Defects and Recombination Kinetics in Copper Indium Gallium Sulfide Thin Films With Spatially Resolved Luminescence in t
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1268-EE03-07
Defects and recombination kinetics in copper indium gallium sulfide thin films with spatially resolved luminescence in the µm-scale Florian Heidemann and Gottfried H. Bauer Institute of Physics, CvO University Oldenburg, 26111 Oldenburg, Germany ABSTRACT Chalcopyrite Cu(In,Ga)S2 is a promising absorber in thin film solar cells, although the comparable high band gaps so far do not correspond to equivalent high open circuit voltages. We have performed photoluminescence studies on CdS passivated absorber layers deposited on Mo coated soda lime glass. From spectrally and spatially resolved (≤ 1µm) room temperature photoluminescence measurements we have extracted the local splitting of quasi-Fermi levels (EFn-EFp) and local absorption (A(ω)) particularly in the sub bandgap-regime via Planck’s generalized law. We observe a substantial negative correlation coefficient between the local sub bandgap/defect absorption and the local (EFn-EFp), which we interpret in terms of the recombination of photogenerated minority carriers (here electrons) via sub bandgap states/deep defects. Moreover we have correlated local PL yields with corresponding values at neighbor sites versus distance (increment analysis). As we find lateral correlation distances in the vicinity of average grain sizes we conclude grains with PL yield and according different splitting of (EFnEFp) to be independent from one another and be laterally distributed randomly. INTRODUCTION Besides the more established Cu(In,Ga)Se2, the sulfidic counterparts Cu(In,Ga)S2 (CIGS) and CuInS2 (CIS) have the potential of a nominal higher open circuit voltage Voc due to its higher band gap [1, 2]. However, an equivalent higher Voc has not been achieved so far and cell efficiencies reported for CIGS and CIS have reached 13% and 11.4% [3 – 5] whereas CIGSe cell efficiencies are as high as 20% [6]. Due to their manufacturing process CIS thin films like others also show a high degree of lateral inhomogeneities in optical, electronic and structural properties that lower the efficiencies of cells [7, 8]. A detailed analysis of such inhomogeneities can be carried out by spatially resolved photoluminescence (PL) of absorber layers. THEORETICAL BACKGROUND To determine the opto-electronic properties of semiconductors relevant for photovoltaics from stationary PL measurements the generalization of Planck’s radiation law can be applied [9 – 11]. In order to describe spectral PL of polycrystalline absorber layers with locally fluctuating optical and electronical properties, measurements have to be carried out with a sufficient lateral resolution, which lies in the length scale of the polycrystalline grains or even below. Consequently the magnitudes quasi-Fermi level splitting EFn(x,y) – EFp(x,y), temperature T(x,y) and absorbance A(ℏω,x,y) in Planck’s generalized law for the photon flux jγ reads
(1) in which k, ℏω and represent the Boltzmann constant, the photon energy, and the constant C
=(4π2ℏ3c2)-1. For sufficiently high photon energies the absorption approaches unity (A(x
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