Near Infrared Broad Band Emitting Cr3+ Phosphors for pc-LEDs

Abstract

Broad band near infrared (NIR) light sources are currently of great interest in many application areas, especially in near infrared spectroscopy, biomedical imaging and phototherapy. Commonly used NIR emitters are incandescent lamps, Globars and (Al,Ga)As light emitting diodes (LEDs). However, the NIR sources available on the market have shortcomings such as low efficiency and poor stability, that limit their application. Near infrared phosphor converted (pc)-LED is one of the most reliable alternatives for traditional NIR light sources in terms of cost and performance. An infrared pc-LED is based on a blue or near UV emitting (Ga,In)N semiconductor chip and a suitable broad band NIR emitting phosphor. In the development of new broad band NIR pc-LEDs a huge challenge is to find stable luminescent materials, that efficiently convert light of the pumping source (blue LED) into broad band NIR radiation. The temperature of high power (Ga,In)N LEDs can reach temperature up to 450 K during operation, which means that the on-chip NIR converter (phosphor) should not suffer from thermal quenching up to this temperature. One of the most promising materials for the development of NIR pc-LEDs are Cr3+ doped crystals. Due to the strong interaction of Cr3+ ions with the crystal field and lattice vibrations, the Cr3+ photoluminescence can be tailored by a variation of the crystal compositions. Depending on the crystal field of a host lattice, Cr3+ ions show narrow R-line emission (2E → 4A2) in a strong crystal field or/and broad band emission (4T2→ 4A2) in a weak crystal field. In this thesis, we focus on the synthesis and optical properties of Cr3+ phosphors emitting between 650 and 1100 nm upon excitation in the blue spectral range. Materials we investigate are borates, garnets, scandates, and phosphates, in which Cr3+ ion occupies intermediate and weak crystal field sites. The aim is to develop Cr3+ phosphors showing an efficient broad band emission with a bandwidth of ~100 nm and low emission loss at higher temperatures. The optical properties of Cr3+ ions in crystals are examined in terms of spectroscopic parameters such as crystal field strength 10Dq, Racah parameters B and C, and electron phonon-coupling parameters. Additionally, we investigate the influence of the host lattice on the energy gap between the two low-lying 2E and 4T2 excited states of Cr3+. The research provides insight into factors that contribute to thermal quenching of the 2E/4T2 emission of trivalent chromium and this can be used in the development of new efficient NIR converters for pc-LEDs. We show that a small relaxation in the excited state, i.e. a small Stokes shift has to be coupled to a relatively high energy emission of the 4T2 level to realize an efficient NIR converter. Similarly, reduction of the Stokes shift will increase the luminescence quenching temperature but at the same time narrow the spectral width of the NIR emission where a broader bandwidth is desired. Clearly, the development of the ‘ideal’ NIR broad band phosphor should involve a balance between optimization of different parameters.