Tag: optoelectronic properties

HZB patent for semiconductor characterisation goes into serial production

HZB patent for semiconductor characterisation goes into serial production

An HZB team has developed together with Freiberg Instruments an innovative monochromator that is now being produced and marketed. The device makes it possible to quickly and continuously measure the optoelectronic properties of semiconductor materials with high precision over a broad spectral range from the near infrared to the deep ultraviolet. Stray light is efficiently suppressed. This innovation is of interest for the development of new materials and can also be used to better control industrial processes.

Electronics, power electronics, light-emitting diodes, sensors, photocatalysis and photovoltaics – these technologies are based on semiconductors with band gaps ranging from the near infrared to the deep ultraviolet. New semiconductor materials with interesting optoelectronic properties are constantly being developed. In particular, the spectrally dependent photoelectric characterisation of semiconductor materials requires light sources whose photon energy can be continuously varied. Such light sources consist of a lamp, which emits light over a broad spectral range, and a monochromator, which filters out light in narrow spectral ranges. Until now, only diffraction grating monochromators have been used commercially, requiring up to five different diffraction gratings to cover a wide spectral range.

Mirrorless double prism monochromator

At the HZB, a team led by Dr. Thomas Dittrich, in collaboration with HEREON, has now developed a mirrorless double prism monochromator based on fused silica (quartz glass). Since fused silica is transparent in a spectral range from about 0.4 to over 7.3 eV, light can be spectrally dispersed over this range with just one fused silica prism. A first prototype was realised together with Freiberg Instruments. The novel, now patented, monochromator consists of a fused silica optics with two prisms and some lenses, where in addition to the dispersion-dependent rotation of the prisms, a precise adjustment of the lenses is done via stepper motors. A laser-driven xenon lamp provides high light intensities even in the deep ultraviolet.

Fast characterisation

The new monochromator makes it possible to determine the optoelectronic and optical properties of semiconductor materials in a single continuous measurement over a very wide spectral range from the near infrared to the deep ultraviolet. An additional advantage: stray light is suppressed very strongly (by more than eight orders of magnitude), which makes the monochromator particularly suitable for the photoelectric characterisation of defects in semiconductors. Due to its high intensity even in the deep ultraviolet, the monochromator is also excellently suited for the characterisation of semiconductor materials with wide or ultra-wide band gaps, such as silicon carbide and gallium oxide for high-performance electronics, diamond for IT technologies and gallium nitride for optoelectronics. With the new compact monochromator, for example, it is now possible for the first time to characterise defect states across almost the entire band gap of aluminium nitride in just a few minutes.

Read more on HZB website

Image: The patented monochromator consists of quartz glass optics with two prisms and a few lenses. The picture shows the central slit, the intermediate lens and the second prism with green reflections in the background.

Credit: T. Dittrich / HZB

A simpler way to inorganic perovskite solar cells

A simpler way to inorganic perovskite solar cells

Inorganic perovskite solar cells made of CsPbI3 are stable over the long term and achieve good efficiencies. A team led by Prof. Antonio Abate has now analysed surfaces and interfaces of CsPbI3 films, produced under different conditions, at BESSY II. The results show that annealing in ambient air does not have an adverse effect on the optoelectronic properties of the semiconductor film, but actually results in fewer defects. This could further simplify the mass production of inorganic perovskite solar cells.

Metal halide perovskites have optoelectronic properties that are ideally suited for photovoltaics and optoelectronics. When they were discovered in 2009, halide perovskites in solar cells achieved an efficiency of 3.9 per cent, which then increased extremely fast. Today, the best perovskite solar cells achieve efficiencies of more than 26 per cent. However, the best perovskite semiconductors contain organic cations such as methylammonium, which cannot tolerate high temperatures and humidity, so their long-term stability is still a challenge. However, methylammonium can be replaced by inorganic cations such as Cesium (Cs). Inorganic halide perovskites with the molecular formula CsPbX3 (where X stands for a halide such as chloride, bromide and iodide) remain stable even at temperatures above 300 °C. CsPbI3 has the best optical properties for photovoltaics (band gap ∼1.7 eV).

Production in glove boxes

Perovskite semiconductors are produced by spin coating or printing from a solution onto a substrate and are typically processed in glove boxes under a controlled atmosphere: There, the solvent is evaporated by heating, after which a thin layer of perovskite crystallizes. This ‘controlled environment’ significantly increases the cost and complexity of production.

…or ambient conditions

In fact, CsPbI3 layers can also be annealed under ambient conditions without loss or even with an increase in efficiency of up to 19.8 per cent, which is even better than samples annealed under controlled conditions.

What happens at the interfaces?

“We investigated the interfaces between CsPbI3 and the adjacent material in detail using a range of methods, from scanning electron microscopy to photoluminescence techniques and photoemission spectroscopy at BESSY II,” says Dr. Zafar Iqbal, first author and postdoctoral researcher in Antonio Abate’s team.

Read mpre on HZB website

Image: Under the scanning electron microscope, the CsPbI3 layer (large blocks in the upper part of the image) on the FTO substrate looks almost exactly the same after annealing in ambient air as after annealing under controlled conditions.

Credit: HZB