Ga gradients in Cu(In,Ga)Se2: Formation, characterization, and consequences

Abstract

We report on the influence of the substrate temperature during the 2nd and 3rd stage of the Cu(In,Ga)Se2 3-stage co-evaporation process on the in-depth Ga and In concentrations and correlate these with the solar cell parameters and external quantum efficiency of soda-lime glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al devices. An increased homogenization of the [Ga]/[III] fraction ([III] refers to the total concentration of the group 3 elements Ga and In) with temperature is found. In the investigated temperature range, the highest efficiency was measured for the lowest temperature and the steepest Ga-profile. The tendency of the short-circuit current density matches well with the notch-deepness. Surprisingly, the open-circuit voltage decreases for higher substrate temperatures, even though the Ga-concentration in the space-charge region increases. We propose back-grading variations and reduced back-interface recombination to explain this observation. For the highest of the tested temperatures of 540 °C, a homogenization of the Ga and In concentrations close to the surface is found. We explain this by the appearance of a liquid Cu2-xSe phase at the end of stage 2 for this high temperature. Break-off experiments at this point are conducted and reveal morphological and compositional lateral inhomogeneities for Tsub < 540 °C.

Interface engineering of ultrathin Cu(In,Ga)Se2 solar cells on reflective back contacts

“…Figure 3 compares SEM cross‐section images of ultrathin CIGS layers coevaporated on Mo and RBC for substrate temperatures of 550°C and 500°C. It reveals that large and columnar CIGS grains are grown at 550°C on Mo while smaller CIGS grains are formed at 500°C, as expected with a lower deposition temperature 35 . The observed CIGS grains are smaller when CIGS is deposited on top of the RBC and similarly to the case of a Mo back contact their size also decreases for a coevaporation temperature of 500°C.…”

Improved Ga grading of sequentially produced Cu(In,Ga)Se2 solar cells studied by high resolution X-ray fluorescence

“…Besides this structural inhomogeneity including grain boundaries, 3 the absorber typically also shows compositional variations as a function of depth, e.g., a gradient of gallium. [4][5][6] The amount of Ga present in the absorber directly affects the band gap and hence the device efficiency. Furthermore, there are even chemical fluctuations on the nanometer scale which also lead to potential fluctuations 7 and Maxwell-Wagner polarization 8 while structural inhomogeneity on the sub-nanometer scale affects the material band gap.…”

Chemistry and Dynamics of Ge in Kesterite: Toward Band-Gap-Graded Absorbers

“…In CIGSe, this type of gradient leads to substantial improvements of open-circuit voltage (V oc ) and charge carrier collection. 4,5 Current efficiency levels of kesterites are below 13%, 6 because of short minority carrier lifetimes in the sub-nano-second regime. 7 Kesterite compounds like Cu 2 Zn(Sn,Ge)(S,Se) 4 as absorber for thin-film solar cells have the potential to contribute to the increase of solar energy in large volumes using earthabundant elements such as Sn and Zn.…”