Cu(In,Ga)Se₂ solar cell grown on flexible polymer substrate with efficiency exceeding 17%

Abstract

Development of a Cu(In,Ga)Se 2 thin film solar cell on a polyimide film with a conversion efficiency of 17.1%, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process.

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

“…The computed a*(Na) values form two groups, confirming our earlier conclusion that (at least) two different Na species would be present at the CIGSe sample surfaces. The a*(Na) values of "species (1)" ["species (2) 6,43 ] or NaF. Although in the latter case, a*(Na) of species (2) agrees with the upper limit of the reported a*(Na) values of NaF, we can exclude this contribution due to the absence of any F-derived XPS/XAES signal (see discussion above).…”

“…Recently, the efficiency of CIGSe/Mo/ PI-based solar cells could be improved to 18.7%. 2 The small efficiency difference compared to the highest CIGSe-based devices on rigid glass substrates (g > 20%, 3 ) can be explained by the process temperature which needs to be well below 500 C when PI is used as a substrate. 4 In comparison high-efficiency devices on soda-lime glass substrates are typically obtained using growth temperatures of $550 C. Furthermore, it is well known that in a conventional sodalime glass substrate based CIGSe solar cell structure, Na from the soda-lime glass substrate diffuses into the absorber layer due to the elevated temperature during CIGSe formation.…”

“…[5][6][7][8] Na incorporation into the CIGSe absorber results in a significant improvement of solar cell efficiency, [9][10][11][12] and so for devices based on alternative (i.e., Na-free) substrates, Na must be added deliberately as part of the solar cell manufacturing process. Na can be incorporated prior to, 13,14 during, 12,15 or after 2,16 CIGSe growth. (Note that the latter approach requires a subsequent annealing process.)…”

Grading and metastable effects in admittance spectroscopy of CIGS-based solar cells

“…Under reverse measurement voltages, the intersection of the Fermi level with this energy level occurs in a region where the acceptor configuration is present instead of the donor configuration, and the defect level can not be detected. The change in defect level energy can be explained by the band gap grading, due to a large Ga grading in the sample near the interface [4]. The defect density for measurements with an initial state at 0.0V is much larger than for the measurements with a reverse voltage during the initial state.…”

“…Some of these are even intentionally introduced, e.g. band gap grading by varying the In/Ga ratio [4]. Hence it is interesting to measure the defect density at various positions in the absorber.…”