Gallium gradients in chalcopyrite thin films: Depth profile analyses of films grown at different temperatures

Abstract

Cu(In,Ga)Se 2 films are used as absorber layers in chalcopyrite thin film solar cells. As the gallium concentration in the absorber can be used to control the band gap, there have been many efforts to vary the gallium concentration in depth to gain an optimum balance of light absorption, carrier collection, and recombination at different depths of the absorber film, leading to improved quantum efficiency. In this study, we investigate the effect of the maximum substrate temperature during film growth on the depth dependent gallium concentration. For the in-depth gallium concentration analyses, we use two techniques, covering complementary depth ranges. Angle dependent soft x-ray emission spectroscopy provides access to information depths between 20 and 470 nm, which covers the depth range of the space charge region, where most of the photoexcited carriers are generated. Therefore, this depth range is of particular interest. To complement this investigation we use secondary neutral mass spectrometry, which destructively probes the whole thickness of the absorber (%2 lm). The two methods show increasingly pronounced gallium and indium gradients with decreasing maximum substrate temperature. The probing of the complementary depth ranges of the absorbers gives a consistent picture of the in-depth gallium distribution, which provides a solid basis for a comprehensive discussion about the effect of a reduced substrate temperature on the formation of gallium gradients in Cu(In,Ga)Se 2 and the device performance of the corresponding reference solar cells.

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

“…), or by at the surface . Schmid et al , Mönig et al , as well as Bröker et al provided indications for Cu depletion and In/Ga enrichment, but not for Se depletion or Cu enrichment, at Cu(In,Ga)Se 2 thin‐film surfaces.…”

Enhanced performance of ultra-thin Cu(In,Ga)Se2 solar cells deposited at low process temperature

“…In spite of this, there has been no experimental comparison of the efficacy of back recombination passivation between a higher back Ga grading and a back passivation structure. In the 3-stage co-evaporation processes [13], lowering process temperature (substrate temperature) was proved to be able to reduce the inter-diffusion of Ga-In [14,15] and therefore provides the possibility to create a higher back Ga grading by engineering the deposition sequence of Ga-Se precursor and In-Se. At the same time, the low process temperature can enable the preparation of CIGSe solar cells on flexible polymer substrates [16,17], thus offering a potential to further reduce the manufacturing cost and widen the applications of the solar cells.…”

Hall effect and transient surface photovoltage (SPV) study of Cu3BiS3 thin films Abstract

“…original article values from 1.15 to 2.73 eV, which makes this kind of compounds potentially useful and attractive candidates for photovoltaic applications (Bar et al 2008, Kawamura et al 2009, Meiss et al 2011, Mönig et al 2011, Tadjarodi et al 2012.…”