[18p-P02-1~30] 6.3 Oxide electronics

English Presentation

[18p-P02-30] Modification of band alignment of Zn_1-xSn_xO alloy buffer/Cu(In,Ga)Se₂ for high- efficiency Cu(In,Ga)Se₂ solar cell

〇(P)CheukKaiGary Kwok¹, Genchi Inohana¹, Yuta Sato¹, Muhammad Monirul Islam¹, Takeaki Sakurai¹ (1.Tsukuba Univ.)

Keywords:

Cu(In,Ga)Se2,Zinc-Tin-oxide,optoelectronic properties

In this work, we explore the optoelectronic properties of Zn_1-xSn_xO alloys with varying composition x in parallel with optimization of their growth conditions and post-annealing treatment. In particular, Zn_1-xSn_xO films were first grown by room temperature RF co-sputtering on both Si and glass substrate using ceramic ZnO and SnO₂ targets, where the Sn content x is controlled by adjusting the sputter power of each target. Annealing at elevated temperatures is also performed to study the improvement on film crystallinity and conductivity. We found that all as-grown films are amorphous/nanocrystalline, but Zn-rich and Sn-rich films crystallize into wurtzite ZnO and rutile SnO₂ phase, respectively, after 300^oC annealing in N₂ for 1h, while films with mid-compositions of 0.15<x<0.55 remain amorphous even after annealing at 500^oC. Nevertheless, a low n-type resistivity in the order of low 10^-2 W-cm can be achieved for the film with 0.3<x<0.4. This is possibly ascribed to the formation of energetically favorable complex donor defects near the conduction band edge (CBE). Moreover, a high transparency of ~90% with a wide transmission window of >2500 nm is achieved for all films, while the bandgap varies from 3.7 eV to 3.1 eV with a small positive bowing, as confirmed by spectroscopic ellipsometry measurement. Since this bandgap variation is mainly due to the shift in CBE as measured and obtained by UV photoelectron spectroscopy and synchrotron x-ray spectroscopy at KeK, we expect that the valence band edge can also be tuned due to p-type nature of SnO. With the modification of CBE, the band offset of buffer to our CIGSe absorber can be alternated from type II to type I, so that the interfacial recombination can be diminished.