Cu(In,Ga)Se₂ Solar Cells with Amorphous In₂O₃-Based Front Contact Layers

Amorphous ( a -) In 2 O 3 -based front contact layers composed of transparent conducting oxide (TCO) and transparent oxide semiconductor (TOS) layers were proved to be effective in enhancing the short-circuit current density ( J sc ) of Cu(In,Ga)Se 2 (CIGS) solar cells with a glass/Mo/CIGS/CdS/TOS/TCO structure, while maintaining high fill factor (FF) and open-circuit voltage ( V oc ). An n -type a -In–Ga–Zn–O layer was introduced between the CdS and TCO layers. Unlike unintentionally doped ZnO broadly used as TOS layers in CIGS solar cells, the grain-boundary(GB)-free amorphous structure of the a -In–Ga–Zn–O layers allowed high electron mobility with superior control over the carrier density ( N ). High FF and V oc values were achieved in solar cells containing a -In–Ga–Zn–O layers with N values broadly ranging from 2 × 10 15 to 3 × 10 18 cm –3 . The decrease in FF and V oc produced by the electronic inhomogeneity of solar cells was mitigated by controlling the series resistance within the TOS layer of CIGS solar cells. In addition, a -In 2 O 3:H and a- In–Zn–O layers exhibited higher electron mobilities than the ZnO:Al layers conventionally used as TCO layers in CIGS solar cells. The In 2 O 3 -based layers exhibited lower free carrier absorption while maintaining similar sheet resistance than ZnO:Al. The TCO and TOS materials and their combinations did not significantly change the V oc of the CIGS solar cells and the mini-modules.