Photovoltaic properties of n-type amorphous In–Ga–Zn–O and p-type single crystal Si heterojunction solar cells: Effects of Ga content

Abstract: Photovoltaic properties and electronic structures of n-type amorphous In–Ga x –Zn–O/p-type Si heterojunction solar cells (x =1, 2, and 3) were investigated focusing on the effects of Ga content based on expectation that Ga-rich films have larger band gaps and improve open circuit voltages (VOC) of solar cells. To know the electronic structures such as the conduction band minimums (CBMs) and the valence band maximums (VBMs) of these materials, hard x-ray photoemission spectroscopy (HX-PES) was performed. Contrary to the above expectation, the best result was obtained for x =1 with an energy conversion efficiency of 5.3%. Although the Ga-rich films had larger optical band gaps and higher CBMs, VOC were remained low and poorer fill factors were obtained due to larger densities of defects. The low VOCwere partly resulted from the deep VBM levels of the Ga-rich films. The defect densities are discussed in relation also to near-VBM states and near-CBM states observed in HX-PES and subgap optical absorptions. [Copyright &y& Elsevier]