Sol-gel processed niobium oxide thin film suitable for a scaffold layer of perovskite solar cells

Increasing stringent mitigation measures on the use of fossil fuels have favored the development of renewable energy as an alternative energy source. This has resulted in a keen interest in solar energy. In particular, organic-inorganic halide perovskite solar cells have increasingly gained popularity as a low-cost energy device since their interception more than a decade ago. For instance, a relatively high increase in power conversion efficiency has recently improved to 25.2% up from 3.81%. This can be attributed to their excellent optoelectronic properties and high mobility charge carriers. However, there is still a great need to develop more efficient strategies for improving the performance of the devices and utilizing the full potential of perovskite materials through fabricating highly homogenous and crystalline planar perovskite thin-films.

Presently, several processes for synthesizing perovskite materials have been developed. These processes majorly involve the use of different solvents, addition of additives and varying the compositions. Despite the good progress, optimizing the crystallization and formation of thin-film kinetics of the perovskite materials has remain a big challenge. Recently, substrate surface modification has been identified as a promising approach for effective control of the perovskite films. Specifically, the scaffold layer of the perovskite film is of great significance in enhancing crystallization and morphological evolutions as well as ensuring selective extraction of the photo-excited electrons. Among the available metal oxides, niobium oxide-based scaffold layer has exhibited higher blocking effects as compared to other metal oxides such as titanium oxide. This has further led to its application in perovskite solar cells. Despite the effectiveness of the currently available methods for niobium oxide films fabrication such as magnetron sputtering, the development of more cost-effective methods is still highly desirable.

To this end, Hosei University researchers: Dr. Eiichi Inami (Associate Professor at Kochi University of Technology at present), Professor Takamasa Ishigaki, and Professor Hironori Ogata investigated the optoelectronic and morphological properties as well as the performance of a niobium oxide scaffold layer synthesized using the sol-gel methods. The main objective was to decrease further the processing cost, much lower than that of the magnetron sputtering technique. Fundamentally, they fabricated three different types of film: niobium oxide, m-titanium oxide, and compact titanium oxide film. Eventually, they compared their morphological and optoelectronic properties using various characterization techniques such as scanning electron microscopy and X-ray diffraction. The work is currently published in the research journal, Thin Solid Films.

The authors observed that the niobium oxide scaffold layer facilitated efficient crystallization of the perovskite film as compared to other conventional mesoporous titanium oxide scaffold layers thus may lead to the high performance of the perovskite solar cell. This was attributed to the high crystallization of the perovskite film and efficient electron transport layer. Generally, the sol-gel processed niobium oxide scaffold layers are a promising solution for design and fabrication of high performance and cost-effective perovskite solar cells. Furthermore, the study by Kochi University of Technology and Hosei University scientists provides essential information that will pave way for the synthesis of the various structure resulting from the niobium oxide i.e. nanotube, nanosheet, and nanochannel, which may, in turn, be used as scaffold layers to enhance the device performance.