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

Abstract

Amorphous (a-) InO-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) of Cu(In,Ga)Se (CIGS) solar cells with a glass/Mo/CIGS/CdS/TOS/TCO structure, while maintaining high fill factor (FF) and open-circuit voltage (V). 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 values were achieved in solar cells containing a-In-Ga-Zn-O layers with N values broadly ranging from 2 × 10 to 3 × 10 cm. The decrease in FF and V 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-InO: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 InO-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 of the CIGS solar cells and the mini-modules.

Impact of front contact layers on performance of Cu(In,Ga)Se2 solar cells in relaxed and metastable states

“…Furthermore, the electron affinities and band gaps of amorphous TOS films can be controlled by choosing appropriate posttransition metals and compositions . Therefore, when incorporated in the front contact layers, In 2 O 3 ‐based materials can potentially generate higher conversion efficiency than ZnO‐based materials, and we demonstrate this concept with a ‐In‐Ga‐Zn‐O ( a ‐IGZO)/ a ‐In 2 O 3 :H front contact layers . Furthermore, several experimental studies revealed that In 2 O 3 ‐based TCOs reduced the electrical loss caused by series resistance and optical loss because of free carrier absorption within the TCO layer while improving the stability of solar cells and mini‐modules.…”

“…A 50‐nm‐thick a ‐IGZO layer was deposited with no intentional substrate heating via rf magnetron sputtering deposition using an In‐Ga‐Zn‐O (In:Zn:Ga = 1:1:1) ceramic target with an O 2 /(Ar + O 2 ) flow ratio of 0.08%. In 2 O 3 :H films with thicknesses of 50, 70, and 150 nm were deposited with no intentional substrate heating via in‐line reactive plasma deposition using In 2 O 3 ceramic tablets and Ar, O 2 , and H 2 O gases at a total pressure of ca. 0.4 Pa and an H 2 O partial pressure of ca.…”

“…The O 2 /(Ar + O 2 ) flow ratios were 24% for 50 and 70‐nm‐thick films and 30% for 150‐nm‐thick films. The electrical properties of the TOS and TCO films are described elsewhere . The sheet resistance of ZnO:Al, ZnO:B (160°C), ZnO:B (170°C), a ‐In 2 O 3 :H (50 nm), a ‐In 2 O 3 :H (70 nm), and a ‐In 2 O 3 :H (150 nm) grown on SLG was 39, 19, 15, 69, 51, and 27 Ω sq −1 , respectively.…”

“…The sheet resistance of ZnO:Al, ZnO:B (160°C), ZnO:B (170°C), a ‐In 2 O 3 :H (50 nm), a ‐In 2 O 3 :H (70 nm), and a ‐In 2 O 3 :H (150 nm) grown on SLG was 39, 19, 15, 69, 51, and 27 Ω sq −1 , respectively. Figure shows optical constants (refractive index [ n ] and extinction coefficient [ k ]) of the TOS and TCO layers evaluated from spectroscopic ellipsometry . To evaluate the optical constants, the TOS and TCO layers were deposited on SiO 2 ‐coated Si.…”

“…Optical constants of ZnO:Al, ZnO:B (160°C), a ‐In 2 O 3 :H, ZnO, a ‐IGZO, and CdS layers [Colour figure can be viewed at wileyonlinelibrary.com]…”