Novel cathode buffer layer of Al(acac)3 enables efficient, large area and stable semi-transparent organic solar cells

Electrode buffer layer plays a more crucial in semi-transparent organic solar cells (ST-OSCs) than that of the opaque devices due to the additional requirements of the average visible transmittance (AVT) and color rendering index (CRI) besides the power conversion efficiency (PCE). Herein, we developed a novel cathode buffer layer Aluminum(III) acetylacetonate (Al(acac)3) via mild spin-coating with post low temperature heat treatment process. Studies show that Al(acac)3 film possess outstanding optical features and suitable energy level. Namely, the absorption of Al(acac)3 film with thickness of 10 nm is as low as 1% and the transmittance is high up to 95% in the visible and near infrared regions, guarantying full absorption of the photoactive layer and high CRI for ST-OSCs; and the suitable energy level ensures a smooth electron transport and collection process. By introducing Al(acac)3 film in ST-OSCs, the devices based on PM6:Y6 yield an efficiency of 12.41%, an AVT of 25.33% (from 370 nm to 740 nm) and a CRI of 94.6. Encouragingly, the large-area ST-OSC of 100 mm2 with Al(acac)3 buffer layer yields an efficiency of 11.28%. Further long-term stability of ST-OSCs show that the device with encapsulation retains its 60% of the initial PCE over 60 hours continuous illumination. The results indicate that Al(acac)3 is a promising cathode buffer layer for fabricating efficient, large area and stable semi-transparent organic solar cells.