The development of novel materials for use in optoelectronics applications requires two intertwined threads of research: the search for materials with the requisite photoelectric properties and efforts to find suitable ways to lay down films of the materials for the fabrication of experimental devices and ultimately commercial systems. There has always been compromise wherein weaving these two threads has to balance those electronic properties and the morphology of the materials being investigated. Now, researchers at Osaka University have developed a two-step process that can produce materials with good morphological properties in addition to excellent photoresistor performance.

Osaka chemists Ryosuke Nishikubo and Akinori Saeki have turned to bismuth sulfide (Bi2S3) in their studies. This substance belongs is one of the metal chalcogenide class of materials that are known to have promising optical and electronic properties. Unfortunately, their application comes up against compromise in that the performance of photoresponsive devices based on these materials is strongly dependent on exactly how the necessary thin film is processed. Earlier efforts to exploit bismuth sulfide have been hampered by low crystallinity in the films formed. Moreover, even for those films that have high crystallinity, the nature of the grains within often has a negative effect on performance. As such, films with low surface roughness and large grain size are desirable are very much needed, which is where the Osaka team's work comes to the fore. [R. Nishikubo and A. Saeki, J Phys Chem Lett, (2018) DOI: 10.1021/acs.jpclett.8b02218]

"We searched more than 200 materials using a unique, ultra high-speed screening method that can evaluate performance, even when only powdered samples are available," explains Saeki who is also based at the Precursory Research for Embryonic Science and Technology (PRESTO) in Saitama. "We found that bismuth sulfide, which is inexpensive and less toxic than conventional inorganic solar cell materials, can be processed in a way that does not compromise its excellent photoelectrical properties."

The team used solution spin-coating followed by crystallization to form a two-dimensional, layered film of the material in two treatment steps. They examined the photo response performance of the resulting film and found it to be at least six times better than that observed with films processed by other methods and even up to 100 times better.

"We believe that solution-processable bismuth-based semiconductors are viable alternatives to commercially available inorganic solar cells and show promise for widespread future use," adds Nishikubo. The team has also demonstrated that other metal sulfide semiconductors can be prepared as responsive films using the same approach.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.