Finn Babbe

Potassium Fluoride Ex Situ Treatment on Both Cu-Rich and Cu-Poor CuInSe2 Thin Film Solar Cells

In this paper, we show that CuInSe2 (CIS) absorbers grown under Cu-excess have better collection efficiencies compared to Cu-poor ones. We also show that an ex situ potassium fluoride postdeposition treatment leads to an improvement in VOC for CIS absorbers grown under both Cu-excess and Cu-poor conditions. Additionally, for absorbers grown under Cu-excess, the junction breakdown, which is observed in reverse bias of untreated cells, is removed. This improvement is based mainly on improving the interface of the CIS absorber grown under Cu-excess to the cadmium sulphide buffer layer through moving the dominant recombination from the interface to the bulk. In contrast to observations in the literature, the treated surface is not completely Cu-free.

Quasi Fermi level splitting of Cu-rich and Cu-poor Cu(In,Ga)Se2 absorber layers

The quasi Fermi level splitting is measured for Cu(In,Ga)Se2 absorber layers with different copper to (indium + gallium) ratios and for different gallium contents in the range of 20%–40%. For absorbers with a [Cu]/[In + Ga] ratio below one, the measured quasi Fermi level splitting is 120 meV higher compared to absorbers grown under copper excess independent of the gallium content, contrary to the ternary CuInSe2 where the splitting is slightly higher for absorber layers grown under copper excess. Possible explanations are the gallium gradient determined by the secondary ion mass spectrometry measurement which is less pronounced towards the surface for stoichiometric absorber layers or a fundamentally different recombination mechanism in the presence of gallium. Comparing the quasi Fermi level splitting of an absorber to the open circuit voltage of the corresponding solar cell, the difference for copper poor cells is much lower (60 meV) than that for copper rich cells (140 meV). The higher loss in V  OC in...

Correction to “Potassium Fluoride ex-situ Treatment on both Cu-rich and Cu-poor CuInSe2 Thin Film Solar Cells” [Mar 17 684-689]

Presents corrections to the paper, “Potassium fluoride ex situ treatment on both Cu-Rich and Cu-Poor CuInSe2 thin film solar cells,” (Elanzeery, H., et al), IEEE J. Photovolt., vol. 7, no. 2, pp. 684–689, Mar. 2017.

Interdiffusion and Doping Gradients at the Buffer/Absorber Interface in Thin-Film Solar Cells

An accurate determination of the net dopant concentration in photovoltaic absorbers is critical for understanding and optimizing solar cell performance. The complex device structure of multilayered thin-film solar cells poses challenges to determine the dopant concentration. Capacitance-voltage ( C- V) measurements of Cu(In,Ga)Se2 thin-film solar cells typically yield depth-dependent apparent doping profiles and are not consistent with Hall measurements of bare absorbers. We show that deep defects cannot fully explain these discrepancies. We instead find that the space charge region capacitance follows the model of a linearly graded junction in devices containing a CdS or Zn(O,S) buffer layer, indicating that elemental intermixing at the absorber/buffer interface alters the dopant concentration within the absorber. For absorbers covered with MgF2, C- V measurements indeed agree well with Hall measurements. Photoluminescence measurements of Cu(In,Ga)Se2 absorbers before and after deposition of a CdS layer provide further evidence for a significant reduction of the near-surface net dopant concentration in the presence of CdS. We thus demonstrate that interdiffusion at the absorber/buffer interface is a critical factor to consider in the correct interpretation of doping profiles obtained from C- V analysis in any multilayered solar cell and that the true bulk dopant concentration in thin-film devices might be considerably different.

Study on the quasi Fermi level splitting of Cu(In, Ga)Se 2 absorber layers with Cu-rich and Cu-poor composition

With in this study several Cu(In,Ga)Se2 absorber layers with varying gallium content and different copper over indium + gallium ratios are investigated by photoluminescence. The quasi Fermi level splitting of absorber layers grown under copper excess is 120 meV lower compared to absorbers with a [Cu]/([In]+[Ga]) ratio below one. Possible explanations for this are the gallium gradient within the absorber layer or a gallium related defect. Comparing the quasi Fermi level splitting of an absorber to the open circuit voltage of the corresponding solar cell, the difference for copper poor cells is much lower (60 meV) than for copper rich cells (140 meV).With in this study several Cu(In, Ga)Se2 absorber layers with varying gallium content and different copper over indium + gallium ratios are investigated by photoluminescence. The quasi Fermi level splitting of absorber layers grown under copper excess is 120 meV lower compared to absorbers with a [Cu]/([In]+[Ga]) ratio below one. Possible explanations for this are the gallium gradient within the absorber layer or a gallium related defect. Comparing the quasi Fermi level splitting of an absorber to the open circuit voltage of the corresponding solar cell, the difference for copper poor cells is much lower (60 meV) than for copper rich cells (140 meV).