Downconversion for the Er[super]3+, Yb[super]3+ couple in KPb sub(2Cl) sub(5) - 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 (Er[super]3+, Yb[super]3+) doped into a host with low (200 cm[super]-1) maximum phonon energy (KPb sub(2Cl) sub(5)). The intermediate energy level in both the upconversion and downconversion processes is the [super]4F sub(7/2 level around 490 nm. While fast multi-phonon relaxation to the lower energy [super]2H) sub(1)1/2/[super]4S sub(3/2 levels is beneficial for upconversion, it prevents efficient downconversion. To reduce multi-phonon relaxation, a low-phonon energy host (KPb) sub(2)Cl sub(5) was doped with Er[super]3+ and varying amounts of Yb[super]3+ co-dopant. The results show that downconversion from the [super]4F) sub(7)/2 level occurs, exciting two neighboring Yb[super]3+ ions to the [super]2F sub(5/2 level. The efficiency is however low due to multi-phonon relaxation from the [super]4F) sub(7)/2 to the [super]4S sub(3/2 level via the intermediate [super]2H) sub(1)1/2 level. Based on the results it is clear that efficient downconversion for the (Er[super]3+, Yb[super]3+) couple requires even lower phonon energy hosts (e.g. bromide host lattices). A Cl[super]--Yb[super]3+ 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[super]--Yb[super]3+ charge transfer emission.