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

Amorphous (<i>a</i>-) In₂O₃-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 (<i>J</i>_sc) of Cu­(In,Ga)­Se₂ (CIGS) solar cells with a glass/Mo/CIGS/CdS/TOS/TCO structure, while maintaining high fill factor (FF) and open-circuit voltage (<i>V</i>_oc). An <i>n</i>-type <i>a</i>-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 <i>a</i>-In–Ga–Zn–O layers allowed high electron mobility with superior control over the carrier density (<i>N</i>). High FF and <i>V</i>_oc values were achieved in solar cells containing <i>a</i>-In–Ga–Zn–O layers with <i>N</i> values broadly ranging from 2 × 10¹⁵ to 3 × 10¹⁸ cm^–3. The decrease in FF and <i>V</i>_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, <i>a</i>-In₂O₃:H and <i>a-</i>In–Zn–O layers exhibited higher electron mobilities than the ZnO:Al layers conventionally used as TCO layers in CIGS solar cells. The In₂O₃-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 <i>V</i>_oc of the CIGS solar cells and the mini-modules