Tunable excitation source for characterization of perovskite nanocrystals

A team of researchers in Professor A. S. Urban’s Nanospectroscopy Group at the Ludwig-Maximilians University in Munich is researching the optical and electronic properties of novel semiconductor materials.

To gain insight into the properties of perovskite nanocrystals, the team used an NKT Photonics supercontinuum laser, the SuperK EXTREME/FIANIUM with built-in pulse picker, in combination with the spectral filter, SuperK VARIA, as a tunable excitation source.

Perovskite nanocrystals boast high photoluminescence quantum yield

Perovskite nanocrystals have photoluminescence quantum yields as high as 99% despite a very cheap and easy synthesis which make them an ideal candidate for future LEDs and other light-emitting applications.

Bandgap tuning enables emission wavelengths in the visible spectrum

Another exciting aspect is the possibility of bandgap tuning by changing the size and composition of the nanocrystals. This allows for emission wavelengths in the whole visible spectrum but creates the need for different excitation wavelengths. Here, the wavelength tunability of the SuperK EXTREME/FIANIUM in combination with the SuperK VARIA spectral filter proves very useful.

Access to information on individual nanocrystals

In the first setup, the team excited atoms to create charge carriers in the nanocrystal. Then, they applied different measurement techniques to collect photoluminescence with high magnification objectives.

The team obtained a high spatial resolution that enabled them to collect the signals of individual nanocrystals normally inaccessible when measuring large nanocrystal ensembles.

 

Photoluminescence decay curves help investigate light emission behavior

The team used the pulsed nature of the laser and the built-in pulse picker to conduct Time-Correlated Single Photon Counting (TCSPC) measurements to form photoluminescence decay curves. The decay curves reveal important information about the fundamental decay mechanisms which are responsible for the nanocrystals’ light emission behavior.

The team also investigated nanocrystal thin films mounted in a cryostat to acquire all the described data in dependence of temperature.

Diffusion processes between adjacent nanocrystals

In a second setup, the team excited nanocrystal films at one spot while collecting photoluminescence from another spot. This technique reveals diffusion process information mainly governed by transfer processes between adjacent nanocrystals. This knowledge is also crucial for future applications exploiting the remarkable properties of perovskite nanocrystals.

 

Advantages of the SuperK laser and filter

The use of a SuperK EXTREME/FIANIUM laser in different setups is enabled by the easy mounting of the laser output fiber in the collimator holders mounted in the different setups.

The SuperK laser is fully modular, allowing easy operation and service where accessory modules can be added without configuration – all plug & play. Operation is simple and functions can be changed on-the-fly without powering down the system.

The SuperK VARIA filter turns the SuperK laser into a powerful single-line laser with a 440 nm tuning range and variable bandwidth. The center wavelength of the pass band can be tuned anywhere between 400 and 840 nm and the bandwidth is variable between 10 and 100 nm.

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Other work on perovskites

Verena A. Hintermayr et al, Polymer Nanoreactors Shield Perovskite Nanocrystals from Degradation, July 19 2019

 

About perovskites

Perovskites are a vast number of compounds with a perovskite crystal structure using the chemical formula: ABX3 where A and B are cations (e.g. CH3NH3 and Pb), and X is an anion (e.g. I, Cl or O).

Perovskites are typically direct bandgap materials that makes them useful for sensors, solar cells, LEDs, and lasers. Changing the crystal size and ratios of the elements tune the properties, such as bandgap, and allow for tandem layers to get optimum solar absorption.

The first demonstration of a crystalline perovskite solar cell was in 2012. Since then, perovskite has exceeded 20% and even beaten Si in tandem devices.