellingson
Department of Physics and Astronomy, Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo, OH, USA 43606
Photoluminescence (PL) spectroscopy has been used to study the defect levels in thin film copper indium diselenide (CuInSe2, CIS) which we are developing as the absorber layer for the bottom cell of a monolithically grown perovskite/CuInSe2 tandem solar cell. Temperature and laser power dependent PL measurements of thin film CIS for two different Cu/In ratios (0.66 and 0.80) have been performed. The CIS film with Cu/In = 0.80 shows a prominent donor-to-acceptor peak (DAP) involving a shallow acceptor of binding energy ~22 meV, with phonon replica at ~32 meV spacing. In contrast, PL measurement of CIS film for Cu/In = 0.66 taken at 20 K exhibited an asymmetric and broad PL spectrum with peaks at 0.845 eV and 0.787 eV. Laser intensity dependent PL revealed that the observed peaks 0.845 eV and 0.787 eV shift towards higher energy (aka j-shift) at ~11.7 meV/decade and ~ 8 meV/decade with increase in laser intensity respectively. The asymmetric and broad spectrum together with large j-shift suggests that the observed peaks at 0.845 eV and 0.787 eV were related to bandto-tail (BT) and band-to-impurity (BI) transition, respectively. Such a band-tail-related transition originates from the potential fluctuation of defect states at low temperature. The appearance of band related transition in CIS film with Cu/In = 0.66 is the indicator of the presence of large number of charged defect states.
INTRODUCTION: Chalcopyrite-structured copper indium diselenide (CuInSe 2, CIS) thin films have been widely used as absorber layers in thin film solar cells because of large absorption coefficient and controllable p-type conduction. [1-4] The CIS or CIGS absorber layers used in CIS or CIGS (copper indium gallium diselenide) solar cells are usually Cu-poor. Lower interfacial recombination sites in Cu-poor CIS or CIGS is the reason behind better performance of the CIS or CIGS solar cells in terms of power conversion efficiency. [5, 6] On the other hand, Cu-poor CIS or CIGS films are highly
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compensated giving rise to fluctuating defect potentials at low temperature. [7] These fluctuating potentials are the reason behind band-to-tail (BT) and band-to-impurity (BI) transitions in Cu-poor CIS or CIGS. [8-10] An asymmetric and broad photoluminescence spectrum, with a tail on the lower energy side, together with larg
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