Efficient Narrow Band Gap Cu(In,Ga)Se₂ Solar Cells with Flat Surface

In this study, the influences of bromine-based etching (Br etching) of narrow band gap CuInSe₂ (CIS) absorbers and Cu(In,Ga)Se₂ absorbers with various single Ga gradings (CIS:Ga) on the properties of solar cells were investigated. Absorbers with narrow absorption edge energies (E_abs) of 1.0-1.02 eV, ideal for the application as a bottom cell in a tandem device, were fabricated using a modified three-stage process and subjected to Br etching. The evolution of surface flatness and their optical and electrical properties upon Br etching were investigated. Br etching typically reduced the root-mean-square deviation of the surface roughness height (R_q) for a CIS:Ga absorber from several hundreds to several tens of nanometers, whereas for some CIS absorbers, R_q reduction was limited by the remaining voids. Moreover, Br etching reduced the leakage current across the pn junction. The high shunt resistances (R_sh) typically up to >10 kΩ·cm² were obtained by introduction of Br etching. However, etching sometimes adversely increased the V_OC deficit. The investigation of the minority carrier lifetime and diode parameters revealed that back-surface recombination in CIS and low-Ga CIS:Ga solar cells increased as the absorber layer thickness decreased. A higher Ga grading significantly reduced back-surface recombination. Narrow band gap CIGS solar cells with improved surface flatness and high V_OC were achieved by introducing Br etching and proper Ga grading.