Wide‐gap (Ag,Cu)(In,Ga)Se₂ solar cells with different buffer materials—A path to a better heterojunction

Abstract

This contribution concerns the effect of the Ag content in wide‐gap AgwCu1‐wIn1‐xGaxSe2 (ACIGS) absorber films and its impact on solar cell performance. First‐principles calculations are conducted, predicting trends in absorber band gap energy (Eg) and band structure across the entire compositional range (w and x). It is revealed that a detrimental negative conduction band offset (CBO) with a CdS buffer can be avoided for all possible absorber band gap values (Eg = 1.0–1.8 eV) by adjusting the Ag alloying level. This opens a new path to reduce interface recombination in wide‐gap chalcopyrite solar cells. Indeed, corresponding samples show a clear increase in open‐circuit voltage (VOC) if a positive CBO is created by sufficient Ag addition. A further extension of the beneficial compositional range (positive CBO at buffer/ACIGS interface) is possible when exchanging CdS with Zn1‐ySnyOz, because of its lower electron affinity (χ). Nevertheless, the experimental results strongly suggest that at present, residual interface recombination still limits the performance of solar cells with optimized CBO, which show an efficiency of up to 15.1% for an absorber band gap of Eg = 1.45 eV.

Thermodynamic stability, phase separation and Ag grading in (Ag,Cu)(In,Ga)Se2 solar absorbers

“…The absorber lms were deposited following a three-stage co-evaporation process described in our previous works. 5,8,50 In short, the elemental precursors were co-evaporated and deposited on two types of substrates, i.e. standard soda-lime glass (SLG) and high-strainpoint K-rich/Na-poor glass (KRG), in each case coated with DC-sputtered Mo back contact layers.…”

“…6 In addition, shiing band edges by alloying allows tailoring band offsets at the absorber/buffer interface to minimize interface recombination. 7,8 From the other side, formation of a compositional gradient can further promote photogenerated carrier separation. An effective utilization of these effects is best exemplied by CIGS itself, which combines optimum [Ga]/([Ga] + [In]) (GGI) level of 0.2-0.4 in the main absorbing region and GGI grading promoting carrier selectivity at the back surface to achieve efficiencies over 20%.…”

“…With the addition of Ag, it was made possible to fabricate wide band gap ACIGS-based solar cells that had reduced interface recombination and smaller V oc decit as compared to the CIGS devices with high GGI. 5,6,8 For single-junction ACIGS devices, the optimum performance has been achieved at [Ag]/([Ag] + [Cu]) z 0.2 and GGI z 0.4, with recently reported efficiencies of 19.9% 21 and 21%. 5 Among wide-gap ACIGS absorbers for tandem applications, prominent results in terms of efficiency are 13.0% for a band gap of 1.6 eV, 22 14.8% for a band gap of 1.5 eV, 6 and 15.1% for a band gap of 1.45 eV.…”

“…5 Among wide-gap ACIGS absorbers for tandem applications, prominent results in terms of efficiency are 13.0% for a band gap of 1.6 eV, 22 14.8% for a band gap of 1.5 eV, 6 and 15.1% for a band gap of 1.45 eV. 8 One explanation for these improvements is that co-alloying Ga and Ag allows engineering conduction band offset at the absorber/buffer interface in addition to the band gap tuning achieved by the Ga-In intermixing. 8 Furthermore, since Ag-based ternary chalcopyrites have lower melting temperatures than their Cu-based counterparts, 23 Ag alloying is also considered as a way to promote crystallisation of the absorbers and open a realm for developing lower-temperature routes for fabricating solar cells on exible polymeric substrates.…”

“…8 One explanation for these improvements is that co-alloying Ga and Ag allows engineering conduction band offset at the absorber/buffer interface in addition to the band gap tuning achieved by the Ga-In intermixing. 8 Furthermore, since Ag-based ternary chalcopyrites have lower melting temperatures than their Cu-based counterparts, 23 Ag alloying is also considered as a way to promote crystallisation of the absorbers and open a realm for developing lower-temperature routes for fabricating solar cells on exible polymeric substrates. 24 For alloying to be effective in tuning the semiconductor properties, the relevant alloy compositions should be (a) miscible during the deposition and (b) stable (either kinetically or thermodynamically) during the post-deposition processing and/or storage.…”