Embedded UV Down-Conversion Layer for Organic Solar Cells with Improved Performance and Stability

High-energy photons arising from the UV component of the solar spectrum are considered as a primary cause of photoinduced degradation observed in organic solar cells (OSCs). We herein demonstrate the scheme of harvesting these high-energy photons via a UV down-conversion (UVDC) layer embedded in thin polymeric substrates as an effective strategy for simultaneously enhancing efficiency and improving the photostability of OSCs. Transparent UV-curable photopolymer resin (NOA 61, Norland, Inc.) is employed to prepare a polymer substrate and to sandwich the UVDC layer comprised of Ir(dmppy-ph)2tmd─a highly efficient yellow phosphorescent emitter, which exhibits a high degree of spectral overlap between its emission spectra and the absorption spectra of the PM6, a donor polymer used to prepare photoactive blend along with non-fullerene acceptor Y7. The PM6:Y7 bulk heterojunction OSCs fabricated on the substrates with the UVDC layer are found to show a nearly 7.2% higher power conversion efficiency (PCE). The study reveals that observed improvement in performance is essentially due to the ability of the UVDC layer to absorb some high-energy photons and make them available for absorption by the photoactive layer by down-converting them to photons in the visible region and due to the optical scattering effect that leads to a longer effective optical path length within the active layer. A study done by monitoring the PV performance as a function of UV exposure time indicates that, with the proposed UVDC approach, it is possible to achieve nearly a 3-fold enhancement in T97, or the time it takes for PCE to become 97% of the initial value, by alleviating the UV-induced photochemical degradation. Statistical analysis is also performed for the wide range of sample distributions, verifying the reliability and repeatability of the results.