Composition and bandgap control in Cu(In,Ga)Se₂ -based absorbers formed by reaction of metal precursors

Abstract

The control of composition and bandgap in chalcopyrite thin-film absorber layers formed by a metal precursor reaction is addressed. Two processes using reaction with either H 2 Se or H 2 S as the final step of a three-step reaction process were compared as follows: a three-step H 2 Se/Ar/H 2 S reaction and a three-step H 2 Se/Ar/H 2 Se reaction. In both processes, significant Ga homogenization was obtained during the second-step Ar anneal, but the third-step selenization resulted in Ga depletion near the Cu(InGa)Se 2 surface, whereas the third-step sulfization did not. Solar cells were fabricated using absorbers formed using each method, and the surface Ga depletion significantly affected device performances. The solar cell incorporating the sulfization yielded a better device performance, with an efficiency of 14.4% (without an anti-reflection layer) and an open-circuit voltage of 609 mV. The bandgap control in the metal precursor reaction is discussed in conjunction with the device behavior.

Deposition Technologies of High-Efficiency CIGS Solar Cells: Development of Two-Step and Co-Evaporation Processes

“…Figure 7b shows the absolute values of the first derivative of EQE with respect to the wavelength, namely |dEQE/dλ|. The bandgap energies of the CIGS films were estimated from the local maxima of |dEQE/dλ| [30][31][32]. The local maxima of |dEQE/dλ| determined by using a quadratic fit were at the wavelengths of 1072 nm, 1073 nm, and 1082 nm for the CIGS films prepared by the single-stage, bi-layer, and three-stage processes, respectively, as illustrated in Figure 7a [33].…”

A highly efficient Cu(In,Ga)(S,Se)2 photocathode without a hetero-materials overlayer for solar-hydrogen production

“…Moreover, CuInSe 2 ’s formation temperature is relatively lower than that of CuInGaSe 2 31 . Since the bandgap increase is directly correlated with increases in Ga and S, the bottom layer should have a bandgap higher than the middle layer 22–24,32 . On the other hand, the layer near surface also has a higher bandgap due to the high content of S, which was achieved by continuing sulfurization even after selenization 33–36 .…”

Investigation of Surface Sulfurization in CuIn1−xGaxS2−ySey Thin Films by Using Kelvin Probe Force Microscopy

“…This was mainly due to a significant increase in open‐circuit voltage ( V OC ) in the S‐rich film device. Several studies have shown that the sulfurization of CIGSe films substantially enhances the V OC and fill factor ( FF ) of solar cell devices compared to selenization‐only CIGSe cells . The increase in the band gap at the surface of the absorber layer (the absorber/buffer layer interface) was attributed to a reduction in recombination at the interface resulting in the increase in the V OC and the fill factor of sulfur‐incorporated CIGSSe solar cells .…”

“…its direct band gap and high absorption coefficient) . CIGSSe also has a band gap that is easily tunable by controlling its elemental ratios, which is helpful for both band‐gap grading of the absorber film and tandem solar cell design …”