Solar cells based on n+-AZO/p-BaSi2 heterojunction: Advanced opto-electrical modelling and experimental demonstration

Abstract

We performed advanced opto-electrical simulations on thin-film BaSi 2 solar cells. First, absorption spectra of BaSi 2 - pn homojunction solar cells on Si substrate were calculated based on flat and/or pyramidally-textured surfaces, wherein 20-nm-thick n + -BaSi 2 was the topmost electron transport layer. By changing the front surface structure from flat to texture, the reflectance decreased in the wavelength ( λ ) range 700–1200 nm and the photocurrent density ( J ph ) delivered by the photogenerated carriers in the 500-nm thick p -BaSi 2 layer increased by 1.2 mA/cm 2 . Simulations revealed that the key factor inhibiting light absorption in the p -BaSi 2 layer was parasitic absorption in the n + -BaSi 2 and in the c-Si substrate. To solve these optical issues, we propose a new device structure, Al-doped n + -ZnO (AZO, 50 nm)/ i -ZnO (20 nm)/ p -BaSi 2 (500 nm) heterojunction solar cell (HJSC). In this device structure, the parasitic absorption reduced drastically, and J ph reached 30.23 mA/cm 2 . Furthermore, by replacing the Si substrate with a glass substrate, the light trapping worked more effectively, and the absorber layer thickness required for J ph to saturate was reduced to 1 μm, yielding 32.06 mA/cm 2 . Based on these simulation results, we manufactured n + -AZO/ p -BaSi 2 HJSC. The internal quantum efficiency exceeded 30% at λ = 600 nm, meaning that we demonstrated the operation of n + -AZO/ p -BaSi 2 HJSC for the first time. We investigated origins of small efficiencies compared to those simulated, and found that the passivation of defects in the p -BaSi 2 layer and the reduction of carrier recombination at the i -ZnO/ p -BaSi 2 interface would significantly improve the solar cell performance. This paper presents n + -AZO/p-BaSi 2 heterojunction solar cells (HJSC) to solve parasitic absorption in the n + -BaSi 2 and in the c-Si substrate of a BaSi 2 -pn homojunction solar cell. By replacing the Si substrate with a glass substrate, the light trapping works more effectively, and the absorber layer thickness required for photocurrent density to saturate can be reduced to 1 μm, yielding 32.06 mA/cm 2 . We manufactured n + -AZO/p-BaSi 2 HJSC, and demonstrated its operation for the first time. • We performed opto-electrical simulation on BaSi 2 pn-junction solar cells with flat and/or pyramidally-textured structure. • Parasitic absorption in the topmost layer significantly decreases the photocurrent density. • A new device structure, Al-doped n-ZnO/p-BaSi 2 , is proposed and demonstrated. • The importance of the carrier accumulation reduction at the ZnO/BaSi 2 interface is discussed.