Downconversion for the Er3+, Yb3+ couple in KPb2Cl5: a low-phonon frequency host

Downconversion of a single blue/green photon to two near-infrared photons offers a promising route to increase the efficiency of photovoltaic cells. Here we report on downconversion for the well-known upconversion couple (Er3+, Yb3+) doped into a host with low (∼200 cm−1) maximum phonon energy (KPb2Cl5). The intermediate energy level in both the upconversion and downconversion processes is the 4F7/2 level around 490 nm. While fast multi-phonon relaxation to the lower energy 2H11/2/4S3/2 levels is beneficial for upconversion, it prevents efficient downconversion. To reduce multi-phonon relaxation, a low-phonon energy host (KPb2Cl5) was doped with Er3+ and varying amounts of Yb3+ co-dopant. The results show that downconversion from the 4F7/2 level occurs, exciting two neighboring Yb3+ ions to the 2F5/2 level. The efficiency is however low due to multi-phonon relaxation from the 4F7/2 to the 4S3/2 level via the intermediate 2H11/2 level. Based on the results it is clear that efficient downconversion for the (Er3+, Yb3+) couple requires even lower phonon energy hosts (e.g. bromide host lattices). A Cl−–Yb3+ charge transfer absorption band is observed between 300 and 400 nm. Excitation in this band results in two broad emission bands centered around 430 and 700 nm at temperatures below 30 K, which are assigned to Cl−–Yb3+ charge transfer emission.