Atomic Layer Deposition of Vanadium Oxide to Reduce Parasitic Absorption and Improve Stability in n-i-p Perovskite Solar Cells for Tandems

Two critical issues associated with semi-transparent, n-i-p perovskite solar cells for 2-terminal tandem devices are parasitic absorption and long-term instability associated with the widely used spiro-OMeTAD and MoO_x hole transport and buffer layers, respectively. In this work, we present an alternative hole contact bilayer that consists of a 30 nm undoped layer of spiro-TTB in conjunction with 9 nm of air-stable vanadium oxide (VO_x) deposited via atomic layer deposition. The low absorption of UV and visible light in this bilayer results in the fabrication of a semi-transparent perovskite cell with 18.9 mA cm^-2 of photocurrent, a 14% increase compared to the 16.6 mA cm^-2 generated in a control device with 150 nm of doped spiro-OMeTAD. The ALD VO_x buffer layer shows promise as a stable alternative to MoO_x; an unencapsulated Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃ device with ALD VO_x and ITO as the top contact maintains its efficiency following 1000 hours at 85 °C in a N₂ environment. Lastly, we use transfer matrix modeling of the optimized perovskite stack to predict its optical performance in a monolithic tandem cell with heterojunction silicon.